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Chromium Extraction from Sewage Sludge Using Polyepoxysuccinic Acid
Pedosphere 22(1): 131–136, 2012 ISSN 1002-0160/CN 32-1315/P c 2012 Soil Science Society of China Published by Elsevier B.V. and Science Press
Chromium Extraction from Sewage Sludge Using Polyepoxysuccinic Acid∗1 ZHANG Li-Hua1,∗2 and ZHU Zhi-Liang2 1 2
Department of Chemistry and Biology Engineering, Sanming University, Sanming 365004 (China) State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai 200092 (China)
(Received April 27, 2011; revised November 15, 2011)
ABSTRACT An environmentally benign biodegradable chelant, polyepoxysuccinic acid (PESA), was used to separate heavy metals from sewage sludge from the Shanghai Taopu Wastewater Treatment Plant, China, based on chemical extraction technology. The extraction of chromium (Cr) from sewage sludge with an aqueous solution of PESA was studied under various conditions. It was found that the extraction of Cr using PESA was more efficient than that using ethylenediaminetetraacetic acid (EDTA) and S,S-ethylenediaminedisuccinic acid (EDDS) under similar conditions. PESA was capable of extracting Cr from the sewage sludge, and the extraction efficiency was obviously dependent on both the pH and the concentration of the chelating reagent. The extraction efficiency decreased gradually with increasing pH, and the dependence on pH decreased as the concentration of PESA increased. The extraction efficiency reached 58% under conditions of pH = 4 and a ratio of PESA to total heavy metals of 10:1. The extraction efficiency was maintained above 40% within the pH range from 1 to 7 at the high ratio of PESA to total heavy metals of 10:1. Comparing the contents of heavy metals in the sewage sludge before and after the extraction, it was found that the extracted Cr came mainly from the reducible and oxidizable fractions. Key Words:
ethylenediaminetetraacetic acid, extraction efficiency, heavy metals, pH, S,S-ethylenediaminedisuccinic acid
Citation: Zhang, L. H. and Zhu, Z. L. 2012. Chromium extraction from sewage sludge using polyepoxysuccinic acid. Pedosphere. 22(1): 131–136.
Heavy metal pollution of soil, sediment, and sewage sludge is widespread across the globe, and the decontamination of heavy metals remains a difficult task (Sun et al., 2001; Vandeviere et al., 2001; Tandy et al., 2004). One of the most attractive potential sludge disposal methods is land application. However, sewage sludge contains not only valuable components such as nutrients but also heavy metals, which are nonbiodegradable and accumulate in the environment. Land application of sewage sludge containing harmful metals may adversely affect human health and cause phytotoxicity (Veeken and Hamelers, 1999; Yoshizaki and Tomida, 2000; Nogueira et al., 2010). Recently, research has been directed to find an efficient method to remove heavy metals from sewage sludge (Veeken and ∗1
Hamelers, 1999; Vandeviere et al., 2001; Tandy et al., 2004; Zhang et al., 2008; Zhu et al., 2009). Chromium (Cr) is a toxic element that occurs in highly variable oxidation states. Cr(III) and Cr(VI) have completely different behaviors, and their toxicities can differ by an order of magnitude. Cr is one of the most “difficult” soil elements. Environmental Cr pollution, especially soil contamination with Cr, has become one of the focuses of the environmental science (Pueyo et al., 2004; Kocher et al., 2005; Banks et al., 2006; Boyd, 2010). However, there has been little significant progress made even though various methods to remove heavy metals from sewage sludge have been investigated (Yoshizaki and Tomida, 2000). One possible remediation technique is ex-situ was-
Supported by the State Key Laboratory of Pollution Control and Resource Reuse Foundation, China (No. PCRRF09004) and the Natural Science Foundation of the Education Department of Fujian Province of China (Nos. JK2011055 and HX200805). ∗2 Corresponding author. E-mail: [email protected].
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hing using chelating agents that offer high efficiency and specificity for the extraction of heavy metals (Van Benschoten et al., 1997; Barona et al., 2001; Sun et al., 2001; Tandy et al., 2004; Lei et al., 2005; Wang et al., 2006). Because some chelating agents would remain in the sewage sludge after the extraction, they may cause secondary pollution. Therefore, degradable chelants are highly favored (Jones and Willams, 2001; Vandeviere et al., 2001; Tandy et al., 2004; Qian et al., 2006; Tandy et al., 2006; Zhu et al., 2009). Polyepoxysuccinic acid (PESA), which is an environmentally benign water-soluble polymer containing no nitrogen or phosphorus, shows good chelating ability for a variety of metal ions (Fuknmoto and Tangiuchi, 1991; Carter et al., 1994) and excellent biodegradability (Wei et al., 2001). The study of PESA as a green chemical product has received great attention in recent years (Rodney and Stephen, 1987; Xiong et al., 1999). Previous work on PESA has concentrated on its synthesis, properties, and application to inhibit corrosion and scale formation in the water recycling field (Fuknmoto and Tangiuchi, 1991; Carter et al., 1994; Zhang et al., 2006). There has been no report on the use of PESA as a chelating reagent to remove Cr from sewage sludge or soils, and thus, its performance with respect to Cr extraction is unknown. Here, we report the investigation of the decontamination of Cr-containing sewage sludge using PESA as an environmentally benign extractant. Hopefully, the results of this study will be helpful in the development of green remediation technology for soils and sewage sludge contaminated with heavy metals. MATERIALS AND METHODS Sewage sludge Solid sewage sludge taken from the Shanghai Taopu Municipal Wastewater Plant in China was used in this study. The sewage sludge samples were air dried naturally, ground, and sieved to a size less than 2 mm. The tested sludge contained 279.7 g kg−1 total organic carbon (TOC), 82.3 g kg−1 water, and heavy metals including Zn, Cu, Cd, Ni, Pb, and Cr. The molar sum of the heavy metals in the sewage sludge was 255.4 mmol kg−1 . The content of Cr was 2 376 mg kg−1 , which exceeded the third-level limit value set by the Environmental Quality Standard for Soils in China (GB 15618-1995) (300–400 mg kg−1 ) and the limit level of the Sludge Agricultural Utilization Standard for soil (GB 18918-2002) (1 000 mg kg−1 for alkali soils of pH
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≥ 6.5 and 600 mg kg−1 for acid soils of pH < 6.5). Chelating agents Polyepoxysuccinic acid (PESA) was obtained from the Meijing Environmental Protection Material Company Limited in Shanghai, China. This material, whose structure is shown in Fig. 1, is highly biodegradable and has an average molecular weight of 1 500, a solid content of ≥ 380 g kg−1 , and a density of ≥ 1.2 g cm−3 . The other reagents used were all of analytical grade and were purchased from the National Chemical Reagent Company Limited in Shanghai, China.
