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ARTICLE IN PRESS WAT E R R E S E A R C H
40 (2006) 3375 – 3384
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Metal-doped carbon aerogels as catalysts during ozonation processes in aqueous solutions M. Sa´nchez-Poloa,b, J. Rivera-Utrillab, U. von Guntena, a
Swiss Federal Institute of Aquatic Science and Technology (EAWAG), Ueberlandstrasse, 133, CH-8600, Du¨bendorf, Switzerland Departamento de Quı´mica Inorga´nica, F. Ciencias, Universidad de Granada, 18071, Granada, Spain
b
art i cle info
A B S T R A C T
Article history:
The efficiency of Co(II)-, Mn(II)-, and Ti(IV)-doped carbon aerogels for the transformation of
Received 13 March 2006
ozone into dOH radicals was investigated. The carbon aerogels had a markedly acid surface
Received in revised form
character (pHPZCffi3–4) with very high surface oxygen concentrations (Offi20%). X-ray
13 July 2006
photoelectron spectroscopy (XPS) analyses of the samples showed the oxidation state of
Accepted 19 July 2006
the metals was +2 for Co and Mn and +4 for Ti. The presence of Mn(II)-doped carbon aerogel
Available online 12 September 2006
enhanced ozone transformation into dOH radicals, whereas the presence of Co(II) and Ti(IV)
Keywords:
carbon aerogels presented no activity in this process. Moreover, it was observed that an
Ozonation
increase in the concentration of Mn in the surface of the aerogel increases its efficiency to
Carbon aerogels
transform ozone into dOH radicals, with an Rct value ([OH]/[O3]) of 5.36 108 for the
Transition metals
aerogel doped with 16% of surface Mn(II) compared to an Rct of 2.68 109 for conventional
p-chlorobenzoate (pCBA)
ozonation. Regardless of the aerogel used, XPS analysis of the ozonated aerogel samples
d
OH radicals
showed an increase in the concentration of surface oxygen when the exposure to ozone was longer. However, presence of oxidized metal species after ozone treatment was only detected in the case of the Mn-doped aerogel, (Mn(III) and Mn(IV)). CO2 activation of carbon aerogel produced a marked increase in its efficiency to transform ozone into dOH radicals compared with non-activated sample. The efficiency of Mn activated carbon aerogel to transform ozone into dOH radicals was greater than that of Witco commercial activated carbon or H2O2 in the ozonation of water from Lake Zurich (Zurich, Switzerland). & 2006 Elsevier Ltd. All rights reserved.
1.
Introduction
Rising concerns about water quality have prompted the investment of considerable human and financial resources by public and private agencies in the development of new and effective water treatment processes to remove organic micropollutants that pose a threat to human health. Their removal by the use of ozone was proposed (von Gunten, 2003a, b; Hoigne´, 1998) because of its high reactivity with certain organic compounds. However, this process is limited by the chemical kinetics and by the generation of degradation products (Rivera-Utrilla et al., 2002; Zaror, 1997). Advanced Corresponding author. Tel.: +41 44 8235270; fax: +41 44 8235210.
E-mail address: [email protected] (U. von Gunten). 0043-1354/$ - see front matter & 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2006.07.020
oxidation processes (AOPs) were developed to increase the degree of transformation of ozone-resistant micropollutants by the way of dOH radical oxidation (Brunuy et al., 1994; Paillard et al., 1988; Prengle and Mank, 1978). These processes can be based on the transformation of ozone into dOH radicals that which attack most organic compounds with constants of 108–1010 M1 s1. However, because of their low selectivity a large fraction of the dOH radicals will be lost to the water matrix. A further development in AOPs resulted from the addition of transition metals (Mn, Co, Zn, Fe) to the system which enhances the transformation of ozone into dOH radicals
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(Abdo et al., 1988; Andreozzi et al., 1992; Sa´nchez-Polo and Rivera-Utrilla, 2004). These methods have proven highly effective to oxidize organic compounds. However, their practical implementation has been hindered by lack of knowledge of the reactions involved in these processes and the need to add these metals to the system and separating them after treatment. To avoid these problems, research is under way into the use of solid catalytic materials that accelerate the transformation of ozone into dOH radicals. The combined use of ozone and granular activated carbon (GAC) in a single treatment process was recently identified as an attractive option for the enhanced oxidation of micropollutants (Beltra´n et al., 2002; Jans and Hoigne´, 1998; Ma et al., 2004; Sa´nchez-Polo et al., 2005, l. 2006) It was also reported (Rivera-Utrilla and Sa´nchez-Polo, 2002; Sa´nchez-Polo and Rivera-Utrilla, 2003) that GAC acts as initiator and/or promoter of ozone transformation into dOH radicals, increasing the range of applicability of this process. Besides activated carbon, new heterogeneous catalytic processes have been developed involving different metal oxides (TiO2, MnO2) and supported metal catalysts (Mn, Cu) (Andreozzi et al., 1996; Karpel vel Leitner et al., 1999; Einaga and Futamura, 2004; Qiu et al., 2004; Beltra´n et al., 2004), although their use is not very widespread because of lack of knowledge of the mechanisms involved. Carbon aerogels (Pekala, 1989; Pekala et al., 1990), first developed at the end of the 1980s, are now used as the basis of numerous industrial applications because of their chemical and textural properties and easy preparation (Wencui et al., 2002; Moreno-Castilla et al., 1999; Rotter et al., 2004). They are prepared from resorcinol/formaldehyde gels by supercritical drying methods. The skeletal structure of wet aerogels is maintained by supercritical drying, obtaining solids with high porosity and specific surface areas. A potentially important feature of these materials is that metal-doped aerogels can be easily prepared by adding a soluble metal salt to the initial resorcinol/formaldehyde mixture (Maldonado-Ho´dar et al., 1999, 2000; Fu et al., 2002). After gelation, the metal salt is trapped within the gel structure and the metal ions are chelated by functional groups of the polymer matrix. These properties make carbon aerogels highly promising materials to enhance ozone transformation into dOH radicals, because advantage can be taken of the properties of transition metals with contrasting catalytic activities in the ozonation process, avoiding their dissolution and the separation from the system. Ozonation in presence of carbon aerogels can lead to oxidation of micropollutants either by a direct reaction of the compounds with ozone or by dOH radicals that are produced by transformation of ozone at the surface of carbon aerogels. The concentration of both oxidants must be known to define and calibrate this process with respect to its oxidation capacity. An experimental approach to determine the concentration of ozone and dOH radicals during conventional ozonation or the AOP O3/H2O2 has been developed in a previous study (Elovitz and von Gunten, 1999). As described by Elovitz and von Gunten a Rct value can be defined as the ratio of the exposure of dOH radicals and ozone (i.e., concentration of oxidant integrated over the reaction time Eq (1)); R d ½ OH dt . Rct ¼ R ½O3 dt
(1)
The Rct can be calculated from the measurement of the decrease of a probe compound, which reacts fast with dOH but not with ozone, and a simultaneous determination of the ozone concentration (Elovitz and von Gunten, 1999). The objectives of this study were to determine the efficacy of Co-, Mn-, and Ti-doped carbon aerogels in the transformation of ozone into dOH radicals and to investigate the mechanism involved in this process. The effects of the CO2 activation on the carbon aerogel characteristics as well as the behavior of the activated carbon aerogel on the above ozone transformation process were also studied.
2.
Experimental
The model compound selected for the study was sodium para-chlorobenzoate (pCBA). pCBA is characterized by a low reactivity against ozone (kO3 ¼ 0:15 M1 s1 ) (Yao and Haag, 1991) and a high reactivity against OH radicals (kOH ¼ 5.2 109 M1 s1) (Yao and Haag, 1991). These properties making it the ideal compound to detect the presence of d OH radicals in the system.
2.1.
Materials
All chemicals (pCBA, indigo, resorcinol, formaldehyde, cobalt acetate, manganese acetate, titanium tetrabutyl ammonium, tert-butanol (t-BuOH), phosphoric acid, hydrochloric acid and sodium hydroxide) were reagent grade or analytical grade when available, and were used without further purification. Stock solutions were prepared in Milli-Q water. Concentrated O3 stock solutions were produced by continuously bubbling O3-containing oxygen through Milli-Q water that was cooled in an ice bath. The concentration of the resulting stock solution was approximately 1.2 mM (60 mg L1). The commercial activated carbon used was supplied by Witco.
2.2.
Experimental methods
2.2.1.
Aerogel preparation
Preparation of metal-doped carbon aerogels was reported elsewhere (Pekala, 1989; Pekala et al., 1990). Briefly, resorcinol (R) and formaldehyde (F) were dissolved in water (W) containing cobalt acetate, manganese acetate or titanium tetra butyl ammonium as catalyst (C). The stoichiometric R/F and R/W molar ratios were 0.15 and 0.13, respectively. The stoichiometric R/C ranged from 15 to 200. Another aerogel was prepared in the same way but adding sodium carbonate for use as a blank, referred to in the text as A. The mixtures were stirred to obtain homogeneous solutions that were cast into glass molds (25 cm length 0.5 cm internal diameter) and cured for a certain period of time. The gel rods were then cut into 5 mm pellets and supercritically dried with carbon dioxide to form the corresponding aerogels. Samples will be referred to in the text as A followed by the metal present and the R/C ratio used. The carbon aerogel sample A-Mn(II)-15, selected for obtaining activated carbon aerogel, was first heated in N2 flow of 100 cm3 min1 to 1173 K, left to soak for 5 h, and then
ARTICLE IN PRESS WAT E R R E S E A R C H
activated at 1173 K in a CO2 flow (100 cm3 min1) for 1 h. This sample was designated A-Mn(II)-15C.
2.2.2.
Aerogel characterization
Aerogel samples and Witco activated carbon were texturally and chemically characterized using N2 and CO2 adsorption at 77 and 273 K, respectively, mercury porosimetry, determination of the pH of the point of zero charge (pHPZC), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). These techniques have been described in detail elsewhere (Rivera-Utrilla and Sa´nchez-Polo, (2002); Valde´s et al., (2002)). The particle size used was 0.05–0.08 mm.
2.2.3.
Methodology
Experiments with model systems were performed in Milli-Q water that was pretreated with ozone to remove any significant O3 or dOH-depleting impurities. The solutions were adjusted to the desired pH with H3PO4 (10 mM) and NaOH (0.01 M). Small aliquots of stock solutions of the model scavengers (t-BuOH) and dOH-probe compound (pCBA) were added to reach the desired concentrations. Reaction bottles were fitted with a dispenser system and were submerged in a thermostated bath at 2270.2 1C. Ozonation reactions were initiated by adding aerogel and an aliquot of O3 stock solution to the reaction bottles. At selected reaction times, samples were withdrawn and dispensed directly into the indigo solution thereby quenching the residual ozone. Similar experiments were performed using natural water. Samples of natural water were collected from Lake Zurich, filtered through a 0.45 mm filter made of cellulose nitrate and stored at 4 1C until use. This water has a concentration of dissolved organic matter (DOC) of 1.4 mg L1 and carbonate alkalinity of 2.6 mM.
2.2.4.
