Oil well–produced water pollutant adsorption and photodegradation using an innovative double Z-scheme ternary heterostructure of MIL-101(Cr)/Fe3O4-SiO2/nanorod-graphitic carbon nitride: adsorption isotherm and degradation kinetic study

An innovative ternary heterostructure, MIL-101(Cr)/Fe₃O₄-SiO₂/nanorod-graphitic carbon nitride (MIL-Cr/F@S/nr-GCN), was synthesized by hydrothermal technique. Comprehensive physiochemical characterizations were conducted to elucidate the structural and optical properties. The synthesized photocatalysts were evaluated for adsorption and photodegradation of oil well–produced water pollutants. Remarkably, the ternary heterostructure composite with 20 wt% of nr-GCN exhibited superior photocatalytic performance compared to nr-GCN and the MIL-Cr/F@S binary composite. Under visible-light illumination, the maximum removal efficiency of chemical oxygen demand for synthetic oil well–produced water reached 97.4% under optimized conditions (pH 4, illumination time 90 min, photocatalyst dosage 0.6 g/L, and pollutant initial concentration 754 mg/L). Adsorption studies revealed adherence to the pseudo-second-order kinetic and Freundlich isotherm models The ternary composite displayed degradation rates 2.8 and 2 times higher than nr-GCN and MIL-Cr/F@S, respectively. This enhanced activity was attributed to the double Z-scheme configuration, providing high specific surface area (653 m²/g), appropriate bandgap energy (1.6 eV), and efficient charge carrier separation. Moreover, the ternary photocatalysts demonstrated excellent reusability over five cycles without Cr ions leaching into the water. These findings underscore the potential of the novel ternary heterostructure as a green and robust photocatalyst for oil well–produced water treatment.