Use of Mercury Isotopes to Quantify Mercury Exposure Sources in

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Ecotoxicology and Human Environmental Health

Use of mercury isotopes to quantify mercury exposure sources in inland populations, China Buyun Du, Xinbin Feng, Ping Li, Runsheng Yin, Ben Yu, Jeroen E. Sonke, Benjamin Guinot, Christopher William Noel Anderson, and Laurence Maurice Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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Environmental Science & Technology

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Use of mercury isotopes to quantify mercury exposure sources in inland populations, China

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Buyun Du,†, ‡ Xinbin Feng,*,† Ping Li,*, † Runsheng Yin,§ Ben Yu,† Jeroen E. Sonke,# Benjamin Guinot,∇

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Christopher W. N. Anderson,∂ and Laurence Maurice#

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†

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Sciences, Guiyang 550081, China

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‡

University of Chinese Academy of Sciences, Beijing 100049, China

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§

State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of

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Sciences, Guiyang 550081, China

State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of

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#

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Sabatier, 14 Avenue Edouard-Belin, 31400 Toulouse, France

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∇Observatoire

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Avenue Edouard-Belin, 31400 Toulouse, France

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∂

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New Zealand

Observatoire Midi-Pyrénées, Géosciences Environment Toulouse (GET), CNRS, IRD, Université Paul

Midi-Pyrénées, Laboratoire d’Aérologie (LA), Université de Toulouse, CNRS, UPS, 14

Soil and Earth Sciences, Institute of Natural Resources, Massey University, Palmerston North, 4442,

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Co-corresponding Authors:

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Xinbin Feng, Phone: +86 851 85891356; Fax: +86 851 85891609; E-mail: [email protected].

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Ping Li, Phone: +86 851 84391375; Fax: +86 851 85891609; E-mail: [email protected].

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1

ACS Paragon Plus Environment


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ABSTRACT: Mercury (Hg) isotopic compositions in hair and dietary sources from Wanshan (WS) Hg

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mining area, Guiyang (GY) urban area, and Changshun (CS) rural area were determined to identify the

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major Hg exposure sources of local residents. Rice and vegetables displayed low δ202Hg and small

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negative to zero ∆199Hg, and are isotopically distinguishable from fish which showed relatively higher

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δ202Hg and positive ∆199Hg. Distinct isotopic signatures were also observed for human hair from the

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three areas. Shifts of 2 to 3‰ in δ202Hg between hair and dietary sources confirmed mass dependent

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fractionation of Hg isotopes occurs during metabolic processes. Near zero ∆199Hg of hair from WS and

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CS suggested rice is the major exposure source. Positive ∆199Hg of hair from GY was likely caused by

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consumption of fish. A binary mixing model based on ∆199Hg showed that rice and fish consumption

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accounted for 59% and 41% of dietary Hg source for GY residents, respectively, whereas rice is the

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major source for WS and CS residents. The model output was validated by calculation of probable daily

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intake of Hg. Our study suggests that Hg isotopes can be a useful tracer for quantifying exposure

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sources and understanding metabolic processes of Hg in humans.

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INTRODUCTION

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Mercury (Hg) is a globally distributed trace-element pollutant.1 In the environment, inorganic

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mercury (IHg) is readily transformed to methylmercury (MeHg), a toxin that causes neurological

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damages such as visual, auditory, and sensory disturbances, numbness, and difficulty in walking.2 MeHg

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has a strong bioaccumulation capability. Methylation of Hg readily occurs microbially in aquatic

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ecosystems, and bioaccumulation of MeHg can result in high levels of MeHg in fish that are long-lived

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and/or at the top of the food chain.3 Fish consumption is considered as the major exposure pathway of

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Hg to humans.4 Mercury exposure due to non-fish dietary sources is thought to be limited due to the

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short nature of food chain length. However, in Hg mining areas and many extreme Hg-contaminated

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sites, crops (especially rice) at the base of the food chain can also contain high levels of IHg and MeHg.