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Pretreatment of metal-doped carbon aerogels
To determine changes in metal-doped carbon aerogel activity due to ozonation, aerogels were pretreated with ozone; 0.5 g
of aerogel was suspended in 0.5 L of 1 mg L1 ozone aqueous solution (pH 7) for 1 h. The aerogel was then dried at 100 1C for 24 h; Mn-doped aerogel was subjected to the same ozone treatment three times. Samples were designated A-Co(II)-151, A-Ti(IV)-15-1, A-Mn(II)-15-1, A-Mn(II)-15-2, A-Mn(II)-15-3, A-Mn(II)-15C-1, A-Mn(II)-15C-2 and A-Mn(II)-15C-3.
2.3.
Analytical methods
Dissolved O3 was analyzed using the indigo method (Bader and Hoigne´, 1982). Solution pH was measured using a combination pH electrode, which was calibrated with standard buffers. pCBA was determined by HPLC with an eluent containing 45% 10-mM H3PO4 and 55% methanol at 1 mL min1 and detected at 234 nm. The quantification limit was 0.025 mM with a 250 mL injection loop. DOC was measured using a Shimadzu TOC-5000A unit.
3.
Results and discussion
3.1. Chemical and textural characterization of Co(II), Mn(II) and Ti(IV) carbon aerogels The textural characteristics of the original aerogels (A, A-Co(II)-15, A-Mn(II)-15 and A-Ti(IV)-15) are compiled in Table 1; all are mainly meso- and macroporous materials. Thus, both the volume of pores with diameter in between 6.6 and 50 nm (V2) and the macropore volume (V3) are much greater than the micropore volume (Vmic). Moreover, their pore size distributions showed the maxima located at around 12 nm in diameter. N2 surface area (SN2 ), CO2 surface area (SCO2 ) and Vmic values were relatively low for sample A (blank) and were slightly higher in aerogels doped with a transition metal (Co(II), Mn(II) or Ti(IV)). However, comparisons between results for A-Co(II)-15, A-Mn(II)-15 and A-Ti(IV)-15 (Table 1)
Table 1 – Textural characterization of the aerogel samples Sample A A-Co(II)-15 A-Ti(IV)-15 A-Mn(II)-15 A-Mn(II)-50 A-Mn(II)-200 A-Mn(II)-15-1 A-Mn(II)-15-2 A-Mn(II)-15-3 A-Co(II)-15-1 A-Ti(IV)-15-1 A-Mn(II)-15C Witco
SN2 (m2 g1)
SCO2 (m2 g1)
Vmic (cm3 g1)
V2 (cm3 g1)
V3 (cm3 g1)
500730 562730 550730 554730 593730 646730 540730 534730 546730 560730 538730 880730 808730
200730 206730 203730 210730 285730 321730 200730 204730 206730 210730 200730 701730 669730
0.0770.01 0.0770.01 0.0770.01 0.0770.01 0.1070.01 0.1170.01 0.0770.01 0.0770.01 0.0770.01 0.0770.01 0.0770.01 0.2570.01 0.2470.01
0.3670.01 0.4370.01 0.4070.01 0.4170.01 0.3570.01 0.2870.01 0.4170.01 0.4070.01 0.4170.01 0.4170.01 0.3870.01 0.4370.01 0.0470.01
0.6870.01 0.9770.01 0.9270.01 0.9570.01 0.9070.01 0.8670.01 0.9470.01 0.9370.01 0.9570.01 0.9270.01 0.9070.01 0.8670.01 0.0570.01
SN2 ¼ apparent surface area determined applying BET equation to N2 adsorption isotherm. SCO2 ¼ apparent surface area determined applying Dubinin–Raduskevich equation to CO2 adsorption isotherm. Vmic ¼ micropore volume determined applying Dubinin–Raduskevich equation to CO2 adsorption isotherm. V2 ¼ volume of pores with diameter of 50–6.6 nm. V3 ¼ volume of pores with diameter above 50 nm.
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indicated that the textural properties of the samples did not differ according to the metal added, with all of them showing values close to 550 m2 g1, 0.4 and 0.9 cm3 g1 for SN2 , V2 and V3, respectively. The value of SN2 was higher than that of SCO2 in these samples indicating that a large fraction of the surface of these samples corresponded to meso- and macropores (Rodrı´guez-Reinoso and Linares-Solano, 1989). Results presented in Table 1 show that when the R/C relationship was higher (samples A-Mn(II)-15, A-Mn(II)-50 and A-Mn(II)-200), the SN2 , SCO2 and Vmi values were greater, however, the V2 and V3 values were lower; this indicates that a decrease in the amount of Mn(II) in the aerogel led to an enhancement of its surface area and microporosity, as well as to a decrease in its meso- and macroporosity. The carbon aerogels (A, A-Co(II)-15, A-Ti(IV)-15 and A-Mn(II)-15) were chemically characterized by determination of the pHPZC, XRD and XPS. Results obtained are shown in Tables 2 and 3. The pHPZC results (Table 2) showed the aerogels to have a high surface acidity (pHPZC ¼ 3.5–4.3). The XPS results (Table 2) showed a high concentration of surface oxygen with values greater than 20%. To determine the nature of the oxygenated surface groups, deconvolution of the O1s XPS spectrum was performed for each sample (Table 3), observing that the surface oxygen was distributed almost equally between ether (–C–O–C) and carbonyl (4CQO) groups, samples A, A-Co(II)-15 and A-Ti(IV)-15. However, in sample A-Mn(II)-15 the amount of 4CQO (76%) is much higher than that of–C–O–C (24%) (Table 3). XRD results for samples A-Co(II)-15, A-Mn(II)-15 and A-Ti(IV)-15 showed a wide dispersion of the metal on the aerogel surface, since no diffraction peaks were observed in any case (results not shown). Importantly, XPS analysis of the same samples revealed that the metals on the aerogel surface were in oxidation state +2 in the case of Mn and Co, and +4 in the case of Ti, and that the surface percentage of each metal was around 15% in all cases (Table 2). The results obtained in aerogel samples doped with decreasing concentrations of Mn(II), i.e., samples A-Mn(II)15, A-Mn(II)-50 and A-Mn(II)-200, showed no clear influence of the R/C ratio used on the surface acidity of the aerogel, since the pHPZC and oxygen percentage were similar among the samples (Table 2). However, deconvolution of the O1s spectra
(Table 3) of these samples showed a slight increase in the percentage of oxygenated functional groups in –C–O form with a reduction in the R/C ratio used (Table 3). Textural analyses of the aerogel samples treated with ozone (A-Mn(II)-15-1, A-Mn(II)-15-2, A-Mn(II)-15-3, A-Co(II)-15-1 and A-Ti(IV)-15-1) showed that this treatment did not significantly affect the SN2 , SCO2 , V2 and V3 values obtained, which were similar to those in the non-pretreated samples (Table 1). XPS results (Table 2) showed that, regardless of the sample considered, an increase was produced in the percentage of surface oxygen with an increase in the number of ozonation cycles to which the samples were subjected. No such increase was detected in the pHPZC values of these samples, which remained close to initial values (pHPZC ¼ 3–4). In all aerogel samples the surface oxygen created during their ozonation was mainly in the form of –CQO (Table 3). Interestingly, in the O3-pretreated Mn-doped aerogel samples Mn(III) and Mn(IV) are formed; the percentage of Mn in oxidation state +4 increased with a larger number of ozonation cycles in the pretreatment. However, the results for samples A-Co(II)-15-1 and A-Ti(IV)-15-1 showed that the oxidation state of the Co
Table 3 – Deconvolution of O1s spectra of the aerogel samples Sample
(–CQO) 53270.2 eV (%)
(–C–O) 533.970.2 eV (%)
5271 5571 4971 7671 6471 5771 8371 9171 9671 6871 6571 371 3971
4871 4571 5171 2471 3671 4371 2771 971 471 3271 3571 9771 6171
A A-Co(II)-15 A-Ti(IV)-15 A-Mn(II)-15 A-Mn(II)-50 A-Mn(II)-200 A-Mn(II)-15-1 A-Mn(II)-15-2 A-Mn(II)-15-3 A-Co(II)-15-1 A-Ti(IV)-15-1 A-Mn(II)-15C Witco
Table 2 – Chemical characterization of the aerogel samples Sample
pHPZC
C (%)
O (%)
Co(II) (%)
Mn(II) (%)
Mn(III) (%)
Mn(IV) (%)
Ti(IV) (%)
A A-Co(II)-15 A-Ti(IV)-15 A-Mn(II)-15 A-Mn(II)-50 A-Mn(II)-200 A-Mn(II)-15-1 A-Mn(II)-15-2 A-Mn(II)-15-3 A-Co(II)-15-1 A-Ti(IV)-15-1 A-Mn(II)-15C Witco
3.570.1 3.870.1 4.370.1 4.270.1 3.770.1 3.270.1 4.070.1 4.170.1 3.970.1 3.970.1 4.270.1 7.570.1 6.870.1
6871 6471 6471 6271 7171 7771 5471 4971 4271 5571 5371 8071 7471
3271 2271 2171 2271 2271 2171 3071 3571 4271 3171 3271 371 2671
— 1471 — — — — — — — 1471 — — —
— — — 1671 771 271 1071 871 671 — — 1771 —
— — — — — — 471 471 471 — — — —
— — — — — — 271 471 671 — — — —
— — 1571 — — — — — — — 1571 — —
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and Ti remained at +2 and +4, respectively (Table 2) after the ozonation pretreatment (see Section 2.2.4.). Sample A-Mn(II)-15 was subjected to physical activation treatment with CO2 following the procedure described in the Experimental Section (sample A-Mn(II)-15C). Results of its textural and chemical characterization are also presented in Tables 1–3. They indicate that the activation process increased its microporosity with an increase in its micropore volume from 0.075 to 0.250 cm3 g1 (Table 1); this led to an enhancement in its surface area, mainly in the value obtained by using CO2, which varied from 210 to 701 m2 g1. Moreover, the activation treatment produced a marked increase in the surface basicity of the sample, which showed a pHPZC value of 7.5 (Table 2), mainly because of the removal of surface oxygenated functional groups by the high temperatures used during the activation process (Valde´s et al., 2002). The low content of surface oxygen observed on sample A-Mn(II)-15C (Table 2) supports this hypothesis. Interestingly, most of the surface oxygen on this sample was in –C–O form (Table 3) and the Mn in it (Table 2) remained in oxidation state +2. XRD analysis of this sample again showed no diffraction peaks, indicating a high dispersion of Mn on the activated aerogel surface.
