Magnetic biochar aluminum cross-linked composite beads for preferential phosphate separation from phosphate-rich effluents

Abstract

This study explores the development of magnetite-biochar-alginate beads cross-linked with aluminum chloride (MBA/AlCl₃) for enhancing phosphate removal and recovery from a wide range of wastewater with easy separation properties. This study investigates how metallic Ca2 +, Cu2+ and Al3+ cross-linkers influence the gelation mechanism and phosphate removal efficiency. Among these, biochar-alginate beads cross-linked with Al3+ exhibited the best phosphate adsorption capability. Multilayer gelation within the three-dimensional covalent bonding structure was assumed for the Al3+ cross-linking. The phosphate adsorption process followed pseudo-second-order kinetics (R2 = 0.99) and the Langmuir isotherm (R2 > 0.99), with the adsorption capacities increasing from 56 mg/g at 298 K to 74 mg/g at 318 K, indicating temperature-enhanced chemisorption. Phosphate adsorption was pH-dependent, with an optimal pH of 3 (100 % removal in 6 h), decreasing to 51 % at pH 9 owing to reduced protonation. Wet beads outperformed oven-dried beads due to enhanced diffusion and greater accessibility of active sites. Competitive anions such as Cl⁻, NO₃⁻, and SO42- showed minimal interference, maintaining 85 % removal efficiency in both single- and multi-component solutions. The beads retained 97 % of their efficiency after regeneration with a 3 % W/V AlCl3 solution during the second cycle, while magnetic properties (25.9 emu/g) enabled easy separation. Phosphate removal is associated with electrostatic attraction, complexation, ion exchange, and ligand exchange. In real wastewater applications, MBA/AlCl₃ beads achieved complete phosphate removal from activated sludge and anaerobic membrane bioreactor effluents (20–40 mg/L PO43-), and 86 % removal from electrodialysis concentrates (84 mg/L PO43-).