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

Area of Science:

• Environmental Science
• Environmental Engineering
• Microbiology

Background:

• Swine manure contains hazardous metals, posing environmental risks.
• Anaerobic bioleaching (AB) is effective for metal removal from sewage sludge but unstudied for swine manure.

Purpose of the Study:

• To evaluate metal solubilization from liquid swine manure using AB.
• To compare AB with pH-stat chemical leaching (CL) and identify metal mobilization mechanisms.

Main Methods:

• Triplicate anaerobic bioleaching reactors operated at 15, 9, and 5 days hydraulic retention time (HRT) for 88 days.
• Parallel pH-stat chemical leaching (CL) test to differentiate acidification and organic matter degradation effects.
• Analysis of metal concentrations in liquid and solid phases, volatile fatty acids (VFAs), and microbial communities.

Main Results:

• AB solubilized up to 61% Mn, 41% Ni, and 33% Co when acidification exceeded 60%.
• Soluble fractions of Cr, Cu, Zn, Mo, and Pb remained below 10%.
• Zn, Cu, and Ni concentrations in the solid digestate exceeded EU limits, restricting commercialization.
• Volatile fatty acids (VFAs) production strongly correlated with metal solubilization.
• AB achieved equal or higher metal solubility than CL at pH < 8.30, except for Pb.

Conclusions:

• Organic matter degradation and microbial dynamics, particularly VFA production by hydrolytic and fermentative bacteria, control metal mobilization in AB.
• AB is a promising sustainable approach for mitigating metal contamination and recovering resources from swine manure without chemical additives or aeration.