beneficially utilization of sewage sludge incineration ash
BENEFICIALLY UTILIZATION OF SEWAGE SLUDGE INCINERATION ASH FOR THERMAL IMMOBILIZATION OF ZINC IN SPINEL STRUCTURE Y. TANG, X. ZHENG, P. WU School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China
SUMMARY: Sewage sludge (municipal wastewater sludge) is generated with huge quantity in urban environments, resulting from the accumulation of solids through wastewater treatment. With complex and variable organic and inorganic substances, sewage sludge may contain viable pathogens and parasites as well as a variety of potentially toxic elements and compounds. Incineration has become an alternative to largely reduce the sludge volume, destruct pathogen agents, and remove organic pollutants for easier and safer handling and disposal. Nevertheless, approximately 30% of the solids remain as residues after sludge incineration. With further development of the incineration technology, the subsequent disposal of incineration residues is becoming a serious concern. Sewage sludge ash always contains aluminum, silicon, and iron as the main components, and as a waste-to-resource technology, the use of sludge resulting from wastewater treatment processes has attracted much attention. By adding aluminum-rich materials into hazardous metal waste, the hazardous metals can be immobilized in a variety of ceramic products such as inorganic pigments, aggregates, bricks. Therefore, in this study, the sewage sludge ash was reused for effectively stabilizing hazardous zinc during the sintering process. Zinc oxide was used as simulated zinc-laden sludge and blended with sewage sludge ash, and the mixture was sintered at 650-1250°C for 3 h. A spinel solid solution was generated with chemical formula of ZnAlxFe2-xO4 confirmed by XRD technique, and the characteristic peaks of the spinel were found to increase with elevated sintering temperature. In the subsequent leaching test, three types of extraction solution with pH values of 2.9, 6.2 and 13.0 were applied to roundly evaluate the efficiency of zinc immobilization under acidic, neutral and alkaline environment. The superior immobilization efficiency of zinc was confirmed by the dramatic decrease in zinc leachability after sintering zinc oxide with the sewage sludge ash. With the incorporation of hazardous zinc into spinel structure, the beneficial use of sewage sludge incineration ash as ceramic materials can both generate a new material resource and reduce the hazard of metal pollutants in the environment.
1. INTRODUCTION Mining, metal coating and battery production are identified as the main industrial processes discharging heavy metal-containing wastewater which is detrimental to human health and
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
environment. It is well known that heavy metals, unlike organic pollutants, cannot be biodegraded at all and have accumulating characteristic in essence which means the contamination is permanent and devastating (Friesl et al., 2006; Fu et al., 2012; Veli et al., 2007). Zinc is one of the most common heavy metals in the discharged wastewater, and several sorption methods have been implemented for zinc removal from the industrial wastewater (Alyuz et al., 2009; Kumpiene et al., 2008). Therefore, a large amount of hazardous metal-laden sludge after adsorption processes needs to be disposed of by cost effective and high-efficiency alternatives other than landfilling. Our previous studies have successfully incorporated copper into ceramic matrices by using kaolinite, mullite and iron oxide as ceramic precursors, and kept the leachability at a reasonably low level under acidic environment. The studies revealed that Al and Fe ions play a vital role in the copper incorporation by forming spinel structure CuAl(or Fe)2O4 after well-controlled thermal reactions (Tang et al., 2011; Tang et al., 2016). Huge amounts of sewage sludge are generated due to rapid industrialization and urbanization (Guerrero et al., 2013), and further accelerated by the application of enhanced sewage treatment techniques to meet the more stringent regulations (He et al., 2007). The disposal of sewage sludge is one of the most difficult problems to be solved, and the need to achieve a sustainable sludge management strategy has become of global concern (Spinosa et al., 2011). Incineration has become an alternative to largely reduce the sludge volume, destruct pathogen agents, and remove organic pollutants for easier and safer handling and disposal (Fullana et al., 2004). Sludge incineration technology has been recognized as one of the most attractive disposal methods in the world (Fytili et al., 2008; Murakami et al., 2009). Nevertheless, approximately 30% of the solids remain as residues after sludge incineration (Malerius et al., 2003). With further development of the incineration technology, the subsequent disposal of incineration residues is becoming a serious concern (Rulkens, 2008). Since the inorganic flocculants used in the water treatment process are rich in Al and Fe elements, incinerated sewage sludge ash (ISSA) can be a potential substitute for kaolinite and mullite to provide Al and Fe elements for metal incorporation (Lin et al., 2015). Employing ISSA to stabilize industrial sludge with high content of heavy metals will save the useful minerals or ceramic materials that are used in the previous studies as aluminum-rich or iron-rich precursors, and further turn the solid waste into applicable resources for brick or tile industries. In this study, ZnO was used as simulated zinc-laden sludge and incinerated sewage sludge ash (ISSA) was treated as Al- and Fe-rich ceramic precursors after being fired at 900 °C.The sintering process was carried out under reasonable firing temperature (650-1250 °C) and sintering time (3 h). The phase transformation and compositions after 3-h sintering were examined while the stabilization effect of zinc in sintered products was evaluated under different environment conditions with pH value from 2.9 to 13.0.
