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Extractable Fractions of Metals in Sewage Sludges from Five Typical Urban Wastewater Treatment Plants of China1
Pedosphere 16(6): 756-761,2006 ISSN 10020160/CN 32-1315/P 0 2006 Soil Science Society of China Published by Elsevier Limited and Science Press
PEDOSPHERE www.elsevier.com/locate/pedosphere
Extractable Fractions of Metals in Sewage Sludges from Five Typical Urban Wastewater Treatment Plants of China*l WANG Chao1t2,LI Xiao-Chen2, WANG Pei-Fang2, ZOU Li-Min2 and MA Hai-Tm2 'Key Labomtory of Integmted Regulation and Resource Department on Shallow Lakes, Ministry of Education, Nanjing 210098 (China). E-mail: [email protected] 'School of Environmental Science and Engineering, Hohai University, Nanjing 21 0098 (China)
(Received April 26, 2006; revised September 12, 2006)
ABSTRACT Metal content and bioavailability are often the limiting factors for application of sewage sludge in agricultural fields. Sewage sludge samples were collected from five typical urban Wastewater treatment plants in China to investigate their contents and distribution of various chemical fractions of Cu, Zn, Ni, Cr, P b and Mo by using the BCR (Community Bureau of Reference) sequential extraction procedure. The sludges contained considerable amounts of organic matter (31.8%48.0%), total N (16.3-26.4 g kg-') and total P (15.1-23.9 g kg-'), indicating high potential agricultural benefits of their practical applications. However, total Zn and Ni contents in the sludge exceeded the values permitted in China's control standards for pollutants in sludges from agricultural use (GB 4284-1984). The residual fraction was the predominant fraction for Mo, Ni and Cr, the oxidiable fraction was the primary fraction for Cu and Pb, and the exchangeable and reducible fractions were principal for Zn. The distribution of different chemical fractions among the sludge samples reflected differences in their physicochemical properties, especially pH. The sludge pH was negatively correlated with the percentages of reducible fraction of Cu and exchangeable fraction of Zn. The sludges from these plants might not be suitable for agricultural applicatiok due to their high contents of Zn, Ni and Cr, as well as high potential of mobility and bioavailability of Zn.
Key Words:
fractions, heavy metals, sequential extraction, sewage sludge
INTRODUCTION As the most common urban wastewater treatment technology in China, activated sludge processes produce a large amount of sludge, which is equivalent to 1%of the wastewater treated (Lester e t al., 1983). Because of a large number of new wastewater treatment plants constructed in China in recent years, it is expected that about four million tons of dry matter will be produced in coming years (Lee et al., 2002). Therefore, the disposal of sludge generated from the wastewater treatment plants is becoming an important environmental issue (Solis et al., 2002; Fuentes et al., 2004). One of the most attractive potential methods for sludge disposal is agricultural application because it can recycle valuable components, such as organic matter, N, P and some other plant nutrients (Zdiaurre et al., 1998; Wong et al., 2001; Fuentes et al., 2004). However, the environmental impact of this practice needs to be investigated carefully. A major restrictive factor for agronomic application of sewage sludge is heavy metals concentrated in sludge during wastewater bio-treatment process (Amir et al., 2005). These heavy metals are often toxic at a certain concentration, and they tend to accumulate along the food chain, where human being is the last in the link (Dudka and .Miller, 1999; Zhang and Ke, 2004). Consequently, the investigation of heavy metals in sewage sludge received increasing attention in recent decades (Logan et al., 1997; Fang and Wong, 1999). At present, it is widely accepted that heavy metal contents in sewage sludge cannot provide sufficient information of its potential hazardous effect on environment, because the mobility and eco-toxicity of heavy metals depend strongly on their specific *'Project supported by the National Key Basic Reaearch Support Foundation of China (No.2002CB412303) and the Natural Science Foundation of Jiangsu Province (No. KB2004115).
