| China enriches in coal resources,and the production and use of coal ranks first in the world in terms of both mining and use.Mercury（Hg）is one of the harmful trace elements commonly associated with coal,which is an important source of Hg in the environment.The long-term large-scale of coal mining activities have produced a large amount of coal slag and acid mine drainage（AMD）,both of which play important roles in the biogeochemical cycle of Hg in the environment of coal mining areas.Guizhou Province is a typical area with developed karst topography and landforms in the southwest.It knows as"the coal sea of coal of the south Yangtze River".Most of the coal mines are located in trench zones of the karst mountainous area.Under the influence of the circ-Pacific Hg mineralization zone,the Hg concentration in coals is generally high,leading to prominent environmental pollution problems in coal mining areas.However,studies on the behavior and production of Hg,particularly methylmercury（Me Hg）as well as its biological effects are rarely reported.In this study,five typical coal mining areas of Guizhou were selected for the investigation of Hg and Me Hg in the supergene environment to systematically elucidate their distribution,transportation,and transformation characteristics in acidic environment.And a preliminary Hg exposure health risks assessment of residents related to the coal mining was also discussed.The main results were as following:1.Concentration and distribution of Hg in the supergene environment of typical karst coal mining areasThe total Hg（THg）concentrations in coals ranged between 10μg·kg-1 and 473μg·kg-1 with an average of 165±92μg·kg-1.The average THg concentration was higher than the national average of THg in coal(100μg·kg-1).THg in coal gangue ranged from 41 to 34800μg·kg-1 with an average of 3565±6594μg·kg-1.THg concentrations in AMD was in a range of 0.13-86ng L-1 with an average of 10.4±15.8ng L-1,while Me Hg concentrations ranged from 0.008-1.21ng L-1 with an average of0.17±0.16 ng L-1.THg concentrations in sediment exhibited large range,ranging from0.15 to 19mg·kg-1,and Me Hg ranged from 0.056 to 7.8μg·kg-1,with the average of1.5±1.9μg·kg-1.THg in surface soil ranged from 98.67 to 1801.06μg·kg-1,with an average of 499.93±320.53μg·kg-1,and Me Hg ranged from 0.36 to 50.64μg·kg-1,with an average of 4.24±5.26μg·kg-1.THg in moss ranged from 35 to 137.3μg·kg-1,with elevated values in flower-shaped wetland moss Hyophila rosea being recorded near the acid mine drainage.Significant difference of Hg concentrations was observed in different coal mining areas.The average concentration of Hg in coal gangue in Jiaole mining area was 11mg kg-1 and the highest concentration of 34.8mg·kg-1.The highest average THg concentration in AMD was 26.1±23.7ng·L-1 with a maximum value of 86 ng·L-1,which was collected from Anlong coal mining area.The high concentration of Hg in coal gangue leads to the rise of Hg concentration in sediment,soil and water in Jiaole and Anlong areas.THg concentrations in surface soil from coal mining areas were in the order of Jiaole>Anlong>Anshun>Panjiazhuang>Zhijin.Compared with the soil THg and Me Hg in background areas,various degrees of accumulation of THg and Me Hg,especially Me Hg in the soil of coal mining areas have been observed,and the Me Hg concentration in the soil of coal mining areas already exceed that of Hg mining areas.THg concentrations of rice in the Jiaole coal mining area can reach to 22μg·kg-1,which exceeded the limit of 20μg·kg-1 recommended by the National Standard for food security.Meanwhile,THg concentrations in edible parts of vegetables also exceeded the limit of 10μg·kg-1 recommended by the National Standard for food security.Overall,the coal mining is the main source of Hg in the surrounding environment,and the difference of Hg concentration in environmental media was also affected by the Hg concentrations in coal and coal gangue.2.Influencing factors of Hg transportation and transformation in the supergene environment of karst coal mining areasSequential extraction results showed that Hg in coal gangues was mainly in nitric acid-extractable speciation F4,accounting for 82.5%of the total Hg,followed by humic acid speciation F3（10.1%）,residual speciation F5（5.46%）,fulvic acid speciation F2（1.6%）and water-soluble speciation F1（0.37%）.The main occurrences speciation of Hg in sediments were mainly in nitric acid-extractable speciation F4（49.2%）,followed by humic acid speciation F3（25.7%）,residual speciation F5（21.7%）,fulvic acid speciation F2（3.44%）and water-soluble speciation F1（0.057%）.Meanwhile,the occurrence speciation of Hg in soil were mainly in humic acid speciation F3（68.2%）,followed by the nitric acid-extractable speciation F4（14.7%）,fulvic acid speciation F2（8.55%）,residual speciation F5（8.15%）and water-soluble speciation F1（0.36%）.Coal gangue is the main source of Hg in coal mining areas,the main Hg fractions changed when He was released into sediments and soil.Significant difference of Hg occurrences speciation in coal gangues,sediments and soil were observed,exhibiting a decreasing trend of the ratio of nitric acid-extractable speciation,while an increasing trend of coal gangues,sediments,and soils was observed in