3.2. Ozonation of pCBA in presence of metal-doped carbon aerogels Fig. 1 depicts the results of pCBA ozonation in presence of the original carbon aerogels and the corresponding adsorption kinetics of this compound on the samples. The adsorption kinetics of pCBA on the aerogel samples studied was very slow, and no adsorption of pCBA was observed during 60 min of contact. The Rct values obtained in the different experiments and the corresponding ozone decomposition constants (kD), determined by a first-order kinetic model (Elovitz and von Gunten, 1999), are listed in Table 4. The presence of the aerogel prepared without metal (sample A) did not increase the pCBA removal rate (Fig. 1),
and the Rct and kD values were close to those observed in absence of aerogel (Table 4). These findings indicate that the organic matrix used to generate the aerogel does not positively contribute to the transformation of ozone into d OH radicals. Results presented in Fig. 1 also show that the pCBA removal rate was increased in presence of Mn aerogel, whereas the presence of Co- or Ti-doped carbon aerogels had virtually no effect on this rate. Because of the low reactivity of pCBA against ozone (Yao and Haag, 1991) and the slow adsorption kinetics of pCBA on the aerogel samples studied (Fig. 1), the increase in its oxidation rate in presence of Mn aerogel would mainly result from the generation of dOH radicals in the system. Moreover, the fact that the presence of aerogel A (blank) did not increase the rate of removal of pCBA from
1.0 [pCBA]/[pCBA]0
WAT E R R E S E A R C H
A
0.8
Without aerogel
A-Co(II)-15
0.6
A-Ti(IV)-15
0.4 A-Mn(II)-15
0.2 0.0 0
20
40
60
t (min)
Fig. 1 – Ozonation of pCBA in presence of the aerogels. pH 7, T 25 1C, [O3] ¼ 2 105 M, [t-BuOH] ¼ 8 105 M, [Aerogel] ¼ 2.5 mg L1. (x), Without aerogel; (K), A; (’), ACo(II)-15; (E), A-Ti(IV)-15; (m), A-Mn(II)-15. Open symbols represent the adsorption kinetics on samples A (J) and AMn(II)-15 (n).
Table 4 – Determination of Rct value in the different ozonation experiments performed pH 7, T 25 1C, [O3] ¼ 2 105 M, [tBuOH] ¼ 8 105 M) Experiment 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Sample
Carbon dose (mg)
kD (s1)
Rct
Without aerogel A A-Co(II)-15 A-Ti(IV)-15 A-Mn(II)-15 A-Mn(II)-15 A-Mn(II)-15 A-Mn(II)-50 A-Mn(II)-200 A-Mn(II)-15-1 A-Mn(II)-15-2 A-Mn(II)-15-3 A-Mn(II)-15C A-Mn(II)-15C-1 A-Mn(II)-15C-2 A-Mn(II)-15C-3
0 2.5 2.5 2.5 2.5 5.0 10.0 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
(6.070.3) 104 (6.270.3) 104 (5.870.3) 104 (6.170.3) 104 (4.270.2) 103 (8.470.4) 103 (1.770.1) 102 (1.770.1) 103 (5.070.2) 104 (2.670.1) 103 (2.170.2) 103 (1.470.1) 103 — — (1.870.1) 102 (2.470.1) 103
(2.6870.13) 109 (2.7470.13) 109 (2.5670.12) 109 (2.7370.13) 109 (5.3670.26) 108 (1.1770.06) 107 (2.2270.11) 107 (2.6870.13) 108 (3.3870.16) 109 (3.3570.16) 108 (2.6870.13) 108 (1.7870.08) 108 — — (2.6870.13) 107 (6.5270.32) 10 8
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the medium (Fig. 1) confirms that the activity of the aerogels in the transformation of ozone into dOH radicals is directly related to the presence of the metal on their surface. The presence of Mn aerogel during pCBA ozonation increased Rct values 20-fold, whereas the Rct values obtained in presence of Co(II) and Ti(IV) were very similar to those obtained in their absence. Likewise, the presence of Mn(II) aerogel during pCBA ozonation increased the kD value, whereas the presence of Co(II) or Ti(IV) aerogels had practically no effect on this parameter (Table 4, Experiments 2–5). These findings confirm that the presence of Mn-doped carbon aerogel during this ozonation process accelerates ozone transformation into dOH radicals. Results obtained in a previous study (Sa´nchez-Polo and Rivera-Utrilla, 2004) showed that the activity of the metals dissolved in the process of the transformation of ozone into dOH radicals is directly related to their reduction potential. Thus, metals susceptible to oxidation by ozone are potential initiators of this process. The same study also showed that the oxidation kinetics of the metal with ozone determines the efficacy of the process. Therefore, under the conditions studied, Co(II) and Ti(IV) are not accelerating ozone transformation into dOH radicals. In order to determine the mechanism by which the Mn(II) aerogel accelerates the pCBA removal rate, samples A-Co(II)15, A-Mn(II)-15 and A-Ti(IV)-15 were studied by XPS after their prior ozonation treatment as described in the Experimental section (samples A-Co(II)-15-1, A-Mn(II)-15-1 and A-Ti(IV)-151). Results of the textural and chemical characterization of these samples were discussed in detail in the previous section (Tables 1–3). However, it is of interest to note that the aerogel sample that enhanced ozone transformation into dOH radicals (A-Mn(II)-15) is also the one in which the oxidation state increased after the ozonation process (Table 2). Thus, 10% of the surface Mn was present in Mn(II) form, 4% in Mn(III) form and 2% in Mn(IV) form on sample A-Mn(II)-15-1. For Co and Ti (samples A-Co(II)-15 and A-Ti(IV)-15), the oxidation state of the metal was unchanged at +2 and +4, respectively. These results indicate that the mechanism by which A-Mn(II)-15 aerogel enhances transformation of ozone into dOH radicals is based on oxidation–reduction reactions. Hence, and in agreement with the results presented in Fig. 1 and Tables 1–4 possible reactions responsible for accelerating the transformation of ozone into dOH radicals due to the presence of A-Mn(II)-15 aerogel during pCBA ozonation may be: O3 þ Mn2þ ! MnO2þ þ O2 ;
(1)
MnO2þ þ H2 O ! Mn3þ þ d OH þ OH ;
(2)
2Mnþ3 þ 2H2 O ! MnO2 þ Mn2þ þ 4Hþ :
(3)
In this way, oxidation of surface Mn(II) to Mn(III) and Mn(IV) during the oxidation process results in a transformation of ozone into dOH radicals (reaction (2)), explaining the increase in the pCBA removal rate (Fig. 1) and the Rct and kD values observed (Table 4, Experiment 5). This mechanism is in agreement with the proposed by other authors (Andreozzi et al., 1996; Ma and Graham, 1999; Ma and Graham, 2000) where
the ozonation of atrazine and oxalic acid in presence of dissolved Mn(II) and MnO2 were studied.
3.3. Influence of Mn(II)-doped organic aerogel dose on pCBA oxidation process The minimum aerogel dose required to initiate and/or promote the transformation of dissolved ozone into dOH radicals is an essential parameter in the design of this novel treatment process. Fig. 2 depicts the evolution of pCBA concentrations in presence of increasing concentrations of aerogel in the system. It can be observed that the pCBA degradation rate increased with a higher dose of added aerogel. Determination of the Rct parameter (Table 4, Experiments 5–7) demonstrated a more efficient generation of dOH radicals when the dose of aerogel in the system was increased, and a linear relationship was observed between the Rct value and the aerogel dose (Fig. 3). In the treatment of drinking waters typical doses of powdered activated carbon range from 1 to 10 mg/L depending on the water matrix (Laine et al., 2000). The results shown in Fig. 2 and Table 4 indicated that the presence of A-Mn(II)-15
1.0 [pCBA]/[pCBA]0
WAT E R R E S E A R C H
0.8
Without aerogel
0.6 2.5 mg/L
0.4 0.2
5 mg/L 10 mg/L
0.0 0
20
40
60
t (min)
Fig. 2 – Influence of dose of A-Mn(II)-15 aerogel on pCBA oxidation. pH 7, T 25 1C. [O3] ¼ 2 105 M, [t-BuOH] ¼ 8 105 M. (x), Without aerogel; (m), 2.5 mg L1; (’), 5 mg L1; (K), 10 mg L1.
2.5 2.0 Rct x 108
3380
1.5 1.0 0.5 0.0 0
2
4
6
8
10
Aerogel dose (mg/L) Fig. 3 – Relationship between Rct value and dose of added aerogel A-Mn(II)-15.
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aerogel in the system, in amounts in between the above range, produced an increase in the pCBA removal rate and a marked rise in the Rct value, indicating an acceleration of ozone transformation into dOH radicals.
3.4. Influence of surface concentration of Mn(II) in carbon aerogels on pCBA oxidation process To determine the minimal surface concentration of Mn(II) able to initiate ozone transformation into dOH radicals, the ozonation of pCBA was carried out in presence of aerogels prepared with decreasing concentrations of Mn(II) (A-Mn(II)15, A-Mn(II)-50 and A-Mn(II)-200). Results obtained are listed in Table 4, experiments 5, 8, and 9. Results of the textural and chemical characterization of these samples (Tables 1–3) were discussed in Section 3.1. An increase in the pCBA removal rate was observed with an increased concentration of surface Mn(II) (figure not shown). kD and Rct values (Table 4) also increased with a higher concentration of Mn on the aerogel surface. These results may confirm that the efficacy of Mn(II)doped aerogels for the transformation of ozone into dOH radicals is mainly due to the presence of Mn(II) on their surface. A linear relationship was found between the kD and Rct values obtained (Table 4) and the concentration of surface Mn(II) (Table 2), similar to the observation reported in the previous section. From the results presented in Tables 2 and 4 it is interesting to note that the Mn surface concentration in A-Mn(II)-200 sample could be almost the limit where still a small effect on ozone transformation can be observed.
3.5. Influence of aerogel preozonation on ozone transformation into dOH radicals In order to confirm the mechanism involved in the transformation of ozone into dOH radicals due to the presence of Mn(II) carbon aerogel in the system, pCBA ozonation were performed in presence of A-Mn(II)-15 aerogel samples subjected to ozonation pretreatment (Fig. 4). The Rct and kD values found are listed in Table 4, Experiments 5, 10–12. The influence of the ozone treatment on the chemical and textural properties of these samples was discussed in greater detail in Section 3.1. The pCBA oxidation rate (Fig. 4) was reduced and the Rct and kD were markedly decreased when the pCBA ozonation was carried out in presence of preozonated aerogel (Table 4). Thus, sample A-Mn(II)-15-3 showed a reduction of around 85% in their values. These results indicate that the ability of the A-Mn(II)-15 aerogel to initiate ozone transformation into dOH radicals is reduced with greater exposure of the aerogel to ozone. Surface characterization of these samples (Tables 1–3) showed that the ozonation treatment applied produced no major change in their textural or chemical properties (only an increase in their surface oxygenated groups). However, XPS analysis of these samples indicated a gradual oxidation of the surface Mn present (Table 2). Thus, the concentration of surface Mn in oxidation states +2 progressively decreased with more ozonation cycles. These results corroborate the mechanism proposed in Eqs. (1)–(3). Thus, the capacity to initiate the transformation of ozone into dOH radicals was reduced when the concentration of surface Mn in oxidation state +2 was lower.
1.0 [pCBA]/[PCBA]0
WAT E R R E S E A R C H
Without aerogel
0.8
A-Mn(II)-15-3
0.6
A-Mn(II)-15-2
0.4
A-Mn(II)-15-1 A-Mn(II)-15
0.2 0.0 0
20
40
60
t (min) Fig. 4 – Ozonation of pCBA in presence of preozonated A-Mn(II)-15 aerogel samples. pH 7, T 25 1C, [O3] ¼ 2 105 M, [t-BuOH] ¼ 8 105 M, [Aerogel] ¼ 2.5 mg L1. (x), Without aerogel; (m), A-Mn(II)-15; (K), A-Mn(II)-15-1; (E) , A-Mn(II)-15-2; (’), A-Mn(II)-15-3.
