Efficient and Sustainable Removal of Phosphates from Wastewater Using Autoclaved Aerated Concrete and Pumice

Abstract

Phosphates are key pollutants involved in the eutrophication of water bodies, creating the need for efficient and low-cost strategies for their removal in order to meet environmental quality standards. This study presents a comparative thermodynamic evaluation of phosphate ion adsorption from aqueous solutions using two sustainable and readily available materials: autoclaved aerated concrete (AAC) and pumice stone (PS). Batch experiments were conducted under acidic (pH 3) and alkaline (pH 9) conditions to determine equilibrium adsorption capacities, and kinetic experiments were carried out for the best-performing adsorbent. Adsorption data were fitted to the Langmuir and the Freundlich isotherm models, while kinetic data were evaluated using pseudo-first-order and pseudo-second-order models. The Freundlich model showed the best correlation (R2 = 0.90 − 0.97), indicating the heterogeneous nature of the adsorbent surfaces, whereas the Langmuir parameters suggested monolayer adsorption, with maximum capacities of 1006.69 mg/kg for PS and 859.20 mg/kg for AAC at pH 3. Kinetic results confirmed a pseudo-second-order behavior, indicating chemisorption as the main mechanism and the rate-limiting step in the adsorption process. To the best of our knowledge, this is the first study to compare the thermodynamic performance of AAC and PS for phosphate removal under identical experimental conditions. The findings demonstrate the potential of both materials as efficient, low-cost, and thermodynamically favorable adsorbents. Furthermore, the use of AAC, an industrial by-product, and PS, a naturally abundant volcanic material, supports resource recovery and waste valorization, aligning with the principles of the circular economy and sustainable water management.