Fig. 1
Structure of polyepoxysuccinic acid.
Extraction experiments The extraction was performed in batch experiments using aqueous suspensions in centrifuge tubes. Different concentrations of the chelating agents were applied, and the molar ratio of the chelant to the sum of the total heavy metals in the sewage sludge was kept in the range from 1:1 to 10:1. The influence of the solution pH on the extraction efficiency of Cr from the sewage sludge was examined at pH from 1 to 10. For each batch, 0.5 g of solid sewage sludge was suspended in 25 mL of the solution (1:50), and the reaction time was fixed at 24 h. The suspension system pH was adjusted with HNO3 or NaOH two days prior to the addition of the chelating agent. The system pH was monitored and readjusted if necessary. The suspensions were shaken at 200 r min−1 for 24 h at room temperature. All suspensions were centrifuged at 4 000 r min−1 for 20 min and passed through 0.45-μm paper filter before analysis of the metal content. After digesting the sludge sample by HF-HClHNO3 in a microwave digestion system, the total content of heavy metals was measured. To determine the species distribution of Cr in the sewage sludge, the samples were analyzed before and after the extraction using the modified BCR (Community Bureau of Reference) sequential extraction method (Quevauviller et al., 1997). Analytical methods The TOC of the sewage sludge was determined
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with a total organic carbon analyzer (SSM-5000A, SHIMADZU, Japan). The total heavy metal content was determined after digestion in a microwave extraction/digestion system (ETHOS E, Mile Stone s.r.l., Italy). The concentrations of dissolved heavy metals in the digestion solutions and extraction solutions were measured with an inductively coupled plasma optical emission spectrometer (Optima 2100 DV, PerkinElmer, USA). The pH measurements for the extraction solutions were conducted with a pH Meter (PP-15 Professional, Sartorius, Germany). Triplicate extractions and analyses were performed for all samples to evaluate the reproducibility of the results, and then the data from the three parallel experiments were averaged. The standard deviation was within 5% of the mean for all the tested heavy metals.
The extraction of Cr with PESA showed a stronger dependence on pH at higher concentrations of the chelant. For example, when the molar ratio of PESA to total heavy metals was 10:1, the extraction efficiency was 35% at pH 10. When pH increased to 4, at the same ratio of 10:1, the extraction efficiency increased to 58%. With the ratio of 10:1, the extraction efficiency remained above 40% in the pH range of 1–7. Influence of PESA concentration on Cr extraction
The system pH is a crucial factor affecting the extraction efficiency. The influence of solution pH on Cr extraction from sludge was examined at pH 1–10. As shown in Fig. 2, the pH of the extraction solution had a significant influence on the extraction efficiency of Cr. In the case where PESA was absent, the extraction efficiency decreased rapidly as the pH increased. The extracted amount of Cr was less than 1% at pH > 3.0. The addition of PESA significantly improved the extraction efficiency in a wide pH range of 3–10 although increasing the pH, in general, reduced the extraction efficiency. The Cr extraction was maximized under weak acidic conditions (pH = 4.0) when the molar ratio of PESA to total heavy metals was greater than 1:1.
When washing soil with the chelant solution, a ratio >1 of the chelant to toxic metals is required for good toxic metal extraction. An excess of chelating agent is needed because major cations in the soil, such as Mn, Mg, Fe, and Ca, along with toxic metals present in smaller amounts compete with the metals being studied for the chelating agent and are also extracted (Tandy et al., 2004). The influence of the PESA concentration on the Cr extraction efficiency was investigated at pH 4 and 7. The molar ratios of PESA to total heavy metals ranged from 1:1 to 10:1. The results, shown in Fig. 3, indicated that the influence of the PESA concentration on the extraction efficiency of Cr was similar at pH 4 and 7. Under both pH conditions, increasing the concentration of PESA improved the Cr extraction at the ratio of PESA to total heavy metals from 1:1 to 10:1. At the ratio of PESA to total heavy metals of > 10:1, the increase in the extracted fraction of Cr was very small. Within the range of the ratios of PESA to total heavy metals from 5 to 10 and at pH 4, approximately 43%–58% of the Cr in the sample was extracted, whereas in the same ratio range at pH 7, the percentage of extracted Cr was only 23%– 35%.