METALS IN SEWAGE SLUDGES
757
chemical forms or binding patterns (Zufiaurre et al., 1998;SEanEar et al., 2000; Zhou et al., 2002; Su and Wong, 2003; Fuentes et al., 2004). Both single and sequential extraction techniques had been developed to evaluate the metal mobility or bioavailability in soils, sediments and sewage sludges during recent decades (Tessier et al., 1979; Forstner and Salomons, 1980)and were thoroughly reviewed by Rauret (1998)and Kot and Namiesriik (2000). Some of the techniques are largely identical but with minor differences in chemical extractants or operating conditions, making the results seldom comparable. For this reason, the Community Bureau of Reference (BCR) developed a sequential extraction program in 1987 to harmonize the methodology for determining metals in soils and sediments (Ure et al., 1993). This procedure has also been successfully applied to determination of metals in sewage sludges (SEanEar et al., 2000; Wong et al., 2001; Soliset al., 2002; Klose and Makeschin, 2005). In the present work, the BCR sequential extraction procedure was employed to extract fractions of Cu, Zn, Ni, Cr, Pb and Mo in sewage sludge samples collected from five typical wastewater treatment plants in China. The aims were to investigate the total contents of heavy metals in the sludge samples, to characterize the chemical fractions of the metals, and to assess their potential mobility or bioavailability. MATERIALS AND METHODS Dewatered sewage sludge samples were collected from five wastewater treatment plants, including sewage sludge sample 1 (Sl)from Beijing Gaobeidian Wastewater Treatment Plant , sample 2 (S2) from Jinan Second Wastewater Treatment Plant, sample 3 (S3)from Nanjing Jiangxinzhou Wastewater Treatment Plant, sample 4 (S4) from Tai'an Wastewater Treatment Plant, and sample 5 (55) from Xiamen Wastewater Treatment Plant. The domestic and industrial wastewater was the sources of all five wastewater treatment plants. Samples of sewage sludge were air-dried at room temperature, ground and homogenized in an agate mortar, then passed through a 1-mmnylon sieve, and kept in polyethylene bags for measurement. The pH of sludge samples was measured with the extract at a sludge/deionized water ratio of 1:5 (w/v) using a digital pH meter. The organic matter content was determined by the procedure of loss on ignition (LOI) at 450 "C for 3 hours. Total nitrogen (N) in the sludge sample was measured using the Kjeldhal method, and NH4-N by the indophenol-blue method. Total phosphorus (P) and potassium (K) were determined by the inductively-coupled plasma atomic emission spectrometry (ICP-AES). To determine the metal contents of the sludge, 0.1 g of powdered sample was wet-digested with 15 mL of concentrated HN03-HC104 (3:l)mixture in a 25 mL Teflon PFA (Perfluoroalkoxy) vial. Three drops of HF acid was added to the vial, and the mixtures were then heated to a clear solution, and continued until near dryness. The cooled residue was dissolved in 5 mL 10% HN03 and the solution volume was added to 25 mL with deionized water. The sequential extraction was carried out by using the BCR procedure described by Ure et al. (1993),with 4 steps as follows: Step 1 (extraction of exchangeable fraction). Twenty mL acetic acid (0.11 mol L-') was added in a 50-mL polypropylene centrifuge tube containing 0.5000 f 0.0001 g sludge sample. The tubes were shaken on an end-over-end shaker at 40 r min-' for 16 h at ambient temperature ( 2 0 f 1 "C). The extract was separated from the solid residue by centrifugation (4000 r min-'), decanted into a polyethylene container, and stored at -4 "C until analysis. Step 2 (extraction of reducible-fraction). The residue after the step 1 was shaken with 20-mL 0.1 mol L-' hydroxylamine hydrochloride after acidification to pH 2 with nitric acid. The extraction process was the same as in the step 1. The residue after step 2 was digested with 20-mL 30% Step 3 (extmction of Oxidizable fraction). hydrogen peroxide after acidification to pH 2 with nitric acid. The extraction was continued with 25-mL 1 mol L-' ammonium acetate after acidification to pH 2 with nitric acid. The extraction process was the same as in the step 1.
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Step 4 (extraction of residual fraction). The residue after step 3 was digested using the same method as the digestion process for metal contents mentioned above. All glassware and plastic containers were previously soaked overnight in super pure nitric acid (Merck) and then rinsed with deionized water thoroughly. The contents of different fractions of Cd, Cr, Pb, Cu, Ni and Zn were determined by using ICP-AES. RESULTS AND DISCUSSION
The physicochemical properties of sewage sludges The physicochemical properties of sewage sludge samples are shown in Table I. Metal mobility and bio-availability increase with decreasing pH below pH 6.5 and decrease with increasing pH above pH 6.5 (Kazi et al., 2005). The pH values of sewage sludge samples were found between 6.5 and 7.4. Organic matter is important because it can either form soluble complexes with the toxic heavy metals or retain metal ions on the surfaces. The organic matter contents in these sludge samples ranged from 31.8% to 48.0%. Total N, total K and total P were in the range of 16.3-26.4 g kg-’, 5.8-12.2 g kg-’ and 15.1-23.9 g kg-’, respectively. On average, soils in China contain 1.0%-4.0% organic matter, 1.0-2.0 g kg-’ total N, 0.44-0.85 g kg-’ total P and about 16 g kg-’ total K (Bao et al., 2000). The relatively high contents of total N, total P, and organic matter in the sludge suggested their high potential agricultural benefits of their practical applications. TABLE I Some physicochemical properties of sludge samples from five typical wastewater treatment plants ~
Sludge sample”)
PH
Organic matter
s1 s2 s3 s4 s5
7.4 7.3 6.6 6.5 6.5 6.8 f0.4
37.2 31.8 33.9 48.0 32.8 36.7 f 6.6
~
~~~~~~~
Total P
26.2 19.1 20.1 26.4 16.3 21.6 f 4.5
15.1 17.9 20.0 23.9 16.9 18.7 f 3.3
%
MeanfSDb)
~
Total N
Total K
g kg-’ 5.8 12.2 7.5 8.2 6.1 7.9 f 2.5
a)Sl, S2, S3, S4 and S5 are sludge samples collected from five wastewater treatment plants located in Beijing, Jinan, Nanjing, Tai’an and Xiamen, respectively. b)SD = standard deviation.
