Physi-chemical mechanism and control effect of CaO2 inhibiting phosphorus release from sediments under different dosing modes

CaO2 is known as an outstanding restoration agent to control phosphorus (P) release from sediments, and its mechanism is believed to depend on chemical passivation. However, we found that the physical actions might also be involved in inhibiting endogenous P release induced by CaO2. To further explore the mechanism of CaO2 controlling P release and optimize the dosing method, a 94-day incubation experiment was conducted under different CaO2 dosing modes. The results showed that CaO2 could form a dense passivation layer near its dosing position by reducing the median diameter of sediments, thereby inhibiting P release through physical obstruction. At the same time, the increase in the specific surface area and Ca content of sediments induced by CaO2 could synchronously enhance the physical and chemical adsorption properties of sediments to P. In addition, CaO2 could significantly reduce the P concentration in sediment interstitial water and the mobile-P and BAP contents in sediments through chemical oxidation and chemical precipitation. Under the combined actions of physical obstruction, physi-chemical adsorption, chemical oxidation, and chemical precipitation, CaO2 effectively inhibited endogenous P release. Finally, the P release flux in each reactor showed that multiple coverage and shallow injection had the optimal effect on inhibiting P release, and the former is recommended for the water systems with shallow sediments, and the latter is suitable for the water systems with deep sediments. In general, this experiment proposed the physi-chemical mechanism of P immobilization mediated by CaO2, studied the formation characteristics of the passivation layer, and optimized the dosing mode, which can provide valuable reference for the research and application of CaO2 controlling P release.