| Hotan area is located in the southern part of the Tarim Basin in Xinjiang.Groundwater is the main source of water supply for the people of all ethnic groups.However,the concentration of fluoride and arsenic in the local groundwater has exceeded the standards and caused serious health risks.The research on the mechanism and efficient co-removal method of arsenic and fluoride is very important.The research results can provide a certain theoretical basis for the reasonable development of local groundwater resources and the prevention and improvement of water.This paper analyzed the formation mechanism of high arsenic and high fluoride in shallow groundwater in the study area based on local hydrogeological conditions,groundwater hydrochemical characteristics,combined with mathematical statistics,hydrogeochemistry and other methods.At the same time,lanthanum-containing activated carbon materials were prepared to study the performance of arsenic and fluorine removal.The main research results were as follows:(1)The chemical type of surface water was mainly HCO3·SO4·Cl-Na·Ca.The fluorine concentration of only one surface water sample in Hotan County exceeded the standard,and arsenic concentrations of all samples were not detected.The groundwater hydrochemical types were mainly HCO3·SO4·Cl-Na·Ca·Mg,SO4·Cl-Na·Ca-Mg,and HCO3·SO4·Cl-Na-Ca.The fluorine concentration of 41.4%groundwater samples exceeded the standard,and arsenic of 5.4%samples exceeded the standard.The concentration of arsenic and fluorine coexistence of 4.0%samples exceeded the standard.(2)High fluoride and high arsenic coexisting groundwater were mainly distributed in the northeast of Minfeng County.The high-arsenic groundwater hydrochemical type was mainly SO4·Cl-Na,and the high-fluorine groundwater hydrochemical type was mainly HCO3·SO4·Cl-Na-Mg.The high-arsenic and high-fluorine coexisting groundwater hydrochemical type was mainly SO4·Cl-Na.As the depth of the well increased,the concentrations of arsenic and fluorine in the groundwater also increased.The alkaline reduction environment in groundwater,strong evaporation and concentration,the dissolution of iron and manganese oxides,and the alkaline chemical environment of soda water were all conducive to the co-enrichment of arsenic and fluorine in groundwater.Evaporation and concentration,weathering of silicate and evaporite,dissolution of minerals such as fluorite,and cation exchange were main factors controlling high arsenic and high fluoride groundwater.The health risk assessment results of arsenic and fluoride in groundwater showed that both carcinogenic and non-carcinogenic risks exceed allowable limits,and arsenic and fluoride through drinking water pathways can adversely affect human health.(3)Correlation analysis of arsenic,fluorine and other groundwater chemical components showed that As had a significant positive correlation with K++Na+,Mg2+,Cl-,SO42-,HCO3-,TDS,F-,and Fe.F-and K++Na+,Ca2+,Mg2+,Cl-,SO42-,HCO3-,TDS,As,NH4+showed significant positive correlations.(4)Hydrogeochemical simulation results of shallow groundwater in the study area showed that dissolution reactions had occurred in the dolomite,fluorite,rock salt,gypsum,and CO2.Precipitation of calcite occurred.At the same time,Na+desorption and Ca2+adsorption occurred.Na+entered the groundwater from the mineral phase,and Ca2+entered the mineral phase from the groundwater.The dissolution of rock salt,dolomite,gypsum and other minerals increased the Na+and Ca2+concentrations in the groundwater,and calcite reached saturation and precipitates.Fluoride in groundwater mainly came from the dissolution of fluorite minerals.(5)The SEM,FTIR and XRD results showed that the lanthanum-loaded activated carbon with modified pores increased and the specific surface area increased.The modified O-H stretching vibration peak,N-O stretching vibration peak,and C-O stretching vibration peak intensity weakened,and La-OH stretching vibration peak appeared at 694 cm-1.Lanthanum was supported on the surface of activated carbon in an amorphous form(amorphous lanthanum oxide).(6)Increasing the amount of lanthanum-loaded activated carbon was beneficial to improve the removal rate of arsenic and fluorine coexistence.Lanthanum-loaded activated carbon was suitable for co-removal of arsenic and fluorine under weakly acidic conditions.When the pH was 4-6,the co-adsorption effect of lanthanum-loaded activated carbon on arsenic and fluorine was the best.The removal rate of fluorine was more than 20%,and the maximum adsorption amount was 9.01 mg/g.The removal rate of arsenic was about 18%,and the maximum adsorption amount was 0.67 mg/g.The effect of pH value on the adsorption of fluorine was more significant than that on arsenic.When the adsorption time reached 24 h,the lanthanum-loaded activated carbon basically reached the co-adsorption equilibrium of arsenic and fluorine.The adsorption amounts of fluorine and arsenic in the coexisting solution of the initial fluoride concentration of 10 mg/L and arsenic concentration of 1 mg/L were 2.69 mg/g and 2.3 mg/g.The adsorption of arsenic by lanthanum-loaded activated carbon conformed to a quasi-first-order kinetic model,and the adsorption of fluorine more conformed to a quasi-second-order kinetic model.Isothermal adsorption studies showed that the adsorption conformed to the Langmuir adsorption model,and the maximum theoretical adsorption amounts for arsenic and fluorine were 2.25 mg/g and 1.84 mg/g.(7)Studies on the effects of coexisting ions showed that arsenic adsorption was more affected by coexisting anions.The effects of anions on the fluoride removal effect were HCO3->PO43->Cl->SO42-,and the effects of arsenic removal effect were HCO3->PO43->SO42->Cl-.The effects of cations on the adsorption of arsenic on lanthanum-loaded activated carbon were Ca2+>Mg2+.Mg2+had less effect on the removal of fluorine,and Ca2+promoted the adsorption of fluorine.The interaction of target ions showed that the presence of fluorine did not affect the adsorption of arsenic,but the presence of arsenic had a greater impact on the adsorption of fluorine.(8)Response surface optimization experiments showed that the significant relationship between the amount of fluoride adsorption was pH value>initial concentration>dosage,and the significant relationship between the total arsenic adsorption amount was pH value>initial concentration>dosage.The optimal fluoride adsorption conditions were:when the dosage was 67.3 mg,the initial concentration was 7.1 mg/L,and the pH was 4,the fluoride ion adsorption was 13 mg/g.The optimal adsorption conditions for total arsenic were:when the dosage was 25 mg,the initial concentration was 10 mg/L,and the pH was 3,the total arsenic adsorption was 3.2 mg/g.The study of the effect of different influent concentrations on the dynamic co-adsorption of arsenic and fluoride on the adsorption column showed that at a given flow rate,the initial concentration of influent increased,and the penetration time and saturation time decreased. |
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