Science of the Total Environment 499 (2014) 349–355
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Distribution of heavy metal pollution in sediments from an acid leaching site of e-waste Sheng-Xiang Quan a,b, Bo Yan a,⁎, Chang Lei a,b, Fan Yang a, Ning Li a,b, Xian-Ming Xiao a, Jia-Mo Fu a a b
State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China University of Chinese Academy of Sciences, Beijing 100049, China
H I G H L I G H T S • • • • •
Distribution of heavy metal in sediments from an acid leaching site was studied. In addition to common pollutants, these sediments were polluted with Sn and Sb. Sn and Sb may exert environmental risks because of their high total concentrations. Pollution level of sediments decreased in the order of Sb N Sn N Cu N Cd N Pb N Zn N Ni. The dominant source of heavy metal in sediments was e-waste recycling activities.
a r t i c l e
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Article history: Received 27 May 2014 Received in revised form 8 August 2014 Accepted 23 August 2014 Available online 7 September 2014 Editor: F.M. Tack Keywords: E-waste Sediment contamination Heavy metal Chemical speciation Risk assessment
a b s t r a c t The spatial distribution, bioavailability, potential risks and emission sources of 12 heavy metals in sediments from an acid leaching site of e-waste were investigated. The results showed that the sediments from the acid leaching site were significantly contaminated with Cu, Zn, Cd, Sn, Sb and Pb, especially in the middle sediments (30– 50 cm), with average concentrations of 4820, 1260, 10.7, 2660, 5690 and 2570 mg/kg, respectively. Cu, Cd and Pb were mainly present in the non-residual fractions, suggesting that the sediments from the acid leaching site may exert considerable risks. Mn, Ni, Zn, Sn and Sb were predominantly associated with the residual fraction. Despite their low reactivity and bioavailability, uncommon pollutants, such as Sn and Sb, may exert environmental risks due to their extremely elevated total concentrations. All of these results indicate that there is an urgent need to control the sources of heavy metal emission and to remediate contaminated sediments. Capture abstract: In addition to Ni, Cu, Zn, Cd and Pb, the sediments from an acid leaching site in Guiyu were heavily polluted with uncommon heavy metal pollutants, such as Sn and Sb. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Electronic waste (e-waste) comes from discarded electrical and electronic equipment, including computers, televisions, air conditioners, washing machines and their components, such as printed circuit boards (Tang et al., 2010; Wong et al., 2007a). A large number of electronic products are abandoned due to their short lifespan and the expansion of the global market. Approximately 20–50 million tons of e-waste is generated annually in the world, one third of this waste is from the US and one quarter of this waste is from the EU (UNEP, 2005). Furthermore, this global rate of e-waste production is increasing by 4% per year (Peng et al., 2009). In China, 1.1 million tons of e-waste was generated in 2003 and the production of e-waste has been
⁎ Corresponding author. Tel.: +86 20 85290335; fax: +86 20 85290706. E-mail address: [email protected] (B. Yan).
http://dx.doi.org/10.1016/j.scitotenv.2014.08.084 0048-9697/© 2014 Elsevier B.V. All rights reserved.
increasing at the rate of 5%–10% per year (Wang, 2008). E-waste is a complex mixture of metals, plastics, glasses and ceramics (M.H. Wong et al., 2007). If disposed of improperly, e-waste can become a source of metal and organic contaminants, and may impose great threats to local ecosystems and human health (Wong et al., 2007b). However, driven by profits, unregulated recycling methods/technologies, such as melting plastic to reduce waste volume, open-burning of wires and cables to recycle Cu, and acid leaching of waste printed circuit boards to recover precious metals (e.g. Au, Ag, Pt and Pd), are usually used (Fu et al., 2008; Wong et al., 2007b; Zheng et al., 2008). These primitive methods for processing e-waste have led to the release of numerous toxic metals (e.g., Cu, Zn, Cd and Pb) and persistent organic pollutants (e.g., polycyclic aromatic hydrocarbons and polybrominated diphenyl ethers) into local aquatic and terrestrial ecosystems (Fink et al., 2000; Wang et al., 2011). Heavy metal contamination in sediments is a significant environmental problem because of the toxicity, non-degradation and easy
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bio-accumulation of heavy metals (Bozkurt et al., 2000). Common heavy metal pollutants in the sediments include Cr, Ni, Cu, Zn, Cd and Pb (Monikh et al., 2013; Nithya et al., 2011). Heavy metals are easily accumulated on fine-grained particles of sediments (Mendiguchía et al., 2006; Zhang et al., 2012). These metals that are fixed to sediments may be returned to water bodies via chemical and biological processes and subsequently transported to downstream rivers (Liu et al., 2009). Thus, sediment is the most important sink for heavy metals and may act as a carrier and source of heavy metals in estuary systems (Luo et al., 2008; Yang et al., 2012). The acid leaching sites are always located in fields adjacent to rivers. The acid waste and wastewater generated during acid leaching processes are discharged into nearby streams, while solid wastes are deposited on-site with no or very few pollution control measures. Therefore, a comprehensive evaluation of heavy metal contamination in sediments from acid leaching sites is important for the environmental management and pollution control of rivers. Several studies have reported some common heavy metal pollutants from surface soil/sediments (b 40 cm) in e-waste recycling sites. Previous soil/sediment pollution investigation of heavy metals in e-waste processing areas had included open-burning sites (Luo et al., 2008), pond areas (Luo et al., 2011), paddy fields (Zhang and Min, 2009), deserted areas, etc. (Luo et al., 2011). It has been reported that the topsoil in the e-waste incineration sites of Longtang were significantly polluted with Cu (11,100 mg/kg), Zn (3690 mg/kg), Cd (17.1 mg/kg) and Pb (4500 mg/kg; Luo et al., 2011). Furthermore, the paddy soils in the Luqiao district, which is the city center of Taizhou, were heavily polluted with Cd (6.4 mg/kg), and weakly polluted with Cu (256 mg/kg) and Zn (210 mg/kg) (Zhang and Min, 2009). However, available information on the status of heavy metal contaminations in deep sediments from e-waste processing sites, especially in acid leaching sites, is limited. In addition, uncommon potential heavy metal contaminants, such as Be, Sn and Sb in sediments from e-waste recycling areas have also rarely been reported. It has been reported that Be is widely used in printed circuit boards and may be a human carcinogen (Peng et al., 2009). Sn may be released from solders within electronic equipment (Li et al., 2011), and Sb is used in semiconductor components and flame retardants within electronic devices (Bi et al., 2011). It should be noted that SnCl2 may induce tumor generation in the thyroid gland (Ferancová et al., 2007). Sb and its compounds are listed as priority pollutants by the Environmental Protection Agency of the United States (US-EPA, 1979). Therefore, these uncommon pollutants may be the important pollutants released from uncontrolled e-waste processing activities. The aims of this study are (1) to further investigate the distribution of 12 heavy metals (Be, V, Cr, Mn, Co, Ni, Cu, Zn, Cd, Sn, Sb and Pb) in sediments from an acid leaching site related with the disposal of e-waste; (2) to obtain more available information on the bioavailability of heavy metals in these sediments; (3) to evaluate the potential risks of heavy metals in these sediments to human health and their surrounding environment; and (4) to explore the relationships among different heavy metals and to identify emission sources of heavy metals in these sediments. It is expected that our findings will provide scientific evidence for preventing heavy metal pollution in sediments from e-waste processing sites. 2. Materials and methods 2.1. Reagents All of the chemicals used in our experiments were of analytical reagent grade or better and were also examined for possible heavy metal pollution. Milli-Q water with a resistance of 18.25 MΩ · cm at 25 °C was prepared using a water purification system. The 500 mg/L Sn (GSB G 62042-90, China) and 500 mg/L Sb (GSB G 62043-90, China) stock solution were obtained from the China Iron and Steel Research Institute. A multi-element standard stock solution (cat# CLMS2AN, USA) containing 10 mg/L Be, V, Cr, Mn, Co, Ni, Cu, Zn, Cd and Pb
