Spatial distribution of polycyclic aromatic hydrocarbons in soil ...

Environ Sci Pollut Res (2015) 22:8786–8801 DOI 10.1007/s11356-013-1465-8

BIOAVAILABILITY - THE UNDERLYING BASIS FOR RISK BASED LAND MANAGEMENT

Spatial distribution of polycyclic aromatic hydrocarbons in soil, sediment, and combusted residue at an e-waste processing site in southeast China Anna O. W. Leung & Kwai Chung Cheung & Ming Hung Wong

Received: 5 September 2012 / Accepted: 3 January 2013 / Published online: 22 January 2013 # Springer-Verlag Berlin Heidelberg 2013

Abstract The environmental pollution and health impacts caused by the primitive and crude recycling of e-waste have become urgent global issues. Guiyu, China is a major hotspot of e-waste recycling. In this study, the levels and distribution of polycyclic aromatic hydrocarbons in soil in Guiyu were determined to investigate the effect of e-waste activities on the environment and to identify possible sources of these pollutants. Sediment samples from a local duck pond, water gullies, a river tributary, and combusted residue from e-waste burning sites were also investigated. The general trend found in soil (Σ16 PAHs) was acid leaching site>duck pond>rice field>printer roller dump site>reservoir (control site) and ranged from 95.2±54.2 to 5,210±89.6 ng/g (dry wt). The highest average total PAH concentrations were found in combusted residues of wires, cables, and other computer electrical components located at two e-waste open burning sites (18,600 and 10,800±3,940 ng/g). These were 195- and 113-fold higher than the PAH concentrations of soil at the control site. Sediment PAH concentrations ranged from 37.2± 6 to 534±271 ng/g. Results of this study provide further evidence of significant input of PAHs to the environment attributed to crude e-waste recycling. Keywords Electronic-waste . Guiyu . Open burning . Acid leaching . Risk assessment Responsible editor: Zhihong Xu A. O. W. Leung : M. H. Wong (*) Croucher Institute for Environmental Sciences, and Department of Biology, Hong Kong Baptist University, Kowloon Tong, Hong Kong, People’s Republic of China e-mail: [email protected] K. C. Cheung Department of Applied Science, Hong Kong Institute of Vocational Education, Chai Wan, Hong Kong, People’s Republic of China

Introduction The crude recycling of e-waste in developing countries continues to be a serious problem due to the release of toxic pollutants, such as polycyclic aromatic hydrocarbons, polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans, polybrominated diphenyl ethers (PBDEs), and heavy metals into the environment, which can cause adverse impacts on human health. These pollutants could affect the health of e-waste workers and local residents via ingestion, inhalation, and dermal exposure pathways. In Guiyu, a traditionally rice-growing community in Guangdong Province, China, mountains of e-waste have been processed (“recycled”) since the mid-1990s by primitive and crude methods. Printed circuit boards are heated over makeshift grills in small family-run workshops to melt solder and to remove reusable electrical components which are collected and sold. Portable household fans placed in front of the grills are the only precautionary measures taken to reduce exposure of e-waste workers to the toxic lead fumes. Circuit boards devoid of reusable electrical components are then acid stripped in large plastic buckets to remove gold and other precious metals. Cables are burned in open fields to separate the copper, and unwanted materials are then dumped and burned. A previous study of polycyclic aromatic hydrocarbon (PAHS) in soil of Guiyu reported an average concentration of 582 ng/g, and an average concentration of 2,065 ± 1,062 ng/g from open burning sites, with the highest concentration being 3,206 ng/g (Wong et al. 2007; Yu et al. 2006). Open burning of e-waste generates PAHs which can induce dioxin-like responses and potentially estrogenic responses (Villeneuve et al. 2002). Various other pollutants such as polychlorinated biphenyls (PCBs), PBDEs, and heavy metals have also been found in soil and sediment from Guiyu (Leung et al. 2006; Wang et al. 2005).

