Competitive adsorption of phosphate and phosphonates onto ...
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Competitive adsorption of phosphate and phosphonates onto goethite Bernd Nowacka,!, Alan T. Stoneb a
Institute of Terrestrial Ecology, ETH Zu¨rich, Universita¨tstrasse 16, 8092 Zu¨rich, Switzerland Department of Geography and Environmental Engineering, The Johns Hopkins University, Baltimore, MD 21218, USA
b
art i cle info
A B S T R A C T
Article history:
Phosphate and phosphonates are both strongly adsorbed onto mineral surfaces and their
Received 26 January 2006
removal during wastewater treatment is mainly due to adsorptive processes. We have
Received in revised form
conducted experiments to study the mutual influence of phosphate and six different
17 March 2006
phosphonates on each other in buffered medium at pH 7.2. We have used phosphonates
Accepted 22 March 2006
having one to five phosphonic acid groups (HMP, IDMP, HEDP, NTMP, EDTMP and DTPMP).
Available online 3 May 2006
The presence of phosphonates suppressed the adsorption of phosphate. The monopho-
Keywords:
sphonate HMP had the smallest and the polyphosphonates the largest effect on phosphate
Phosphate
adsorption. The presence of phosphate lowered phosphonate adsorption. The competition
Phosphonates
in the multicomponent system can reasonably well be predicted using a surface
Competition
complexation model developed for single component systems. The competitive model
Adsorption
only failed in systems containing the polyphosphonate DTPMP. With this approach we can
Iron oxides
predict the behavior of both compounds during wastewater treatment. The calculations
Water
show that phosphonates have a small effect on phosphate adsorption at the actual
Water treatment
concentrations in observed wastewater. Adsorption of low concentrations of phosphonates was calculated to be significantly reduced by phosphate concentrations as observed in wastewater. & 2006 Elsevier Ltd. All rights reserved.
1.
Introduction
Phosphate and phosphonates are interesting substances from a water treatment point of view. Phosphate discharged into surface waters can stimulate plant growth, resulting in a eutrophication of rivers and lakes. An efficient removal of phosphate during wastewater treatment is therefore an important factor in preserving the water quality of eutrophic lakes. Phosphonates are complexing agents containing one or more C–PO(OH)2 groups (Nowack, 2003). Phosphonates have three main properties: they are effective chelating agents for two- and tri-valent metal ions (Lacour et al., 1998), they inhibit crystal growth and scale formation and they are quite !Corresponding author. Tel.: +41 44 633 61 60; fax: +41 44 633 11 23.
E-mail address: [email protected] (B. Nowack). 0043-1354/$ - see front matter & 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2006.03.018
stable under harsh chemical conditions. Important industrial uses of phosphonates are in cooling waters, desalination systems and in oil fields to inhibit scale formation. In pulp and paper manufacturing and in textile industry they are used as peroxide bleach stabilizers, acting as chelating agents for metals that could inactivate the peroxide. In detergents they are used as a combination of chelating agent, scale inhibitor and bleach stabilizer. Around 16,000 tons of phosphonates are used per year in Europe and 30,000 tons in the USA (Nowack and VanBriesen, 2005). Phosphonates are not biodegraded during wastewater treatment (Steber and Wierich, 1987; Horstmann and Grohmann, 1988) but can be rapidly oxidized in the presence of
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manganese (Nowack and Stone, 2000) or are photodegraded by sunlight (Lesueur et al., 2005). Phosphate and phosphonate can influence the behavior of each other. It has been shown that high concentrations of phosphonates have an adverse effect on phosphate elimination (Mu¨ller et al., 1984; Horstmann and Grohmann, 1986). Similar to phosphate, phosphonates adsorb strongly onto mineral surfaces such as iron oxides (Mu¨ller et al., 1984; Fischer, 1991; Nowack and Stone, 1999a, b). Iron dosage for phosphate elimination during wastewater treatment also removes most of the phosphonates (Mu¨ller et al., 1984). This has been verified in field studies for wastewater treatment plants (WWTPs) that operated with chemical phosphate elimination (Nowack, 1998, 2002). Removal rates of 90% and more were observed regularly. Influent concentrations of 0.5–2 mM phosphonates were reduced to less than 0.05 mM in most cases. During phosphate precipitation by iron salts, both ironphosphates and iron (hydr)oxides are formed (Parsons and Berry, 2004) on which both phosphate and phosphonates adsorb. The adsorption of phosphonates onto the iron (hydr)oxide goethite has been described by a surface complexation model (Nowack and Stone, 1999a). Phosphate adsorption onto goethite is well studied and several models have been used to successfully describe the adsorption (Nilsson et al., 1992; Geelhoed et al., 1997). While adsorption studies have mainly been done in single component systems, natural environments always include a variety of different anions and cations that compete for surface sites and change the adsorption of each other by influencing the surface charge. When a single component system can be modeled successfully, this does not necessarily imply that a more complex multicomponent system can be described by applying the results from different single adsorbate systems. The ultimate goal of adsorption modeling is to come up with a set of stability constants that are able to describe multicomponent systems. Surface complexation models have been used to successfully describe multicomponent systems, e.g. Mesuere and Fish (1992), Goldberg (2002). This study investigated phosphonate–phosphonate and phosphonate–phosphate competitive adsorption in pH 7.2 suspensions containing the iron (hydr)oxide goethite. We have used the polyphosphonates HEDP, NTMP, EDTMP and DTPMP (see Table 1 for names and structures), which are the most widely used in technical applications. Additionally, we have used the monophosphonate HMP (hydroxymethylphos-phonic acid) and the bisphosphonate IDMP (iminodimethylenephosphonic acid), a breakdown product of NTMP (Steber and Wierich, 1987). The data were modeled using experimentally determined phosphate surface complexation constants and the phosphonate surface complexation model presented by Nowack and Stone (1999a). Calculations were performed to examine whether phosphonates interfere with efforts to chemically remove phosphate during wastewater treatment. The behavior of phosphonates in WWTPs was then compared to the behavior of the aminocarboxylate chelating agent EDTA.
2.
Materials and methods
2.1.
Materials
The goethite used in this study was synthesized and characterized as described by Coughlin and Stone (1995) and stored as a slurry containing 44 g/L goethite. The reported surface area of this goethite preparation was 47.5 m2/g. The phosphonates HEDP, IDMP and NTMP were obtained in the acid form from Fluka with 497% purity. EDTMP in the acid form was obtained from Monsanto (Dequest 2041, 95%). DTPMP and HMP in the acid form were provided by Monsanto (St. Louis, USA). Phosphate solution was prepared from Na2HPO4.
2.2.