Total heavy metal contents Total contents of Cu, Zn, Ni, Cr, Pb and Mo in the sewage sludge samples are presented in Table 11. The total metal contents ranged from 47.5 to 751.6 mg kg-’ for Cu, 581.7 to 3982.3 mg kg-’ for Zn, 64.2 to 2 176.7mg kg-l for Ni, 81.0 to 1414.8 mg kg-’ for Cr, 42.9 to 122.2 mg kg-’ for Pb, and 33.4 to 56.7 mg kg-’ for Mo. It was evident from the data of Table I1 that the total heavy metal contents varied greatly with the sewage sludges from different wastewater treatment plants, which might be due to the Werent characteristics of wastewater or the different ratios of domestic wastewater/industrial wastewater these treatment plants received. The maximum contents of heavy metals in sewage sludges permitted by China’s “control standards for pollutants in sludge for agricultural use (GB 4284-1984)” are listed in Table 11. The Pb contents in the sludge samples were lower than the values permitted by the Chinese standards (GB 4284-1984), and for most samples, the Cu and Cr contents were also below the permitted values, except for S4. The contents of Ni exceeded the permitted values for S3,S4 and S5. Zn contents in all samples exceeded the permitted values, with especially high contents (1 189.8 and 3 982.3 g kg-’) in S4 and S5. The highest contents of Cu, Ni and Cr were found in S4,and those of Zn and Pb in S5. From Table 11, it was evident that these sludges, especially S4 and S5,were not suitable for agricultural use, particularly due to their
METALS IN SEWAGE SLUDGES
759
high Zn and Ni contents. TABLE I1 Total contents of heavy metals in the sewage sludges and the China’s permitted values (PV) for agricultural use cu
Sludge sample’)
s1 s2 53 s4 s5 Meanf SDb, PV for soil pH < 6.5‘) PV for soil pH 2 6.5‘)
58.9 47.5 67.0 751.6 187.1 222.4f 301.1 250 500
Zn
Ni
Cr
Pb
745.4 581.7 646.5 1189.8 3982.3 1429.1 f 1446.9 500 1000
mg kg-’ dry 64.2 85.5 192.1 2176.7 119.2 527.6f 923.2 100 200
weight 81.0 177.6 272.8 1414.8 195.9 428.4 f 555.6 600 1000
42.9 33.4 50.8 54.6 63.9 56.7 63.5 46.6 122.2 50.9 68.6f31.2 48.4 f 9.2 300 1000 -
Mo
’)Sl, S2, S3, S4 and S5 are sludge samples collected from five wastewater treatment plants located in Beijing, Jinan, Nanjing, Tai’an and Xiamen, respectively. b)SD = standard deviation. ‘)F’rom China’s “control standards for pollutants in sludges from agricultural use (GB 42841984)”.
Fraction distribution of heavy metals in sewage sludge
The sequential chemical extraction technique can provide information on the origin, the mode of occurrence, the biological and physicochemical availability, the mobilizatiofi and the transport of metals in sewage sludge (Kazi et al., 2005). The distribution of various fractions of heavy metals in the sludge samples obtained using the BCR sequential extraction scheme, as given in Fig. 1, showed that for Pb in all the sludge samples, 48%62% was in the oxidizable fraction, 26%-38% in the residual fraction, and no more than 20% in the exchangeable and reducible fractions. Except for S4, Ni and Cr are mainly found in the residual fraction, accounting for 77%-84% and 70%-91%, respectively. Mo was mostly found in the residual fraction (38%64%), followed by the oxidizable fraction (20%-43%) and the exchangeable fraction (< 2%). For Cu, 39%-61%was in the oxidizable fraction. Metals were regarded to be less mobile and bioavailable in the 100 80
60 40
g
20
g
o
8 F4
Y
H F3
‘3100
El F2
0
80
NF l
60 40
20 0 -
s1
s2
s3
s4
s5
s1
s2
s3
s4
s5
s1
s2
s3
s4
s5
Sample Fig. 1 The distribution percentages of various fractions of heavy metala in five sludge samples (S1 to S5) collected from five wastewater treatment plants located in Beijing, Jinan, Naqjing, Tai’an and Xiamen, respectively. F1 = exchangeable fraction; F2 = reducible fraction; F3 = oxidizable fraction; F4 = residual fraction.
