Layered double hydroxides vs. metal-organic frameworks: Which enables more sustainable phosphate fertilizer production from sludge?

Removal of phosphorus (P) via adsorption from wastewater primarily relies on selective surface interactions, such as electrostatic forces and complexation. The study explores the potential of adsorbents synthesized from drinking water treatment sludge (DWTS), with a focus on octahedral layered double hydroxides (LDH) and La-Al-Fumarate (Fum) metal-organic frameworks (MOFs), evaluating their properties and P adsorption performance. X-ray photoelectron spectroscopy (XPS) revealed that terminal hydroxyl groups on the adsorbent surface acted as critical active sites, facilitating the formation of La/Al-O-P coordination complexes. Compared to LDH-based composites, La-Al-Fum MOFs exhibited a 71.6 % increase in phosphate adsorption capacity. This enhancement was attributed to optimized surface coordination environments, mediated by multiple mechanisms: predominantly ligand exchange, supplemented by electrostatic attraction, ion exchange, van der Waals interactions, and hydrogen bonding. In contrast, LDHs initially relied primarily on ion exchange, while inner-sphere complexation became active after regeneration. Furthermore, the superior physiochemical properties of La-Al-Fum MOFs, stemming from strong carboxylate-metal coordination and enhanced structural stability compared to LDH-based composites, also contributed to their higher P adsorption capacity. The spent La-Al-Fum MOF adsorbent demonstrated superior loading capacity as a type of sustained P slow-release fertilizer, showing minimal risk of metal leaching effects and avoiding other negative environmental impacts (such as acidification, eutrophication, and global warming). Sludge-derived P fertilizers (specifically, the spent La-Al-Fum MOF) reduced environmental impacts better than traditional mineral P fertilizers in most impact categories. These findings offer practical alternatives for recycling DWTS into value-added MOF adsorbents/fertilizers for P recovery, advancing global food security and environmental sustainability.