Fly ash converting into ordered mesoporous silica materials for water purification from azo dyes

Extensively used azo dyes enter water bodies, posing a serious threat to the environment. Their removal is of great importance, hence, methods based on low-cost adsorbents derived from waste materials are currently being developed. In this study, fly ash-based silica materials (MCM-41 and SBA-15) were subjected to modifications with 3-aminopropyltrimethoxysilane (APTMS) and 3-aminopropyltriethoxysilane (APTES) to obtain novel and cheap adsorbents for the elimination of sunset yellow FCF and tartrazine from water. The functionalization significantly altered the samples’ porosity, structure ordering, and acid-base properties. The introduction of amine groups into the samples was confirmed by the increase in nitrogen content and the presence of new bands on FT-IR spectra. The influence of pH (2–9), temperature (RT, 35 °C, 45 °C), contact time (0–240 min), and initial azo dyes concentration (12.5–900 mg L⁻¹) on sorption capacities of the obtained adsorbents was evaluated. The pH value of 2 was the most favorable for dye adsorption as a consequence of electrostatic interactions between their SO₃⁻ groups and NH₃⁺ groups of adsorbents. APTMS-modified MCM-41 exhibited the highest sorption capacities toward sunset yellow FCF (531 mg g⁻¹) and tartrazine (360 mg g⁻¹). The adsorption kinetics followed the pseudo-second-order kinetic model, and the experiments on the regeneration of silica materials showed that modified adsorbents have potential for multiple reuses, thereby reducing the generation of waste.