Transforming sewage sludge into calcium-enhanced biochar for phosphate capture in batch and continuous-flow systems with application to food industry wastewater

Phosphate (PO 4 3− ) discharge from industrial wastewater is a major contributor to eutrophication, highlighting the need for efficient, low-cost, and sustainable treatment technologies. In this study, sewage sludge derived biochar was functionalized with calcium (Ca-BC) via an impregnation-pyrolysis approach and evaluated for phosphate removal under batch and continuous-flow conditions, with validation using food industry wastewater. Physicochemical characterization confirmed Ca enrichment and presence of Ca-containing phases that can facilitate phosphate uptake. The optimized Ca-BC 10 (10 wt. % Ca) achieved 93.28% phosphate removal from a 50 mg/L phosphate solution at 1 g/L dosage, with a maximum phosphate uptake capacity of 147.48 mg/g predicted by the Langmuir model. High phosphate removal was maintained across pH 4-10, with equilibrium and kinetic data best described by the Sips isotherm and pseudo-second-order kinetic models, respectively. Fixed-bed column experiments achieved 81.18% maximum removal efficiency ([PO 4 3− ] 0 = 10 mg/L, bed height = 5 cm, flow rate = 2.6 mL/min), with a breakthrough time of 396.79min and phosphate uptake capacity of 58.99 mg/g, demonstrating reduced axial dispersion coefficient, and enhanced mass transfer. Ca-BC 10 also demonstrated excellent practical applicability, achieving 99.89% PO 4 3− removal from food industry wastewater containing 72.78 mg/L phosphate, with negligible interference from competing anions. Mechanistic analyses supported calcium-phosphate precipitation and inner-/outer-sphere complexation as contributing phosphate removal pathways. Overall, this study highlights the potential of calcium-modified sewage sludge derived biochar as a sustainable and effective material for phosphate removal, offering a promising approach that combines sewage sludge valorization with efficient phosphate remediation from aqueous solutions and industrial wastewater.