was purchased from SPEX CertiPrep. All plastic tubes and bottles were strictly cleaned, soaked in a 10% (v/v) diluted nitric acid solution for at least 24 h and rinsed 3 times with Milli-Q water prior to the experiments. 2.2. Study area and sample collection Guiyu (116.40°E, 23.33°N) is the largest e-waste recycling center in China. The average annual rainfall is 1386 mm. E-waste processing activities have been ongoing in Guiyu since 1995. There are 3207 dismantling and recycling workshops (including companies and individual households) in this town according to the estimations from the Statistical Bureau of the Chaoyang district. Moreover, most e-waste dismantling processes are conducted in family-run workshops where primitive and traditional recycling methods (e.g., acid leaching and open-burning of e-waste) are always used. The acid waste, wastewater and solid wastes containing multiple heavy metals and organic compounds might severely contaminate local rivers. Sediment sampling was performed in May of 2013. The locations of the sampling sites are shown in Fig. 1. A total of 15 sediment cores were collected using a professional heavy metal sampler equipped with a polyvinyl chloride (PVC) liner. To comprehensively assess heavy metal pollution of sediments at various depths, each sediment core was divided into three parts: surface sediment (0–20 cm, SS), middle sediment (30–50 cm, MS) and deep sediment (60–80 cm, DS). All 45 subsamples from the 15 sediment cores were placed in polyethylene bags (Ziploc) and stored at 4 °C prior to pretreatment. 2.3. Pretreatment and analysis All sediment samples were dried for 48 h in an 80 °C oven, finely powdered using an agate mortar and sieved to pass through a 100mesh nylon sieve, thoroughly mixed and placed into glass bottles. The pH (water/solid = 2.5:1) of the sediments was measured using a Leichi pH meter (pHS-3C, China). The C, H, N and S contents of all sediments were analyzed using an elemental analyzer (Vario EL-III, Germany). After removing inorganic carbon with 5% (v/v) HCl, the total organic carbon (TOC) in the sediments was measured using a CM250 TOC analyzer (USA). Approximately 0.2 g of each sample was digested with a mixture of concentrated acids (HF/HNO3/HCl = 5:5:2) (Li et al., 2011) using a hot plate (ML-2-4, Beijing, China) at 200 °C for the determination of 12 heavy metals. The digestion liquids were filtered through 0.45 μm micro-porous membranes and diluted to 10 mL. The total concentrations of heavy metals were determined using an Agilent 7700X inductively coupled plasma-mass spectrometer (ICP-MS). The instrument was calibrated with 0.05 mg/L internal standards (4Li, 42Sc, 72Ge, 103 Rh, 115In and 219Bi). The detection limits for Be, V, Cr, Mn, Co, Ni, Cu, Zn, Cd, Sn, Sb and Pb were 0.001, 0.006, 0.078, 0.041, 0.012, 0.008, 0.081, 0.112, 0.010, 0.005, 0.011 and 0.042 μg/L, respectively. In addition to reagent blanks and replicates, a Standard Reference Materials Estuarine Sediment (SRM 1646a) was used for evaluating the reliability of the analytical methods. The percentages of recoveries of the 12 heavy metals from SRM 1646a were reasonably good (84.2%–120.5%). Chemical speciation of heavy metals in an average sediment sample, which was prepared by mixing an equal amount of each of the 45 subsamples, was conducted based on the modified BCR three-step sequential extraction procedure (Baig et al., 2009; Pueyo et al., 2008). The extraction procedure used in this study classified heavy metals into four chemical fractions: acid-soluble fraction, reducible fraction, oxidizable fraction and residual fraction. The residual fraction was digested using the same procedure used for analyzing the total heavy metal concentration in the sediment described above. Triplicates and procedure blanks were conducted for quality assurance and quality control purposes. The sum of four fractions of Be, V, Cr, Mn, Co, Ni, Cu, Zn, Cd, Sn,
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351
Fig. 1. Sampling map of sediments from Guiyu.
Sb and Pb accounted for 78.9%–118.4% of the total heavy metal concentration determined from independent analyses.
3. Results and discussion 3.1. Physicochemical characteristics of sediments
2.4. Data analysis The Nemerow pollution index (PN) can reflect the impact of heavy metal pollutants on sediment environment and is widely used for assessing the overall pollution status of sediments with heavy metals (Cheng et al., 2007; Hu et al., 2013). PN was calculated as:
Pi ¼
Ci Cb
PN ¼
ð1Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P i max 2 þ P iave 2 2
ð2Þ
where Pi is the pollution index for a single pollutant; Ci and Cb are the measured concentration of a heavy metal in sediment and its background value, respectively. The national soil background values in China, which are 1.95, 82.4, 61, 583, 12.7, 26.9, 22.6, 74.2, 0.097, 2.6, 1.21 and 26 mg/kg for Be, V, Cr, Mn, Co, Ni, Cu, Zn, Cd, Sn, Sb and Pb, respectively, were adopted in this study due to a lack of relevant background data in the study area (Wei et al., 1991). Pimax and Piave are the maximum and average pollution indices of an individual heavy metal, respectively. The degrees of heavy metal pollutions in the sediments can be classified into the following categories: Not polluted: PN ≤ 1.0 Slightly polluted: 1.0 b PN ≤ 2.0 Moderately polluted: 2.0 b PN ≤ 3.0 Heavily polluted: PN N 3.0 A correlation matrix was used to identify possible sources of heavy metals in sediments from the acid leaching site. Pearson correlation coefficients were calculated to quantitatively investigate the relationships among different heavy metals in these sediments.
The basic physicochemical characteristics of sediments from an acid leaching site in Guiyu are summarized in Table 1. A preliminary survey revealed that these sediments were dark-green in color and released an offensive odor. This could partly be explained by the fact that the sediments contained numerous fragments of waste printed circuit boards. The experimental results showed that the average pH of sediments collected from the acid leaching site was 3.97, which was apparently lower than those from downstream of the acid leaching site (pH 6.44) and from Lianjiang (pH 6.40) (Wong et al., 2007b). Heavy metals in sediments from the acid leaching site may be more soluble and easily taken up by plants because the pH of sediment plays a major role in the solubility and hydrolysis of metal hydroxides, carbonates and phosphates (Wuana et al., 2010). These sediments from the acid leaching site (39.3%) were considerably richer in TOC (total organic carbon) than those from downstream of the acid leaching site (11.3 mg/kg) and from Lianjiang (37.2 mg/kg) (Wong et al., 2007b). In addition, the elemental carbon (40.0%) of sediments from the acid leaching site was mainly in the form of organic carbon (39.3%), suggesting the need for further investigations of organic pollutants in this site. 3.2. Heavy metals in sediments The spatial distribution of heavy metals in sediments from the acid leaching site is depicted in Fig. 2. Extremely high concentrations of Cu, Zn, Cd, Sn, Sb and Pb were found in these sediments, especially in the
Table 1 Physical and chemical characteristics of sediments (depth, 0–80 cm; number of samples, 45) from an acid leaching site in Guiyu. Sediment
pH
TOC a
C
H
N
S
Average values Maximum values Minimum values
3.97 4.35 3.68
39.3% 41.6% 35.7%
40.0% 44.1% 37.4%
4.66% 5.06% 3.99%
1.42% 1.78% 1.21%
0.80% 1.02% 0.63%
a
TOC: total organic carbon.
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Fig. 2. Heavy metal concentrations of sediments from an acid leaching site in Guiyu. Circles in the top and bottom of box plots correspond to the maximum and minimum, respectively. The pentagram in the box plot is the average value. The horizontal lines in the top, middle and bottom of the box plot correspond to 75% percentile, median and 25% percentile, respectively.
