Effects Of Hydrochemical Factors On Arsenic Migration And Transformation In Groundwater-surface Water Interaction Zone

Hydrochemical conditions are one of the important factors that influence the transfer and conversion of arsenic in the groundwater and surface water interaction zone.It is of great realistic connotation to strengthen the study of various complex hydrochemical factors of groundwater and surface water on the migration and transformation of arsenic.In this study,we prepare As(III)and As(V)imitation solutions and collect river sand from riparian zone as adsorption medium.Carry out the static adsorption experiments to study the adsorption characteristics of arsenic on river sand.Investigate the effects of different hydrochemical factors(including ph,redox conditions,coexisting ions)on arsenic adsorption behavior on river sand.The mechanism were revealed by XRD,XPS,FTIR,SEM-EDS and other material characterization techniques.On this basis,we design dynamic sand column experiment to explore the migration and transformation of arsenic under the condition of surface water and groundwater cycling alternately.The following conclusions are drawn:(1)The pseudo-second-order model can fit the adsorption process of As(III)and As(V)by river sand better;the isothermal adsorption experiments show that both Langmuir and Freundlich models can well fit the isotherm adsorption curve of As(III)and As(V)by river sand(R²>0.99).The maximum single-layer adsorption capacity of As(III)and As(V)on river sand is 348.19?g/g and 442.76?g/g,respectively.(2)As(III)and As(V)adsorption capacity of river sand decreases with the increase of pH.As(III)and As(V)adsorbed by river sand gradually decreased with the experimental conditions from aerobic to anoxic to reduction,and the reduction conditions were not conducive to arsenic adsorption.On the whole,the coexisting anions have inhibitory effect on arsenic adsorption by river sand,and the order is:PO₄^3->Si O₃^2->S^2->HCO₃^->SO₄^2->NO₃^-.The coexisting cations can promote arsenic adsorption by river sand in the order of Fe³⁺>Fe²⁺.(3)The pH,REDOX environment,PO₄^3-,Si O₃^2-,HCO₃^-,Fe³⁺and Fe²⁺were identified as the main hydrochemical factors that influence the adsorption of arsenic by river sand.pH affects the adsorption and desorption of arsenic by changing the electrostatic attraction (repulsion)force between river sand surface and arsenic and the competitive adsorption between OH^-and arsenic;redox environment affects arsenic migration by changing the form of arsenic;PO₄^3-,Si O₃^2-and HCO₃^-compete with arsenic to inhibit arsenic adsorption;Fe³⁺and Fe²⁺are beneficial to arsenic adsorption by forming iron(hydrogen)oxide or colloid attached.(4)The results of dynamic sand column experiment show that the arsenic concentration in effluent gradually reaches penetration with the increase of the number of alternate cycles between surface water and groundwater.In the second and third alternate cycles,the arsenic-containing simulated liquid flows down into the surface water.After the arsenic concentration in the effluent stabilizes,the arsenic-containing simulated liquid flows upwardly into the groundwater.Due to the difference in chemical composition of groundwater and surface water,the entrance caused the river sand to reduce the adsorption of arsenic,and even the originally adsorbed arsenic was desorbed,which significantly increased the arsenic concentration in the effluent.Compared with surface water,the water chemical composition of groundwater is more unfavorable for river sand to adsorb arsenic.(5)The river sand with sulfur(S2-)and iron(Fe²⁺,Fe3+)added was characterized by FTIR,XRD,XPS and SEM-EDS.The results showed that:in the reaction system with S^2-addition,the O/Si ratio value increased significantly and a small peak of As-S mineral occurred after adsorption.These indicated that the sorbed arsenic reacted with S^2-to form As-S precipitation on the surface of river sand and thereby the adsorption sites for arsenic decreased.In addition,the decrease of Fe and O contents suggested that iron oxides reduction dissolution,which was caused by the strong reducing environment by adding S^2-,was also related to arsenic adsorption decrease.In the system with Fe²⁺and Fe³⁺addition,no iron oxide but a new peak of As-Fe were formed on the river sand after adsorption.These indicated that the increase of arsenic adsorption may be attributed to the facts that the formed Fe(OH)₃ in solution sorbed part of arsenic from aqueous solutions and part of aqueous arsenic reacted with iron to form As-Fe precipitation on the surface of sand.