contamination investigation and risk assessment of molybdenum on an industrial site in china

9
Contamination investigation and risk assessment of molybdenum on an industrial site in China Chunnu Geng a,b,c,d,1 , Yangjun Gao d,2 , Dan Li d,2 , Xuping Jian b,1 , Qinhong Hu e,f, a The Ecological Technique and Engineering College, Shanghai Institute of Technology, No. 100 Haiquan Road, Fengxian District, Shanghai 201418, China b Institute of Urban Environment, Chinese Academy of Sciences, 1799 Jimei Road, Xiamen 361021, PR China c Institute of Soil Science, Chinese Academy of Sciences, 71 East Beijing Road, Nanjing 210008, PR China d Shanghai Academy of Environmental Sciences, 508 Qinzhou Road, Shanghai 200233, China e College of Environmental Studies, China University of Geosciences, Wuhan 430074, PR China f Department of Earth and Environmental Sciences, The University of Texas at Arlington, 500 Yates Street, Arlington, TX 76019, USA abstract article info Article history: Received 13 April 2013 Accepted 29 December 2013 Available online xxxx Keywords: Mo contamination Soil Groundwater Risk assessment Transport Because certied reference standards on molybdenum (Mo) in soil are not available, and drafted risk assessment procedures in China have not included Mo. Assessment of the potential risks posed by Mo in soil and groundwa- ter is the key to establishing the extent of the contamination and deriving achievable remedial targets, should re- mediation deemed to be necessary. This paper reports the rst investigation of Mo contamination in soil and groundwater from an incandescent light-bulb manufacturing facility in China. Though the plant was built in 1971, the contamination of Mo in the site received little attention until the 1994 and 2008 monitoring campaigns; soil Mo concentrations ranged from not-detectable to 15 mg/kg in 1994, and from 0.25 to 252 mg/kg in 2008. In addition, groundwater Mo concentrations ranged from not-detectable to 362 μg/L in 1994, and 1 to 32, 500 μg/L in 2008. Simulation by Visual MINTEQ software showed that Mo speciation in the groundwater would be limited to MoO 4 2. Detailed site investigation showed that the high concentrations of Mo in groundwater could be adequately explained by the predominant presence of anionic MoO 4 2at the measured average soil pH of 8.65, and given the small adsorption coefcient 112 L/kg for Mo onto eld soil samples. A modied three-step sequen- tial extraction procedure showed that residual percentages of soil Mo at this industrial site ranged from 69.8 to 81.0%, indicating that Mo in the soil was mostly present in the mineral lattice. Bounding of Mo onto the mineral lattice is not available to living organisms and thus the risk from soil contamination was considered minimal. A conceptual site model developed for quantitative risk assessment indicated that the main exposure pathways would be consumption of the groundwater by local residents, and lateral Mo migration to bounding rivers. Only non-carcinogenic risk was assessed, because Mo has no known carcinogenic effect on living organisms, as indicated from the toxicity data for Mo. Among the 35 assessment locations, Mo in 14 locations was determined to pose un- acceptable non-cancer risk for on-site children, with a hazard quotient from 1.2 to 433 when children directly drink groundwater. In addition, Mo in groundwater which is transported off-site by lateral migration will pose unaccept- able non-cancer risks for off-site children, with a hazard quotient of up to 45.8 from direct water drinking. Although Mo concentrations in the bounding rivers ranged from 4 to 6,053 μg/L, toxicity data indicated that the ecological risk is minimal for aquatic biota in the surface water. A site-specic target level for Mo in groundwater was established as 75.1 μg/L. Further work will be conducted regarding remediation feasibility at this site; permeable reactive bar- riers may be an effective option given the predominant distribution of Mo in the groundwater in the site. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Due to the rapid urbanization in China, many factories have been relocated from the urban area to the suburb, where the factory sites are redeveloped for residential or municipal use. Some regulations have been drafted in China for site investigation, monitoring, risk as- sessment and remediation. However, most of these regulations are di- rectly taken from other countries, not adapted to specic Chinese conditions. Some case studies have been addressing these open issues (Chen et al., 2006; Geng et al., 2010). Journal of Geochemical Exploration xxx (2014) xxxxxx Corresponding author at: College of Environmental Studies, China University of Geosciences, Wuhan 430074, PR China. Tel.: +1 86 27 6788 3512; fax: +1 86 27 8743 6235. E-mail addresses: [email protected] (C. Geng), [email protected] (Y. Gao), [email protected] (D. Li), [email protected] (X. Jian), [email protected] (Q. Hu). 1 Tel./fax: +86 592 6190 560. 2 Tel.: +86 21 6408 5119; fax: +86 21 021 64840680. GEXPLO-05275; No of Pages 9 0375-6742/$ see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gexplo.2013.12.014 Contents lists available at ScienceDirect Journal of Geochemical Exploration journal homepage: www.elsevier.com/locate/jgeoexp Please cite this article as: Geng, C., et al., Contamination investigation and risk assessment of molybdenum on an industrial site in China, J. Geochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2013.12.014

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Journal of Geochemical Exploration xxx (2014) xxx–xxx

GEXPLO-05275; No of Pages 9

Contents lists available at ScienceDirect

Journal of Geochemical Exploration

j ourna l homepage: www.e lsev ie r .com/ locate / jgeoexp

Contamination investigation and risk assessment of molybdenum on an industrial sitein China

Chunnu Geng a,b,c,d,1, Yangjun Gao d,2, Dan Li d,2, Xuping Jian b,1, Qinhong Hu e,f,⁎a The Ecological Technique and Engineering College, Shanghai Institute of Technology, No. 100 Haiquan Road, Fengxian District, Shanghai 201418, Chinab Institute of Urban Environment, Chinese Academy of Sciences, 1799 Jimei Road, Xiamen 361021, PR Chinac Institute of Soil Science, Chinese Academy of Sciences, 71 East Beijing Road, Nanjing 210008, PR Chinad Shanghai Academy of Environmental Sciences, 508 Qinzhou Road, Shanghai 200233, Chinae College of Environmental Studies, China University of Geosciences, Wuhan 430074, PR Chinaf Department of Earth and Environmental Sciences, The University of Texas at Arlington, 500 Yates Street, Arlington, TX 76019, USA

⁎ Corresponding author at: College of EnvironmentaGeosciences, Wuhan 430074, PR China. Tel.: +1 86 27 66235.

