An efficient g-C3N4@ZnO@ZrO2 heterostructure for heavy metal ions removal – Experimental and computational modelling

Photodegradation and adsorption using metal oxide and carbon-based semiconductors have emerged as cost-effective and efficient methods for removing hazardous organic dyes and heavy metal ions from aquatic environments. In this work, a new CN@ZnO@ZrO₂ heterostructure was successfully fabricated through a simple ultrasonication method to enhance physicochemical performance for Cd(II) ion adsorption. X-ray diffraction (XRD) confirmed the formation of hexagonal g-C₃N₄, hexagonal ZnO, and monoclinic ZrO₂ phases. Scanning and transmission electron microscopy (SEM and TEM) showed a uniform distribution of ZnO and ZrO₂ nanoparticles on the g-C₃N₄ nanosheets, while energy-dispersive X-ray (EDX) analysis confirmed the elemental purity. The ternary composite exhibited a relatively high specific surface area of 87.4 m²·g⁻¹, with a pore volume of 0.207 cm³·g⁻¹ and an average size of 14.53 nm. Adsorption experiments conducted under optimal conditions (1400 min at 25 °C, pH: 7, and 0.4 g·L⁻¹ of adsorbent with 60 mg·L⁻¹ of Cd²⁺ concentration) yielded a maximum adsorption capacity of 272.4 mg·g⁻¹. kinetic modelling and isotherm analysis indicated that the process follows pseudo-second-order kinetics and Langmuir adsorption model, respectively. The adsorption mechanism was investigated using XRD, XPS, FTIR, and SEM-EDS mapping, while density functional theory (DFT) calculations provided theoretical confirmation, showing excellent agreement with the experimental results. Specifically, DFT revealed a a strongly negative Cd²⁺ adsorption energy of −3.84 eV and substantial charge transfer (1.16 |e|) on the CN@ZnO@ZrO₂ (100) surface, supporting the observed superior uptake performance. Overall, the CN@ZnO@ZrO₂ heterostructure demonstrated significant potential for use as an adsorbent in the removal of pollutants from contaminated water.