Marine macro-alga Sargassum cymosum as electron donor for hexavalent chromium reduction to trivalent state in aqueous solutions

Abstract

In this study, the brown macro-alga Sargassum cymosum was used as an electron donor for the reduction of hexavalent chromium to its trivalent form, through the oxidation of the biomass, and as a natural cation exchanger for the subsequent trivalent chromium sequestration from aqueous solutions. The reaction kinetics for hexavalent chromium reduction in aqueous solutions was evaluated as a function of the pH and initial concentrations of Cr(VI) and biomass. Hexavalent chromium reduction was found to be highly dependent on the pH of the solution, since protons are involved in the reduction mechanism. The concentration of organic compounds per unit of biomass able to reduce the hexavalent chromium was 3.0mmol/g, i.e., 1g of biomass was able to reduce 3mmol of Cr(VI). FTIR and potentiometric titration techniques showed that trivalent chromium removal was mainly associated with weak acidic carboxylic groups present at the surface of the biomass. The oxidation of the biomass during Cr(VI) reduction generated new binding sites responsible for trivalent chromium removal. A kinetic model based on a pseudo-first-order reaction successfully fitted the hexavalent chromium concentration profiles. Trivalent chromium concentration was predicted based on a mass balance of the total chromium in solution, considering the instantaneous binding of trivalent chromium to the negatively charged functional groups present at the surface of the biomass.