Fig. 2 Influence of pH on Cr extraction from sludge with polyepoxysuccinic acid (PESA) at pH 1–10 and molar ratios of PESA to total heavy metals of 0:1, 1:1, 5:1 and 10:1.
Fig. 3 Influence of polyepoxysuccinic acid (PESA) concentration on Cr extraction at pH 4 or 7 and molar ratios of PESA to total heavy metals from 1:1 to 10:1.
RESULTS AND DISCUSSION Influence of pH on Cr extraction
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Relationship between extraction efficiency and species distribution of Cr in sewage sludge The content and species distribution of the heavy metals in sewage sludge also affect the extraction efficiency of Cr. We are unable to evaluate the environmental impact using the total concentrations of heavy metals because information on potential environmental mobility or bioavailability is not available (Tandy et al., 2004). Cr can occur in soil samples as Cr(III) and Cr(VI). Cr(III) and Cr(VI) have completely different behaviors, and the toxicity of the two species can differ by an order of magnitude (Chen et al., 1994). Cr(III) is considered to be essential for the maintenance of glucose, lipid, and protein metabolism, but Cr(VI) is reported to be toxic due to its oxidizing potential and easy permeation of biological membranes (BeceiroGonzalez et al., 1992). However, the difference in the Cr valence can not reflect the real existence fractions because soil is a complicated system. Various bound species of Cr exist in soil with different toxicity and bioavailability (Chen et al., 1994). Sequential extractions could provide the information needed to explain the different extraction efficiencies of different heavy metals (Tandy et al., 2004). Among various sequential extraction techniques, Tessier’s sequential extraction (Tessier et al., 1979) and the BCR sequential extraction (Quevauviller et al., 1997; Davidson et al., 1998) are the two commonly used techniques to determine the species distribution of heavy metals in soils, atmospheric deposits, and sewage sludge. Compared with other methods, the BCR method has better reproducibility and precision and comparable performance (Quevauviller et al., 1997). In the BCR sequential extraction method, the metal elements are divided into acid-soluble, reducible, and oxidizable fractions. In the
L. H. ZHANG AND Z. L. ZHU
modified BCR sequential extraction method, two additional fractions, the water-soluble and residual fractions, are considered. The distribution of Cr fractions in the sewage sludge samples before and after the extraction with PESA was examined using the modified BCR sequential extraction method. The experiments were performed at pH 4 using a molar ratio of PESA to total heavy metals of 4:1. The results are represented in Fig. 4. The results showed that before extraction, the Cr in the sludge was mainly found in the oxidizable and residual fractions. These two fractions were 49% and 39%, respectively. A lower amount of Cr, approximately 11%, was found in the reducible fraction. After the extraction with PESA, the species distribution of Cr changed, and the majority was found in the residual fraction, whereas the contents of the other fractions were markedly reduced. This result indicated that the extracted Cr from the tested sewage sludge with PESA came mainly from the reducible and oxidizable fractions. Under the given experimental conditions, the extraction efficiency of Cr reached 35%. Comparison of PESA with EDTA and EDDS According to previously reported studies, some chelating agents, such as ethylenediaminetetraacetic acid (EDTA) and S,S-ethylenediaminedisuccinic acid (EDDS), offer good extraction efficiency of heavy metals from soils (Vandeviere et al., 2001; Tandy et al., 2004). EDTA is a very effective chelating agent that is used widely for heavy metal decontamination (Barona et al., 2001; Lei et al., 2005; Zeng et al., 2006). Unfortunately, it has the disadvantage of being persistent in the environment due to its low biodegradability. Recently, the easily biodegradable chelating agent EDDS
Fig. 4 Species distribution of Cr in sewage sludge before and after extraction with polyepoxysuccinic acid (PESA) at pH = 4 and a molar ratio of PESA to total heavy metals of 4:1.