E-mail addresses: [email protected] (C. [email protected] (D. Li), [email protected] (X(Q. Hu).

1 Tel./fax: +86 592 6190 560.2 Tel.: +86 21 6408 5119; fax: +86 21 021 64840680.

0375-6742/$ – see front matter © 2014 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.gexplo.2013.12.014

Please cite this article as: Geng, C., et al., CoGeochem. Explor. (2014), http://dx.doi.org/1

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 April 2013Accepted 29 December 2013Available online xxxx

Keywords:Mo contaminationSoilGroundwaterRisk assessmentTransport

Because certified reference standards onmolybdenum (Mo) in soil are not available, and drafted risk assessmentprocedures in China have not includedMo. Assessment of the potential risks posed byMo in soil and groundwa-ter is the key to establishing the extent of the contamination and deriving achievable remedial targets, should re-mediation deemed to be necessary. This paper reports the first investigation of Mo contamination in soil andgroundwater from an incandescent light-bulb manufacturing facility in China. Though the plant was built in1971, the contamination ofMo in the site received little attention until the 1994 and 2008monitoring campaigns;soil Mo concentrations ranged from not-detectable to 15 mg/kg in 1994, and from 0.25 to 252 mg/kg in 2008. Inaddition, groundwater Mo concentrations ranged from not-detectable to 362 μg/L in 1994, and 1 to 32, 500 μg/Lin 2008. Simulation by Visual MINTEQ software showed thatMo speciation in the groundwaterwould be limitedto MoO4

2−. Detailed site investigation showed that the high concentrations of Mo in groundwater could beadequately explained by the predominant presence of anionic MoO4

2− at the measured average soil pH of 8.65,and given the small adsorption coefficient 112 L/kg forMo onto field soil samples. Amodified three-step sequen-tial extraction procedure showed that residual percentages of soil Mo at this industrial site ranged from 69.8 to81.0%, indicating that Mo in the soil was mostly present in the mineral lattice. Bounding of Mo onto the minerallattice is not available to living organisms and thus the risk from soil contamination was considered minimal.A conceptual site model developed for quantitative risk assessment indicated that the main exposure pathwayswould be consumption of the groundwater by local residents, and lateral Mo migration to bounding rivers. Onlynon-carcinogenic riskwas assessed, becauseMo has no known carcinogenic effect on living organisms, as indicatedfrom the toxicity data for Mo. Among the 35 assessment locations, Mo in 14 locations was determined to pose un-acceptable non-cancer risk for on-site children,with a hazard quotient from 1.2 to 433when children directly drinkgroundwater. In addition, Mo in groundwater which is transported off-site by lateral migrationwill pose unaccept-able non-cancer risks for off-site children, with a hazard quotient of up to 45.8 fromdirectwater drinking. AlthoughMo concentrations in the bounding rivers ranged from4 to6,053 μg/L, toxicity data indicated that the ecological riskis minimal for aquatic biota in the surface water. A site-specific target level for Mo in groundwater was establishedas 75.1 μg/L. Further work will be conducted regarding remediation feasibility at this site; permeable reactive bar-riers may be an effective option given the predominant distribution of Mo in the groundwater in the site.

© 2014 Elsevier B.V. All rights reserved.

l Studies, China University of788 3512; fax: +1 86 27 8743

), [email protected] (Y. Gao),. Jian), [email protected]

ghts reserved.

ntamination investigation a0.1016/j.gexplo.2013.12.014

1. Introduction

Due to the rapid urbanization in China, many factories have beenrelocated from the urban area to the suburb, where the factory sitesare redeveloped for residential or municipal use. Some regulationshave been drafted in China for site investigation, monitoring, risk as-sessment and remediation. However, most of these regulations are di-rectly taken from other countries, not adapted to specific Chineseconditions. Some case studies have been addressing these open issues(Chen et al., 2006; Geng et al., 2010).

nd risk assessment of molybdenum on an industrial site in China, J.

2 C. Geng et al. / Journal of Geochemical Exploration xxx (2014) xxx–xxx

Molybdenum (Mo) is a necessary plant and human micronutrient(O'Connor et al., 2001; Sequi, 1973), but Mo in the high concentrationmay cause copper deficiency in cattle (Elliott & Taylor, 2000; O'Connoret al., 2001). Receiving less attention in contaminated sites, the regula-tion of Mo in soil and water has not been a priority in the past due tothe relatively low concentrations found in most groundwater and soils(Ecometrix Inc., 2007). However, Mo has been found at elevated con-centrations in the irrigation drainage of arid agricultural soils (Lemly,1994; O'Connor et al., 2001). Though Mo concentration in soils andgroundwater near, and in contact with, Mo ore has been investigatedin China, these investigations have produced no risk assessment forMo (Cong et al., 2009; Qu et al., 2007) largely because certified referencestandards on soil were not available in the current laws and regulationsin China (MEP, 1995).

Furthermore, Mo concentrations can be high at sites where Mo isused as the main industrial material and subsequently discharged intowastewater. For example, during the manufacturing of tungsten fila-ments for electric lamps, tungsten coils are double-wound around Momandrel wire or rods, which are then separated from the coil by the dis-solution of Mo in a reagent composed of nitric and sulfuric acids. Suchoperation results in the generation of large volumes of spent acid con-taining dissolved Mo (Mukherjee et al., 1988). Although some effortshave been extended to recover Mo from such spent acid, much Mohas been discharged into the environment as wastewater (Liu, 2006;Mukherjee et al., 1988).

However, little or no information is currently available on Mo con-centrations in soils and groundwater of lamp-making plants, especiallyfor plants built in the 1990s in China. Moreover, in the current Chinesestandards on Mo, the only available certified reference standards arefor groundwater and drinking water; no certified reference standardsare available for soil and surface water. Therefore, the objectives of thepresent work are to: 1) investigate the occurrence and concentrationsof Mo in soils and groundwater in an industrial site in Shanghai whereMo was historically and widely used during incandescent light-bulbmanufacturing processes; 2) identifyMo fractions in soil and speciationin groundwater; 3) propose and analyze the possible mechanism of Moaccumulation in groundwater; 4) undertake quantitative risk assess-ments to evaluate the potential risks of the site to human health andecological receptors, and to identify exposure pathways that may besubject to possible groundwater remediation; and5) derive site-specifictarget level (SSTL) for remediation validation.

