The "metamorphosis" of alum sludge for phosphate adsorption: Deciphering the interlayer microenvironment construction and molecular mechanism of intercalated layered double hydroxides.

To address the urgent need for synergy between eutrophication control and solid waste resource recovery, and to overcome the limitations of conventional phosphorus removal materials-such as high cost, low selectivity, and unclear mechanisms, this study innovatively proposes a circular-economy-based "treating waste with waste" framework centered on "aluminum sludge valorization-interlayer physicochemical microenvironment construction-phosphorus removal mechanistic analysis." Using a water treatment plant's aluminum sludge as raw material, effective aluminum was recovered via acid leaching and coupled with Zn2+ and pyromellitic acid (PMA) intercalation to modify hydrotalcite-like compounds, successfully preparing Zn-Al-PMA-LDHs. Results show that PMA intercalation not only physically expands the interlayer spacing but also introduces carboxyl-rich anions, creating a favorable chemical microenvironment. This optimizes phosphate mass transfer, enabling efficient recognition and enhanced immobilization, thereby raising the experimental adsorption capacity to 35.84 mg/g which is 4.7 times that of raw sludge (7.5 mg/g). Integrated multi-scale characterization and molecular simulations, including model construction, molecular dynamics, and adsorption energy calculations, elucidated the microscopic 3D adsorption configuration and bonding mechanisms. The process is synergistically driven by bidentate binuclear inner-sphere complexation, hydrogen bonding, and electrostatic attraction. Dynamic adsorption experiments confirmed the strong engineering potential of both adsorbent and process regarding operational stability and cost-effectiveness, while the material also exhibited excellent anti-interference performance and pH adaptability. Addressing the need for deep synergy between solid waste valorization and advanced phosphorus control, this study establishes an integrated closed-loop system covering "material preparation-performance evaluation-mechanistic analysis-engineering application," offering a green, economical, and scalable new material and solution for high-value utilization of water-treatment-plant sludge and aquatic eutrophication control.