MS. The average concentrations of Cu, Zn, Cd, Sn, Sb and Pb in the MS from the acid leaching site were 4820, 1260, 10.7, 2660, 5690 and 2570 mg/kg, respectively. These six heavy metal concentrations the in the MS were clearly higher than those in the SS and DS, indicating that these heavy metals in the MS from the acid leaching site were enriched during the period when primitive e-waste processing activities were the most active. Traditional e-waste recycling methods (mainly acid leaching and open-burning of e-waste) have been abandoned by the local government (Li et al., 2011). This might partly explain why the concentrations of Cu, Zn, Cd, Sn, Sb and Pb in the SS from the acid leaching site were lower than those in the MS. Another reason for this phenomenon might be that heavy metals in the SS may be recycled back to water bodies through chemical and biological processes (Tessier and Campbell, 1987; Yang et al., 2012). In contrast, the highest concentration of Mn (1000 mg/kg) was found in the SS from the acid leaching site. Furthermore, the concentrations of Mn in sediments from the acid leaching site decreased with increasing sediment depth. These results indicated that Mn pollution in these sediments may be caused by other industrial activities, such as metalliferous mining and smelting, sewage sludge discharge and fossil fuel combustion (Hu and Chen, 2013), not e-waste processing activities. The contents of Be, V, Cr and Co in sediments at various depths from the acid leaching site were low. This result indicated that the concentrations of these four heavy metals may primarily depend on the local soil background values. In summary, it was the MS in the acid leaching site that was most seriously contaminated with heavy metals, and the dominant pollutants in the MS from the acid leaching site were Cu, Zn, Cd, Sn, Sb and Pb. Because China does not have an available sediment standard for some uncommon heavy metal pollutants, such as Sn and Sb, both Chinese standards (SEPA, 1995) and new Dutch standards (VROM, 2000) were used to assess the level of heavy metal contamination in
sediments. In addition, in the present study, only heavy metal pollutions in the SS from other areas were compared with our results due to the lack of appropriate comparable information in the MS and DS in similar areas. The heavy metal concentrations in the SS collected from the acid leaching site (present study), downstream of the acid leaching site, Lianjiang and reservoir of Guiyu, and from a pond of Qingyuan, are listed in Table 2. When compared with the new Dutch standards, the concentration of Sn in the SS from the acid leaching site (2310 mg/kg) was considerably elevated than the indicative level for serious soil contamination of Dutch standard (900 mg/kg). Furthermore, the concentration of Sb in the SS from the acid leaching site (3990 mg/kg) exceeded the intervention value of Dutch standards (15 mg/kg) by 266-fold. These results suggested that the sediments in the acid leaching site were extremely contaminated with Sn and Sb. It should be noted that SnCl2 could inhibit immune response in rodent, induce tumor formation in thyroid gland and alter gene expression (Ferancová et al., 2007; Silva et al., 2002). Besides, antimonate [Sb(OH)6]− may be more toxic and more mobile in soil than Pb. Some Sb compounds might be potential carcinogens (Ferancová et al., 2007; Tschan et al., 2009). Therefore, specific attention must be paid to Sn and Sb in future heavy metal pollution investigations of e-waste processing sites. Elevated concentrations of Ni, Cu, Zn, Cd and Pb were also found in the SS from the acid leaching site, with average values of 216, 3370, 1190, 5.66 and 2170 mg/kg, respectively. The concentrations of Ni, Cu, Zn, Cd and Pb in the SS from the acid leaching site exceeded Grade III of the Chinese standards by 1.08, 8.43, 2.38, 5.66 and 4.35 times, respectively, and were obviously higher than those from downstream of the acid leaching site (with concentrations of 181 mg/kg Ni, 1070 mg/kg Cu, 324 mg/kg Zn, 4.09 mg/kg Cd and 230 mg/kg Pb) (Wong et al., 2007b), Lianjiang (with concentrations of 25.2 mg/kg Ni, 65.1 mg/kg Cu, 107 mg/kg Zn and 47.3 mg/kg Pb)
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Table 2 Concentrations of heavy metals in surface sediments from e-waste recycling areas.
Guiyu
Qingyuan Chinese d
Dutch e
Acid leaching site Downstream of acid leaching site a Lianjiang a Reservoir b Pond c I II III Target values Intervention values
Depth
Be
V
Cr
Mn
Co
Ni
Cu
Zn
Cd
Sn
Sb
Pb
0–20 0–15 0–15 0–10 0–40 – – – – –
1.18 – – – – – – – 1.1 30 f
15.2 62.6 54.3 – – – – – 42 250 f
10.2 – – 3.4 1.89 90 250 400 100 380
100 – – – – – – – – –
7.66 12.7 15.5 – – – – – 9 240
216 25.2 181 11.3 – 40 40 200 35 210
3370 65.1 1080 6.3 760 35 50 400 36 190
1190 107 324 45.2 181 100 200 500 140 720
5.66 b0.57 4.09 0.1 1.12 0.2 0.3 1 0.8 12
2310 – – – – – – – – 900 f
3990 – – – – – – – 3 15
2170 47.3 230 39.4 130 35 250 500 85 530
cm cm cm cm cm
I, natural background values of metals in soil in China. II, the maximum allowable concentrations of metals in agricultural soils in China. III, the upper limit values for regular growing of plants. a (Wong et al., 2007b). b (Leung et al., 2006). c (Luo et al., 2008). d (SEPA, 1995). e (VROM, 2000). f The indicative level for soil/sediment serious contamination of Dutch standards.
(Wong et al., 2007b) and the reservoir of Guiyu (with concentrations of 11.3 mg/kg Ni, 6.3 mg/kg Cu, 45.2 mg/kg Zn, 0.1 mg/kg Cd and 39.4 mg/kg Pb) (Leung et al., 2006). Besides, the concentrations of Ni, Cu, Zn, Cd and Pb in the SS from the acid leaching site were higher than those from a large-scale e-waste plant in Wenling, Taizhou (with concentrations of 48.0 mg/kg Ni, 296 mg/kg Cu, 433 mg/kg Zn, 2.60 mg/kg Cd and 502 mg/kg Pb) (Tang et al., 2010) and from a pond of Qingyuan (Table 2). These results further demonstrated that, in addition to Sn and Sb, the sediments from the acid leaching site were heavily polluted with Ni, Cu, Zn, Cd and Pb released from irregular e-waste recycling activities. Correlation analysis has been widely used to investigate the associations among various pollutants (Chen and Yan, 2012; Zhang et al., 2006). The correlation analyses of Be, V, Cr and Co were not performed in this part based on the results described above (the sediments were not polluted with Be, V, Cr or Co). The results of the correlation analysis are presented in Table 3. Significant correlations were found among Ni, Cu, Zn, Cd and Pb at the level of 0.01 (two-tailed), suggesting that these metals may originate from some common sources. Strong correlations among Sn, Sb and Pb were also found, while the correlation coefficients of Ni-Sn, Cu-Sn and Cu-Sb were 0.429, 0.554 and 0.495, respectively. The results showed that Ni, Cu, Sn, Sb and Pb may have a common source as well. We could further infer that Ni, Cu, Zn, Cd, Sn, Sb and Pb were released from traditional e-waste recycling activities because e-waste was primarily composed of metals and plastics (M.H. Wong et al., 2007).
their chemical forms (Cuong and Obbard, 2006; Morillo et al., 2007). In general, heavy metals in non-residual fractions, including the acidsoluble fraction, reducible fraction and oxidizable fraction, may reflect various degrees of mobility and bioavailability, whereas heavy metals in residual fraction are relatively stable (Tessier et al., 1979; Wong et al., 2007b) The sequential chemical distribution of 8 heavy metals (Mn, Ni, Cu, Zn, Cd, Sn, Sb and Pb) in the average sediment sample is depicted in Fig. 3. As shown in Fig. 3, Cu in the average sediment collected from the acid leaching site was predominately associated with the oxidizable fraction (39.8%, effectively representing as much as 1700 mg Cu/kg). The result was consistent with those from Lianjiang sediments (Wong et al., 2007b). Physical and geochemical changes, such as the oxidation of anaerobic sediments, may lead to an increase in dissolved Cu concentration in river water (Lu and Allen, 2001). Therefore, particular concern was associated with Cu in the oxidizable fraction of sediments from the acid leaching site. In the average sediment sample, Cd was mainly present in the acid-soluble fraction (38.4%) and in the residual fraction (42.6%), effectively representing as much as 3.00 mg Cd/kg and 3.50 mg Cd/kg, respectively. This result was consistent with the sediments from a typical mariculture zone in Hailing Bay, south China, in which the sediments had the highest proportion (N 50% on average) in the acid-soluble fraction (Zhang et al., 2012). Therefore, Cd in the sediments from the acid leaching site may pose a significant risk due to its high total concentration and high percentages of non-residual fractions. In addition, the dominant proportions of Pb in the sediments from the
3.3. Chemical speciation Simple determination of the total heavy metal concentrations cannot provide sufficient information regarding their bioavailability and potential risks to human health and the environment because the mobility and toxicity of heavy metals in sediments largely depend on
Table 3 Pearson correlation coefficients for heavy metals in sediments.