2. Materials and methods

2.1. Environmental settings of the industrial site

The general area inwhich the site is located includes both residentialand industrial zones in Shanghai, China. The site and its surroundingarea are generally flat. The east side of the site is immediately boundedby a paved-road, and the south is bounded by residential areas, beyondwhich is a surface water body; both the west and the north sides arebounded by a surface water body (Fig. 1). Site reconnaissance revealsthat no groundwater abstraction well is present onsite, but ground-water has been used in surrounding residential areas for washingpurposes.

2.2. Field sampling and chemical measurements

The site was used for incandescent light-bulb manufacturing from1971 to 2008; the operational layout of the site is shown in Fig. 1. Theprimary operations at the site were coil production and light-bulb as-sembly. Detailed site investigations were performed in 1994 and 2008.

The selection of soil borings and groundwater monitoring wellswere based on the detailed phase I investigation of due diligence andthe characterization of contamination sources, such as near wastedumping areas or leaching areas. In 1994, 6 soil bores were drilled,

Please cite this article as: Geng, C., et al., Contamination investigation aGeochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2013.12.014

and 3 groundwater monitoring wells were installed to the depth of5 m. In 2008, 70 soil borings were drilled to the groundwater levelwith a maximum depth of 6 m, and one soil sample was collectedfrom each soil boring at the depth of 0.1–1.5 m based on the results ofX-ray fluorescent analyzers. Following the drilling, 35 of the soil boringswere converted to groundwater monitoring wells with 75-mm diame-ter PVC casing, in accordance with the U.S. Environmental ProtectionAgency's RCRA Orientation Manual (USEPA, 2003). The layout of these35 monitoring wells is shown in Fig. 1. During the 2008 site investiga-tion, one groundwater sample was collected from each monitoringwell. In 2010, groundwater samples were collected again from eachmonitoring well and analyzed for Mo.

Soil sampleswere digested by acid and the concentrations ofMo andarsenic were analyzed by inductively coupled plasma-mass spectrome-try (ICP-MS, Agilent 7500cx, Agilent Inc., USA); the concentrations ofMo and arsenic in groundwater were directly analyzed by ICP-MSafter filtration with 0.45 μm membrane. Soil was extracted by Milli-Qwater, and measured by pH meter (Lu, 1999), while groundwater pHvalue was directly analyzed by pH meter (MEP, 2002b). In addition,two additional soils (#1 and #2)were sampled from the site by the dig-ging bucket when the sitewas remediated, with the depth of 0.5–1.0 m.Samples #1 and #2 were near MW31 and MW19, respectively, whichare located in the north-east and south-west areas of the site, respec-tively (shown in Fig. 1). The soil samples were air-dried, sieved through2 mm screens, and analyzed for basic properties (results are shownin Table 1). Carbon (C), nitrogen (N), and sulfur (S) were analyzed byVario Max CNS analyzer (Elementar Inc., Germany), following ap-proaches of Guo et al. (2011), Lin et al. (2004) and Zhang (2004). Solu-ble bromine (Br) and chlorine (Cl) were extracted byMilli-Q water andanalyzed by ICP-MS according to Lu and Liu (2011) and SBQTS (2010).Soil texture was analyzed, according to Li et al. (2011) and Jiang et al.(2005), by the following procedure: the soilwasfirst treatedwith sodiumhexametaphosphate for dispersion, then carbon was removed by adding6% H2O2 and dilute HCl in succession, and finally the treated soil was an-alyzed by the laser particle sizer (Mastersizer 2000,Malvern Inc., UK). Themeasurement of cation exchange capacity (CEC) followed the method ofLu (1999). CEC ranged from 17.4 ± 0.6 to 21.7 ± 0.1 cmol/kg and soilswere predominantly silty in texture.

2.3. Geology and hydrogeology

The surface and subsurface profile encountered in the boreholesconsisted of a layer of fill material (including concrete surface base),consisting of silty clay with gravel and/or cobbles/boulders or coal ash,extending to depths of 0.5–3.0 m below ground surface (bgs). The filllayer was underlain by clay to depth of at least 6 m, the maximumdepth of the boreholes. The clay can be further divided into upper andlower clay layers. The upper clay is brown to gray-brown, firm, of highplasticity, and appears saturated or nearly so. The lower clay layer isgenerally gray, soft, saturated and of high plasticity.

Groundwater levels encountered in the boreholes ranged from0.6 to2.7 m bgs. Static groundwater levels in the monitoring wells gauged on16 June 2008 ranged from 0.35 to 2.91 m bgs. Based on the staticgroundwater elevations in the wells, the shallow groundwater flow inthe immediate vicinity of the site appears to be toward the surroundingareas and local rivers. More regional groundwater and surface waterflow is assumed to be from southwest to northeast, towards the YangtzeRiver about 100 km away from the site.

2.4. Mo fractionation in soil

Amodified three-step sequential extraction procedure recommend-ed by the Commission of European Communities Bureau of Reference(BCR), and proposed by Žemberyová et al. (2010), was used to deter-mine Mo fractions in the two industrial soil samples (soils #1 and #2).In addition, pseudo-total metal contents, both for the original soils and

nd risk assessment of molybdenum on an industrial site in China, J.

North

Fig. 1. The site layout of monitoring wells for the light-bulb plant. Circles are themonitoring wells based on the year 2008monitoring campaign; rectangles and triangles are the soil bor-ings andmonitoring wells based on the year 1994 monitoring campaign. Arrows near the monitoring wells MW19 andMW31 indicate the locations where soils #2 and #1 are sampled.(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3C. Geng et al. / Journal of Geochemical Exploration xxx (2014) xxx–xxx

for the residue after the three-step sequential extraction, were deter-mined by digestion with aqua regia using the following steps: a soilamount of 0.15 g was weighed into a reaction vessel; 5 ml aqua regiawas then added and allowed to stand for 16 h (overnight) at room tem-perature, to permit slow oxidation of the organic matter in the soil; thesamples were then digested at 120 °C for 13 h; after cooling, 4 ml con-centrated HClO4 was added and digested at 140 °C for another 13 h;

Table 1Properties of the soils.