This rapid deactivation of the aerogel samples during ozonation may limit their applicability in water treatment. Therefore, new treatments of these aerogels must be sought that avoid their deactivation. Powdered activated carbon is commonly used in treatment of drinking waters because of its high adsorbent properties. This material can also enhance ozone transformation into d OH radicals (Beltra´n et al., 2002; Jans and Hoigne´, 1998; Ma et al., 2004; Rivera-Utrilla et al., 2002; Sa´nchez-Polo et al., 2005; Sa´nchez-Polo et al., 2006). It was demonstrated that the metal centers of its mineral matter, the electrons of the basal plane of the activated carbon, and the basic groups on its surface are the activated carbon characteristics largely responsible for the transformation of ozone into dOH radicals (Rivera-Utrilla et al., 2002). Therefore, in order to increase the capacity of sample A-Mn(II)-15 to transform ozone into dOH radicals it was activated with CO2, obtaining the activated aerogel denominated sample A-Mn(II)-15C. The influence of the treatment applied on chemical and textural properties was discussed in detail in Section 3.1. Fig. 5 depicts the results obtained from the ozonation of pCBA in presence of the activated aerogel. The pCBA oxidation rate increased significantly in presence of sample A-Mn(II)15C compared to the presence of the non-activated sample. In fact, Rct and kD could not be determined for the system O3/AMn(II)-15C because of the very high rate of transformation of the ozone into dOH radicals (Table 4, Experiment 13). These results are mainly due to an increase in surface basicity (Rivera-Utrilla et al., 2002). The reduction in the oxygen concentration (Table 2) produced an increase in the surface electronic density, thereby increasing the extent of the transformation of ozone into dOH radicals.
3.6. Ozonation of natural surface waters in presence of activated carbon prepared from Mn(II) aerogel. Comparison between O3/H2O2 and O3/activated carbon processes The applicability of this new O3/activated aerogel process for water treatment was tested by comparing it with AOPs commonly used for this purpose (O3, O3/H2O2, and
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O3/activated carbon) in pCBA ozonation experiments in Lake Zurich water (Fig. 6). The doses of O3, H2O2 and activated carbon selected were those commonly used for treating drinking waters. The results obtained are shown in Fig. 6 and Table 5. The pCBA removal rate was especially increased when ozonation of this contaminant was performed in
1.0 [pCBA]/[pCBA]0
Without aerogel 0.8 0.6 A-Mn(II)-15 0.4 0.2 A-Mn(II)-15C 0.0 0
20
40
60
t (min) Fig. 5 – Influence of activation of A-Mn(II)-15 aerogel on pCBA ozonation. pH 7, T 25 1C, [O3] ¼ 2 105 M, [tBuOH] ¼ 8 105 M, [Aerogel] ¼ 2.5 mg L1. (x), Without aerogel; (m), A-Mn(II)-15; (’), A-Mn(II)-15C.
[pCBA]/[pCBA]0
1.0 0.8 0.6
O3
0.4
O3/Activated carbon
0.2
O3/A-Mn(II)-15
O3/H2O2 O3/A-Mn(II)-15C
0.0 0
20
40
60
t (min) Fig. 6 – Ozonation of Lake Zurich water in presence of the different processes studied. pH 7, T 25 1C, [O3] ¼ 2 105 M, [H2O2] ¼ 1 105, [Aerogel] ¼ 2.5 mg L1, [Activated carbon] ¼ 2.5 mg L1. (x), O3; (K), O3/Activated carbon; (m), O3/A-Mn(II)-15; (E) , O3/H2O2; (’), O3/A-Mn(II)-15C.
presence of the activated Mn-doped aerogel A-Mn(II)-15C (Fig. 6). Moreover, the Rct and kD values obtained (Table 5) were markedly higher than those observed in the other AOPs studied (O3/H2O2 and O3/activated carbon). pCBA ozonation was also performed in presence of A-Mn(II)-15C subjected to ozone pretreatment (see Experimental section). The pCBA oxidation rate was reduced and the Rct and kD were markedly decreased in presence of preozonated activated aerogel. These results indicate that the ability of the A-Mn(II)-15C sample to initiate the ozone transformation process into dOH radicals is reduced with greater exposure of the aerogel to ozone (Table 4, Experiments 13–16), confirming the results presented in the previous section. It is interesting to compare the Rct and kD values observed using the O3/activated carbon and the O3/activated aerogel systems. The activated carbon selected was Witco commercial carbon because it has SN2 and pHPZC values very close to those found for sample A-Mn(II)-15C. Moreover, this carbon has a low ash content (0.3%). The volume of meso- and macropores in the Witco carbon was notably lower than that in sample A-Mn(II)-15C (Tables 1 and 2). The results shown in Fig. 6 and Table 5 indicate the greater efficiency of sample A-Mn(II)-15C for the transformation of ozone into dOH radicals compared with activated carbon Witco. These findings confirm that the activity of carbon materials in the transformation of ozone into dOH radicals, is favored by: (i) an elevated meso- and macroporosity, which reduces diffusion problems and facilitates access of the ozone to active surface sites; and (ii) the presence of metals susceptible to surface oxidation by ozone. A highly important parameter to assess the efficacy of a treatment process is its capacity to remove natural organic matter (NOM) from the natural water. Fig. 7 depicts the evolution of the DOC concentration as a function of the treatment time for each of the processes studied. It can be observed that the systems based on O3 and O3/H2O2, do not lead to a reduction in DOC concentration after 60 min of treatment. This is because none of the processes have sufficient oxidizing capacity to transform NOM into CO2. In contrast, there was a reduction of 70% after 60 min for the O3/activated aerogel system (Fig. 7). This reduction in DOC is decreased to around 50% and 20% when using O3/aerogel and O3/activated carbon, respectively. This reduction of NOM is mainly due to adsorption of the DOC on the surface of the
Table 5 – Determination of Rct value in the different experiments performed using Lake Zurich waters (pH 7, T 25 1C, [O3] ¼ 2 105 M, [H2O2] 1 105, [Aerogel] ¼ 2.5 mg L1, [Activated carbon] ¼ 2.5 mg L1) Experiment 1 2 3 4 5 6
Sample
kD (s1)
Rct
Without carbon A A-Mn(II)-15 A-Mn(II)-15C Witco H2O2
(7.070.3) 104 (6.870.3) 104 (6.870.3) 103 (9.270.4) 103 (6.770.3) 104 (8.570.4) 103
(5.0170.25) 109 (4.9670.25) 109 (8.5270.42) 108 (6.1570.30) 107 (4.9270.25) 109 (1.1770.06) 107
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1.0 [DOC]/[DOC]0
4 0 (200 6) 337 5 – 338 4
O3/Activated carbon
0.8 0.6
O3/A-Mn(II)-15
0.4
O3/A-Mn(II)-15C
0.2
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treatment produced a marked increase in the surface basicity of the sample, which showed a pHPZC value of 7.5, mainly because of the removal of surface oxygenated functional groups by the high temperatures used during the activation process. Ozonation of drinking waters in presence of activated aerogel leads to (i) an acceleration of ozone transformation into dOH radicals, and (ii) a reduction in the concentration of dissolved organic material.
0.0 0
10
20
30
40
50
60
t (min) Fig. 7 – Evolution of DOC during ozonation of lake Zurich water for the different processes studied. pH 7, T 25 1C, [O3] ¼ 2 105 M, [H2O2] ¼ 1 105, [Aerogel] ¼ 2.5 mg L1, [Activated carbon] ¼ 2.5 mg L1. (x), O3; (K), O3/ Activated carbon; (m), O3/A-Mn(II)-15; (E), O3/H2O2; (’), O3/A-Mn(II)15C.
Acknowledgments The authors are grateful for the financial support provided by MCyT and FEDER (Project: CTQ2004-07783-C02-01) and Junta de Andalucia (Project: RNM547). The authors would also like to acknowledge Elisabeth Salhi for her assistance with the experimental work. M. Sa´nchez-Polo expresses his gratitude to the Junta de Andalucı´a for providing a research fellowship. R E F E R E N C E S
solid material. The higher adsorption capacity of activated aerogel is principally due to a larger volume of meso- and macropores, increasing the surface area accessible to adsorb organic matter components. The results presented in Fig. 7 indicate that, besides markedly accelerating the transformation of ozone into dOH radicals, the presence of low concentrations of Mn-doped carbon aerogels or Mn-doped activated carbon aerogels during the ozonation of drinking waters also lead to a reduction in the organic matter present in the system, which is desirable to improve the water quality.
4.
Conclusions
Addition of metals does not have a marked influence on the textural and chemical properties of aerogel samples. However, the addition of increasing concentrations of metals produces a substantial reduction in surface area and an increase in the volume of meso- and macropores, although the surface chemistry is not modified. The capacity of metal-doped carbon aerogels to accelerate the transformation of ozone into dOH radicals is related to the metal on their surface. Only metals susceptible to oxidation by ozone are effective. The presence of Mn(II)-doped carbon aerogels during ozonation accelerates ozone transformation into dOH radicals. The capacity to accelerate this process depends on the dose of aerogel and the concentration of Mn(II) on its surface. The capacity of the Mn-doped carbon aerogel to accelerate ozone transformation into dOH radicals decreases with longer exposure of the aerogel to ozone. The oxidation of surface Mn(II) to higher oxidation states is responsible for this behavior and leads to an inactivation of this material. The pCBA oxidation rate increased significantly in presence of Mn aerogel activated sample (A-Mn(II)-15C) compared to the presence of the non-activated sample. These results are mainly due to an increase in surface basicity. Activation
Abdo, M.S.E., Shaban, H., Bader, M.S.H., 1988. Decolorization by ozone of direct dyes in presence of some catalysts. J. Environ. As soc. Health A 23, 697–708. Andreozzi, R., Insola, A., Caprio, V., D0 amore, M.G., 1992. The kinetics of Mn(II)-catalysed ozonation of oxalic acid in aqueous solution. Water Res. 26, 917–925. Andreozzi, R., Insola, A., Caprio, V., Marotta, R., Tufano, V., 1996. The use of manganese dioxide as a heterogeneous catalyst for oxalic ozonation in aqueous solution. Appl. Catal. A: Gen. 138, 75–81. Bader, H., Hoigne´, J., 1982. Determination of ozone in water by the indigo method. A submitted standard method. Ozone Sci. Eng. 4, 169–176. Beltra´n, F.J., Rivas, J., Alvarez, P., Montero de Espinosa, R., 2002. Kinetics of heterogeneous catalytic ozone decomposition in water with activated carbon. Ozone Sci. Eng. 24, 227–237. Beltra´n, F.J., Rivas, F.J., Montero de Espinosa, R., 2004. A TiO2/Al2O3 catalyst to improve the ozonation of oxalic acid in water. Appl. Catal. B: Environ. 47, 101–109. Brunuy, R., Bourbigot, M.M., Dore´, M., 1994. Oxidation of organic compounds through the combination of ozone–hydrogen peroxide. Ozone Sci. Eng. 6, 163–183. Einaga, H., Futamura, S., 2004. Catalytic oxidation of benzene with ozone over aluminia-supported manganese oxides. J. Catal 227, 304–312. Elovitz, M.S., von Gunten, U., 1999. Hydroxyl radical/ozone ratios during ozonation processes. I. The Rct concept. Ozone Sci. Eng. 21, 239–260. Fu, R., Yoshizawa, N., Dresselhaus, M.S., Dresselhaus, G., Joe, H., Satcher Jr., A., Bauman, T.F., 2002. XPS study of copper-doped carbon aerogels. Langmuir 18, 10100–10104. Hoigne´, J., 1998. Chemistry of aqueous ozone and transformation of pollutants by ozonation and advanced oxidation processes. In: Hubrec, J. (Ed.), The Handbook of Environmental Chemistry Quality and Treatment of drinking water. Berlin, Springer. Jans, U., Hoigne´, J., 1998. Activated carbon and carbon black catalyzed transformation of aqueous ozone into dOH radicals. Ozone Sci. Eng. 20, 67–87. Karpel vel Leitner, N., Delouane, B., Legube, B., Luck, F., 1999. Effects of catalysts during ozonation of salicylic acid, peptides and humic substances in aqueous solution. Ozone Sci. Eng. 21, 261–276.
