Tuning Electronic and Pore Structures of Biochar via Nitrogen and Magnesium Doping for Superior Methylene Blue Adsorption: Synergistic Mechanisms and Kinetic Analysis

Developing high-performance porous biochar materials aims to meet the growing demand for efficient adsorbent solutions in addressing environmental pollution issues, such as dye wastewater treatment. In this study, Lycium chinensis stalks were utilized as a precursor, and high-temperature pyrolysis was employed to incorporate nitrogen and magnesium (Mg) ions, resulting in the production of effective porous biochars designated as GPC-N and GPC-Mg. The surface morphology and physicochemical properties of these activated carbons were characterized using various techniques including Brunauer-Emmett-Teller (BET) analysis, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The results indicate that, under a mass ratio of 1:2.5 for Lycium chinensis stalks to H₃PO₄, the optimal preparation conditions for magnesium-modified activated carbon (GPC-Mg) involve a GPC-to-Mg chloride mass ratio of 1:0.75, a holding time of 1 h, and a pyrolysis temperature of 800 °C. For ammonium chloride-modified activated carbon (GPC-N), the optimal preparation conditions consist of a GPC-to-ammonium chloride mass ratio of 1:3, a holding time of 1.5 h, and a pyrolysis temperature of 900 °C. The specific surface areas of GPC-Mg and GPC-N are 281.53 m²/g and 730.63 m²/g, respectively, with total pore volumes of 0.20 cm³/g and 0.41 cm³/g, and average pore diameters of 4.82 and 4.03 nm, respectively. Both materials are biomass-derived activated carbons predominantly characterized by mesoporous structures. Notably, GPC-N displayed a higher adsorption capacity for methylene blue at 496.53 mg/g, significantly surpassing that of GPC-Mg at 48.06 mg/g. The GPC-N biochar underwent in situ nitrogen doping that significantly reconstructed the electronic structure of the carbon matrix, forming abundant nitrogen-containing functional groups, primarily pyridinic nitrogen. In contrast, GPC-Mg optimized its pore structure through the template effect induced by magnesium ions; however, it exhibited a lower density of surface active chemical sites compared to GPC-N. Consequently, the nitrogen-doped biochar demonstrated superior surface area and chemical morphology. The adsorption process was best described by Langmuir isotherm models along with pseudo-second-order kinetic models and intraparticle diffusion models. These findings indicate that chemical adsorption serves as the primary mechanism involved in this process while highlighting synergistic effects between surface adsorption and pore diffusion during adsorption. This research provides both theoretical foundations and technical support for designing high-performance biochar materials with significant potential applications in dye wastewater treatment.