Solidification of Heavy Metals in Sludge Ceramsite and

Jul 6, 2009 - The solidifying efficiencies of heavy metals are improved by crystallization, and the main compounds in ceramsite are crocoite, chrome o...
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Environ. Sci. Technol. 2009, 43, 5902–5907

To investigate stabilization of heavy metals in ceramsite made from wastewater treatment sludge (WWTS) and drinking water treatment sludge (DWTS), leaching tests were conducted to find out the effect of SiO2:Al2O3, acidic oxides (SiO2 and Al2O3), Fe2O3: CaO:MgO, and basic oxides (Fe2O3, CaO, and MgO) on the binding ability of heavy metals. Results show that as ratios of SiO2: Al2O3 decrease, leaching contents of Cu and Pb increase, while leaching contents of Cd and Cr first decrease and then increase; under the variation of Fe2O3:CaO:MgO (Fe2O3 contents decrease), leaching contents of Cd, Cu, and Pb increase, while leaching contents of Cr decrease. Acidic and basic oxide leaching results show that higher contents of Al2O3, Fe2O3, and MgO are advantageous to improve the stability of heavy metals, while the binding capacity for Cd, Cu, and Pb is significantly reduced at higher contents of SiO2 and CaO. The solidifying efficiencies of heavy metals are improved by crystallization, and the main compounds in ceramsite are crocoite, chrome oxide, cadmiumsilicate,andcopperoxide.Theseresultscanbeconsidered as a basic understanding for new technologies of stabilization of heavy metals in heavily polluted WWTS.

nology for sludge disposal, but some of the final products still have to be deposited in landfills. Continuous increases in the quantity of WWTS call for efficient and environmentally friendly approaches to solve its disposal problems. The utilization of WWTS for making ceramic or ceramsite is a potential solution for the disposal problem due to the fact that it adds value to the sludge by transforming it into useful materials, which can provide a real potential for the application of wastes in significant quantities (11-18). The resulting ceramsite is usually made with WWTS and clay. To avoid more consumption of clay and protect the earth’s surface environment, the searching of other materials to replace clay in ceramsite production is to be prompted. Drinking water treatment sludge (DWTS) (major inorganic components in DWTS, such as SiO2, Al2O3, Fe2O3, CaO, and MgO, and typical gas-producing materials such as carbonates, hydrates, and sulfates (19, 20), to a great extent are similar to those in clay) has successfully been utilized as a substitute for clay for the production of ceramsite (21). Now, the main concern is whether it is safe to use ceramsite made with WWTS containing heavy metals. Leaching of toxic heavy metals from ceramsite into water may be critically affected by metal compounds, components of raw materials, and the surrounding environment. Thus, the short-term and longterm durability of the metal compounds should be evaluated, especially in this case where the heavy metals are not separated but stabilized in the product (17, 18). Although much work has reported the leaching behaviors of heavy metals in sludge and the derived products (17, 18, 22), few results have been reported on the relationship between leaching behaviors and chemical composition (such as SiO2, Al2O3, Fe2O3, CaO, and MgO, the major acidic and basic oxides in DWTS and WWTS, which profoundly affect the bloating behavior and crystal formation of the ceramsite during the sintering process). The aim of the present work is (i) to obtain valuable information about the potential environmental risks of the use of ceramsite by studying the leachability of heavy metals, (ii) to demonstrate the effect of SiO2:Al2O3, acidic oxides (SiO2 and Al2O3), Fe2O3:CaO:MgO, and basic oxides (Fe2O3, CaO, and MgO) on the solidification of heavy metals in ceramsite, and (iii) to investigate the forms of heavy metals in ceramsite as well as to analyze the solidification mechanism and to establish effective parameters for evaluation.

Introduction

Experimental Section

Disposal of wastewater treatment sludge (WWTS) is a pressing environmental problem at this time, which is aggravated by its accumulation around the world (1, 2). Improper sludge disposal can pose risks to both public health and the environment because most sludge contains heavy metals, disease-causing pathogens, and organic and inorganic compounds. It is, therefore, of great significance to find a proper way to dispose WWTS to avoid secondary pollution (3-6). The generally adopted sludge disposal is landfilling, but this option takes up valuable space and may generate methane that contributes to the greenhouse effect. Another commonly used method is thermal treatment (7-9) involving incineration, gasification, and pyrolysis, which can reduce the leachability of heavy metals in the obtained materials with a dramatic decrease in the volume of sludge (10). This method proposes an alternative waste management tech-