2. MATERIALS AND METHODS The sewage sludge was collected from a wastewater treatment plant in Shenzhen of Guangdong Province in China. The collected sludge was dried at 60 °C, and then incinerated at 900 °C for 1 h in a muffle oven to obtain ISSA. The elemental compositions of the ISSA was analyzed via X-ray fluorescence (XRF, SHIMADZU XRF-1800 wavelength-dispersive X-ray fluorescence spectrometer). To intensively study the mechanisms of zinc stabilization, the samples were simulated from the mixture of zinc oxide (ZnO) and ISSA at a molar ratio of Zn:Al around 1:2. The mixing
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
process was carried out by ball milling 60 g of the powder mixture in water slurry for 18 h, and then the water slurry was transferred and dried in the oven at 105 °C. The dried mixture was homogenized by mortar grinding and each time 2 g powder sample was pressed into 20 mm pellets under the pressure of 156.8 MPa with the holding time of 2 minutes. The pellets were respectively sintered in a high-temperature furnace at 650-1250 °C for 3 h with a heating rate of 10 °C /min. Then the sintered pellets were broken and ground into fine powders for the subsequent X-ray diffraction (XRD, Rigaku Smartlab) scanning and leaching tests. Phase composition and transformation were investigated by powder XRD in continuous scanning mode with 2θ range of 10°-90° and a scan speed of 10° per minute. Qualitative phase identification was executed by matching powder XRD patterns with those retrieved from the standard powder diffraction database of the International Centre for Diffraction Data (ICDD PDF2, Release 2008). The leaching experiments were carried out in both acidic (pH=2.9, TCLP extraction fluid #2, US Environmental Protection Agency’s SW-846 Method 1311), neutral (deionized water, pH=6.2) and alkaline (pH=13.0, 0.1M NaOH solution) environment. Each leaching vial was filled with 4 mL of the as-prepared extraction solution and 0.2 g powder sample, and then rotated end-over-end at 60 rpm. The 18-h leaching solution was collected after being filtered with 0.45-μm syringe filters, and the pH values of the leachate were measured instantly. All leachates were measured by an inductively coupled plasma-optical emission spectrometer (ICP-OES, Optima 8000DV PerkinElmer) for concentrations of zinc and phosphorus.
3. RESULTS AND DISCUSSION 3.1 Characterization of the sewage sludge The sewage sludge was collected from one wastewater treatment plant in Shenzhen, and experiments were carried out to further characterize the sewage sludge samples. A thermal analysis of the sludge was carried out with thermogravimetry and differential scanning calorimetry at temperatures up to 1200 °C (NETZSCH STA 449F3), and the results were shown in Figure 1. The raw dried sludge was placed in an Al2O3 crucible and heated in an Ar atmosphere with a heating rate of 10 K min−1 to 1200 °C. An obvious weight loss (~ 30 wt %) of the sewage sludge was observed up to 550 °C, with one endothermic peak detected at around 125 °C caused by the removal of adsorbed and bounded water, and one huge exothermic peak at about 610 °C, mainly due to the decomposition of organic components. Furthermore, two more exothermic peaks were observed at around 850 and 1100 °C, which might have been caused by the decomposition of inorganic minerals.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 1. Thermogravimetry (TG) and differential scanning calorimetry (DSC) curves of the thermally dried sewage sludge collected from Shenzhen.
