Circular Economy Pathway: Valorization of Cotton Stalk into Biochar for Textile Wastewater Treatment

Abstract

Agricultural residues, often burned openly pose environmental challenges but offers opportunities for valorization into functional materials. Converting such residues into biochar supports circular economy principles. Here, cotton stalk (CS) was converted into phosphate-modified biochar (PMCS) via pyrolysis at 350, 550, and 800°C. Response Surface Methodology (RSM) was applied to the adsorption process, treating pyrolysis temperature as a categorical factor in a rotatable CCD. PMCS was characterized by Brunauer-Emmett-Teller (BET), Scanning Electron Microscopy (SEM), and Fourier Transform Infrared Spectroscopy (FTIR), Point of Zero Charge while adsorption was evaluated through isotherm and kinetic modeling for synthetic textile wastewater containing Eriochrome Black T (EBT), starch, and salts. PCS800 exhibited a BET analysis of 750 m² per gram of biochar and achieved nearly 77% COD reduction at 33 min and 6.43 g/L for synthetic wastewater, while 60% with complete decolorization for real effluent. The optimum removal followed Langmuir behaviour (qmax=90.2 mg/g, KL= 0.049 L/mg) with pseudo-second-order kinetics, reflecting micropore filling by starch. Overall, this study establishes a circular economy pathway by valorizing CS into an efficient adsorbent, mitigating residue burning while offering scalable potential for textile wastewater treatment.