Preparation, optimization, and application of sustainable ceramsite substrate from coal fly ash/waterworks sludge/oyster shell for phosphorus immobilization in constructed wetlands

This study has prepared a sustainable and efficient ceramsite substrate for phosphorus immobilization in constructed wetlands by adopting coal fly ash (CFA) and waterworks sludge (WS) as the main materials, and oyster shell (OS) as the additive. The optimal parameters for CFA/WS/OS ceramsite preparation were determined as follows: mass ratio of CFA:WS:OS = 6:4:0.8, preheating temperature = 600 °C, preheating time = 30 min, calcining temperature = 1050 °C, and calcining time = 8 min. The CFA/WS/OS-op ceramsite, obtained under the optimal condition, had a microporous structure with crystal strength components and active calcium components. Static adsorption experiments indicated neutral-alkaline conditions and/or higher temperature were conducive to improve the phosphorus removal by CFA/WS/OS-op; the adsorption kinetics and isotherm could be well described by the second-order model and Langmuir model, respectively. The theoretical maximum phosphorus adsorption capacity of CFA/WS/OS-op at 35 °C reached up to 4.51 mg/g. Speciation analysis on the adsorbed phosphorus revealed chemisorption by calcium in CFA/WS/OS-op was the dominant adsorption mechanism. In dynamic column experiments, as much as 90% of the phosphorus in raw water collected from the secondary effluent of a sewage treatment plant was removed by CFA/WS/OS-op ceramsite with a hydraulic retention time of 12 h.