Feasibility Study of TiO2-Coal Fly Ash Photocatalysts for Simultaneous Organic Pollutant Removal and Hydrogen Evolution from Greywater

Abstrak

Indonesia's rapid industrialization has led to a significant increase in coal fly ash generation from coal-fired power plants, while simultaneously, domestic greywater from laundry activities remains largely discharged without adequate treatment. Both challenges present substantial environmental burdens that demand integrated and resource-efficient solutions. This study proposes the valorization of coal fly ash as a low-cost support material for TiO2-based photocatalysts synthesized via a sol–gel method, evaluated for simultaneous photocatalytic organic pollutant degradation and hydrogen evolution using real laundry greywater as the reaction medium. Coal fly ash was characterized prior to use through Loss on Ignition (LOI), X-ray fluorescence (XRF), and slurry pH measurements to establish its physicochemical suitability as a catalyst support. A pretreatment protocol comprising sequential washing, mild acid neutralization, drying, and calcination was applied to stabilize surface chemical conditions before sol–gel synthesis. The synthesized TiO2–coal fly ash composite photocatalyst was immobilized on stainless steel substrates via brush coating and calcined at 400°C, then evaluated in a closed batch reactor under UV irradiation using real laundry greywater conditioned to a pH range of 6.0–7.5 through the addition of 0.1 N NaOH or 0.1 N HCl, followed by coarse filtration to remove large suspended solids prior to treatment. Characterization results confirmed the suitability of the coal fly ash as a support material, with a low LOI value of 1.66%, a silica–alumina-dominated composition (SiO2: 47.64%, Al2O3: 18.17%), and a slurry pH reduced from 11.25 to approximately 8.0 following pretreatment. Photocatalytic treatment over six hours reduced the organic content of greywater from 97.9 mg/L to approximately 45.1 mg/L, corresponding to a removal efficiency of approximately 54%. Qualitative evidence of reducing gas formation, consistent with photocatalytic hydrogen evolution, was detected using a semiconductor-based MQ-2 gas sensor under nitrogen-purged anaerobic conditions, suggesting the concurrent occurrence of oxidative and reductive pathways within the photocatalytic system. These findings demonstrate the technical feasibility of valorizing coal fly ash as a functional TiO2 photocatalyst support for integrated greywater treatment and photocatalytic hydrogen evolution, providing baseline performance data for the development of a low-cost, dual-function photocatalytic system applicable at household or communal scales in Indonesia. Quantitative hydrogen production analysis and advanced catalyst characterization are identified as priorities for subsequent research stages.