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40 (2006) 3375 – 3384
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Metal-doped carbon aerogels as catalysts during ozonation processes in aqueous solutions M. Sa´nchez-Poloa,b, J. Rivera-Utrillab, U. von Guntena, a
Swiss Federal Institute of Aquatic Science and Technology (EAWAG), Ueberlandstrasse, 133, CH-8600, Du¨bendorf, Switzerland Departamento de Quı´mica Inorga´nica, F. Ciencias, Universidad de Granada, 18071, Granada, Spain
b
art i cle info
A B S T R A C T
Article history:
The efficiency of Co(II)-, Mn(II)-, and Ti(IV)-doped carbon aerogels for the transformation of
Received 13 March 2006
ozone into dOH radicals was investigated. The carbon aerogels had a markedly acid surface
Received in revised form
character (pHPZCffi3–4) with very high surface oxygen concentrations (Offi20%). X-ray
13 July 2006
photoelectron spectroscopy (XPS) analyses of the samples showed the oxidation state of
Accepted 19 July 2006
the metals was +2 for Co and Mn and +4 for Ti. The presence of Mn(II)-doped carbon aerogel
Available online 12 September 2006
enhanced ozone transformation into dOH radicals, whereas the presence of Co(II) and Ti(IV)
Keywords:
carbon aerogels presented no activity in this process. Moreover, it was observed that an
Ozonation
increase in the concentration of Mn in the surface of the aerogel increases its efficiency to
Carbon aerogels
transform ozone into dOH radicals, with an Rct value ([OH]/[O3]) of 5.36 108 for the
Transition metals
aerogel doped with 16% of surface Mn(II) compared to an Rct of 2.68 109 for conventional
p-chlorobenzoate (pCBA)
ozonation. Regardless of the aerogel used, XPS analysis of the ozonated aerogel samples
d
OH radicals
showed an increase in the concentration of surface oxygen when the exposure to ozone was longer. However, presence of oxidized metal species after ozone treatment was only detected in the case of the Mn-doped aerogel, (Mn(III) and Mn(IV)). CO2 activation of carbon aerogel produced a marked increase in its efficiency to transform ozone into dOH radicals compared with non-activated sample. The efficiency of Mn activated carbon aerogel to transform ozone into dOH radicals was greater than that of Witco commercial activated carbon or H2O2 in the ozonation of water from Lake Zurich (Zurich, Switzerland). & 2006 Elsevier Ltd. All rights reserved.
1.
Introduction
Rising concerns about water quality have prompted the investment of considerable human and financial resources by public and private agencies in the development of new and effective water treatment processes to remove organic micropollutants that pose a threat to human health. Their removal by the use of ozone was proposed (von Gunten, 2003a, b; Hoigne´, 1998) because of its high reactivity with certain organic compounds. However, this process is limited by the chemical kinetics and by the generation of degradation products (Rivera-Utrilla et al., 2002; Zaror, 1997). Advanced Corresponding author. Tel.: +41 44 8235270; fax: +41 44 8235210.
E-mail address: [email protected] (U. von Gunten). 0043-1354/$ - see front matter & 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2006.07.020
oxidation processes (AOPs) were developed to increase the degree of transformation of ozone-resistant micropollutants by the way of dOH radical oxidation (Brunuy et al., 1994; Paillard et al., 1988; Prengle and Mank, 1978). These processes can be based on the transformation of ozone into dOH radicals that which attack most organic compounds with constants of 108–1010 M1 s1. However, because of their low selectivity a large fraction of the dOH radicals will be lost to the water matrix. A further development in AOPs resulted from the addition of transition metals (Mn, Co, Zn, Fe) to the system which enhances the transformation of ozone into dOH radicals
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(Abdo et al., 1988; Andreozzi et al., 1992; Sa´nchez-Polo and Rivera-Utrilla, 2004). These methods have proven highly effective to oxidize organic compounds. However, their practical implementation has been hindered by lack of knowledge of the reactions involved in these processes and the need to add these metals to the system and separating them after treatment. To avoid these problems, research is under way into the use of solid catalytic materials that accelerate the transformation of ozone into dOH radicals. The combined use of ozone and granular activated carbon (GAC) in a single treatment process was recently identified as an attractive option for the enhanced oxidation of micropollutants (Beltra´n et al., 2002; Jans and Hoigne´, 1998; Ma et al., 2004; Sa´nchez-Polo et al., 2005, l. 2006) It was also reported (Rivera-Utrilla and Sa´nchez-Polo, 2002; Sa´nchez-Polo and Rivera-Utrilla, 2003) that GAC acts as initiator and/or promoter of ozone transformation into dOH radicals, increasing the range of applicability of this process. Besides activated carbon, new heterogeneous catalytic processes have been developed involving different metal oxides (TiO2, MnO2) and supported metal catalysts (Mn, Cu) (Andreozzi et al., 1996; Karpel vel Leitner et al., 1999; Einaga and Futamura, 2004; Qiu et al., 2004; Beltra´n et al., 2004), although their use is not very widespread because of lack of knowledge of the mechanisms involved. Carbon aerogels (Pekala, 1989; Pekala et al., 1990), first developed at the end of the 1980s, are now used as the basis of numerous industrial applications because of their chemical and textural properties and easy preparation (Wencui et al., 2002; Moreno-Castilla et al., 1999; Rotter et al., 2004). They are prepared from resorcinol/formaldehyde gels by supercritical drying methods. The skeletal structure of wet aerogels is maintained by supercritical drying, obtaining solids with high porosity and specific surface areas. A potentially important feature of these materials is that metal-doped aerogels can be easily prepared by adding a soluble metal salt to the initial resorcinol/formaldehyde mixture (Maldonado-Ho´dar et al., 1999, 2000; Fu et al., 2002). After gelation, the metal salt is trapped within the gel structure and the metal ions are chelated by functional groups of the polymer matrix. These properties make carbon aerogels highly promising materials to enhance ozone transformation into dOH radicals, because advantage can be taken of the properties of transition metals with contrasting catalytic activities in the ozonation process, avoiding their dissolution and the separation from the system. Ozonation in presence of carbon aerogels can lead to oxidation of micropollutants either by a direct reaction of the compounds with ozone or by dOH radicals that are produced by transformation of ozone at the surface of carbon aerogels. The concentration of both oxidants must be known to define and calibrate this process with respect to its oxidation capacity. An experimental approach to determine the concentration of ozone and dOH radicals during conventional ozonation or the AOP O3/H2O2 has been developed in a previous study (Elovitz and von Gunten, 1999). As described by Elovitz and von Gunten a Rct value can be defined as the ratio of the exposure of dOH radicals and ozone (i.e., concentration of oxidant integrated over the reaction time Eq (1)); R d ½ OH dt . Rct ¼ R ½O3 dt
(1)
The Rct can be calculated from the measurement of the decrease of a probe compound, which reacts fast with dOH but not with ozone, and a simultaneous determination of the ozone concentration (Elovitz and von Gunten, 1999). The objectives of this study were to determine the efficacy of Co-, Mn-, and Ti-doped carbon aerogels in the transformation of ozone into dOH radicals and to investigate the mechanism involved in this process. The effects of the CO2 activation on the carbon aerogel characteristics as well as the behavior of the activated carbon aerogel on the above ozone transformation process were also studied.
2.
Experimental
The model compound selected for the study was sodium para-chlorobenzoate (pCBA). pCBA is characterized by a low reactivity against ozone (kO3 ¼ 0:15 M1 s1 ) (Yao and Haag, 1991) and a high reactivity against OH radicals (kOH ¼ 5.2 109 M1 s1) (Yao and Haag, 1991). These properties making it the ideal compound to detect the presence of d OH radicals in the system.
2.1.
Materials
All chemicals (pCBA, indigo, resorcinol, formaldehyde, cobalt acetate, manganese acetate, titanium tetrabutyl ammonium, tert-butanol (t-BuOH), phosphoric acid, hydrochloric acid and sodium hydroxide) were reagent grade or analytical grade when available, and were used without further purification. Stock solutions were prepared in Milli-Q water. Concentrated O3 stock solutions were produced by continuously bubbling O3-containing oxygen through Milli-Q water that was cooled in an ice bath. The concentration of the resulting stock solution was approximately 1.2 mM (60 mg L1). The commercial activated carbon used was supplied by Witco.
2.2.
Experimental methods
2.2.1.
Aerogel preparation
Preparation of metal-doped carbon aerogels was reported elsewhere (Pekala, 1989; Pekala et al., 1990). Briefly, resorcinol (R) and formaldehyde (F) were dissolved in water (W) containing cobalt acetate, manganese acetate or titanium tetra butyl ammonium as catalyst (C). The stoichiometric R/F and R/W molar ratios were 0.15 and 0.13, respectively. The stoichiometric R/C ranged from 15 to 200. Another aerogel was prepared in the same way but adding sodium carbonate for use as a blank, referred to in the text as A. The mixtures were stirred to obtain homogeneous solutions that were cast into glass molds (25 cm length 0.5 cm internal diameter) and cured for a certain period of time. The gel rods were then cut into 5 mm pellets and supercritically dried with carbon dioxide to form the corresponding aerogels. Samples will be referred to in the text as A followed by the metal present and the R/C ratio used. The carbon aerogel sample A-Mn(II)-15, selected for obtaining activated carbon aerogel, was first heated in N2 flow of 100 cm3 min1 to 1173 K, left to soak for 5 h, and then
ARTICLE IN PRESS WAT E R R E S E A R C H
activated at 1173 K in a CO2 flow (100 cm3 min1) for 1 h. This sample was designated A-Mn(II)-15C.
2.2.2.