Environ Sci Pollut Res (2015) 22:8786–8801

PAHs are widespread harmful compounds, which consist of two or more fused aromatic (benzene) rings. There are over 100 known PAH compounds, 16 of which have been listed by the US Environmental Protection Agency (EPA) as priority pollutants and seven of these have been classified by the US EPA as probable human carcinogens under the B2 classification. These include chrysene, benzo(a)anthracene, benzo(k)fluoranthene, benzo(b)fluoranthene, benzo(a)pyrene (B(a)P), indeno(1,2,3-cd)pyrene, and dibenz(a,h)anthracene (US EPA 2007). Other agencies include benzo(g,h, i)perylene as a potential carcinogen (ATSDR 1995). Benzo (a)pyrene is generally recognized as the most potent carcinogen among the PAHs. Several PAHs such as benzo(a)pyrene, chrysene, indeno(1,2,3-cd)pyrene, and benzo (b)fluoranthene have demonstrated carcinogenic, mutagenic, and teratogenic effects on animals (Thyssen et al. 1981; Deutsch-Wenzel et al. 1983). PAHs usually are generated by incomplete combustion of organic matter containing carbon and hydrogen. They can arise from natural combustion such as forest fires and volcanic eruptions and from anthropogenic activities such as vehicle exhausts, power and heat generation plants, residential heating, incinerators, and several industrial processes (e.g., coke production, aluminum smelting) (Harvey 1997). In addition to these anthropogenic sources which traditionally contribute to the majority of PAH input into the environment, the handling of e-waste in Southeast Asia is an important factor. PAHs can be generated as a result of the open burning of cables to extract metal wiring or the open burning of unwanted electronic and plastic components of computers. Plastics are readily combustible and generate thick black smoke and decomposition and volatization products, including PAHs, under open burning conditions (Fu et al. 1997; Simoneit et al. 2005). Richter et al. (1997) have shown that computers (i.e., printed circuit boards) do not contain much PAHs, only small amounts of naphthalene. PAHs have been found in liquid crystal of monitors. However, the pyrolysis and combustion of plastics including polypropylene have been shown to result in the emission of considerable amounts of PAHs (Levin 1987; Wheatley et al. 1993). The incineration of plastics generates different emission factors. Li et al. (2001) reported that incinerating polyvinyl chloride (PVC) would result in an emission factor of 195.4 mg of PAH/kg PVC in bottom ash. The incineration of PVC plastic waste produced a larger percentage of higher-ringed PAHs in bottom ash compared to incineration of high-density polyethylene and polypropylene plastic wastes (Li et al. 2001). The incineration of e-waste results in contamination of the ambient and terrestrial environment and leads to human exposure. As there is limited information available on the extent of PAH contamination in soil and sediment caused by e-waste activities, the major aim of this study was to investigate the

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level, distribution, and source of PAHs in soil at various ewaste sites in Guiyu, in particular acid leaching, printer roller dump site, and open burning sites and also at nearby environs, such as a duck pond and rice fields. PAH levels in sediment were also investigated. It is hoped that the information collected could be utilized in assessing potential health risks associated with soil and sediment contaminated by PAHs.

