Investigation of Sewage Sludge–Derived Biochar for Enhanced Pollutant Adsorption: Effect of Particle Size and Alkali Treatment

Sewage sludge (SS) holds promise for environmental, agricultural, and energy applications. However, its direct use is limited due to contaminant concerns. Pyrolysis can turn SS into beneficial products like bio-oil and biochar. This study explores biochar production from SS pyrolysis and its potential for pollutant adsorption. The effects of pyrolysis temperature (500, 650, 850 °C) and SS particle size (800–1000 µm, 400–800 µm, 100–400 µm, ≤100 µm) on biochar yield and adsorption capacity for methylene blue and mercury were investigated. Regardless of particle size and temperature, SS-derived biochar exhibited second-order adsorption kinetics. Biochar with a particle size of 100–400 µm displayed the highest potential for methylene blue adsorption. Subsequent alkali treatment (biochar:NaOH = 3:4) of these particles significantly increased specific surface area from 27.5 m²/g to 144.27 m²/g and further enhanced adsorption capacities for both methylene blue (from 9 mg/g to 35 mg/g) and mercury (from 17 mg/g to 36 mg/g). These findings suggest that SS-derived biochar, particularly the 100–400 µm fraction with alkali treatment, presents a promising cost-effective adsorbent for water treatment, aligning with circular economy principles.