Phosphorus Removal Efficiency of Magnesium Chloride-Activated Biochar Produced from Industrial Wastes

This study evaluates phosphorus adsorption using biochars produced from three industrial residues—biological sludge, physicochemical sludge, and spent bleaching earth—activated with 2% magnesium chloride and pyrolyzed at 450°C. The materials were characterized through ash content, pH, electrical conductivity, and by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS) to elucidate surface morphology, porosity, and elemental composition before and after adsorption. Physicochemical sludge biochar exhibited a highly porous and irregular structure, whereas biological sludge biochar showed a fibrous matrix with moderate compaction; the spent bleaching earth biochar displayed a smoother and less porous surface. Postadsorption SEM images revealed pore obstruction and surface deposition, suggesting interaction between phosphate species and the biochar surface, while EDS indicated increased phosphorus, magnesium, and iron contents—most notably in the biological sludge biochar. Response surface methodology was applied to quantify the effects of pH, initial phosphorus concentration, and biochar dosage, identifying optimal conditions that yielded adsorption efficiencies approaching 100%, with maximum adsorption capacities of 64.04 and 40.80 mg P g ⁻¹ for biological and physicochemical sludge biochars, respectively, in contrast to the limited performance of spent bleaching earth, which exhibited phosphorus adsorption efficiencies below 35% and a maximum adsorption capacity of only 3.10 mg P g ⁻¹ . These findings demonstrate the potential of residue-derived biochars as functional sorbents for nutrient recovery, highlighting their relevance for circular-material design and sustainable wastewater treatment systems.