The dried were then fired at 900 °C for 30 min to remove the organic contents and then ground into powder for elemental composition analysis. Normalization into metal oxides (Table 1) shows that the top three elements were Al, Si, and P for the sludge ash. The XRD patterns in Figure 2 further indicates complicated compositions in the calcined sludge, which demonstrates the existence of mullite and hematite with considerable peak intensities. Therefore, it is also feasible for the sludge ash to be used as an iron- and aluminum-rich raw material for heavy metal stabilization.
Table 1. Basic oxide forms of major elemental compositions of the 900 °C and 1-h incinerated sewage sludge ash (ISSA) detected by X-ray fluorescence spectrometry. Elements Weight Percentages (%)
Al2O3
SiO2
P 2O 5
Fe2O3
CaO
SO3
K 2O
MgO
Others
35.1
28.0
14.0
10.2
3.2
3.1
1.7
1.2
3.5
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 2. XRD patterns of 900 °C and 30 min calcined sewage sludge.
3.2 Phase transformation after sintering at different temperatures The phase transformation during the sintering processes was shown in Figure 3. The successful formation of zinc spinel after thermal treatment was verified by the XRD patterns. From the increase in the peak intensities, a higher sintering temperature favored the crystallization of zinc spinel within the temperature range from 650 to 1250 °C. At the highest sintering temperature (1250 °C) of this sintering scheme, the spinel phase was observed as the predominant phase with the highest peak intensity. Moreover, the spinel phase was identified as a solid solution with a chemical formula of ZnAlxFe2-xO4 rather than ZnAl2O4 or ZnFe2O4, due to the coexistence of Al and Fe elements in the incinerated sewage sludge ash and the further competition when forming the spinel structure. According to Bragg’s equation—2dsinθ=nλ (d represents interplanar spacing, θ represents the inclined angle of the incident ray and the reflecting plane, n represents a positive integer, and λ represents the wavelength of incident ray) - the value of 2θ was inversely proportional to the value of d. As the interplanar spacing of ZnAlxFe2-xO4 was between that of ZnAl2O4 and ZnFe2O4, the XRD pattern of ZnAlxFe2-xO4 would be similar to theirs, but the diffraction peaks would fall between the corresponding peaks of ZnAl2O4 and ZnFe2O4. From two sets of spinel peaks from the XRD patterns of samples sintered at 650 and 750 °C, there might be two different x values in the ZnAlxFe2-xO4 spinel solid solution, probably because at lower temperatures such as 650 and 750 °C, the driving forces of solid-state reactions are usually small and the supplied energy was not enough for the homogenization of Zn, Al, and Fe ions. As the sintering temperature rose from 850 to 1250 °C, the peaks of two spinel solid solutions began to converge into one identical spinel, as shown in Figure 3, indicating the homogenized solid reactions between Zn, Al, and Fe ions at higher temperatures. Zinc oxide was distinguishable at 650 and 750 °C but eventually diminished with elevated sintering temperatures, as reflected from the XRD patterns. Moreover, at 650 and 750 °C, the major Al-containing crystalline phase was identified as mullite (Al4.54Si1.46O9.73), whereas Fe
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
mainly existed as hematite (Fe2O3). Like zinc oxide, mullite and hematite disappeared when the sintering temperature reached 850 °C, followed by an increase in the spinel peaks. Therefore, the formation of ZnAlxFe2-xO4 spinel was energetically favorable in this reaction system. Once the temperature or the supplied energy was high enough, the Zn, Al, and Fe elements would transform from other phases, for example, zinc oxide, mullite, and hematite, into the ZnAlxFe2xO4 spinel.