C. WANG et aL
760
oxidizable and residual fractions than in the other fractions, suggesting that Pb, Ni, Cr, Mo and Cu in these sludges were not easily mobilized. The predominant portion (61%-93%) of Zn was in the exchangeable and reducible fractions, indicating high potential mobility and bioavailability of Zn. These were in consistent with the conclusions of Alvarez et al. (2002). Considering the high Zn content and high potential mobility in environment, it could be concluded that these sludges should not be directly applied to agricultural fields prior to further treatment. As seen in Fig. 2, the percentage of exchangeable fraction plus reducible fraction increased significandy with decreasing sludge pH for both Cu and Zn. Sludge properties such as pH, electrical conductivity, potentially mineralizable N and ionic strength indirectly affect the availability of sludge-borne metals (Merrington et al., 2003). A significant correlation was found between the sludge pH values and the percentages of exchangeable fraction and reducible fraction of Cu and Zn. The sludge pH value was negatively correlated with the percentages of reducible fraction of Cu and exchangeable fraction of Zn, with correlation coefficients being -0.962 and -0.916, respectively. Therefore, higher percentages of exchangeable fraction and reducible fraction for most metals in S4 and S5 than in the other sludge samples could be attributed to the lower pH of S4 and S5. 7.6
100
2 2 .-c0. 0
!!
LL
80
7.2
0
60 6.8 40
Zn
A PH 6.4
20
0
5
cu
s1
s2
S3 Sample
s4
s5
6.0
Fig. 2 The percentages of exchangeable fraction plus reducible fraction for Cu and Zn in relation to the pH values of sludges.
CONCLUSIONS The sludge samples contained considerable amounts of organic matter (31.8%-48.0%),total N (16.326.4 g kg-') and total P (15.1-23.9 g kg-I), indicating high potential agricultural benefits of their practical applications. The contents of Cu and Pb were below the values permitted by the Chinese standards (GB 4284-1984), but Cr in S4, Ni in S3, S4, and S5, and Zn in all the sludges exceeded the values permitted by the standards. Most of Mo, Ni and Cr were in the residual fraction, Cu and Pb were mainly in the oxidizable fraction, and Zn was dominated by the exchangeable fraction and reducible fraction. Significant correlation between pH value of sludge and the percentage of the exchangeable fraction and reducible fraction in total Cu and Zn indicated that pH.-played an important role in the chemical fraction distribution of metals in the sewage sludges. The sewage sludges were not suitable for agricultural use without further treatments due to their high Zn, Ni and Cr contents as well as high Zn potential mobility and bioavailability. REFERENCES Alvarez, E. A., Mochbn, M. C., JimCnez Shchez, J. C. and Rodriguez, M. T. 2002. Heavy metal extractable forms in sludge from wastewater treatment plants. Chemosphere. 47: 765-75. Amir, S., Hafidi, M., Merlina, G., Revel, J. C. 2005. Sequential extraction of heavy metals during composting of sewage sludge. Chemosphere. 59: 801-810. Bao, S., Wang, R. and Yang, C. 2000. Soil and Agricultural Chemistry Analysis. Third ed., Chinese Agricultural Press, Beijing.
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Dudka, S. and Miller, W. P. 1999. Accumulation of potentially toxic elements in plants and their transfer to human food chain. 1. Environ. Sci. Health. B34: 681-708. Fang, M. and Wong, J. W. C. 1999. Effects of lime amendment on availability of heavy metals and maturation in sewage sludge composting. Environ. Pollut. 106: 83-89. Forstner, U. and Salomons, W. 1980. Tkace metal analysis on polluted sediments. Part 11. Evaluation of environmental impact. Environ. Technol. Lett. 1: 506-517. Fuentes, A., Llorhns, M., S k z , J., Soler, A., Aguilar, M. I., Ortuiio, J. F. and Meseguer, V. F. 2004. Simple and sequential extractions of heavy metals from different sewage sludges. Chemosphere. 64: 1039-1 047. Kazi, T. G., Jamali, M. K., Kazi, G. H., Arain, M. B., Afridi, H. I. and Siddiqui, A. 2005. Evaluating the mobility of toxic metals in untreated industrial wastewater sludge using a BCR sequential extraction procedure and a leaching test. Anal Bioanal Chem. 383: 297-304. Klose, S. and Makeschin, F. 2005. Soil properties in coniferous forest stands along a fly ash deposition gradient in eastern Germany. Pedosphere. 16(6): 681-694. Kot, A. and Namieshik, J. 2000. The role of speciation in analytical chemistry. %rids in Analytical Chemistry. 19(2-3): 69-79.
