Investigations of Phosphorus Removal Using an Eco-Friendly Modified Biochar: Batch and Continuous Stirred Reactor Studies

In this study, a sustainable calcium-rich biochar was synthesized via co-pyrolysis at 800 °C of marble waste, animal manure, and lignocellulosic biomass. This biochar (MWM–B) was comprehensively characterized and then valorized for phosphorus (P) removal from real effluent and synthetic solutions in both batch and continuous stirred tank reactor (CSTR) modes. Characterization results confirm the formation and deposition of significant amounts of calcium oxides and calcium hydroxides on the biochar surface, which enhance the biochar’s surface chemistry and textural properties. In batch mode, MWM–B efficiently removes P with a removal capacity (108.4 mg g−1) that is 5.3 times higher than that observed in the CSTR system. This efficiency drop is due to the limited contact time between phosphate species and the biochar particles. Interestingly, the presence of calcium and magnesium in the continuously renewed real effluent in the CSTR system increases P removal efficiency by approximately 136% compared with synthetic solutions. A detailed analysis of MWM–B before and after P removal suggests that this process occurs mainly through precipitation as hydroxyapatite, complexation with hydroxyl functional groups, electrostatic interactions, and hydrogen bonding. This work confirms that MWM–B generated at 800 °C is an attractive material for P removal from effluents.