Soil #1 #2

C (g/kg) 19.7 ± 0.2 9.7 ± 0.6N (g/kg) 0.93 ± 0.03 0.87 ± 0.07S (g/kg) 0.88 ± 0.00 0.25 ± 0.00pH 8.5 ± 0.04 8.3 ± 0.01CEC (cmol/kg) 21.7 ± 0.1 17.4 ± 0.6Cl (mg/kg) 140 ± 10.6 5.67 ± 1.31Br (mg/kg) 0.45 ± 0.11 0.30 ± 0.00K (mg/kg) 302 ± 9.96 326 ± 22.8Clay (%) 12 10Silt (%) 86 87Sand (%) 2 3

Please cite this article as: Geng, C., et al., Contamination investigation aGeochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2013.12.014

after another cooling period, 2 ml concentrated H2O2 was added anddigested until the supernatant became transparent.

2.5. Mo speciation and potential transport in groundwater

Visual Minteq V 4.0 (HydroGeoLogic Inc. & Allison GeoscienceConsultants, Inc., 1998) was used to calculate Mo speciation in ground-water at the site. The conductivity measured in the monitoring wellsranged from 757 to 1665 μs/cm, indicating that electrolyte concentra-tion in the groundwater was high due to leaching from the waste.There is no available relationship between the conductivity and theelectrolyte concentration. A background electrolyte concentration of0.01 M was assumed, and a measured average pH of 8.65 from thissite was used for calculation.

Based on the 2008 monitoring results, it was recognized that Moconcentrations in the groundwater were extremely high, and that Momight be transported offsite, posing a potential risk to off-site receptors.The possible off-site receptors are the local streams or off-site points ofexposure (POEs). Three POEs were identified during the site reconnais-sance.Mo concentrationswere calculated based on theDomenico solutetransport model (Connor et al., 2008) and the parameters provided inASTM E-2081 (ASTM, 2000).

nd risk assessment of molybdenum on an industrial site in China, J.

4 C. Geng et al. / Journal of Geochemical Exploration xxx (2014) xxx–xxx

2.6. Quantitative risk assessment

Many countries, including the USA and the UK, utilize a multi-tieredrisk-basedmethodology in regulating andmanaging contaminated sites(ASTM, 2000; UKEA, 2009). The risk-based methodology providesquantitative methods for the estimation of human and ecological risksusing analytical models of contaminant fate and transport, and assess-ments of human and ecological exposure. Common to these approachesare the adaptation of amulti-tiered framework, and the requirement fordeveloping risk-based screening levels in an earlier tier, and site-specifictarget levels in a later tier, thus allowing risk assessment to be undertak-en in a progressive and cost-effective manner.

The RBCA (Risk Based Corrective Action) toolkit for chemical re-leases, largely based on the ASTM RBCA Guidance (American Societyfor Testing & Materials, 2000) and accepted worldwide, was used inthis work to assess human health and environmental risks for the site.For exposure assessment, a bodyweight of 14.4 kg for children up to6 years old, an exposure frequency of 365 days/year and an exposureduration of 6 years were used to calculate non-carcinogenic hazard(Geng et al., 2010; UKEA, 2009). The water ingestion rate was taken as1 L/day for children, according to USEPA exposure factors handbook(USEPA, 1989). The reference dose for the Mo oral pathway was takenas 0.005 mg/kg/day, based on the USEPA Integrated Risk InformationSystem (USEPA, 2013a). A threshold limit of one for non-carcinogenichazard is compared with the calculated hazard index.

3. Results and discussion

3.1. Mo concentrations in soil and groundwater

In 1994, Mo concentrations in 6 soil boreholes ranged from ND (notdetectable) to 15 mg/kg. In 1994, the highest Mo concentration ingroundwater was 362 μg/L detected in SB6, and this may have beendue to the fact that SB6 was situated in an area of waste dumping, and

Fig. 2.Mo concentration contours (μg/L) in groundwater (based on the year 2008monitoring cgroundwater monitoring wells.

Please cite this article as: Geng, C., et al., Contamination investigation aGeochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2013.12.014

that near-surface groundwater had been impacted by leaching ofmetalsfrom the waste materials (Fig. 1).

Because certified reference standards are not available in China, theDutch intervention value of 190 mg/kg was selected as the assessmentcriterion. In 2008, Mo concentrations in soils ranged from 0.25 to252 mg/kg among 70 soil samples. Most of the soil samples werebelow theDutch intervention value of 190 mg/kg. However, one samplefrommonitoring well MW31was found to contain 252 mg/kg, exceed-ing the Dutch intervention value, though it was still below the regionalscreening level (5,100 mg/kg; USEPA, 2013b). During the 2008 moni-toring campaign, Mo concentrations in 13 out of 35 groundwater sam-ples ranged between 126 μg/L and 32, 500 μg/L, exceeding the Class IIIlimit (100 μg/L) of the Chinese Quality Standards for Groundwater(CQSG). In comparison, the Dutch intervention value for Mo in ground-water is 300 μg/L. Fig. 2 shows the Mo contours in groundwater as of2008. Using the CQSG standard (100 μg/L), groundwater contaminatedwithMo occurred over about 60% of the site area at that time. A hot areaoccurs in the middle of the site, aroundMW33; another hot area is evi-dent in the southern part of the site, around MW6. No contaminationwas found in the western part of the MW23 and MW24 area. This canbe related to the fact that the paved road may be built after the soilwas contaminated, and this area was unpaved, and therefore was prob-ably an area of high infiltration and recharge. In 2010, Mo concentra-tions in groundwater monitoring wells ranged from ND to 19,300 μg/L. Concentrations of Mo in most groundwater monitoring wells werelower than those in 2008, with some exceptions. For example, com-pared to 2008, Mo concentrations in MW8, MW30 and MW31 were al-most doubled in 2010.

Incandescent light-bulb manufacturing on the site began in 1971and ceased in 2008. It is probable that Mo contamination was mainlyfromproduction during the period from1994 to 2008, as themonitoringresults in 1994 showed much less contamination than those in 2008.Soil Mo concentrations ranged from ND to 15 mg/kg in 1994, andfrom 0.25 to 252 mg/kg in 2008; groundwater Mo concentrationsranged from ND to 362 μg/L in 1994 and from 1 to 32,500 μg/L in

ampaign). The Kriging interpolationmethod was used to estimate the contours among the

nd risk assessment of molybdenum on an industrial site in China, J.

