Experimental application of micro/nano bubbles to control P release and separate P particles from benthic lake sediment

Abstract

Under persistent hypoxic conditions in the bottom layer of a lake, phosphorus (P) can be released from benthic sediments, significantly impacting the lake ecosystem. Hypoxia reduces iron ions, leading to P release through biogeochemical reactions associated with organic matter degradation, which in turn depletes dissolved oxygen (DO) in the sediment. The application of bubble technology is beneficial, as microbubbles (MBs) effectively remove P and enhance DO levels in both the overlying water and sediment surface. To identify the optimal bubble size for application, we conducted a series of flotation experiments and P fractionation studies using MBs and nanobubbles (NBs). Results indicate that NBs achieved a flotation efficiency of 12 %, MBs reached 44 %, and the combined use of NBs and MBs attained the highest flotation efficiency at 62 %, confirming that a combination of bubble sizes can enhance flotation efficiency. Furthermore, the introduction of air bubbles to mitigate hypoxia in the overlying water increased DO concentrations from 1 to 5 mg/L, followed by a gradual decline. P release in the bubble-injected groups was up to three times lower than in the control group. Water quality profiling of P distribution in sediments revealed a significant increase of approximately 10 % in iron-bound P under aerobic conditions, highlighting its critical role in regulating P release. Our findings suggest that utilizing a combination of bubble sizes, including NBs, enhances P removal efficiency and improves the anaerobic conditions of the lake's bottom layer, thereby further reducing P release from sediments.