Mobilization Of Arsenic During Light-induced Dissolution And Transformation Of Arsenic Contained Iron (Hydr) Oxides

The light-induced dissolution of iron(hydroxy)oxides in natural environment is of great importance.However,light-induced dissolution of iron(hydroxy)oxides with As(?)has rarely been reported in literature,and in that process the redistribution mechanism of As(?)between aqueous environment and iron(hydroxy)oxides is also poorly understood.In this study,we simulated the light-induced dissolution of ferrihydrite and schwertmannite with As(?)in acidic mine drainage.The reactivity of ferrihydrite and schwertmannite with adsorbed As(?)[Fhy*-As(?)and Sch*-As(?)] toward light-induced dissolution by oxalate was investigated.We also compared the effect of adsorbed and structurally-incorporated As(?)[Sch*-As(?)and Sch-As(?)] on the reactivity of schwertmannite towards light-induced dissolution by oxalate.By means of XRD,FTIR and TEM analyses,effect of secondary minerals formed during these processes on the mobilization of As(?)was elucidated.The light-induced dissolution of ferrihydrite and schwertmannite can occur at pH = 3.0 in the absence of any dissolved organic ligands.However,in the presence of oxalate,a large amount of Fe(III)and trace amounts of Fe(II)were formed in the solution during the dark reaction,indicating that the dissolution was mainly controlled by the ligand-promoted dissolution.Under U? irradiation,the concentration of Fe(II)in the solution initially increased rapidly,reached a maximum value quickly,and then decreased slowly.Experimental results have shown that in the presence of oxalate,light-induced dissolution of ferrihydrite and schwertmannite caused a significant increase in the pH value of the solution.High pH values are not only unfavorable to the light-induced dissolution,but also cause the oxidation of Fe(II)in the solution and subsequent hydrolysis of Fe(III).Goethite was found in the solid product after the light-induced dissolution of both ferrihydrite and schwertmannite in the presence of oxalate.The dissolving process of the ferrihydrite and the schwertmannite with adsorbed As(?)is similar to that of without As(?)respectively,but the amount of released Fe is relatively small.It shows that adsorbed As(?)inhibits the dissolution of ferrihydrite and schwertmannite.Structurally-incorporated As(?)made the schwertmannite exhibit a higher activity toward light-induced dissolution.In the absence of oxalate,U? irradiation promoted the mobilization of As(?)into aqueous environment.While in the presence of oxalate,U? irradiation can significantly inhibit the mobilization of As(?)into aqueous environment.In the presence of oxalate,for Fhy*-As(?)and Sch*-As(?),the concentration of As(?)in solution in the dark was 7.75-13.0 and 14.0-36.5 times respectively as that under U? irradiation under the same condition.For Sch-As(?),the concentration of As(?)in the dark reaction is 6.30-8.75 times that of under U? irradiation under the same conditions.The significance of this study is to reveal the light-induced dissolution of iron(hydroxy)oxides with As(?)by oxalate,and the mobilization mechanisms of As(?).It provides a theoretical basis for effectively controlling arsenic contamination in aqueous environments.