Optimization of nitrogen doping via microwave irradiation for improved adsorptive performance of pomegranate peel-derived biochar

Abstract

This study presents a novel strategy to enhance the adsorption performance of biochar derived from pomegranate peel waste through nitrogen doping facilitated by microwave irradiation. The raw biochar was first synthesized via pyrolysis and subsequently modified using nitric acid under controlled microwave-assisted conditions. Critical parameters, including microwave power, irradiation time, and acid concentration, were systematically optimized to achieve effective nitrogen incorporation. The microwave-assisted treatment not only accelerated the doping process but also promoted the development of a more porous structure and the introduction of abundant nitrogen- and oxygen-containing functional groups, which increase surface polarity and provide additional active sites for metal ion binding. The structural and surface properties of the nitrogen-doped biochar were comprehensively characterized by Brunauer–Emmett–Teller (BET) surface area analysis, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). Batch adsorption experiments confirmed that these physicochemical enhancements significantly improved the affinity and selectivity for Cd(II) ions through mechanisms such as electrostatic interactions, complexation, and ion exchange. At 30 °C and an initial Cd(II) concentration of 100 ppm, the adsorption capacities of the raw biochar, nitric acid-treated biochar, and microwave-assisted nitrogen-doped biochar were determined to be 16.24 mg/g, 35.25 mg/g, and 42.82 mg/g, respectively, clearly demonstrating the synergistic effect of the combined modification. The adsorption behavior was further analyzed using kinetic and thermodynamic models to elucidate the underlying mechanisms. The nitrogen-doped biochar exhibited a notably enhanced maximum adsorption capacity of 106.84 mg/g for Cd(II), demonstrating that microwave-assisted nitrogen functionalization effectively tailors the biochar's pore structure and surface chemistry, making it a sustainable, cost-effective, and efficient adsorbent for heavy metal remediation in wastewater treatment applications.