LDH-TiO₂ Composite for Selenocyanate (SeCN⁻) Photocatalytic Degradation: Characterization, Treatment Efficiency, Reaction Intermediates and Modeling

Selenium as a nutrient has a narrow margin between safe and toxic limits. Hence, wastewa-ter discharges from selenium-containing sources require appropriate treatment that considers health concerns and stringent selenium-related water treatment standards. This work examined the use of a photocatalysis-cum-adsorption system based on a layered double hydroxide coupled with TiO₂ (LDH-TiO₂) to remove aqueous phase selenocyanate (SeCN⁻), which is difficult to treat and requires specific treatment procedures. The synthesized LDH and LDH-TiO₂ composite samples were charac-terized using the X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and thermogravimetry analysis (TGA) methods. The XRD results for the uncalcined LDH indicated a hydrotalcite mass with a rhombohedral structure, whereas increasing the calcination temperature indicated transition to an amorphous state. FESEM results for the LDH-TiO₂ matrix indicated round titanium dioxide particles and LDH hexagonal layers. The TGA findings for uncalcined LDH showed a gradual decrease in weight up to 250^◦C, followed by a short plateau and then a sharp decrease in LDH weight from 320^◦C, with a net weight loss around 47%. Based on the characterization and initial selenocyanate adsorption results, the 250^◦C calcined LDH-TiO₂ matrix was used for the se-lenocyanate photocatalysis. A ~100% selenium removal was observed using LDH:TiO₂ at a 1.5:1 w/w ratio with a 2 g/L dose, whereas up to 80% selenium removal was noted for LDH:TiO₂ at a 0.5:1 w/w ratio. The respective difference in the efficiency of selenium treatment was attributed to enhanced LDH-based adsorption sites in the enhanced LDH:TiO₂ w/w ratio. Furthermore, the selenite and selenate that occurred during SeCN⁻ photocatalytic degradation (PCD) were also nearly completely removed via adsorption. An optimization exercise using response surface methodology (RSM) for total selenium removal showed R² values of more than 0.95, with a prediction accuracy of more than 90%. In summary, the present findings show that the use of a photocatalysis-cum-adsorption system based on LDH-TiO₂ is a promising technique to treat industrial wastewater discharges for selenocyanate and also remove the resulting intermediates.