A magnetically recyclable 3D hierarchically porous La(OH)₃/Fe₃O₄–loofah-derived carbon for phosphate removal: Linking structure, mechanism, and cost analysis

Loofah sponge carbon (LOC) was employed as a three-dimensional scaffold and magnetized through alkaline hydrothermal treatment followed by co-precipitation to obtain magnetic loofah sponge carbon (MLOC). Subsequent lanthanum incorporation produced La₃-MLOC, which integrates a hierarchically porous architecture with magnetic separability for phosphate removal from complex waters. La₃-MLOC delivered a Langmuir capacity of 117.32 mg/g at pH 3 and showed rapid uptake across pH 3–6, achieving > 90% removal within 15–40 min while maintaining good tolerance to common coexisting ions. Kinetic fitting (PSO and Avrami, n ≈ 0.82) suggests a predominantly site-specific chemisorption pathway. Microscopic and spectroscopic analyses support La–OH-mediated hydroxyl displacement/inner-sphere complexation, accompanied by partial LaPO₄ precipitation. The sorbent retained high performance over four adsorption–desorption cycles (removal about 92%; desorption about 77%). When evaluated using real effluent from a coal-mining subsidence outfall, La₃-MLOC enabled low-level phosphate polishing to Class III targets at low dosage and short contact time, with an estimated treatment cost of 0.341 USD/g, indicating its potential for end-of-pipe phosphate control and resource-oriented management.