Development of high-silica fly ash-based adsorbents for efficient dye removal from wastewater: a comparative study of MSAM and SDS/FA

Solid waste fly ash is challenged by accumulation, storage, low comprehensive utilization, insufficient high-value use technologies, and environmental and ecological risks. Owing to its high silicon content and superior adsorption capability, two novel adsorbents—mesoporous silicon aluminum material (MSAM) and sodium dodecyl sulfate modified fly ash (SDS/FA)—were prepared using ultrasonic-assisted, alkali fusion–hydrothermal, and surface modification methods. Their ability to enhance the adsorption of dyes (MB, MV) on high-silica fly ash through various modification strategies was explored. The effects of the alkali-to-ash ratio, ultrasonic time, hydrothermal time, and hydrothermal temperature on MSAM adsorption were evaluated, and the optimal preparation conditions were determined using Box–Behnken response surface methodology. Likewise, the impact of particle size, SDS dosage, ultrasonic time, and oscillation time on the SDS/FA system was analyzed, and optimal conditions were established. XRD, SEM, FTIR, and BET were used for characterization. Dynamic column experiments assessed the performance of SDS/FA in removing MB and MV from dye wastewater. Results showed that: (1) MSM and SDS/FA optimal preparation conditions were determined, with MSAM (alkali-to-ash ratio 1.2 : 1, ultrasonic 20 min, hydrothermal 8 h, 100 °C) achieving 94.70% and 80.05% removal for MB and MV, respectively; SDS/FA (0.25–0.38 mm, 3 g SDS, 20 min ultrasound, 8 h oscillation) achieved 85.33% and 95.38%. Characterization revealed significantly enhanced surface area and active sites. (2) Dynamic experiments demonstrated that SDS/FA columns increased MB and MV removal by 34.41% and 37.92% compared to high-silica fly ash, with stable effluent pH over time. The static adsorption of MSAM supports its application in dye wastewater treatment, and the structure–property relationship provides a new pathway for the high-value use of fly ash.