Biological Treatment Methods for Chromium in Water Bodies

Heavy metal pollution, particularly from chromium (Cr), poses significant risks to aquatic ecosystems and human health. Industrial activities such as plating, tanning, and dyeing release hexavalent chromium [Cr(VI)], a highly toxic and carcinogenic form, into water bodies. 

Traditional physicochemical methods for chromium removal often involve high costs, chemical consumption, and sludge generation. In contrast, biological treatment strategies offer eco-friendly, efficient, and cost-effective alternatives. This article summarizes key biological approaches for chromium removal from water, focusing on microbial and algal biosorption, bioreduction, and plant-microbe interactions.

1. Microbial Biosorption and Bioreduction

Microorganisms, including bacteria, fungi, and yeast, can adsorb or reduce Cr(VI) to the less toxic trivalent form [Cr(III)]. For instance:

Bacterial Strains: Specific strains like Pseudomonas spp. (e.g., strain SCU20) and Providencia burhodogranariea (strain T91) exhibit high Cr(VI) removal efficiency. Under optimal conditions (pH 7–8, 26°C), T91 achieves up to 98.7% removal of Cr(VI) by combining biosorption with enzymatic reduction.

Mechanisms: Adsorption involves ion exchange, where Cr(VI) binds to functional groups (e.g., -COOH, C=O) on cell walls, releasing K+, Mg2+, or Ca2+ ions. Concurrently, enzymes or cellular metabolites facilitate reduction to Cr(III).

Engineering Applications: Inoculating activated sludge with engineered yeast fusion strains can remove 70–80% of nickel and chromium under controlled dissolved oxygen (DO: 2.5–4.5 mg/L).

2. Algal Biosorption

Marine macroalgae, such as Ulva fasciata, serve as low-cost biosorbents due to their high surface area and functional groups. Studies show:

Ulva fasciata achieves complete Cr(VI) removal at low concentrations (10–20 mg/L) within 5 minutes. Adsorption follows pseudo-second-order kinetics and is spontaneous and exothermic.

Efficiency depends on pH, biosorbent dose, and temperature, with acidic conditions favoring adsorption.

3. Fungal Biosorbents

Fungi like Mucor LH3 demonstrate high adsorption capacity in acidic environments (pH 1, 28°C), achieving 99% removal within 8 hours. Adsorbents can be regenerated using NaOH (98.6% desorption), allowing reuse for multiple cycles.

4. Plant-Microbe Synergistic Systems

Combining plants and bacteria enhances chromium remediation through rhizosphere interactions. For example:

Waxy bacillus (Bacillus cereus) and Lee grass (Leersia hexandra) form a synergistic system where bacteria enhance plant uptake and reduction of Cr(VI), achieving high removal at concentrations of 8–12 mg/L.

5. Composite Microbial Reagents and Nanomaterials

Composite Bacteria: Strains like Desulfovibrio, Desulfotomaculum, and Desulfobacter produce nanoscale FeSx under anaerobic conditions. In acidic environments, FeSx dissociates into S2− and Fe2+, reducing Cr(VI) to Cr(III) and precipitating it as insoluble sulfides.

Bioaugmentation: Mixed bacterial consortia (e.g., Thauera and Agrobacterium) effectively treat co-contaminated waters (e.g., chromium and phenanthrene) by simultaneously degrading organics and immobilizing metals.

Factors Influencing Efficiency

pH: Acidic conditions favor Cr(VI) adsorption, while neutral-alkaline conditions support microbial activity.

Temperature and Flow: Optimal temperatures range from 26–37°C; moderate flow (0.15–0.30 m/s) improves mass transfer and degradation rates.

Genetic Regulation: Genes encoding heavy metal resistance (e.g., Kdp operon) and chemotaxis (e.g., CheA) are upregulated under chromium stress, enhancing bacterial survival and removal capacity.