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has been proposed as a safe and environmentally benign replacement for EDTA in soil washing and for chelant-enhanced phytoremediation (Jones and Willams, 2001; Vandeviere et al., 2001; Tandy et al., 2004; Qian et al., 2006; Tandy et al., 2006; Zhang et al., 2008). However, EDDS may have a potential risk of secondary water pollution as the result of eutrophication because this compound contains nitrogen. Obviously, this risk of eutrophication limits the ability of EDDS to meet the increasingly stringent environmental policies and laws. To evaluate the extractability of Cr with PESA, the extraction of Cr from the sewage sludge with EDDS and EDTA was conducted simultaneously under the same conditions. The extraction conditions and the extraction efficiencies are shown in Fig. 5. It was found that PESA gave better Cr extraction performance than EDTA or EDDS under similar conditions. The efficiency of Cr extraction with PESA was nearly constant at approximately 8% over the pH range from 3 to 10 at the ratio of PESA to total heavy metals of 1:1, whereas the efficiency of Cr extraction with EDTA or EDDS
was lower. Furthermore, the efficiency of Cr extraction with PESA was obviously higher than that with EDTA or EDDS at a ratio of PESA to total heavy metals of 10:1 over the pH range of 1–10. The efficiencies of Cr extraction with PESA, EDTA, and EDDS reached 58%, 45%, and 28%, respectively, under conditions of pH = 4 and a ratio of PESA to total heavy metals of 10:1, whereas the extraction efficiencies were 40%, 31%, and 4%, respectively, under conditions of pH = 7 and a ratio of PESA to total heavy metals of 10:1. Compared with EDTA or EDDS, the new chelating agent, PESA, gave better performance in extracting Cr, in addition to the other advantages, such as ready biodegradability and the lack of nitrogen and phosphorus components. Thus, the potential application of PESA as a novel chelating agent for the extraction of Cr from sewage sludge was very promising. CONCLUSIONS Polyepoxysuccinic acid, an environmentally benign biodegradable chelant, was able to remove Cr from the sludge effectively. The extraction efficiency of Cr with PESA reached 58% under the given experimental conditions of pH = 4 and a ratio of PESA to total heavy metals of 10:1, and this extracted Cr came mainly from the reducible and oxidizable fractions. The pH and the concentration of PESA in the system were the two major factors that affected the efficiency of Cr extraction. The Cr extraction efficiency decreased gradually with pH increasing; however, the pH dependence decreased as the PESA concentration increased. PESA had a better Cr extraction performance than EDTA or EDDS under similar conditions. Considering the slow biodegradation of EDTA and the cost of nitrogencontaining EDDS, the potential application of PESA as a novel chelating agent for the extraction of Cr from sewage sludge was very promising. REFERENCES Banks, M. K., Schwab, A. P. and Henderson, C. 2006. Leaching and reduction of chromium in soil as affected by soil organic content and plants. Chemosphere. 62: 255–264.
Fig. 5 Comparison of Cr extractions with polyepoxysuccinic acid (PESA), ethylenediaminetetraacetic acid (EDTA), and S,S-ethylenediaminedisuccinic (EDDS) at pH 1–10 and a molar ratio of chelating agent to total heavy metals of 1:1 (a) and at pH 1–10 and a molar ratio of chelating agent to total heavy metals of 10:1 (b).
Barona, A., Aranguiz, I. and El´ıas, A. 200l. Metal associations in soils before and after EDTA extractive decontamination: implications for the effectiveness of further clean-up procedures. Environ. Pollut. 113: 79–85. Beceiro-Gonzalez, E., Barciela-Garcia, J., Bermejo-Barrera, P. and Bermejo-Barrera, A. 1992. Separation of Cr(III) and Cr(VI) using complexation of Cr(III) with
136 8-hydroxyquinoline and determination of both species in waters by ETA-AAS. Fresen. J. Anal. Chem. 344: 301–305. Boyd, R. S. 2010. Heavy metal pollutants and chemical ecology: exploring new frontiers. J. Chem. Ecol. 36: 46–58. Carter, Charles, G. F. and Lai-Dulen. 1994. Method of inhibiting corrosion of metals using polytartaric acid. European Patent No. 0609590. European Patent Office, Munich. Chen, Y. X., He, Z. Y. and Wu, J. P. 1994. Speciation and transformation of chromium in soil. Environ. Sci. (in Chinese). 15(3): 53–56. Davidson, C. M., Duncan, A. L., Littlejohn, D., Ure, A. M. and Garden, L. M. 1998. A critical evaluation of the three-stage BCR sequential extraction procedure to assess the potential mobility and toxicity of heavy metals in industrially contaminated land. Anal. Chim. Acta. 363: 45–55. Fuknmoto, Y. and Tangiuchi, T. 1991. Water Treating Agents for Prevention of Metal Corrosion and Scale Generation. Japan Patent No. 04166298. Japan Patent Office, Tokyo. Jones, P. W. and Willams, D. R. 2001. Chemical speciation used to assess [S, S]-ethylenediaminedisuccinic acid (EDDS) as a readily-biodegradable replacement for EDTA in radiochemical decontamination formulations. Appl. Radiat. Isotopes. 54: 587–593. Kocher, B., Wessolek, G. and Stoffregen, H. 2005. Water and heavy metal transport in roadside soils. Pedosphere. 15: 746–753. Lei, M., Liao, B. H., Zeng, Q. R., Zhou, X. H. and Liu, L. 2005. EDTA extraction and changes of Pb, Cd and Zn fractions in two contaminated soils. J. Agro-Environ. Sci. (in Chinese). 24: 1233–1237. Nogueira, T. A. R., Melo, W. J., Fonseca, I. M., Marcussi, S. A., Melo, G. M. P. and Marques, M. O. 2010. Fractionation of Zn, Cd and Pb in a tropical soil after nine-year sewage sludge applications. Pedosphere. 20: 545–556. Pueyo, M., L´ opez-Sanchez, J. F. and Ruret, G. 2004. Assessment of CaCl2 , NaNO3 and NH4 NO3 extraction procedures for the study of Cd, Cu, Pb and Zn extractability in contaminated soils. Anal. Chim. Acta. 504: 217–226. Qian, M., Shen, Z. G. and Wei, L. 2006. Comparison of EDDS- and EDTA-assisted uptake of heavy metals by Elsholtzia haichowensis. J. Agro-Environ. Sci. (in Chinese). 25: 113–118. Quevauviller, P., Rauret, G., L´ opez-S´ anchez, J. F., Rubio, R., Ure, A. and Muntau, H. 1997. Certification of trace metal extractable contents in a sediment reference material (CRM601) following a three-step sequential extraction procedure. Sci. Total Environ. 205: 223–234. Rodney, D. B. and Stephen, W. H. 1987. Ether Hydroxylpolycarboxylate Detergency Builders. US Patent Publication No. 4654159. USPTO, Alexandria.
