Adsorption of phosphorus in water by metal-modified large-size biochar: Realizing the recovery and recycling of phosphorus

Biochar finds extensive use in adsorbing phosphorus (P) from wastewater abundant in P content. Most biochars carry negative surface charges, causing electrostatic repulsion towards anions. However, as P commonly exists as anions in water, the P adsorption capacity of biochar remains limited. Moreover, conventional biochar used for P adsorption presents challenges in recycling due to its small particle size. This study presented a novel approach wherein metal-modified large-size (1–2 mm) walnut shell biochars were created using Fe, Mg, and Ca additives at varying ratios. This innovation aims to enhance P adsorption capacity and recyclability. Experimental findings revealed that unmodified biochar (BC) exhibited minimal P adsorption capacity (0.34 mg/g), while all metal-modified biochars (MBCs) demonstrated elevated P adsorption capabilities (1.06–25.60 mg/g). Among the distinct MBCs, Ca-MBCs displayed the highest P adsorption capacity (25.60 mg/g), trailed by Mg-MBCs (23.58 mg/g) and Fe-MBCs (5.99 mg/g). Furthermore, both isotherm and kinetic adsorption analyses suggested that the P adsorption mechanism for MBCs predominantly involves chemical adsorption. However, desorption studies post-adsorption indicated suboptimal desorption efficiencies for P (19.16–26.07%) in MBCs, posing challenges for their application as fertilizers. Therefore, it is essential to enhance simultaneous P adsorption and desorption capabilities of biochar.