Graphitic Carbon Nitride and Nickel-Doped Graphitic Carbon Nitride for Doxycycline Removal: Adsorption Efficiency and Mechanistic Insights

Adsorption is a simple yet effective technique for the removal of emerging contaminants. In this study, pure graphitic carbon nitride (g-C₃N₄) and nickel-doped g-C₃N₄ (Ni-<i>g</i>-C₃N₄) were synthesized via a thermal polycondensation method and evaluated as adsorbents for doxycycline removal. Structural and morphological characterization was performed using X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, and transmission electron microscopy. The results confirmed the successful incorporation of Ni²⁺ into the g-C₃N₄ matrix, with homogeneous Ni²⁺ distribution and no secondary phases. Morphological analysis indicated that nickel doping induced only slight changes in the g-C₃N₄ morphology. Adsorption experiments revealed that both materials exhibited enhanced adsorption capacity at pH 9, with Ni-<i>g</i>-C₃N₄ achieving a significantly higher removal efficiency (53.4%) compared to g-C₃N₄ (14.2%). This improvement was attributed to Ni²⁺-induced positively charged regions, facilitating stronger adsorbate-adsorbent interactions. Kinetic analysis demonstrated that doxycycline adsorption onto Ni-<i>g</i>-C₃N₄ followed a pseudo-second-order model. Among the tested isotherm models, the Sips model provided the best fit, yielding maximum adsorption capacities of 37.889 mg·g^-1 for g-C₃N₄ and 116.265 mg·g^-1 for Ni-<i>g</i>-C₃N₄. Theoretical calculations corroborated experimental findings, confirming that Ni²⁺ incorporation in the g-C₃N₄ structure enhances adsorption capacity by facilitating strong chemical bonds between doxycycline and the Ni-<i>g</i>-C₃N₄ adsorbent surface.