Oxygen nanobubbles used to prevent algae problems
A research team of Chinese and British academics have come up with a new technique for combating eutrophication and ‘dead zones' created by algal growth in nutrient-rich water.
The team led by Dr Gang Pan of Nottingham Trent University brought together researchers from the university’s Centre of Integrated Water-Energy-Food Studies (iWEF) with those from the Research Center for Eco-Environmental Sciences at the Chinese Academy of Sciences.
Eutrophication is the main cause of harmful algal blooms (HABs) which lead to the death of fish and other major aquatic living beings worldwide. HABs are caused by excessive nutrients that come from terrestrial runoff due to increasing human activities and polluted sediment. If levels of dissolved oxygen in water fall below 30 per cent, then fish cannot survive, and dead zones – or anoxia – can then develop where dissolved oxygen falls to zero.
So far, there is no satisfactory method to control HABs because there is a lack of safe and cost-effective technology to tackle the globally difficult problems of eutrophication, hypoxia and dead zones.
In order to save the aquatic ecosystem, Dr Gang Pan and his team have used an Oxygen Nanobubble technique to mitigate hypoxia and anoxia. This technology provides a potentially promising principle to tackle the dead zones. When the oxygen nanobubble-modified zeolite is used and sinking by gravity onto the anoxic sediment, it can effectively deliver oxygen to the sediment surface and to the entire water column. The materials are cheap and natural, which can be delivered to deep waters using natural hydraulic forces such as river current flows and gravity to avoid the huge energy costs of mixing that has challenged existing methods. The percentage of dissolved oxygen will be critical for the sustenance of aquatic living beings.
The researchers conducted a column experiment, in which they created an oxygen-locking surface sediment layer on the sediment which prevents the further depletion of dissolved oxygen in the water. This oxygen-locking surface sediment layer was formed after capping with oxygen nanobubble-modified zeolites (ONMZ) and local soils (ONMS) particles. The synergy of diffusion of oxygen nanobubbles and retention of oxygen in this layer leads to both the increase of dissolved oxygen and reversal of hypoxic conditions. Finally, the water in the treatment system had significantly higher dissolved oxygen (DO) values (4-7.5 mg/L) over the experimental period of 127 days compared with the control systems (around 1 mg/L).
The oxygen-locking capping layer was very important in preventing oxygen consumption caused by the reduced substances coming from the anoxic sediment. Dr Gang Pan’s experiment has helped in mitigating sediment anoxia and controlling nutrient release from sediments, which could help in solving the eutrophication problem and restoring the aquatic ecosystem. The finding has already triggered new studies related to hypoxia and eutrophication. For instance, Dr Gang Pan and his team found that a thin capping layer of oxygen nanobubble-modified zeolite to the anoxic sediment can significantly reduce greenhouse gas emission such as methane from eutrophic waters.
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