Selenocyanate removal using combined system of TiO₂- photocatalysis and Fe(III)/SiO₂ adsorption {UV-TiO₂/[Fe(III)/SiO₂]}: Complex destruction and simultaneous selenium species adsorption

Selenocyanate (SeCN‾) species, commonly found in industrial effluents from crude oil refineries and mining operations, present substantial health risks to humans and animals. This study investigates the treatment of selenocyanate-contaminated streams using a combination of TiO₂ photocatalysis and Fe(III)/SiO₂ binary oxide adsorption {UV-TiO₂/[Fe(III)/SiO₂]}. The Fe(III)/SiO₂ binary oxide was synthesized and characterized using X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy. XRD analysis revealed non-crystalline Fe-oxide phases, while FTIR indicated the presence of various Fe and O functional groups on the Fe(III)/SiO₂ surface. Adsorption studies using Fe(III)/SiO₂ showed its selenocyanate uptake capacity ∼ 2.1 mg/g, with the respective data closely aligning with the Langmuir isotherm model. This Fe(III)/SiO₂ adsorption feature was successfully combined in the {UV-TiO₂/[Fe(III)/SiO₂]} system in which the hydroxyl radicals (^●OH) generated by the UV-TiO₂ facilitated the breakdown of the selenocyanate complex, while the resulting selenite and selenate were simultaneously adsorbed by the Fe(III)/SiO₂ within a 240-min reaction time. A mathematical model developed using a face-centered central composite design-response surface methodology (FCD-RSM) showed significant predictive capability with an insignificant lack of fit. Optimal conditions for selenium removal were determined to be 20 mg/L selenocyanate, 1 g/L TiO₂, and 1 g/L Fe(III)/SiO₂, at pH 5, within a 240-min reaction time. Kinetic analysis demonstrated that selenocyanate degradation followed pseudo-first-order kinetics, while total selenium removal adhered to pseudo-second-order kinetics, confirming the proposed selenium species removal mechanism. Overall, this integrated approach offers a promising solution for effective selenocyanate remediation in industrial wastewater.