Figure 3. XRD patterns of ISSA treated from 650 °C to 1250 °C for 3 h. (zinc oxide: ZnO, PDF#79-2205; α-quartz: SiO2, PDF#77-1060; mullite: Al4.54Si1.46O9.73, PDF# 79-1456; hematite: Fe2O3, PDF#85-0987)
3.3 Evaluation on the effect of zinc stabilization after sintering processes Figure 4 shows the variations in pH value and zinc concentrations in leachates with an initial pH value of 2.9. Both the pH value and the zinc concentration of the leachates decreased as the sintering temperatures increased. After the 18-h leaching process, the pH value increased from 2.9 to 6.2 due to the dramatic extraction of zinc from the untreated ZnO + ISSA ash sample. When the sample was thermally treated at 650 °C, the zinc concentration was still considerable, elucidating insufficient incorporation of zinc into the spinel structure, which was in accord with the XRD results that exhibited a dominant zinc oxide phase with intense peaks at 650 °C. However, the zinc concentration dropped significantly with a further increase of 200 °C and decreased continuously with the elevated sintering temperature. From Figure 4, the zinc concentration in the leachate of untreated ZnO + ISSA was about 3274 mg/L, whereas the lowest zinc leachability (264 mg/L) was achieved from samples after being sintered at 1250 °C. The leachability of zinc from 1250 °C-sintered ZnO + ISSA was around 8% of the untreated sample. Given that the leachate of the unsintered sample was nearly neutral, with a pH value of 6.2, and the hydrogen ions in the extraction solution were almost depleted, the amount of
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
leachable zinc in the untreated incinerated sewage sludge ash might be greater than that leached out in this experiment. The pH values and zinc concentrations demonstrated the feasibility of thermal treatment for zinc stabilization in the sewage sludge ash system. Furthermore, the zinc leachability was also evaluated using leaching liquid with initial pH value of 6.2, and 13.0, respectively. With an intial pH value of 6.2, the zinc concentration is extremely low and was measured to be around zero for samples sintered at temperatures ≥ 750 °C, but ~ 5 mg/L in leachates of unsintered mixture. Meanwhile, in alkaline environment with intial pH value of 13.0, the zinc concentration is around 70 mg/L in leachates of the mixture before and after being sintered at 650 °C. However, the zinc concentration decreased obviously to 30 mg/L when the mixture was further heated at 750 °C, and reached a minimum value of around 10 mg/L at high temperatures. From the leachability of zinc under conditions of acidic, neutral, and alkaline, it can be concluded that the stabilization efficiency was generally improved at higher temperatures due to the development of the spinel phase, which was in agreement with the XRD results displayed in Figure 3.
Figure 4. Zinc concentrations and pH values of the leachates of ZnO + ISSA samples before and after being sintered at 650-1250 °C for 3 h.
4. CONCLUSIONS The feasibility was verified in this study to beneficially use ISSA as Al- and Fe-cotnaining ceramic precursor for zinc stabilization by incorporating zinc in a highly stable spinel structure. The spinel structure was detected and identified to be zinc spinel soild solution due to the coexistence of Al and Fe in the sintered mixture. Moreover, the peaks of the spinel solid solution became sharper and intensified with elevated firing temperatures, which indicated the enhance of zinc incorporation favoured by higher sintering temperatures. Furthermore, zinc concentration in the leachates were decreased with the increase of sintering temperature, which confirmed that a higher stabilization efficiency can be achieved with the formation of the spinel phase.
AKNOWLEDGEMENTS This work is supported financially by the Shenzhen Science and Technology Innovation
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Committee (JCYJ20150601155130432; JCYJ20160429191618506).
