Photocatalytic Reduction of Cr(VI) in the Presence of NO3- and CI- Electrolytes as Influenced by Fe(III)

Abstract

Photoreduction of Cr(VI) involving Fe is strongly affected by the presence of organic or inorganic compounds in an acidic environment. In this study, we have found a new pathway of Cr(VI) photoreduction in the presence of Fe(III) that is influenced by two inorganic electrolytes (i.e., NO₃^- and Cl^-) and the pH. In NO₃^- and Cl^- systems without Fe(III), Cr(VI) photoreduction could occur and was independent of the Cr(VI) concentration. The zero-order rate constant of the photoreduction reaction increased when the solution pH was decreased from 2 to 1; the reaction rate was higher in the NO₃^- system than in the Cl^- system. The higher reaction rate in the NO₃^- system was attributed to the photolysis of NO₃^-, which resulted in the formation of NO₂^- for reduction of Cr(VI). Conversely, the effect of Fe(III) addition on the increase in Cr(VI) photoreduction rate in the Cl^- system was more significant than that in the NO₃^- system. The addition of Fe(III) to the Cl^- system caused the formation of [Fe(OH₂)₅Cl]²⁺, the photolysis of which subsequently resulted in the formation of Fe(II) for reduction of the Cr(VI). This study suggests that the photolysis of NO₃^- and Fe−Cl complex may contribute significantly to Cr(VI) reduction in surface water that receives electroplating wastewater containing high levels of NO₃^-, Cl^-, and Fe(III). Therefore, under the acidic conditions that are favorable for Fe−Cl complex formation or in the presence of NO₃^-, the effects of inorganic components on Cr(VI) photoreduction cannot be ignored for the precise evaluation of the transformation of Cr in the environment.