Sustainable restoration of anoxic freshwater using environmentally-compatible oxygen-carrying biochar: Performance and mechanisms

The long-term decline in dissolved oxygen (DO) levels in freshwater systems including rivers and lakes has become a worldwide concern, which can threaten biodiversity, nutrient biogeochemistry, water quality and ultimately human health. Herein, we report a sustainable restoration strategy for anoxic freshwater using local sediment-based biochar as novel oxygen nanobubble carriers. Column incubation experiments were conducted with water and sediment samples from an urban tributary of the Yangtze River. The oxygen-carrying sediment-based biochar (O-SBC) showed long-lasting re-oxygenation performance for anoxic river waters during 28-day period, in which DO was rapidly elevated from ∼0.14 to ∼7.87 mg/L and gradually maintained at ∼4.78 mg/L until the end. O-SBC with multiple functions switched the sediments from a source to a sink of nutrients, and its release of oxygen nanobubbles contributed further decrements of 66.3% NH4+-N and 142.9% PO43--P except for physical blocking and physicochemical adsorption. Notably, a comprehensive focus on restoration mechanism was explored in view of microbial community response. The re-oxygenation was followed by a ∼5.05% increase of bacterial diversity (Shannon index) in water, but a ∼2.40% decrease in sediments. A proliferation of some specific aerobic populations was observed, of which the nitrifying Nitrospira abundances were ∼10-fold higher in the water from O-SBC than the control. Additionally, functional genes involved in nitrous oxide reduction, polyphosphate synthesis/degradation, and thiosulfate oxidation were also enriched. Taken together, our findings can not only expand the promising candidates for oxygen nanobubble carriers based on sediment recycling, but also highlight the microbial molecular mechanisms for anoxic freshwater restoration based on nutrient cycle regulation.