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<p>This study comprehensively investigates the bio-neutralization of bauxite residue (BR) using <i>Aspergillus niger</i> fungus. However, the extremely high pH (&gt; 11) poses a distinctive challenge to neutralize BR, as it creates a non-conducive environment for microbe’s proliferation. Additionally, the pH of BR undergoes rebound over time post neutralization. To circumvent these limitations, citric acid (CA) is introduced as a mediating agent that creates a favourable environment to enhance microbial viability. Two sets of bio-neutralization experiments are conducted: fungus alone and fungus mediated with CA. In each experiment, inoculant (broth containing fungi) at concentrations of 10, 15, 20, and 25% is added to BR, and the pH of resultant mixture is measured over prolonged incubation periods up to 270-days. The novelty of the study lies in identifying the mediating agent (i.e. citric acid: 0.5, 0.75, 1 and 1.5&#xa0;M) and adopting a multi-cycle approach (2 cycles) for the BR neutralization. The results emphasize that both these steps are indispensable for declining pH of BR to an acceptable value of 8.5, as per the IRC standard, while accounting for pH rebound. Importantly, the pH rebound did not reach 8.5 threshold even after 2nd cycle, that is, 270 days of incubation period for the fungus with CA-mediation case, whereas it crossed 8.5 in just 20 days in both cycles for the fungus-only case. Analyses of mineralogical and microstructural studies reveal dissolution of alkali-causing minerals, prime contributors to pH abate, only after two multi-cycles of tests. Comprehensive investigations indicate that 15% broth addition with 1&#xa0;M CA-mediation is the optimal dosage. The findings suggest that treated BR is environmentally benign and that CA-mediated fungal bio-neutralization is feasible, sustainable, and eco-friendly, making in-situ rehabilitation of BR storage facilities practicable.</p>

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Role of citric acid mediation in Aspergillus niger fungus performance for mitigating extreme alkalinity in bauxite residue: a multi-cycle strategy to mitigate pH rebound

  • 期刊论文

  • 发表日期：

    2026年4月28日

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  • Benazeer Sultana,
  • Bendadi Hanumantha Rao

摘要

This study comprehensively investigates the bio-neutralization of bauxite residue (BR) using Aspergillus niger fungus. However, the extremely high pH (> 11) poses a distinctive challenge to neutralize BR, as it creates a non-conducive environment for microbe’s proliferation. Additionally, the pH of BR undergoes rebound over time post neutralization. To circumvent these limitations, citric acid (CA) is introduced as a mediating agent that creates a favourable environment to enhance microbial viability. Two sets of bio-neutralization experiments are conducted: fungus alone and fungus mediated with CA. In each experiment, inoculant (broth containing fungi) at concentrations of 10, 15, 20, and 25% is added to BR, and the pH of resultant mixture is measured over prolonged incubation periods up to 270-days. The novelty of the study lies in identifying the mediating agent (i.e. citric acid: 0.5, 0.75, 1 and 1.5 M) and adopting a multi-cycle approach (2 cycles) for the BR neutralization. The results emphasize that both these steps are indispensable for declining pH of BR to an acceptable value of 8.5, as per the IRC standard, while accounting for pH rebound. Importantly, the pH rebound did not reach 8.5 threshold even after 2nd cycle, that is, 270 days of incubation period for the fungus with CA-mediation case, whereas it crossed 8.5 in just 20 days in both cycles for the fungus-only case. Analyses of mineralogical and microstructural studies reveal dissolution of alkali-causing minerals, prime contributors to pH abate, only after two multi-cycles of tests. Comprehensive investigations indicate that 15% broth addition with 1 M CA-mediation is the optimal dosage. The findings suggest that treated BR is environmentally benign and that CA-mediated fungal bio-neutralization is feasible, sustainable, and eco-friendly, making in-situ rehabilitation of BR storage facilities practicable.