Sustainable Phosphate Removal with Acid-Modified Fly Ash: Kinetic, Isothermal, and Thermodynamic Insights

The removal of pollutants from water bodies has emerged as a pressing global concern. Discharging untreated wastewater into the environment poses a significant threat due to the presence of hazardous substances like nitrate and phosphate, contributing to the widespread issue of eutrophication. This study focused on investigating the adsorption of phosphate from a synthetic solution using fly ash, an industrial by-product. To enhance the efficiency of coal fly ash, acid treatment was employed. Batch experiments were conducted to examine the influence of different factors, including pH, adsorbent dosage, initial phosphate ion concentration, contact time, and temperature. Surface electron microscopy (SEM) explained the morphology of the adsorbent, and Fourier Transform Infrared Spectroscopy (FTIR) analysis was performed to analyze the adsorbent pre and post-adsorption, allowing for the identification of functional groups tangled in the adsorption process. The major functional groups observed were hydroxyl, carboxylic acid, amines, and nitrile groups, all contributing to the adsorption process. Acid-modified fly ash (AMFA) demonstrated favorable results in terms of phosphate removal, particularly at a pH of 5.0 and an initial phosphate concentration of 50 ppm. Equilibrium in adsorption was achieved within 30 min at a temperature of 15°C with constant stirring of 100 rpm, resulting in a high phosphate removal rate of 91%. Freundlich isotherm was found to contribute a better fit for the adsorption data compared to the Langmuir isotherm. Pseudo-second-order kinetic model, with a high R2 value of 0.998, exhibited excellent agreement with the adsorption data for acid-modified fly ash. Thermodynamic study indicated that the adsorption process was heat absorbing (endothermic) and non-spontaneous at low temperatures. Overall, the results of the experimental study highlighted the promising adsorption potential of acid-modified fly ash as an effective adsorbent for phosphate removal in water treatment applications.