Research On Adsorption And Desorption Characteristics Of As(?) In Groundwater By B Horizon Of The Soil Weathered From Cretaceous Sandstone In Hunan Province

Hunan Province is one of the main areas with arsenic-rich groundwater in China.Arsenic removal from groundwater has become an important task for the restoration of groundwater environment in the province.Red soil with the rich iron,manganese and clay minerals is an important material for removal of arsenic in water.Cretaceous sandstone with high contents of Fe-Mn are widely distributed in Hunan.In the weathered soil profiles,the B horizon tends to have higher Fe-Mn compositions.Therefore,it is of great theoretical and practical significance for the treatment of arsenic polluted groundwater in our province and enrichment of the practice of arsenic removal to select it as arsenic removal material of groundwater,and discuss its mechanism of adsorption and desorption of arsenic in groundwater.Based on the analysis of mineral types,contents,chemical compositions and basic physicochemical properties of the weathered red soil sedimentary layers of the cretaceous sandstone,single factor experiments such as particle size of red soil,initial concentration of arsenic in solution,pH,temperature and reaction time,and the desorption experiments of arsenic were implemented to explore the adsorption and desorption mechanism of sedimentary layers on As(V)in the groundwater.The main results were:(1)The adsorption rate of arsenic in solution decreased from 58.70%to 42.33%as the size of red soil sample increased from 200 mesh to 80 mesh.The initial concentration of arsenic in the solution increased from 0.04 mg/L to 20 mg/L,and the adsorption rate decreased from 82.7%to 24.0%.The decline of adsorption efficiency grows slow and tends to be gentle at the beginning of initial concentration of 1.5mg/L.The adsorption rate shows the decreased tendency in general from 85.0%to 48.0%with the increase of pH when changing the system pH.(2)In the first 60 min of the adsorption test,the adsorption rate is very fast,which is a rapid reaction.After that,the reaction tends to be gentle and reaches the adsorption equilibrium after 20 h.The kinetic model fitting results show that the Elovich kinetic equation and Second degree dynamic model fit well to the experimental data,which indicate there exist other reaction mechanisms in addition to the diffusion between the particles in the reaction.(3)When the temperature of the system increased from 5? to 35 ?,the adsorption rate increased from 62.20%to 85.0%,representing that the increase of temperature was in favor of the adsorption reaction.Thermodynamic analysis shows that Langmuir,Freundlich and linear isothermal adsorption models can fit the experimental data well,and all their R2 can exceed 0.90.(4)Desorption experiment results of five-step extraction showed that the five kinds of arsenic reserve states in the sedimentary layer were as follows:residual state>organic bound state>ferromanganese oxide bound state>ion exchangeable state>carbonate bound state.