Stereoselective degradation of chiral drugs during wastewater treatment

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Funder: Engineering and Physical Research Council

Project No: EP/I038608/1 (project details)

Summary: Pharmacologically active compounds (PACs) constitute a vast and diverse group of chemicals. Despite low ppt concentrations of these compounds in environmental matrices, PACs pose considerable environmental concern as many of them are active at very low concentrations. Additionally, PACs are present in the environment as multi-residue mixtures, and therefore synergistic effects of PACs should also be considered. Due to the non-volatile nature of the majority of PACs, and their continuous introduction into the environment, their environmental impact cannot be underestimated. PACs have also been detected in drinking water, which poses a direct risk to humans and raises the issue of contaminated water sources and especially water reuse.

Water is a limited resource in an expanding global economy and population. In the future its accessibility will be significantly impacted by changing climate. Reclaiming water for non-potable (irrigation, urban, industrial) or potable purposes is therefore considered to be an important element of sustainable water resource management. There are, however, certain risks associated with water reuse including microbiological and chemical risks (e.g. the presence of PACs in reclaimed water). They have to be verified before water reuse is to be implemented on a wider scale. Several groups of PACs such as beta-blockers, antibiotics or analgesics have been studied before in the environment but surprisingly their chiral character, despite its great importance in the pharmaceutical industry, has been overlooked by environmental researchers.

More than half of the drugs currently in use are chiral compounds and many of those are distributed as racemates consisting of an equimolar mixture of two enantiomers. Enantiomers of the same drug have the same chemical formula and physicochemical properties but they differ in interactions with chiral environments such as enzymes. Therefore in biological systems they have to be recognised as two different substances that elicit different responses: one enantiomer of the same drug may produce the desired therapeutic activity, while the other may be inactive or even toxic. The ratio of active/inactive enantiomer of the chiral drug can change significantly after its administration, metabolism in and excretion from the body. It can be subsequently altered during wastewater treatment and when the drug is already present in the environment. This is because degradation of enantiomers can be stereo-specific and can in some cases lead to an increase in the drug’s toxicity. As a result chiral PACs might reveal different environmental persistence, fate and toxicity. Currently, the environmental fate and toxicity of chiral drugs are assessed without taking into consideration their enantiomeric form. This might lead to a significant under or overestimation of toxicity of chiral drugs and to inaccurate environmental risk assessment.

As a consequence there is an urgent need to identify chiral drugs, which are resistant to currently used wastewater treatment technologies, are characterised by stereoselective degradation during wastewater treatment and can as a result be released into the environment or remain in reclaimed water in the form of non-racemic mixtures. This project will tackle this issue as it aims to test the hypothesis that degradation of chiral drugs during wastewater treatment and in receiving waters is stereoselective and will be undertaken at the laboratory scale with the usage of microcosm protocol and in a full scale wastewater treatment plant.

Project Objectives: 

Objective 1: To investigate the hypothesis that stereoselective biodegradation of chiral drugs during biological sewage treatment takes place.

Objective 2: To investigate the hypothesis that stereoselective biodegradation of chiral drugs can also take place in receiving waters.

Objective 3: To verify stereoselectivity of degradation of chiral drugs in a full scale WWTP utilising activated sludge treatment.

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