Response of sediment with Ca/Al composites capping to cyanobacterial bloom decline: Blocking the formation and the release of sediment iron-bound phosphorus (Fe-P)

The immobilization of phosphorus (P) in sediments plays a pivotal role in managing lake eutrophication over the long term. Therefore, key factors that may cause uncertainties in P fixation are of increasing interest to researchers. Calcium‑aluminum composites (CA) can passivate sediment P well; however, the effect of cyanobacterial bloom decline on their sediment P remediation remains unclear. In this study, CA addition significantly reduced P equilibrium concentration as well as augmented P adsorption capacity of sediment characterized as cyanobacterial dominance zone (CDZ). The results of the simulated experiments on cyanobacterial bloom decline indicated that the algae decomposition led to a rapid decrease in dissolved oxygen (DO) level, and to release amounts of P, thus increasing the P concentration in the overlying water. The released algal P into the sediment primarily encouraged the formation of iron-bound phosphorus (Fe-P), followed by calcium-bound phosphorus (Ca-P). The subsequent anaerobic incubation led to a notable release of the newly formed Fe-P, strengthening the anaerobic P release from sediments. Conversely, CA-capping accelerated the adsorption of algal P by sediments, and promoted the formation of Ca-P in sediment from cyanobacterial P, hindering the generation of reactive Fe-P. Moreover, during subsequent anaerobic incubation, the P forms in sediments capped with CA remained stable, showing no obvious P release. These findings suggested that CA capping induced the formation of stable P from algal P and disrupted the positive feedback effect between P contamination in sediments and cyanobacterial blooms, which would provide valuable insights for the remediation of sediments in CDZ.