the ratio of humic acid and fulvic acid speciation.Meanwhile,significant negative correlations of percentages between humic acid speciation（F3）and strongly complexed speciation（F4）were observed in gangues（r=-0.99,p<0.0001）,sediments（r=-0.87,p<0.0001）and soil（r=-0.57,p<0.0001）respectively.This phenomenon indicates a clear trend that the strong complexed speciation F4 was converted to organic binding Hg（F2 and F3）under acidic condition in coal mining areas,suggesting the rise of Hg bioavailability.The concentration of THg and Me Hg in rice is related to the content of Hg in soil and organic matter.The Hg combined with organic matter in the soil has strong bioavailability and strong mobility.Total organic matter was positively correlated with soil THg（r=0.34,p<0.01）,soil Me Hg（r=0.51,p<0.01）,rice IHg（r=0.37,p<0.05）,and rice Me Hg（r=0.59,p<0.01）.Further analysis found that weak negative correlations were observed between concentration of humic acid in soil and rice IHg（r=-0.16,p=0.51）and rice Me Hg（r=-0.26,p=0.27）,while the concentration of humic acid in soil between soil fulvic acid and rice IHg showed positive correlations（r=0.35,p=0.13）,rice Me Hg（r=0.27,p=0.26）,and between concentration of non-humic acid（n-Has,the difference between the total organic carbon and humic acids）in soil and rice IHg（r=0.39,p=0.04）,rice Me Hg（r=0.63,p=0.0027）.Although organic matter is easy to combine with Hg,it will also affect the occurrence forms of humic acid in the acidic environment of coal mining areas.Humic acid in organic matter could combine with Hg and formed a complex,thereby hindering its migration and the absorption of Hg in the soil by rice.However,in coal mining areas,the high content of organic matter in the soil has combined a large amount of Hg,which will also increase the bioavailability of Hg.This characteristics indicates that the occurrence of organic matter significantly affects the bioavailability of Hg.The acid mine drainage with a low p H value,which affects the migration and transformation of Hg in the mining area.Due to the impact of acid mine drainage,the Hg in sediments and soils is negatively correlated with p H,indicating that low p H promotes the release of Hg.The impact of acid mine drainage in coal mines on the transportation and transformation of Hg is mainly manifested in two ways.On the one hand,low p H in acid mine drainage promotes the release of Hg in coal gangue,sediment and soil.However,on the other hand,acid mine drainage will affect the organic matter in sediment and soil,and different forms of organic matter combined with Hg,which in turn affects the transportation and methylation of Hg.3.Risk assessment of Hg exposure in typical karst coal mining areasThe ecological risk assessment of Hg and other heavy metals caused by coal mine activities showed that Hg in sediments is highly enriched at a serious pollution level,while Hg in soil is slightly or moderately enriched.The results of potential ecological hazard index（HI）showed that HI of Hg in sediment is 1323.4 with a high risk.The average value of HI of Hg in the soil of the five coal mining areas is 163,also showed high ecological risk.These results showed that coal mining activities have posed a significant impact on the surrounding environment.The tailings,coal gangue,scattered coal,and coal mine wastewater have increased the risk of Hg in the surrounding soil.The results of human health risk assessment showed that,the THg concentrations in local resident hair samples ranged between 297μg·kg-1 and 1485μg·kg-1,with an average of 627±284μg·kg-1,some of which exceeded the threshold of 1000μg·kg-1set by the United States environmental protection agency（USEPA）.THg concentrations in urine samples ranged between 0.05 mg kg-1 Cr and 1.42 mg kg-1 Cr,with an average of 0.43±0.32 mg kg-1 Cr,which were much lower than the threshold value of 5 mg kg-1 Cr set by the United Nations industrial development organization.Human health risk assessment of Hg in coal mining area showed that exposure risk of children exposure to inorganic mercury（IHg）and Me Hg through rice consumption is higher than that of adults,and the average Me Hg daily exposure dose of children through rice consumption is greater than the reference dose of Me Hg,suggesting that significant Me Hg exposure risk through rice consumption exists in coal mining areas.The contribution proportion of IHg exposure of residents in coal mining areas was in the order of vegetables>rice>inhalation>drinking water,and the contribution proportions were 59.26%,38.46%,2.24%,and 0.04%,respectively.Meanwhile,the contribution proportion of Me Hg exposure is rice>vegetables>drinking water>atmospheric inhalation,and the contribution ratios are 98.67%,1.31%,0.02%,respectively.And the source of breathing was ignored.In coal mining areas,the main exposure pathways of IHg is consumption of vegetables and rice,while main exposure pathways of Me Hg is rice consumption.Although the average daily exposure dose of IHg and Me Hg of residents in coal mining areas is lower than the reference dose of IHg and Me Hg,and the total exposure hazard index is 0.78,was almost 1.Suggesting that there is a potential non-carcinogenic health risk. |
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