Application of ionic mesoporous silica in selective recovery of tungstate ions through column adsorption and subsequent photocatalytic degradation of an organic dye

Abstract

Designing tungsten (W) containing mesoporous photocatalysts has involved excessive usage of rare W salts. The present study depicts the role of ionic mesoporous organosilica (IMOS) constituting a polyethylene glycol (PEG)-linked bis-imidazolium chloride framework for the selective recovery of tungstate ions [WO₄²⁻] from wastewater and its subsequent use as a photocatalyst. The ion-exchange capacity of IMOS was investigated by varying the bed height of the column and flow-rate of wastewater. A maximum recovery of tungstate ions (q₀ = 123.2 mg g⁻¹) was observed with a bed height of 3 cm and a flow rate of 3 mL min⁻¹. By using W(VI)-immobilized IMOS (W-IMOS), Rhodamine B dye (RhB) was oxidized through successive adsorption (30%) in the dark and photocatalytic degradation (66%) under UV-vis light irradiation. The results are attributed to better interfacial charge transfer between localized WO₄²⁻ and the organic moieties of IMOS, which lowered the band gap (E_g) and enhanced the photodegradation rate (k_app) from 0.330 to 0.485 h⁻¹ for RhB within 6 h. Quenching experiments for active radicals were performed by EPR analysis to discover the degradation mechanism. PL spectra of the catalysts exhibited electronic behaviour. The surface charge density was recorded with the help of the pH drift method and the PZC was estimated. The XPS studies demonstrated the chemical stability of IMOS after adsorption and subsequent photocatalytic degradation. The investigation reveals the approach of fabricating a photocatalyst by merely using nanomaterials and a secondary source of precious metals and simultaneously assisting in the destruction of organic pollutants.