Biological neutralization of bauxite residue with fermented waste sludge and bio-acid, and the microbial ecological restoration

The bioneutralization of alkaline bauxite residues (BR) presents a significant environmental challenge. However, this can be mitigated by generating organic acids in situ through the fermentative decomposition of carbohydrate-rich organic matter. This process is driven by organophilic and heterotrophic prokaryotes that are tolerant to extreme saline and alkaline conditions. In this study, we conducted a comprehensive evaluation on the effectiveness of various carbon sources and microbial inocula substrates, each with differing complexities, in modifying the physicochemical properties of BR. Our laboratory-scale bioreactors, integrating waste sludge and acid-producing bacteria for BR neutralization, achieved a significantly lower minimum pH (<8) in a considerably reduced timeframe (<20 days) compared to bioreactors using other inocula or carbon sources. This outcome was attributed to the fact that incorporation of waste sludge and acid-producing bacteria promoted fermentation pathways, yielding higher amounts of acetate and formate compared to other inocula. Overall, our findings indicate that the use of cost-effective carbon sources and acid-producing bacteria is a key component of an optimal system for BR treatment and effective bioneutralization. These results provide a comprehensive framework for creating more sustainable and cost-efficient strategies for managing BR waste, while mitigating environmental risks.