Experimental section Study area Guiyu, a town in Guangdong Province, China, is located about 250 km northeast of Hong Kong. It has a population of 150,000 and has been involved in e-waste “recycling” for approximately since the mid-1990s. Major recycling activities include the extraction of electrical components and solder recovery from printed circuit boards which are mainly conducted in family-run workshops in the southwestern part of Guiyu, the acid leaching of printed circuit boards, cathode ray tube cracking, plastic chipping and melting, and the open burning of cables for metal recovery. The terrain at the study site is basically flat except for the northern area near the reservoir which consists of hills. The climate at Guiyu is sub-tropical climate with an annual average temperature of 21.5 °C, relative humidity of 80 %, and a mean annual rainfall of 1,721 mm. The prevailing wind is northeast except for in the summertime when the wind blows from the southwest. Sample collection and preparation Soil, sediment, and combusted residue sludge were collected in February 2004 from various e-waste sites which included sites for acid leaching of printed circuit boards, dumping of printer rollers, and e-waste open burning sites. Samples were also collected from a duck pond, rice fields, river tributaries, and water gullies which were located in the vicinity of e-waste open burning sites. Soil and sediment samples taken from a reservoir located in the northern part of Guiyu approximately 6 km from the central e-waste processing area, and soil from Shantou University (30 km from Guiyu), served as control samples. Samples were collected from each of the sites using a stainless steel shovel at a uniform depth of 0–10 cm. To obtain representative samples, a composite sampling strategy of four to five subsamples was used (30×30 m) where possible. All collected samples were wrapped in aluminum foil and stored at −20 °C in the laboratory prior to analysis. Following, they were freeze-dried, sieved (R-1. The sediment in Guiyu was found to be generally lower than PAH concentrations in surface sediment of the Meiliang Bay, Taihu Lake, China (1,207 to 4,754 ng/g, average 2,563) (Qiao et al. 2006) and in sediments in the coastal region off Macao, China (294 to 12,741 ng/g) (Mai et al. 2003), but the average PAH concentration at AL-2 was higher than the average PAH concentration (290 ng/g) of the Yalujiang River (Wu et al. 2003). Also, for comparison, the transect measurement of PAHs in the sediment from the edge of the Mai Po mangroves, Hong Kong, to Inner Deep Bay showed a decrease in PAH concentration from 832 to 425 ng/g (Zheng et al. 2002). The New Dutch List guidelines for soil can also be applied to sediment. There were no exceedances of the Dutch guidelines as the PAH concentrations were approximately 500 ng/g or less. There were also no exceedances of the MPCs (Kalf et al. 1997); however, there were slight exceedances of the Canadian Council of Ministers of the Environment interim sediment quality guideline for naphthalene, phenanthrene, pyrene, and benzo(a)pyrene at AL-2.

Combusted residue


Soil

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Table 7 Carcinogenic PAHs in sediment (nanograms per gram) US EPA PAH

Σ7 carcinogenic PAHsa % Carcinogenic PAHs Swedish EPA (carcinogenic) compounds B(a)P equivalents

Sediment AL-2

DP

RT

WG-1

WG-2

R-1

R-2

257±123a 48.2 316 103±55.0

126±35.3b 41.2 145 30.9±10.4

67.1±20.4bc 47.0 67 13.4±6.2

139±34b 39.9 159 29.3±7.3

121±33.7b 41.0 131 25.1±9.2

2.6±0.8c 6.9 2.6 0.11±0.03

1.9±0.7c 3.7 1.9 0.11±0.05

a

US EPA B2 classification compounds: chrysene, benzo(a)anthracene, benzo(k)fluoranthene, benzo(b)fluoranthene, benzo(a)pyrene, indeno(123cd)pyrene, dibenzo(ah)anthracene b Swedish EPA (carcinogenic) compounds: benzo(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(j)fluoranthene, benzo(k)fluoranthene, dibenz(a,h)anthracene, indeno(1,2,3-cd)pyrene

percentage of carcinogenic PAHs in the combusted residue samples (OB-1, 5.5 %; OB-2, 48.3 %). This may be explained by the fact that since the combusted residue at OB-1 site was collected within a short period after the emission of PAHs, much of the more volatile compound, naphthalene, still remained in the residue. Naphthalene accounted for 37 % of the total PAH concentration at the OB-1 site. For OB-2, naphthalene only accounted for 2.8 % of the total PAH concentration. Benzo(a)pyrene, the most toxic of the PAHs, was not detectable in soils from RF-1, RF-2, R-1, and G; however, it accounted for 15.6, 9.5, 11.5, and 4.6 % of the Σ7 carcinogenic PAHs for AL-1, AL-2, PR, and DP, respectively. For OB-1 and OB-2, the percentages were 7.4 % (75 ng/g) and 22.4 % (1,169±508 ng/g), respectively. The New Dutch List does not have any guidelines pertaining to carcinogenic PAHs exclusively. When comparing the PAH levels to the more stringent Swedish EPA generic guidelines for PAHs in soil for sensitive land use, soils from the AL1, DP, and also G exceeded the guideline of 300 ng/g for carcinogenic PAHs (benzo(a)anthracene, chrysene, benzo

