The treatment of wastewater and potable water typically uses ultrafiltration (UF) methods, or membrane bioreactors (MBR), where membrane filtration is combined with biological treatment techniques. While these methods can remove suspended solids, microorganisms, and large contaminant molecules, they are ineffective at removing smaller organic molecules, such as pharmaceuticals, pesticides, hormones, surfactants, and plasticisers. Complicating the issue is the biorefractory nature of these pollutants, meaning they are often resistant to biological degradation.

To remove biorefractory contaminants from water, alternative methods are required. One solution is the use of Advanced Oxidation Processes (AOP), which are chemical-based treatment methods that involve the generation of highly reactive hydroxyl or sulfate radicals capable of oxidising and breaking down organic pollutants into less harmful byproducts.

Despite the effectiveness of AOP in degrading small organic pollutants, there are some limitations. For instance, hydroxyl radicals have very short lifespans, restricting both their diffusion range and duration of action. Hydroxyl radicals are also typically generated through the activation of an oxidising agent (e.g. hydrogen peroxide, ozone, ultraviolet radiation) or catalyst (e.g. metal oxides) which adds complexity and cost to the treatment process. The metals used as catalysts may also damage membranes, or leech into the water, becoming pollutants themselves.

To overcome these challenges, Associate Professor Zhang Sui from NUS Chemical and Biomolecular Engineering, turned to an alternative source of free radicals that could be safely combined with membrane technology to essentially combine AOP methodology with traditional filtration. This would result in the simultaneous removal of larger molecules, and the degradation of smaller organic molecules, from wastewater.

As described in their paper, which was published in the Proceedings of the National Academy of Sciences (PNAS) in February, Assoc Prof Zhang and her team investigated the inclusion of radical polymers (RP) in a filtration membrane. Not only did the membrane filter larger molecules, but it provided an alternative source of free radicals for AOP. Importantly, unlike hydroxyl radicals, which are highly reactive chemical species, free radicals originating from radical polymers are persistent and stable.

Mechanistic investigations revealed that peroxyl radicals, which were created from the radical polymer, could directly trigger oxidative degradation of sulfamethoxazole (SMX), one of the most prevalent antimicrobial contaminants detected in groundwater in the United States.

Molecular stimulations verified that the radicals could be regenerated in the presence of oxygen, which is key to the continuation of reactions for degradation of organic molecules. Replacing oxygen by nitrogen drastically decreased degradation efficiency.

Using this information, Assoc Prof Zhang and her team designed a RP-based membrane that could filter and degrade pollutants in the same treatment cycle. To improve the mechanical strength and stability of the membrane, they fabricated an interpenetrating membrane network comprising of polyvinyl alcohol (PVA) and RP.

To further demonstrate the effectiveness of their technology, the team employed a model wastewater solution containing 2000 ppm of humic acid and 20 ppm of SMX. Humic acids are complex molecules that exist naturally in soils, peats, oceans and fresh waters.

The membrane could filter large humic acid molecules and subsequently degrade SMX. With humic acid rejection rate exceeding 98.5%, their membrane is comparable to an ultrafiltration membrane. Furthermore, the SMX degradation rate remained high. In addition, the degradation efficiency remained constant throughout the 120-min tests, proving the stability of the membrane.

The findings from this work highlights the potential for novel innovative membranes that function not only in filtration, but can drive other processes like radical polymer-based AOP. Such an approach offers an energy-efficient solution for the degradation of organic compounds and water filtration in wastewater treatment.