Enhancement of heterogeneous Cr(VI) reduction using clay minerals in the presence of organic carboxylic acids under UV irradiation

Chromium is a well-known highly toxic metal, considered as a priority pollutant. In this study, a series of experiments were conducted to investigate the effect of the properties of clay minerals and organic carboxylic acids on the reduction of Cr(VI) in aqueous solution under UV illumination. Two common clay minerals (montmorillonite, kaolinite) and two typical organic carboxylic acids (oxalate and citrate) were utilized in our system. The experimental results confirmed that the existence of oxalate or citrate can markedly accelerate the Cr(VI) photoreduction reaction on the clay minerals in aqueous solution compared with clay minerals alone. This process of photo-reduction follows approximately zero-order kinetics. The effects of pH, dissolved oxygen, and initial Cr(VI) concentration, and clay minerals dose on removal ratio were also investigated. The photoreduction ratio of chromium was favorable under acidic pH than in the weakly alkaline pH. Furthermore, it was found that the dissolved oxygen exerted an inhibition effect on Cr(VI) removal.