Study On The Blocking Mechanism Of Modified Montmorillonite On Mercury And Methylmercury Pollution In Rice Fields

As one of the toxic and harmful heavy metal pollutants,mercury has been listed as one of the key heavy metal control institutions.Soil as the "source" and "sink" of mercury makes remediation of mercury-contaminated soil more difficult.Mercury can also react in soil to produce more toxic methylmercury.Rice,as one of the main grains,has much higher enrichment capacity for mercury,especially methylmercury,than other crops.The special wetland environment of paddy fields makes paddy fields bec-ome a hot research area for methylmercury.Therefore,it is necessary to explore an effective control and remediation measure for mercury-contaminated rice.In this study,mercapto-montmorillonite and chitosan-montmorillonite were successfully prepared by modifying montmorillonite as insitu remediation materials for mercury-contaminated soils.FTIR,X-ray diffraction?XRD? and scanning electron microscopy ?SEM? were used to characterize the properties of montmorillonite before and after modification.The modified and modified montmorillonite were added to the polluted soil of mercury mining area in 2% proportion.The efficiency of reducing total mercury and methyl mercury in rice by montmorillonite,mercapto montmorillonite and chitosan-montmorillonite was studied through soil immersion experiment and rice pot experiment.The results show that:?1?The characterization results of FTIR,XRD and SEM showed that the characteristic peaks of montmorillonite modified by thiol and chitosan changed significantly,indicating that thiol and chitosan were successfully loaded on montmorillonite.At the same time,the interlayer spacing of the modified montmorillonite did not change significantly,suggesting that thiol and chitosan may mainly adhere to the surface of montmorillonite and react with hydroxyl groups on the surface.?2?Soil immersion experiments showed that the concentration of methylmercury in the soil after adding remediation materials decreased in varying degrees compaed with that in the blank group under the conditions of flooding,alternating drying and wetting,and drought.After adding montmorillonite,soil methylmercury can be reduced by 33.73%,while after adding modified mercapto-montmorillonite,soil methylmercury can be reduced by 82.10%,followed by chitosanmontmorillonite,and soil methylmercury can be reduced by 49.79%.In addition,the addition of remediation materials has a certain degree of passivation effect on exchangeable and water-soluble mercury in soil.The passivation effect of montmorillonite is the best under drought conditions,with a reduction rate of 10.12%,while that of mercapto-montmorillonite reaches the highest reduction rate of 63.86%under flooding conditions,and that of chitosan-montmorillonite reaches the highest reduction rate of 31.51%under flooding conditions.At the beginning of the experiment,the concentration of exchangeable and water-soluble mercury was significantly correlated with the concentration of methyl mercury;at the end of the experiment,there was only a significant correlation between them under the condition of alternating wet and dry.?3?The results of rice pot experiment showed that compared with common montmorillonite,the effect of mercapto and chitosan modified montmorillonite on controlling mercury in rice was significantly improved.The total mercury and methylmercury concentrations in rice grown in Wanshan mining area were 38.06±4.64 ?g·kg^-1 and 27.40±5.55 ?g·kg^-1?MeHg?,respectively.After adding montmorillonite the mercury concentrations in rice decreased to 34.03±3.27?g·kg^-1 ?THg?,21.44±2.11?g·kg^-1 ?MeHg?,with an average reduction rate of 10.59% and 21.75%, respectively.After adding mercapto-montmorillonite,the reduction rate can reach67.34% ?THg?,79.10% ?MeHg?,and after adding chitosan-montmorillonite,the reduction rate can reach 18.52% ?THg?,52.76% ?MeHg?.The total mercury and methyl mercury concentrations in rice grown in Shaanxi mining area soil were 83.53±1.88?g·kg^-1?THg?,53.34⁵5.99?g·kg^-1 ?MeHg?,respectively.After adding montmorillonite,the reduction rates of mercury concentration in rice were 17.09% ?THg?,22.86% ?MeHg?,and 40.25% ?THg? and 64.27% ?MeHg? respectively.