Aerogel characterization
Aerogel samples and Witco activated carbon were texturally and chemically characterized using N2 and CO2 adsorption at 77 and 273 K, respectively, mercury porosimetry, determination of the pH of the point of zero charge (pHPZC), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). These techniques have been described in detail elsewhere (Rivera-Utrilla and Sa´nchez-Polo, (2002); Valde´s et al., (2002)). The particle size used was 0.05–0.08 mm.
2.2.3.
Methodology
Experiments with model systems were performed in Milli-Q water that was pretreated with ozone to remove any significant O3 or dOH-depleting impurities. The solutions were adjusted to the desired pH with H3PO4 (10 mM) and NaOH (0.01 M). Small aliquots of stock solutions of the model scavengers (t-BuOH) and dOH-probe compound (pCBA) were added to reach the desired concentrations. Reaction bottles were fitted with a dispenser system and were submerged in a thermostated bath at 2270.2 1C. Ozonation reactions were initiated by adding aerogel and an aliquot of O3 stock solution to the reaction bottles. At selected reaction times, samples were withdrawn and dispensed directly into the indigo solution thereby quenching the residual ozone. Similar experiments were performed using natural water. Samples of natural water were collected from Lake Zurich, filtered through a 0.45 mm filter made of cellulose nitrate and stored at 4 1C until use. This water has a concentration of dissolved organic matter (DOC) of 1.4 mg L1 and carbonate alkalinity of 2.6 mM.
2.2.4.
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Pretreatment of metal-doped carbon aerogels
To determine changes in metal-doped carbon aerogel activity due to ozonation, aerogels were pretreated with ozone; 0.5 g
of aerogel was suspended in 0.5 L of 1 mg L1 ozone aqueous solution (pH 7) for 1 h. The aerogel was then dried at 100 1C for 24 h; Mn-doped aerogel was subjected to the same ozone treatment three times. Samples were designated A-Co(II)-151, A-Ti(IV)-15-1, A-Mn(II)-15-1, A-Mn(II)-15-2, A-Mn(II)-15-3, A-Mn(II)-15C-1, A-Mn(II)-15C-2 and A-Mn(II)-15C-3.
2.3.
Analytical methods
Dissolved O3 was analyzed using the indigo method (Bader and Hoigne´, 1982). Solution pH was measured using a combination pH electrode, which was calibrated with standard buffers. pCBA was determined by HPLC with an eluent containing 45% 10-mM H3PO4 and 55% methanol at 1 mL min1 and detected at 234 nm. The quantification limit was 0.025 mM with a 250 mL injection loop. DOC was measured using a Shimadzu TOC-5000A unit.
3.
Results and discussion
3.1. Chemical and textural characterization of Co(II), Mn(II) and Ti(IV) carbon aerogels The textural characteristics of the original aerogels (A, A-Co(II)-15, A-Mn(II)-15 and A-Ti(IV)-15) are compiled in Table 1; all are mainly meso- and macroporous materials. Thus, both the volume of pores with diameter in between 6.6 and 50 nm (V2) and the macropore volume (V3) are much greater than the micropore volume (Vmic). Moreover, their pore size distributions showed the maxima located at around 12 nm in diameter. N2 surface area (SN2 ), CO2 surface area (SCO2 ) and Vmic values were relatively low for sample A (blank) and were slightly higher in aerogels doped with a transition metal (Co(II), Mn(II) or Ti(IV)). However, comparisons between results for A-Co(II)-15, A-Mn(II)-15 and A-Ti(IV)-15 (Table 1)
Table 1 – Textural characterization of the aerogel samples Sample A A-Co(II)-15 A-Ti(IV)-15 A-Mn(II)-15 A-Mn(II)-50 A-Mn(II)-200 A-Mn(II)-15-1 A-Mn(II)-15-2 A-Mn(II)-15-3 A-Co(II)-15-1 A-Ti(IV)-15-1 A-Mn(II)-15C Witco
SN2 (m2 g1)
SCO2 (m2 g1)
Vmic (cm3 g1)
V2 (cm3 g1)
V3 (cm3 g1)
500730 562730 550730 554730 593730 646730 540730 534730 546730 560730 538730 880730 808730
200730 206730 203730 210730 285730 321730 200730 204730 206730 210730 200730 701730 669730
0.0770.01 0.0770.01 0.0770.01 0.0770.01 0.1070.01 0.1170.01 0.0770.01 0.0770.01 0.0770.01 0.0770.01 0.0770.01 0.2570.01 0.2470.01
0.3670.01 0.4370.01 0.4070.01 0.4170.01 0.3570.01 0.2870.01 0.4170.01 0.4070.01 0.4170.01 0.4170.01 0.3870.01 0.4370.01 0.0470.01
0.6870.01 0.9770.01 0.9270.01 0.9570.01 0.9070.01 0.8670.01 0.9470.01 0.9370.01 0.9570.01 0.9270.01 0.9070.01 0.8670.01 0.0570.01
SN2 ¼ apparent surface area determined applying BET equation to N2 adsorption isotherm. SCO2 ¼ apparent surface area determined applying Dubinin–Raduskevich equation to CO2 adsorption isotherm. Vmic ¼ micropore volume determined applying Dubinin–Raduskevich equation to CO2 adsorption isotherm. V2 ¼ volume of pores with diameter of 50–6.6 nm. V3 ¼ volume of pores with diameter above 50 nm.
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indicated that the textural properties of the samples did not differ according to the metal added, with all of them showing values close to 550 m2 g1, 0.4 and 0.9 cm3 g1 for SN2 , V2 and V3, respectively. The value of SN2 was higher than that of SCO2 in these samples indicating that a large fraction of the surface of these samples corresponded to meso- and macropores (Rodrı´guez-Reinoso and Linares-Solano, 1989). Results presented in Table 1 show that when the R/C relationship was higher (samples A-Mn(II)-15, A-Mn(II)-50 and A-Mn(II)-200), the SN2 , SCO2 and Vmi values were greater, however, the V2 and V3 values were lower; this indicates that a decrease in the amount of Mn(II) in the aerogel led to an enhancement of its surface area and microporosity, as well as to a decrease in its meso- and macroporosity. The carbon aerogels (A, A-Co(II)-15, A-Ti(IV)-15 and A-Mn(II)-15) were chemically characterized by determination of the pHPZC, XRD and XPS. Results obtained are shown in Tables 2 and 3. The pHPZC results (Table 2) showed the aerogels to have a high surface acidity (pHPZC ¼ 3.5–4.3). The XPS results (Table 2) showed a high concentration of surface oxygen with values greater than 20%. To determine the nature of the oxygenated surface groups, deconvolution of the O1s XPS spectrum was performed for each sample (Table 3), observing that the surface oxygen was distributed almost equally between ether (–C–O–C) and carbonyl (4CQO) groups, samples A, A-Co(II)-15 and A-Ti(IV)-15. However, in sample A-Mn(II)-15 the amount of 4CQO (76%) is much higher than that of–C–O–C (24%) (Table 3). XRD results for samples A-Co(II)-15, A-Mn(II)-15 and A-Ti(IV)-15 showed a wide dispersion of the metal on the aerogel surface, since no diffraction peaks were observed in any case (results not shown). Importantly, XPS analysis of the same samples revealed that the metals on the aerogel surface were in oxidation state +2 in the case of Mn and Co, and +4 in the case of Ti, and that the surface percentage of each metal was around 15% in all cases (Table 2). The results obtained in aerogel samples doped with decreasing concentrations of Mn(II), i.e., samples A-Mn(II)15, A-Mn(II)-50 and A-Mn(II)-200, showed no clear influence of the R/C ratio used on the surface acidity of the aerogel, since the pHPZC and oxygen percentage were similar among the samples (Table 2). However, deconvolution of the O1s spectra
(Table 3) of these samples showed a slight increase in the percentage of oxygenated functional groups in –C–O form with a reduction in the R/C ratio used (Table 3). Textural analyses of the aerogel samples treated with ozone (A-Mn(II)-15-1, A-Mn(II)-15-2, A-Mn(II)-15-3, A-Co(II)-15-1 and A-Ti(IV)-15-1) showed that this treatment did not significantly affect the SN2 , SCO2 , V2 and V3 values obtained, which were similar to those in the non-pretreated samples (Table 1). XPS results (Table 2) showed that, regardless of the sample considered, an increase was produced in the percentage of surface oxygen with an increase in the number of ozonation cycles to which the samples were subjected. No such increase was detected in the pHPZC values of these samples, which remained close to initial values (pHPZC ¼ 3–4). In all aerogel samples the surface oxygen created during their ozonation was mainly in the form of –CQO (Table 3). Interestingly, in the O3-pretreated Mn-doped aerogel samples Mn(III) and Mn(IV) are formed; the percentage of Mn in oxidation state +4 increased with a larger number of ozonation cycles in the pretreatment. However, the results for samples A-Co(II)-15-1 and A-Ti(IV)-15-1 showed that the oxidation state of the Co
Table 3 – Deconvolution of O1s spectra of the aerogel samples Sample
(–CQO) 53270.2 eV (%)
(–C–O) 533.970.2 eV (%)
5271 5571 4971 7671 6471 5771 8371 9171 9671 6871 6571 371 3971
4871 4571 5171 2471 3671 4371 2771 971 471 3271 3571 9771 6171
A A-Co(II)-15 A-Ti(IV)-15 A-Mn(II)-15 A-Mn(II)-50 A-Mn(II)-200 A-Mn(II)-15-1 A-Mn(II)-15-2 A-Mn(II)-15-3 A-Co(II)-15-1 A-Ti(IV)-15-1 A-Mn(II)-15C Witco
Table 2 – Chemical characterization of the aerogel samples Sample
pHPZC
C (%)
O (%)
Co(II) (%)
Mn(II) (%)
Mn(III) (%)
Mn(IV) (%)
Ti(IV) (%)
A A-Co(II)-15 A-Ti(IV)-15 A-Mn(II)-15 A-Mn(II)-50 A-Mn(II)-200 A-Mn(II)-15-1 A-Mn(II)-15-2 A-Mn(II)-15-3 A-Co(II)-15-1 A-Ti(IV)-15-1 A-Mn(II)-15C Witco
3.570.1 3.870.1 4.370.1 4.270.1 3.770.1 3.270.1 4.070.1 4.170.1 3.970.1 3.970.1 4.270.1 7.570.1 6.870.1
6871 6471 6471 6271 7171 7771 5471 4971 4271 5571 5371 8071 7471
3271 2271 2171 2271 2271 2171 3071 3571 4271 3171 3271 371 2671
— 1471 — — — — — — — 1471 — — —
— — — 1671 771 271 1071 871 671 — — 1771 —
— — — — — — 471 471 471 — — — —
— — — — — — 271 471 671 — — — —
— — 1571 — — — — — — — 1571 — —
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and Ti remained at +2 and +4, respectively (Table 2) after the ozonation pretreatment (see Section 2.2.4.). Sample A-Mn(II)-15 was subjected to physical activation treatment with CO2 following the procedure described in the Experimental Section (sample A-Mn(II)-15C). Results of its textural and chemical characterization are also presented in Tables 1–3. They indicate that the activation process increased its microporosity with an increase in its micropore volume from 0.075 to 0.250 cm3 g1 (Table 1); this led to an enhancement in its surface area, mainly in the value obtained by using CO2, which varied from 210 to 701 m2 g1. Moreover, the activation treatment produced a marked increase in the surface basicity of the sample, which showed a pHPZC value of 7.5 (Table 2), mainly because of the removal of surface oxygenated functional groups by the high temperatures used during the activation process (Valde´s et al., 2002). The low content of surface oxygen observed on sample A-Mn(II)-15C (Table 2) supports this hypothesis. Interestingly, most of the surface oxygen on this sample was in –C–O form (Table 3) and the Mn in it (Table 2) remained in oxidation state +2. XRD analysis of this sample again showed no diffraction peaks, indicating a high dispersion of Mn on the activated aerogel surface.
