Phosphorus (P) is an essential nutrient for aquatic ecosystems. However, excessive phosphorus discharge into surface water is one of the primary causes of eutrophication, thus triggering algal blooms, degrading water quality, and threatening aquatic life as well as human health. It is widely recognized that even low concentrations of phosphorus can significantly accelerate eutrophication processes, making efficient phosphorus removal a critical issue in water pollution control. Among the existing treatment technologies, adsorption has attracted increasing attention due to its operational simplicity, high efficiency for low-concentration phosphorus, and limited risk of secondary pollution.

Coal fly ash is one of the most abundant industrial solid wastes that are generated in large quantities in coal-fired power plants. Although its comprehensive utilization rate is increased, a considerable fraction of fly ash is still disposed of by landfilling or stockpiling, posing long-term environmental risks. Fly ash is a promising precursor for the preparation of ceramic materials due to its high contents of SiO₂ and Al₂O₃. Moreover, the presence of Ca, Mg, and other alkaline components endows fly-ash-derived materials with a potential chemical affinity toward phosphate species. Transforming fly ash into functional ceramsite for water treatment therefore represents a typical "waste-to-resource" strategy.

Previous studies explored fly-ash-based ceramsite or related materials for phosphorus removal. However, most of them focused primarily on adsorption performance evaluation, while systematic optimization of preparation parameters and in-depth clarification of phosphorus removal mechanisms remained insufficient. Such limitations hinder the rational design and engineering application of these materials. In this work, fly ash was used as a main raw material, supplemented with municipal sludge, furnace slag, and cement to prepare porous ceramsite for phosphorus removal. The adsorption behavior, comprehensive physicochemical characterization, preparation conditions, and removal mechanism were systematically investigated.

Fly-ash-based ceramsite was prepared by a disc granulation method. Fly ash was mixed with municipal sludge as a pore-forming component, furnace slag as a functional additive, and cement as a binder. After granulation with deionized water, green pellets with a controlled particle size were obtained and subjected to preheating and high-temperature sintering. To optimize the preparation process, a Taguchi L25 (5⁶) orthogonal experimental design was employed, considering six factors, i.e., fly ash-to-sludge ratio, preheating temperature, preheating time, sintering temperature, sintering time, and heating rate. Phosphate removal efficiency was selected as an evaluation index to determine the optimal preparation parameters.

Batch adsorption experiments were conducted using simulated phosphate solutions prepared from potassium dihydrogen phosphate. The effects of dosage, initial pH value, coexisting ions, and humic acid were systematically investigated to evaluate adsorption adaptability under different water chemistry conditions. Adsorption isotherms were analyzed using the Langmuir, the Freundlich, the Sips, and the Dubinin-Radushkevich models, while adsorption kinetics were interpreted using pseudo-first-order, pseudo-second-order models, i.e., Elovich, and intraparticle diffusion models.

The physicochemical properties and adsorption mechanisms of the ceramsite were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). In addition, the leaching risk of heavy metals was also assessed using standard toxicity characteristic leaching procedures to evaluate environmental safety.

The results of orthogonal experimental analysis reveal that sintering temperature is the most dominant factor affecting phosphate removal performance, following by sintering time and heating rate. The excessively high sintering temperature leads to pore collapse and crystallization of stable mineral phases, thereby reducing adsorption capacity. The optimal preparation conditions obtained are a fly ash-to-municipal sludge ratio of 7∶3, preheating at 600 ℃ for 5 min, sintering at 1050 ℃ for 5 min, and a heating rate of 5 ℃/min. Under the optimal conditions, the ceramsite achieves a phosphate removal efficiency of 90.77%, which is significantly higher than that of all orthogonal experimental groups.

The SEM images show that the optimized ceramsite has a rough surface with abundant interconnected pores, originating from the thermal decomposition of organic matter in municipal sludge and gas evolution during high-temperature reactions. The XRD patterns indicate that mullite and anorthite are the dominant crystalline phases, while Ca- and Mg-containing components are retained in reactive forms. The results of batch experiments demonstrate that phosphate removal efficiency increases with increasing dosage but decreases under strong alkaline conditions. The ceramsite maintains effective phosphorus removal in a wide range of pH values, with optimal performance under weakly acidic to neutral conditions.

The coexisting anions exhibit varying degrees of inhibition on phosphate removal, following a decreasing order CO₃^2- >HCO₃^- > SO₄^2- > NO₃^- > Cl^-, whereas common monovalent cations show a negligible influence. In contrast, the presence of Ca²⁺ and Mg²⁺ significantly enhances phosphate removal due to additional precipitation reactions. Humic acid notably suppresses adsorption via competing for active sites and altering phosphate speciation.

Adsorption isotherm analysis shows that the Langmuir and Sips models both fit the experimental data, while the Sips model provides a better physical interpretation, indicating a heterogeneous surface adsorption. The kinetic analysis reveals that the pseudo-first-order model can describe the adsorption process, indicating that surface reactions are dominant the rate-controlling step. The XPS spectra confirm the formation of Ca- and Mg-phosphate species on the ceramsite surface after adsorption, showing that phosphate removal occurs based on a synergistic mechanism involving physical adsorption and chemical precipitation.

The results of leaching tests indicate that the concentrations of heavy metals released from the ceramsite are well below regulatory limits, having its environmental safety for water treatment applications.

A fly-ash-based ceramsite was prepared using municipal sludge and furnace slag as auxiliary components for efficient phosphate removal in water. The ceramsite exhibited a high removal efficiency, a broad pH value adaptability, and a stable performance under complex water chemistry conditions via systematic optimization of preparation parameters and comprehensive adsorption studies. The phosphate removal mechanism was dominated due to the synergistic effect of surface adsorption and Ca/Mg-induced chemical precipitation. Moreover, the ceramsite showed a negligible heavy-metal leaching risk, indicating a good environmental compatibility. This study could provide a feasible approach for the large-scale resource utilization of fly ash and offer a promising adsorbent for phosphorus control in aquatic environments.