Rethinking the dewatering mechanisms underlying bioleaching from the perspective of extracellular polymeric substances and biogenic Schwertmannite interaction.

The acclimated acidified sludge (AS) inoculated during the bioleaching process significantly influenced its performance. However, the underlying mechanism of this effect remains unclear. The iron-containing substances in AS were separated and purified through a simulated cultivation method. Various spectral analysis techniques and density functional theory (DFT) calculations were used to investigate the compositional characteristics of the AS and the interaction mechanisms between key components and extracellular polymeric substances (EPS). The results showed that Schwertmannite (Sch) was the main mineral formed after 24 h, while jarosite was dominant in AS after 96 h. The specific surface area of biogenic Sch was significantly higher at 52.90 m/g, leading to its significant adsorption capacity. Scanning electron microscopy images revealed that the compact and high-strength floc structure formed between Sch and EPS functioned as a skeleton, facilitating the formation of more porous structures and channels that enhance the dewatering process. The results from 2D-COS showed that the effect of Sch on protein substances was greater than that on humic acid substances. The quenching of fluorescence intensity by Sch to proteins was mainly due to static quenching, with no less than two binding sites involved, indicating that biogenic Sch was efficiently and rapidly bound with protein-like substances. Furthermore, the DFT calculations showed that the protein exhibited a strong affinity for Sch due to the enhanced electrostatic force, evidenced by an adsorption energy of 9.44 eV. These findings show the critical role of the biogenic Sch generated in the AS in the bioleaching conditioning of the sludge.