Long-term Effect Of Selenium And Biochar On Reducing The Methylmercury Accumulation In Rice Grain

Soil mercury pollution caused by the emission of waste gas,waste residue and wastewater in mercury(Hg)mining areas or industrial areas has been widely concerned.Due to lack of land,and affected by planting habits of residents,mercury contaminated soil is still used for agricultural production which is the main cause for accumulation of mercury in rice and other crops.The flooded environment of paddy field is favorable to the formation of methylmercury(Me Hg)in soil,and thus Me Hg is easy to accumulate in rice,which has become one of the important ways of Me Hg exposure for residents.Currently,there is still no effective and environmental methods or technologies to control soil mercury pollution.Therefore,it is necessary to explore and develop new in-situ remediation materials and technologies for Hg-contaminated soil to improve the environmental quality.Based on a one-year experiment in which selenium(Se)and biological addition were used to reduce the accumulation of Me Hg in soil and rice,we used new microcosm experiments and 3-year pot experiments to explore the long-term effects of adding selenium(Se)and biochar,or a combination of both Se and biochar,on Me Hg bioaccumulation in rice to mitigate Hg pollution in paddy soils resulting from Hgcontaining wastewater discharge.The main results are:1.The microcosm experiments with Se addition showed that Se can reduce the concentrations of Me Hg in paddy soil,and the effect is positively correlated with the doses of Se which futher supports the previous research results.The reason of Me Hg decreasing may be the interaction between Se and Hg to form difficult selenium mercury compounds,such as Hg Se,and then inhibit the methylation of Hg.However,the results of 3 years pot experiments showed that whether adding low dose(3.0 mg/kg)Se or high dose(6.0 mg/kg)Se,Me Hg in brown rice decreased significantly in the first year(62%～73%),and decreased about 9%～36% in the second,but increased in third years,which indicate that the inhibitory effect of Se on the accumulation of Me Hg in rice decreased with time and had no long-term effect,even had a negative effect.This needs to futher study.2.The results of pot experiments showed biochar or a co-application of Se and biochar for three years pot experiments showed effective reduce of Me Hg in brown rice as biochar added(52%～88%).And with increasing time,the change of effect tended to be stable.Considering the daily intake of rice Me Hg and Se,the addition of biochar can effectively reduce the health risk of human exposure to Me Hg,while the addition of high-dose selenium may bring risk of human exposure to selenium.Comparing the research reports on mercury pollution remediation,it has great potential to apply appropriate Se and biochar in the remediation of mercury contaminated rice fields.3.The 50-day microcosm experiments with biochar addition or a combination of both Se and biochar showed that after three different types of biochar(wheat straw-,corn straw-and bamboo-derived biochar)added(0.5%,w/w),the formation of Me Hg in whether biochar groups or Se+biochar groups decreased significantly,and the effect of co-application was more effective.At the same time,biochar addition reduced the bioavailability of Me Hg in soil,but with the enhancement of oxidation conditions,the bioavailability of Me Hg increased,which is also reflected in pot experiments.These results indicate that the change of redox conditions in soil can affect the bioavailability of Me Hg when biochar added,which means that water management should be taken into account in actual field when applying biochar to control the accumulation of Me Hg in rice,and further study is needed.In conclusion,this study provides basic technical remediation for reducing the risk of mercury pollution in paddy fields,indicating that Se or biochar,especially the coapplication of the two,has obvious advantages in the long-term treatment and remediation of mercury pollution in paddy fields.When biochar is applied in combination with other environment-friendly remediation agents,it may be expected to achieve good remediation effects and cut the cost,which has practical significance for agricultural safety production.