Table 2Mo concentration ranges detected in soil and groundwater during 1994 and 2008monitoring campaigns and the corresponding environmental quality standards.

Soil (mg/kg) Groundwater (μg/L)

1994 ND–15a ND–362a

2008 0.25–252 1–32,5002010 ND–19,300China n/ab 100c

Dutch 190d 300d

a ND: not detectable.b n/a: not available.c Class III category of Chinese Quality Standards for Groundwater (CBTS, 1994).d Dutch intervention value (VROM, 2009).

5C. Geng et al. / Journal of Geochemical Exploration xxx (2014) xxx–xxx

2008 (Table 2). For purposes of comparison, Mo concentrations in thesoils of a molybdenum mining, ore collection, and ore transportationarea have been measured in the range 721 to 21,367 mg/kg (Qu et al.,2007).Mo concentrations in both soil and groundwater have also been in-vestigated in an agricultural area close to the mining area describedabove, and provide a further basis for comparison. Soil concentrations inthe agricultural area were measured as 305 mg/kg, and groundwaterconcentrations as 13.2 mg/L (Cong et al., 2009). These results fall in theranges measured at the industrial site of this study.

3.2. Soil pH and Kd value of Mo sorption onto soil

Soil pH values were measured to range from pH 7.3 to 12.3, with anaverage soil pH of 8.65. The sorption coefficients Kd (in liters per kilo-gramdry soil) were determined asKd = Qs/Qw,where Qw (inmilligramper liter) was taken as the Mo concentration in groundwater from a

Table 3Soil pH, Mo concentrations in soils and field Kd values in 35 monitoring wells.

Monitoringwell

SoilpH

Soil Mo(mg/kg)

Kd

(L/kg)Soil arsenic(mg/kg)

Kd

(L/kg)

MW1 9.2 1.28 13.9 18.1 1508MW2 8.3 0.53 132.5 7.9 1580MW3 8.6 0.96 137.1 9.8 1960MW4 12.3 33.5 62.0 4.1 820MW5 8.4 0.72 240.0 11.2 2240MW6 8.3 1.75 0.2 13 813MW7 8.6 1.15 104.5 12.6 2520MW8 9.1 16.2 4.5 25.8 1290MW9 8.4 9.19 1.8 10.6 2120MW10 7.9 0.25 2.0 9.4 1880MW11 8.5 0.51 72.9 7.5 1500MW12 7.6 0.91 13.8 11.5 2300MW13 8.5 0.75 57.7 7.6 1520MW14 9.1 0.73 243.3 6.9 1380MW15 8.5 16.5 54.5 9.8 188MW16 8.4 0.53 2.0 10.6 2120MW17 8.7 0.43 0.4 9.6 1920MW18 8.4 0.52 86.7 10.1 2020MW19 8.2 0.51 39.2 14.1 2820MW20 8.5 0.7 350.0 10.2 2040MW21 9.2 0.46 230.0 11.2 2240MW22 8.6 0.94 470.0 11.1 2220MW23 8.5 0.56 560.0 13.3 2660MW24 8.8 0.37 185.0 8.5 1700MW25 8.8 0.31 77.5 8.4 1680MW26 9.1 0.38 54.3 7.9 1580MW27 9.1 0.73 12.0 9.7 1940MW28 8.0 1.52 126.7 8.8 629MW29 8.2 127 77.0 66.3 13,260MW30 8.8 10.5 4.1 11.3 84MW31 8.3 252 52.6 14 2800MW32 8.1 0.35 0.4 6 133MW33 9.0 25.7 0.8 10.1 721MW34 8.2 0.48 160.0 9 1800MW35 8.6 0.91 303.3 10.8 982Average 8.65 14.57 112 12 1971

Please cite this article as: Geng, C., et al., Contamination investigation aGeochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2013.12.014

given monitoring well, and Qs (in milligram per kilogram dry soil)was taken as the Mo concentration in soil from the soil boring corre-sponding to that monitoring well. Values of Kd for Mo on the site werefound to range from 0.2 to 560 L/kg, with a mean value of 112 L/kg(Table 3). The RBCA toolkit chemical database gives the value of Kd forMo as 20.0 L/kg (Connor et al., 2008), which is lower than the averageKd determined for our site. Again at our site, the mean value of field Kd

for arsenic was 1, 971 L/kg, as determined by using 35 pairs of soiland groundwater data, where in each case the monitoring well supply-ing the water sample corresponded to the soil boring supplying the soilsample (Table 3). These results indicate that Mo at this industrial sitehas a greater tendency to sorb than the RBCA database would predict,but a much lower tendency to sorb than does arsenic at the same site.

Adsorption of Mo onto soils and soil minerals has been well studied(e.g., Goldberg & Forster, 1998; Goldberg et al., 1996, 1998; Manning &Goldberg, 1996; O'Connor, 2001). These studies report an adsorptionmaximum at pH 3–5 and significantly less adsorption at higher pHvalues (up to pH 8). In acidic soils, Mo exists in soil pore water as aweak acid (H2MoO4), but is likely to exist primarily as MoO4

2− in sys-tems with pH N4. MoO4

2− is increasingly soluble at high pH; for exam-ple, elevated concentrations of Mo were reported in limed and/oralkaline streams (Borg et al., 2001; Sjöstedt et al., 2009). These findingscan be used to explainwhyMowashighly concentrated in groundwateron our contaminated site because soil pH values ranged between pH 7.3and 12.3.

This hypothesis is further supported by our laboratory results, (Genget al., in preparation). These results show a maximum capacity of0.01 mg/kg Mo adsorption onto soil #2 when a solution containing27 mg/L Mo was used in flow-cell experiments, and Kd was only0.19 L/kg by batch-sorption equilibration experiments. Similarly,Carroll et al. (2006) measured Mo adsorption in an agricultural soiland the biosolid-amended soil, with a small Kd value of 0.35 versus1.3 L/kg.