3.2. Ozonation of pCBA in presence of metal-doped carbon aerogels Fig. 1 depicts the results of pCBA ozonation in presence of the original carbon aerogels and the corresponding adsorption kinetics of this compound on the samples. The adsorption kinetics of pCBA on the aerogel samples studied was very slow, and no adsorption of pCBA was observed during 60 min of contact. The Rct values obtained in the different experiments and the corresponding ozone decomposition constants (kD), determined by a first-order kinetic model (Elovitz and von Gunten, 1999), are listed in Table 4. The presence of the aerogel prepared without metal (sample A) did not increase the pCBA removal rate (Fig. 1),
and the Rct and kD values were close to those observed in absence of aerogel (Table 4). These findings indicate that the organic matrix used to generate the aerogel does not positively contribute to the transformation of ozone into d OH radicals. Results presented in Fig. 1 also show that the pCBA removal rate was increased in presence of Mn aerogel, whereas the presence of Co- or Ti-doped carbon aerogels had virtually no effect on this rate. Because of the low reactivity of pCBA against ozone (Yao and Haag, 1991) and the slow adsorption kinetics of pCBA on the aerogel samples studied (Fig. 1), the increase in its oxidation rate in presence of Mn aerogel would mainly result from the generation of dOH radicals in the system. Moreover, the fact that the presence of aerogel A (blank) did not increase the rate of removal of pCBA from
1.0 [pCBA]/[pCBA]0
WAT E R R E S E A R C H
A
0.8
Without aerogel
A-Co(II)-15
0.6
A-Ti(IV)-15
0.4 A-Mn(II)-15
0.2 0.0 0
20
40
60
t (min)
Fig. 1 – Ozonation of pCBA in presence of the aerogels. pH 7, T 25 1C, [O3] ¼ 2 105 M, [t-BuOH] ¼ 8 105 M, [Aerogel] ¼ 2.5 mg L1. (x), Without aerogel; (K), A; (’), ACo(II)-15; (E), A-Ti(IV)-15; (m), A-Mn(II)-15. Open symbols represent the adsorption kinetics on samples A (J) and AMn(II)-15 (n).
Table 4 – Determination of Rct value in the different ozonation experiments performed pH 7, T 25 1C, [O3] ¼ 2 105 M, [tBuOH] ¼ 8 105 M) Experiment 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Sample
Carbon dose (mg)
kD (s1)
Rct
Without aerogel A A-Co(II)-15 A-Ti(IV)-15 A-Mn(II)-15 A-Mn(II)-15 A-Mn(II)-15 A-Mn(II)-50 A-Mn(II)-200 A-Mn(II)-15-1 A-Mn(II)-15-2 A-Mn(II)-15-3 A-Mn(II)-15C A-Mn(II)-15C-1 A-Mn(II)-15C-2 A-Mn(II)-15C-3
0 2.5 2.5 2.5 2.5 5.0 10.0 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
(6.070.3) 104 (6.270.3) 104 (5.870.3) 104 (6.170.3) 104 (4.270.2) 103 (8.470.4) 103 (1.770.1) 102 (1.770.1) 103 (5.070.2) 104 (2.670.1) 103 (2.170.2) 103 (1.470.1) 103 — — (1.870.1) 102 (2.470.1) 103
(2.6870.13) 109 (2.7470.13) 109 (2.5670.12) 109 (2.7370.13) 109 (5.3670.26) 108 (1.1770.06) 107 (2.2270.11) 107 (2.6870.13) 108 (3.3870.16) 109 (3.3570.16) 108 (2.6870.13) 108 (1.7870.08) 108 — — (2.6870.13) 107 (6.5270.32) 10 8
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the medium (Fig. 1) confirms that the activity of the aerogels in the transformation of ozone into dOH radicals is directly related to the presence of the metal on their surface. The presence of Mn aerogel during pCBA ozonation increased Rct values 20-fold, whereas the Rct values obtained in presence of Co(II) and Ti(IV) were very similar to those obtained in their absence. Likewise, the presence of Mn(II) aerogel during pCBA ozonation increased the kD value, whereas the presence of Co(II) or Ti(IV) aerogels had practically no effect on this parameter (Table 4, Experiments 2–5). These findings confirm that the presence of Mn-doped carbon aerogel during this ozonation process accelerates ozone transformation into dOH radicals. Results obtained in a previous study (Sa´nchez-Polo and Rivera-Utrilla, 2004) showed that the activity of the metals dissolved in the process of the transformation of ozone into dOH radicals is directly related to their reduction potential. Thus, metals susceptible to oxidation by ozone are potential initiators of this process. The same study also showed that the oxidation kinetics of the metal with ozone determines the efficacy of the process. Therefore, under the conditions studied, Co(II) and Ti(IV) are not accelerating ozone transformation into dOH radicals. In order to determine the mechanism by which the Mn(II) aerogel accelerates the pCBA removal rate, samples A-Co(II)15, A-Mn(II)-15 and A-Ti(IV)-15 were studied by XPS after their prior ozonation treatment as described in the Experimental section (samples A-Co(II)-15-1, A-Mn(II)-15-1 and A-Ti(IV)-151). Results of the textural and chemical characterization of these samples were discussed in detail in the previous section (Tables 1–3). However, it is of interest to note that the aerogel sample that enhanced ozone transformation into dOH radicals (A-Mn(II)-15) is also the one in which the oxidation state increased after the ozonation process (Table 2). Thus, 10% of the surface Mn was present in Mn(II) form, 4% in Mn(III) form and 2% in Mn(IV) form on sample A-Mn(II)-15-1. For Co and Ti (samples A-Co(II)-15 and A-Ti(IV)-15), the oxidation state of the metal was unchanged at +2 and +4, respectively. These results indicate that the mechanism by which A-Mn(II)-15 aerogel enhances transformation of ozone into dOH radicals is based on oxidation–reduction reactions. Hence, and in agreement with the results presented in Fig. 1 and Tables 1–4 possible reactions responsible for accelerating the transformation of ozone into dOH radicals due to the presence of A-Mn(II)-15 aerogel during pCBA ozonation may be: O3 þ Mn2þ ! MnO2þ þ O2 ;
(1)
MnO2þ þ H2 O ! Mn3þ þ d OH þ OH ;
(2)
2Mnþ3 þ 2H2 O ! MnO2 þ Mn2þ þ 4Hþ :
(3)
In this way, oxidation of surface Mn(II) to Mn(III) and Mn(IV) during the oxidation process results in a transformation of ozone into dOH radicals (reaction (2)), explaining the increase in the pCBA removal rate (Fig. 1) and the Rct and kD values observed (Table 4, Experiment 5). This mechanism is in agreement with the proposed by other authors (Andreozzi et al., 1996; Ma and Graham, 1999; Ma and Graham, 2000) where
the ozonation of atrazine and oxalic acid in presence of dissolved Mn(II) and MnO2 were studied.
3.3. Influence of Mn(II)-doped organic aerogel dose on pCBA oxidation process The minimum aerogel dose required to initiate and/or promote the transformation of dissolved ozone into dOH radicals is an essential parameter in the design of this novel treatment process. Fig. 2 depicts the evolution of pCBA concentrations in presence of increasing concentrations of aerogel in the system. It can be observed that the pCBA degradation rate increased with a higher dose of added aerogel. Determination of the Rct parameter (Table 4, Experiments 5–7) demonstrated a more efficient generation of dOH radicals when the dose of aerogel in the system was increased, and a linear relationship was observed between the Rct value and the aerogel dose (Fig. 3). In the treatment of drinking waters typical doses of powdered activated carbon range from 1 to 10 mg/L depending on the water matrix (Laine et al., 2000). The results shown in Fig. 2 and Table 4 indicated that the presence of A-Mn(II)-15
1.0 [pCBA]/[pCBA]0
WAT E R R E S E A R C H
0.8
Without aerogel
0.6 2.5 mg/L
0.4 0.2
5 mg/L 10 mg/L
0.0 0
20
40
60
t (min)
Fig. 2 – Influence of dose of A-Mn(II)-15 aerogel on pCBA oxidation. pH 7, T 25 1C. [O3] ¼ 2 105 M, [t-BuOH] ¼ 8 105 M. (x), Without aerogel; (m), 2.5 mg L1; (’), 5 mg L1; (K), 10 mg L1.
2.5 2.0 Rct x 108
3380
1.5 1.0 0.5 0.0 0
2
4
6
8
10
Aerogel dose (mg/L) Fig. 3 – Relationship between Rct value and dose of added aerogel A-Mn(II)-15.
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aerogel in the system, in amounts in between the above range, produced an increase in the pCBA removal rate and a marked rise in the Rct value, indicating an acceleration of ozone transformation into dOH radicals.
3.4. Influence of surface concentration of Mn(II) in carbon aerogels on pCBA oxidation process To determine the minimal surface concentration of Mn(II) able to initiate ozone transformation into dOH radicals, the ozonation of pCBA was carried out in presence of aerogels prepared with decreasing concentrations of Mn(II) (A-Mn(II)15, A-Mn(II)-50 and A-Mn(II)-200). Results obtained are listed in Table 4, experiments 5, 8, and 9. Results of the textural and chemical characterization of these samples (Tables 1–3) were discussed in Section 3.1. An increase in the pCBA removal rate was observed with an increased concentration of surface Mn(II) (figure not shown). kD and Rct values (Table 4) also increased with a higher concentration of Mn on the aerogel surface. These results may confirm that the efficacy of Mn(II)doped aerogels for the transformation of ozone into dOH radicals is mainly due to the presence of Mn(II) on their surface. A linear relationship was found between the kD and Rct values obtained (Table 4) and the concentration of surface Mn(II) (Table 2), similar to the observation reported in the previous section. From the results presented in Tables 2 and 4 it is interesting to note that the Mn surface concentration in A-Mn(II)-200 sample could be almost the limit where still a small effect on ozone transformation can be observed.