L. H. ZHANG AND Z. L. ZHU Sun, B., Zhao, F. J., Lombi, E. and McGrath, S. P. 2001. Leaching of heavy metals from contaminated soils using EDTA. Environ. Pollut. 113: 111–120. Tandy, S., Ammann, A., Schulin, R. and Nowack, B. 2006. Biodegradation and speciation of residual SSethylenediaminedisuccinic acid (EDDS) in soil solution left after soil washing. Environ. Pollut. 142: 191–199. Tandy, S., Bossart, K., Mueller, R., Ritschel, J., R., Hauser, L., Schulin, R. and Nowack, B. 2004. Extraction of heavy metals from soils using biodegradable chelating agents. Environ. Sci. Technol. 38: 937–944. Tessier, A., Campbell, P. G. C. and Bisson, M. 1979. Sequential extraction procedure for the speciation of particulate trace metals. Anal. Chem. 51: 884–851. Van Benschoten, J. E., Matsumoto, M. R. and Young, W. H. 1997. Evaluation and analysis of soil washing for seven lead-contaminated soils. J. Environ. Eng. l23: 217–224. Vandeviere, P., Hammes, F., Verstraete, W., Feijtel, T. and Schowanek, D. 2001. Metal decontamination of soil, sediment, and sewage sludge by means of transition metal chelant [S,S]-EDDS. J. Environ. Eng. 127: 802–811. Veeken, A. H. M. and Hamelers, H. V. M. 1999. Removal of heavy metals from sewage sludge by extraction with organic acid. Water Sci. Technol. 40: 129–136. Wang, X. H., Liu, Y. G., Zeng, G. M., Zhou, C. H., Li, X., Fan, T. and Zuo, M. 2006. Extraction of heavy metals from contaminated soils with EDTA and their redistribution of fractions. Environ. Sci. (in Chinese). 27: 1008–1012. Wei, G., Xu, Y. N. and Xiong, R. C. 2001. Study on biodegradability of inhibitor. J. Beijing Technol. (in Chinese). 28: 59–62. Xiong, R. C., Wei, G., Zhou, D., Zhao, X., Yang, J., Xu, B. and Dong, X. L. 1999. Synthesis of green scale inhibition polyepoxysuccinic acid. Ind. Water Treat. (in Chinese). 19(3): 11–13. Yoshizaki, S. and Tomida, T. 2000. Principle and process of heavy metal removal from sewage sludge. Environ. Sci. Technol. 34: 1572–1575. Zeng, M., Liao, B. H., Zeng, Q. R., Zhang, Y. and Ou, Y. B. 2006. Effects of three extractants on removal and availabilities of heavy metals in the contaminated soil. J. Agro-Environ. Sci. (in Chinese). 25: 979–982. Zhang, B. R., Li, H., Li, F. T. and Zheng, J. W. 2006. Study of synergistic effect of polyepoxysuccinic acid on corrosion inhibition of carbon steel. Ind. Water Treat. (in Chinese). 26(2): 53–56. Zhang, L. H., Zhu, Z. L., Zhang, R. H., Zheng, C. S., Zhang, H., Qiu, Y. L. and Zhao, J. F. 2008. Extraction of copper from sewage sludge using biodegradable chelant EDDS. J. Environ. Sci. 20: 970–974. Zhu, Z. L., Zhang, L. H., Zhang, H., Qiu, Y. L., Zhang, R. H. and Zhao, J. F. 2009. Extraction of cadmium from sewage sludge using polyepoxysuccinic acid. Pedosphere. 19: 137–142.
Chromium Extraction from Sewage Sludge Using Polyepoxysuccinic Acid∗1 ZHANG Li-Hua1,∗2 and ZHU Zhi-Liang2 1 2
Department of Chemistry and Biology Engineering, Sanming University, Sanming 365004 (China) State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai 200092 (China)
(Received April 27, 2011; revised November 15, 2011)
ABSTRACT An environmentally benign biodegradable chelant, polyepoxysuccinic acid (PESA), was used to separate heavy metals from sewage sludge from the Shanghai Taopu Wastewater Treatment Plant, China, based on chemical extraction technology. The extraction of chromium (Cr) from sewage sludge with an aqueous solution of PESA was studied under various conditions. It was found that the extraction of Cr using PESA was more efficient than that using ethylenediaminetetraacetic acid (EDTA) and S,S-ethylenediaminedisuccinic acid (EDDS) under similar conditions. PESA was capable of extracting Cr from the sewage sludge, and the extraction efficiency was obviously dependent on both the pH and the concentration of the chelating reagent. The extraction efficiency decreased gradually with increasing pH, and the dependence on pH decreased as the concentration of PESA increased. The extraction efficiency reached 58% under conditions of pH = 4 and a ratio of PESA to total heavy metals of 10:1. The extraction efficiency was maintained above 40% within the pH range from 1 to 7 at the high ratio of PESA to total heavy metals of 10:1. Comparing the contents of heavy metals in the sewage sludge before and after the extraction, it was found that the extracted Cr came mainly from the reducible and oxidizable fractions. Key Words:
ethylenediaminetetraacetic acid, extraction efficiency, heavy metals, pH, S,S-ethylenediaminedisuccinic acid
Citation: Zhang, L. H. and Zhu, Z. L. 2012. Chromium extraction from sewage sludge using polyepoxysuccinic acid. Pedosphere. 22(1): 131–136.
