Effects Of Dissolved Organic Matter On Migration And Transformation Of Soil Mercury Speciation In Critical Zones

Mercury is a highly toxic heavy metal contaminant that is persistent,migratory,and cumulative in the environment.It is transferred to the food chain through the environment-plant system,causing accumulation and amplification of toxicity,thus affecting human health.In addition,the mercury cycle is typical of heavy metals circulating in ecosystems.Mercury will migrate and transform in the state of elements in water,soil,atmosphere,and biosphere.Therefore,the study on the migration and transformation of mercury in critical zone and the treatment of mercury pollution are of great significance to the current natural ecology and human health.For the restoration of mercury in soil,we must understand its bioavailability,and the bioavailability is closely related to the adsorption-desorption behavior of mercury and the form of mercury in soil.In addition,dissolved organic matter,as an important component of terrestrial ecosystems and aquatic ecosystems,also has an important impact on the environmental chemistry of mercury in soils.Based on the results of environmental geological survey in the Pilot Area of the Zhujiang-Xijiang Economic Belt,this study will conduct sampling analysis and risk assessment of the areas with severe mercury pollution in Guangzhou City(part of Foshan City).The representative soil samples was selected and indoor simulation experiments were conducted to study the adsorption-desorption behavior and kinetic adsorption characteristics of Hg(II)in soil,and the effects of changing the initial pH and addition of dissolved organic matter on the soil adsorption process.The effect of dissolved organic matter addition on the migration and transformation of mercury in soil has also been studied,providing a basis for the treatment of mercury pollution in soil.The main research contents and results of this paper are as follows:(1)Sampling analysis and risk assessment were conducted in areas with high mercury pollution in Guangzhou.The results show that the average Hg content in the soil was 0.69 mg/kg,which is higher than most of the southern cities in China.According to the division of functional areas,the mercury in the soil of the planting area exists mainly in the form of residuals and is not easily released under normal conditions.The potential ecological risk level of the entire sampling area was represented by the medium ecological risk level.(2)The results of adsorption and desorption of Hg(II)in tested soils at 25°C showed that with the increase of Hg(II)concentration,the amount of mercury adsorbed by soil increased,but the adsorption capacity of different soil samples is different.The sorption amount of soil-desorption amount always shows a positive correlation,but the desorption rate is low.After the addition of humic acid,the amount of adsorbed and desorption of the soil decreased,indicating that the increase of humic acid inhibited the adsorption of Hg(II)by the soil.(3)The Langmuir equation can better describe the isothermal adsorption process of Hg(II)in soil.The comparison of correlation coefficients showed that the adsorption strength of soil samples with large adsorption capacity is not necessarily large.The theoretical maximum adsorption amount after addition of HA to the soil became smaller,indicating that the ability of the soil to adsorb mercury was decreased after the addition of humic acid.(4)The experimental results of the adsorption kinetics of Hg(II)in the soil under test at 25°C showed that the adsorption process is mainly divided into two stages,the rapid adsorption stage and the slow adsorption stage,and the rapid adsorption stage occurs at the initial adsorption stage.It is possible to complete 90% of the entire adsorption process and then start the slow adsorption phase.(5)With the change of the initial pH value of the soil,the amount of mercury adsorbed on the soil first gradually increases and then slowly decreases.The maximum amount of adsorption occurs when it approaches the pH value of the soil itself.(6)Add different concentrations of exogenous mercury to the soil,observe the transformation of mercury form after three months of culture,and the results showed that,with the increase of time,the soil will change from the exchangeable state to the carbonate bound state.As well as the transformation of the bound state of organic matter,and gradually through the adsorption,complexation and other effects to further transform the iron-manganese oxide bound state and residue state.(7)Add a certain amount of humic acid to the soil to study its effect on the migration and transformation of mercury in the soil.The results showed that the addition of HA promotes the formation of the exchangeable and the organic bound state,while inhibiting the formation of carbonate bound states,iron-manganese oxide bound state and residual state,but this promotion and inhibition also has a great relationship with the physical and chemical properties of the soil itself.In summary,the increase of dissolved organic matter in the soil will reduce the ability of the soil to adsorb mercury.For the mercury form,the addition of organic matter can promote the formation of the exchangeable state and organic matter binding state,and inhibit the formation of carbonate bound state,iron-manganese oxide bound state and residual state.