Synthetic Dyes a Threat to the Environment
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| Prof. Wong Po-keung obtained his B.Sc. and M.Phil. from The Chinese University in 1977 and 1979 respectively, and his Ph.D. from the University of California in 1983. He joined CUHK as lecturer in biology in 1986 and was promoted to senior lecturer rank in 1994. Prof. Wong's research interest is in environmental biotechnology and environmental toxicology. He is concurrently associate director of the University's Centre for Environmental Studies. |
Synthetic dyes are indispensable to the textile and dyeing industries. Fashion would not have so much colour and arouse so much interest were it not for the effects of such dyes. Among all synthetic dyes, azo dyes are the most common, being used up to 90 per cent of the time, because they are versatile and easy to synthesize. Yet many azo dyes are toxic and may cause genetic mutations. And because they are synthetic, the natural environment cannot recognize them or degrade their toxicity. Though man has invented sewage treatment plants to deal with different kinds of man-made sewage, technology as it is now is unable to degrade the toxic components of azo dyes. Even a very low concentration of these dyes in industrial effluents is enough to do great damage to the environment.
More worrying is the fact that current legislation only governs the amount of biochemical oxides in, and the level of alkalinity and acidity of industrial effluents, but not the dye concentration. If polluted fluids are discharged directly into the aquatic environment, their toxicity will be absorbed by aquatic creatures and will eventually find its way through the food chain into human beings. The crucial thing right now is to find a way to degrade azo dyes and this is precisely what Prof. Wong Po-keung of the Department of Biology has been working at since 1989.
Current Methods of Degradation Less
Than Ideal
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| Drawing dye-contaminated water samples from a dyeing factory for research |
Prof. Wong said methods of treating azo dyes fall into three broad categories: physical, chemical, and biological.
Physical methods include flocculation, membrane filtration, electrolysis, and electroflotation.
Chemical methods involve adding chemicals into industrial effluents to break up azo dyes into tiny spheres that float or sink, or using ion exchange to enable the dyes to stick to resin. Irradiation and oxidation are also used.
Yet none of these methods is ideal. Physical methods require high energy input, while the effectiveness of chemical methods and whether these processes generate further wastes are still questions that remain unanswered.
Biological methods involve the use of microorganisms such as bacteria to turn pollutants into non-toxic, harmless substances. Prof. Wong pointed out that biodegradation is the most environment friendly as it does not require large amounts of energy and does not generate toxic substances. He has been using biosorption to remove heavy metals from contaminated waste water and biodegradation to treat such metals successfully since the late 1980s. Can the same methods be used for azo dyes?
'The drawback of using biosorption to remove synthetic dyes from industrial effluents is that there is no satisfactory treatment for the microorganisms that have absorbed the dye. It's not practical for treating large volumes of contaminated water,' Prof. Wong said.
He further pointed out that researchers had tried to degrade azo dyes anaerobically using microorganisms. However it only turned the water colourless, and failed to degrade the toxic and mutagenic aromatic amines therein. Some other researchers also discovered that azo dyes could be degraded aerobically using bacteria and fungi, but the method has yet to be improved.
In Search of an Ideal Method
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| Figure 1: Structure of (a) methyl red (MR) and (b) procion red Mx-5B |
Prof. Wong was determined to overcome these obstacles to find the perfect treatment method. His research project was allocated a total of HK$1.29 million by the Research Grants Council in 1991 and 1999.
First he contacted some dyeing factories in Hong Kong to request them to supply dye-contaminated water samples for the project. However, as most of them had moved north, he could only lay his hands on a limited number of samples and that, in turn, narrowed down the scope of his research. Among the limited samples, Prof. Wong picked methyl red (MR, see Figure 1a) and procion red MX-5B (PR, see Figure 1b) for his study because their structures are the simplest.
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| Figure 2: Colonies of Pseudomonas sp. K-1 stained by procion red MX-5B |
Prof. Wong isolated a bacterium called Pseudomonas sp. K-1 (Figure 2) from dye-contaminated sludge and cultured large amounts of it. Pseudomonas sp. K-1 could absorb large quantities of MR and PR very quickly, yet it could not degrade the toxic substances in these dyes, not even given time.
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| Figure 3: Methyl red decoloured (degraded) by Klebsiella pneumonia RS-13 |
Then he isolated another bacterium, Klebsiella pneumonia RS-13 (Figure 3), from the sludge samples, and discovered that this bacterium could decolour MR aerobically through reductive cleavage of the dye into aminobenzoic acid and N,N'-dimethyl-p-phenylene diamine (DMPD, Figure 4), a toxic aromatic amine. Moreover, the bacterium could metabolize DMPD into non-toxic compounds. Prof. Wong's experiments showed also that this bacterium could degrade other azo dyes such as PR, procion red H-E3B and procion yellow H-E4R.
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| Figure 4: Degradation of methyl red by Klebsiella pneumonia RS-13 |
Besides bacteria, Prof. Wong isolated a fungus called Geotrichum candidum CU-1 (Figure 5) whose extracellular oxidative enzymes could degrade PR and MR in only 12 to 24 hours --- much more quickly than Klebsiella pneumonia RS-13.
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| Complete medium | Procion red MX-5B |
Incubation time: 6 days |
| Figure 5: Decolourization (degradation) of procion red MX-5B by Geotrichum candidum CU-1 | ||