Mn Ni Cu Zn Cd Sn Sb
Ni
Cu
Zn
Cd
Sn
Sb
Pb
0.494⁎⁎
0.417⁎⁎ 0.660⁎⁎
0.910⁎⁎ 0.553⁎⁎ 0.530⁎⁎
0.568⁎⁎ 0.396⁎⁎ 0.484⁎⁎ 0.786⁎⁎
0.211 0.429⁎⁎ 0.554⁎⁎ 0.287 0.100
0.353⁎ 0.132 0.495⁎⁎ 0.379⁎ 0.327⁎ 0.469⁎⁎
0.609⁎⁎ 0.442⁎⁎ 0.575⁎⁎ 0.645⁎⁎ 0.412⁎⁎ 0.762⁎⁎ 0.735⁎⁎
⁎⁎ Significant at a level of 0.01 (two-tailed). ⁎ Significant at a level of 0.05 (two-tailed).
Fig. 3. Relative distribution of heavy metals in sediments.
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Table 4 Nemerow pollution indices of surface sediments from the Nanyang River.
Be V Cr Mn Co Ni Cu Zn Cd Sn Sb Pb Nemerow pollution index Pollution level a
Background values a Nemerow pollution Pollution level index
Heavy metal concentration (mg/kg)
Single pollution index
SS
MS
DS
SS
MS
DS
1.18 15.2 10.2 1000 7.66 216 337 1190 5.66 2310 3990 2160
0.93 13.4 14.5 829 10.3 237 4820 1260 10.7 2660 5690 2570
2.61 24.7 10.8 760 13.0 270 4740 1070 8.48 2410 4000 2060
0.61 0.18 0.17 1.72 0.60 8.03 149 16.0 58.4 887 3300 83.0 1660 Heavily polluted
0.48 0.16 0.24 1.42 0.81 8.80 213 17.0 110 1020 4700 99/0 2360 Heavily polluted
1.34 1.95 0.30 82.4 0.18 61 1.30 583 1.03 12.7 10.0 26.9 210 22.6 14.4 74.2 87.4 0.097 926 2.6 3300 1.21 79.3 26 1660 Heavily polluted
0.78 0.14 0.15 1.14 0.65 6.72 143 11.6 69.8 697 3010 65.9
Not polluted Not polluted Not polluted Slightly polluted Not polluted Heavily polluted Heavily polluted Heavily polluted Heavily polluted Heavily polluted Heavily polluted Heavily polluted
(Wei et al., 1991).
acid leaching site were reducible fraction (42.6%) and residual fraction (27.3%), effectively representing as much as 900 mg Pb/kg and 600 mg Pb/kg, respectively. Similar results were reported in the Pearl River Estuary and adjacent shelf in China (Yang et al., 2012). These results showed that Cu (81.6%), Cd (57.5%) and Pb (72.7%) in sediments from the acid leaching site were predominantly associated with the non-residual fractions. Therefore, Cu, Cd and Pb in the sediment from the acid leaching site may present considerable risks to local environments and human health. In addition, Mn (61.2%), Ni (82.8%), Zn (73.6%), Sn (84.8%) and Sb (85.0%) in the sediment were mainly present in the residual fraction. It should be noted that, in spite of their low mobility and bioavailability in sediments from the acid leaching site, Sn (with concentrations of 2300 mg/kg for SS, 2660 mg/kg for MS and 2410 mg/kg for DS) and Sb (with concentrations of 3990 mg/kg for SS, 5690 mg/kg for MS and 4000 mg/kg for DS; Table 2) may still pose environmental risks due to their extremely high total concentrations.
4. Conclusion Traditional e-waste recycling activities have led to the release of heavy metal pollutants. High levels of heavy metal pollution were found in the SS, MS and DS from acid leaching sites, and the pollution levels in the MS from the acid leaching sites were higher than those in the SS and DS. Cu, Cd and Pb in sediment from the acid leaching site may pose considerable risks to the local environments and human health due to their high total concentrations and high percentages of non-residual fractions. Despite their low reactivity and potential bioavailability, Sn and Sb may exert environmental risks because of their extremely elevated total concentrations. Therefore, much attention must be paid to Sn and Sb in future studies of heavy metal pollutions in soils and sediments in similar areas. The Pearson correlation analysis results showed that the dominant source of heavy metal pollutants in sediments from the acid leaching site was primitive e-waste processing activities. Overall, further investigations are required to optimize and improve e-waste processing technologies.
3.4. Risk assessment The results of Nemerow pollution index are shown in Table 4. On one hand, the Nemerow pollution indices for the SS, MS and DS from the acid leaching site were 1660, 2360 and 1660, respectively. The results showed that the pollution levels of the SS, MS and DS from the acid leaching site were heavily polluted and decreased in the order of MS N SS ≈ DS. The results were similar to the above findings. On the other hand, the acid leaching site was the most polluted with heavy metals with the exception of Be, V, Cr and Co. The acid leaching site was slightly contaminated with Mn, with a Nemerow pollution index of 1.14. The Nemerow pollution indices of Ni, Cu, Zn, Cd, Sn, Sb and Pb were 6.72, 143, 11.6, 69.8, 697, 3010 and 65.9, respectively. These Nemerow pollution indices were considerably higher than 3, suggesting that the acid leaching site was heavily polluted with Ni, Cu, Zn, Cd, Sn, Sb and Pb, and its pollution level decreased in the following order: SbNSnNCuNCdNPbNZnNNi. It should be pointed out that, in this study, only heavy metal pollutants were selected for the risk assessment of sediments from the acid leaching site. However, organic contaminants, such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polybrominated biphenyl ethers (PBDEs) (Liu et al., 2013) and dichlorodiphenyltrichloroethane (DDT) (Yu et al., 2011), may also be present in these sediments. Thus, the actual pollution level of sediments in the acid leaching site might be higher than that indicated by the above results. All in all, efforts have to be taken immediately to control the emission of pollutants and to remediate the sediments in similar areas.
