Oxidative damage of RO membranes
One of the particular challenges with thin-film RO and NF membranes is that they are susceptible to oxidative damage. These membranes are made by casting very thin films of polyamide polymers onto a range of supporting substrates. The thickness of this film is key to both minimising the pressure, and maximizing separate dissolved salts from the water. This thin polyamide layer is susceptible to oxidation by a number of pathways but the most common include:
- Hypochlorite oxidation: Hypochlorite (bleach) solutions are commonly used to clean ultra-filtration (UF) equipment and other systems, and errors in these cleaning procedures can lead to membrane damage. Hypochlorite is also commonly used as a biocide in water treatment and if residual levels are too high, can damage membranes
- Peroxide damage: Hydrogen peroxide is a much weaker oxidant than hypochlorite and some manufacturers do allow the use of 0.1 wt.% hydrogen peroxide and peracetic acid as sterilizing agents. However, peroxide will oxidize iron and will then precipitate from the water and can catalyse oxidative damage
- Oxygen damage: Membranes are susceptible to damage by oxygen if they are left outside operation for long periods. Generally, if a membrane plant is going to be down for over a week or two then the system should be flushed with permeate and then preserved with sodium metabisulphate to scavenge any atmospheric oxygen.
There are a number of mechanisms for oxidative damage but they generally follow a set pathway; Firstly, the amide group is oxidized and halogenated. There is then a rearrangement that moves the chlorine to the benzene group. Further oxidation can result in break-down and scission of the polymer chains, and loss of mechanical integrity.
What are the impacts of oxidative damage?
Oxidative damage usually manifests in a loss of salt rejection as the membrane starts to break down. You will start to see an increase in permeate conductivity and transmission of different salts, especially monovalent salts like chloride. If the damage is very bad then you may start to see increases in flux, as more water is passed through the membrane.
The unfortunate impact of oxidative damage is that it cannot be reversed. If the degraded performance level of the membranes is unacceptable then they must be replaced. As such, an oxidative event can be an expensive incident.
Preventing oxidative damage
There are a number of ways to prevent oxidative damage and we recommend that as many as possible be implemented to create layers of defence.
Administrative Measures: The cheapest to implement are administrative measures. If possible, avoid the use of oxidizing agents in the plant and make sure that operations teams are aware of the risks. A complete review of operational procedures should include checking steps to avoid incorrectly exposing membranes to oxidizing agents.
Engineering Controls: A number of engineering controls exist to help prevent oxidative damage. If clean-in-place procedures for UF membranes can be automated, then pump interlocks and piping systems can eliminate the risk of exposure. ORP and free-chlorine tests are available in hand-held instruments, paper test kits and also online measurements. These can be used to lock-out a system if oxidizing agents are present.
Chemical dosing: One of the more common methods of controlling oxidizing agents is to dose a reducing agent like sodium metabisulphate into the water to neutralize any residual hypochlorite prior to the RO system.
With these controls in place, most plants successfully avoid oxidative damage.
The Fujiwara tests
The Fujiwara test is one of the first ways we look for oxidative damage. It works by detecting the organic halides that form after oxidation by chlorinated or brominated oxidizing agents. This test involves exposing a section of the membrane to a mix of pyridine and sodium hydroxide. The organic halide reacts with the pyridine to produce a bright pink colour when the highly conjugated ring-openign reaction occurs.
While there are a few interferences that can occur but generally a red or pink colour indicates that oxidative damage has occurred.
Other options to look for oxidative damage
If we get a positive result in the Fujiwara test, then we will often use other techniques to delve further. The two main methods are:
FTIR – Using FTIR we can look at the decline in the key amide I and Amide II peaks that are indicative of oxidative damage. As we are looking at the polymer here, it can give an indication of non-halogen-based oxidation also. Unfortunately, if there is a lot of contamination of the membrane then this approach can become harder.
XPS – XPS is a very accurate method for looking at the existence and type of halides present on the membrane surface. It is also possible to look at loss of the nitrogen peaks that would show break-down of the polyamide.