Natural Feldspar as a Low-Cost Adsorbent and Solar Photocatalyst for Dye Removal

Abstract

This study evaluates a low-cost natural potassium feldspar from Chihuahua, Mexico, for removing the cationic dyes methylene blue (MB) and rhodamine 6G (R6G) from water. Rietveld refinement identified the raw mineral as a polymineralic material composed of sanidine (49 vol%), muscovite (27 vol%), calcite (16 vol%), and anorthoclase (7 vol%). The mineral exhibited dual adsorptive and photocatalytic functionality. Under dark conditions, it achieved removal efficiencies of 98% for MB and 76% for R6G after 120 min, with equilibrium data best described by the Langmuir isotherm. Under solar irradiation, the mineral promoted dye degradation, as demonstrated by the rapid decrease and shifts in their characteristic absorption bands. The addition of hydrogen peroxide (H₂O₂) produced a synergistic advanced oxidation process, achieving 98% degradation of MB and 93% degradation of R6G within 15 min, compared with 97% and 65%, respectively, under solar irradiation alone. The kinetics followed a pseudo-second-order model, suggesting a surface-controlled process influenced by dye concentration and active-site availability. Elovich and Avrami modeling further indicated surface heterogeneity and multistep pathways involving adsorption, surface-mediated photocatalytic reactions, and oxidation by reactive oxygen species. Scavenger experiments showed that superoxide radicals (O₂•⁻) predominated without H₂O₂, whereas hydroxyl radicals (•OH) dominated under H₂O₂-assisted conditions. Photoluminescence spectroscopy confirmed structural changes in the dyes and the formation of fluorescent intermediates. Overall, natural potassium feldspar shows considerable potential as an abundant, low-cost, and environmentally sustainable material for solar-driven wastewater treatment.

Keywords: feldspar; dye; photocatalysis