Acknowledgement This work presented in this study was supported by the Key Project of the Chinese Academy of Sciences Comprehensive Strategic Cooperation with Guangdong Province (No. 2012B090400030), Guangdong Natural Science Funds for Distinguished Young Scholar (No. S2013050014122) and the GIGCAS 135 project Y234021001. This in contribution No. IS-1947 from GIGCAS. References Baig JA, Kazi TG, Arain MB, Shah AQ, Sarfraz RA, Afridi HI, et al. Arsenic fractionation in sediments of different origins using BCR sequential and single extraction methods. J Hazard Mater 2009;167:745–51. Bi X, Li Z, Zhuang X, Han Z, Yang W. High levels of antimony in dust from e-waste recycling in southeastern China. Sci Total Enviro 2011;409:5126–8. Bozkurt S, Moreno L, Neretnieks I. Long-term processes in waste deposits. Sci Total Environ 2000;250:101–21. Chen T, Yan B. Fixation and partitioning of heavy metals in slag after incineration of sewage sludge. Waste Manag 2012;32:957–64. Cheng Jl, Shi Z, Zhu YW. Assessment and mapping of environmental quality in agricultural soils of Zhejiang Province, China. J Environ Sci 2007;19:50–4. Cuong DT, Obbard JP. Metal speciation in coastal marine sediments from Singapore using a modified BCR-sequential extraction procedure. Appl Geochem 2006;21:1335–46. Ferancová A, Adamovski M, Gründler P, Zima J, Barek J, Mattusch J, et al. Interaction of tin (II) and arsenic (III) with DNA at the nanostructure film modified electrodes. Bioelectrochemistry 2007;71:33–7. Fink H, Panne U, Theisen M, Niessner R, Probst T, Lin X. Determination of metal additives and bromine in recycled thermoplasts from electronic waste by TXRF analysis. Fresenius J Anal Chem 2000;368:235–9.
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Contents lists available at ScienceDirect
Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Distribution of heavy metal pollution in sediments from an acid leaching site of e-waste Sheng-Xiang Quan a,b, Bo Yan a,⁎, Chang Lei a,b, Fan Yang a, Ning Li a,b, Xian-Ming Xiao a, Jia-Mo Fu a a b
State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China University of Chinese Academy of Sciences, Beijing 100049, China
H I G H L I G H T S • • • • •
Distribution of heavy metal in sediments from an acid leaching site was studied. In addition to common pollutants, these sediments were polluted with Sn and Sb. Sn and Sb may exert environmental risks because of their high total concentrations. Pollution level of sediments decreased in the order of Sb N Sn N Cu N Cd N Pb N Zn N Ni. The dominant source of heavy metal in sediments was e-waste recycling activities.
a r t i c l e
i n f o
Article history: Received 27 May 2014 Received in revised form 8 August 2014 Accepted 23 August 2014 Available online 7 September 2014 Editor: F.M. Tack Keywords: E-waste Sediment contamination Heavy metal Chemical speciation Risk assessment
a b s t r a c t The spatial distribution, bioavailability, potential risks and emission sources of 12 heavy metals in sediments from an acid leaching site of e-waste were investigated. The results showed that the sediments from the acid leaching site were significantly contaminated with Cu, Zn, Cd, Sn, Sb and Pb, especially in the middle sediments (30– 50 cm), with average concentrations of 4820, 1260, 10.7, 2660, 5690 and 2570 mg/kg, respectively. Cu, Cd and Pb were mainly present in the non-residual fractions, suggesting that the sediments from the acid leaching site may exert considerable risks. Mn, Ni, Zn, Sn and Sb were predominantly associated with the residual fraction. Despite their low reactivity and bioavailability, uncommon pollutants, such as Sn and Sb, may exert environmental risks due to their extremely elevated total concentrations. All of these results indicate that there is an urgent need to control the sources of heavy metal emission and to remediate contaminated sediments. Capture abstract: In addition to Ni, Cu, Zn, Cd and Pb, the sediments from an acid leaching site in Guiyu were heavily polluted with uncommon heavy metal pollutants, such as Sn and Sb. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Electronic waste (e-waste) comes from discarded electrical and electronic equipment, including computers, televisions, air conditioners, washing machines and their components, such as printed circuit boards (Tang et al., 2010; Wong et al., 2007a). A large number of electronic products are abandoned due to their short lifespan and the expansion of the global market. Approximately 20–50 million tons of e-waste is generated annually in the world, one third of this waste is from the US and one quarter of this waste is from the EU (UNEP, 2005). Furthermore, this global rate of e-waste production is increasing by 4% per year (Peng et al., 2009). In China, 1.1 million tons of e-waste was generated in 2003 and the production of e-waste has been
⁎ Corresponding author. Tel.: +86 20 85290335; fax: +86 20 85290706. E-mail address: [email protected] (B. Yan).
http://dx.doi.org/10.1016/j.scitotenv.2014.08.084 0048-9697/© 2014 Elsevier B.V. All rights reserved.
increasing at the rate of 5%–10% per year (Wang, 2008). E-waste is a complex mixture of metals, plastics, glasses and ceramics (M.H. Wong et al., 2007). If disposed of improperly, e-waste can become a source of metal and organic contaminants, and may impose great threats to local ecosystems and human health (Wong et al., 2007b). However, driven by profits, unregulated recycling methods/technologies, such as melting plastic to reduce waste volume, open-burning of wires and cables to recycle Cu, and acid leaching of waste printed circuit boards to recover precious metals (e.g. Au, Ag, Pt and Pd), are usually used (Fu et al., 2008; Wong et al., 2007b; Zheng et al., 2008). These primitive methods for processing e-waste have led to the release of numerous toxic metals (e.g., Cu, Zn, Cd and Pb) and persistent organic pollutants (e.g., polycyclic aromatic hydrocarbons and polybrominated diphenyl ethers) into local aquatic and terrestrial ecosystems (Fink et al., 2000; Wang et al., 2011). Heavy metal contamination in sediments is a significant environmental problem because of the toxicity, non-degradation and easy
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bio-accumulation of heavy metals (Bozkurt et al., 2000). Common heavy metal pollutants in the sediments include Cr, Ni, Cu, Zn, Cd and Pb (Monikh et al., 2013; Nithya et al., 2011). Heavy metals are easily accumulated on fine-grained particles of sediments (Mendiguchía et al., 2006; Zhang et al., 2012). These metals that are fixed to sediments may be returned to water bodies via chemical and biological processes and subsequently transported to downstream rivers (Liu et al., 2009). Thus, sediment is the most important sink for heavy metals and may act as a carrier and source of heavy metals in estuary systems (Luo et al., 2008; Yang et al., 2012). The acid leaching sites are always located in fields adjacent to rivers. The acid waste and wastewater generated during acid leaching processes are discharged into nearby streams, while solid wastes are deposited on-site with no or very few pollution control measures. Therefore, a comprehensive evaluation of heavy metal contamination in sediments from acid leaching sites is important for the environmental management and pollution control of rivers. Several studies have reported some common heavy metal pollutants from surface soil/sediments (b 40 cm) in e-waste recycling sites. Previous soil/sediment pollution investigation of heavy metals in e-waste processing areas had included open-burning sites (Luo et al., 2008), pond areas (Luo et al., 2011), paddy fields (Zhang and Min, 2009), deserted areas, etc. (Luo et al., 2011). It has been reported that the topsoil in the e-waste incineration sites of Longtang were significantly polluted with Cu (11,100 mg/kg), Zn (3690 mg/kg), Cd (17.1 mg/kg) and Pb (4500 mg/kg; Luo et al., 2011). Furthermore, the paddy soils in the Luqiao district, which is the city center of Taizhou, were heavily polluted with Cd (6.4 mg/kg), and weakly polluted with Cu (256 mg/kg) and Zn (210 mg/kg) (Zhang and Min, 2009). However, available information on the status of heavy metal contaminations in deep sediments from e-waste processing sites, especially in acid leaching sites, is limited. In addition, uncommon potential heavy metal contaminants, such as Be, Sn and Sb in sediments from e-waste recycling areas have also rarely been reported. It has been reported that Be is widely used in printed circuit boards and may be a human carcinogen (Peng et al., 2009). Sn may be released from solders within electronic equipment (Li et al., 2011), and Sb is used in semiconductor components and flame retardants within electronic devices (Bi et al., 2011). It should be noted that SnCl2 may induce tumor generation in the thyroid gland (Ferancová et al., 2007). Sb and its compounds are listed as priority pollutants by the Environmental Protection Agency of the United States (US-EPA, 1979). Therefore, these uncommon pollutants may be the important pollutants released from uncontrolled e-waste processing activities. The aims of this study are (1) to further investigate the distribution of 12 heavy metals (Be, V, Cr, Mn, Co, Ni, Cu, Zn, Cd, Sn, Sb and Pb) in sediments from an acid leaching site related with the disposal of e-waste; (2) to obtain more available information on the bioavailability of heavy metals in these sediments; (3) to evaluate the potential risks of heavy metals in these sediments to human health and their surrounding environment; and (4) to explore the relationships among different heavy metals and to identify emission sources of heavy metals in these sediments. It is expected that our findings will provide scientific evidence for preventing heavy metal pollution in sediments from e-waste processing sites. 