Study On Mechanochemical Preparation Of Tricalcium Aluminate Composite Material And Removal Of Arsenic From Groundwater

Arsenic pollution in groundwater has always been a global environmental problem,which seriously threatens the lives and health of people in areas where groundwater is used as a source of water for production and living.It is necessary to develop simple and economical methods to treat groundwater contaminated by arsenic.At present,arsenic treatment methods in groundwater include ex-situ treatment and in-situ treatment.The core lies in the development of high-efficiency and low-cost arsenic-fixing materials.Tricalcium aluminate（Katoite）is a fast-hardening cement C₃A（3CaO·Al₂O₃）hydration product,which has an excellent curing effect on heavy metal ions.The zero-valent iron has a good performance in adsorbing and fixing arsenic,and it has been widely studied and applied to in-situ removal of arsenic in groundwater.In this paper,Katoite and its composite material with zero-valent iron（Katoite-Fe composite material）were prepared by mechanochemical one-step dry grinding.Explore its removal performance of As（Ⅲ）and As（Ⅴ）in groundwater,and explore the removal mechanism,which provides a theoretical basis for removal of arsenic in groundwater.First,Ca（OH）₂ and Al（OH）₃ were jointly ball-milled to prepare Katoite for the ex-situ treatment process of arsenic in groundwater.XRD,FT-IR and SEM characterizations showed that Katoite was successfully synthesized.Adsorption kinetics and isotherm adsorption studies show that the maximum adsorption capacity of Katoite for As（Ⅲ）is 455.86 mg/L,As（Ⅴ）is 347.51 mg/L.The Freundlich model can better simulate the adsorption behavior of As（Ⅲ）（R²:0.96>0.92）,and the adsorption behavior of As（Ⅴ）is more in line with the Langmuir model（R²:0.96>0.79）.At low concentrations,As（Ⅲ）and As（Ⅴ）can enter the Katoite layered space to form Ca-Al LDH intercalated with arsenite ions and arsenate ions in the solution,and the arsenic is fixed and removed by the intercalation mechanism.At high concentrations,the Ca²⁺in Katoite plays a major role in generating calcium arsenate and calcium arsenite,so that arsenic can be removed.Coexisting anions have a certain negative effect on the adsorption and fixation of arsenic by Katoite,the order is SO₄^2->Cl^->NO₃^-.Secondly,the composite material of Katoite and zero-valent iron is prepared by mechanochemistry for in-situ treatment of arsenic in groundwater.Ca（OH）₂,Al（OH）₃and iron powder were jointly ball-milled to prepare Katoite-Fe composites and the synthesis process parameters are optimized through static adsorption experiments.The optimal ratio of Fe powder for As（Ⅲ）is 50%（Katoite-50%Fe composite material）,and As（Ⅴ）is 1%（Katoite-1%Fe composite material）.Both ball mill speeds are 450 rpm.The removal of As（Ⅲ）by Katoite-50%Fe composites is mainly through the formation of Ca-Al LDH and calcium arsenite,and the presence of iron hydroxide enhances the removal of As（Ⅲ）.The removal mechanism of As（Ⅴ）by Katoite-1%Fe composite material is divided into two stages:low concentration and high concentration.At low concentration,Ca-Al LDH is formed with arsenate intercalation.At high concentration,As（Ⅴ）is fixed by Ca element.The formation of calcium arsenate can be removed.Through dynamic column adsorption to simulate in-situ processing,explore the adsorption performance under different adsorbent quality,different column height,different initial concentration and different flow rate.Both Thomas model and Yoon-Nelson model can simulate the adsorption behavior of Katoite-Fe composite material to arsenic.The results show that the adsorption capacity of the material for arsenic is（As（Ⅲ）:8327.91 mg/L;As（Ⅴ）:7725.2 mg/L）;the 50%breakthrough time is（As（Ⅲ）:277.6 min;As（Ⅴ））:386.3 min).The above studies have proved that Katoite and Katoite-Fe composite materials have excellent pentavalent and trivalent arsenic removal performance,and the residual arsenic concentration in the actual groundwater after a single adsorption is lower than the World Health Organization drinking water standard（<10 ppb）.According to the synthesis conditions and parameters,the feasibility and economic cost analysis of the large-scale production of Katoite and Katoite-Fe composites show that the cost of Katoite is 1.17 usd/m³,and that of Katoite-Fe composites is 0.22～0.33 usd/m³.Compared with other treatment technologies（0.3～20 usd/m³）,it has a certain cost advantage and can be used for the remediation of actual arsenic groundwater.