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-g-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-g-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-g-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-g-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-g-C3N4 adsorbent surface.