Materials. The WWTS used in this study was obtained from Wen-chang Wastewater Treatment Plant, Harbin, China. The dewatering of WWTS was performed with a belt filter press, and cationic polymeric flocculants were used for the flocculation and dewatering of the activated sludge. The DWTS was collected from the chemical coagulation/flocculation unit of the third drinking water treatment plant, Harbin, China. The coagulant is aluminum sulfate (Al2(SO4)3). Major components of WWTS and DWTS were analyzed using a Philips PW 4400 XR spectrometer (X-ray fluorescence-XRF, Amsterdam, The Netherlands) as shown in Tables S1 and S2 (Supporting Information). The parameters for the production of ceramsite were obtained and presented as follows: DWTS/ WWTS ) 45/55, sodium silicate (Na2O · (SiO2)x · (H2O)y)/ (DWTS + WWTS) ) 20%, sintering temperature ) 1000 °C, and sintering time ) 35 min (21). The original ratio of SiO2: Al2O3:Fe2O3:CaO:MgO in the mixture of DWTS, WWTS, and sodium silicate for production of ceramsite is 27.2:15.8:6.0: 3.5:1.8. The simulated contents (wt%) of tested oxide (SiO2, Al2O3, Fe2O3, CaO, or MgO) were adjusted by adding the oxide or the other four oxides to the raw materials. All of the used

Stabilization/Solidification of Heavy Metals in Sludge Ceramsite and Leachability Affected by Oxide Substances G U O R E N X U , * ,† J I N L O N G Z O U , †,‡ A N D GUIBAI LI† State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China, and School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China

Received February 26, 2009. Revised manuscript received June 1, 2009. Accepted June 3, 2009.

* Corresponding author phone: +86-451-86282559; fax: +86-45186282559; e-mail: [email protected]. † Harbin Institute of Technology. ‡ Heilongjiang University. 5902

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10.1021/es900139k CCC: $40.75

 2009 American Chemical Society

Published on Web 07/06/2009

FIGURE 1. Effect of SiO2:Al2O3 on the leaching contents of heavy metals. oxides (SiO2, Al2O3, Fe2O3, CaO, and MgO) with particle sizes below 10 µm were of analytical grade. The reference WWTS sample was made with heavy metals by adding metal solutions (Cd(NO3)2, K2CrO4, Pb(NO3)2, and CuSO4 were of analytical grade) into the dried WWTS, mixing the components, and allowing them to react for 30 days. The contents of Cd, Cr, Cu, and Pb were designed according to the basic data obtained from the analysis of activated sludge at different places in China as shown in Table S3 (Supporting Information) (17). The synthetic metal solutions were prepared by dissolving 0.05 g L-1 Cd2+, 0.1 g L-1 Cr6+, 0.1 g L-1 Pb2+, and 0.5 g L-1 Cu2+ in deionized water. Simulated heavy metal concentrations were prepared by adding the tested heavy metal compounds into sludge. The purpose for this simulation was to investigate whether it is harmless to utilize a different region’s sludge for production of ceramsite even at high concentrations of hazardous metals. The contents of heavy metals added into the reference WWTS samples are shown in Table S4 (Supporting Information). Methods. The WWTS containing heavy metals and DWTS were treated by the air-dry method and were ground into sizes below 100 µm that are sufficiently fine to be homogeneously mixed. The ceramsite for determination of the stabilization of Cd, Cr, Cu, and Pb was made with DWTS, WWTS, and sodium silicate. The raw materials were mixed and pelletized to particle sizes of 5-8 mm and left in a room with a temperature of about 20 °C for about 5 days, and then the samples were dried at 110 °C in a DHG-9070A blast roaster (Shanghai Shenxian Thermostatic Equipment Factory, Shanghai, China) for 24 h. The heating of samples started at 20 °C. The samples were heated at a rate of 8 °C/min in a SX2-10-12 muffle furnace (Harbin Songjiang Electric Co., LTD, Harbin, China), and the samples were soaked at 200, 600, and 800 °C for a duration of 10 min and at 1000 °C for a duration of 35 min, and then these samples were naturally cooled until they reached room temperature (21).