Lee, D. J., Spinosa, L. and Liu, J. C. 2002. Towards sustainable Sludge management. Water. 21: 22-23. Lester, J. N., Sterrnt, R. M. and Kirk, P. W. W. 1983. Significance and behaviour of heavy metals in wastewater treatment process. 11. Sludge treatment and disposal. Sci. Total Environ. 30: 45-83. Logan, T.J., Lindasy, B. J., Goins, L. E. and Ryan, J. A. 1997. Field assessment of sludge metal bioavailability to crops: Sludge rate response. J Environ Qual. 26: 534-550. Merrington, G., Oliver, I., Smernik, R. J. and McLaughlin, M. J. 2003. The influence of sewage sludge properties on sludgeborne metal availability. Adu. Environ. Res. 8: 21-36. Rauret, G. 1998. Extraction procedures for the determination of heavy metals in contaminated soil and sediment. Talanta. 46: 449-455. SEanEar, J., MilaEiE, R., Straiar, M. and Burica, 0. 2000. Total metal concentrations and partitioning of Cd, Cr, Cu, Fe, Ni and Zn in sewage sludge. Sci. Total Environ. 260(1-3): 9-19. Solis, G. J., Alonso, E. and Riesco, P. 2002. Distribution of metal Extractable Fractions during anmrobic sludge treatment in Southern Span WWTPs. Water, Air, and Soil Pollution. 140: 139-156. Su, D. C. and Wong, J. W. C. 2003. Chemical speciation and phytoavailability of Zn, Cu, Ni and Cd in soil amended with fly ash-stabilized sewage sludge. Environ. Int. 29: 895-900. Tessier, A., Campbell, P. G. C. and Bisson M. 1979. Sequential extraction procedure for the speciation of particulate trace metals. Anal. Chem. 61: 844-851. Ure, A. M., Quevauviller, P. H., Muntau, H. and Griepink, B. 1993. Speciation of heavy metals in soils and sediments. An account of the improvement and harmonization of extraction techniques undertaken under the auspices of the BCR of the Comission of European Communities. fnt. J . Environ. Anal. Chem. 61: 135-151. Wong, J. W. C., Li, K., Fang, M. and Su, D. C. 2001. Toxicity evaluation of sewage sludges in Hong Kong. Environ. Int. 27: 373-380.
Zhang, M. K. and Ke, Z. X. 2004. Heavy metals, phosphorus and some other elements in urban soils of Hangzhou City, China. Pedosphere. 14(2): 177-185. Zhou, D. M., Chen, H. M., Hao, X. Z. and Wang, Y.J. 2002. Fractionation of heavy metals in soils as affected by soil types and metal load quantity. Pedosphere. 12 (4): 309-319. Zufiaurre, R., Olivar, A., Chamorro, P., Nerin, C. and Callizo, A. 1998. Speciation of metals in sewage sludge for agricultural uses. Analyst. 123: 255-259.
PEDOSPHERE www.elsevier.com/locate/pedosphere
Extractable Fractions of Metals in Sewage Sludges from Five Typical Urban Wastewater Treatment Plants of China*l WANG Chao1t2,LI Xiao-Chen2, WANG Pei-Fang2, ZOU Li-Min2 and MA Hai-Tm2 'Key Labomtory of Integmted Regulation and Resource Department on Shallow Lakes, Ministry of Education, Nanjing 210098 (China). E-mail: [email protected] 'School of Environmental Science and Engineering, Hohai University, Nanjing 21 0098 (China)
(Received April 26, 2006; revised September 12, 2006)
ABSTRACT Metal content and bioavailability are often the limiting factors for application of sewage sludge in agricultural fields. Sewage sludge samples were collected from five typical urban Wastewater treatment plants in China to investigate their contents and distribution of various chemical fractions of Cu, Zn, Ni, Cr, P b and Mo by using the BCR (Community Bureau of Reference) sequential extraction procedure. The sludges contained considerable amounts of organic matter (31.8%48.0%), total N (16.3-26.4 g kg-') and total P (15.1-23.9 g kg-'), indicating high potential agricultural benefits of their practical applications. However, total Zn and Ni contents in the sludge exceeded the values permitted in China's control standards for pollutants in sludges from agricultural use (GB 4284-1984). The residual fraction was the predominant fraction for Mo, Ni and Cr, the oxidiable fraction was the primary fraction for Cu and Pb, and the exchangeable and reducible fractions were principal for Zn. The distribution of different chemical fractions among the sludge samples reflected differences in their physicochemical properties, especially pH. The sludge pH was negatively correlated with the percentages of reducible fraction of Cu and exchangeable fraction of Zn. The sludges from these plants might not be suitable for agricultural applicatiok due to their high contents of Zn, Ni and Cr, as well as high potential of mobility and bioavailability of Zn.
