Enhanced synergistic adsorption of ammonium and phosphate by metal-modified corn stalk: adsorption performance and mechanisms

Abstract

In order to solve the eutrophication caused by nitrogen and phosphorus, magnesium-modified corn stover biochar (Mg–BC) was prepared by pyrolysis in this study to achieve synergistic adsorption and resource utilization of nitrogen and phosphorus. The experimental results showed that the calcination temperature and calcination time altered the physicochemical properties of Mg–BC, and also had a significant effect on NH₄⁺–N and PO₄³⁻–P adsorption. Compared with unmodified BC, the adsorption of ammonium and phosphate by Mg–BC increased by 8 and 9 times, respectively. Mg–BC exhibits superior adsorption performance. The adsorption of ammonium and phosphate by Mg–BC is more consistent with the pseudo-second-order kinetics and Sips model, with maximum adsorption capacities of 153.87 mg g⁻¹ for ammonium and 315.67 mg g⁻¹ for phosphate. The adsorption behavior is mainly controlled by chemisorption, and the adsorption process is exothermic. The primary mechanisms of phosphate and ammonium adsorption are electrostatic attraction, ion exchange, complexation, surface precipitation, and ligand exchange, with struvite crystallization being the predominant form of precipitation. It serves as an effective adsorbent for the co-recovery of NH₄⁺–N and PO₄³⁻–P. More importantly, in pot experiments, NP@Mg–BC promoted the growth of ryegrass and achieved green recovery of adsorbate byproducts.