Adsorption experiments
All experiments were performed in 30 mL glass vials and employed 0.42 g/L goethite and 0.01 M NaNO3 (Baker Chemical Co.). After addition of appropriate amounts of phosphate and phosphonate stock solutions, suspensions were stirred with Teflon-coated magnetic stir bars and maintained at 2572 1C for 24 h before pH measurement and sample collection. Phosphonate adsorption occurs on a timescale of 1 min (Nowack and Stone, 1999a), while phosphate adsorption was slower and required 24 h equilibration. Samples were filtered through 0.2 mm polycarbonate filters (Nuclepore Corp.) prior to analysis. No dissolved iron was detected, indicating that ligand controlled dissolution of goethite by phosphonates was negligible. Phosphate adsorption as a function of pH was examined without using an added buffer; suspension pH was varied by adding different amounts of HNO3 or NaOH. Competitive adsorption experiments and all other experiments performed at pH 7.2 employed a 1 mM MOPS-buffer (4-morpholinepropane sulfonic acid, Aldrich). MOPS was found to have no influence on phosphonate adsorption (Nowack and Stone, 1999a). In one set of competitive adsorption experiments, the concentration of each phosphonate was increased at a fixed phosphate concentration of 40 mM. In another set, phosphate concentrations were increased at fixed phosphonate concentration (HMP: 40 mM, HEDP and IDMP: 25 mM, NTMP, EDTMP and DTPMP: 20 mM). Additionally the effect of increasing EDTMP concentrations on adsorption of 12 mM NTMP was measured.
2.3.
Analytical methods
Using the derivatization plus ion-pair HPLC technique described by Nowack (1997), detection limits of 0.1 mM for NTMP, EDTMP and DTPMP and 0.5 mM for HEDP could be achieved. The technique involves amending samples with Fe(III) at low pH, which generates anionic Fe(III)–phosphonate complexes; these complexes are then separated on a polymer-based reversed-phase HPLC column (Polymer Laboratories PLRP-S) using tetrabutylammonium as counterion and a bicarbonate–acetonitrile eluent at pH 8.3. We have tested that at the concentrations employed in this study, phosphate did not interfere with phosphonate analysis.
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Table 1 – Abbreviations, names, and structures of the phosphonates used in this study Acronym
Name
HMP
Hydroxymethylphosphonic acid
IDMP
Iminodimethylenephosphonic acid
Structure
OH
(HO)2OP
(HO)2OP (HO)2OP
NH PO(OH)2
H3C HEDP
1-Hydroxyethane (1,1-diylbis-phosphonic acid)
NTMP
Nitrilotrismethylenephosphonic acid
EDTMP
1,2-Diaminoethanetetrakismethylene phosphonic acid
OH C PO(OH)2
(HO)2OP (HO)2OP
PO(OH)2
N
(HO)2OP N
(HO)2OP
PO(OH)2
N
PO(OH)2 (HO)2OP DTPMP
(HO)2OP
Diethylenetriaminepentakismethylene phosphonic acid
PO(OH)2 N
N
N
PO(OH)2
PO(OH)2
Phosphate was determined by the molybdenum blue method and spectrophotometric detection at 720 nm (American Public Health Association, 1989). We have tested that the presence of phosphonates did not interfere with the analysis of phosphate. HMP and IDMP were quantified by measuring phosphate concentrations before (phosphate alone) and after digestion with potassium peroxodisulfate at 100 1C for 2 h (phosphate plus phosphonate).
2.4.
Table 2 – Equilibrium constants describing the acid–base properties of goethite from Lo¨vgren et al. (1990) RFeOH+H+3RFeOH+2 RFeOH3RFeO"+H+ Total specific capacitance Site density BET surface areaa a
log K: 7.47 log K: "9.51 1.28 F m2 1.7 site/nm2 47.6 m2/g
From Coughlin and Stone (1995).
Adsorption and aqueous speciation modeling
ChemEQL (Mu¨ller, 2004) was used to calculate chemical speciation and adsorption. Stability constants for calculating the protonation speciation of phosphate and phosphonate in aqueous solution were taken from the CRITICAL database (Martell et al., 1997) and calculated for an ionic strength of 0.01 M. The basic surface parameters of the goethite are listed in Table 2 and were taken from Lo¨vgren et al. (1990). It is reasonable to assume that these data apply since both Lo¨vgren et al. (1990) and Coughlin and Stone (1995) prepared the goethite according to the same method. Acid–base properties of surface sites and electrostatic aspects of the (hydr)oxide–water interface were modeled using the 2-pK constant capacitance model (CCM). The ionic medium in the present work is a factor of ten less concentrated that that
used in Lo¨vgren et al. (1990) but the assumption is made that the values still apply. The adsorption of phosphonates was calculated using the surface complexation model of Nowack and Stone (1999a) without further modification. According to the model, phosphonates are adsorbed as mononuclear complexes with two or more distinct protonation levels. The following generalized reaction represents adsorption stoichiometries: RFeOH þ La" þ ðn þ 1ÞHþ 3RFe2L2Hn
ða"n"1Þ"
þ H2 O
(1)
where RFeOH is a surface site and La" the phosphonate in the completely deprotonated form. The number of protonation levels is analogous to the number of protonation levels
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observed for dissolved species. This equation allows for mononuclear complex formation and disallows multinuclear complex formation. It makes no distinction between monodentate and multidentate–mononuclear surface complexes. The goethite preparation used to determine competitive adsorption with phosphate was the same as that was used in our previous study of phosphonate adsorption (Nowack and Stone, 1999a). Measurements of the extent of adsorption of 40 mM total phosphate as a function of pH provided the basis for our modeling effort because in this data set phosphate was in excess of surface sites which allows an accurate determination of log K values at all pH values. The total surface site concentration used in the model was the same as that used to model phosphonate adsorption. It was assumed that all adsorbates occupy only one surface site and that the charge accompanying species adsorption lies at the zero plane. log K values of the phosphate surface complexes were fitted using FITEQL 3.1 (Herbelin and Westall, 1994). Although the actual form of precipitated iron (hydr)oxides within WWTPs is not known, it is likely to be quite similar to the hydrous iron (hydr)oxide (HFO) described by Dzombak and Morel (1990), i.e. with a surface area of 600 m2/g, a site density of 0.2 mol/mol Fe and 89 g HFO/mol Fe. If a WWTP uses an average dosage of 5 mg/L iron, this corresponds to a WWTP surface site concentration of 18 mM. Our adsorption experiments have been conducted with the crystalline iron (hydr)oxide goethite because information on phosphonate and EDTA adsorption onto goethite was available. As noted in Table 2, Lo¨vgren et al. (1990) reported 1.7 site/ nm2 for goethite, while HFO has 2.26 site/nm2 (Dzombak and Morel, 1990), so quite comparable when normalized to the surface area. For the sake of discussion we could assume that the surface chemistry of goethite and HFO are comparable and then we only need to make a surface area correction. A goethite loading of 134 mg/L has the same concentration of surface sites than HFO precipitated from 5 mg/L iron and all calculations for WWTPs have therefore been performed with this solids loading. Additionally one calculation for WWTPs conditions was performed including adsorption of EDTA. The extent of EDTA adsorption onto goethite was previously measured and modeled by Nowack and Sigg (1996). By applying adsorption stoichiometries and log K values from this previous study, we were able to perform additional calculations that explore competitive adsorption in the presence of EDTA.
3.
Results
3.1.
