734 Table 12), their differing inhibition patterns highlight the influence of other physicochemical factors, including ionic strength, exchangeable sodium percentage, and multi-metal interactions, on fungal tolerance. While most metals were present at levels well below the reported minimal inhibitory concentrations, iron concentrations exceeded these values, ranging from 3,949 to 10,734 ppm, compared with inhibitory concentrations of 2,000 ppm and 1,117 ppm for A. niger and P. oxalicum, respectively (Anahid, Yaghmaei, & Ghobadinejad, 2011; Levinskaitė et al., 2009). The known synergistic inhibitory effects of multi metal exposure (Le, Tang, Ryan, & Valix, 2006; Tzeferis, 1994; Valix & Loon, 2003) likely contributed to the reduced tolerance observed at higher BR concentrations. Despite the BR stress, A. niger showed substantial neutralisation capability, decreasing BR surface pH by 0.6-2.2 units, values consistent with earlier reports for submerged fermentations, where pH reductions of 1.5-2.5 units were observed at pulp densities of 1-10% (w/v) (Kiskira et al., 2023), and demonstrating the capability to decrease pH from 10 to 3.6. Such strong organic-acid production is essential for both proton-driven mineral dissolution in bioleaching and the neutralisation of alkaline residues during bioremediation. P. oxalicum exhibited lower tolerance overall, with maximum tolerated concentrations of 15% (BR-A), 2% (BR-B), and 20% (BR-C), with increased lag phases and growth inhibition. Although strains of P. oxalicum isolated directly from BR disposal areas have shown tolerance up to 20% (w/v) and rapid neutralisation, reducing BR from pH 10.26 to 6.48 within 11 days (Liao et al., 2018), this strain decreased pH by only 0.6 units at 20% BR-C. This contrast underscores the potential of using BR-adapted isolates that have evolved to withstand combined alkaline, saline, and multi-metal stress. Nevertheless, P. oxalicum effectively neutralised BR to near-neutral conditions in several treatments, though consistently less than A. niger. This confirms that strain selection must align with the specific aim of the process as neutralization to near-neutral pH is advantageous for revegetation, whilst acidic conditions enhance metal dissolution (Gräfe & Klauber, 2011; Soto-Montandon, Gillane, Marcellin, Harrison, & Villa-Gomez, 2025). The ability of these filamentous fungi to produce organic acids and reduce pH under BR conditions has important implications for its valorization, bioremediation, and site restoration. Neutralization facilitates the release of exchangeable sodium, dissolution of alkaline buffering minerals, and immobilization of toxic metals, processes essential for converting BR from a hazardous waste into a substrate capable of supporting vegetation. Nonetheless, BR’s high alkalinity, elevated sodium content, limited nutrient availability, and strong acid-buffering capacity continue to impede the establishment of stable plant communities (Jones & Haynes, 2011). The sustained metabolic activity of filamentous fungal strains has been shown to contribute to long-term stabilization by promoting gradual microscale pH modification via organic acid and CO2 production, enhancing nutrient accumulation, improving physicochemical and biochemical properties, and fostering the formation and persistence of micro- and macroaggregates, key features of a sustainable soil structure (Babu & Reddy, 2011; Jiang, Chen, et al., 2023; Krishna et al., 2005). Overall, the tolerance and neutralisation capacities observed in A. niger and P. oxalicum demonstrate their potential roles in integrated circular processes in which bioremediation and bioleaching operate synergistically: fungi neutralise alkaline residues, remove alkaline phases, stabilise structure, and promote ecological recovery, while simultaneously producing organic acids capable of solubilizing metals of economic interest. The results provided in this research reinforce the necessity of evaluating fungal tolerance on a residue-specific basis and support the strategic selection or isolation of robust fungal strains tailored to both bioleaching efficiency and long-term BR rehabilitation.
RkJQdWJsaXNoZXIy MTM0Mzk2