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.