3.5. Influence of aerogel preozonation on ozone transformation into dOH radicals In order to confirm the mechanism involved in the transformation of ozone into dOH radicals due to the presence of Mn(II) carbon aerogel in the system, pCBA ozonation were performed in presence of A-Mn(II)-15 aerogel samples subjected to ozonation pretreatment (Fig. 4). The Rct and kD values found are listed in Table 4, Experiments 5, 10–12. The influence of the ozone treatment on the chemical and textural properties of these samples was discussed in greater detail in Section 3.1. The pCBA oxidation rate (Fig. 4) was reduced and the Rct and kD were markedly decreased when the pCBA ozonation was carried out in presence of preozonated aerogel (Table 4). Thus, sample A-Mn(II)-15-3 showed a reduction of around 85% in their values. These results indicate that the ability of the A-Mn(II)-15 aerogel to initiate ozone transformation into dOH radicals is reduced with greater exposure of the aerogel to ozone. Surface characterization of these samples (Tables 1–3) showed that the ozonation treatment applied produced no major change in their textural or chemical properties (only an increase in their surface oxygenated groups). However, XPS analysis of these samples indicated a gradual oxidation of the surface Mn present (Table 2). Thus, the concentration of surface Mn in oxidation states +2 progressively decreased with more ozonation cycles. These results corroborate the mechanism proposed in Eqs. (1)–(3). Thus, the capacity to initiate the transformation of ozone into dOH radicals was reduced when the concentration of surface Mn in oxidation state +2 was lower.
1.0 [pCBA]/[PCBA]0
WAT E R R E S E A R C H
Without aerogel
0.8
A-Mn(II)-15-3
0.6
A-Mn(II)-15-2
0.4
A-Mn(II)-15-1 A-Mn(II)-15
0.2 0.0 0
20
40
60
t (min) Fig. 4 – Ozonation of pCBA in presence of preozonated A-Mn(II)-15 aerogel samples. pH 7, T 25 1C, [O3] ¼ 2 105 M, [t-BuOH] ¼ 8 105 M, [Aerogel] ¼ 2.5 mg L1. (x), Without aerogel; (m), A-Mn(II)-15; (K), A-Mn(II)-15-1; (E) , A-Mn(II)-15-2; (’), A-Mn(II)-15-3.
This rapid deactivation of the aerogel samples during ozonation may limit their applicability in water treatment. Therefore, new treatments of these aerogels must be sought that avoid their deactivation. Powdered activated carbon is commonly used in treatment of drinking waters because of its high adsorbent properties. This material can also enhance ozone transformation into d OH radicals (Beltra´n et al., 2002; Jans and Hoigne´, 1998; Ma et al., 2004; Rivera-Utrilla et al., 2002; Sa´nchez-Polo et al., 2005; Sa´nchez-Polo et al., 2006). It was demonstrated that the metal centers of its mineral matter, the electrons of the basal plane of the activated carbon, and the basic groups on its surface are the activated carbon characteristics largely responsible for the transformation of ozone into dOH radicals (Rivera-Utrilla et al., 2002). Therefore, in order to increase the capacity of sample A-Mn(II)-15 to transform ozone into dOH radicals it was activated with CO2, obtaining the activated aerogel denominated sample A-Mn(II)-15C. The influence of the treatment applied on chemical and textural properties was discussed in detail in Section 3.1. Fig. 5 depicts the results obtained from the ozonation of pCBA in presence of the activated aerogel. The pCBA oxidation rate increased significantly in presence of sample A-Mn(II)15C compared to the presence of the non-activated sample. In fact, Rct and kD could not be determined for the system O3/AMn(II)-15C because of the very high rate of transformation of the ozone into dOH radicals (Table 4, Experiment 13). These results are mainly due to an increase in surface basicity (Rivera-Utrilla et al., 2002). The reduction in the oxygen concentration (Table 2) produced an increase in the surface electronic density, thereby increasing the extent of the transformation of ozone into dOH radicals.
3.6. Ozonation of natural surface waters in presence of activated carbon prepared from Mn(II) aerogel. Comparison between O3/H2O2 and O3/activated carbon processes The applicability of this new O3/activated aerogel process for water treatment was tested by comparing it with AOPs commonly used for this purpose (O3, O3/H2O2, and
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O3/activated carbon) in pCBA ozonation experiments in Lake Zurich water (Fig. 6). The doses of O3, H2O2 and activated carbon selected were those commonly used for treating drinking waters. The results obtained are shown in Fig. 6 and Table 5. The pCBA removal rate was especially increased when ozonation of this contaminant was performed in
1.0 [pCBA]/[pCBA]0
Without aerogel 0.8 0.6 A-Mn(II)-15 0.4 0.2 A-Mn(II)-15C 0.0 0
20
40
60
t (min) Fig. 5 – Influence of activation of A-Mn(II)-15 aerogel on pCBA ozonation. pH 7, T 25 1C, [O3] ¼ 2 105 M, [tBuOH] ¼ 8 105 M, [Aerogel] ¼ 2.5 mg L1. (x), Without aerogel; (m), A-Mn(II)-15; (’), A-Mn(II)-15C.
[pCBA]/[pCBA]0
1.0 0.8 0.6
O3
0.4
O3/Activated carbon
0.2
O3/A-Mn(II)-15
O3/H2O2 O3/A-Mn(II)-15C
0.0 0
20
40
60
t (min) Fig. 6 – Ozonation of Lake Zurich water in presence of the different processes studied. pH 7, T 25 1C, [O3] ¼ 2 105 M, [H2O2] ¼ 1 105, [Aerogel] ¼ 2.5 mg L1, [Activated carbon] ¼ 2.5 mg L1. (x), O3; (K), O3/Activated carbon; (m), O3/A-Mn(II)-15; (E) , O3/H2O2; (’), O3/A-Mn(II)-15C.
presence of the activated Mn-doped aerogel A-Mn(II)-15C (Fig. 6). Moreover, the Rct and kD values obtained (Table 5) were markedly higher than those observed in the other AOPs studied (O3/H2O2 and O3/activated carbon). pCBA ozonation was also performed in presence of A-Mn(II)-15C subjected to ozone pretreatment (see Experimental section). The pCBA oxidation rate was reduced and the Rct and kD were markedly decreased in presence of preozonated activated aerogel. These results indicate that the ability of the A-Mn(II)-15C sample to initiate the ozone transformation process into dOH radicals is reduced with greater exposure of the aerogel to ozone (Table 4, Experiments 13–16), confirming the results presented in the previous section. It is interesting to compare the Rct and kD values observed using the O3/activated carbon and the O3/activated aerogel systems. The activated carbon selected was Witco commercial carbon because it has SN2 and pHPZC values very close to those found for sample A-Mn(II)-15C. Moreover, this carbon has a low ash content (0.3%). The volume of meso- and macropores in the Witco carbon was notably lower than that in sample A-Mn(II)-15C (Tables 1 and 2). The results shown in Fig. 6 and Table 5 indicate the greater efficiency of sample A-Mn(II)-15C for the transformation of ozone into dOH radicals compared with activated carbon Witco. These findings confirm that the activity of carbon materials in the transformation of ozone into dOH radicals, is favored by: (i) an elevated meso- and macroporosity, which reduces diffusion problems and facilitates access of the ozone to active surface sites; and (ii) the presence of metals susceptible to surface oxidation by ozone. A highly important parameter to assess the efficacy of a treatment process is its capacity to remove natural organic matter (NOM) from the natural water. Fig. 7 depicts the evolution of the DOC concentration as a function of the treatment time for each of the processes studied. It can be observed that the systems based on O3 and O3/H2O2, do not lead to a reduction in DOC concentration after 60 min of treatment. This is because none of the processes have sufficient oxidizing capacity to transform NOM into CO2. In contrast, there was a reduction of 70% after 60 min for the O3/activated aerogel system (Fig. 7). This reduction in DOC is decreased to around 50% and 20% when using O3/aerogel and O3/activated carbon, respectively. This reduction of NOM is mainly due to adsorption of the DOC on the surface of the
Table 5 – Determination of Rct value in the different experiments performed using Lake Zurich waters (pH 7, T 25 1C, [O3] ¼ 2 105 M, [H2O2] 1 105, [Aerogel] ¼ 2.5 mg L1, [Activated carbon] ¼ 2.5 mg L1) Experiment 1 2 3 4 5 6
Sample
kD (s1)
Rct
Without carbon A A-Mn(II)-15 A-Mn(II)-15C Witco H2O2
(7.070.3) 104 (6.870.3) 104 (6.870.3) 103 (9.270.4) 103 (6.770.3) 104 (8.570.4) 103
(5.0170.25) 109 (4.9670.25) 109 (8.5270.42) 108 (6.1570.30) 107 (4.9270.25) 109 (1.1770.06) 107
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1.0 [DOC]/[DOC]0
4 0 (200 6) 337 5 – 338 4
O3/Activated carbon
0.8 0.6
O3/A-Mn(II)-15
0.4
O3/A-Mn(II)-15C
0.2
3383
treatment produced a marked increase in the surface basicity of the sample, which showed a pHPZC value of 7.5, mainly because of the removal of surface oxygenated functional groups by the high temperatures used during the activation process. Ozonation of drinking waters in presence of activated aerogel leads to (i) an acceleration of ozone transformation into dOH radicals, and (ii) a reduction in the concentration of dissolved organic material.
0.0 0
10
20
30
40
50
60
t (min) Fig. 7 – Evolution of DOC during ozonation of lake Zurich water for the different processes studied. pH 7, T 25 1C, [O3] ¼ 2 105 M, [H2O2] ¼ 1 105, [Aerogel] ¼ 2.5 mg L1, [Activated carbon] ¼ 2.5 mg L1. (x), O3; (K), O3/ Activated carbon; (m), O3/A-Mn(II)-15; (E), O3/H2O2; (’), O3/A-Mn(II)15C.
Acknowledgments The authors are grateful for the financial support provided by MCyT and FEDER (Project: CTQ2004-07783-C02-01) and Junta de Andalucia (Project: RNM547). The authors would also like to acknowledge Elisabeth Salhi for her assistance with the experimental work. M. Sa´nchez-Polo expresses his gratitude to the Junta de Andalucı´a for providing a research fellowship. R E F E R E N C E S
solid material. The higher adsorption capacity of activated aerogel is principally due to a larger volume of meso- and macropores, increasing the surface area accessible to adsorb organic matter components. The results presented in Fig. 7 indicate that, besides markedly accelerating the transformation of ozone into dOH radicals, the presence of low concentrations of Mn-doped carbon aerogels or Mn-doped activated carbon aerogels during the ozonation of drinking waters also lead to a reduction in the organic matter present in the system, which is desirable to improve the water quality.
4.
Conclusions
Addition of metals does not have a marked influence on the textural and chemical properties of aerogel samples. However, the addition of increasing concentrations of metals produces a substantial reduction in surface area and an increase in the volume of meso- and macropores, although the surface chemistry is not modified. The capacity of metal-doped carbon aerogels to accelerate the transformation of ozone into dOH radicals is related to the metal on their surface. Only metals susceptible to oxidation by ozone are effective. The presence of Mn(II)-doped carbon aerogels during ozonation accelerates ozone transformation into dOH radicals. The capacity to accelerate this process depends on the dose of aerogel and the concentration of Mn(II) on its surface. The capacity of the Mn-doped carbon aerogel to accelerate ozone transformation into dOH radicals decreases with longer exposure of the aerogel to ozone. The oxidation of surface Mn(II) to higher oxidation states is responsible for this behavior and leads to an inactivation of this material. The pCBA oxidation rate increased significantly in presence of Mn aerogel activated sample (A-Mn(II)-15C) compared to the presence of the non-activated sample. These results are mainly due to an increase in surface basicity. Activation
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