Repurposing of municipal solid waste incineration fly ash for phosphate removal in batch and continuous flow column experiments

• Municipal solid waste incineration fly ash (MSWI-FA) served as phosphate adsorbent. • In batch tests, the < 75 µm MSWI-FA, pH 6, and 48 h had the highest removal. • 25 mm thick MSWI-FA in column tests showed a more rapid removal rate. • 15 mL/min flow rate removed phosphate faster, while 10 mL/min gained a higher rate. Municipal solid waste incineration fly ash (MSWI-FA), a by-product of burning wastes at high temperatures for volume and mass reduction, is a promising, cost-effective, and sustainable adsorbent for phosphate (PO 4 3– ) removal due to its significant calcium oxide (CaO), silicon dioxide (SiO 2 ), aluminum oxide (Al 2 O 3 ), and iron oxide (Fe 2 O 3 ) contents, which facilitate phosphate removal via adsorption and chemical precipitation reactions. This study investigates the efficiency of MSWI-FA for phosphate removal from wastewater through batch and continuous flow column experiments, with a particular focus on the influence of particle size, pH, contact time, and hydraulic retention time (HRT). Batch experiments showed that the –75 µm fraction achieved the highest phosphate removal (∼70%) at pH 6, with adsorption behavior controlled by chemisorption. Continuous flow column experiments revealed that a 25 mm bed provided 91.3% removal, while a 15 mL/min flow rate achieved ∼ 99% removal within 480 min. Fourier transform infrared spectroscopy (FTIR) analysis supported a dual removal mechanism of phosphate: (i) surface adsorption through hydroxyl and metal oxide functional groups (e.g., Al–OH, Fe–OH, Si–OH) (IR bands at ∼ 1050, ∼560 cm –1 ; ∼3000–3700 cm –1 ), and (ii) chemical precipitation with calcium ions forming calcium phosphate compounds (IR band at ∼ 1300–1600 cm –1 ). These findings underscore the practical potential of MSWI-FA in wastewater treatment and support its alignment with circular economy goals through waste valorization.