Potential of <i>Aspergillus niger</i> for the Neutralization of Highly Alkaline Bauxite Residue

Abstract

Biological methods are well-documented as efficient and eco-friendly techniques to convert waste into a green sustainable material. This study demonstrates the potential of organic acid–secreting filamentous fungi such as Aspergillus niger in mitigating the alkalinity of bauxite residue (BR). A series of bioneutralization experiments by adding broth containing Aspergillus niger in different percentages (10%, 15%, 20%, and 25%) with inoculation intervals of 1, 2, 3, 4, and 5 days to BR are performed. The efficacy of fungi is monitored by measuring the pH of BR over an incubation period of 50 days. The analyses of the results reveal that broth additions at all percentages of a 1-day inoculation interval, 15%, 20%, and 25% of 2 and 3-day intervals, and 20% and 25% of a 4-day interval successfully reduce the pH of BR to an acceptable value of 8.5, as prescribed by the Indian Road Congress standard. When considering the economy, it is perceived that 20% broth addition with a 4-day inoculation interval is optimal. The underlying mechanism for pH decline is corroborated by the reaction of H+ ions, dissociated from the organic acids secreted by filamentous fungi, with various alkaline substances of BR. The performance of fungi is further assessed by measuring the rate of decline parameter, which is found to be dependent on percent broth addition and inoculation interval. The outcome of this study gives an insight that microbial-driven remediation could become a practical, sustainable, and eco-friendly method for in situ rehabilitation of BR and thereby convert it into green material.