Bioleaching of Heavy Metals from Pig Manure Employing Indigenous Sulfur-Oxidizing Bacteria: Effects of Solid Content

The effects of solid content (10–80 g/L) on the bioleaching of heavy metals (HMs) from pig manure (PM) were investigated using indigenous sulfur-oxidizing bacteria. The results showed that an increase in solid content increased the PM buffering capacity, which slowed the rates of pH reduction, ORP increase, and sulfur oxidation and decreased the solubilization efficiency of HMs from PM. Approximately 75–99% of Cu, 76–99% of Zn, and 55–88% of Mn were leached from PM with solid contents of 10–80 g/L after 28 days of bioleaching. However, the content of HMs in bioleached manure did not meet the requirement for agricultural application when the solid content was ≥ 60 g/L after 28 days of bioleaching. The solubilization of HMs from PM was well-described by a kinetic equation. Regression analysis showed that Cu solubilization was primarily controlled by ORP, and pH seemed to be the sole factor responsible for the solubilization of Zn and Mn. Additionally, nutrient (N, P, K, and organic matter) loss significantly increased when PM solid content decreased from 40 to 20 g/L. Therefore, the recommended solid content for the bioleaching of HMs from PM is 40 g/L.