Efficiency and mechanism of a novel La-based hydrogel designed for controlling lake eutrophication: Insight from phosphorus release characteristics of sediment, sulfur-driven autotrophic denitrification and cyanobacterial bloom response.

Reported nutrient passivators often target single-nutrient control and require complex, energy-intensive processes. In this study, we developed a mesoporous network-structured spherical La-based hydrogel for dual nitrogen and phosphorus control. The hydrogel framework, cross-linked by sodium alginate and lanthanum, encapsulates free La³⁺. The preparation process avoids high-energy techniques and produces no waste, with recyclable reagents. Through adsorption tests, cryo-electron microscopy, and surface area and pore size analysis, we found that the mesoporous structure shields internal free La³⁺, preventing rapid release and minimizing waste of La³⁺. The cross-linked La³⁺ in the spherical framework is released by microbial degradation of the polysaccharide skeleton, inducing anaerobic conditions in the sediment and passivating soluble reactive phosphorus (SRP) simultaneously. The G-units in sodium alginate act as a carbon source, enhancing sulfur-nitrogen cycle coupling, denitrification and reducing NO₃⁻ release. Three-month laboratory simulations and nine-month field in-situ experiments demonstrated significant nutrient control. In the field trials, one month after application, the SRP concentration in the water column decreased to 0.006 mg/L (a 75 % reduction), and the NO₃⁻ concentration decreased to 0.15 mg/L (an 87.5 % reduction), with greatly improved water transparency and no cyanobacterial blooms. This study provides a novel design concept for a nutrient passivator that involves clean production, balanced input-output, and full mobilization of microbial metabolism. The properties, mechanisms, and effects of the material were comprehensively explored and verified from various aspects. This material can provide a new option for controlling endogenous pollution in eutrophic lakes.