The leachability of ceramsite samples was determined with a revised method derived from a toxicity characteristic leaching procedure (TCLP) (17). The leaching test was conducted with the solution prepared at a liquid-solid ratio of 1 L/200 g (pH ) 4.93) and stirred at 110 rpm for 24 h or 30 days. The supernatant was analyzed with a PerkinElmer Optima 5300DV Inductively Coupled Plasma Atomic Emission spectrometer (ICP-AES, Waltham, MA). The total contents of heavy metals in WWTS or sintered ceramsite were extracted by acid digestion (using HNO3/HClO4/HF) according to U.S. E.P.A. SW3050 and were examined by ICP-AES. The standard toxicity of heavy metals leached from hazardous waste in China requires the maximum leaching contents of 10, 150, 1000, and 50 ug g-1 for Cd, Cr(VI), Cu, and Pb, respectively (GB 5085.3-2007). Powder X-ray diffraction (XRD) patterns for main crystalline phases and forms of heavy metals in ceramsite were recorded on a D/max-γ β X-ray diffractometer with 50 mA and 40 kV Cu KR radiation (Japan).

Results and Discussion Effect of SiO2:Al2O3. In our investigation, it was found that the solidification characteristics of heavy metals in sintered ceramsite were strongly influenced by the mass ratios of (CaO + Fe2O3 + MgO)/(SiO2 + Al2O3) (defined as F/SA ratios, an important controlling parameter to optimize the components for production of ceramsite) (Figure S1, Supporting Information). In this study, to indicate the actual binding capacity for heavy metals provided by the structures of ceramsite, the effects of SiO2:Al2O3 and Fe2O3:CaO:MgO on the solidification of heavy metals under the condition of F/SA ratio ) 0.275 were investigated. Simulated ratios of SiO2:Al2O3 (or Fe2O3: CaO:MgO) were prepared by adding the oxides (SiO2, Al2O3, Fe2O3, CaO, and MgO) to the raw materials. The mass ratios of SiO2:Al2O3 and Fe2O3:CaO:MgO are shown in Tables S5 and S6 (Supporting Information), respectively. VOL. 43, NO. 15, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. XRD analyses of the main crystalline phases (A) in ceramsite with SiO2:Al2O3 of 1:0.23, 1:0.58, and 1:1.92 and the forms of heavy metals (band labeling: A, albite-anorthite; K, kyanite; Q, quartz; S, sillimanite) and (B) in ceramsite with SiO2:Al2O3 of 1:0.58 (band labeling: 9, CuO; 1, PbCrO4; 2, CdSiO3; b, Cr2O3). It can be seen from Figure 1 that, as ratios of SiO2:Al2O3 decrease, the leaching contents of Cu and Pb at the 24th h or on the 30th day decrease, while the leaching contents of Cd and Cr first decrease and then increase. It seems that higher contents of Al2O3 may enhance the (Cu and Pb) substitution of parent ions (Al3+ or Ca2+) in ceramsite and may restrain the replacement of parent ions by Cd and Cr. This phenomenon indicates that heavy metals can be significantly solidified in ceramsite for a long period of time because the heavy metals with stable forms in the porous surface cannot be easily washed out. It should be noted that Cu leached from ceramsite with lower Cu contents on the 30th day is higher than those with higher Cu contents. Because of the enhancement of diffusion for the ions of Cu in the sintering processes, cation-exchange reactions between the ions of Cu and the crystals in ceramsite occur easily, and consequently, cation exchange is partly responsible for the immobilizations of such heavy metal ions (Cu). The higher Cu contents in ceramsite may enhance their substitution of parent ions (Al3+ and Ca2+) and may, therefore, enable the leaching contents of Cu to decrease (23). In this study, XRD analyses were applied to obtain mineral compositions of powder ceramsite specimens with an XRD pattern database (International Centre for Diffraction Data, ICDD). It can be seen from Figure 2A that the main crystalline phases of the three kinds of ceramsite with SiO2:Al2O3 of 1:0.23, 1:0.58, and 1:1.92 are similar. Ceramsite with SiO2: Al2O3 of 1:1.92 and 1:0.58 consists of many crystalline materials, which are attributable to kyanite (Al2SiO5), quartz (SiO2), and Na-Ca feldspars [albite (NaAlSi3O8) and anorthite (CaAl2Si2O8)] with small amounts of sillimanite [Al1.98Fe0.2SiO5]. Quartz, kyanite, and Na-Ca feldspars are the identified crystalline phases of ceramsite with SiO2:Al2O3 5904

