Sewage sludge-derived biochar modified with MgCl2: Used as adsorbent material for phosphorus removal from aqueous medium

Sewage sludge (SS) is produced in wastewater treatment plants due to the removal of solid and liquid waste. SS is rich in organic matter and can contain pathogens and high levels of heavy metals, posing a significant environmental challenge. The search for effective and sustainable solutions to treat this waste has intensified in this context. Transforming SS into biochar is emerging as a useful solution for effectively managing this waste. Biochar is a carbonaceous material obtained from biomass pyrolysis, having porosity and specific surface area that vary depending on the raw material used. This study aimed to produce MgCl₂-modified biochar using SS as biomass to be used as an adsorbent for P in an aqueous medium. SS containing or not MgCl₂ incorporated was pyrolyzed at different temperatures and the obtained materials were characterized by X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS), specific surface area (BET) and point of zero charge (PZC). Kinetic and equilibrium adsorption studies of P on the biochar were evaluated, in which a 50.0 mg constant mass of biochar was added to 25.0 mL solutions containing P concentrations ranging from 0 to 350.0 mg∙L⁻¹. The solutions were swirled for 48 h and was conducted in triplicate. The process of modifying the biochar with magnesium was effective. Additionally, the kinetic data showed that the pseudo-second-order model better fitted the experimental data. The adsorption mechanism occurs through the chemical precipitation of P species with Mg²⁺ ions or ionic exchange with anions such as Cl^- on the surface of the material pyrolyzed at 500°C. The P adsorption isotherm followed the Langmuir model, with a maximum adsorption capacity of 176 mg·g⁻¹ after 48 h. Magnesium-functionalized biochar produced from SS has a high capacity for P removal from an aqueous medium, which represents a promising alternative for applying this material as a P fertilizer.