Sediment The total US EPA B2 classification carcinogenic PAHs in sediment ranged from 67.1±20.4 (RT) to 257±123 ng/g

100%

Percentage of different ring numbers

Fig. 2 Percentage of two-to six-ring PAH compounds in soil and combusted residue

(b)fluoranthene, benzo(j)fluoranthene, benzo(k)fluoranthene, dibenz(a,h)anthracene, indeno(1,2,3-cd)pyrene. The acid leaching site, duck pond, and the rice field may be regarded as sensitive land use. The acid leaching site was located along a river inhabited by wild fish. The calculated B(a)P equivalents for the soils and the combusted residues are shown in Table 6 and were the highest at AL-1 and OB-2. For AL-1, the contribution to the total B(a)P equivalents decreased in the following order: benzo(a)pyrene (60 %)>dibenz(a,h)anthracene (15 %)> benzo(b+k)fluoranthene (12 %)>indeno(1,2,3-cd)pyrene (6 %) > benzo(a)anthracene (4.3 %) > chrysene (0.7 %) whereas for OB-2, the order was benzo(a)pyrene (75 %)> benzo(b+k)fluoranthene (7.6 %)>indeno(1,2,3-cd)pyrene (6 %)>benzo(a)anthracene (5 %)>dibenz(a,h)anthracene (4 %)>chrysene (0.7 %).

90% 6-rings

80%

5-rings

70%

4-rings

60%

3-rings 2-rings

50% 40% 30% 20% 10% 0%

AL-1 AL-2 PR DP RF-1 RF-2 R-1 R-2 G OB-1 OB-2

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PAH profiles

6,905 ng/g and the three-ring compounds ranged from 511 ng/g (acenaphthylene) to 5,478 ng/g (phenanthrene). OB-1 contained mainly low molecular weight (two- to three-ring) PAHs (naphthalene, 37 %; phenanthrene, 29 %) whereas the high molecular weight compounds (four to six rings) were all less than 3.5 % of the total PAHs. Combusted residue at OB-2 contained the highest average concentrations for each of the four- to six-ring PAH compounds measured among the sampling sites. Therefore, OB-2 contained higher quantities of higher molecular weight compounds (85.6 %) than the lower molecular weight compounds (14.4 %). In general, naphthalene, phenanthrene, pyrene, and benzo(b+k)fluoranthene were dominant in the soil samples and in combusted residue.

Soil and combusted residue

Sediment

Soil collected from AL-1, AL-2, PR, DP, RF-1, and RF-2 contained two- to six-ring PAHs whereas R-1 only contained two- to four-ring compounds (five- to six-ring compounds were not detectable). Soil at R-2 did not contain any six-ring compounds. In general, high molecular weight PAHs dominate the PAH profiles of urban soils whereas rural soils have been shown to contain predominantly low molecular weight PAHs (Zhang et al. 2006). As shown in Table 3 and Fig. 2, four-ring compounds dominated the soils collected from RF-2 (49 %), RF-1 (45 %), DP (38 %), AL-1 (31 %), and AL-2 (29 %). At AL-1, the dominant compounds were phenanthrene, benzo(b+k)fluoranthene, naphthalene, and pyrene accounting for 14, 14, 12, and 12 % of the total PAHs, respectively. At AL-2, the dominant compounds were naphthalene, benzo(b+k)fluoranthene, phenanthrene, and fluoranthene accounting for 23, 15, 11, and 9 %, respectively. Combusted residue from OB-1 contained the highest concentration of each of the two- to three-ring PAH compounds among all the sampling, whereby naphthalene was

