Study On Remediation And Mechanism Of Mercury Pollution In Paddy Field Soil In Tongren Mercury Mining Area

Mercury（Hg）is a toxic heavy metal.The toxicity of Hg is related to its occurrence forms,while methylmercury（Me Hg）is the most toxic form.Recent studies have found that rice grain can bioaccumulate Me Hg,and it has become the main source of human Me Hg exposure in Hg mining areas.The Hg mining areas in China are mainly distributed in southwest area,which is also karst area with fragile ecology.The long-term Hg mining and smelting activities have caused serious pollution in the local environment and the Hg contents in farmland soil and agricutural crops are seriously elevated.In this study,Siqiandaba,an important grain production base in Tongren City,Guizhou Province,was selected for the remediation study.The remediation effects of biochar on Hg contaminated farmland were evaluated,and the adsorption and passivation mechanism of Hg by biochar was discussed.（1）In order to clarify the suitable method for extracting the bioactive Hg from soil,four kinds of available extractants were selected for the extraction of Hg in contaminated paddy soil.Results showed that the extraction rates of bioactive Hg from soil by 4 reagents were in the order:HCl（0.036%）>DTPA（0.018%）>H₂O（0.005%）>Ca Cl₂（0.003%）.Significant positive correlations were found between soil bioavailable Hg extracted by water and total mercury（THg）contents in rice roots（r=0.742,p=0.01）,stems（r=0.752,p=0.01）,and soils（r=0.69,p=0.01）,respectively,indicating that the water extractant was suitable for assessing soil bioavailable Hg levels in paddy fields in Hg mining areas.The obtained results provided important scientific basis for risk assessment and remediation of Hg contaminated soils.（2）In this study,biochar was used as passivator for remediation.The results showed that the restoration effect was remarkable.Compared with the control group,the ratio of THg in rice/THg in soil decreased 32.0%on average,and the ratio of bioavailable Hg/THg in soil decreased 51.4%on average after remediation.The results from two years’continuous monitoring showed that the ratios of THg content in rice and bioavailable Hg to soil THg remained stable,indicating that the passivator had good stability in farmland restoration.At the same time,the weight of single plant in the experimental group increased 6.34%than the control group,and the yield production increased by 10.2%,indicating that the addition of passivating agent could also promote rice yield.（3）Morphological characteristics of biochar before and afetr adding to the paddy field were determined by Fourier transform,X ray diffraction,SEM and TEM spectroscopy.It indicated that the particle diameters of biochar are between 10～40μm,in which more than 95%are<20μm.The biochar’s surfaces are enriched with oxygen-containing functional groups such as OH、-COOH、-CHO,which can enhance soil adsorption capacity for Hg ions.The surface of biochar after application will adsorb a large number of oxygen-containing crystals,which can promote the adsorption of Hg.The surface of biochar is irregular,rough and porous,which provides a binding point for the adsorption of Hg ions.Some biochar particles have skeleton structure and obvious macropores on the surface,which is beneficial to the adsorption of Hg-containing particles.