REFERENCES Alyuz B., Veli S. (2009). Kinetics and equilibrium studies for the removal of nickel and zinc from aqueous solutions by ion exchange resins. J. Hazard. Mater., vol. 167, 482-488. Friesl W., Friedl J., Platzer K., Horak O., Gerzabek M.H. (2006). Remediation of contaminated agricultural soils near a former Pb/Zn smelter in Austria: Batch, pot and field experiments. Environ. Pollut., vol. 144, 40-50. Fu F.L., Xie L.P., Tang B., Wang Q., Jiang S.X. (2012). Application of a novel strategyadvanced fenton-chemical precipitation to the treatment of strong stability chelated heavy metal containing wastewater. Chem. Eng. J., vol. 189, 283-287. Fullana A., Conesa J.A., Font R, Sidhu S. (2004). Formation and destruction of chlorinated pollutants during sewage sludge incineration. Environ. Sci. Technol.,vol. 38, 2953-2958. Fytili D., Zabaniotou A. (2008). Utilization of sewage sludge in EU application of old and new methods - A review. Renew. Sustain. Energ. Rev., vol. 12, 116-140. Guerrero L.A., Maas G., Hogland W. (2013). Solid waste management challenges for cities in developing countries. Waste Manage., vol. 33, 220-232. He P.J., Lü F., Zhang H., Shao L.M., Lee D.J. (2007). Sewage sludge in China: challenges toward a sustainable future. Water Pract. Technol., vol. 2, 1-8. Kumpiene J., Lagerkvist A., Maurice C. (2008). Stabilization of As, Cr, Cu, Pb, and Zn in soil using amendments-A review. Waste Manage., vol. 28, 215-225. Lin X., Li X., Lu S. (2015). Influence of organic and inorganic flocculants on the formation of PCDD/Fs during sewage sludge incineration. Environ. Sci. Pollut. R., vol. 22, 14629-14636. Malerius O., Werther J. (2003). Modeling the adsorption of mercury in the flue gas of sewage sludge incineration. Chem. Eng. J., vol. 96, 197-205. Murakami T., Suzuki Y., Nagasawa H., Yamamoto T., Koseki T., Hirose H., Okamoto S. (2009). Combustion characteristics of sewage sludge in an incineration plant for energy recovery. Fuel Process Technol., vol. 90, 778-783. Rulkens W. (2008). Sewage sludge as a biomass resource for the production of energy: Overview and assessment of the various options. Energ. Fuel, vol. 22, 9-15. Spinosa L., Ayol A., Baudez J.C., Canziani R., Jenicek P., Leonard A., Rulkens W., Xu G., van Dijk L. (2011). Sustainable and innovative solutions for sewage sludge management. IWA Publishing, vol. 3, 702-717. Tang Y., Chui S.S.Y., Shih K., Zhang L. (2011). Copper stabilization via spinel formation during the sintering of simulated copper-laden sludge with aluminum-rich ceramic precursors. Environ. Sci. Technol., vol. 45, 3598-3604. Tang Y., Shih K., Liu C., Liao C. (2016). Cubic and tetragonal ferrite crystal structures for copper ion immobilization in an iron-rich ceramic matrix. RSC Adv., vol. 6, 28579-28585 Veli S., Alyüz B. (2007). Adsorption of copper and zinc from aqueous solutions by using natural clay. J. Hazard. Mater., vol. 149, 226-233.
SUMMARY: Sewage sludge (municipal wastewater sludge) is generated with huge quantity in urban environments, resulting from the accumulation of solids through wastewater treatment. With complex and variable organic and inorganic substances, sewage sludge may contain viable pathogens and parasites as well as a variety of potentially toxic elements and compounds. Incineration has become an alternative to largely reduce the sludge volume, destruct pathogen agents, and remove organic pollutants for easier and safer handling and disposal. Nevertheless, approximately 30% of the solids remain as residues after sludge incineration. With further development of the incineration technology, the subsequent disposal of incineration residues is becoming a serious concern. Sewage sludge ash always contains aluminum, silicon, and iron as the main components, and as a waste-to-resource technology, the use of sludge resulting from wastewater treatment processes has attracted much attention. By adding aluminum-rich materials into hazardous metal waste, the hazardous metals can be immobilized in a variety of ceramic products such as inorganic pigments, aggregates, bricks. Therefore, in this study, the sewage sludge ash was reused for effectively stabilizing hazardous zinc during the sintering process. Zinc oxide was used as simulated zinc-laden sludge and blended with sewage sludge ash, and the mixture was sintered at 650-1250°C for 3 h. A spinel solid solution was generated with chemical formula of ZnAlxFe2-xO4 confirmed by XRD technique, and the characteristic peaks of the spinel were found to increase with elevated sintering temperature. In the subsequent leaching test, three types of extraction solution with pH values of 2.9, 6.2 and 13.0 were applied to roundly evaluate the efficiency of zinc immobilization under acidic, neutral and alkaline environment. The superior immobilization efficiency of zinc was confirmed by the dramatic decrease in zinc leachability after sintering zinc oxide with the sewage sludge ash. With the incorporation of hazardous zinc into spinel structure, the beneficial use of sewage sludge incineration ash as ceramic materials can both generate a new material resource and reduce the hazard of metal pollutants in the environment.