Phosphate adsorption
Phosphate adsorption onto goethite can be described by a surface complexation model with three different surface complexes of different protonation level. The log K values were fitted to the pH-edge data and the best-fit values are presented below: RFeOH þ PO4 3" þ Hþ 3RFePO4 2" þ H2 O log K 18:86 % 0:21; (2)
RFeOH þ PO4 3" þ 2 Hþ 3RFePO4 H" þ H2 O log b 25:68 % 0:25; (3) RFeOH þ PO4 3" þ 3 Hþ 3RFePO4 H2 þ H2 O log b 31:64 % 0:1: (4) These three phosphate surface complexes with different protonation level have been frequently used to describe phosphate adsorption, e.g. Goldberg and Sposito (1984; Nilsson et al. (1992). Fig. 1 shows the adsorption of 40 and 10 mM phosphate onto goethite together with the model fit. The distribution of the three surface species is shown for the 40 mM experiment. The two experiments represent an excess of phosphate over surface sites (40 mM) and an excess of surface sites over phosphate (10 mM). In the latter case complete adsorption of phosphate is observed at low to
Fig. 1 – Adsorption of phosphate as a function of pH at 10 and 40 lM total phosphate. The lines represent model calculations based upon log K values from Table 2 and Eqs. (2)–(4). The calculated distribution of the surface species is given for the 40 lM experiment.
Fig. 2 – Single adsorbate experiments. Isotherms for the adsorption of phosphate and the six phosphonates HMP, HEDP, IDMP, NTMP, EDTMP and DTPMP onto goethite at pH 7.2 are shown. Lines represent model calculations based upon log K values from Table 2 and from Nowack and Stone (1999a). Data redrawn from Nowack and Stone (1999a).
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neutral pH. The same log K values obtained from the pH-edge were also able to describe the adsorption isotherm of phosphate at pH 7.2 (Fig. 2).
phonates was observed. The multicomponent modeling gives an accurate description of EDTMP adsorption while NTMP adsorption is a little bit underestimated.
3.2.
3.4. Simulation of phosphonate adsorption under treatment plant conditions
Phosphonate adsorption
The adsorption of phosphonates onto goethite is characterized by a very strong affinity to the surface. Adsorption is greatest at low pH, but all phosphonates yield significant adsorption at pH above 8 (Nowack and Stone, 1999a). The amount adsorbed decreases with increasing number of phosphonic acid groups from the monophosphonate HMP to the pentaphosphonate DTPMP. The adsorption of the monophosphonates HMP is very similar to that of phosphate. Fig. 2 shows the adsorption of the phosphonates used in this study at pH 7.2 (redrawn from Nowack and Stone (1999a)) and shows that even at very low dissolved concentration of phosphonates high surface coverage is achieved. The lines in Fig. 2 are model calculations based on the surface complexation model developed by Nowack and Stone (1999a).
3.3. Competitive adsorption of phosphate and phosphonates Fig. 3 shows that with increasing phosphate addition, there is a decrease in phosphonate adsorption. The presence of phosphonates suppresses the adsorption of phosphate, as shown by the model calculation for phosphate alone in Fig. 3. The monophosphonate HMP has the smallest and the polyphosphonate DTPMP the largest effect on phosphate adsorption. Increasing the number of phosphonic acid groups in the phosphonates increases the effect on phosphate adsorption. The lines in Fig. 3 are model calculations based on Table 2, phosphate adsorption as described in Eqs. (2)–(4) and the log K values for phosphonate adsorption from Nowack and Stone (1999a). No fitting was therefore involved in calculating the competition, neither for phosphate nor for the phosphonates. The competitive adsorption can be modeled very reasonably with the constants from the single component system. There is a good prediction for the influence of phosphate on phosphonate adsorption with the exception of DTPMP. The adsorption of phosphate is represented less well by the model. The phosphate adsorption was always underestimated to some extent. The best agreement was obtained for the competition between phosphate and the monophosphonate HMP. The worst agreement was obtained for the competition between phosphate and the polyphosphonate DTPMP. Much more phosphate was adsorbed than predicted by the model. If a constant amount of phosphate was brought in contact with increasing concentrations of phosphonates, a decrease in phosphate adsorption was observed (Fig. 4). Again the polyphosphonates had a stronger influence on phosphate adsorption than the monophosphonate HMP. The lines are again the model prediction based on log K values derived from the single component system and then used to model the two-component system. Fig. 5 shows the effect of increasing EDTMP on NTMP adsorption. Competitive adsorption between the two phos-
The effect of NTMP on phosphate adsorption under WWTP conditions is shown in Fig. 6 for three different phosphate concentrations. For the average phosphonate concentration normally found in WWTPs of less than 0.5 mM, there is only a small effect on phosphate adsorption. However, at the maximum concentration of phosphonates observed in WWTPs and a low phosphate concentration of 10 mM, a reduction in phosphate adsorption of up to 40% is predicted. At 100 mM phosphate however, almost no effect of the phosphonate is found. Phosphate, of course, also has an effect on phosphonate adsorption. Fig. 7 shows the influence of increasing phosphate on NTMP adsorption (0.5 and 2 mM) under WWTP conditions. To have a significant influence on NTMP adsorption, 50 mM phosphate are expected with an expected decrease of about 50%. The effect of phosphate is more pronounced at higher NTMP concentration.
3.5.
Comparison of phosphonates and EDTA
Table 3 shows the result of modeling single and multicomponent systems with simultaneous presence of EDTA, NTMP and phosphate. The chosen conditions were pH 7, 1 mM EDTA, 0.5 mM NTMP, 20 mM phosphate and 134 mg/L goethite. Calculations were first performed for the three single component systems and then for the combined system with all three compounds competing for surface sites. If only adsorption of EDTA is considered, 95% of EDTA is predicted to be adsorbed onto the surface. However, if we also consider competition with phosphate and phosphonate for surface sites in our calculation, the amount of adsorbed EDTA becomes negligible. The same calculation for phosphate and the phosphonate NTMP shows that the reduction in the adsorbed amount is 8% for NTMP (relative to the amount adsorbed without competition) and 9% for phosphate in the calculation with competition.
4.
Discussion
We observed as expected competition between phosphate and phosphonates for available surface sites. The surface complexation model based on single component systems was able to accurately describe phosphonate adsorption in the presence of phosphate (with the exception of DTPMP). The influence of phosphonates on phosphate adsorption was less well predicted, especially for polyphosphonates. Again the agreement between model and experimental data was worst for DTPMP. However, for the other phosphonates the disagreement between model and data was never more than 35% (for HEDP) and for the other compounds about 20% or less. The results of modeling show that the assumption of competition between phosphate and phosphonates for the
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Fig. 3 – Effect of increasing phosphate concentration on the adsorption of six phosphonates at fixed concentration (pH 7.2). The lines represent model calculations based upon log K values from Table 2 and from Nowack and Stone (1999a). The thin line represents the modeled phosphate adsorption in the absence of phosphonate, the thick line is the modeled phosphate adsorption with competition. HMP: 40 lM, HEDP and IDMP 25 lM, NTMP, EDTMP, and DTPMP 20 lM.
same surface sites is justified and that we can predict the effect of one compound on the other reasonable well by using independently determined surface complexation constants. Measured phosphate adsorption was always slightly higher than the predicted adsorption. This may indicate that phosphate had access to surface sites that were not accessible to phosphonates, especially polyphosphonates with higher molecular mass. Our model has assumed that all charges associated with the adsorbed phosphonate are deposited at the zero plane. If these charges were instead distributed in multi-layers, there would be less electrostatic repulsion that serves to lower the amount of phosphate adsorbed. Competition with HMP containing just one func-
tional group, was well predicted. The worst agreement between model and experimental data was obtained for DTPMP, which contains five phosphonic acid groups. The model predicted that DTPMP would have a larger effect on phosphate adsorption than was actually observed. The model also predicted that phosphate would have a smaller effect on DTPMP adsorption than was actually observed. The accessibility of surface sites for the two compounds was clearly not the same. Charge distribution further away from the surface would be most significant for DTPMP and may be a reason for the lower competition than modeled. It is important to note that our experiments were performed using a NaNO3 medium. Under real treatment
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Fig. 4 – Competition experiments involving increasing phosphonate concentrations at fixed phosphate concentration (40 lM). The lines represent model calculations for the two-component systems based upon log K values from Table 2 and from Nowack and Stone (1999a).