2. Materials and methods 2.1. Reagents All of the chemicals used in our experiments were of analytical reagent grade or better and were also examined for possible heavy metal pollution. Milli-Q water with a resistance of 18.25 MΩ · cm at 25 °C was prepared using a water purification system. The 500 mg/L Sn (GSB G 62042-90, China) and 500 mg/L Sb (GSB G 62043-90, China) stock solution were obtained from the China Iron and Steel Research Institute. A multi-element standard stock solution (cat# CLMS2AN, USA) containing 10 mg/L Be, V, Cr, Mn, Co, Ni, Cu, Zn, Cd and Pb
was purchased from SPEX CertiPrep. All plastic tubes and bottles were strictly cleaned, soaked in a 10% (v/v) diluted nitric acid solution for at least 24 h and rinsed 3 times with Milli-Q water prior to the experiments. 2.2. Study area and sample collection Guiyu (116.40°E, 23.33°N) is the largest e-waste recycling center in China. The average annual rainfall is 1386 mm. E-waste processing activities have been ongoing in Guiyu since 1995. There are 3207 dismantling and recycling workshops (including companies and individual households) in this town according to the estimations from the Statistical Bureau of the Chaoyang district. Moreover, most e-waste dismantling processes are conducted in family-run workshops where primitive and traditional recycling methods (e.g., acid leaching and open-burning of e-waste) are always used. The acid waste, wastewater and solid wastes containing multiple heavy metals and organic compounds might severely contaminate local rivers. Sediment sampling was performed in May of 2013. The locations of the sampling sites are shown in Fig. 1. A total of 15 sediment cores were collected using a professional heavy metal sampler equipped with a polyvinyl chloride (PVC) liner. To comprehensively assess heavy metal pollution of sediments at various depths, each sediment core was divided into three parts: surface sediment (0–20 cm, SS), middle sediment (30–50 cm, MS) and deep sediment (60–80 cm, DS). All 45 subsamples from the 15 sediment cores were placed in polyethylene bags (Ziploc) and stored at 4 °C prior to pretreatment. 2.3. Pretreatment and analysis All sediment samples were dried for 48 h in an 80 °C oven, finely powdered using an agate mortar and sieved to pass through a 100mesh nylon sieve, thoroughly mixed and placed into glass bottles. The pH (water/solid = 2.5:1) of the sediments was measured using a Leichi pH meter (pHS-3C, China). The C, H, N and S contents of all sediments were analyzed using an elemental analyzer (Vario EL-III, Germany). After removing inorganic carbon with 5% (v/v) HCl, the total organic carbon (TOC) in the sediments was measured using a CM250 TOC analyzer (USA). Approximately 0.2 g of each sample was digested with a mixture of concentrated acids (HF/HNO3/HCl = 5:5:2) (Li et al., 2011) using a hot plate (ML-2-4, Beijing, China) at 200 °C for the determination of 12 heavy metals. The digestion liquids were filtered through 0.45 μm micro-porous membranes and diluted to 10 mL. The total concentrations of heavy metals were determined using an Agilent 7700X inductively coupled plasma-mass spectrometer (ICP-MS). The instrument was calibrated with 0.05 mg/L internal standards (4Li, 42Sc, 72Ge, 103 Rh, 115In and 219Bi). The detection limits for Be, V, Cr, Mn, Co, Ni, Cu, Zn, Cd, Sn, Sb and Pb were 0.001, 0.006, 0.078, 0.041, 0.012, 0.008, 0.081, 0.112, 0.010, 0.005, 0.011 and 0.042 μg/L, respectively. In addition to reagent blanks and replicates, a Standard Reference Materials Estuarine Sediment (SRM 1646a) was used for evaluating the reliability of the analytical methods. The percentages of recoveries of the 12 heavy metals from SRM 1646a were reasonably good (84.2%–120.5%). Chemical speciation of heavy metals in an average sediment sample, which was prepared by mixing an equal amount of each of the 45 subsamples, was conducted based on the modified BCR three-step sequential extraction procedure (Baig et al., 2009; Pueyo et al., 2008). The extraction procedure used in this study classified heavy metals into four chemical fractions: acid-soluble fraction, reducible fraction, oxidizable fraction and residual fraction. The residual fraction was digested using the same procedure used for analyzing the total heavy metal concentration in the sediment described above. Triplicates and procedure blanks were conducted for quality assurance and quality control purposes. The sum of four fractions of Be, V, Cr, Mn, Co, Ni, Cu, Zn, Cd, Sn,
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Fig. 1. Sampling map of sediments from Guiyu.
Sb and Pb accounted for 78.9%–118.4% of the total heavy metal concentration determined from independent analyses.
3. Results and discussion 3.1. Physicochemical characteristics of sediments
2.4. Data analysis The Nemerow pollution index (PN) can reflect the impact of heavy metal pollutants on sediment environment and is widely used for assessing the overall pollution status of sediments with heavy metals (Cheng et al., 2007; Hu et al., 2013). PN was calculated as:
Pi ¼
Ci Cb
PN ¼
ð1Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P i max 2 þ P iave 2 2
ð2Þ
where Pi is the pollution index for a single pollutant; Ci and Cb are the measured concentration of a heavy metal in sediment and its background value, respectively. The national soil background values in China, which are 1.95, 82.4, 61, 583, 12.7, 26.9, 22.6, 74.2, 0.097, 2.6, 1.21 and 26 mg/kg for Be, V, Cr, Mn, Co, Ni, Cu, Zn, Cd, Sn, Sb and Pb, respectively, were adopted in this study due to a lack of relevant background data in the study area (Wei et al., 1991). Pimax and Piave are the maximum and average pollution indices of an individual heavy metal, respectively. The degrees of heavy metal pollutions in the sediments can be classified into the following categories: Not polluted: PN ≤ 1.0 Slightly polluted: 1.0 b PN ≤ 2.0 Moderately polluted: 2.0 b PN ≤ 3.0 Heavily polluted: PN N 3.0 A correlation matrix was used to identify possible sources of heavy metals in sediments from the acid leaching site. Pearson correlation coefficients were calculated to quantitatively investigate the relationships among different heavy metals in these sediments.
The basic physicochemical characteristics of sediments from an acid leaching site in Guiyu are summarized in Table 1. A preliminary survey revealed that these sediments were dark-green in color and released an offensive odor. This could partly be explained by the fact that the sediments contained numerous fragments of waste printed circuit boards. The experimental results showed that the average pH of sediments collected from the acid leaching site was 3.97, which was apparently lower than those from downstream of the acid leaching site (pH 6.44) and from Lianjiang (pH 6.40) (Wong et al., 2007b). Heavy metals in sediments from the acid leaching site may be more soluble and easily taken up by plants because the pH of sediment plays a major role in the solubility and hydrolysis of metal hydroxides, carbonates and phosphates (Wuana et al., 2010). These sediments from the acid leaching site (39.3%) were considerably richer in TOC (total organic carbon) than those from downstream of the acid leaching site (11.3 mg/kg) and from Lianjiang (37.2 mg/kg) (Wong et al., 2007b). In addition, the elemental carbon (40.0%) of sediments from the acid leaching site was mainly in the form of organic carbon (39.3%), suggesting the need for further investigations of organic pollutants in this site. 3.2. Heavy metals in sediments The spatial distribution of heavy metals in sediments from the acid leaching site is depicted in Fig. 2. Extremely high concentrations of Cu, Zn, Cd, Sn, Sb and Pb were found in these sediments, especially in the
Table 1 Physical and chemical characteristics of sediments (depth, 0–80 cm; number of samples, 45) from an acid leaching site in Guiyu. Sediment
pH
TOC a
C
H
N
S
Average values Maximum values Minimum values
3.97 4.35 3.68
39.3% 41.6% 35.7%
40.0% 44.1% 37.4%
4.66% 5.06% 3.99%
1.42% 1.78% 1.21%
0.80% 1.02% 0.63%
a
TOC: total organic carbon.