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Rice paddies have been shown to enhance Hg methylation, explaining high MeHg levels in rice.5-8 In

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Guizhou province, China, rice as a staple food can be an important source of human Hg exposure.9-10

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Mercury exposure from non-dietary sources is also important to specific population, such as Hg miners

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and gold miners.11-12 Identifying Hg exposure pathways to a specific population is important for the

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health risk assessment, however this is often difficult because a comprehensive investigation of Hg

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concentration and speciation in numerous dietary sources is often needed to make the evaluation

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representative.5

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Mercury stable isotope geochemistry has been demonstrated as a useful approach to unravel sources

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and bio-geochemical processes of Hg in the environment.13-14 Mercury's seven stable isotopes (196-204

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amu), can undergo mass-dependent isotope fractionation (MDF, reported as δ values) and

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mass-independent fractionation (MIF, reported as ∆ values) during various biogeochemical processes.13

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MDF occurs under both kinetic and equilibrium conditions, while Hg MIF for odd-mass numbers

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(∆199Hg and ∆201Hg) is affected by magnetic isotope effect and nuclear volume effect,15-16 and the

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even-mass-number isotopes (∆200Hg and ∆204Hg) is likely a result of gas phase reactions in the

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atmosphere.17 Combining "MDF-MIF" of Hg isotopes can provide multi-dimensional information about

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sources and geochemical fates of Hg in the environment. Hg isotopes have been used as tracers to

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understand exposure sources and bioaccumulation processes in food chains18-21 and humans.22-28 No

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significant MDF and MIF of Hg isotopes have been observed during consumption of dietary MeHg by

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fish.19-21,29 However, an offset of ~2‰ in δ202Hg was observed between fish and hair of fish-consumers

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such as humans and whales, indicating that MDF of Hg isotopes may occur during metabolic processes 3



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in mammals.22-27 Significant MIF signatures have also been reported in fish and fish-consumers,

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however the MIF was mainly explained by Hg photo-degradation in ecosystems prior to trophic transfer,

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not by metabolic and trophic transfer processes.18,30-34 The absence of MIF during metabolic and trophic

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transfer processes suggests that MIF can be a robust tracer to identify Hg sources in food webs.

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In previous studies, Hg isotopes in hair were successfully used to identify exposure sources of Hg

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for gold miners23,35 and fish-consumers.22,24-27 Recently, the isotopic composition of Hg in rice from the

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Wanshan Mercury Mine in China was investigated, and rice showed more negative δ202Hg and ∆199Hg

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than fish.28,36-38 We hypothesized that populations from Guizhou Province, China may have distinct hair

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isotopic signatures compared to previous results, since rice consumption is the most important pathway

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for human Hg exposure in Guizhou.9-10 To test this hypothesis, we investigated the isotopic composition

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of Hg in human hair samples from local residents from three areas of the Guizhou Province, China, in a

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mining, an urban and a rural area. The isotopic composition of potential dietary Hg sources (e.g., rice,

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fish and vegetables consumed by the study populations) were investigated as well.

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EXPERIMENTAL SECTION

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Site description and sample collection. Three primary schools at Guiyang (GY), Changshun (CS)

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and Wanshan (WS) in Guizhou Province, SW China, were selected in our study (Figure S1 of supporting

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information (SI)). GY primary school is located in Guiyang (N 26º34'30.29"/E 106º3'21.66", 1080 a.s.l.),

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the capital of Guizhou. Coal combustion is the major Hg emission source in GY.39 The average of

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gaseous elemental Hg (9.72 ng m-3) in ambient air in GY in 2009 was slightly higher than most of the

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cities in China such as Beijing, Shanghai, Chongqing and Nanjing, but much lower than Changchun and

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Guangzhou.40 WS primary school (N 26º11'48.61"/E 106º32'57.91", 1244 a.s.l.) is located in a small

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village in the Wanshan Hg mine area, the largest Hg mine in China. Mercury mining in WS has been

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documented for thousand years, and Hg mining and retorting activities intensified from the 1950s to

