Isotope fractionation and spectroscopic analysis as an evidence of Cr(VI) reduction during biosorption

•Cr isotope fractionation analyses during Cr(VI) biosorption are presented.•Brewers draff, grape waste and humic acids were tested as Cr(VI) biosorbents.•Significant isotope shifts were observed after Cr(VI) interaction with the sorbents.•XPS analyses confirmed that adsorbed Cr is mostly or completely reduced.•The reduction extent can be quantified using the Rayleigh fractionation model.

This work investigates the mechanisms behind Cr(VI) biosorption/reduction on three biomaterials (brewers draff, grape waste and synthetic humic acid). Coupled Cr isotope analysis with ICP-OES, XPS and SEM was tested as a novel approach to study the reduction of Cr(VI) by the biomaterials. The Cr(VI) biosorption process was accompanied with heavier Cr isotopes enrichment in the remaining Cr(VI) fraction. A significant fractionation of Cr stable isotopes was observed with no significant pH effect; δ53Cr of the remaining fraction ranged from 0.2‰ to 1.9‰ while δ53Cr of the product (sorbed Cr) ranged from −1.2‰ to −2.8‰. The Rayleigh fractionation model fitted well the measured data and Cr isotope analysis provides thus an efficient tool to quantify Cr(VI) reduction by different biomaterials. In general, the sorption/reduction potential of the three studied biomaterials decreased in the following order: grape waste > humic acids > brewers draff.