Magnesium-modified sewage sludge biochar as an efficient adsorbent for phosphorus recovery

The recovery and reuse of phosphorus (P) is of great scientific and technological interest to ensure the long-term sustainability of this vital resource, which plays a crucial role in plant biological and biochemical processes. This work aimed to evaluate the effect of hydrofluoric acid (HF) pretreatment of sewage sludge (LL) prior to pyrolysis in the production of a MgCl 2 -modified biochar for P adsorption and release. Dried LL was washed with a HF, dried again, and subsequently treated with a MgCl 2 solution. The material was then pyrolyzed at 500 °C for 4 h under an N 2 atmosphere (LLN/Mg500). The results showed that P adsorption was effective after treatment with MgCl 2 and pyrolysis process. Characterization techniques confirmed the presence of P on the surface of LLN/Mg500, indicating the formation of MgHPO 4 . Analysis of the adsorption mechanisms identified chemical precipitation and ion exchange as the main processes controlling the P adsorption. The adsorption isotherm best fit the Langmuir model ( q e,max = 221.37 mg g −1 ), and the pseudo-second-order kinetic models were the most appropriate to fit the data, indicating that the adsorption process occurs in a monolayer and is predominantly chemical. Thermodynamic analysis showed that P adsorption on LLN/Mg500 was entropically driven. The LLN/Mg500 presents a potential application perspective for treating P-containing wastewater, contributing to environmental and agricultural sustainability. Furthermore, the biochar produced shows strong potential as a phosphate fertilizer for agricultural use. These findings highlight the feasibility of modified biochar as an effective, sustainable approach for wastewater treatment and nutrient recycling in agriculture.