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2001 when the mine was officially closed.41 Long-term Hg mining and retorting activities have resulted

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in serious contamination of Hg to the surrounding environment (soil, sediment, food, water and

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atmosphere).42-43 CS primary school (N26º11'16.19", E106º33'34.84", 1282m a.s.l.) is located in a rural

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area, in a small residential community, 70 km far from Guiyang. CS has been used as a control site by

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previous study9 due to lack of significant industrial activities, and a reliance on agriculture (rice and

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tobacco planting) as the main source of income for the local residents. Previously results showed that 4


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the average concentrations for gaseous elemental Hg in CS was only 7.9 ng m-3, which was lower than

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GY (unpublished data).

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Pupils and their supervision were selected for investigation. Sampling work was conducted from

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November 2013 to October 2014. Regarding human sampling, 26, 21, and 9 participants in WS, GY and

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CS respectively, were randomly selected for hair sample collection. Ages, gender, weight, heights,

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dental Hg amalgam using, and dietary information about each participant were investigated and

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summarized in Table S1 of SI. As shown in Table S1, people from GY consume more fish than WS and

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CS residents. Every participant signed a consent agreement before starting the survey. The present study

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obtained ethics approval from the Institute of Geochemistry, Chinese Academy of Sciences.

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Hair samples (0.2 to 0.5 g) were cut with stainless steel scissors from the occipital region of the

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scalp, bundled together by stapler, placed and sealed in polythene bags, according the method by Li et

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al.44 Fillet fish (15 to 20 g), rice (15 to 20 g), and vegetables (10 to 25 g) were collected from the kitchen

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of each participant. According to our investigation, rice and vegetables collected from WS and CS were

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grown and harvested locally, but food samples collected from GY were bought from the market. The

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fish samples were collected directly from the participant's house to ensure that these species were

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consumed regularly by the local residents. The fish species were not identified but Guizhou Province is

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an inland area and most of fish obtained the market are caught from freshwater aquaculture. All the hair

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and food samples were kept in a cooler before being delivered to the laboratory. The samples were

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washed with distilled water. The fillet fish and vegetables samples were freeze-dried (-55 °C, 72 hours),

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and the rice samples were air-dried (72 days). The dried samples were then powdered and stored in

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polyethylene bags in room temperature until chemical analyses.

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Total mercury concentration analysis. The total Hg concentrations (THg) in all hair and fillet

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fish samples, as well as vegetable and rice samples from WS, was measured by atomic absorption

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spectroscopy (RA915+ with a pyrolysis unit Pyro-915+, Lumex, Russia) following a method reported

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previously,45. The limit of detection is 0.5 ng g-1. For rice samples from GY and CS, approximately

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0.1-0.2 g of each sample was digested with 5 ml of acid mixture (HNO3:H2SO4=4:1, v:v) at 95 °C for

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three hours. Then the THg concentrations in digestive solutions were determined by cold vapor atomic

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fluorescence spectroscopy (CVAFS) following the Method 1631E,46 with limit of detection of 0.3 ng g-1.

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The internal calibration of RA915+ was verified prior to sample measurement using a saturated 5



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Hg0 vapor source. When the relative standard deviation (RSD) was 0.05). Significant differences in rice MeHg proportions as THg (%MeHg) were observed between

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WS and GY, as well as between WS and CS, which indicated different MeHg production potential at

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these sites. The THg and MeHg concentrations, and %MeHg in rice and hair samples collected from WS

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were comparable with previous results reported by Feng et al.9 and Li et al,54 at the same sampling sites.

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The mean THg concentration in vegetables in WS was 139±280 ng g-1 (2SD, n=8), which was

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comparable with previous results (87 to 436 ng g-1) by Zhang et al10 and Feng et al9 at the same site.

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Generally, MeHg constitutes a small fraction (0.1% to 0.2%) of THg in vegetables, and human MeHg

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exposure through vegetable consumption can be negligible.9-10 The mean THg and MeHg concentration

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in fillet fish samples collected from GY was 43±70 ng g-1 and 27±32ng g-1 (2SD, n=10), respectively.

