Enhancement of Microbial-Mediated Cr(VI) Reduction Processes for Recovering Contaminated Groundwater

Groundwater Cr (VI) pollution largely depends on industrial applications in the field of energy production, manufacturing of metals and chemicals, tanning, and subsequent waste and wastewater management. Cr(VI) is a hazardous environmental pollutant, because of its mobility and toxicity, especially for mammalian cells, which do not have efficient chromate detoxifying pathways. On the contrary, chromate-reducing genes are widely present in prokaryotes which can reduce Cr(VI) to the less toxic and less mobile Cr(III). Bioremediation represents a valid alternative to chemical methods which are ineffective at lower concentrations of Cr(VI) in large volumes of wastewater and could generate secondary pollution. Nevertheless, the biological treatment of Cr(VI) contaminated groundwaters is limited, because most known species capable of chromate reduction are heterotrophic, and need extra nutrition addition during the reduction process, especially in an oligotrophic groundwater environment. The aim of this work was to evaluate the extent of Cr(VI) reduction by groundwater indigenous microorganisms at a laboratory scale, aimed at potential application for in situ recovery of the industrial area of Barletta Municipality (Southern Italy). In 2020, two monitoring campaigns carried out on the study site revealed a Cr(VI) concentration in groundwater above the legal limit (Italian environmental law, D.Lgs. 152/06), ranging from 50 to 102 µg L -1 . Microcosms have been set up with groundwater and water-saturated soil, collected from the above-mentioned industrial area. Different experimental conditions have been tested, as follows: biological batch, biological batch with two different amendments and, the corresponding sterile controls. Specifically, the amendments employed in the microcosms' set up, were: 1) yeast extract, that resulted efficient in promoting a quick Cr(VI) bioreduction in our previous laboratory study; 2) polyhydroxybutyrate, that is well-known in scientific literature as a slowrelease substrate for enhancing biological reduction in bioremediation processes. A multidisciplinary approach has been performed to elucidate microbial-mediated Cr(VI) reduction processes. Changings in microbial abundance have been evaluated via Real-Time PCR in parallel with chemical and spectrophotometric analyses for monitoring Cr(VI) concentration. FTIR spectroscopy has been used to investigate chromium forms in microcosms solid phase.