Characterization and invitro toxicity assay of bio-reduced hexavalent chromium by Acinetobacter sp. isolated from tannery effluent

Chromium, a transition metal element widely distributed in the earth's crust, with the valence from −2 to +6, but exists mainly in the form of trivalent and hexavalent states. Hexavalent chromium Chromium (VI) is often found in soil and ground water due to its widespread industrial use such as tannery, electroplating, and steel industries. Chromium (VI) is toxic, mutagenic, carcinogenic, and teratogenic and much more toxic to many plants, animals, and bacteria inhabiting aquatic environments because it is motile, highly toxic, soluble in water and it is a strong oxidizing agent that causes severe damage to cell membranes while the trivalent chromium Cr (III) is the most stable form of chromium and relatively immobile in the aquatic system due to its lower solubility. This study therefore, aims at identifying a bacterium that can reduce Chromium (IV) to (III) using a pour plate technique on Luria Bertani medium amended with K2Cr2O7. Graphs and tables were used for the data analysis. The effects of incubation time, pH, temperature, inoculum size, and potassium dichromate (K2Cr2O7) concentration were then examined to characterize the bacterium based on one factor at a time. To assess Chromium (VI) reduction, a spectrophotometric study of the 1, 5-diphenyl carbazide test (DPC) at 370 nm was utilized. By using molecular phylogenetic analysis and partial 16s ribosomal RNA analysis, the isolate was found to be Acinetobacter sp. strain BUK_BCH_BTE 5. The findings demonstrated that tryptic soy broth (TSB), with an optimal K2Cr2O7 level of 200 mg/L, pH of 6.5, 35 °C temperature, inoculum size of 2%, as well as incubation time of 48 h, was the most successful approach for decreasing chromium (VI). Following Nickel (Ni), the isolate was found to withstand the highest doses of Lead (Pb), Mercury (Hg), Zink (Zn), Iron (Fe), Cadmium (Cd), Arsenic (As), and Cupper, in that order. A reduced culture's (supernatant) larvacidal bioassay showed a 30% decrease in toxicity in just 48 hours, indicating that the isolate is a promising candidate, and that the reduction method is less dangerous for decontaminating chromium-contaminated environments.