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On average, the MeHg accounted for 70%±38% of THg in the fillet fish samples.

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Hg isotope compositions of dietary sources. Hg isotopes compositions of all samples were shown

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in Table S2 of SI, and, plotted for comparison in Figure 1. Rice samples were isotopically 8


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δ202Hg and ∆199Hg values in rice samples collected

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distinguishable among the three sites. The mean

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from WS were −2.59±0.98‰ and 0.03±0.06‰ (2SD, n=15) respectively. The δ202Hg were consistent

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with previous results from Yin et al. (−2.38±0.32‰, 2SD, n=6),38 but lower than those from Feng et al.

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(−1.60±2.80‰, 2SD, n=14)36 and Rothenberg et al. (−1.23±1.38‰, 2SD, n=8)28. The ∆199Hg values

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were within the range of previous studies.28,36,43 Rice samples collected from GY showed relatively

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higher mean δ202Hg values (−1.35±0.38‰, 2SD, n=11) but similar ∆199Hg values with WS rice

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(0.02±0.08‰; 2SD; n=11). Rice samples collected from CS showed intermediate δ202Hg values

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(−1.83±0.24‰, 2SD, n=14), but characterized with slightly negative ∆199Hg (−0.07±0.08‰, 2SD, n=14).

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Vegetables collected from WS showed negative δ202Hg (−2.34±1.12‰, 2SD, n=9) and ∆199Hg

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(−0.05±0.14‰, 2SD; n=9). Fillet fish samples collected from GY had mean δ202Hg of −0.48±1.04‰,

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and mean ∆199Hg of 0.96±1.30‰ (2SD; n=10), which are distinct from rice and vegetable samples

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(Figure 1).

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As shown in Figure 1, rice and vegetables showed negative δ202Hg values, which have been

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explained by the significant MDF of −3 to −1‰ during incorporation of Hg by plants.55-56 Plants receive

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Hg mainly from atmosphere and soil. Previous studies showed that the metabolic effects are unlikely to

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cause MIF.23,32,38 The near-zero ∆199Hg values in rice samples were similar to the previous results of

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soils in WS and GY, which showed limited MIF signals. This was consistent with previous studies,

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which demonstrated that Hg in rice is mainly derived from soils.38 Lichens55 and plant leaves38,57-59

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showed negative ∆199Hg values, which have been explained by the uptake of atmospheric Hg0 by foliage.

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Vegetables from WS showed variable ∆199Hg values with a range of -0.10 to 0.08‰. Few of our

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vegetables have negative ∆199Hg values, indicating that atmospheric Hg(0) is the major source. However,

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most of the vegetables showed limited MIF signals, which may be explained by the contribution of soil

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Hg in these samples.

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The higher δ202Hg and ∆199Hg in GY fillet fish are in general agreement with previous results for

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freshwater and marine fish (Figure 2).21,34 Previous studies have shown the absence of both MDF and

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MIF during Hg metabolism and trophic transfer in aquatic food chains,19-20 and the higher δ202Hg and

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∆199Hg values in fish have been explained by photo-degradation of MeHg in the water column.60

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∆199Hg/∆201Hg ratios of 1.20 to 1.36 were previously reported for aquatic organisms and human hair

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samples, which have been attributed to the MeHg photo-degradation processes in the aquatic

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environment before MeHg was transferred and bio-accumulated in food chains.34,60-62 In addition, a ratio 9



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of about 1.0 for ∆199Hg/∆201Hg was observed for samples with lower MeHg fractions (e.g., soils, plants,

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and atmosphere), and this MIF was mainly a result of aqueous Hg2+ photo-reduction.18,63 In our study,

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rice and vegetables showed a ∆199Hg/∆201Hg ratio of 0.96±0.24 (2SD, r2=0.55, p

Use of Mercury Isotopes to Quantify Mercury Exposure Sources in

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