The distribution of the percentages of two- to six-ring PAH compounds is shown in Fig. 3. It was observed that while acenaphthene was low for AL-2, DP, RT, WG-1, and WG-2 (0 to 0.5 %), this compound accounted for 9 and 15 % of the total PAHs for R-1 and R-2, respectively. It was also noted that indeno(1,2,3-cd)pyrene and benzo(g,h,i)perylene (sixring compounds) were significantly higher at AL-2 than the other sites. These compounds accounted for 21 % of the total PAHs at AL-2 and ranged from 6.5 to 13 % for the other sites. The control sites did not contain any detectable five- to six-ring compounds. In general, the dominant compounds in the sediment at all the sites were naphthalene and phrenanthrene, in addition to fluoranthene, chrysene, and pyrene. Benzo(b+k)fluoranthene was predominant in sediment at most sites and ranged from 35 to 97 % of the total PAHs and was non-detectable at R-1 and R-2. With the exception of phenanthrene, the three-ring compounds accounted for only a small portion of the total PAHs. Like the soil samples, the four-ring compounds accounted for the majority of PAHs and ranged from 33 to 40 % (13.4 to

(AL-2) (Table 7) and accounted for 40 to 48 % of the total PAH concentration. Carcinogenic PAHs at the reservoirs were less than 3 ng/g, accounting for 4 to 7 % of the total PAHs. Benzo(a)pyrene was the highest at AL-2 (35.8± 26.1 ng/g) and ranged from non-detectable to 2.7 ± 3.8 ng/g at the other sites. Based on B(a)P equivalents, the toxicity varied from 0.11 to 103 ng/g and was the highest at the acid leaching site. Among the different PAHs, the contribution to the total B(a)P equivalents decreased in the following order: dibenz(a,h)anthracene (53 %) > benzo (a)pyrene (35 %)>benzo(b+k)fluoranthene (9.3 %)>benzo (a)anthracene (1.7 %) and benzo(g,h,i)perylene (0.57 %).

Fig. 3 Percentage of two- to six-ring PAH compounds in sediment

100%

Percentage of different ring numbers

90% 80% 70% 6-rings 60%

5-rings

50%

4-rings 3-rings

40%

2-rings 30% 20% 10% 0%

AL-2

DP

RT

WG-1

WG-2

R-1

R-2


174 ng/g). The proportions of two- and three-ring compounds were the highest at R-1 (64 %) and R-2 (66.3 %).

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(a) Cross plot for the ratio Ant/(Ant+Phe) vs. Fla/(Fla+Pyr)

Sources of PAHs in soil and sediment PAHs generated from a petrogenic-source generally have lower molecular weights with an absence of higher molecular weight compounds, while pyrogenic sources are abundant in higher molecular weight PAHs (Zakaria et al. 2002). Parent PAH ratios have been widely used to detect combustionderived PAH. The source of PAHs may be determined by investigating several PAH isomer ratios. PAHs with the molecular mass 178 (phenanthrene, anthracene) and molecular mass 202 (pyrene, fluoranthene) are commonly used to distinguish between combustion and petroleum sources. The isomer ratio anthracene/(anthracene+phenanthrene) [Ant/(Ant+Phe)] which is 0.10 indicates a combustion source (Budzinski et al. 1997). A fluoranthene/(fluoranthene+pyrene) [Fla/(Fla+Pyr)] isomer ratio 0.50 indicates kerosene, grass, coal, and wood combustion (Yunker et al. 2002). Moreover, other isomer ratios have been used. A benzo (a)anthracene/(benzo(a)anthracene+chrysene), BaA/(BaA+ Chr) ratio less than 0.20 indicates a petroleum source. BaA/(BaA+Chr) ratio from 0.20 to 0.35 indicates either petroleum or combustion, and >0.35 implies combustion. An indeno(1,2,3-cd)pyrene/[indeno(1,2,3-cd)pyrene+benzo(g,h, i)perylene] [IcdP/(IcdP+BghiP)] ratio 0.50 for grass combustion, wood soot, creosote, almost all wood, and coal combustion aerosols. In contrast, combustion products of gasoline, kerosene, diesel, and crude oil all have ratios 0.5) indicated that most of the sites were affected by kerosene, grass, coal, or wood combustion. The ratio at the

