In situ reduction of hexavalent chromium in groundwater by injecting organic substrates from food industry

In situ bioremediation has a significant role in the remediation techniques for its general sustainability in terms of environmental impact and costs. While organics can be mineralized, inorganics can only change their oxidation state, hopefully reducing their toxicity and mobility in the environment. Innovative technologies for Cr(VI) in groundwater include bio-induced reduction, that is Cr(VI) reduction to Cr (III) by injection of organic substrates that are readily degraded by heterotrophic microorganisms in the aquifer, depleting the different electron acceptors and causing reducing conditions. Lab scale tests for Cr(VI) reduction were performed to have preliminary information about different organic substrates and iron involvement in Cr precipitation. The experiments were carried out in batch systems (1.25 L), with a soil (sand) to groundwater ratio of 25% w/w. Two different initial Cr(VI) concentrations were investigated (5000 and 10000 ug/L), as well as two different organic substrates (cheese whey permeate at 5 mg/L, beer distillation residue at 300 mg COD/L). After 7 days of incubation and groundwater sampling for Cr(VI) analysis, Fe(II) at 10 mg/L was added in some bottles; after 1 additional month of incubation, groundwater was analyzed again. In all bottles, the dissolved oxygen decreased from 6.5 mg/L to values <0.5 mg/L over 1 d incubation. The redox potential decreased from approximately +250 mV to –500 mV in 7 days. At this time, however, the concentration of Cr(VI) was similar to the initial values. After 38 days, significant Cr(VI) abatements in water were observed where iron had been added and Cr(VI) had the lower initial concentration (71% with cheese whey permeate, and 79% with beer distillation residue); the abatements were lower (38% with cheese whey permeate, and 48% with beer distillation residue) where iron had been used and the initial Cr(VI) was at 10000 ug/L. Removal between 6% and 24% were obtained in the remaining samples. Fe(II) availability was a key factor in Cr(III) coprecipitation, which was not an instantaneous process.