Metal solubilization from liquid swine manure through fermentation-based anaerobic bioleaching

Swine manure contains various hazardous metals which pose environmental risks when applied to agricultural soils. While fermentation-based anaerobic bioleaching (AB) showed to be efficient for metal removal from sewage sludge, its application to swine manure remains unexplored. This study evaluated metal solubilization from liquid swine manure via AB in triplicate reactors operated at hydraulic retention times (HRT) of 15, 9, and 5 days over 88 days. A parallel pH-stat chemical leaching (CL) test was conducted to differentiate acidification effects from organic matter degradation. Results showed that when acidification yields exceeded 60 %, up to 61 % of total Mn, 41 % of total Ni and 33 % of total Co, were transferred to the soluble phase. In contrast, soluble fractions of Cr, Cu, Zn, Mo, and Pb remained below 10 % of their respective total concentrations. Furthermore, total concentrations of Zn, Cu, and Ni in the solid digestate fraction exceeded EU Fertilizing Products Regulation limits by days 44, 52, and 72, respectively, thereby restricting its commercialization. Volatile fatty acids (VFAs) production (1.1-3.8 g L^-1), driven by hydrolytic and fermentative bacteria (i.e. Bacteroidota, Firmicutes and Proteobacteria), strongly positively correlated with metal solubilization. AB achieved equal or higher metal solubility than CL at pH < 8.30, except for Pb, indicating that organic matter degradation and microbial dynamics control metal mobilization. These findings demonstrate the potential of AB for mitigating metal contamination from manure without external chemical additives or aeration, positioning it as a promising sustainable approach for resource recovery.