Bioreduction of Chromium in Water

Hexavalent chromium (Cr(VI)) is a highly toxic water pollutant. This study explores a biological method using Bacillus subtilis to reduce Cr(VI) to less toxic trivalent chromium (Cr(III)). Batch experiments showed that under optimal conditions (pH 7.0, 35°C, 2% inoculum), 92% of Cr(VI) (initial concentration 50 mg/L) was removed within 48 hours. The process follows first-order kinetics. This microbial reduction offers an eco-friendly and cost-effective strategy for chromium detoxification.

1. Introduction

Chromium contamination from industrial effluents (e.g., tanning, electroplating) poses severe health risks. Conventional chemical reduction methods generate secondary sludge. Biological reduction using metal-resistant bacteria presents a sustainable alternative. This study investigates the Cr(VI)-reducing capacity of Bacillus subtilis isolated from contaminated soil.

2. Materials and Methods

B. subtilis was cultured in Luria-Bertani broth. Cr(VI) solutions (10–100 mg/L) were prepared from K₂Cr₂O₇. Bacterial cells (10⁸ CFU/mL) were added to 100 mL synthetic wastewater and incubated at varying pH (5–9), temperature (25–40°C), and contact time. Residual Cr(VI) was measured by diphenylcarbazide method (UV-Vis at 540 nm).

3. Results and Discussion

Maximum reduction (92%) occurred at pH 7.0 and 35°C after 48 h (Figure 1). At 100 mg/L Cr(VI), reduction efficiency dropped to 68%, indicating substrate inhibition. The process fitted the pseudo-first-order model (R²=0.97). Control experiments without bacteria showed <5% abiotic reduction. 

FTIR analysis confirmed involvement of amino and carboxyl groups in chromium binding. Compared to chemical methods, this biological approach requires no hazardous reagents and produces minimal sludge.

Bacillus subtilis effectively reduces Cr(VI) in water, achieving >90% removal under optimized conditions. This microbial method is promising for in situ bioremediation of chromium-polluted water bodies. Future work should focus on immobilization of cells for continuous treatment.