Heavy metal pollution of soil, sediment, and sewage sludge is widespread across the globe, and the decontamination of heavy metals remains a difficult task (Sun et al., 2001; Vandeviere et al., 2001; Tandy et al., 2004). One of the most attractive potential sludge disposal methods is land application. However, sewage sludge contains not only valuable components such as nutrients but also heavy metals, which are nonbiodegradable and accumulate in the environment. Land application of sewage sludge containing harmful metals may adversely affect human health and cause phytotoxicity (Veeken and Hamelers, 1999; Yoshizaki and Tomida, 2000; Nogueira et al., 2010). Recently, research has been directed to find an efficient method to remove heavy metals from sewage sludge (Veeken and ∗1
Hamelers, 1999; Vandeviere et al., 2001; Tandy et al., 2004; Zhang et al., 2008; Zhu et al., 2009). Chromium (Cr) is a toxic element that occurs in highly variable oxidation states. Cr(III) and Cr(VI) have completely different behaviors, and their toxicities can differ by an order of magnitude. Cr is one of the most “difficult” soil elements. Environmental Cr pollution, especially soil contamination with Cr, has become one of the focuses of the environmental science (Pueyo et al., 2004; Kocher et al., 2005; Banks et al., 2006; Boyd, 2010). However, there has been little significant progress made even though various methods to remove heavy metals from sewage sludge have been investigated (Yoshizaki and Tomida, 2000). One possible remediation technique is ex-situ was-
Supported by the State Key Laboratory of Pollution Control and Resource Reuse Foundation, China (No. PCRRF09004) and the Natural Science Foundation of the Education Department of Fujian Province of China (Nos. JK2011055 and HX200805). ∗2 Corresponding author. E-mail: [email protected].
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hing using chelating agents that offer high efficiency and specificity for the extraction of heavy metals (Van Benschoten et al., 1997; Barona et al., 2001; Sun et al., 2001; Tandy et al., 2004; Lei et al., 2005; Wang et al., 2006). Because some chelating agents would remain in the sewage sludge after the extraction, they may cause secondary pollution. Therefore, degradable chelants are highly favored (Jones and Willams, 2001; Vandeviere et al., 2001; Tandy et al., 2004; Qian et al., 2006; Tandy et al., 2006; Zhu et al., 2009). Polyepoxysuccinic acid (PESA), which is an environmentally benign water-soluble polymer containing no nitrogen or phosphorus, shows good chelating ability for a variety of metal ions (Fuknmoto and Tangiuchi, 1991; Carter et al., 1994) and excellent biodegradability (Wei et al., 2001). The study of PESA as a green chemical product has received great attention in recent years (Rodney and Stephen, 1987; Xiong et al., 1999). Previous work on PESA has concentrated on its synthesis, properties, and application to inhibit corrosion and scale formation in the water recycling field (Fuknmoto and Tangiuchi, 1991; Carter et al., 1994; Zhang et al., 2006). There has been no report on the use of PESA as a chelating reagent to remove Cr from sewage sludge or soils, and thus, its performance with respect to Cr extraction is unknown. Here, we report the investigation of the decontamination of Cr-containing sewage sludge using PESA as an environmentally benign extractant. Hopefully, the results of this study will be helpful in the development of green remediation technology for soils and sewage sludge contaminated with heavy metals. MATERIALS AND METHODS Sewage sludge Solid sewage sludge taken from the Shanghai Taopu Municipal Wastewater Plant in China was used in this study. The sewage sludge samples were air dried naturally, ground, and sieved to a size less than 2 mm. The tested sludge contained 279.7 g kg−1 total organic carbon (TOC), 82.3 g kg−1 water, and heavy metals including Zn, Cu, Cd, Ni, Pb, and Cr. The molar sum of the heavy metals in the sewage sludge was 255.4 mmol kg−1 . The content of Cr was 2 376 mg kg−1 , which exceeded the third-level limit value set by the Environmental Quality Standard for Soils in China (GB 15618-1995) (300–400 mg kg−1 ) and the limit level of the Sludge Agricultural Utilization Standard for soil (GB 18918-2002) (1 000 mg kg−1 for alkali soils of pH
L. H. ZHANG AND Z. L. ZHU
≥ 6.5 and 600 mg kg−1 for acid soils of pH < 6.5). Chelating agents Polyepoxysuccinic acid (PESA) was obtained from the Meijing Environmental Protection Material Company Limited in Shanghai, China. This material, whose structure is shown in Fig. 1, is highly biodegradable and has an average molecular weight of 1 500, a solid content of ≥ 380 g kg−1 , and a density of ≥ 1.2 g cm−3 . The other reagents used were all of analytical grade and were purchased from the National Chemical Reagent Company Limited in Shanghai, China.
Fig. 1
Structure of polyepoxysuccinic acid.
