Chlorine: friend and foe

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  • Published: Oct 19, 2009
  • Author: Jon Evans
  • Channels: Detectors
thumbnail image: Chlorine: friend and foe

Adding chlorine to drinking water may be a very effective way to reduce the spread of disease, but it also has its drawbacks. For the water chlorination process can generate a range of toxic by-products, which most countries require to be present in drinking water at minimal concentrations.

Unfortunately, detecting these by-products and determining their concentrations has always been quite tricky. But now US chemists have come up with a simple detection technique, which, for the first time, offers the possibility of detecting these by-products in near real time.

In most Western countries, chlorine is added to drinking water to control a wide range of disease-causing pathogens, including amoeba, bacteria and viruses. It does this by reacting with the water to form hypochlorous acid and hydrochloric acid, both of which are effective at killing microbial pathogens. More recently, some water companies have replaced chlorine with chloramine, which works in a similar way but persists in the water for a longer period of time.

The problem is that both chlorine and chloramine can react with various naturally-occurring organic compounds in the water to produce toxic halogen-containing compounds, including nine different haloacetic acids (HAAs). In the US, five of the nine HAAs are not allowed to exceed a combined concentration of 0.06mg/L.

The US Environmental Protection Agency (EPA) has developed a number of methods for determining the concentration of these five HAAs in drinking water, all based on gas chromatography (GC). But these methods are all fairly complicated and time-consuming, requiring numerous extraction and chemical modification steps.

Furthermore, the monitoring requirements in the US have recently changed. Previously, water companies were required to show that the average HAA concentration over their entire distribution network didn't exceed the set concentration. Now, they have to show that this concentration isn't exceeded at a number of set locations within the distribution system. This is a much more onerous task and essentially requires the water companies constantly to monitor HAA concentrations in their water supplies.

To help with this effort, a team of chemists led by Gary Emmert at the University of Memphis decided to develop a simpler and quicker method for determining HAA concentrations. This would be based on their finding that HAA reacts with the water-soluble vitamin nicotinamide to form a fluorescent compound that can be picked up by fluorescent-based detection methods.

They came up with a first attempt in 2005, with a capillary membrane sample-flow injection analysis method that could determine total HAA concentrations in water. But now they have combined the nicotinamide-based detection method with anion-exchange chromatography to produce a method for detecting each of the HAA species individually.

They began by testing this novel method on water containing varying concentrations of HAAs. The method was able to detect each of the nine HAAs at concentrations ranging from 0.6µg/L to 10.1µg/L and took around 2 hours to process one sample. This makes the method both fast enough and sensitive enough to conduct continuous, near real time monitoring of water supplies.

Emmert and his team also compared this method with one of the EPA GC methods on two samples of drinking water, one of which had been disinfected with chlorine and one with chloramine. Both methods detected broadly similar concentrations for all the HAAs, although there was more variation between the methods for the chloraminated sample than the chlorinated sample.

The scientists are now exploring the reasons for this difference in variation.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Drinking water

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