Treating waste with waste: New insight into phosphorus removal by graphitic carbon nitride modified water treatment sludge-based ceramsite

This study developed a novel ceramsite adsorbent, combining water treatment sludge (WTS) and graphite carbon nitride, which aimed at removing phosphate from wastewater to realize the sustainable development strategy of ‘treating waste with waste’. The optimal conditions for the preparation of N-modified WTS (NWTS)-based ceramsite were determined as follows: NWTS: bentonite: CaCO 3 ​= ​1:0.4:0.5, preheating at 450 ​°C for 15 ​min, and calcinating at 1100 ​°C for 15 ​min. The study examined the effects of pH, ceramsite dosage, initial phosphorus concentration, and temperature on phosphate removal, and it assessed the potential risk of heavy metal release from the ceramsite during application. The result indicated that phosphorus adsorption of the ceramsite reached 0.5 ​mg/g at pH ​= ​6, T ​= ​303 ​K, P initial ​= ​1 ​mg/L and dosage of 2 ​g/L, achieving 100 ​% removal efficiency. The adsorption process of phosphate by ceramsite followed pseudo-second order kinetic and the Sips model, suggesting that the adsorption mechanism involved multilayer chemisorption. Ligand exchange reaction was the primary driver of adsorption, with hydrogen bonding further augmenting the adsorption efficiency. Additionally, the heavy metal content released by the ceramsite was significantly below the relevant standard limits, confirming its excellent feasibility and safety. These characteristics suggested that the ceramsite adsorbent had broad application potential in actual wastewater treatment process. • Porous ceramsite prepared from carbon nitride and water treatment sludge. • Phosphorus adsorption of ceramsite reached 0.5 ​mg/g, realizing 100 ​% removal efficiency. • Adsorption mechanism of N-modified water treatment sludge-based ceramsite. • Phosphorus removal by ceramsite had excellent safety and feasibility.