Organic contaminants removal and carbon sequestration using pig manure solid residue-derived biochar: A novel closed-loop strategy for anaerobic liquid digestate

Abstract

Anaerobic digestion of livestock manure as a carbon-negative practice yields bioenergy and biofertilizer. However, substantial antibiotics and benzenes contained in liquid and solid digestate hinder its large-scale applications. In this study, a novel closed-loop treatment strategy was proposed to integrate the synthesis benefits of anaerobic digestion, pyrolysis of solid digestate, and adsorption. The solid digestate-derived biochar (S-biochar) was successfully prepared by coupling pyrolysis and H2SO4 activation. The S-biochar possessed a polycyclic aromatic structure, abundant micropores (0.8–1.0 nm), various functional groups (–COOH, Aro–OH), and a higher absolute zeta potential value. These characteristics contributed to the excellent adsorption of tetracycline (51.3 mg/L), oxytetracycline (32.9 mg/L), benzene (86.0 mg/L) and toluene (74.1 mg/L), wherein pore filling, electrostatic interactions, H-bonding, surface partition, and π-π conjugation mechanisms were involved. The adsorption of antibiotics was more sensitive to pH, in comparison with benzenes, and displayed a declining trend (pH>7). Under actual liquid digestate conditions, >95% adsorption of antibiotics and benzenes were reached at the dosage of 1.5 g/L, in a company with >80% of nitrogen and phosphorus. In the end, a carbon and nitrogen flow analysis was performed to elucidate that more than 80% of carbon in pig manure can be settled as biochar and biogas, and up to 94.7% of nitrogen remained in the biochar and liquid digestate. This study not only developed a S-biochar with excellent performance in removing organic pollutants from liquid digestate, but also established a closed-loop strategy as an alternative carbon sequestration approach using anaerobic digestion side-products.