Synthesis of nano-sized Zn–Mn ferrite from the resulting bioleachate of obsolete Zn–Mn batteries at a high pulp density of 5.0% enhanced by the added Fe³⁺

Synthesis of Zn–Mn ferrite from spent Zn–Mn batteries using a multi-step process of bioleaching and co-precipitation represents a promising means in waste management of the spent batteries. However, the low pulp density of 1.0% in bioleaching process means a low yield of Zn–Mn ferrite. In this work, the cheap and benign Fe³⁺ was used to replace dangerous H₂SO₄ or expensive Cu²⁺ to promote bioleaching performance of spent batteries at a high pulp density of 5.0% for synthesis of Zn–Mn ferrite for the first time. The results displayed the addition of Fe³⁺ greatly enhanced bioleaching of spent batteries. The extraction efficiency of Zn and Mn increased from 34.5% to 29.4% to the maximum of 85.1% and 83.2%, respectively, when the concentration of added Fe³⁺ increased from 0 to 5.0 g/L. The added Fe³⁺ motivated more generation of both Fe²⁺ and sulfur to promote the growth of both Leptospirillum ferriphilum and Acidithiobacillus thiooxidans, respectively, attaining a higher bioleaching rate. The electro - chemical analysis also revealed the highest J_Corr of 0.571 mA cm⁻² occurred when the concentration of added Fe³⁺ was 5.0 g/L. The addition of Fe³⁺ at 5.0 g/L witnessed the maximum synthesis yield of Zn–Mn ferrite (52.6 g/L).