3.3. Mo distribution in soil and groundwater

To explain the difference ofMo distribution in soil and groundwater, amodified BCR extraction approach was used to characterize the Mo frac-tion in soil. Pseudo-total concentrations of Mo in #1 and #2 soils were8.1 ± 0.1 and 260.2 ± 10.2 mg/kg, both of which are much higher thanthe Mo background value in Shanghai soils, 0.8 ± 0.15 mg/kg (MEP,1990), indicating that the site soil was seriously contaminated by light-bulb manufacturing processes. To assess the accuracy of the modifiedBCR three-step extraction procedure, it is useful to compare the sum ofthe Mo content in the extracts and aqua-regia leachable contents in theresidue with the pseudo-total aqua-regia leachable contents in the origi-nal sample (Žemberyová et al., 2010). These results are included inTable 4. Good agreement was found, with recoveries ranging from 113to 126%.

The content of extractable Mo from all steps of the sequential extrac-tion procedurewas comparedwith the aqua-regia leachable contents andthe distribution ofMo in the following respective fractions: (1) exchange-able and associated with carbonates; (2) associated with easily andmod-erately reducible iron (Fe) and manganese (Mn) oxyhydroxides; (3)associated with organic matter and sulfides; (4) molybdenum in theresidue-bound in mineral lattice. Residual percentages in #2 and #1soils were 81.0 ± 15.4 and 69.8 ± 1.7%, indicating that Mo in the soilwas mostly present in the mineral lattice, in line with the findings ofŽemberyová et al. (2010), who have reported that nearly 100% Mo insoil was bounded into the mineral lattice.

The percentage of Mo associated with organic matter and sulfidesexhibited the same range from 11.6 to 12.2% in both soils. However,total Mo concentration in Soil #1 was 27.7 times higher than that inSoil #2, so Mo concentration associated with organic matter and sul-fides in Soil #1 was much higher than that in Soil #2. As shown inTable 1, C and N contents in Soil #1 were 1.0 and 2.5 times higher

nd risk assessment of molybdenum on an industrial site in China, J.

Table 4Summary of results ofMo (mg/kg)a in soils obtained by sequential extraction (1–3), aqua-regia digestion of residue (4) and aqua-regia digestion of original samples (determined pseudo-total content).

Soil Fraction percentage (%) ∑ 1 + 2 + 3 + 4 Pseudo-total concentrations Recoveryb %

1 2 3 4

Step 1 Step 2 Step 3 Residual

1# 4.6 ± 0.3 13.3 ± 1.1 12.2 ± 1.0 69.8 ± 1.7 293 ± 21.0 260 ± 10.2 1132# 2.6 ± 0.8 4.9 ± 0.5 11.6 ± 14.3 81.0 ± 15.4 10.2 ± 0.4 8.1 ± 0.1 126

a Mean values ± standard deviation of 4 replicate analyses (4 repeated extraction procedures).b Recovery = (∑ 1 + 2 + 3 + 4 / pseudo-total concentration from original sample) ∗ 100%.

6 C. Geng et al. / Journal of Geochemical Exploration xxx (2014) xxx–xxx

than those in Soil #2, respectively, which could explain the higher Moconcentration associated with organic matter and sulfides for Soil #1.In addition, a higher percentage of Mo associated with easily and mod-erately reducible Fe and Mn oxyhydroxides was found in Soil #1, andthus Fe and Mn oxyhydroxides in both soils should be measured in fu-ture research.

In groundwater, Mo speciation is determined by pH and solute com-position. Using PHREEQC software, Carroll et al. (2006) reported thatMo speciation as MoO4

2− was 75.8% of the total Mo in groundwater atpH 7.8. Our simulation by Visual MINTEQ software showed that an av-eraged pH of 8.65 in our site would result in almost 100%Mo speciationas MoO4

2− (Fig. 3). In the following discussion of Mo lateral transportand quantitative risk assessment in groundwater, total Mo concentra-tions detected were directly used, due to this conclusion that most ofMo speciation occurred as MoO4

2−.

3.4. Mo lateral transport in groundwater

Mo can be transported from the source area to off-site areas by later-al groundwatermigration. Due to the fact that the values ofKd fromfieldsoil samples and the batch equilibrium experiments were low (Table 3and Geng et al., in preparation), the Domenico solute transport modelwithout decay parameters (Connor et al., 2008) was used to simulateMo transport from the source areas to POEs and the surrounding rivers.Parameter inputs into the Domenico model are shown in Table 5. Thecalculated results of Mo lateral transport are shown in Tables 6 and 7.The distance of POE1, POE2 and POE3 was 30 m, 65 m and 20 m awayfrom the respective source areas at MW9, MW33 and MW17. Sourceconcentrations in the source of POE1, POE2 and POE3 were 5.19 mg/L,32.5 mg/L and 1.18 mg/L, and Mo concentrations in POE1, POE2 andPOE3 were 1.35 mg/L, 3.44 mg/L and 0.45 mg/L, respectively. There-fore, Mo concentrations in the three POEs were lower than those ofthe corresponding sources, with dilution attenuation factors between

pH

2 3 4 5 6 7 8 9 10 11 12

Rel

ativ

e co

ncen

trat

ion

0.0

0.2

0.4

0.6

0.8

1.0

HMoO4-

MoO3(H2O)3(aq)

MoO42-

Fig. 3. Visual MINTEQ model results for aqueous species distribution for a range of pHvalues.

Please cite this article as: Geng, C., et al., Contamination investigation aGeochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2013.12.014

2.6 and 9.4, indicating that Mo attenuation was fast during its ground-water transport.

In spite of much decrease during transport, Mo concentrations inPOEs still exceed Class III limit (100 μg/L) of CQSG. Therefore, furtherquantitative risk assessment is needed for these three POEs.

Because the local streams are considered to be in hydraulic continu-ity with shallow groundwater, surface water standards should also beconsidered when screening contamination sources from groundwater.Though the Class III limit for groundwater is 100 μg/L, the standardlimit for drinking water from surface sources in China is 70 μg/L. There-fore, 70 μg/Lwas used as the screening level for selecting contaminationsources for groundwater. Based on the 2008 monitoring campaign, Moconcentrations in 14 out of 35 groundwater samples exceeded 70 μg/Land thus the locations of these 14 groundwater wells were chosen asthe potential source areas of lateral contaminant migration to the localstreams. The concentrations of Mo in these 14 monitoring wells rangedfrom 92 to 32,500 μg/L (Table 7). The rivers bound the site on threesides, and the nearest distance to one of the three rivers among the 14monitoring wells ranges from 9.4 to 219 m. Based on calculationswith the Domenico solute transport model, concentrations of Mo inthe local streams, due to contaminated groundwater transported fromthepotential source areas, ranged from4 to 6,053 μg/L (Table 7). The re-sults of these calculations are considered extremely conservative in thatno effect of dilution by the stream flow was considered.