Key Words:
fractions, heavy metals, sequential extraction, sewage sludge
INTRODUCTION As the most common urban wastewater treatment technology in China, activated sludge processes produce a large amount of sludge, which is equivalent to 1%of the wastewater treated (Lester e t al., 1983). Because of a large number of new wastewater treatment plants constructed in China in recent years, it is expected that about four million tons of dry matter will be produced in coming years (Lee et al., 2002). Therefore, the disposal of sludge generated from the wastewater treatment plants is becoming an important environmental issue (Solis et al., 2002; Fuentes et al., 2004). One of the most attractive potential methods for sludge disposal is agricultural application because it can recycle valuable components, such as organic matter, N, P and some other plant nutrients (Zdiaurre et al., 1998; Wong et al., 2001; Fuentes et al., 2004). However, the environmental impact of this practice needs to be investigated carefully. A major restrictive factor for agronomic application of sewage sludge is heavy metals concentrated in sludge during wastewater bio-treatment process (Amir et al., 2005). These heavy metals are often toxic at a certain concentration, and they tend to accumulate along the food chain, where human being is the last in the link (Dudka and .Miller, 1999; Zhang and Ke, 2004). Consequently, the investigation of heavy metals in sewage sludge received increasing attention in recent decades (Logan et al., 1997; Fang and Wong, 1999). At present, it is widely accepted that heavy metal contents in sewage sludge cannot provide sufficient information of its potential hazardous effect on environment, because the mobility and eco-toxicity of heavy metals depend strongly on their specific *'Project supported by the National Key Basic Reaearch Support Foundation of China (No.2002CB412303) and the Natural Science Foundation of Jiangsu Province (No. KB2004115).
METALS IN SEWAGE SLUDGES
757
chemical forms or binding patterns (Zufiaurre et al., 1998;SEanEar et al., 2000; Zhou et al., 2002; Su and Wong, 2003; Fuentes et al., 2004). Both single and sequential extraction techniques had been developed to evaluate the metal mobility or bioavailability in soils, sediments and sewage sludges during recent decades (Tessier et al., 1979; Forstner and Salomons, 1980)and were thoroughly reviewed by Rauret (1998)and Kot and Namiesriik (2000). Some of the techniques are largely identical but with minor differences in chemical extractants or operating conditions, making the results seldom comparable. For this reason, the Community Bureau of Reference (BCR) developed a sequential extraction program in 1987 to harmonize the methodology for determining metals in soils and sediments (Ure et al., 1993). This procedure has also been successfully applied to determination of metals in sewage sludges (SEanEar et al., 2000; Wong et al., 2001; Soliset al., 2002; Klose and Makeschin, 2005). In the present work, the BCR sequential extraction procedure was employed to extract fractions of Cu, Zn, Ni, Cr, Pb and Mo in sewage sludge samples collected from five typical wastewater treatment plants in China. The aims were to investigate the total contents of heavy metals in the sludge samples, to characterize the chemical fractions of the metals, and to assess their potential mobility or bioavailability. MATERIALS AND METHODS Dewatered sewage sludge samples were collected from five wastewater treatment plants, including sewage sludge sample 1 (Sl)from Beijing Gaobeidian Wastewater Treatment Plant , sample 2 (S2) from Jinan Second Wastewater Treatment Plant, sample 3 (S3)from Nanjing Jiangxinzhou Wastewater Treatment Plant, sample 4 (S4) from Tai'an Wastewater Treatment Plant, and sample 5 (55) from Xiamen Wastewater Treatment Plant. The domestic and industrial wastewater was the sources of all five wastewater treatment plants. Samples of sewage sludge were air-dried at room temperature, ground and homogenized in an agate mortar, then passed through a 1-mmnylon sieve, and kept in polyethylene bags for measurement. The pH of sludge samples was measured with the extract at a sludge/deionized water ratio of 1:5 (w/v) using a digital pH meter. The organic matter content was determined by the procedure of loss on ignition (LOI) at 450 "C for 3 hours. Total nitrogen (N) in the sludge sample was measured using the Kjeldhal method, and NH4-N by the indophenol-blue method. Total phosphorus (P) and potassium (K) were determined by the inductively-coupled plasma atomic emission spectrometry (ICP-AES). To determine the metal contents of the sludge, 0.1 g of powdered sample was wet-digested with 15 mL of concentrated HN03-HC104 (3:l)mixture in a 25 mL Teflon PFA (Perfluoroalkoxy) vial. Three drops of HF acid was added to the vial, and the mixtures were then heated to a clear solution, and continued until near dryness. The cooled residue was dissolved in 5 mL 10% HN03 and the solution volume was added to 25 mL with deionized water. The sequential extraction was carried out by using the BCR procedure described by Ure et al. (1993),with 4 steps as follows: Step 1 (extraction of exchangeable fraction). Twenty mL acetic acid (0.11 mol L-') was added in a 50-mL polypropylene centrifuge tube containing 0.5000 f 0.0001 g sludge sample. The tubes were shaken on an end-over-end shaker at 40 r min-' for 16 h at ambient temperature ( 2 0 f 1 "C). The extract was separated from the solid residue by centrifugation (4000 r min-'), decanted into a polyethylene container, and stored at -4 "C until analysis. Step 2 (extraction of reducible-fraction). The residue after the step 1 was shaken with 20-mL 0.1 mol L-' hydroxylamine hydrochloride after acidification to pH 2 with nitric acid. The extraction process was the same as in the step 1. The residue after step 2 was digested with 20-mL 30% Step 3 (extmction of Oxidizable fraction). hydrogen peroxide after acidification to pH 2 with nitric acid. The extraction was continued with 25-mL 1 mol L-' ammonium acetate after acidification to pH 2 with nitric acid. The extraction process was the same as in the step 1.