8798 Table 8 Exposure factors for a child and an adult (US EPA 1997)

a

US EPA (2004b)


Exposure factors

Child

Adult

Ingestion rate, IngR (mg/day) Inhalation rate, InhR (m3/day) Exposed skin area, SA (cm2) (face, forearms, hands, lower legs, 50th percentile) Skin adherence factor, AFsoil (mg/cm2/day) Body weight, BW (kg) Exposure frequency, EF (days/year) Exposure duration, ED (years) Averaging time For noncarcinogens, AT (days)=(ED×365 days/year) For carcinogens, AT (days)=(70 years×365 days/year)

200 7.6 2,800a 0.2 15 350 6

50 20 5,700a 0.07 70 350 6

2,190 25,550

2,190 25,550

sampling sites ranged from 0.34 to 0.65. In Fig. 4b, the BaA/(BaA+Chr) ratios were between 0.11 and 0.45. Most of the samples, except for RF-1and RF-2, indicated that the PAHs originated from a mixture of petroleum and combustion sources or from combustion alone. IcdP/(IcdP+BghiP) ratios (Fig. 4c) which may serve as a more reliable indicator of PAH sources than the Ant/(Ant+Phe) and BaA/(BaA+ Chr) ratios showed that many of the sampling sites were affected by grass, wood, and coal combustion. Table 9 Calculation of lifetime average daily dose of B(a)P equivalents and cancer risk due to soil ingestion and inhalation exposure pathways

From Khalili et al. (1995), diesel engines and wood combustion are characterized by a high percentage of three-ring PAHs (58.2 to 69.8 %) while gasoline engines and coke ovens have a high percentage of two-ring PAHs (75.8 to 89.7). Only diesel engines generate six-ring PAHs while coke ovens are dominated by two to three rings (49.6 %). When comparing the PAH profiles in Table 3 with those of selected emissions from Khalili et al. (1995), there did not appear to be any strong similarities. Therefore, PAHs

Max B(a)P concentration (ng/g, dry wt)

