Dual-alkali Modified Fly Ash for High-Performance Phosphate Adsorption: Synergistic Effects of Structural Activation and Calcium-Mediated Immobilization

Excessive phosphate discharge into aquatic environments causes severe eutrophication, posing significant threats to ecological balance and human health. Adsorption is a promising technology for phosphate removal, and fly ash (FA), an industrial solid waste, has attracted attention as a low-cost adsorbent. However, raw FA suffers from low adsorption capacity and pH-dependent performance, limiting its practical application. In this study, a dual-alkali (NaOH and Ca(OH) 2 ) modified FA adsorbent (FA-NC) was developed to address these limitations. The synthesis parameters, including the alkali composition, total alkali dosage, and calcination temperature, were systematically optimized. The optimal FA-NC was prepared by mixing 5.0 g of fly ash with 7.0 g of NaOH and Ca(OH) 2 at a mass ratio of 1:1, followed by calcination at 600 °C for 2 h. An adsorbent dosage of 1.0 g L −1 was selected for the subsequent phosphate adsorption experiments. The physicochemical properties of FA-NC were characterized by XRD, SEM, EDS, XPS, and nitrogen adsorption–desorption techniques, and its phosphate adsorption performance was evaluated through batch and dynamic experiments. The results showed that under the standard batch condition (1.0 g L −1 FA-NC, 20.0 mg P L⁻ 1 , 240 min), phosphate removal exceeded 97% over an initial pH range of 2–12, with a maximum adsorption capacity of 33.0 mg·g −1 . The adsorption process followed the pseudo-second-order kinetic model and the Langmuir isotherm model, indicating chemisorption and monolayer adsorption. Thermodynamic analysis confirmed that the adsorption was spontaneous and endothermic. FA-NC also exhibited excellent selectivity against coexisting anions, good regenerability (removal efficiency > 82% after four cycles), and effective performance in dynamic adsorption, meeting the Class II water quality standard in China. Mechanistic studies revealed that phosphate removal by FA-NC was achieved through the synergistic effects of electrostatic attraction, surface precipitation, and anion exchange, with calcium sites playing a central role. These results demonstrate that the principal novelty of FA-NC lies in the integration of structural activation and calcium-mediated phosphate immobilization within a simple, rare-earth-free modification route, providing a potentially cost-competitive approach for fly ash valorization and phosphate removal.