Microwave-Assisted Sulfur Functionalization of Biochar: A Sustainable Route for Cadmium Remediation

Abstract

The conversion of agricultural residues into value-added adsorbents offers a sustainable strategy for waste management and water remediation. In this study, pomegranate peel waste was transformed into sulfur-functionalized biochar (MT@S-doped biochar) through sulfuric acid-assisted microwave carbonization and evaluated for the removal of Cd(II) ions from aqueous solutions. The effects of microwave power, irradiation time, solution pH, contact time, and temperature on adsorption performance were systematically investigated. Structural characterization by XRD, FTIR, and SEM confirmed the formation of a porous carbonaceous structure enriched with oxygen- and sulfur-containing functional groups. The optimum preparation conditions were identified as 750 W microwave power and 20 min irradiation time. Adsorption performance was strongly influenced by solution pH, with maximum Cd(II) removal observed at pH 6. Equilibrium data were best described by the Langmuir isotherm model (R2 = 0.994-0.996), yielding maximum adsorption capacities of 29.32, 31.74, and 38.46 mg g(-)1 at 303, 313, and 323 K, respectively. Kinetic studies showed excellent agreement with the pseudo-second-order model (R2 = 0.998), indicating that chemisorption-related interactions play a dominant role in the adsorption process. Thermodynamic analysis revealed that Cd(II) adsorption was spontaneous and endothermic, with Delta G degrees values ranging from - 4.70 to - 6.68 kJ mol(-)1 and a positive Delta H degrees value of 25.44 kJ mol(-)1. Notably, enhanced Cd(II) adsorption was achieved despite a reduction in BET surface area after activation, demonstrating that the improvement was governed primarily by microwave-induced surface chemical functionalization and surface-charge modification rather than by physical surface-area development alone. The enhanced adsorption performance was attributed to the combined effects of sulfur-containing functional groups, electrostatic attraction, surface complexation, and porous structure development. These findings demonstrate that sulfur-functionalized pomegranate peel biochar is an effective, low-cost, and environmentally sustainable adsorbent for Cd(II) removal and represents a promising approach for the valorization of agricultural waste in water treatment applications.