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of 1:0.23. The results in Figure 2A suggest that SiO2:Al2O3 plays an important role in the formation process of crystals (22), but the crystalline phases in ceramsite are similar to each other, and the contents of each crystal do not dramatically change under the variation of SiO2:Al2O3. The results also imply that the forms of the heavy metal compounds in ceramsite are not significantly influenced by the content variation of SiO2:Al2O3, and the metal compounds in ceramsite can be investigated by analyzing the discretional one of the ceramsite (SiO2:Al2O3 ) 1:0.58 was selected for analysis). Figure 2B shows that the four heavy metals in ceramsite with SiO2:Al2O3 of 1:0.58 are in steady forms and the main compounds are crocoite (PbCrO4, melting point (tm) ) 844.0 °C), chrome oxide (Cr2O3, tm ) 2266.0 ( 25.0 °C), cadmium silicate (CdSiO3, tm ) 1242.0 °C), and copper oxide (CuO, tm ) 1326.0 °C). The formation of Cr2O3, CuO, and PbCrO4 suggests that the solidification of these heavy metals is caused by the incorporation of these compounds into the aluminosilicates or silicates matrix after the heat-induced transformation. The formation of CdSiO3 suggests that Cd has entered the liquid-solid phases to react with silicates and to stabilize in the crystalline structures of ceramsite during the sintering process. It is, thus, proved by XRD analyses that the solidification efficiencies of these heavy metals and their leachability strongly depend on their specific chemical forms or ways of binding, as reported in other studies (24). In the sintering process, solidification of heavy metals in ceramsite is caused by the rearrangement of particles, transformations of crystal phases, and substitution of parent ions in the ceramsite bodies, which may strongly depend on the ratios of SiO2:Al2O3 in the raw materials. It is, thus, concluded that the capacities for binding heavy metals are improved by the transformation of these heavy metals to the crystalline state and the chemical incorporation of these metals into the aluminosilicate or silicate frameworks. Effect of Acidic Oxides. It can be seen from Figure S2 (Supporting Information) that as the SiO2 contents increase, the leaching contents of Cd and Cu of the three specimens (1 mg kg-1 Cd, 25 mg kg-1 Cd, and 50 mg kg-1 Cd) increase at the 24th h or on the 30th day; the leaching contents of Cr gradually decrease, while the leaching contents of Pb first decrease and then increase. The binding of trace metals (Cd and Cu) changes significantly as the SiO2 contents increase, meaning that most of the heavy metals become more readily available for leaching. The leaching behavior of Pb in this study indicates that as the SiO2 contents are >27%, Pb may be ejected from the bulk structures of ceramsite during the restructuring process and may be relatively enriched on the surface of the particles, which, therefore, leads to higher leaching contents of Pb. At the 24th h or on the 30th day, the leaching contents of Cd, Cr, Cu, and Pb increase as the Al2O3 contents decrease (as shown in Figure S3, Supporting Information). The reason may be that, during the sintering process, the increasing quantities of Al3+ may substitute for the Si4+ in a network tetrahedron (23) and other unreacted Al2O3 present as oxide acts as an accelerator using the readily available cations (such as Cd, Cr, Cu, and Pb) to increase the pozzolanic reactions, which contribute to the stability of the networks and make heavy metals to enclose in the framework of silicates or aluminosilicates and make them difficult to be leached. The results in Figures S2 and S3 (Supporting Information) indicate that the acidic oxides (SiO2 and Al2O3) play an important role in the solidification of heavy metals, which is inevitably related to the formation process of crystals and to forms of heavy metal compounds. The XRD analyses are, thus, needed to test the main crystalline phases and the forms of heavy metals in ceramsite to judge the actual improvement in metal binding resulting from the thermal treatment. It is found that the crystalline phases in ceramsite do not

FIGURE 3. XRD analyses for the forms of heavy metals in ceramsite with SiO2 contents of 35% and Al2O3 contents of 20% (band labeling: 9, CuO; 1, PbCrO4; 2, CdSiO3; b, Cr2O3). dramatically change under the variation of SiO2 or Al2O3 contents (Figure 3) (22). Raw materials with SiO2 content ) 35% and Al2O3 content ) 20% are selected for analyzing the forms of heavy metal compounds in ceramsite. Figure 3 shows that the main compounds of the four heavy metals in ceramsite are crocoite, chrome oxide, cadmium silicate, and