Fig. 5 – Competitive experiments involving increasing EDTMP concentrations at fixed NTMP concentration (12 lM). The line represents model calculations based upon Nowack and Stone (1999a).
plant conditions, calcium and other divalent (and trivalent) cations are likely to exert a strong influence on phosphate and phosphonate adsorption. Nowack and Stone (1999b) have shown that millimolar calcium concentrations can substantially increase phosphonate adsorption onto goethite. A model that can account for divalent metal ion effects would require complex formation constants for the divalent metal ion in the solution, adsorption constants for the divalent metal ion and adsorption constants for possible metal ion–phosphate (or phosphonate) ternary surface complexes. This information is not yet available, hence experiments examining competitive adsorption in the presence of calcium and other divalent metal ions are needed. The concentrations of phosphonates used in this study are much higher than observed in WWTPs. For phosphonates there are only two studies which reported phosphonates in WWTP (Nowack, 1998, 2002). Most of the WWTPs had less than 0.5 mM phosphonates in the influent and no measurable concentrations (o0.05 mM) in the effluent. The maximum
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Fig. 6 – Calculated phosphate and NTMP adsorption under conditions representative for a wastewater treatment plant taking into account competitive adsorption. Influence of increasing NTMP concentrations on the adsorption of 10, 30 and 100 lM phosphate. The range of phosphonates found in WWTPs is indicated by the shaded area, the maximum found by the dotted line. Assumed treatment plant conditions: pH 7.0, 0.134 g/L goethite.
Fig. 7 – Calculated phosphate and NTMP adsorption under conditions representative for a wastewater treatment plant taking into account competitive adsorption. Influence of increasing phosphate concentrations on the adsorption of 0.5 and 2 lM NTMP. Assumed treatment plant conditions: pH 7.0, 0.134 g/L goethite.
influent concentration was 1.8 mM of the phosphonate DTPMP in one particular WWTP. Phosphate can be expected in concentrations between 150 and 300 mM in the influent of WWTP (Parsons and Berry, 2004). The calculations employing realistic concentrations of phosphonates, phosphate and oxide loading show that phosphonates will have only a slight effect on phosphate removal by adsorption at high phosphate concentrations as observed in WWTPs. However, these high concentrations will have some negative effect on phosphonate removal, especially at higher phosphonate concentrations. These calculations have always assumed that all iron that is added to the wastewater forms iron oxides onto which subsequently both
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Table 3 – Calculated competition between NTMP, EDTA and phosphate under conditions representative for wastewater treatment
Phosphate NTMP EDTA
Adsorbed alone (mM)
Adsorbed with competition (mM)
% Reduction
9.36 0.5 0.955
8.48 0.46 0.00036
9 8 99.96
pH 7, 1 mM EDTA, 0.5 mM NTMP, 20 mM phosphate, 0.134 g/L goethite.
phosphate and phosphonates absorb. However, precipitation of phosphate as FePO4 is usually also observed (Parsons and Berry, 2004) and this will reduce the amount of iron oxide formation and also lower the dissolved phosphate concentration that will compete with the phosphonates for surface sites on the oxide. The phosphonates can also be compared to EDTA, another important chelating agent (Nowack and VanBriesen, 2005). EDTA is adsorbed also onto iron oxides (Nowack and Sigg, 1996) and could therefore be expected to be also removed during wastewater treatment. This stands in contrast to field studies which show that EDTA is not significantly retained during wastewater treatment (Kari and Giger, 1995). Metals such as Ca and Zn have no or only a slight influence on EDTA adsorption (Nowack and Sigg, 1996). The disagreement between adsorption observed in simple systems and under treatment conditions can be explained by competition reactions. The model calculations have shown that phosphonates are still efficiently adsorbed when in competition with EDTA and phosphate for surface sites, while EDTA adsorption is completely hindered. The difference in the observed elimination during wastewater treatment can therefore be attributed to different affinities for oxide surfaces.
5.
Conclusions
& Phosphate and phosphonates compete for the same surface sites on goethite.
& Surface complexation models developed in single compo& &
& &
nent systems have been applied to predict the competition in the multicomponent system. The influence of phosphate on phosphonate adsorption is predicted well using the model. Adsorption of phosphate in the presence of phosphonates is underestimated by 10–35% depending on the phosphonate, indicating that part of the surface sites are accessible to phosphate but not to polyphosphonates. The multicomponent surface complexation model is not able to predict the behavior of phosphate and DTPMP under competition conditions. If the analogy between HFO and goethite that we have employed is reasonable, then phosphonates and phosphate should exhibit only limited competitive interactions under wastewater treatment plant conditions.
Acknowledgments B.N. gratefully acknowledges financial support by the Swiss National Science Foundation. Additional funding was provided by Grant R82-6376, US Environmental Protection Agency—National Center of Environmental Research, and Quality Assurance (Office of Exploratory Research). We thank M. Trehy from Monsanto (St. Louis, USA) for providing samples of HMP and DTPMP. R E F E R E N C E S
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Mu¨ller, G., Steber, J., Waldhoff, H., 1984. The effect of hydroxyethane diphosphonic acid on phosphate elimination with FeCl3 and remobilization of heavy metals: results from laboratory experiments and field trials. Vom Wasser 63, 63–78. Nilsson, N., Lo¨vgren, L., Sjo¨berg, S., 1992. Phosphate complexation at the surface of goethite. Chem. Spec. Bioavail. 4, 21–130. Nowack, B., 1997. Determination of phosphonates in natural waters by ion-pair high performance liquid chromatography. J. Chromatogr. A 773, 139–146. Nowack, B., 1998. Behavior of phosphonates in wastewater treatment plants of Switzerland. Water Res. 32, 1271–1279. Nowack, B., 2002. Aminopolyphosphonate removal during wastewater treatment. Water Res. 36, 4636–4642. Nowack, B., 2003. Environmental chemistry of phosphonates. Water Res. 37, 2533–2546. Nowack, B., Sigg, L., 1996. Adsorption of EDTA and metal–EDTA complexes to goethite. J. Colloid. Interface Sci. 177, 106–121. Nowack, B., Stone, A.T., 1999a. Adsorption of phosphonates onto the goethite–water interface. J. Colloid. Interface Sci. 214, 20–30.
