Recovery of ammonium and phosphate from urine as value-added fertilizer using wood waste biochar loaded with magnesium oxides

Sustainability in waste and wastewater treatment is currently drawing extensive attention. This study employed magnesium-oxides-modified biochar (Mg-biochar) yielded from wood waste to recover ammonium and phosphate from human urine. The Mg-biochar demonstrated high removal capacities of ammonium and phosphate, reaching 47.5 mgN/g and 116.4 mgP/g, respectively. Particularly, the removal capacity of ammonium was higher than those of many other carbon-based absorbents. Process mechanisms were discussed by analyzing the chemical composition of nutrient-enriched biochar (NBC), and the adsorption kinetics and isotherms. The struvite precipitation on the biochar surface was the dominant mechanism in the removal of both ammonium and phosphate, whereas the surface adsorption of phosphate also played a role in phosphate removal. The NBC presented competitive effects on growths of maize and ryegrass as a commercial fertilizer diammonium phosphate in pot experiments. However, the relatively low concentration of phosphate in the urine limited the removal efficiency of ammonium to only 1%, although most phosphates were removed. As such, another treatment unit should be combined for the complete removal of ammonium. Results indicated that Mg-biochar from wood waste can effectively remove phosphate from urine while recovering ammonium and phosphate as a biochar-based nutrient-enriched fertilizer.