Chromium(VI) reduction characteristics of acinetobacter haemolyticus immobilized on wood shavings

Abstract

Hexavalent chromium contamination in the environment is a result of the extensive use of chromate and dichromate in numerous industries including electroplating, stainless-steel production and wood preservation. Cr(VI)-reducing biofilms system in the treatment of Cr(VI)-containing wastewaters has been receiving great attention due to its efficiency and cost effectiveness. In this study, a lab-scale bioreactor consisting of Acinetobacter haemolyticus immobilized on wood husk was used to form Cr(VI)-reducing biofilms in packed bed column. The chromium reduction process was carried out at the laboratory–scale bioreactor for 90 days using different batches of electroplating wastewater (EW) containing Cr (VI) ranging from 25-200 mg/L where parameters such as flowrate, nutrient supplementation and initial Cr(VI) concentration in the bioreactor were initially optimized using Response Surface Methodology (RSM). RSM was used to achieve optimum condition for three parameters i.e. flowrate (3-6 mL/min), initial Cr(VI) concentration (40-100 mg/L) and nutrient supplementation (10-20% v/v) for complete reduction of Cr(VI). The attached bacterial cells in the bioreactor were also quantified during the course of Cr(VI) reduction via colony forming unit (CFU/mL) and biofilm development observation in the bioreactor using Field Emission Scanning Electron Microscope (FESEM) analyses. The optimum conditions were determined as flowrate of 3 mL/min, initial Cr(VI) concentration of 100 mg/L and nutrient supplementation of 20%. Under these optimized conditions, the lab-scale bioreactor was able to reduce completely EW at concentration of 100 mg/L in single cycle while two or three cycles were needed for higher Cr(VI) concentrations (110-200 mg/L). The number of A. haemolyticus cells in the bioreactor decreases to 105 from an initial cell concentration of 107 after treatment with 150-200 mg/L Cr(VI). Extracellular Polymeric Substances (EPS) was secreted by the cells in the bioreactor during the course of Cr(VI) reduction as evidenced from FESEM analysis.