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Step 4 (extraction of residual fraction). The residue after step 3 was digested using the same method as the digestion process for metal contents mentioned above. All glassware and plastic containers were previously soaked overnight in super pure nitric acid (Merck) and then rinsed with deionized water thoroughly. The contents of different fractions of Cd, Cr, Pb, Cu, Ni and Zn were determined by using ICP-AES. RESULTS AND DISCUSSION
The physicochemical properties of sewage sludges The physicochemical properties of sewage sludge samples are shown in Table I. Metal mobility and bio-availability increase with decreasing pH below pH 6.5 and decrease with increasing pH above pH 6.5 (Kazi et al., 2005). The pH values of sewage sludge samples were found between 6.5 and 7.4. Organic matter is important because it can either form soluble complexes with the toxic heavy metals or retain metal ions on the surfaces. The organic matter contents in these sludge samples ranged from 31.8% to 48.0%. Total N, total K and total P were in the range of 16.3-26.4 g kg-’, 5.8-12.2 g kg-’ and 15.1-23.9 g kg-’, respectively. On average, soils in China contain 1.0%-4.0% organic matter, 1.0-2.0 g kg-’ total N, 0.44-0.85 g kg-’ total P and about 16 g kg-’ total K (Bao et al., 2000). The relatively high contents of total N, total P, and organic matter in the sludge suggested their high potential agricultural benefits of their practical applications. TABLE I Some physicochemical properties of sludge samples from five typical wastewater treatment plants ~
Sludge sample”)
PH
Organic matter
s1 s2 s3 s4 s5
7.4 7.3 6.6 6.5 6.5 6.8 f0.4
37.2 31.8 33.9 48.0 32.8 36.7 f 6.6
~
~~~~~~~
Total P
26.2 19.1 20.1 26.4 16.3 21.6 f 4.5
15.1 17.9 20.0 23.9 16.9 18.7 f 3.3
%
MeanfSDb)
~
Total N
Total K
g kg-’ 5.8 12.2 7.5 8.2 6.1 7.9 f 2.5
a)Sl, S2, S3, S4 and S5 are sludge samples collected from five wastewater treatment plants located in Beijing, Jinan, Nanjing, Tai’an and Xiamen, respectively. b)SD = standard deviation.
Total heavy metal contents Total contents of Cu, Zn, Ni, Cr, Pb and Mo in the sewage sludge samples are presented in Table 11. The total metal contents ranged from 47.5 to 751.6 mg kg-’ for Cu, 581.7 to 3982.3 mg kg-’ for Zn, 64.2 to 2 176.7mg kg-l for Ni, 81.0 to 1414.8 mg kg-’ for Cr, 42.9 to 122.2 mg kg-’ for Pb, and 33.4 to 56.7 mg kg-’ for Mo. It was evident from the data of Table I1 that the total heavy metal contents varied greatly with the sewage sludges from different wastewater treatment plants, which might be due to the Werent characteristics of wastewater or the different ratios of domestic wastewater/industrial wastewater these treatment plants received. The maximum contents of heavy metals in sewage sludges permitted by China’s “control standards for pollutants in sludge for agricultural use (GB 4284-1984)” are listed in Table 11. The Pb contents in the sludge samples were lower than the values permitted by the Chinese standards (GB 4284-1984), and for most samples, the Cu and Cr contents were also below the permitted values, except for S4. The contents of Ni exceeded the permitted values for S3,S4 and S5. Zn contents in all samples exceeded the permitted values, with especially high contents (1 189.8 and 3 982.3 g kg-’) in S4 and S5. The highest contents of Cu, Ni and Cr were found in S4,and those of Zn and Pb in S5. From Table 11, it was evident that these sludges, especially S4 and S5,were not suitable for agricultural use, particularly due to their
METALS IN SEWAGE SLUDGES
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high Zn and Ni contents. TABLE I1 Total contents of heavy metals in the sewage sludges and the China’s permitted values (PV) for agricultural use cu
Sludge sample’)
s1 s2 53 s4 s5 Meanf SDb, PV for soil pH < 6.5‘) PV for soil pH 2 6.5‘)
58.9 47.5 67.0 751.6 187.1 222.4f 301.1 250 500
Zn
Ni
Cr
Pb
745.4 581.7 646.5 1189.8 3982.3 1429.1 f 1446.9 500 1000
mg kg-’ dry 64.2 85.5 192.1 2176.7 119.2 527.6f 923.2 100 200
weight 81.0 177.6 272.8 1414.8 195.9 428.4 f 555.6 600 1000
42.9 33.4 50.8 54.6 63.9 56.7 63.5 46.6 122.2 50.9 68.6f31.2 48.4 f 9.2 300 1000 -
Mo
’)Sl, S2, S3, S4 and S5 are sludge samples collected from five wastewater treatment plants located in Beijing, Jinan, Nanjing, Tai’an and Xiamen, respectively. b)SD = standard deviation. ‘)F’rom China’s “control standards for pollutants in sludges from agricultural use (GB 42841984)”.