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Nowack, B., Stone, A.T., 1999b. Influence of metals on the adsorption of phosphonates onto goethite. Environ. Sci. Technol. 3, 3627–3633. Nowack, B., Stone, A.T., 2000. Degradation of nitrilotris(methylenephosphonic acid) and related (amino)phosphonate chelating agents in the presence of manganese and molecular oxygen. Environ. Sci. Technol. 34, 4759–4765. Nowack, B., VanBriesen, J.M., 2005. Chelating agents in the environment. In: Nowack, B., VanBriesen, J.M. (Eds.), Biogeochemistry of Chelating Agents. ACS Symposium Series. American Chemical Society, Washington, DC, pp. 1–18. Parsons, S.A., Berry, T.A., 2004. Chemical phosphorous removal. In: Valsami-Jones, E. (Ed.), Phosphorous in Environmental Technologies: Principles and Applications. IWA Publishing, London, pp. 260–271. Steber, J., Wierich, P., 1987. Properties of aminotris(methylenephos-phonate) affecting its environmental fate: degradability, sludge adsorption, mobility in soils, and bioconcentration. Chemosphere 16, 1323–1337.
40 (2006) 2201 – 2209
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Competitive adsorption of phosphate and phosphonates onto goethite Bernd Nowacka,!, Alan T. Stoneb a
Institute of Terrestrial Ecology, ETH Zu¨rich, Universita¨tstrasse 16, 8092 Zu¨rich, Switzerland Department of Geography and Environmental Engineering, The Johns Hopkins University, Baltimore, MD 21218, USA
b
art i cle info
A B S T R A C T
Article history:
Phosphate and phosphonates are both strongly adsorbed onto mineral surfaces and their
Received 26 January 2006
removal during wastewater treatment is mainly due to adsorptive processes. We have
Received in revised form
conducted experiments to study the mutual influence of phosphate and six different
17 March 2006
phosphonates on each other in buffered medium at pH 7.2. We have used phosphonates
Accepted 22 March 2006
having one to five phosphonic acid groups (HMP, IDMP, HEDP, NTMP, EDTMP and DTPMP).
Available online 3 May 2006
The presence of phosphonates suppressed the adsorption of phosphate. The monopho-
Keywords:
sphonate HMP had the smallest and the polyphosphonates the largest effect on phosphate
Phosphate
adsorption. The presence of phosphate lowered phosphonate adsorption. The competition
Phosphonates
in the multicomponent system can reasonably well be predicted using a surface
Competition
complexation model developed for single component systems. The competitive model
Adsorption
only failed in systems containing the polyphosphonate DTPMP. With this approach we can
Iron oxides
predict the behavior of both compounds during wastewater treatment. The calculations
Water
show that phosphonates have a small effect on phosphate adsorption at the actual
Water treatment
concentrations in observed wastewater. Adsorption of low concentrations of phosphonates was calculated to be significantly reduced by phosphate concentrations as observed in wastewater. & 2006 Elsevier Ltd. All rights reserved.
1.
Introduction
Phosphate and phosphonates are interesting substances from a water treatment point of view. Phosphate discharged into surface waters can stimulate plant growth, resulting in a eutrophication of rivers and lakes. An efficient removal of phosphate during wastewater treatment is therefore an important factor in preserving the water quality of eutrophic lakes. Phosphonates are complexing agents containing one or more C–PO(OH)2 groups (Nowack, 2003). Phosphonates have three main properties: they are effective chelating agents for two- and tri-valent metal ions (Lacour et al., 1998), they inhibit crystal growth and scale formation and they are quite !Corresponding author. Tel.: +41 44 633 61 60; fax: +41 44 633 11 23.
E-mail address: [email protected] (B. Nowack). 0043-1354/$ - see front matter & 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2006.03.018
stable under harsh chemical conditions. Important industrial uses of phosphonates are in cooling waters, desalination systems and in oil fields to inhibit scale formation. In pulp and paper manufacturing and in textile industry they are used as peroxide bleach stabilizers, acting as chelating agents for metals that could inactivate the peroxide. In detergents they are used as a combination of chelating agent, scale inhibitor and bleach stabilizer. Around 16,000 tons of phosphonates are used per year in Europe and 30,000 tons in the USA (Nowack and VanBriesen, 2005). Phosphonates are not biodegraded during wastewater treatment (Steber and Wierich, 1987; Horstmann and Grohmann, 1988) but can be rapidly oxidized in the presence of
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manganese (Nowack and Stone, 2000) or are photodegraded by sunlight (Lesueur et al., 2005). Phosphate and phosphonate can influence the behavior of each other. It has been shown that high concentrations of phosphonates have an adverse effect on phosphate elimination (Mu¨ller et al., 1984; Horstmann and Grohmann, 1986). Similar to phosphate, phosphonates adsorb strongly onto mineral surfaces such as iron oxides (Mu¨ller et al., 1984; Fischer, 1991; Nowack and Stone, 1999a, b). Iron dosage for phosphate elimination during wastewater treatment also removes most of the phosphonates (Mu¨ller et al., 1984). This has been verified in field studies for wastewater treatment plants (WWTPs) that operated with chemical phosphate elimination (Nowack, 1998, 2002). Removal rates of 90% and more were observed regularly. Influent concentrations of 0.5–2 mM phosphonates were reduced to less than 0.05 mM in most cases. During phosphate precipitation by iron salts, both ironphosphates and iron (hydr)oxides are formed (Parsons and Berry, 2004) on which both phosphate and phosphonates adsorb. The adsorption of phosphonates onto the iron (hydr)oxide goethite has been described by a surface complexation model (Nowack and Stone, 1999a). Phosphate adsorption onto goethite is well studied and several models have been used to successfully describe the adsorption (Nilsson et al., 1992; Geelhoed et al., 1997). While adsorption studies have mainly been done in single component systems, natural environments always include a variety of different anions and cations that compete for surface sites and change the adsorption of each other by influencing the surface charge. When a single component system can be modeled successfully, this does not necessarily imply that a more complex multicomponent system can be described by applying the results from different single adsorbate systems. The ultimate goal of adsorption modeling is to come up with a set of stability constants that are able to describe multicomponent systems. Surface complexation models have been used to successfully describe multicomponent systems, e.g. Mesuere and Fish (1992), Goldberg (2002). This study investigated phosphonate–phosphonate and phosphonate–phosphate competitive adsorption in pH 7.2 suspensions containing the iron (hydr)oxide goethite. We have used the polyphosphonates HEDP, NTMP, EDTMP and DTPMP (see Table 1 for names and structures), which are the most widely used in technical applications. Additionally, we have used the monophosphonate HMP (hydroxymethylphos-phonic acid) and the bisphosphonate IDMP (iminodimethylenephosphonic acid), a breakdown product of NTMP (Steber and Wierich, 1987). The data were modeled using experimentally determined phosphate surface complexation constants and the phosphonate surface complexation model presented by Nowack and Stone (1999a). Calculations were performed to examine whether phosphonates interfere with efforts to chemically remove phosphate during wastewater treatment. The behavior of phosphonates in WWTPs was then compared to the behavior of the aminocarboxylate chelating agent EDTA.
