Synchronous algal removal and phosphorus immobilization by attapulgite-based Ca-Al-LDH incorporated with nano-CaO₂ for cyanobacterial bloom control

Relocating cyanobacteria from surface waters to sediments using modified clay is an established emergency measure for controlling cyanobacterial bloom in eutrophic systems. However, the accumulation and subsequent decay of settled algal biomass promote sedimentary phosphorus release, compromising the long-term efficacy of this interventions measure against eutrophication and bloom recurrence. This study synthesized attapulgite-based calcium-aluminum layered double hydroxide incorporated with nano-calcium peroxide (nCaO₂/Ca-Al-LDH) via in-situ loading. The simultaneous cyanobacterial removal and inhibition of sediment phosphorus release by nCaO₂/Ca-Al-LDH were investigated through simulated experiments. In controlled sediment-water simulation systems, this material showed dual functionally, simultaneously removing cyanobacteria and suppressing sediment phosphorus release. Notably, the system with nCaO₂/Ca-Al-LDH (1.0 g/L) showed 90.48% chlorophyll-a (Chl-a) removal within 1 day, with the Chl-a concentration remaining consistently low (0.02–0.03 mg/L) throughout the cultivation period. Further, compared with the control system, the nCaO₂/Ca-Al-LDH system exhibited a 42.75% increase in sediment total phosphorus content accompanied by 45.79% and 39.59% decreases in pore water orthophosphate and total phosphorus concentrations, respectively. This composite material also decreased sediment labile phosphorus (NH₄Cl-P) content by 16.91%, while increasing the contents of refractory fractions, Al-bound P and Ca-bound P, by 2.72% and 74.64%, respectively, and collectively, these changes suppressed sediment-water interface (SWI) phosphorus flux by over 50.0%. nCaO₂/Ca-Al-LDH also increased the dissolved oxygen content of overlying water to 4.3–15.2 mg/L within 7 days, preventing algal decay-induced hypoxia and maintaining oxic conditions at the SWI. The observed reduction in the relative abundance of the genus Cyanobacteria in sediments further demonstrates the long-term efficacy of the material in suppressing cyanobacterial growth. Overall, the fabricated composite has great potential for application in preventing cyanobacterial blooms.