1. INTRODUCTION Mining, metal coating and battery production are identified as the main industrial processes discharging heavy metal-containing wastewater which is detrimental to human health and
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
environment. It is well known that heavy metals, unlike organic pollutants, cannot be biodegraded at all and have accumulating characteristic in essence which means the contamination is permanent and devastating (Friesl et al., 2006; Fu et al., 2012; Veli et al., 2007). Zinc is one of the most common heavy metals in the discharged wastewater, and several sorption methods have been implemented for zinc removal from the industrial wastewater (Alyuz et al., 2009; Kumpiene et al., 2008). Therefore, a large amount of hazardous metal-laden sludge after adsorption processes needs to be disposed of by cost effective and high-efficiency alternatives other than landfilling. Our previous studies have successfully incorporated copper into ceramic matrices by using kaolinite, mullite and iron oxide as ceramic precursors, and kept the leachability at a reasonably low level under acidic environment. The studies revealed that Al and Fe ions play a vital role in the copper incorporation by forming spinel structure CuAl(or Fe)2O4 after well-controlled thermal reactions (Tang et al., 2011; Tang et al., 2016). Huge amounts of sewage sludge are generated due to rapid industrialization and urbanization (Guerrero et al., 2013), and further accelerated by the application of enhanced sewage treatment techniques to meet the more stringent regulations (He et al., 2007). The disposal of sewage sludge is one of the most difficult problems to be solved, and the need to achieve a sustainable sludge management strategy has become of global concern (Spinosa et al., 2011). Incineration has become an alternative to largely reduce the sludge volume, destruct pathogen agents, and remove organic pollutants for easier and safer handling and disposal (Fullana et al., 2004). Sludge incineration technology has been recognized as one of the most attractive disposal methods in the world (Fytili et al., 2008; Murakami et al., 2009). Nevertheless, approximately 30% of the solids remain as residues after sludge incineration (Malerius et al., 2003). With further development of the incineration technology, the subsequent disposal of incineration residues is becoming a serious concern (Rulkens, 2008). Since the inorganic flocculants used in the water treatment process are rich in Al and Fe elements, incinerated sewage sludge ash (ISSA) can be a potential substitute for kaolinite and mullite to provide Al and Fe elements for metal incorporation (Lin et al., 2015). Employing ISSA to stabilize industrial sludge with high content of heavy metals will save the useful minerals or ceramic materials that are used in the previous studies as aluminum-rich or iron-rich precursors, and further turn the solid waste into applicable resources for brick or tile industries. In this study, ZnO was used as simulated zinc-laden sludge and incinerated sewage sludge ash (ISSA) was treated as Al- and Fe-rich ceramic precursors after being fired at 900 °C.The sintering process was carried out under reasonable firing temperature (650-1250 °C) and sintering time (3 h). The phase transformation and compositions after 3-h sintering were examined while the stabilization effect of zinc in sintered products was evaluated under different environment conditions with pH value from 2.9 to 13.0.
2. MATERIALS AND METHODS The sewage sludge was collected from a wastewater treatment plant in Shenzhen of Guangdong Province in China. The collected sludge was dried at 60 °C, and then incinerated at 900 °C for 1 h in a muffle oven to obtain ISSA. The elemental compositions of the ISSA was analyzed via X-ray fluorescence (XRF, SHIMADZU XRF-1800 wavelength-dispersive X-ray fluorescence spectrometer). To intensively study the mechanisms of zinc stabilization, the samples were simulated from the mixture of zinc oxide (ZnO) and ISSA at a molar ratio of Zn:Al around 1:2. The mixing
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
process was carried out by ball milling 60 g of the powder mixture in water slurry for 18 h, and then the water slurry was transferred and dried in the oven at 105 °C. The dried mixture was homogenized by mortar grinding and each time 2 g powder sample was pressed into 20 mm pellets under the pressure of 156.8 MPa with the holding time of 2 minutes. The pellets were respectively sintered in a high-temperature furnace at 650-1250 °C for 3 h with a heating rate of 10 °C /min. Then the sintered pellets were broken and ground into fine powders for the subsequent X-ray diffraction (XRD, Rigaku Smartlab) scanning and leaching tests. Phase composition and transformation were investigated by powder XRD in continuous scanning mode with 2θ range of 10°-90° and a scan speed of 10° per minute. Qualitative phase identification was executed by matching powder XRD patterns with those retrieved from the standard powder diffraction database of the International Centre for Diffraction Data (ICDD PDF2, Release 2008). The leaching experiments were carried out in both acidic (pH=2.9, TCLP extraction fluid #2, US Environmental Protection Agency’s SW-846 Method 1311), neutral (deionized water, pH=6.2) and alkaline (pH=13.0, 0.1M NaOH solution) environment. Each leaching vial was filled with 4 mL of the as-prepared extraction solution and 0.2 g powder sample, and then rotated end-over-end at 60 rpm. The 18-h leaching solution was collected after being filtered with 0.45-μm syringe filters, and the pH values of the leachate were measured instantly. All leachates were measured by an inductively coupled plasma-optical emission spectrometer (ICP-OES, Optima 8000DV PerkinElmer) for concentrations of zinc and phosphorus.
