Synergistic and inhibitory reduction of Cr(VI) by montmorillonite, citric acid, and Mn(II)

Cr(VI) could be reduced in nature by citric acid, Mn(II), and montmorillonite. However, the co-presence of each material could enhance or inhibit Cr(VI) reduction. In this study, the synergistic interaction of citric acid, Mn(II), and montmorillonite on reduction of Cr(VI) was evaluated by experimental design. The interaction between two variables was evaluated for a short-time (5 min) and a long-time (300 min) reaction. The amounts of Cr(VI) reduced was statistically simulated to evaluate the interactions quantitatively. Cr(VI) was spontaneously reduced by Fe(II) released from montmorillonite, and citric acid enhanced extraction of Fe(II), which increased the reduction of Cr(VI) compared to montmorillonite without citric acid. The presence of Mn(II) did not affect Cr(VI) reduction within a short time. The co-presence of Mn(II) and citric acid enhanced Cr(VI) reduction even though the reaction rate was slower than that by Fe(II) released from montmorillonite. However, the presence of montmorillonite inhibited Cr(VI) reduction for a long-time reaction. Fe(III) released from montmorillonite suppressed formation of Mn(II)-citrate complexes based on the greater stability constant of Fe(III)-citrate complex. In a short time, aqueous phase Fe(II) is a key factor on the reduction of Cr(VI), but Fe(III) played a more important role on the reduction for a long time.