Fraction distribution of heavy metals in sewage sludge
The sequential chemical extraction technique can provide information on the origin, the mode of occurrence, the biological and physicochemical availability, the mobilizatiofi and the transport of metals in sewage sludge (Kazi et al., 2005). The distribution of various fractions of heavy metals in the sludge samples obtained using the BCR sequential extraction scheme, as given in Fig. 1, showed that for Pb in all the sludge samples, 48%62% was in the oxidizable fraction, 26%-38% in the residual fraction, and no more than 20% in the exchangeable and reducible fractions. Except for S4, Ni and Cr are mainly found in the residual fraction, accounting for 77%-84% and 70%-91%, respectively. Mo was mostly found in the residual fraction (38%64%), followed by the oxidizable fraction (20%-43%) and the exchangeable fraction (< 2%). For Cu, 39%-61%was in the oxidizable fraction. Metals were regarded to be less mobile and bioavailable in the 100 80
60 40
g
20
g
o
8 F4
Y
H F3
‘3100
El F2
0
80
NF l
60 40
20 0 -
s1
s2
s3
s4
s5
s1
s2
s3
s4
s5
s1
s2
s3
s4
s5
Sample Fig. 1 The distribution percentages of various fractions of heavy metala in five sludge samples (S1 to S5) collected from five wastewater treatment plants located in Beijing, Jinan, Naqjing, Tai’an and Xiamen, respectively. F1 = exchangeable fraction; F2 = reducible fraction; F3 = oxidizable fraction; F4 = residual fraction.
C. WANG et aL
760
oxidizable and residual fractions than in the other fractions, suggesting that Pb, Ni, Cr, Mo and Cu in these sludges were not easily mobilized. The predominant portion (61%-93%) of Zn was in the exchangeable and reducible fractions, indicating high potential mobility and bioavailability of Zn. These were in consistent with the conclusions of Alvarez et al. (2002). Considering the high Zn content and high potential mobility in environment, it could be concluded that these sludges should not be directly applied to agricultural fields prior to further treatment. As seen in Fig. 2, the percentage of exchangeable fraction plus reducible fraction increased significandy with decreasing sludge pH for both Cu and Zn. Sludge properties such as pH, electrical conductivity, potentially mineralizable N and ionic strength indirectly affect the availability of sludge-borne metals (Merrington et al., 2003). A significant correlation was found between the sludge pH values and the percentages of exchangeable fraction and reducible fraction of Cu and Zn. The sludge pH value was negatively correlated with the percentages of reducible fraction of Cu and exchangeable fraction of Zn, with correlation coefficients being -0.962 and -0.916, respectively. Therefore, higher percentages of exchangeable fraction and reducible fraction for most metals in S4 and S5 than in the other sludge samples could be attributed to the lower pH of S4 and S5. 7.6
100
2 2 .-c0. 0
!!
LL
80
7.2
0
60 6.8 40
Zn
A PH 6.4
20
0
5
cu
s1
s2
S3 Sample
s4
s5
6.0
Fig. 2 The percentages of exchangeable fraction plus reducible fraction for Cu and Zn in relation to the pH values of sludges.
CONCLUSIONS The sludge samples contained considerable amounts of organic matter (31.8%-48.0%),total N (16.326.4 g kg-') and total P (15.1-23.9 g kg-I), indicating high potential agricultural benefits of their practical applications. The contents of Cu and Pb were below the values permitted by the Chinese standards (GB 4284-1984), but Cr in S4, Ni in S3, S4, and S5, and Zn in all the sludges exceeded the values permitted by the standards. Most of Mo, Ni and Cr were in the residual fraction, Cu and Pb were mainly in the oxidizable fraction, and Zn was dominated by the exchangeable fraction and reducible fraction. Significant correlation between pH value of sludge and the percentage of the exchangeable fraction and reducible fraction in total Cu and Zn indicated that pH.-played an important role in the chemical fraction distribution of metals in the sewage sludges. The sewage sludges were not suitable for agricultural use without further treatments due to their high Zn, Ni and Cr contents as well as high Zn potential mobility and bioavailability. REFERENCES Alvarez, E. A., Mochbn, M. C., JimCnez Shchez, J. C. and Rodriguez, M. T. 2002. Heavy metal extractable forms in sludge from wastewater treatment plants. Chemosphere. 47: 765-75. Amir, S., Hafidi, M., Merlina, G., Revel, J. C. 2005. Sequential extraction of heavy metals during composting of sewage sludge. Chemosphere. 59: 801-810. Bao, S., Wang, R. and Yang, C. 2000. Soil and Agricultural Chemistry Analysis. Third ed., Chinese Agricultural Press, Beijing.
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