The Morphological Distribution And Adsorption Behavior Of Thioarsenate In Groundwater With High Arsenic Content

High arsenic groundwater is a global environmental problem.Thio-arsenite/arsenate,as newly identified arsenic species,have attracted extensive attentions from experts and scholars at home and abroad.In this study,high arsenic groundwater areas of Hetao Plain with the arid and semi-arid climate and Jianghan Plain with humid climate were selected as study areas.Firstly,based on field investigation results and thermodynamic data,arsenic speciation in high arsenic groundwater was simulated and analyzed by phreeqc software,meanwhile,the main factors affecting thio-arsenite/arsenate distribution were identified.Secondly,monothioarsenate of high purity was synthesized,and a novel method of LC-HG-AFS was developed for on-line simultaneous detection of arsenate,arsenite and monothioarsenate concentrations in water.Finally,a series of laboratory experiments were designed to find adsorption kinetics and thermodynamics characteristics of monothioarsenate on sand and soil sediments,and the influences of pH and solid-liquid ratio on its adsorption behavior.The main conclusions of this study are as follows:?1?As???/As???in groundwater of Hetao Plain and Jianghan Plain are redox potential systems that control the distribution characteristics of arsenic species.At the same time,the distribution of arsenic speciation in groundwater with high arsenic content is closely related to the sulfide concentration in groundwater.With the increase of sulfide concentration in groundwater,the arsenite content in groundwater in Hetao Plain and Jianghan Plain decreases obviously,while the arsenite content in groundwater increases obviously.?2?In Hetao Plain and Jianghan Plain,arsenite is the main form of groundwater,followed by thioarsenate.Thioarsenates are dominated by monothioarsenate,trithioarsenate and tetrathioarsenite,and thioarsenites are dominated by monothioarsenite.The content of arsenate in groundwater in Hetao Plain is relatively high,which is second only to that of arsenite and thioarsenate in this area.Thioarsenite content is the least,less than 1%of total arsenic content.The contents of thioarsenite and arsenate in groundwater with high arsenic content in Jianghan Plain are lower than 1%of total arsenic content.?3?In the laboratory,sodium thioarsenate?Na₃AsO₃S·7H₂O?with a purity of 98.16%was synthesized.A new LC-HG-AFS method for simultaneous on-line analysis of arsenite,arsenate and thioarsenate was established.The linear correlation coefficient R2 is 99.98%in the range of 35.80-358.03 ug/L of thioarsenate,the detection limit is 33.03 ug/L,and the method error is-0.11%.?4?The adsorption of monothioarsenate was on sand and sediment reached equilibrium in about 24 hours.However,under the coexistence of arsenite and monothioarsenate was,monothioarsenate and arsenite can be oxidized to form arsenate after 24 hours of adsorption equilibrium on sediments,which leads to the breakdown of the original adsorption equilibrium and the concentration of monothioarsenate in the solution is further reduced.?5?The maximum adsorption capacity of monothioarsenate was under single layer adsorption on sand and soil was 21.64?g/g and 296.00?g/g,respectively.When the solid-liquid ratio?m/v?was within the range of 11.1⁶6.7?g/L?,the equilibrium adsorption capacity?Qe?of thioarsenate was on sand decreased linearly.When m/v is greater than or equal to 66.7,Qe decreases slowly.When the pH is between 7 and 10,Qe is at a low level.Field investigation shows that groundwater with high arsenic content is mostly in a neutral and alkaline environment,which means that the content of thioarsenate was in groundwater with high arsenic content will be at a relatively high level.