Mechanism Of Coexisting Components On The Interaction Between Arsenic And Carbonate In High Arsenic Groundwater

Recently,high arsenic groundwater is one of the serious environmental geological and health problems that the world is facing.Drinking groundwater with high arsenic can cause serious damage to human health and even threaten lives.The migration,transformation and enrichment of arsenic in groundwater are influenced by geological,hydrogeological and hydro-biogeochemical processes.Carbonate is one of the most prevalent and critical hydrogeochemical processes in the evolution of groundwater quality.Carbonate has strong interactions with various pollutants.The latest research had shown that the carbonate system in groundwater plays an indispensable role in the migration transformation,enrichment and fixation of arsenic.So far,many international geological scholars had carried out research work on calcite-arsenic interactions and achieved a large amount of scientific understanding.But research on the behavior of arsenic-carbon system and the influence of key coexisting components in high arsenic groundwater is still lacking.In order to further improve the understanding of the arsenic enrichment mechanism with carbonate in groundwater.This thesis selected Yuncheng Basin in northern China and Jianghan Plain in southern China as the key investigation areas.The geochemical characteristics of arsenic-carbonate systems in groundwater and sediments of these two areas were systematically investigated.On this basis,indoor batch experiment of the effects of representative coexisting components（major component Mg,trace element Fe and dissolved organic matter FA）on carbonatite-arsenic interactions in aqueous solution were simulated.Combined elemental morphological analysis（IC-HG-AFS）,microscopic solid-phase characterization（SEM,TEM,XRD,FTIR,XPS and 1H-NMR）and hydrogeochemical simulation（PHREEQC）,which revealed the mechanism of arsenic uptake in carbonate systems in groundwater under the influence of co-existing components.The main insights obtained in this thesis are as follows:（1）Hydrogeochemical investigation of high arsenic groundwater found that high arsenic water in the study area is generally characterized by Fe enrichment,high bicarbonate and high organic matter（DOC）content.As the carbonate in groundwater reaches supersaturation,the arsenic concentration in groundwater shows a significant decreasing trend.In addition,environmental mineralogical characterization and arsenic grading extraction experiments for aquifer sediments were carried out.The results proved that the percentage of carbonate-arsenic in the sediments was about 9.2%,and it was mainly calcite-arsenic（77%-84%）.Based on the above understanding,it is judged that during the process of carbonate precipitation in groundwater,the uptake of arsenic in groundwater would through adsorption and co-precipitation.The SEM-EDS results of secondary Mg-carbonate in sediments further verify this inference that arsenic uptake by Mg-carbonate in the form of interfacial adsorption and co-precipitation during carbonate precipitation.（2）Interfacial adsorption experiments of arsenic on Mg-calcite were carried out,results showed that the arsenic adsorption capacity of Mg-calcite was significantly stronger compared to that of pure calcite.When the initial arsenic concentration was300μM,the adsorption capacities of calcite(Ca₁₀Mg₀)and Mg-calcite（Ca₈Mg₂）for As（V）were 0.21 mg/g and 0.44 mg/g,respectively.While those for As（III）were 0.07mg/g and 0.12 mg/g.The adsorption mechanism of Ca₁₀Mg₀ for arsenic mainly includes:1)electrostatic adsorption;2)As-O anion is adsorbed by ion exchange with CO₃^2-on the calcite surface.The adsorption mechanism of Mg-calcite for As（V）and As（III）is complicated,including:1)Mg-calcite has more surface positive charge than pure calcite,which is conducive to the electrostatic adsorption.2)Mg bring in the shrinking of calcite lattice,which provides more space for the exchange of As-O anion.3)More OH groups on Mg-calcite surface because of the hydrolysis of Mg,which helps to form bidentate complexes with As（V）,and also facilitates the formation of hydrogen bonds with As（III）.（3）The presence of Mg in groundwater significantly affects the co-precipitation behavior of carbonatite and As（III）.It was found that the content of As（III）in carbonate is as high as 0.20 mg/g in Mg co-precipitation system,which is much higher than that of0.11 mg/g in the system without Mg.But the presence of Mg has less influence on the co-precipitation of As（V）.The main mechanisms of the effect of Mg on arsenic co-precipitation were obtained:1)As O₄^3-and H₂As O₃^-can easily replace CO₃^2-and binding with Ca²⁺,then incorporated into the carbonate lattice.2)As（V）and As（III）would also be immobilized on Mg binding sites by adsorption.3)The incorporation of Mg promotes the dissociation of As（III）bring in more H₂As O₃^-to be immobilized.4)The presence of Mg leads to more HCO₃^-in solution,which will generate the easily adsorbed complex:As（OH）₂CO₃^-.