Abiotic reduction of Cr(VI) by humic acids: kinetics and removal mechanism

The contamination of groundwater by hexavalent chromium is a worldwide problem that has resulted mainly from the poorly managed disposal of hexavalent chromium wastes. Numerous different sources of wastes can produce Cr(VI)-contaminated groundwater with a wide range of pH values. The reduction of Cr(VI) to Cr(III) is of great importance because the hazard of Cr(VI) decreases when it is reduced to the less toxic form, Cr(III). Humic acids obtained from different sources are capable of Cr(VI) reduction. This study compared Cr(VI) reduction over a wide range of pH values (3-11) using humic acids extracted from lignite, peat, soil, and sludge phyto conditioning residue. The reduction of Cr(VI) by all humic acids was pH dependent.
All humic acids reduced Cr(VI) to Cr(III) in a first-order reaction with respect to Cr(VI) concentration but exhibited a partial order of approximately 0.5 with respect to [H+]. Among the humic acids, peat humic acid showed the highest rates of reactions, where Cr(VI) reduction was observed at all pH values investigated (3-11). 13C NMR and pyrolysis GC-MS spectroscopy indicated that the reduction of Cr(VI) into Cr(III) by the humic acids resulted in a loss of aromatic compounds and hydroxyl groups. X-ray absorption spectroscopy indicated that at all pH values the resulting Cr(III) was associated with the partially degraded humic acid in an inner sphere adsorption complex. The reaction mechanism of Cr(VI) by the humic acid’s functional groups is likely to be similar to that by simple organic moieties, which controlled by chromate ester formation followed by the electron transfer decomposition step. The initial step is rapid in acidic systems but far less favourable in alkaline conditions. The finding of this study highlights the potential use of organic matter rich with humic acids for the in-situ reduction of Cr(VI) from aqueous systems over a broad range of environmental conditions.