copper oxide. These results imply that the effect of the variation of SiO2 or Al2O3 contents on the forms of metal compounds in ceramsite is minor and the contents of each compound do not dramatically change under the content variation of the heavy metal. Effect of Fe2O3:CaO:MgO. To investigate the effects of Fe2O3:CaO:MgO on the leaching characteristics of heavy metals solidified in ceramsite, leaching tests were conducted with the ceramsite made with different contents of Fe2O3, CaO, and MgO (as shown in Figure 4). As the contents of Fe2O3 decrease (i.e., the ratios of Fe2O3 in Fe2O3:CaO:MgO decrease), the leaching contents of Cd, Cu, and Pb at the 24th h or on the 30th day increase, while the leaching contents of Cr decrease. It can be concluded that higher contents of Fe2O3 enhance the solidification of Cd, Cu, and Pb in ceramsite and reduce the mobility of these heavy metals. One of the reasons for this phenomenon may be attributed to a better physical and chemical interaction between the heavy metals and the silicate or aluminosilicate matrix. Fe2O3 can react with silicates or aluminosilicates to form mineral groups with relatively lower eutectic points around 1000 °C, and the reaction can effectively lower the sintering point of the materials and enhance the formation of the liquid phase (26). Formation of liquid phases reduces the number of pores in ceramsite and hinders the initiation of any crack, which, thus, decreases the available space for water ingress and improves the solidification efficiencies of Cd, Cu, and Pb in ceramsite. As the CaO contents increase, the crystalline particles are easily formed at a given temperature, and its chemical reactivity in the silicate networks is also increased, which may restrain the substitution of Ca2+ by heavy metals (23) and lead to the increase of the leaching contents of heavy metals (Cd, Cu, and Pb). The variation trend of the leaching contents of Cr is quite different from the other three metals as shown in Figure 4. The leaching contents of Cr are fully

FIGURE 4. Effect of Fe2O3:CaO:MgO on the leaching contents of heavy metals. VOL. 43, NO. 15, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. XRD analyses of the main crystalline phases (A) in ceramsite with Fe2O3:CaO:MgO of 1:0.27:0.14, 1:0.58:0.30, and 1:1.13:0.30 and the forms of heavy metals (band labeling: A, albite-anorthite; K, kyanite; Q, quartz; S, sillimanite) and (B) in ceramsite with Fe2O3:CaO:MgO of 1:1.13:0.30 (band labeling: 9, CuO; 1, PbCrO4; 2, CdSiO3; b, Cr2O3). controlled by hexavalent chromium compounds in the materials (18, 27), and the leaching contents are, therefore, greatly affected by the changes in the redox state upon heating. During the sintering process, Cr(VI) can be partly deoxidized to Cr(III), which may dramatically influence its solidification and leaching behaviors (27). Kyanite, Na-Ca feldspars, and quartz with small amounts of sillimanite can be identified in XRD patterns for the ceramsite with Fe2O3:CaO:MgO of 1:1.13:0.30, 1:0.58:0.30, and 1:0.27:0.14 (as shown in Figure 5A). As ratios of CaO increase, the amounts of Na-Ca feldspars increase in the ceramsite while the peaks of quartz and kyanite crystals tend to decrease. The results indicate that there is little difference in the XRD analyses for the three ceramsites. Therefore, Fe2O3: CaO:MgO of 1:1.13:0.30 is selected for analyzing the forms of heavy metal compounds in ceramsite made with different contents of heavy metals. Figure 5B shows that the main compounds in ceramsite are crocoite, chrome oxide, cadmium silicate, and copper oxide. The main differences in the compounds in ceramsite with different contents of heavy metals are the intensity of their peaks. Effect of Basic Oxides. It can be seen from Figure S4 (Supporting Information) that as Fe2O3 contents increase, the leaching contents of Cd, Cu, and Pb decrease at the 24th h or on the 30th day, while the leaching contents of Cr first decrease and then increase. Fe2O3 can react with silicates to enhance the formation of the liquid phase (26), and more liquid phases improve the solidification efficiencies of Cd, Cu, and Pb in ceramsite. When the Fe2O3 contents are e6%, the decrease in leaching contents is very likely caused by the reducing environment formed locally while heating, thus, causing a reduction of Cr(VI) to Cr(III). As the contents of 5906

