Biochar production using biogas residue and their adsorption of ammonium nitrogen and chemical oxygen demand in wastewater

Modified biochars prepared from straw, pig manure, and chicken manure biogas residues were used to adsorb and remove ammonium nitrogen (NH₄⁺-N) and chemical oxygen demand (COD) in wastewater. The results showed that at 400–600°C, the adsorption performance of the biogas residue biochars increased with pyrolysis temperature, reaching the maximum at 550°C. Biochar from chicken manure biogas residue exhibited strong NH₄⁺-N (20.3%) and COD (28.1%) adsorption capacity. Through FeCl₃ modification, the Fe content on the surface of the biochar increased, and the adsorption data of the modified biochar conformed well to the pseudo-second-order kinetics and Langmuir model, and the reaction was dominated by chemical adsorption. The fitting calculation revealed that the FeCl₃-modified biochar from chicken manure biogas residue showed maximum NH₄⁺-N (55.29 mg/g) and COD (186.24 mg/g) absorption capacity. These results indicated that modified biochar from biogas residue could have potential application for the treatment and recycling of biogas slurry. Graphical abstract: [Figure not available: see fulltext.].