Photoreductive dissolution of schwertmannite induced by oxalate and the mobilization of adsorbed As(V).

Schwertmannite (Sch), a poorly crystalline iron mineral, shows high sorption capacity to As(V). In this study, the effects of UV irradiation and oxalate on the dissolution of pure Sch, Sch with adsorbed As(V) [Sch*-As(V)] and subsequent mobilization of As(V) were investigated at pH 3.0. Under UV irradiation, the dissolved Fe(II) took the majority of the total dissolved Fe during the dissolution of Sch and Sch*-As(V). In the presence of oxalate, Fe(III)-oxalate complexes formed on Sch [or Sch*-As(V)] could be converted into Fe(II)-oxalate by photo-generated electrons under UV illumination, and more total dissolved Fe produced compared to that without oxalate. In the dark, total dissolved Fe reached the maximum value (42.64 mg L-1 for Sch) rapidly and existed as Fe(III) predominately. In addition, UV irradiation has almost no effect on the mobilization of As(V) in Sch*-As(V) in the absence of oxalate. However, in the presence of oxalate, UV irradiation resulted in the mobilization of As(V) declined by 14-36.5 times compared to that in the dark. This study enhanced our understanding on the mobilization of As(V), and UV irradiation could contribute to the immobilization of As(V) on Sch in the aquatic environments containing oxalate.