2.
Materials and methods
2.1.
Materials
The goethite used in this study was synthesized and characterized as described by Coughlin and Stone (1995) and stored as a slurry containing 44 g/L goethite. The reported surface area of this goethite preparation was 47.5 m2/g. The phosphonates HEDP, IDMP and NTMP were obtained in the acid form from Fluka with 497% purity. EDTMP in the acid form was obtained from Monsanto (Dequest 2041, 95%). DTPMP and HMP in the acid form were provided by Monsanto (St. Louis, USA). Phosphate solution was prepared from Na2HPO4.
2.2.
Adsorption experiments
All experiments were performed in 30 mL glass vials and employed 0.42 g/L goethite and 0.01 M NaNO3 (Baker Chemical Co.). After addition of appropriate amounts of phosphate and phosphonate stock solutions, suspensions were stirred with Teflon-coated magnetic stir bars and maintained at 2572 1C for 24 h before pH measurement and sample collection. Phosphonate adsorption occurs on a timescale of 1 min (Nowack and Stone, 1999a), while phosphate adsorption was slower and required 24 h equilibration. Samples were filtered through 0.2 mm polycarbonate filters (Nuclepore Corp.) prior to analysis. No dissolved iron was detected, indicating that ligand controlled dissolution of goethite by phosphonates was negligible. Phosphate adsorption as a function of pH was examined without using an added buffer; suspension pH was varied by adding different amounts of HNO3 or NaOH. Competitive adsorption experiments and all other experiments performed at pH 7.2 employed a 1 mM MOPS-buffer (4-morpholinepropane sulfonic acid, Aldrich). MOPS was found to have no influence on phosphonate adsorption (Nowack and Stone, 1999a). In one set of competitive adsorption experiments, the concentration of each phosphonate was increased at a fixed phosphate concentration of 40 mM. In another set, phosphate concentrations were increased at fixed phosphonate concentration (HMP: 40 mM, HEDP and IDMP: 25 mM, NTMP, EDTMP and DTPMP: 20 mM). Additionally the effect of increasing EDTMP concentrations on adsorption of 12 mM NTMP was measured.
2.3.
Analytical methods
Using the derivatization plus ion-pair HPLC technique described by Nowack (1997), detection limits of 0.1 mM for NTMP, EDTMP and DTPMP and 0.5 mM for HEDP could be achieved. The technique involves amending samples with Fe(III) at low pH, which generates anionic Fe(III)–phosphonate complexes; these complexes are then separated on a polymer-based reversed-phase HPLC column (Polymer Laboratories PLRP-S) using tetrabutylammonium as counterion and a bicarbonate–acetonitrile eluent at pH 8.3. We have tested that at the concentrations employed in this study, phosphate did not interfere with phosphonate analysis.
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Table 1 – Abbreviations, names, and structures of the phosphonates used in this study Acronym
Name
HMP
Hydroxymethylphosphonic acid
IDMP
Iminodimethylenephosphonic acid
Structure
OH
(HO)2OP
(HO)2OP (HO)2OP
NH PO(OH)2
H3C HEDP
1-Hydroxyethane (1,1-diylbis-phosphonic acid)
NTMP
Nitrilotrismethylenephosphonic acid
EDTMP
1,2-Diaminoethanetetrakismethylene phosphonic acid
OH C PO(OH)2
(HO)2OP (HO)2OP
PO(OH)2
N
(HO)2OP N
(HO)2OP
PO(OH)2
N
PO(OH)2 (HO)2OP DTPMP
(HO)2OP
Diethylenetriaminepentakismethylene phosphonic acid
PO(OH)2 N
N
N
PO(OH)2
PO(OH)2
Phosphate was determined by the molybdenum blue method and spectrophotometric detection at 720 nm (American Public Health Association, 1989). We have tested that the presence of phosphonates did not interfere with the analysis of phosphate. HMP and IDMP were quantified by measuring phosphate concentrations before (phosphate alone) and after digestion with potassium peroxodisulfate at 100 1C for 2 h (phosphate plus phosphonate).
2.4.
Table 2 – Equilibrium constants describing the acid–base properties of goethite from Lo¨vgren et al. (1990) RFeOH+H+3RFeOH+2 RFeOH3RFeO"+H+ Total specific capacitance Site density BET surface areaa a
log K: 7.47 log K: "9.51 1.28 F m2 1.7 site/nm2 47.6 m2/g
From Coughlin and Stone (1995).
Adsorption and aqueous speciation modeling
ChemEQL (Mu¨ller, 2004) was used to calculate chemical speciation and adsorption. Stability constants for calculating the protonation speciation of phosphate and phosphonate in aqueous solution were taken from the CRITICAL database (Martell et al., 1997) and calculated for an ionic strength of 0.01 M. The basic surface parameters of the goethite are listed in Table 2 and were taken from Lo¨vgren et al. (1990). It is reasonable to assume that these data apply since both Lo¨vgren et al. (1990) and Coughlin and Stone (1995) prepared the goethite according to the same method. Acid–base properties of surface sites and electrostatic aspects of the (hydr)oxide–water interface were modeled using the 2-pK constant capacitance model (CCM). The ionic medium in the present work is a factor of ten less concentrated that that
used in Lo¨vgren et al. (1990) but the assumption is made that the values still apply. The adsorption of phosphonates was calculated using the surface complexation model of Nowack and Stone (1999a) without further modification. According to the model, phosphonates are adsorbed as mononuclear complexes with two or more distinct protonation levels. The following generalized reaction represents adsorption stoichiometries: RFeOH þ La" þ ðn þ 1ÞHþ 3RFe2L2Hn
ða"n"1Þ"
þ H2 O
(1)
where RFeOH is a surface site and La" the phosphonate in the completely deprotonated form. The number of protonation levels is analogous to the number of protonation levels
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observed for dissolved species. This equation allows for mononuclear complex formation and disallows multinuclear complex formation. It makes no distinction between monodentate and multidentate–mononuclear surface complexes. The goethite preparation used to determine competitive adsorption with phosphate was the same as that was used in our previous study of phosphonate adsorption (Nowack and Stone, 1999a). Measurements of the extent of adsorption of 40 mM total phosphate as a function of pH provided the basis for our modeling effort because in this data set phosphate was in excess of surface sites which allows an accurate determination of log K values at all pH values. The total surface site concentration used in the model was the same as that used to model phosphonate adsorption. It was assumed that all adsorbates occupy only one surface site and that the charge accompanying species adsorption lies at the zero plane. log K values of the phosphate surface complexes were fitted using FITEQL 3.1 (Herbelin and Westall, 1994). Although the actual form of precipitated iron (hydr)oxides within WWTPs is not known, it is likely to be quite similar to the hydrous iron (hydr)oxide (HFO) described by Dzombak and Morel (1990), i.e. with a surface area of 600 m2/g, a site density of 0.2 mol/mol Fe and 89 g HFO/mol Fe. If a WWTP uses an average dosage of 5 mg/L iron, this corresponds to a WWTP surface site concentration of 18 mM. Our adsorption experiments have been conducted with the crystalline iron (hydr)oxide goethite because information on phosphonate and EDTA adsorption onto goethite was available. As noted in Table 2, Lo¨vgren et al. (1990) reported 1.7 site/ nm2 for goethite, while HFO has 2.26 site/nm2 (Dzombak and Morel, 1990), so quite comparable when normalized to the surface area. For the sake of discussion we could assume that the surface chemistry of goethite and HFO are comparable and then we only need to make a surface area correction. A goethite loading of 134 mg/L has the same concentration of surface sites than HFO precipitated from 5 mg/L iron and all calculations for WWTPs have therefore been performed with this solids loading. Additionally one calculation for WWTPs conditions was performed including adsorption of EDTA. The extent of EDTA adsorption onto goethite was previously measured and modeled by Nowack and Sigg (1996). By applying adsorption stoichiometries and log K values from this previous study, we were able to perform additional calculations that explore competitive adsorption in the presence of EDTA.
