Removal of Cr(VI) by corn stalk based anion exchanger: the extent and rate of Cr(VI) reduction as side reaction

Abstract Reduction of Cr (VI) often happens simultaneously with biosorption, making it difficult to exactly understand its removal mechanism. Quantitative description of Cr (VI) reduction is a key point to identify the two mechanisms. Herein, corn stalk based anion exchanger (CS-AE), capable of removing Cr (VI) rapidly and effectively, was used as model biosorbent to investigate Cr (VI) reduction. Batch experiments confirmed Cr (VI) was mainly removed via ion exchange with chloride ion on CS-AE. X-ray photoelectron spectroscopy (XPS) was employed to characterize Cr-laden CS-AE. Result showed the presence of Cr (III) on CS-AE surface and indicated N-contained group especially quaternary ammonium participated in the reduction reaction as electron donor. The effects of initial concentration, pH and sorbent dose on Cr (VI) reduction were studied by quantifying Cr (III) content both in liquid and solid phase. It was found that Cr (III) formation increased with initial Cr (VI) concentration because high concentrations of Cr (VI) enhanced oxidation-reduction potential (ORP) as well as the reactivity of Cr (VI). Acidic conditions facilitated the reduction reaction to form more Cr (III), and the positive relationship between H + depletion and Cr (III) formation was observed. Increase of sorbent dose also can promote the reduction to some extent. Kinetic study showed that reduction of Cr (VI) could reach near equilibrium in about 20∼30 min under experimental conditions. Rate equations based on the change of Cr (III) amount were proposed to describe the reduction reaction, and better fittings were obtained with zero-order kinetic equation. Moreover, activated energy of Cr (VI) reduction reaction was evaluated as about 36.8 kJ/mol. It was also found that increase of temperature favored the reduction of Cr (VI).