Natural Minerals as Support of Silicotungstic Acid for Photocatalytic Degradation of Methylene Blue in Wastewater

Abstract

In this paper, silicotungstic acid (HSiW) was impregnated on natural minerals such as clinoptilolite (HSiW/Clin), mordenite (HSiW/Mord), bentonite (HsiW/Bent) and kaolinite (HSiW/Kaoln) and were evaluated towards photocatalytic degradation of methylene blue in wastewater. The as-prepared photocatalysts were characterized by X-ray powder diffraction (XRD), Field Emission Scanning Microscopy (FESEM), Fourier transform infrared (FTIR) and UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS). The results showed that the crystallinty of HSiW was different on above mentioned minerals and HSiW on clinoptilolite had the most crystallinty and the smallest crystallite size. FESEM showed that the supported HSiW had nanometric spherical particles with narrow and uniform distribution. FTIR revealed Keggin structure of HSiW in all photocatalysts and a strong interaction between HSiW and minerals. DRS studies showed HSiW/Clin and HSiW/Mord had photoactivity in both visible and ultraviolet regions while HSiW/Bent and HSiW/Kaoln photocatalysts were just active in UV region. More studies indicated that HSiW/Clin had the lowest band gap of 3.1 eV among studied nano photocatalysts. This sample could degrade 92% of methylene blue after 90 min of UV irradiations and HSiW/Mord, HSiW/Bent and HSiW/Kaoln were in the next positions. Kinetic studies illustrated photocatalytic degradation of methylene blue followed a pseudo first order reaction and the highest rate constant belonged to HSiW/Clin and other photocatalysts had much lower rate constants. Effect of operation parameters like wastewater pH, initial dye concentration, photocatalyst loading and successive runs of HSiW/Clin was investigated and at the end a new mechanism for this photocatalyst was proposed.