3.
Results
3.1.
Phosphate adsorption
Phosphate adsorption onto goethite can be described by a surface complexation model with three different surface complexes of different protonation level. The log K values were fitted to the pH-edge data and the best-fit values are presented below: RFeOH þ PO4 3" þ Hþ 3RFePO4 2" þ H2 O log K 18:86 % 0:21; (2)
RFeOH þ PO4 3" þ 2 Hþ 3RFePO4 H" þ H2 O log b 25:68 % 0:25; (3) RFeOH þ PO4 3" þ 3 Hþ 3RFePO4 H2 þ H2 O log b 31:64 % 0:1: (4) These three phosphate surface complexes with different protonation level have been frequently used to describe phosphate adsorption, e.g. Goldberg and Sposito (1984; Nilsson et al. (1992). Fig. 1 shows the adsorption of 40 and 10 mM phosphate onto goethite together with the model fit. The distribution of the three surface species is shown for the 40 mM experiment. The two experiments represent an excess of phosphate over surface sites (40 mM) and an excess of surface sites over phosphate (10 mM). In the latter case complete adsorption of phosphate is observed at low to
Fig. 1 – Adsorption of phosphate as a function of pH at 10 and 40 lM total phosphate. The lines represent model calculations based upon log K values from Table 2 and Eqs. (2)–(4). The calculated distribution of the surface species is given for the 40 lM experiment.
Fig. 2 – Single adsorbate experiments. Isotherms for the adsorption of phosphate and the six phosphonates HMP, HEDP, IDMP, NTMP, EDTMP and DTPMP onto goethite at pH 7.2 are shown. Lines represent model calculations based upon log K values from Table 2 and from Nowack and Stone (1999a). Data redrawn from Nowack and Stone (1999a).
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neutral pH. The same log K values obtained from the pH-edge were also able to describe the adsorption isotherm of phosphate at pH 7.2 (Fig. 2).
phonates was observed. The multicomponent modeling gives an accurate description of EDTMP adsorption while NTMP adsorption is a little bit underestimated.
3.2.
3.4. Simulation of phosphonate adsorption under treatment plant conditions
Phosphonate adsorption
The adsorption of phosphonates onto goethite is characterized by a very strong affinity to the surface. Adsorption is greatest at low pH, but all phosphonates yield significant adsorption at pH above 8 (Nowack and Stone, 1999a). The amount adsorbed decreases with increasing number of phosphonic acid groups from the monophosphonate HMP to the pentaphosphonate DTPMP. The adsorption of the monophosphonates HMP is very similar to that of phosphate. Fig. 2 shows the adsorption of the phosphonates used in this study at pH 7.2 (redrawn from Nowack and Stone (1999a)) and shows that even at very low dissolved concentration of phosphonates high surface coverage is achieved. The lines in Fig. 2 are model calculations based on the surface complexation model developed by Nowack and Stone (1999a).
3.3. Competitive adsorption of phosphate and phosphonates Fig. 3 shows that with increasing phosphate addition, there is a decrease in phosphonate adsorption. The presence of phosphonates suppresses the adsorption of phosphate, as shown by the model calculation for phosphate alone in Fig. 3. The monophosphonate HMP has the smallest and the polyphosphonate DTPMP the largest effect on phosphate adsorption. Increasing the number of phosphonic acid groups in the phosphonates increases the effect on phosphate adsorption. The lines in Fig. 3 are model calculations based on Table 2, phosphate adsorption as described in Eqs. (2)–(4) and the log K values for phosphonate adsorption from Nowack and Stone (1999a). No fitting was therefore involved in calculating the competition, neither for phosphate nor for the phosphonates. The competitive adsorption can be modeled very reasonably with the constants from the single component system. There is a good prediction for the influence of phosphate on phosphonate adsorption with the exception of DTPMP. The adsorption of phosphate is represented less well by the model. The phosphate adsorption was always underestimated to some extent. The best agreement was obtained for the competition between phosphate and the monophosphonate HMP. The worst agreement was obtained for the competition between phosphate and the polyphosphonate DTPMP. Much more phosphate was adsorbed than predicted by the model. If a constant amount of phosphate was brought in contact with increasing concentrations of phosphonates, a decrease in phosphate adsorption was observed (Fig. 4). Again the polyphosphonates had a stronger influence on phosphate adsorption than the monophosphonate HMP. The lines are again the model prediction based on log K values derived from the single component system and then used to model the two-component system. Fig. 5 shows the effect of increasing EDTMP on NTMP adsorption. Competitive adsorption between the two phos-
The effect of NTMP on phosphate adsorption under WWTP conditions is shown in Fig. 6 for three different phosphate concentrations. For the average phosphonate concentration normally found in WWTPs of less than 0.5 mM, there is only a small effect on phosphate adsorption. However, at the maximum concentration of phosphonates observed in WWTPs and a low phosphate concentration of 10 mM, a reduction in phosphate adsorption of up to 40% is predicted. At 100 mM phosphate however, almost no effect of the phosphonate is found. Phosphate, of course, also has an effect on phosphonate adsorption. Fig. 7 shows the influence of increasing phosphate on NTMP adsorption (0.5 and 2 mM) under WWTP conditions. To have a significant influence on NTMP adsorption, 50 mM phosphate are expected with an expected decrease of about 50%. The effect of phosphate is more pronounced at higher NTMP concentration.
3.5.
Comparison of phosphonates and EDTA
Table 3 shows the result of modeling single and multicomponent systems with simultaneous presence of EDTA, NTMP and phosphate. The chosen conditions were pH 7, 1 mM EDTA, 0.5 mM NTMP, 20 mM phosphate and 134 mg/L goethite. Calculations were first performed for the three single component systems and then for the combined system with all three compounds competing for surface sites. If only adsorption of EDTA is considered, 95% of EDTA is predicted to be adsorbed onto the surface. However, if we also consider competition with phosphate and phosphonate for surface sites in our calculation, the amount of adsorbed EDTA becomes negligible. The same calculation for phosphate and the phosphonate NTMP shows that the reduction in the adsorbed amount is 8% for NTMP (relative to the amount adsorbed without competition) and 9% for phosphate in the calculation with competition.
4.