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Fig. 2. Heavy metal concentrations of sediments from an acid leaching site in Guiyu. Circles in the top and bottom of box plots correspond to the maximum and minimum, respectively. The pentagram in the box plot is the average value. The horizontal lines in the top, middle and bottom of the box plot correspond to 75% percentile, median and 25% percentile, respectively.
MS. The average concentrations of Cu, Zn, Cd, Sn, Sb and Pb in the MS from the acid leaching site were 4820, 1260, 10.7, 2660, 5690 and 2570 mg/kg, respectively. These six heavy metal concentrations the in the MS were clearly higher than those in the SS and DS, indicating that these heavy metals in the MS from the acid leaching site were enriched during the period when primitive e-waste processing activities were the most active. Traditional e-waste recycling methods (mainly acid leaching and open-burning of e-waste) have been abandoned by the local government (Li et al., 2011). This might partly explain why the concentrations of Cu, Zn, Cd, Sn, Sb and Pb in the SS from the acid leaching site were lower than those in the MS. Another reason for this phenomenon might be that heavy metals in the SS may be recycled back to water bodies through chemical and biological processes (Tessier and Campbell, 1987; Yang et al., 2012). In contrast, the highest concentration of Mn (1000 mg/kg) was found in the SS from the acid leaching site. Furthermore, the concentrations of Mn in sediments from the acid leaching site decreased with increasing sediment depth. These results indicated that Mn pollution in these sediments may be caused by other industrial activities, such as metalliferous mining and smelting, sewage sludge discharge and fossil fuel combustion (Hu and Chen, 2013), not e-waste processing activities. The contents of Be, V, Cr and Co in sediments at various depths from the acid leaching site were low. This result indicated that the concentrations of these four heavy metals may primarily depend on the local soil background values. In summary, it was the MS in the acid leaching site that was most seriously contaminated with heavy metals, and the dominant pollutants in the MS from the acid leaching site were Cu, Zn, Cd, Sn, Sb and Pb. Because China does not have an available sediment standard for some uncommon heavy metal pollutants, such as Sn and Sb, both Chinese standards (SEPA, 1995) and new Dutch standards (VROM, 2000) were used to assess the level of heavy metal contamination in
sediments. In addition, in the present study, only heavy metal pollutions in the SS from other areas were compared with our results due to the lack of appropriate comparable information in the MS and DS in similar areas. The heavy metal concentrations in the SS collected from the acid leaching site (present study), downstream of the acid leaching site, Lianjiang and reservoir of Guiyu, and from a pond of Qingyuan, are listed in Table 2. When compared with the new Dutch standards, the concentration of Sn in the SS from the acid leaching site (2310 mg/kg) was considerably elevated than the indicative level for serious soil contamination of Dutch standard (900 mg/kg). Furthermore, the concentration of Sb in the SS from the acid leaching site (3990 mg/kg) exceeded the intervention value of Dutch standards (15 mg/kg) by 266-fold. These results suggested that the sediments in the acid leaching site were extremely contaminated with Sn and Sb. It should be noted that SnCl2 could inhibit immune response in rodent, induce tumor formation in thyroid gland and alter gene expression (Ferancová et al., 2007; Silva et al., 2002). Besides, antimonate [Sb(OH)6]− may be more toxic and more mobile in soil than Pb. Some Sb compounds might be potential carcinogens (Ferancová et al., 2007; Tschan et al., 2009). Therefore, specific attention must be paid to Sn and Sb in future heavy metal pollution investigations of e-waste processing sites. Elevated concentrations of Ni, Cu, Zn, Cd and Pb were also found in the SS from the acid leaching site, with average values of 216, 3370, 1190, 5.66 and 2170 mg/kg, respectively. The concentrations of Ni, Cu, Zn, Cd and Pb in the SS from the acid leaching site exceeded Grade III of the Chinese standards by 1.08, 8.43, 2.38, 5.66 and 4.35 times, respectively, and were obviously higher than those from downstream of the acid leaching site (with concentrations of 181 mg/kg Ni, 1070 mg/kg Cu, 324 mg/kg Zn, 4.09 mg/kg Cd and 230 mg/kg Pb) (Wong et al., 2007b), Lianjiang (with concentrations of 25.2 mg/kg Ni, 65.1 mg/kg Cu, 107 mg/kg Zn and 47.3 mg/kg Pb)
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Table 2 Concentrations of heavy metals in surface sediments from e-waste recycling areas.
Guiyu
Qingyuan Chinese d
Dutch e
Acid leaching site Downstream of acid leaching site a Lianjiang a Reservoir b Pond c I II III Target values Intervention values
Depth
Be
V
Cr
Mn
Co
Ni
Cu
Zn
Cd
Sn
Sb
Pb
0–20 0–15 0–15 0–10 0–40 – – – – –
1.18 – – – – – – – 1.1 30 f
15.2 62.6 54.3 – – – – – 42 250 f
10.2 – – 3.4 1.89 90 250 400 100 380
100 – – – – – – – – –
7.66 12.7 15.5 – – – – – 9 240
216 25.2 181 11.3 – 40 40 200 35 210
3370 65.1 1080 6.3 760 35 50 400 36 190
1190 107 324 45.2 181 100 200 500 140 720
5.66 b0.57 4.09 0.1 1.12 0.2 0.3 1 0.8 12
2310 – – – – – – – – 900 f
3990 – – – – – – – 3 15
2170 47.3 230 39.4 130 35 250 500 85 530
cm cm cm cm cm
I, natural background values of metals in soil in China. II, the maximum allowable concentrations of metals in agricultural soils in China. III, the upper limit values for regular growing of plants. a (Wong et al., 2007b). b (Leung et al., 2006). c (Luo et al., 2008). d (SEPA, 1995). e (VROM, 2000). f The indicative level for soil/sediment serious contamination of Dutch standards.
(Wong et al., 2007b) and the reservoir of Guiyu (with concentrations of 11.3 mg/kg Ni, 6.3 mg/kg Cu, 45.2 mg/kg Zn, 0.1 mg/kg Cd and 39.4 mg/kg Pb) (Leung et al., 2006). Besides, the concentrations of Ni, Cu, Zn, Cd and Pb in the SS from the acid leaching site were higher than those from a large-scale e-waste plant in Wenling, Taizhou (with concentrations of 48.0 mg/kg Ni, 296 mg/kg Cu, 433 mg/kg Zn, 2.60 mg/kg Cd and 502 mg/kg Pb) (Tang et al., 2010) and from a pond of Qingyuan (Table 2). These results further demonstrated that, in addition to Sn and Sb, the sediments from the acid leaching site were heavily polluted with Ni, Cu, Zn, Cd and Pb released from irregular e-waste recycling activities. Correlation analysis has been widely used to investigate the associations among various pollutants (Chen and Yan, 2012; Zhang et al., 2006). The correlation analyses of Be, V, Cr and Co were not performed in this part based on the results described above (the sediments were not polluted with Be, V, Cr or Co). The results of the correlation analysis are presented in Table 3. Significant correlations were found among Ni, Cu, Zn, Cd and Pb at the level of 0.01 (two-tailed), suggesting that these metals may originate from some common sources. Strong correlations among Sn, Sb and Pb were also found, while the correlation coefficients of Ni-Sn, Cu-Sn and Cu-Sb were 0.429, 0.554 and 0.495, respectively. The results showed that Ni, Cu, Sn, Sb and Pb may have a common source as well. We could further infer that Ni, Cu, Zn, Cd, Sn, Sb and Pb were released from traditional e-waste recycling activities because e-waste was primarily composed of metals and plastics (M.H. Wong et al., 2007).
