Effect of Hardness Ions on Cr(VI) Reduction with Organic Reducing Substances

Abstract

Batch laboratory-scale experiments were conducted to investigate the effects of hardness ions (Mg and Ca) on Cr(VI) reduction with organic reducing substances in the form of sugarcane molasses at different conditions. Results illustrated that the reaction could be described as pseudo-first-order with respect to the Cr(VI) concentration. Reaction rate constants of Cr(VI) reduction with sugarcane molasses were in the decreasing order of no hardness ions > Ca(II) > Mg(II). The possible mechanism was that the complex formation between organic reducing substances and hardness ions (Mg or Ca) hampered the electron transfer rate of the organic reducing substances, and further led to a decrease in the reaction rate. For Mg(II), the initial reaction rates at different pH values (2.0, 2.5, 3.0, and 3.5) decreased by 0.0316, 0.0246, 0.0095, and 0.0023/h in the presence of Mg(II) compared with those in the absence of Mg(II). The inhibitory effect of Mg(II) on the reduction of Cr(VI) was present at different temperatures. The activation energy (Ea) was estimated at 57.4 and 65.6 kJ/mol in the presence and absence of Mg(II), respectively. This would be significant in promoting understanding influence mechanism of hardness ions on Cr(VI) reduction with sugarcane molasses.