Discussion
We observed as expected competition between phosphate and phosphonates for available surface sites. The surface complexation model based on single component systems was able to accurately describe phosphonate adsorption in the presence of phosphate (with the exception of DTPMP). The influence of phosphonates on phosphate adsorption was less well predicted, especially for polyphosphonates. Again the agreement between model and experimental data was worst for DTPMP. However, for the other phosphonates the disagreement between model and data was never more than 35% (for HEDP) and for the other compounds about 20% or less. The results of modeling show that the assumption of competition between phosphate and phosphonates for the
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Fig. 3 – Effect of increasing phosphate concentration on the adsorption of six phosphonates at fixed concentration (pH 7.2). The lines represent model calculations based upon log K values from Table 2 and from Nowack and Stone (1999a). The thin line represents the modeled phosphate adsorption in the absence of phosphonate, the thick line is the modeled phosphate adsorption with competition. HMP: 40 lM, HEDP and IDMP 25 lM, NTMP, EDTMP, and DTPMP 20 lM.
same surface sites is justified and that we can predict the effect of one compound on the other reasonable well by using independently determined surface complexation constants. Measured phosphate adsorption was always slightly higher than the predicted adsorption. This may indicate that phosphate had access to surface sites that were not accessible to phosphonates, especially polyphosphonates with higher molecular mass. Our model has assumed that all charges associated with the adsorbed phosphonate are deposited at the zero plane. If these charges were instead distributed in multi-layers, there would be less electrostatic repulsion that serves to lower the amount of phosphate adsorbed. Competition with HMP containing just one func-
tional group, was well predicted. The worst agreement between model and experimental data was obtained for DTPMP, which contains five phosphonic acid groups. The model predicted that DTPMP would have a larger effect on phosphate adsorption than was actually observed. The model also predicted that phosphate would have a smaller effect on DTPMP adsorption than was actually observed. The accessibility of surface sites for the two compounds was clearly not the same. Charge distribution further away from the surface would be most significant for DTPMP and may be a reason for the lower competition than modeled. It is important to note that our experiments were performed using a NaNO3 medium. Under real treatment
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Fig. 4 – Competition experiments involving increasing phosphonate concentrations at fixed phosphate concentration (40 lM). The lines represent model calculations for the two-component systems based upon log K values from Table 2 and from Nowack and Stone (1999a).
Fig. 5 – Competitive experiments involving increasing EDTMP concentrations at fixed NTMP concentration (12 lM). The line represents model calculations based upon Nowack and Stone (1999a).
plant conditions, calcium and other divalent (and trivalent) cations are likely to exert a strong influence on phosphate and phosphonate adsorption. Nowack and Stone (1999b) have shown that millimolar calcium concentrations can substantially increase phosphonate adsorption onto goethite. A model that can account for divalent metal ion effects would require complex formation constants for the divalent metal ion in the solution, adsorption constants for the divalent metal ion and adsorption constants for possible metal ion–phosphate (or phosphonate) ternary surface complexes. This information is not yet available, hence experiments examining competitive adsorption in the presence of calcium and other divalent metal ions are needed. The concentrations of phosphonates used in this study are much higher than observed in WWTPs. For phosphonates there are only two studies which reported phosphonates in WWTP (Nowack, 1998, 2002). Most of the WWTPs had less than 0.5 mM phosphonates in the influent and no measurable concentrations (o0.05 mM) in the effluent. The maximum
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Fig. 6 – Calculated phosphate and NTMP adsorption under conditions representative for a wastewater treatment plant taking into account competitive adsorption. Influence of increasing NTMP concentrations on the adsorption of 10, 30 and 100 lM phosphate. The range of phosphonates found in WWTPs is indicated by the shaded area, the maximum found by the dotted line. Assumed treatment plant conditions: pH 7.0, 0.134 g/L goethite.
Fig. 7 – Calculated phosphate and NTMP adsorption under conditions representative for a wastewater treatment plant taking into account competitive adsorption. Influence of increasing phosphate concentrations on the adsorption of 0.5 and 2 lM NTMP. Assumed treatment plant conditions: pH 7.0, 0.134 g/L goethite.
influent concentration was 1.8 mM of the phosphonate DTPMP in one particular WWTP. Phosphate can be expected in concentrations between 150 and 300 mM in the influent of WWTP (Parsons and Berry, 2004). The calculations employing realistic concentrations of phosphonates, phosphate and oxide loading show that phosphonates will have only a slight effect on phosphate removal by adsorption at high phosphate concentrations as observed in WWTPs. However, these high concentrations will have some negative effect on phosphonate removal, especially at higher phosphonate concentrations. These calculations have always assumed that all iron that is added to the wastewater forms iron oxides onto which subsequently both
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Table 3 – Calculated competition between NTMP, EDTA and phosphate under conditions representative for wastewater treatment
Phosphate NTMP EDTA
Adsorbed alone (mM)
Adsorbed with competition (mM)
% Reduction
9.36 0.5 0.955
8.48 0.46 0.00036
9 8 99.96
pH 7, 1 mM EDTA, 0.5 mM NTMP, 20 mM phosphate, 0.134 g/L goethite.
phosphate and phosphonates absorb. However, precipitation of phosphate as FePO4 is usually also observed (Parsons and Berry, 2004) and this will reduce the amount of iron oxide formation and also lower the dissolved phosphate concentration that will compete with the phosphonates for surface sites on the oxide. The phosphonates can also be compared to EDTA, another important chelating agent (Nowack and VanBriesen, 2005). EDTA is adsorbed also onto iron oxides (Nowack and Sigg, 1996) and could therefore be expected to be also removed during wastewater treatment. This stands in contrast to field studies which show that EDTA is not significantly retained during wastewater treatment (Kari and Giger, 1995). Metals such as Ca and Zn have no or only a slight influence on EDTA adsorption (Nowack and Sigg, 1996). The disagreement between adsorption observed in simple systems and under treatment conditions can be explained by competition reactions. The model calculations have shown that phosphonates are still efficiently adsorbed when in competition with EDTA and phosphate for surface sites, while EDTA adsorption is completely hindered. The difference in the observed elimination during wastewater treatment can therefore be attributed to different affinities for oxide surfaces.
5.
Conclusions
& Phosphate and phosphonates compete for the same surface sites on goethite.
& Surface complexation models developed in single compo& &
& &
nent systems have been applied to predict the competition in the multicomponent system. The influence of phosphate on phosphonate adsorption is predicted well using the model. Adsorption of phosphate in the presence of phosphonates is underestimated by 10–35% depending on the phosphonate, indicating that part of the surface sites are accessible to phosphate but not to polyphosphonates. The multicomponent surface complexation model is not able to predict the behavior of phosphate and DTPMP under competition conditions. If the analogy between HFO and goethite that we have employed is reasonable, then phosphonates and phosphate should exhibit only limited competitive interactions under wastewater treatment plant conditions.
Acknowledgments B.N. gratefully acknowledges financial support by the Swiss National Science Foundation. Additional funding was provided by Grant R82-6376, US Environmental Protection Agency—National Center of Environmental Research, and Quality Assurance (Office of Exploratory Research). We thank M. Trehy from Monsanto (St. Louis, USA) for providing samples of HMP and DTPMP. R E F E R E N C E S
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