The following two reasons indicate groundwater flow and contami-nant transport occur readily in the site aquifer: Mo dilution attenuationfactors ranged between 2.6 and 9.4 for three POEs; during twomonitor-ing campaigns in 2008 and 2010, Mo concentrations in the samemoni-toring wells changed significantly, which suggest permeable reactivebarriers, and pump-and-treat are good options for remediation.

3.5. Quantitative risk assessment

3.5.1. Conceptual site modelDevelopment of a robust conceptual site model (CSM) is an integral

part of a successful risk assessment. It provides a qualitative evaluationof potential contaminant sources, pathways and receptors at the site

Table 5Groundwater parameters used in Domenico solute transport model and exposureparameters used in quantitative risk assessment.

Parameters Value Unit

Ugw Groundwater Darcy velocity 2.16 cm/dayVgw Groundwater seepage velocity 5.4 cm/dayKs Saturated hydraulic conductivity 43.2 cm/dayi Groundwater gradient 0.05 –

Sw Width of groundwater source zone 50 mSd Depth of groundwater source zone 1 mqeff Effective porosity in water-bearing unit 0.4 –

foc-sat Fraction organic carbon in water-bearing unit 0.001 –

pHsat Groundwater pH 8.65 –

BW Body weight 14.4 kgED Exposure duration 6 yearEF Exposure frequency 350 day/yearWI Water intake 1 L/kg

nd risk assessment of molybdenum on an industrial site in China, J.

Table 6GroundwaterMo concentrations in contamination sources and off-site POEs by lateral mi-gration calculated from the Domenico model.

Exposure locations POE1 POE2 POE3

Source name MW9 MW33 MW17

Source concentrations (mg/L) 5.19 32.5 1.18Distance from the point of exposure (m) 30 65 20Mo concentrations in the point of exposure (mg/L) 1.35 3.44 0.45

7C. Geng et al. / Journal of Geochemical Exploration xxx (2014) xxx–xxx

based on plausible contaminant–pathway–receptor linkages under thecurrent and future land use of the site. Based on the historic records ofthe site and field-scale site investigation, a CSM was developed for thesite (Fig. 4). The local government has requested the land to be returnedfor commercial redevelopment. The future land use would be residen-tial. The principal source–pathways–receptors are summarized inFig. 4. These pollution linkages will be subjected to a quantitative riskassessment and have been used as the basis for calculating site specifictarget level (SSTL).

The first step is to choose the contamination source from soil andgroundwater based on relevant standards. Based on the 2008 monitor-ing campaign, just one soil sample exceeded the Dutch interventionvalue, and the concentration in this sample was still below the USEPApreliminary remediation goal (5,100 mg/kg). Therefore, risk from soilshould be minimal and there is no contamination source from soil.

Based on the 2008monitoring campaign,Mo concentrations in 14 outof 35 groundwater samples exceeded 70 μg/L and thus the areas aroundthese 14 groundwater wells were treated as potential sources of contam-ination in screening calculations. Groundwater monitoring wells MW33and MW32, which were located in the northwest corner and east ofNo. 2 workshop (Fig. 1), had Mo concentrations of 32,500 μg/L and905 μg/L, respectively. Located in the east of the process wastewatertreatment station and the dissolvingworkshop,MW29 showed aMo con-centration of 1,650 μg/L. Located in the south side of coiling workshop,MW30 had a Mo concentration of 2,580 μg/L. MW31, located in thenorth of power distribution room and in the south of the coiling work-shop, showed aMo concentration of 4,790 μg/L. Groundwatermonitoringwell MW6, located in the center of the northern boundary of the mainplant and northwest of the lead wire workshop, showed the relativelyhigh Mo concentration of 10,100 μg/L. MW8, located between acidstorage tank and dissolving workshop, had a Mo concentration of3,630 μg/L. In addition, MW9, located in the east of the abandoned pro-cess wastewater treatment station, had aMo concentration of 5,190 μg/L.

The principal human receptors are future site users associated withresidential development on the site and to the south of the site. The crit-ical receptors among residential land users have been identified as

Table 7Mo concentrations in the nearest surface water by lateral migration from 16 monitoring wells

Monitoring wells Mo concentrations (μg/L) Distance from

MW1 92 9.4MW4 540 14.5MW6 10,100 11.9MW8 3630 103.8MW9 5190 116.4MW10 126 136.5MW15 303 148.7MW16 268 104.5MW17 1180 45.4MW29 1650 150.6MW30 2580 163.6MW31 4790 171.3MW32 905 218.7MW33 32,500 181.8

⁎ Dilution by river water is not considered.

Please cite this article as: Geng, C., et al., Contamination investigation aGeochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2013.12.014

female children aged between0 and 6 years old. A female child is select-ed as a sensitive receptor (Table 5, bodyweight of 14.4 kg, exposure du-ration of 6 years, exposure frequency of 350 days/year, and waterintake of 1 L/day) based on a lower bodyweight than hermale counter-part. Risks to constructionworkers are short-termwith 180-day's expo-sure duration. Groundwater is considered a source of potable watersupply in the future, and is thus considered as a receptor. In addition,the groundwater acts as a pollution pathway to the local stream. Risksto the children will be minimal by wading in the local stream due tothe guardrail along the local stream. Aquatic biota in the local streamalso represents potential ecological receptors.

There are a number of potential pathways thatmay allow themigra-tion of contaminants and could potentially impact the identified recep-tors; these include or involve drinking of contaminated groundwater,and lateral migration of contaminated groundwater into the localstreams and POEs. Table 8 shows the receptors and exposure pathways.

3.5.2. Human health riskFor soil, just one sample (from MW31) exceeded the Dutch inter-

vention value, although at 252 mg/kg it was still below the USEPA pre-liminary remediation goal (5,100 mg/kg). In addition, Mo fractionationresults showed that at least 69.8% of theMowas bounded into themin-eral lattice in the soil and was therefore not mobile and available to liv-ing organisms. For higher Mo concentration in soil, just 4.62% wasavailable to living organisms. The available Mo in the MW31 samplewas thus 11.6 mg/kg. Therefore, the risk from soil on this site was con-sidered minimal.