Ingestion AL-1 AL-2 PR DP RF-1 RF-2 R-1 R-2 G OB-1 OB-2 Inhalation AL-1

Child

Adult

LADD

Cancer risk

LADD

Cancer risk

645 46.7 35.7 124 14.7 7.01 1.33 4.5 63.0 168 2,220

7.07E−07 5.12E−08 3.91E−08 1.36E−07 1.61E−08 7.68E−09 1.46E−09 4.93E−09 6.90E−08 1.84E−07 2.42E−06

5.16E−06 3.74E−07 2.86E−07 9.92E−07 1.18E−07 5.61E−08 1.06E−08 3.60E−08 5.04E−07 1.34E−06 1.77E−05

3.79E−08 2.74E−09 2.10E−09 7.28E−09 8.63E−10 4.12E−10 7.81E−11 2.64E−10 3.70E−09 9.86E−09 1.30E−07

2.76E−07 2.00E−08 1.53E−08 5.31E−08 6.30E−09 3.00E−09 5.70E−10 1.93E−09 2.70E−08 7.20E−08 9.48E−07

645

1.98E−11

1.44E−10

1.11E−11

8.13E−11

AL-2 PR DP RF-1 RF-2 R-1 R-2 G OB-1

46.7 35.7 124 14.7 7.01 1.33 4.5 63.0 168

1.43E−12 1.09.E−12 3.80E−12 4.50E−13 2.15E−13 4.07E−14 1.382E−13 1.93E−12 5.14E−12

1.04E−11 7.98E−12 2.77E−11 3.29E−12 1.57E−12 2.97E−13 1.01E−12 1.41E−11 3.76E−11

8.06E−13 6.16E−13 2.14E−12 2.54E−13 1.21E−13 2.30E−14 7.77E−14 1.09E−12 2.90E−12

5.89E−12 4.50E−12 1.56E−11 1.85E−12 8.84E−13 1.68E−13 5.67E−13 7.94E−12 2.12E−11

OB-2

2,220

6.78E−11

4.95E−10

3.82E−11

2.79E−10


from vehicle emissions were not a major contributing factor of PAHs at the e-waste sampling sites. Health risk assessment Exposure in humans to PAHs is mostly through ingestion and inhalation; however, skin toxicity such as dermatitis and keratosis has been demonstrated to be caused by occupational exposures to PAHs. PAHs are generally well absorbed in the body but are stored only briefly mainly in the kidney, liver, and spleen. Most of the PAHs are metabolized in the body making it more water soluble, thus facilitating excretion in feces and, to a lesser extent, in urine (ATSDR 1995). A risk assessment was conducted using the toxic equivalency factors proposed by Nisbet and LaGoy (1992) to calculate the average and maximum B(a)P equivalent daily doses for oral intake, dermal, and inhalation exposure. Exposure factors are shown in Table 8. For the three exposure pathways, soil ingestion would account for the highest daily intake of B(a)P equivalent. For a child, the soil ingestion pathway would be approximately 2.7 times higher than the dermal absorption pathway; however, for an adult, the average daily doses for soil ingestion and dermal intake were approximately the same. Since B(a)P is a carcinogen, the lifetime average daily doses for the different scenarios were also determined in order to calculate the cancer risk. Slope factors are only available for oral [7.3 1/(mg/kg/day)] and inhalation [7.3 1/(mg/kg/day)] (US EPA 2004a) exposures, and the accumulative cancer risks for these exposure routes were calculated as shown in Table 9. The highest cancer risk (1.8×10−5) would be for a child who ingested soil having similar B(a)P value as the combusted residue at OB-2. All of the other calculated risk were deemed acceptable risks since they were smaller than 1 in a million. It should be noted that the use of B(a)P toxicity equivalency factors may not be always suitable because the carcinogenic risk of PAH mixtures is highly dependent on the exposure pathway (Schneider et al. 2002).

Conclusions Soil, sediment, and combusted residues collected from a town with intense e-waste activities and e-waste dumping consisted of a wide concentration range of PAHs. The total PAH concentrations in the soil ranged from 95.2±54.2 ng/g at a reservoir approximately 6 km north of the central e-waste processing center to 1,950±1,320 ng/g at an acid leaching site. Combusted residues contained the highest concentration of PAHs. At a site located beside a rice field where cables and electrical components were openly burnt, the concentration was 18,600 ng/g. Combusted residues at e-waste open burning sites contained the highest concentrations of carcinogenic

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PAHs (i.e., 5,220 and 1,020 ng/g). The level of PAH contamination at the sampling sites followed the trend: open burn sites>acid leaching sites>duck pond>rice fields>printer roller dumpsite>reservoir. In the sediment samples, the highest PAH concentration was found at the acid leaching site (534± 271 ng/g), and the lowest was found at the Guiyu reservoir (37.2±6 ng/g). There was exceedance of the New Dutch List Optimum Value for PAHs in soil collected from one of the acid leaching sites. Most of the PAH concentrations in sediment were found to be lower than in soil at the same sites. Results of this study provide further evidence of the significant input of PAHs to the environment attributed to crude ewaste recycling. Of the e-waste activities investigated, open burning and acid leaching emitted the highest PAH concentrations. Although health risk assessment estimated the cancer risk due to oral intake and inhalation of PAH contaminated soil to be low for these study sites based on B(a)P equivalents, it is recommended that further studies on human health be undertaken. Acknowledgments We thank colleagues and students of the Croucher Institute for Environmental Sciences and Department of Biology, Hong Kong Baptist University for field and technical assistance. Financial support for this work was sponsored by The Research Grants Council of the University Grants Committee of Hong Kong (Central Allocation Group Research Project HKBU 1/03C) and Special Equipment Grant SEG HKBU09).

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