An Investigation Of The Effects Of Sediments Adsorption/desorption On Arsenic Fate And Transport In Groundwater System Of Jianghan Plain

High concentration of arsenic groundwater pollution is nawdays one of the most serious global environmental geological problems and hundreds of millions of people are exposed under arsenic health threats around the world.Jianghan Plain is famous for its intensive agricultural activities and plenty of phosphate fertilizer was used.However,due to the low efficiency of phosphate fertilizer application,large amount of phosphate has entered groundwater system and therefore promoted the arsenic accumulation.At the same time,different states of arsenic have varies bioa-vailabilities in groundwater system and causing different environmental ecological effects.Therefore,in order to provide scientific basis of preventing and controlling the excessive arsenic accumulated in corps and drinking water caused by phosphate and provide certain reference for solving and regulating similar scientific problems in the world.This thesis studied a high arsenic groundwater site by collecting and analyzing groundwater samples,surface water samples,and sediment samples of different depths in Xiantao City,Hubei,China and applied ICP-MS,ICP-OES,IC,XRF,XRD,SEM,TOC and parallel extraction to investigate and characterize the sample collected underground.Based on the hydrogeochemical characteristics of aquifer sediments,the competitive adsorption/desorption process between As（Ⅲ）,As（Ⅴ）,and PO₄^3-in sediments was explored as well as conversion process in the groundwater system of Jianghan Plain.An Investigation of the Effects of Sediments Adsorption/Desorption on Arsenic Fate and Transport in Groundwater System of Jianghan Plain.Mainly conclusions are as follows:（1）The neutral weak alkaline groundwater chemical form of Shahu Field Site is mainly HCO₃-Ca·Mg and the groundwater oxidation-reduction potential is negative,and has large ammountgs of NH₄⁺-N,HS^-and Fe²⁺,indicating a reduction condition.The concentration of As（Ⅲ）is generally higher than that of As（Ⅴ）in all samples.The average concentration of total As in groundwater is 74.39μg·L^-1 with a maxinxium concentration of 580.77μg·L^-1,which occurs at 25 m below the surface.The spatial distribution of arsenic is extremely uneven.（2）Sediments mineral forms are mainly composed of quartz,feldspar,and clay.The arsenic contents in sediments ranged from 5.59 to 21.15 mg·kg^-1,and the mean value was 11.85 mg·kg^-1.The arsenic concentrations in the sediments increased with the increasing depth at beginning and then start to decrease at 17.4 m,which has a maxium arsenic concentration of 21.15 mg·kg^-1.The variation of arsenic concentrations with depth showed the same trend with the change of TOC,Fe₂O₃,and Al₂O₃ concentrations revealed the accumulation of arsenic in organic matter,iron-aluminum oxide,and large specific surface areas in clay layer.The arsenic in sediments mainly exists in the form of crystalline iron oxides,dissolved sulfides,and reactive silicates.It also proved that the fate and transport of arsenic in groundwater in Jianghan Plain is mainly affected by the reduction and dissolution of iron hydroxides.（3）PO₄^3-as a dominant species in Jianghan Plain has a greater affinity than As（Ⅲ）and As（Ⅴ）so that result in ion exchange and accumulate released arsenic in the groundwater.（4）Both of a faster desorption rate of As（Ⅲ）than that of As（Ⅴ）and the inhibitory effect of PO₄^3-on the adsorption of As（Ⅲ）on sediments indicate that As（Ⅲ）is more mobile than As（Ⅴ）.Sediment adsorption of PO₄^3-,As（Ⅲ）,As（Ⅴ）,and adsorption of PO₄^3-under the influence of As（Ⅲ）,As（Ⅴ）,and adsorption of both As（Ⅲ）and As（Ⅴ）under the influence of PO₄^3-are all meets the linear quasi-secondary kinetics.（5）In the groundwater system,the adsorption of arsenic by sediments under different depths are similar when the initial concentrations of As（Ⅴ）or As（Ⅲ）is very low.When the initial concentration of As（Ⅴ）or As（Ⅲ）increased(≥500μg·L^-1),the sediments at a depth of 20.1 m is less capable of adsorbing As（Ⅴ）or As（Ⅲ）than any other depths lead to the most accumulation of arsenic at this level.（6）Sediments below 17.8 m to 21.3 m and 21.3 m to 27.3 m of depth are lithologic fine sand and coarse sand layer,respectively.Both having low specific area so that the absorption capability is not stable.Moreover,the existing phosphate also accelerated the release of absorbed arsenic and therefore causing arsenic accumulation in the groundwater.During the releasing,part of organic compounds are consumed and some iron oxides are reduced and dissolved,however,the phosphate has increased.The phosphate adsorption/desorption experiment results indicates that the sediments has weaker arsenic adsorption capability within the depth of 17.8 m to 21.3m and 21.3 m-27.3 m resulting in high concentrations of arsenic in the groundwater.The higher arsenic adsorption capability was observed within the sediments depth of17.8 m to 21.3 m and 21.3 m to 27.3 m and lead to high concentrations of arsenic in groundwater solution.