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Fe2O3 increase in the raw materials (>6%), the formation of FeO with higher viscosity is apparently increased by consuming more CO (3Fe2O3 + CO f 2Fe3O4 + CO2, at >820 °C; Fe3O4 + CO f 3FeO + CO2, at >820 °C) (28), which accordingly reduces the deoxidization efficiency of Cr(VI) to Cr(III) and increases the leaching chance of Cr(VI). Figure S5 (Supporting Information) shows that as the CaO contents increase, the leaching contents of Cd, Cu, and Pb increase at the 24th h or on the 30th day, while the leaching contents of Cr first decrease and then increase. As the contents of CaO increase from 2.75 to 7%, the leaching contents of Cd, Cu, and Pb increase, which implies that excessive CaO exceeds the needed ions for producing electrical neutrality and restrains the substitution of Ca2+ by the heavy metals (24, 25). As CaO contents increase from 2.75 to 4%, the deoxidization of Cr(VI) to Cr(III) is enhanced by the decrease of another deoxidization reaction, which is reflected by the gradual decrease of leaching contents of Cr(VI); when the CaO contents are >4%, although the deoxidization of Cr(VI) may be still enhanced in ceramsite, the compressive strength of ceramsite is lowered (21), which enables more Cr(VI) to become available to be leached by increasing the pore rates, and more Cr(VI) is easily washed out by distilled water. It can be seen from Figure S6 (Supporting Information) that the leaching contents of Cd, Cr, Cu, and Pb increase at the 24th h or on the 30th day as MgO contents decrease from 1.6 to 4%. The results indicate that the sintering of ceramsite with higher contents of MgO leads to a chemically restructured matrix that may have the potential to stabilize heavy metals by decreasing their availability for leaching (29, 30). This also implies that the change in metals binding over time in aquatic environments is weak and that the toxic metals bound in the ceramsite pose no harmful impact on the environment. It is, therefore, concluded from the above results that as the MgO contents increase, stronger chemical bonds are formed between these heavy metals and the silicates or aluminosilicates in the ceramsite, making it difficult for them to be leached even over a long period. The heavy metal compounds in ceramsite with Fe2O3 contents of 7% (CaO of 4% and MgO of 3%) are selected for analyses. The evidence of chemical immobilization of heavy metals can be gained from the mineralogical characterization, and the heavy metal compounds in ceramsite can be identified by XRD analyses (as shown in Figure S7, Supporting Information). The main compounds in the three ceramsites (Fe2O3of 7%, CaO of 4%, and MgO of 3%) are crocoite, chrome oxide, cadmium silicate, and copper oxide. The transformation of these heavy metals to the crystalline state is advantageous for the long-term stability of the metals, and the crystalline solids are likely to have improved binding capacity. Moreover, it is possible that the heat-induced transformation of crystallization causes most of the heavy metal ions to transfer from the surface to the interior of the particles. Environmental and Technological Implications. This study demonstrates the feasibility of transforming heavy metals into stable forms in ceramsite and the successful reduction of heavy metals mobility after heat treatment. The results show that the stability of heavy metals in ceramsite cannot be easily released into the environment again to cause secondary pollution and that this new technology for production of ceramsite can utilize sludge with different components by adjusting the oxide contents. The results are helpful to understand the leaching behaviors of heavy metals influenced by SiO2:Al2O3, acidic oxides (SiO2 and Al2O3), Fe2O3: CaO:MgO, and basic oxides (Fe2O3, CaO, and MgO) and to make a comprehensive judgment of the environmental impact of ceramsite during its service life or any secondary service life. It is concluded that WWTS and DWTS can be converted to harmless ceramsite and solve the problem of

sludge disposal, in accordance with the concept of sustainable development.

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Acknowledgments The authors gratefully acknowledge the financial support by National Key Technology R&D Program (2006BAC19B04-04), NSFC (50778051), and Program for New Century Excellent Talents in University (NCET-08-0161).

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Supporting Information Available Additional details are available including components of the WWTS and the DWTS (Tables S1 and S2), average contents of heavy metals in the WWTS at different wastewater treatment plants (Table S3), the contents of heavy metals in reference WWTS samples (Table S4), ratios of SiO2:Al2O3 and Fe2O3:CaO:MgO (Tables S5 and S6), effect of F/SA ratios, SiO2, Al2O3, Fe2O3, CaO, and MgO on the leaching behaviors of heavy metals (Figures S1-S6), and XRD analyses for forms of heavy metals in ceramsite with Fe2O3 contents of 7%, CaO contents of 4%, and MgO contents of 3% (Figure S7). This material is available free of charge via the Internet at http:// pubs.acs.org.

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