A team from Nottingham Trent University has developed a new way to combat the globally difficult problems of eutrophication, hypoxia and dead zones which lead to the growth of harmful algal blooms (HABs) using oxygen nanobubble-modified clay.

The team, led by Dr. Gang Pan at the Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences and Centre of Integrated Water-Energy-Food Studies (iWEF), at the university, have used an Oxygen Nanobubble technique to mitigate the states of hypoxia and anoxia which can lead to the growth of harmful algal blooms (HABs) in a wide range of water bodies.

Eutrophication is the main cause of HABs which deplete dissolved oxygen (DO) in the water, leading to the death of fish and other major aquatic creatures worldwide. HABs – also known as phytoplankton - are caused by excessive nutrients that come from terrestrial runoff due to increasing human activities and polluted sediment.

Hypoxia occurs at one to 30 percent saturation of dissolved oxygen in a body of water, while anoxia or dead zones occurs at zero percent saturation of dissolved oxygen.

In the competition for light for photosynthesis by natural organisms, phytoplankton takes priority and can grow very rapidly to spread and cover entire water ponds or very large area of lakes and oceans, causing DO.

As the phytoplankton dies, it will go to the bottom of the waters where the bacteria will decompose it, which further depletes the dissolved oxygen.

If the dissolved oxygen depletes, hypoxia, fish kills and invertebrates like worms and clams will also die - fish cannot live below the 30 percent saturation of dissolved oxygen.

Dead zone or anoxia is often responsible for eutrophication as it creates the conditions for the release of excess nutrients, greenhouse gases and other toxic substances, such as hydrogen sulphide, from the sediment.

To date there has been no satisfactory way of control HABs because there is a lacking of safe and cost-effective technology to tackle the globally difficult problems of eutrophication, hypoxia and dead zone.

Oxygen Nanobubble technology provides potentially promising principle to tackle 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 – the 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 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 team 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. The 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.

Technique could help significantly reduce greenhouse gas emissions such as methane from eutrophic waters

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 also significantly reduce greenhouse gas emission such as methane from eutrophic waters

Dr. Pan has already reported his work on nanobubbles at two major international conferences in 2018 - Nanobubbles 2018 in Suzhou, China and at the American Chemical Society National Meeting in Boston, USA in August as an invited keynote speaker.

A pioneer in developing cost-effective and safe technologies for toxic harmful algal blooms and water-pollution controls, Dr Pan is vice president of the Water Environment Committee, Chinese Society of Environment and chair of the Global Phosphorus Recycling Initiative.

He leads the Centre of Integrated Water, Energy, and Food Studies (iWEF) at Nottingham Trent University, where wastewater and waste industrial gas can be purified and turned into biofuel, electricity and microsphere algae biochar (anti-toxin for agriculture safety) through innovative technologies.