Study Of Methylmercury Abiotic Degradation In Fe-Riching Soil

Elevated mercury deposition resulting from human activities has caused widespread mercury contamination of aquatic and terrestrial systems around the world.Soil are generally considered to be sinks for mercury deposited to the landscape,but also act as biogeochemical reactors wherein inorganic mercury is transformed into bioaccumulative,organic methylmercury（MeHg）.MeHg is a toxic compound.It forms mainly in reducing environments,and then degrades through biogeochemical processes.Recent,photodegradation and microorganism degradation of MeHg are among the processes that have been reported.However,little attention has been focused on the abiotic degradation of MeHg in soil/sediment without light.To demonstrate the abiotic degradation process of MeHg,basing on the geography and hydrogeology characteristics of Guangzhou,this dissertation verify a abiotic degradation process of MeHg in Guangzhou soil.（1）Based on the distribution of functional zones,we thoroughly carried out a survey for soil pollution in Guangzhou.In the results,the percent MeHg（methylation percent of Hg）in Guangzhou soil in southern China was found to be variable and exhibited a significant negative correlation with the content of Fe or Cu.Thus,we degrade MeHg with soil sample,and carried out a series of control experiment as well as using ESR technology.The results prove that the degradation of MeHg is a radical-based and O₂needed chemistry process.（2）To understand the contributors of MeHg degradation by,batch experiments with structural and dissolve Fe/Cu-containing components were done,respectively.Results showed that annite in the soils could activate O₂ to generate·OH and·O₂^-to elevate MeHg degradation under oxic conditions.Meanwhile,Cu components in the soil further enhanced the production of·OH,and was oxidized to Cu（Ⅲ）promoting degradation of MeHg directly.（3）To exam whether reduced iron-bearing clay minerals（annite）oxygenation can degrade MeHg in field soil,we conduct soil column to imitate the water table fluctuation induced by rain percolation,and the results demonstrated that MeHg can be degraded by annite under the water table fluctuation condition.As rainfall is the basic condition,the characteristic of Guangzhou rainwater will have influence on the degradation process,and also be investigated in soil column experiments.Interestingly,the effect of the Guangzhou rainwater main ionic seem had no obvious influence on MeHg degradation in soil column,while the acid rain in Guangzhou positive contribute to the process.To understand the influence mechanism of pH and cupric ion on MeHg degradation,batch experiments were carried out,the results of which prove that Cu content in the soil can improve the production of·OH under acid condition,while be activated as Cu（Ⅲ）under neutral and alkaline condition,both of them can promote the degradation of MeHg.The products of MeHg degradation are also identified in this work,both Hg（Ⅱ）and Hg（0）are detected.