3. RESULTS AND DISCUSSION 3.1 Characterization of the sewage sludge The sewage sludge was collected from one wastewater treatment plant in Shenzhen, and experiments were carried out to further characterize the sewage sludge samples. A thermal analysis of the sludge was carried out with thermogravimetry and differential scanning calorimetry at temperatures up to 1200 °C (NETZSCH STA 449F3), and the results were shown in Figure 1. The raw dried sludge was placed in an Al2O3 crucible and heated in an Ar atmosphere with a heating rate of 10 K min−1 to 1200 °C. An obvious weight loss (~ 30 wt %) of the sewage sludge was observed up to 550 °C, with one endothermic peak detected at around 125 °C caused by the removal of adsorbed and bounded water, and one huge exothermic peak at about 610 °C, mainly due to the decomposition of organic components. Furthermore, two more exothermic peaks were observed at around 850 and 1100 °C, which might have been caused by the decomposition of inorganic minerals.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 1. Thermogravimetry (TG) and differential scanning calorimetry (DSC) curves of the thermally dried sewage sludge collected from Shenzhen.
The dried were then fired at 900 °C for 30 min to remove the organic contents and then ground into powder for elemental composition analysis. Normalization into metal oxides (Table 1) shows that the top three elements were Al, Si, and P for the sludge ash. The XRD patterns in Figure 2 further indicates complicated compositions in the calcined sludge, which demonstrates the existence of mullite and hematite with considerable peak intensities. Therefore, it is also feasible for the sludge ash to be used as an iron- and aluminum-rich raw material for heavy metal stabilization.
Table 1. Basic oxide forms of major elemental compositions of the 900 °C and 1-h incinerated sewage sludge ash (ISSA) detected by X-ray fluorescence spectrometry. Elements Weight Percentages (%)
Al2O3
SiO2
P 2O 5
Fe2O3
CaO
SO3
K 2O
MgO
Others
35.1
28.0
14.0
10.2
3.2
3.1
1.7
1.2
3.5
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 2. XRD patterns of 900 °C and 30 min calcined sewage sludge.
3.2 Phase transformation after sintering at different temperatures The phase transformation during the sintering processes was shown in Figure 3. The successful formation of zinc spinel after thermal treatment was verified by the XRD patterns. From the increase in the peak intensities, a higher sintering temperature favored the crystallization of zinc spinel within the temperature range from 650 to 1250 °C. At the highest sintering temperature (1250 °C) of this sintering scheme, the spinel phase was observed as the predominant phase with the highest peak intensity. Moreover, the spinel phase was identified as a solid solution with a chemical formula of ZnAlxFe2-xO4 rather than ZnAl2O4 or ZnFe2O4, due to the coexistence of Al and Fe elements in the incinerated sewage sludge ash and the further competition when forming the spinel structure. According to Bragg’s equation—2dsinθ=nλ (d represents interplanar spacing, θ represents the inclined angle of the incident ray and the reflecting plane, n represents a positive integer, and λ represents the wavelength of incident ray) - the value of 2θ was inversely proportional to the value of d. As the interplanar spacing of ZnAlxFe2-xO4 was between that of ZnAl2O4 and ZnFe2O4, the XRD pattern of ZnAlxFe2-xO4 would be similar to theirs, but the diffraction peaks would fall between the corresponding peaks of ZnAl2O4 and ZnFe2O4. From two sets of spinel peaks from the XRD patterns of samples sintered at 650 and 750 °C, there might be two different x values in the ZnAlxFe2-xO4 spinel solid solution, probably because at lower temperatures such as 650 and 750 °C, the driving forces of solid-state reactions are usually small and the supplied energy was not enough for the homogenization of Zn, Al, and Fe ions. As the sintering temperature rose from 850 to 1250 °C, the peaks of two spinel solid solutions began to converge into one identical spinel, as shown in Figure 3, indicating the homogenized solid reactions between Zn, Al, and Fe ions at higher temperatures. Zinc oxide was distinguishable at 650 and 750 °C but eventually diminished with elevated sintering temperatures, as reflected from the XRD patterns. Moreover, at 650 and 750 °C, the major Al-containing crystalline phase was identified as mullite (Al4.54Si1.46O9.73), whereas Fe
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
mainly existed as hematite (Fe2O3). Like zinc oxide, mullite and hematite disappeared when the sintering temperature reached 850 °C, followed by an increase in the spinel peaks. Therefore, the formation of ZnAlxFe2-xO4 spinel was energetically favorable in this reaction system. Once the temperature or the supplied energy was high enough, the Zn, Al, and Fe elements would transform from other phases, for example, zinc oxide, mullite, and hematite, into the ZnAlxFe2xO4 spinel.
