Jianquan Luo and colleagues at the Chinese Academy of Sciences combine glucose oxidase and iron oxide nanoparticles to create a system that catalyzes Fenton-based degradation of contaminants, creating an efficient and sensitive purification system for membrane filters.

Removing fouling layers from polyamide nanofiltration membranes

Nanofiltration with polymer membranes may be a suitable method for treating surface water, ground water or pre-cleaned wastewater for producing high-quality process water or even drinking water (the latter requires an additional disinfection step). Due to their typical membrane cut-off, NF membranes are able to simultaneously retain dissolved organic substances and bivalent salts. However, membrane fouling caused by the organic contaminants present in the raw water significantly reduces the stability and increases the cost of nanofiltration processes. Cleaning agents like acids, bases or oxidants must be applied to remove fouling layers from the membrane surface. Chlorine-containing oxidants such as NaClO (bleaching lye) can break down the most stubborn organic debris, but they unfortunately also damage polyamide membranes, which are in most commercial nanofiltration systems, and they produce toxic byproducts. A milder alternative to bleach is hydrogen peroxide, but it decomposes contaminants slowly.

Fenton reaction with enzymatic production of hydrogen peroxide

If hydrogen peroxide is combined with iron oxide, the Fenton oxidation is a strong tool to oxidize organic pollutants. In a classic reaction scheme, the iron oxide catalyzes the building of hydroxyl radicals that act as stronger oxidant. Yet in order for the Fenton reaction to clean filters, extra hydrogen peroxide and acid are needed, increasing financial and environmental costs.

Using the enzyme glucose oxidase

One way to avoid these additional chemicals is to use the enzyme glucose oxidase, which simultaneously forms hydrogen peroxide and gluconic acid from glucose and oxygen. So, the researchers from the Chinese Academy of Sciences wanted to combine glucose oxidase and iron oxide nanoparticles into a system that catalyzes Fenton-based degradation of impurities, creating an efficient and sensitive cleaning system for membrane filters.

A “greener” approach for cleaning nanofiltration membranes

First, the researchers compared the removal of organic contaminants from polyamide filters by the glucose oxidase enzyme and iron oxide nanoparticles to other cleaning methods, including the traditional Fenton reaction. They found this approach was superior at breaking down the common contaminants bisphenol A and methylene blue, while also preserving more of the membrane structure. Encouraged by their initial results, the team combined glucose oxidase and iron oxide into a single nanoparticle, connecting them with an amino bridge. Finally, they tested the new nanoparticle’s ability to clean methylene blue-soaked nanofiltration membranes, which they fouled and cleaned for three cycles. After each cleaning cycle, the nanoparticles were retrieved with a magnet and reused with fresh glucose to activate the catalyst. The nanoparticles were highly effective at cleaning the membranes, returning them to 94% of their initial water filtration capacity. Because the nanoparticles don’t require strong chemicals and are easily recoverable, the researchers say their new system is a “greener” and more cost-effective approach for cleaning nanofiltration membranes.