Recovery of Ni and Co from Nickeliferous Residues with Reducing Agent of Trichoderma harzianum

Microbial bioextraction offers a sustainable strategy for the treatment of mining residues. This study evaluates the efficacy of a biologically derived reducing agent produced by Trichoderma harzianum Rifai in recovering metals (Ni, Co, Cu, Mn, Mg, and Fe) from acid-leached nickel tailings sourced from Moa, Cuba. The reducing agent was tested both independently and in combination with citric acid under varying conditions of temperature (30–60 °C), concentration (0.5–1 M), liquid-to-solid ratio (3–8 mL/g), and reaction time (1–3 hours), using tailings with a particle size of 0.045 mm. Mineralogical characterization was performed via X-ray diffraction (XRD). The biological agent exhibited moderate leaching performance, achieving extraction efficiencies of 42.91% for Mn, 36.86% for Ni, 35.57% for Cu, 34.89% for Mg, 24.56% for Co, and 13.7% for Fe. Selectivity was below 5% for Ni, Co, Fe, and Mn, while Mg and Cu showed selectivity values exceeding 9%. The combined use of the biological reducing agent with citric acid significantly enhanced metal extraction, reaching 66% for Mn, 70.83% for Ni, 97.19% for Cu, 69.54% for Mg, 47.15% for Co, and 56.59% for Fe, accompanied by a reduction in selectivity for all metals to below 3%. XRD analysis confirmed that no new mineral phases formed during the process. These results demonstrate the potential of Trichoderma harzianum as a biological reducing agent for managing nickel-bearing residues, attributed to its organic acid production and minimal alteration of the mineral matrix. Moreover, the synergistic effect observed with citric acid significantly improves metal recovery, providing a technically feasible and environmentally friendly solution for the mining industry. This work highlights the promise of biohydrometallurgical approaches for valorizing industrial residues.