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Breaking it Down: What are the Chemical Byproducts of the Water Treatment Process?

Synthetic chemicals are found everywhere in our modern everyday life. They are in the clothing we wear, as well as in cosmetics, personal care products and medications that we use everyday. But we tend to give little thought to what happens to these chemicals when we flush them away, assuming they are harmless once they disappear down the sink.

The reality is that most wastewater treatment facilities do not have the capacity to remove synthetic organic chemicals such as those used in personal care products, pharmaceuticals and opioids. So, what happens to them?

Because wastewater treatment plants are not capable of removing these chemicals, trace amounts remain in the effluent that wastewater treatment facilities discharge into streams, rivers and lakes. Although these concentrations are extremely minute, mere nanograms or micrograms, very little is known about how the risk they pose to the environment or to human health.

Credit: Sarah Bird/Michigan Tech Credit: Sarah Bird/Michigan Tech

What is more worrying, is that even less is known about the environmental and human health risk posed by chemical byproducts formed during the water treatment process, where thousands of potentially harmful byproducts can be formed in just a few minutes.

A new study, which was recently published in the American Chemical Society's journal Environmental Science and Technology, has sought to shed more light on the mechanisms that enable the formation of chemical byproducts during the wastewater treatment process looking at acetone as a case study to determine the chemical byproducts that are created as acetone breaks down during advanced oxidation wastewater treatment process.

Chemically speaking, acetone has rather a simple structure, which makes it the ideal candidate for modeling chemical reaction pathways — the various ways a chemical can break down into free-radicals and chemical byproducts — in order to predict what byproducts and free-radicals can form.

"When we do water treatment using advanced chemical oxidation, those oxidants destroy target organic compounds but create byproducts," explains Daisuke Minakata, assistant professor of civil and environmental engineering at Michigan Technological University and lead author of the study. "Some byproducts may be more toxic than their parent compound. We need to understand the fundamental mechanisms of how the byproducts are produced and then we can predict what to be produced from many other chemicals. We found more than 200 reactions involved in acetone degradations based on computational work."

The researchers then compared the results predicted by their model to ten byproducts observed in an earlier experimental study, and found that the modeled results were similar to those observed in the experimental study.

Advanced oxidation is an important water treatment method that is effective at removing contaminants. However, many communities, particularly those living in arid regions, are facing water scarcity and are forced to recycle treated wastewater for reuse. Should synthetic organic chemicals together with the byproducts that form during the oxidation process remain in the water, animals and people who consume that water will also consume the chemicals present in the water.

In other areas, wastewater from communities living upstream is discharged into rivers and lakes. Communities living further downstream may depend on that water as a source of drinking water. As conventional water treatment processes are incapable of effectively removing all the organic chemicals, these consumers are exposed to the chemicals that remain in the water.

According to the authors: "Advanced oxidation can effectively target specific organic chemicals to remove them from water. Modeling reaction pathways is critical to help water treatment managers understand how best to wield the knife, as it were."

For the study, the team calculated the chemical reaction pathways using Michigan Tech's Superior supercomputer, however the model is limited to organic contaminants that have a simple structure like acetone. Organic chemicals tend to have much more complex structures, making their reaction pathways nigh impossible for even a supercomputer like Superior to compute.

According to the authors: "Understanding the mechanisms of chemical byproduct formation isn't just important for water treatment; it's also advancing what we know about chemical reactions in the atmosphere and inside our bodies."

Journal Reference:

Divya Kamath, Stephen P. Mezyk, Daisuke Minakata. Elucidating the Elementary Reaction Pathways and Kinetics of Hydroxyl Radical-Induced Acetone Degradation in Aqueous Phase Advanced Oxidation Processes. Environmental Science & Technology, 2018; DOI: 10.1021/acs.est.8b00582

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