（4）The high Fe content in high arsenic groundwater will inevitably precipitate with calcite and form Fe-calcite during the supersaturated precipitation of carbonate.Because of the strong influence of Fe on arsenic migration,it is necessary to investigate the arsenic adsorption on Fe-calcite.Experimental results showed that the adsorption capacity of Fe-calcite（20Fe-calcite）for As（V）was three times higher than that of pure calcite（0Fe-calcite）,and about four times higher than that of 0Fe-calcite for As（III）.The results indicated that the strong adsorption capacity of Fe-calcite for arsenic.The main adsorption mechanisms are:1)Fe-calcite has more surface positive charge than pure calcite,which enhances the electrostatic adsorption.2)The CO₃ group in the complex Fe CO₃OH is more easily released and participate in the ion-exchange interaction.3)Fe also brings abundant Fe-OH on calcite surface,which has strong adsorption properties with arsenic.4)The increase of SSA of Fe-calcite also provides more binding sites for adsorption.（5）The presence of Fe in groundwater may significantly enhance the co-precipitation of arsenic by carbonate.Simulation experiments showed that the contents of As（V）and As（III）in the carbonate reaches 0.78 mg/g and 0.23 mg/g in the co-precipitation system with Fe,respectively.These amounts were almost about twice as much as those in the blank group（without Fe）.The presence of Fe leads to the new mechanism of carbonate-arsenic co-precipitation:1)Fe will bring in iron hydrous oxides（HFO）,and the As-O anion will co-precipitation with HFO.Therefore,the precipitation rate increased,which promotes the precipitation of As（V）.2)The HFO with positively charged under the conditions of this study,so it is easier to bind to As-O anion.3)The co-precipitation process is also accompanied by the generation of Fe OOH with strong adsorption properties,then arsenic uptake through the coordination with Fe OOH.4)For As（III）,Fe-calcite can promote the dissociation of H₃As O₃⁰.5)Fe³⁺in solution will oxidize a portion of As（III）to As（V）,therefore,the precipitation of As（III）is promoted.（6）The presence of FA in calcite significantly enhances the adsorption capacity of arsenic.The adsorption capacity of 20FA-calcite for arsenic is about twice as large as that of 0FA-calcite.In addition to the ion-exchange interaction,the adsorption mechanism includes the coordination between FA on calcite surface and arsenic.Carboxyl and hydroxyl group would coordination with As through deprotonation and dihydroxylation under the bridging effect of Ca²⁺,respectively.In addition,with the coordination reaction,Ca²⁺in solution is continuously consumed.Thus,promoting the dissolution of calcite and the increase of CO₃^2-,which is more favorable for the ion-exchange interaction between As-O anion and CO₃^2-.Finally,FA-calcite causes the occurrence of lattice defects and the change of CO₃ group bond energy,which are more helpful for the ion-exchange interaction.（7）Dissolved organic matter in groundwater significantly enhances the arsenic co-precipitation by carbonate.Compared with the blank group,when the concentration of FA in solution was 20 mg/L,the As（V）and As（III）uptake by carbonate increased to0.70 mg/g and 0.29 mg/g,respectively.The characterization of liquid and solid phase showed that the mechanism of arsenic co-precipitation includes:Firstly,Ca²⁺,FA and As in solution form FA-Ca-As ternary complexes by cation bridging.The presence of FA reduces the precipitation rate of carbonate and leads to more Ca²⁺to form FA-Ca-As complexes.Subsequently,some of these FA-Ca-As complexes gradually enter the lattice of carbonate by lateral squeezing,while others are wrapped around the precipitated of carbonate in the form of aggregates.These special combinations between FA and carbonate provide more ways for arsenic immobilization.For As（III）,the reduced precipitation rate also leads to the generation of more As（OH）₂CO₃^-favoring immobilization.Additionally,As（III）would be more likely to react with FA than dissociation by diffusion onto the surface of carbonate.In summary,this thesis systematically reveals that the coexisting components（such as Mg,Fe and dissolved organic matter（FA））in high arsenic groundwater positively contribute to the arsenic uptake by carbonate.It further explains the source and formation mechanism of high arsenic in carbonate,which has never been solved in the international field of high arsenic water environment.This thesis is of academic value in promoting the research of arsenic in international environmental.These results enrich the theoretical understanding of arsenic uptake by impure carbonate in groundwater systems.Meanwhile,this thesis provides new insights into arsenic cycling in groundwater and environmental.The new theoretical support and technical approach to the treatment of high arsenic groundwater was supplied.