Extraction experiments The extraction was performed in batch experiments using aqueous suspensions in centrifuge tubes. Different concentrations of the chelating agents were applied, and the molar ratio of the chelant to the sum of the total heavy metals in the sewage sludge was kept in the range from 1:1 to 10:1. The influence of the solution pH on the extraction efficiency of Cr from the sewage sludge was examined at pH from 1 to 10. For each batch, 0.5 g of solid sewage sludge was suspended in 25 mL of the solution (1:50), and the reaction time was fixed at 24 h. The suspension system pH was adjusted with HNO3 or NaOH two days prior to the addition of the chelating agent. The system pH was monitored and readjusted if necessary. The suspensions were shaken at 200 r min−1 for 24 h at room temperature. All suspensions were centrifuged at 4 000 r min−1 for 20 min and passed through 0.45-μm paper filter before analysis of the metal content. After digesting the sludge sample by HF-HClHNO3 in a microwave digestion system, the total content of heavy metals was measured. To determine the species distribution of Cr in the sewage sludge, the samples were analyzed before and after the extraction using the modified BCR (Community Bureau of Reference) sequential extraction method (Quevauviller et al., 1997). Analytical methods The TOC of the sewage sludge was determined
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with a total organic carbon analyzer (SSM-5000A, SHIMADZU, Japan). The total heavy metal content was determined after digestion in a microwave extraction/digestion system (ETHOS E, Mile Stone s.r.l., Italy). The concentrations of dissolved heavy metals in the digestion solutions and extraction solutions were measured with an inductively coupled plasma optical emission spectrometer (Optima 2100 DV, PerkinElmer, USA). The pH measurements for the extraction solutions were conducted with a pH Meter (PP-15 Professional, Sartorius, Germany). Triplicate extractions and analyses were performed for all samples to evaluate the reproducibility of the results, and then the data from the three parallel experiments were averaged. The standard deviation was within 5% of the mean for all the tested heavy metals.
The extraction of Cr with PESA showed a stronger dependence on pH at higher concentrations of the chelant. For example, when the molar ratio of PESA to total heavy metals was 10:1, the extraction efficiency was 35% at pH 10. When pH increased to 4, at the same ratio of 10:1, the extraction efficiency increased to 58%. With the ratio of 10:1, the extraction efficiency remained above 40% in the pH range of 1–7. Influence of PESA concentration on Cr extraction
The system pH is a crucial factor affecting the extraction efficiency. The influence of solution pH on Cr extraction from sludge was examined at pH 1–10. As shown in Fig. 2, the pH of the extraction solution had a significant influence on the extraction efficiency of Cr. In the case where PESA was absent, the extraction efficiency decreased rapidly as the pH increased. The extracted amount of Cr was less than 1% at pH > 3.0. The addition of PESA significantly improved the extraction efficiency in a wide pH range of 3–10 although increasing the pH, in general, reduced the extraction efficiency. The Cr extraction was maximized under weak acidic conditions (pH = 4.0) when the molar ratio of PESA to total heavy metals was greater than 1:1.
When washing soil with the chelant solution, a ratio >1 of the chelant to toxic metals is required for good toxic metal extraction. An excess of chelating agent is needed because major cations in the soil, such as Mn, Mg, Fe, and Ca, along with toxic metals present in smaller amounts compete with the metals being studied for the chelating agent and are also extracted (Tandy et al., 2004). The influence of the PESA concentration on the Cr extraction efficiency was investigated at pH 4 and 7. The molar ratios of PESA to total heavy metals ranged from 1:1 to 10:1. The results, shown in Fig. 3, indicated that the influence of the PESA concentration on the extraction efficiency of Cr was similar at pH 4 and 7. Under both pH conditions, increasing the concentration of PESA improved the Cr extraction at the ratio of PESA to total heavy metals from 1:1 to 10:1. At the ratio of PESA to total heavy metals of > 10:1, the increase in the extracted fraction of Cr was very small. Within the range of the ratios of PESA to total heavy metals from 5 to 10 and at pH 4, approximately 43%–58% of the Cr in the sample was extracted, whereas in the same ratio range at pH 7, the percentage of extracted Cr was only 23%– 35%.
Fig. 2 Influence of pH on Cr extraction from sludge with polyepoxysuccinic acid (PESA) at pH 1–10 and molar ratios of PESA to total heavy metals of 0:1, 1:1, 5:1 and 10:1.
Fig. 3 Influence of polyepoxysuccinic acid (PESA) concentration on Cr extraction at pH 4 or 7 and molar ratios of PESA to total heavy metals from 1:1 to 10:1.
