Methylene blue removal by AgCl/ZnS/ZnO composite during dark contact and white LED irradiation

Methylene blue (MB) removal was investigated using AgCl/ZnS/ZnO composite under a commercial white light-emitting diode (LED) source. ZnO, ZnS, ZnS/ZnO and AgCl were prepared using precipitation-based methods, and AgCl was physically mixed with ZnS/ZnO. Three composites were prepared using different initial ZnO masses where as ZnS and AgCl were kept constant. X-ray diffraction (XRD) supported the presence of ZnO and ZnS in the composite, while crystalline AgCl was identified only in the separately prepared AgCl sample. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS) detected Zn, O, S, Ag and Cl within the analysed region, although the Ag and Cl signals were weak. N 2 adsorption–desorption analysis gave a Brunauer–Emmett–Teller surface area of 49.96 m 2 g − 1 , a Barrett–Joyner–Halenda desorption cumulative pore volume of 0.1688 cm 3 g − 1 and a pore diameter of 11.27 nm for the AgCl/ZnS/ZnO composite. Dark adsorption contributed substantially to the overall apparent MB removal. The overall apparent removal increased from 87.99% to 100% as the catalyst loading increased from 10 to 30 g L − 1 . The light-stage MB concentration data were fitted to an apparent pseudo-first-order model, giving preliminary rate constants of 0.0111–0.0399 min⁻¹. The estimated electrical energy per order, calculated using the nominal 30 W lamp rating, increased from 563 to 1971 kWh m − 3 order − 1 , while the average volumetric MB removal rate increased from 4.64 to 5.09 g m − 3 h − 1 . These results describe laboratory-scale apparent MB removal because mineralisation, photon flux, reactive species and catalyst stability were not measured.