Change In Methylmercury And Its Influence Mechanism Under Rice Cultivation In Low Temperature Pyrolysis Remediated Soil

The data in reported literatures showed that mercury（Hg）pollution in the cultivated soil in some areas in China was serious.Hg in soil will be transferred through the food chain and accumulated in human body,posing a potential threat to human health.Methylmercury（Me Hg）in soil is more toxic and tends to accumulate in rice grain,increasing the potential risk of Me Hg exposure through rice consumption.Low temperature pyrolysis technology developed for Hg-contaminated soil can quickly and efficiently achieve the treatment and remediation of severely Hg-contaminated soil,which is of great significance for reducing the environmental risk of Hg-contaminated soil.Low temperature pyrolysis has significant effects on soil physicochemical properties,microbial community structure and abundance,Hg activity,etc.However,the effect of environmental conditions on residual Hg methylation is still unclear.In this work,two typical Hg-contaminated soils in Wanshan Hg mining area and Qingzhen chemical industry area in Guizhou province were selected as experimental objects to optimize the repair parameters of low temperature pyrolysis and study the direction of mercury separation.Further,in the field experiment,soil samples were collected and analyzed in seven different rice growing periods,and the transformation rule of residual Hg into Me Hg in the soil after low temperature pyrolysis was summarized.The Hg fraction changes,microbial diversity,and the corresponding relationship between methylation functional genes and Me Hg as well as the mechanism of action were studied by Hg fractionation analysis,high-throughput sequencing,real-time fluorescence quantitative PCR,and other techniques.The absorption,translocation,and accumulation of total mercury（THg）and Me Hg in rice plant were analyzed.The main research results achieved are as follows:1)Under the optimal pyrolysis temperature and retention time of 350℃and 40 min,THg removal rates of Hg-contaminated soil in Hg mining and chemical industry areas were 89.4%and 93.0%,respectively.The removal rates of Me Hg in Hg mining and chemical industry areas were 95.7%and 99.7%,respectively,and the methylation rates decreased from 0.02‰and 0.48‰to 0.01‰and 0.017‰,respectively.The proportions of organic matter and organic bound Hg in the soil from chemical industry area were higher than those in the soil from Hg mining area,which is an important reason for the high methylation rate and the high removal efficiency of THg and Me Hg by low temperature pyrolysis.2)After low temperature pyrolysis,Me Hg in soil increased with time during rice planting and reached the maximum value at flowering period.However,the concentration of Me Hg in soil from Hg mining area increased greatly,from 0.36±0.07μg/kg in flowering period to 10.89±2.37μg/kg,which increased by 30.3 times.However,the increase of methylmercury in soil in the chemical industry area was relatively gentle,and the concentration of Me Hg in the flowering period increased from 0.2±0.07μg/kg to 3.42±1.54μg/kg,which increased by 17.1times.The methylation rates of the four types soil in seven sampling periods showed the order:pristine soil in the chemical industry area（0.358-0.833‰）>pyrolysis-treated soil in the chemical industry area（0.024-0.387‰）>pristine soil in the Hg mine area（0.011-0.230‰）>pristine soil in the Hg mine area（0.008-0.028‰）.The superimposed factors of high organic matter concentration and strong mercury activity in the pristine soil in the chemical industry area could lead to the high methylation rate.3)In the process of rice planting,the contents of all species of Hg in the soil remediated by low temperature pyrolysis in chemical industry area decreased,while the growth rates of dissolved and exchangeable forms in the soil remediated by Hg mine area were 0.63-2.04 times,which may be one of the reasons why the methylation rate of the soil in Hg mine area was higher than that in the pristine soil.In addition,the low temperature pyrolysis significantly increased the concentrations of available P and K,and decreased the concentrations of soil organic matter,dissolved organic carbon,and alkali-hydrolyzed nitrogen.The ratios of Fe²⁺and Mn²⁺in the soil were significantly increased by pyrolysis remediation,while the concentration of SO₄^2-decreased in Hg mining area and increased in chemical industry area.4)The low temperature pyrolysis process caused serious damage to the soil microbial community,but the soil microbial diversity recovered to the level of the pristine soil in one month after rice planting.The diversity of methylated microorganisms in the chemical zone was more abundant,and the abundance of potential methylated microorganisms in the remediated soil after the jointing period was close to that in the original soil.5)Soil organic matter and dissolved-exchangeable Hg were the key environmental geochemical factors for Hg methylation in soil.At the phylum level,Proteobacteria and Firmicutes are the main factors affecting Hg methylation in the soil from Hg mining area,while Firmicutes was the main Hg methylation microorganism in the soil from the chemical industry area.On the genus level,Desulfosporosinus and Syntrophobacter are the dominant Hg-methylated microorganisms in the pristine soil,after pyrolysis and remediation,it was Desulfuromonas in the Hg mining area.It has obvious correlation with Me Hg concentration in soil in the corresponding period.Desulfosporosinus is the original methylated microbe in the chemical industry zone.However,Desulfuromonas,Desulfobulbus,Desulfitobacterium,Syntrophobacter,and Geobacter are predominant after pyrolysis remediation.There was a significant positive correlation between Me Hg concentration and hgc A gene abundance in soil（R²=0.53,p=0.001）.The methylation process of inorganic Hg in soil was mainly regulated by anaerobic microorganisms owinging hgc A genes.The pyrolysis remediation process significantly reduced the abundance of hgc A genes in the two soils,and affected the distribution of Me Hg in the soil after remediation.hgc A gene played a more significant role in regulating the formation of Me Hg in soil from the chemical industry area.6)THg concentration in rice grain and rice husk decreased to 0.05±0.01 mg/kg and0.12±0.02 mg/kg,respectively,by 81.7%and 63.7%in the mercury mine area.The Me Hg concentration of rice planted in Hg mining area and chemical industry area decreased by 57.14%and 92.9%,respectively.The bioaccumulation factor（BAF）of THg in rice grain was lower those in different parts of rice plant,while the BAF of Me Hg rice grain was higher than those in different tissues.Me Hg translocation in rice plant was stronger than THg.The health risk index of Me Hg in rice produced in Hg mining area and chemical industry area decreased from2.6 and 1.8 to 0.31 and 0.13,respectively.The health risk of rice produced in soil after pyrolysis remediation was low.Overall,soil in the chemical industry area has a higher proportion of organic matter and organic-bound Hg than that in Hg mining area,which resulted in a higher removal efficiency of THg and Me Hg than that in Hg mining area during low temperature pyrolysis.The concentration of Me Hg in soil increased gradually with the growth of rice,which showed that the Me Hg concentration in the soil from chemical industry area was significantly lower than that in the pristine soil in the same period,and the Me Hg concentration in the soil from Hg mining area was the same as that in the pristine soil in the same period.This may be due to the changes in the microbial community structure caused by the reconstruction of soil microorganisms destroyed by low temperature pyrolysis and the methylated microorganisms containing hgc A genes during rice cultivation.We believe that the change in Hg fraction caused by low temperature pyrolysis is also one of the reasons for the inconsistent changes of Me Hg in soil.After low temperature pyrolysis,the contents of THg and Me Hg in the tissue of soil-grown rice decreased significantly,and the health risk of rice reached an acceptable level.