Fabrication of H₂O₂ slow-releasing composites for simultaneous Microcystis mitigation and phosphate immobilization

Hydrogen peroxide (H₂O₂) is a widely accepted algicide in controlling cyanobacterial blooms. However, this method includes two disadvantages: 1) a low H₂O₂ concentration (<5 mg L^-1) is required; 2) H₂O₂-induced cell lysis causes phosphorus (P) contamination. To overcome the drawbacks, a H₂O₂ slow-releasing composite (HSRC) based on calcium peroxide (CaO₂) was fabricated to substitute liquid H₂O₂. According to the results, a higher CaO₂ dose increased H₂O₂ yield and releasing rate. H₂O₂ yield of 160 mg L^-1 CaO₂ in HSRC reached 32.9 mg L^-1 and its releasing rate was 0.407 h^-1. In addition, a higher temperature decreased H₂O₂ yield and increased H₂O₂-releasing rate. Besides, HSRC endowed with a remarkable ability to immobilize P. Higher CaO₂ dose, pH value, and temperature increased the rate of P immobilization. The highest rate was 0.185 h^-1, which occurred with 160 mg L^-1 CaO₂ in HSRC at 25 °C and pH 8.0. Toxicity assays showed that HSRC exerted sustaining oxidative stress on Microcystis aeruginosa. Accumulation of intracellular reactive oxygen species resulted in the disruption of enzymatic systems and inactivation of photosystem. Tracking the variations of cell growth and H₂O₂ concentration during HSRC treatments, it suggested that the lethal effect on Microcystis aeruginosa was achieved with a super-low H₂O₂ concentration (<0.3 mg L^-1). In addition, cell lysis did not cause a sudden rise in P concentration due to the P immobilization by HSRC. Therefore, HSRC successfully offsets the drawbacks of liquid H₂O₂ in mitigating cyanobacterial blooms. It may be a novel and promising algicide that not only kills cyanobacteria but also reduces eutrophication momentarily.