Figure 3. XRD patterns of ISSA treated from 650 °C to 1250 °C for 3 h. (zinc oxide: ZnO, PDF#79-2205; α-quartz: SiO2, PDF#77-1060; mullite: Al4.54Si1.46O9.73, PDF# 79-1456; hematite: Fe2O3, PDF#85-0987)
3.3 Evaluation on the effect of zinc stabilization after sintering processes Figure 4 shows the variations in pH value and zinc concentrations in leachates with an initial pH value of 2.9. Both the pH value and the zinc concentration of the leachates decreased as the sintering temperatures increased. After the 18-h leaching process, the pH value increased from 2.9 to 6.2 due to the dramatic extraction of zinc from the untreated ZnO + ISSA ash sample. When the sample was thermally treated at 650 °C, the zinc concentration was still considerable, elucidating insufficient incorporation of zinc into the spinel structure, which was in accord with the XRD results that exhibited a dominant zinc oxide phase with intense peaks at 650 °C. However, the zinc concentration dropped significantly with a further increase of 200 °C and decreased continuously with the elevated sintering temperature. From Figure 4, the zinc concentration in the leachate of untreated ZnO + ISSA was about 3274 mg/L, whereas the lowest zinc leachability (264 mg/L) was achieved from samples after being sintered at 1250 °C. The leachability of zinc from 1250 °C-sintered ZnO + ISSA was around 8% of the untreated sample. Given that the leachate of the unsintered sample was nearly neutral, with a pH value of 6.2, and the hydrogen ions in the extraction solution were almost depleted, the amount of
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
leachable zinc in the untreated incinerated sewage sludge ash might be greater than that leached out in this experiment. The pH values and zinc concentrations demonstrated the feasibility of thermal treatment for zinc stabilization in the sewage sludge ash system. Furthermore, the zinc leachability was also evaluated using leaching liquid with initial pH value of 6.2, and 13.0, respectively. With an intial pH value of 6.2, the zinc concentration is extremely low and was measured to be around zero for samples sintered at temperatures ≥ 750 °C, but ~ 5 mg/L in leachates of unsintered mixture. Meanwhile, in alkaline environment with intial pH value of 13.0, the zinc concentration is around 70 mg/L in leachates of the mixture before and after being sintered at 650 °C. However, the zinc concentration decreased obviously to 30 mg/L when the mixture was further heated at 750 °C, and reached a minimum value of around 10 mg/L at high temperatures. From the leachability of zinc under conditions of acidic, neutral, and alkaline, it can be concluded that the stabilization efficiency was generally improved at higher temperatures due to the development of the spinel phase, which was in agreement with the XRD results displayed in Figure 3.
Figure 4. Zinc concentrations and pH values of the leachates of ZnO + ISSA samples before and after being sintered at 650-1250 °C for 3 h.
4. CONCLUSIONS The feasibility was verified in this study to beneficially use ISSA as Al- and Fe-cotnaining ceramic precursor for zinc stabilization by incorporating zinc in a highly stable spinel structure. The spinel structure was detected and identified to be zinc spinel soild solution due to the coexistence of Al and Fe in the sintered mixture. Moreover, the peaks of the spinel solid solution became sharper and intensified with elevated firing temperatures, which indicated the enhance of zinc incorporation favoured by higher sintering temperatures. Furthermore, zinc concentration in the leachates were decreased with the increase of sintering temperature, which confirmed that a higher stabilization efficiency can be achieved with the formation of the spinel phase.
AKNOWLEDGEMENTS This work is supported financially by the Shenzhen Science and Technology Innovation
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Committee (JCYJ20150601155130432; JCYJ20160429191618506).
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