Mechanism and Kinetics of Hexavalent Chromium Chemical Reduction with Sugarcane Molasses

摘要

Sugarcane molasses, which is a kind of microbial carbon source, is a viscous by-product of the refining of sugarcane into sugar. However, experiments were designed to ascertain the mechanism and kinetics of Cr(VI) reduction with sugarcane molasses without adding microbes in aqueous solution. Results indicated that sugarcane molasses can reduce Cr(VI) to Cr(III) at pH values that range from 2.0 to 6.1 when no bioreduction occurs in the reaction. Furthermore, the reaction mechanism was proven to be that Cr(VI) acts as an electrophile that readily accepts electrons from the phenolic hydroxyl group of plant polyphenol, and it is then reduced to Cr(III) and in the process oxidizes the phenolic hydroxyl group to a quinone. Meanwhile, the reaction could be described by the pseudo-first-order kinetic model with respect to Cr(VI) concentration. The reaction rate constants were 324.2, 65.9, 21.9, and 14.4 h(-1) when pH values were 2.0, 3.5, 5.0, and 6.1, respectively, at 20 degrees C. The kobs increased 3.36, 7.02, and 13.48 times with the temperature adjusted from 5 to 10, 20, and 30 degrees C.