Biological Approaches for Reducing Chromium Content in Water

Chromium contamination in water arises from industrial activities such as electroplating, tanning, and textile dyeing. In aquatic environments, hexavalent chromium [Cr(VI)] is highly toxic, soluble, and carcinogenic, whereas trivalent chromium [Cr(III)] is less harmful and readily precipitates. 

Conventional physicochemical treatments are effective but costly and generate secondary waste. Biological methods offer sustainable, low-cost alternatives.

Microbial Reduction

Numerous bacteria, including Pseudomonas, Bacillus, and Shewanella species, enzymatically reduce Cr(VI) to Cr(III) under aerobic or anaerobic conditions. Chromate reductase enzymes transfer electrons from endogenous donors (e.g., NADH) to Cr(VI). 

Under anoxia, Cr(VI) can serve as a terminal electron acceptor for dissimilatory metal-reducing bacteria. The resulting Cr(III) forms insoluble hydroxides that settle and are removed via sedimentation. Optimal conditions include pH 6.5–8.5, temperature 25–37°C, and initial Cr(VI) concentrations below 100 mg/L. Immobilized biofilms on activated carbon or zeolite achieve >90% removal in continuous reactors.

Biosorption

Both living and dead biomass bind chromium ions through functional groups (carboxyl, hydroxyl, amino) on cell surfaces. Bacterial, fungal (e.g., Aspergillus niger), and algal (e.g., Sargassum) biomass exhibit Cr(VI) sorption capacities of 30–120 mg/g. 

Agricultural wastes such as rice husk and sugarcane bagasse, after simple pretreatment, also serve as effective low-cost biosorbents. Biosorption is rapid and unaffected by toxicity, but disposal of metal-laden biomass remains a challenge.

Phytoremediation

Aquatic plants like water hyacinth (Eichhornia crassipes), duckweed (Lemna minor), and cattail (Typha latifolia) remove chromium via rhizofiltration and phytoextraction. Water hyacinth can reduce Cr(VI) from 10 mg/L to below 0.5 mg/L within 7–14 days. Mechanisms include root-mediated reduction of Cr(VI) to Cr(III) by exudates or rhizosphere microbes, followed by vacuolar sequestration. Phytoremediation is suitable for shallow, moderately contaminated water bodies but has slow kinetics and seasonal limitations.

Constructed Wetlands

Integrated systems combining microbial reduction, biosorption, and plant uptake, such as horizontal or vertical flow wetlands planted with Phragmites or Typha, achieve 70–95% Cr(VI) removal. They require low energy and minimal maintenance but demand larger land area and longer retention times (2–10 days).

Biological methods provide viable pathways for chromium remediation. Microbial reduction directly converts toxic Cr(VI) to insoluble Cr(III). Biosorption offers rapid, cost-effective uptake. Phytoremediation and constructed wetlands suit passive, long-term management. Hybrid systems may achieve optimal efficiency. Future work should focus on field-scale applications and biomass reuse.