their chemical forms (Cuong and Obbard, 2006; Morillo et al., 2007). In general, heavy metals in non-residual fractions, including the acidsoluble fraction, reducible fraction and oxidizable fraction, may reflect various degrees of mobility and bioavailability, whereas heavy metals in residual fraction are relatively stable (Tessier et al., 1979; Wong et al., 2007b) The sequential chemical distribution of 8 heavy metals (Mn, Ni, Cu, Zn, Cd, Sn, Sb and Pb) in the average sediment sample is depicted in Fig. 3. As shown in Fig. 3, Cu in the average sediment collected from the acid leaching site was predominately associated with the oxidizable fraction (39.8%, effectively representing as much as 1700 mg Cu/kg). The result was consistent with those from Lianjiang sediments (Wong et al., 2007b). Physical and geochemical changes, such as the oxidation of anaerobic sediments, may lead to an increase in dissolved Cu concentration in river water (Lu and Allen, 2001). Therefore, particular concern was associated with Cu in the oxidizable fraction of sediments from the acid leaching site. In the average sediment sample, Cd was mainly present in the acid-soluble fraction (38.4%) and in the residual fraction (42.6%), effectively representing as much as 3.00 mg Cd/kg and 3.50 mg Cd/kg, respectively. This result was consistent with the sediments from a typical mariculture zone in Hailing Bay, south China, in which the sediments had the highest proportion (N 50% on average) in the acid-soluble fraction (Zhang et al., 2012). Therefore, Cd in the sediments from the acid leaching site may pose a significant risk due to its high total concentration and high percentages of non-residual fractions. In addition, the dominant proportions of Pb in the sediments from the
3.3. Chemical speciation Simple determination of the total heavy metal concentrations cannot provide sufficient information regarding their bioavailability and potential risks to human health and the environment because the mobility and toxicity of heavy metals in sediments largely depend on
Table 3 Pearson correlation coefficients for heavy metals in sediments.
Mn Ni Cu Zn Cd Sn Sb
Ni
Cu
Zn
Cd
Sn
Sb
Pb
0.494⁎⁎
0.417⁎⁎ 0.660⁎⁎
0.910⁎⁎ 0.553⁎⁎ 0.530⁎⁎
0.568⁎⁎ 0.396⁎⁎ 0.484⁎⁎ 0.786⁎⁎
0.211 0.429⁎⁎ 0.554⁎⁎ 0.287 0.100
0.353⁎ 0.132 0.495⁎⁎ 0.379⁎ 0.327⁎ 0.469⁎⁎
0.609⁎⁎ 0.442⁎⁎ 0.575⁎⁎ 0.645⁎⁎ 0.412⁎⁎ 0.762⁎⁎ 0.735⁎⁎
⁎⁎ Significant at a level of 0.01 (two-tailed). ⁎ Significant at a level of 0.05 (two-tailed).
Fig. 3. Relative distribution of heavy metals in sediments.
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Table 4 Nemerow pollution indices of surface sediments from the Nanyang River.
Be V Cr Mn Co Ni Cu Zn Cd Sn Sb Pb Nemerow pollution index Pollution level a
Background values a Nemerow pollution Pollution level index
Heavy metal concentration (mg/kg)
Single pollution index
SS
MS
DS
SS
MS
DS
1.18 15.2 10.2 1000 7.66 216 337 1190 5.66 2310 3990 2160
0.93 13.4 14.5 829 10.3 237 4820 1260 10.7 2660 5690 2570
2.61 24.7 10.8 760 13.0 270 4740 1070 8.48 2410 4000 2060
0.61 0.18 0.17 1.72 0.60 8.03 149 16.0 58.4 887 3300 83.0 1660 Heavily polluted
0.48 0.16 0.24 1.42 0.81 8.80 213 17.0 110 1020 4700 99/0 2360 Heavily polluted
1.34 1.95 0.30 82.4 0.18 61 1.30 583 1.03 12.7 10.0 26.9 210 22.6 14.4 74.2 87.4 0.097 926 2.6 3300 1.21 79.3 26 1660 Heavily polluted
0.78 0.14 0.15 1.14 0.65 6.72 143 11.6 69.8 697 3010 65.9
Not polluted Not polluted Not polluted Slightly polluted Not polluted Heavily polluted Heavily polluted Heavily polluted Heavily polluted Heavily polluted Heavily polluted Heavily polluted
(Wei et al., 1991).
acid leaching site were reducible fraction (42.6%) and residual fraction (27.3%), effectively representing as much as 900 mg Pb/kg and 600 mg Pb/kg, respectively. Similar results were reported in the Pearl River Estuary and adjacent shelf in China (Yang et al., 2012). These results showed that Cu (81.6%), Cd (57.5%) and Pb (72.7%) in sediments from the acid leaching site were predominantly associated with the non-residual fractions. Therefore, Cu, Cd and Pb in the sediment from the acid leaching site may present considerable risks to local environments and human health. In addition, Mn (61.2%), Ni (82.8%), Zn (73.6%), Sn (84.8%) and Sb (85.0%) in the sediment were mainly present in the residual fraction. It should be noted that, in spite of their low mobility and bioavailability in sediments from the acid leaching site, Sn (with concentrations of 2300 mg/kg for SS, 2660 mg/kg for MS and 2410 mg/kg for DS) and Sb (with concentrations of 3990 mg/kg for SS, 5690 mg/kg for MS and 4000 mg/kg for DS; Table 2) may still pose environmental risks due to their extremely high total concentrations.
4. Conclusion Traditional e-waste recycling activities have led to the release of heavy metal pollutants. High levels of heavy metal pollution were found in the SS, MS and DS from acid leaching sites, and the pollution levels in the MS from the acid leaching sites were higher than those in the SS and DS. Cu, Cd and Pb in sediment from the acid leaching site may pose considerable risks to the local environments and human health due to their high total concentrations and high percentages of non-residual fractions. Despite their low reactivity and potential bioavailability, Sn and Sb may exert environmental risks because of their extremely elevated total concentrations. Therefore, much attention must be paid to Sn and Sb in future studies of heavy metal pollutions in soils and sediments in similar areas. The Pearson correlation analysis results showed that the dominant source of heavy metal pollutants in sediments from the acid leaching site was primitive e-waste processing activities. Overall, further investigations are required to optimize and improve e-waste processing technologies.
3.4. Risk assessment The results of Nemerow pollution index are shown in Table 4. On one hand, the Nemerow pollution indices for the SS, MS and DS from the acid leaching site were 1660, 2360 and 1660, respectively. The results showed that the pollution levels of the SS, MS and DS from the acid leaching site were heavily polluted and decreased in the order of MS N SS ≈ DS. The results were similar to the above findings. On the other hand, the acid leaching site was the most polluted with heavy metals with the exception of Be, V, Cr and Co. The acid leaching site was slightly contaminated with Mn, with a Nemerow pollution index of 1.14. The Nemerow pollution indices of Ni, Cu, Zn, Cd, Sn, Sb and Pb were 6.72, 143, 11.6, 69.8, 697, 3010 and 65.9, respectively. These Nemerow pollution indices were considerably higher than 3, suggesting that the acid leaching site was heavily polluted with Ni, Cu, Zn, Cd, Sn, Sb and Pb, and its pollution level decreased in the following order: SbNSnNCuNCdNPbNZnNNi. It should be pointed out that, in this study, only heavy metal pollutants were selected for the risk assessment of sediments from the acid leaching site. However, organic contaminants, such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polybrominated biphenyl ethers (PBDEs) (Liu et al., 2013) and dichlorodiphenyltrichloroethane (DDT) (Yu et al., 2011), may also be present in these sediments. Thus, the actual pollution level of sediments in the acid leaching site might be higher than that indicated by the above results. All in all, efforts have to be taken immediately to control the emission of pollutants and to remediate the sediments in similar areas.
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