Suitable conditions and reduction mechanisms of Cr(VI) contaminated groundwater by in-situ bioremediation with molasses

Molasses is an inexpensive and efficient biological carbon source for Cr(VI) remediation. This study explored the application of molasses in the remediation of contaminated aquifers. Under the conditions of 10–30 °C and pH > 6, molasses was effective for the bioremediation of 50 mg L−1 Cr(VI). Under these conditions, Cr(VI) had a faster remediation rate, and the generated Cr(III) had a lower migration risk. The average removal rate of Cr(VI) doubled when the temperature increased from 10 °C to 20 °C. Considering the remediation efficiency of Cr(VI) and total chromium in groundwater, a concentration limit of injected molasses (<5 g L−1) was proposed. When the molasses concentration exceeded 5 g L−1, >40 % of Cr(III) existed in the form of a complex colloid in the solution, which hindered the precipitation removal of Cr(III). Reduction of Cr(VI) by indigenous microorganisms mainly occurs extracellularly. The results of the simulated tank study showed that molasses can rapidly create in-situ biological reaction zones in the aquifer. Molasses migrated faster in deep aquifers thereby enabling a quicker removal of Cr(VI). In addition, it also has a good removal effect on NO3−. This study is of great significance for the application of molasses to reduce Cr(VI).