Oxalic Acid-Induced Photodissolution of Ferrihydrite and the Fate of Loaded As(V): Kinetics and Mechanism

Abstract

The fate of arsenic in the water environment is of great concern. Here, the influences of oxalic acid and UV light illumination on the dissolution of naked ferrihydrite (Fhy), Fhy loaded with As(V) [Fhy*-As(V)], as well as the fate of As(V) at pH 3.0 were studied. With the assistance of oxalic acid, complexes of Fe(III)-oxalic acid produced on Fhy/Fhy*-As(V) were reduced to Fe(II)-oxalic acid by photo-induced electrons under UV light irradiation. UV light has nearly no impact on the release of As(V) in the system of Fhy*-As(V) without the assistance of oxalic acid. Nevertheless, in the existence of oxalic acid, UV light illumination resulted in the contents of liberated As(V) decreased by 775–1300% compared to that without light. Considering the coexistence of As(V), oxalic acid as well as iron oxides in aquatic environments, the present study revealed that UV illumination could enhance the retention of As(V) on Fhy in the acidic water environment containing oxalic acid.

Photoreductive Dissolution of Iron (Hydr)oxides and Its Geochemical Significance

“…On the other hand, Ren et al found that UV irradiation enhanced the immobilization of As­(V) in Schwertmannite-As­(V) in the presence of oxalate, which was attributed to the transformation of dissolved Schwertmannite to goethite and the readsorption of As­(V) on the surface of the freshly formed secondary mineral. Similar oxidation and immobilization of As­(III) by ferrihydrite and goethite as caused by photoreductive dissolution was also reported in previous studies. , …”

“…Similar oxidation and immobilization of As(III) by ferrihydrite and goethite as caused by photoreductive dissolution was also reported in previous studies. 209,210 In the presence of NOM and light, Cr(VI) can be reduced to Cr(III) by Fe(II) produced in the photoreductive dissolution of iron (hydr)oxides, decreasing the mobility of Cr. 211,212 the release of other metal elements, such as U, Cu, Pb, and REE can be significantly enhanced during the photoreductive dissolution of iron (hydr)oxides as well.…”