However, due to low field Kd for Mo on this site, Mo was highly con-centrated in the groundwater, and the risk associated with this shouldnot be neglected. Based on the toxicity data from the database of theInternational Agency for Research on Cancer (IARC, 2013) and the Inte-grated Risk Information System (USEPA, 2013a), Mo has no carcinogen-ic effect on living organisms, and 0.005 mg/kg/day of toxicity value wastaken for risk assessment. Therefore, only non-carcinogenic risk wasassessed; the results are presented in Table 9. Among 35 assessment lo-cations, Mo in 14 locations will pose unacceptable non-cancer risks foron-site children when they directly drink the groundwater. In addition,the hazard quotient (HQ) in MW6 andMW33 exceeds 100, which indi-cates that the risk to children associated with these sites is high. Forthree off-site POEs, where the HQ ranges from 6.0 to 45.8, unacceptablenon-cancer risks will exist for off-site children, if groundwater is thedrinking water source.

By inversing the equation to calculate HQand inputting the same ex-posure parameters, the site-specific target level for Mo at this site wascalculated as 75.1 μg/L, which is below the Class III limit (100 μg/L) for

with Mo concentrations higher than 100 μg/L.

the nearest rivers (m) Mo concentrations in the nearest rivers⁎ (μg/L)

66275

6053182212

49

1319942569611

581

nd risk assessment of molybdenum on an industrial site in China, J.

Fig. 4. Conceptual site model developed for the former light-bulb plant.

Table 8Receptor and its exposure pathways.

Pathways Receptors

On-site resident Off-site resident Construction worker River Aquatic biota

Drinking groundwater/surface water √ √ × × √Lateral migration of groundwater × √ × √ √Dermal contact of groundwater/surface water × × × × √

8 C. Geng et al. / Journal of Geochemical Exploration xxx (2014) xxx–xxx

groundwater in China (CBTS, 1994), but is near the standard value fordrinking water (70 μg/L) in China (MEP, 2002a).

3.5.3. Ecological riskLiterature review showed that the toxicity of Mo to aquatic species,

such as aquatic invertebrates (the water flea Daphnia magna) and fish,was low (Ecometrix Inc., 2007). For example, Diamantino et al. (2000)reported a relatively low toxicity of Mo to water fleas in the laboratory,with an estimated acute 48-h LC50 of 2,848 mg/L. This LC50 concentra-tion is comparable to other studies with this species (e.g., 48-h LC50 of3,220 mg/L reported by Kálmán, 1994). The chronic toxicity of Mo towater fleas in this study was also low, i.e., no observed adverse effectconcentration (NOAEC), and lowest observed adverse effect concentra-tion (LOAEC) for growth and mortality were 50 and 75 mg/L, respec-tively. The high concentrations, needed to cause reduced growth andmortality in the laboratory, suggest little risk to water fleas in the

Table 9Calculated non-carcinogenic hazard quotients for children residents from drinkinggroundwater.

Monitoring well or POEs Mo concentrations (μg/L) Hazard quotients

On site MW1 92 1.2MW4 540 7.2MW6 10,100 134.5MW8 3 630 48.3MW9 5190 69.1MW10 126 1.7MW15 303 4.0MW16 268 3.6MW17 1180 15.7MW29 1650 22.0MW30 2580 34.4MW31 4790 63.8MW32 905 12.1MW33 32,500 432.8

Off site POE1 1350 18.0POE2 3440 45.8POE3 450 6.0

Please cite this article as: Geng, C., et al., Contamination investigation aGeochem. Explor. (2014), http://dx.doi.org/10.1016/j.gexplo.2013.12.014

environment at this site from dissolved Mo concentration. The reviewpaper of Eisler (1989) reports that aquatic biota generally are not verysusceptible to toxic effects of Mo, citing no effect on growth or survivalat concentrations of less than 50 mg/L in the water column.

Based on the calculation of the Domenico solute transport model,concentrations of Mo in the local stream transported from the corre-sponding source ranged from 4 to 6,053 μg/L (Table 7), again with noconsideration of dilution by the stream flow. These estimates aremuch lower than the NOAEC of 50 mg/L. Therefore, Mo in the localstreams due to inflow of polluted groundwater will not pose unaccept-able risks to aquatic biota.

4. Conclusions

Field investigations of a former incandescent light-bulbmanufactur-ing site indicated that the sitewas contaminated by operations between1994 and 2008. Although wastewater discharged at the site containedspent acid, soil pH values ranged from 7.3 to 12.3, which could explainthe high concentrations of Mo in groundwater from speciation model-ing. Most of the Mo in the soil was bounded onto the mineral lattice,making the risk minimal. A conceptual model developed for the siteshows the main exposure pathway to be consumption of groundwaterby future residents after the site is redeveloped as a residential areaand off-site residents. Among 35 assessment locations, Mo in 14 loca-tions was determined to pose unacceptable non-cancer risks for on-site children. In addition, Mo in groundwater which is transportedoffsite by lateral migration will pose unacceptable non-cancer risks foroff-site children from direct water drinking. Ecological risk for aquaticbiota was minimal from Mo lateral migration to the local stream. Asite-specific target level for Mo in groundwater was established as75.1 μg/L. Though the risk assessment guideline (Technical guidelinesfor risk assessment of contaminated sites) has been in public reviewin China, Mo is not included in this guideline. Our case study wouldbridge the knowledge gap of Mo in risk assessment. Further work willbe performed on remediation feasibility of the site, such as pump andtreat and permeable reactive barriers.

nd risk assessment of molybdenum on an industrial site in China, J.

9C. Geng et al. / Journal of Geochemical Exploration xxx (2014) xxx–xxx

Acknowledgments

The authors would like to thank the Environmental Resources Man-agement in China for participating in the site investigation. The authorsthank the Natural Science Foundation of China (grant no. 21107072),the State Key Laboratory of Soil and Sustainable Agriculture (Instituteof Soil Science, Chinese Academy of Sciences, grant no. Y052010026),the China Postdoctoral Science Foundation (grant no. 2012M520408)and the Ministry of Environmental Protection of China (grant no.201109052) for the financial support of this work.

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