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Relationship between extraction efficiency and species distribution of Cr in sewage sludge The content and species distribution of the heavy metals in sewage sludge also affect the extraction efficiency of Cr. We are unable to evaluate the environmental impact using the total concentrations of heavy metals because information on potential environmental mobility or bioavailability is not available (Tandy et al., 2004). Cr can occur in soil samples as Cr(III) and Cr(VI). Cr(III) and Cr(VI) have completely different behaviors, and the toxicity of the two species can differ by an order of magnitude (Chen et al., 1994). Cr(III) is considered to be essential for the maintenance of glucose, lipid, and protein metabolism, but Cr(VI) is reported to be toxic due to its oxidizing potential and easy permeation of biological membranes (BeceiroGonzalez et al., 1992). However, the difference in the Cr valence can not reflect the real existence fractions because soil is a complicated system. Various bound species of Cr exist in soil with different toxicity and bioavailability (Chen et al., 1994). Sequential extractions could provide the information needed to explain the different extraction efficiencies of different heavy metals (Tandy et al., 2004). Among various sequential extraction techniques, Tessier’s sequential extraction (Tessier et al., 1979) and the BCR sequential extraction (Quevauviller et al., 1997; Davidson et al., 1998) are the two commonly used techniques to determine the species distribution of heavy metals in soils, atmospheric deposits, and sewage sludge. Compared with other methods, the BCR method has better reproducibility and precision and comparable performance (Quevauviller et al., 1997). In the BCR sequential extraction method, the metal elements are divided into acid-soluble, reducible, and oxidizable fractions. In the
L. H. ZHANG AND Z. L. ZHU
modified BCR sequential extraction method, two additional fractions, the water-soluble and residual fractions, are considered. The distribution of Cr fractions in the sewage sludge samples before and after the extraction with PESA was examined using the modified BCR sequential extraction method. The experiments were performed at pH 4 using a molar ratio of PESA to total heavy metals of 4:1. The results are represented in Fig. 4. The results showed that before extraction, the Cr in the sludge was mainly found in the oxidizable and residual fractions. These two fractions were 49% and 39%, respectively. A lower amount of Cr, approximately 11%, was found in the reducible fraction. After the extraction with PESA, the species distribution of Cr changed, and the majority was found in the residual fraction, whereas the contents of the other fractions were markedly reduced. This result indicated that the extracted Cr from the tested sewage sludge with PESA came mainly from the reducible and oxidizable fractions. Under the given experimental conditions, the extraction efficiency of Cr reached 35%. Comparison of PESA with EDTA and EDDS According to previously reported studies, some chelating agents, such as ethylenediaminetetraacetic acid (EDTA) and S,S-ethylenediaminedisuccinic acid (EDDS), offer good extraction efficiency of heavy metals from soils (Vandeviere et al., 2001; Tandy et al., 2004). EDTA is a very effective chelating agent that is used widely for heavy metal decontamination (Barona et al., 2001; Lei et al., 2005; Zeng et al., 2006). Unfortunately, it has the disadvantage of being persistent in the environment due to its low biodegradability. Recently, the easily biodegradable chelating agent EDDS
Fig. 4 Species distribution of Cr in sewage sludge before and after extraction with polyepoxysuccinic acid (PESA) at pH = 4 and a molar ratio of PESA to total heavy metals of 4:1.
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has been proposed as a safe and environmentally benign replacement for EDTA in soil washing and for chelant-enhanced phytoremediation (Jones and Willams, 2001; Vandeviere et al., 2001; Tandy et al., 2004; Qian et al., 2006; Tandy et al., 2006; Zhang et al., 2008). However, EDDS may have a potential risk of secondary water pollution as the result of eutrophication because this compound contains nitrogen. Obviously, this risk of eutrophication limits the ability of EDDS to meet the increasingly stringent environmental policies and laws. To evaluate the extractability of Cr with PESA, the extraction of Cr from the sewage sludge with EDDS and EDTA was conducted simultaneously under the same conditions. The extraction conditions and the extraction efficiencies are shown in Fig. 5. It was found that PESA gave better Cr extraction performance than EDTA or EDDS under similar conditions. The efficiency of Cr extraction with PESA was nearly constant at approximately 8% over the pH range from 3 to 10 at the ratio of PESA to total heavy metals of 1:1, whereas the efficiency of Cr extraction with EDTA or EDDS
was lower. Furthermore, the efficiency of Cr extraction with PESA was obviously higher than that with EDTA or EDDS at a ratio of PESA to total heavy metals of 10:1 over the pH range of 1–10. The efficiencies of Cr extraction with PESA, EDTA, and EDDS reached 58%, 45%, and 28%, respectively, under conditions of pH = 4 and a ratio of PESA to total heavy metals of 10:1, whereas the extraction efficiencies were 40%, 31%, and 4%, respectively, under conditions of pH = 7 and a ratio of PESA to total heavy metals of 10:1. Compared with EDTA or EDDS, the new chelating agent, PESA, gave better performance in extracting Cr, in addition to the other advantages, such as ready biodegradability and the lack of nitrogen and phosphorus components. Thus, the potential application of PESA as a novel chelating agent for the extraction of Cr from sewage sludge was very promising. CONCLUSIONS Polyepoxysuccinic acid, an environmentally benign biodegradable chelant, was able to remove Cr from the sludge effectively. The extraction efficiency of Cr with PESA reached 58% under the given experimental conditions of pH = 4 and a ratio of PESA to total heavy metals of 10:1, and this extracted Cr came mainly from the reducible and oxidizable fractions. The pH and the concentration of PESA in the system were the two major factors that affected the efficiency of Cr extraction. The Cr extraction efficiency decreased gradually with pH increasing; however, the pH dependence decreased as the PESA concentration increased. PESA had a better Cr extraction performance than EDTA or EDDS under similar conditions. Considering the slow biodegradation of EDTA and the cost of nitrogencontaining EDDS, the potential application of PESA as a novel chelating agent for the extraction of Cr from sewage sludge was very promising. REFERENCES Banks, M. K., Schwab, A. P. and Henderson, C. 2006. Leaching and reduction of chromium in soil as affected by soil organic content and plants. Chemosphere. 62: 255–264.
Fig. 5 Comparison of Cr extractions with polyepoxysuccinic acid (PESA), ethylenediaminetetraacetic acid (EDTA), and S,S-ethylenediaminedisuccinic (EDDS) at pH 1–10 and a molar ratio of chelating agent to total heavy metals of 1:1 (a) and at pH 1–10 and a molar ratio of chelating agent to total heavy metals of 10:1 (b).
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