GC-HRMS Reveals New Byproducts in Desalination

As populations grow and water becomes scarcer, more places are turning to desalination to produce safe drinking water. However, there is a catch: this process can end up creating higher amounts of certain byproducts containing bromine and iodine that tend to be more toxic than the byproducts found in regular treated water. While desalination does a good job of filtering out most organic matter and the bromine/iodine-containing substances that lead to these byproducts, it does not catch everything. Smaller, uncharged molecules and dissolved salts can still slip through the process.

A research team recently ran a thorough investigation to identify the byproducts created when desalinated water is disinfected. They used gas chromatography-high resolution-mass spectrometry (GC-HRMS) to scan for byproducts across water treated with five different disinfection methods: chlorine, chlorine dioxide, ozone, chloramine, and UV light. Their findings were published in the journal Environment & Health.¹

How does Desalination Work, and What Are the Main Methods Used to Turn Seawater into Drinking Water?

Seawater can be turned into drinking water in a couple of different ways: either by heating it up (distillation) or by pushing it through filters (reverse osmosis, or RO).

There are two common heating-based methods: multistage flash (MSF) and multieffect distillation (MED). MSF works by heating the seawater and then suddenly dropping the pressure, which causes some of the water to instantly flash into steam, like how water can boil almost instantly when pressure drops. This happens in stages, with each stage capturing more fresh water.²

RO works by pushing seawater through special filters that block out germs and most of the salt, leaving cleaner, drinkable water behind.^3-5

In real-world use, each of these desalination methods usually involves adding chemicals early in the process, which helps prevent buildup of algae, bacteria, and other negatives on the equipment (such as the intake pipes or the RO filters). Other chemicals, such as phosphate, lime, and carbonate, are also added along the way. Finally, a small amount of seawater (approximately 0.2%) is mixed back into the finished water to restore minerals and balance the pH, since the desalination process strips those out too.⁶

While MED and MSF have been the go-to approach in the Middle East for decades, reverse osmosis has become the preferred choice for newer desalination plants being built worldwide these days, mainly because it is cheaper to run and uses less energy.⁷

What Byproducts Form When Desalinated Water is Disinfected with Different Methods? How Does the Treated Water Compare in Toxicity with Regular Tap Water?

The researchers studied real desalinated water from three big plants in the United Arab Emirates, testing different disinfection methods side by side in the laboratory. They also looked at how toxic the byproducts of this water treatment are to cells, both individually and as part of the whole treated water. They found 55 different chemical byproducts, many of which had never been seen before in desalinated water, including a brand-new compound and several other chemical groups. Two of these compounds (di- and tribromophenols) turned out to be more harmful to cells than the similar chemicals that are currently regulated by law.¹

How Does the Treated Water Compare in Toxicity with Regular Tap Water?

Water treated with reverse osmosis (a filtering method) was more toxic than water treated by distillation (a heating method). Among the reverse-osmosis samples, the ones disinfected with a combination of ozone and chlorine were the most toxic, followed by UV light, ozone alone, and then chloramine. This is consistent with the fact that the ozone-chlorine combo produced more of certain harmful byproducts.¹

The good news: even though desalination created more of these brominated byproducts, the overall levels (and the actual toxicity) were still lower than what is typically found in regular tap water. Therefore, despite producing more of these chemicals, desalinated water may be safer to drink overall than standard drinking water.¹

While their study raises important questions, the researchers admit that their findings do not completely answer them. They recommend that future research should:

• Measure exact amounts of the byproducts found (this study only identified what is present, not how much).
• Use more sensitive tools to catch iodine-based byproducts that may have been missed.
• Investigate what's driving toxicity in desalinated water more precisely.
• Build a predictive model to estimate toxicity of these new byproducts (and similar undiscovered ones) without testing each individually.
• Search for larger, heavier byproducts using different laboratory methods, since these may be hidden contributors to drinking water toxicity that have not been accounted for.¹

References

1. El-Shorbagy, W.; Cuthbertson, A. A.; Harren, R. et al. Desalination of Seawater: Comprehensive Nontarget Assessment of Disinfection Byproducts. Environ Health (Wash). 2026, 4 (8), 1611-1624. DOI: 10.1021/envhealth.5c00858
2. Semiat, R.; Hasson, D. Water Desalination. Rev. Chem. Eng. 2012, 28, 43-60. DOI. 10.1515/revce-2011-0019
3. Watson, K.; Farre, M. J.; Knight, N. Strategies for the Removal of Halides from Drinking Water Sources, and Their Applicability in Disinfection By-product Minimization: A Critical Review. J. Environ. Manage. 2012, 110, 276-298. DOI: 10.1016/j.jenvman.2012.05.023
4. Duranceau, S. J. Determination of the Total Iodide Content in Desalinated Seawater Permeate.Desalination 2010, 261, 251-254. DOI: 10.1016/j.desal.2010.06.039
5. Bellona, C.; Drewes, J. E.; Xu, P. et al. Factors Affecting the Rejection of Organic Solutes During NF/RO Treatment─A Literature Review. Water Res. 2004, 38, 2795-2809. DOI: 10.1016/j.watres.2004.03.034
6. El Shorbagy, W.; Abdulkarim, M. Chlorination Byproducts in Drinking Water Produced from Thermal Desalination in United Arab Emirates. Environ. Monit. Assess. 2006, 123, 313-331. DOI: 10.1007/s10661-006-9199-4
7. Desalination. European Commission website 2025. https://blue-economy-observatory.ec.europa.eu/eu-blue-economy-sectors/desalination_en#:~:text=In%20terms%20of%20uses%2C%2068%25%20of%20capacity,with%2025%25%20and%20the%20Netherlands%20with%2010%25https://blue-economy-observatory.ec.europa.eu/eu-blue-economy-sectors/desalination_en#:~:text=In%20terms%20of%20uses%2C%2068%25%20of%20capacity,with%2025%25%20and%20the%20Netherlands%20with%2010%25. (accessed 2025-12-22).