Mechanochemical synthesis of a multi-waste-based CaAl layered double hydroxide/biochar composite for phosphate sequestration and recovery

The excessive discharge of phosphorus poses a serious threat to aquatic ecosystems, and developing efficient, low-cost, and green adsorbents is crucial. This study innovatively utilized oyster shells (OS), drinking water treatment residues (WT), and coffee grounds (CG) as raw materials to synthesize a series of multicomponent carbon-supported Ca Al layered double hydroxide composites (WT/OS-LDH@CGB) via a facile mechanochemical method, realizing the concept of using waste to treat waste”. Characterization techniques (XRD, SEM, XPS, BET) confirmed the successful formation of composites containing LDH, biochar, and siliceous particles. The adsorption performance, mechanisms, and potential for phosphorus recovery were systematically investigated. Kinetics adsorption experiments followed the pseudo-second-order model and isotherms best described by the Generalized-Langmuir model, indicating monolayer adsorption on a heterogeneous surface. Site energy distribution analysis revealed that phosphate was preferentially adsorbed by high-energy LDH sites. A multi-mechanism involving ion exchange and surface complexation was elucidated. The composite maintained good adsorption across a broad pH spectrum and showed certain selectivity amidst competing anions. Dynamic column adsorption experiments indicated its stable performance under continuous flow conditions. Furthermore, the phosphate-loaded material displayed slow-release properties in different soils and significantly promoted the growth of water spinach in pot experiments, demonstrating its potential functioning as a controlled-release fertilizer. This work provides a sustainable approach to facilitate phosphate removal and recovery, showing great promise for wastewater treatment and agricultural application.