Science / Microbe of the Month: Germ of an idea to stop industry creating a stink: Bernard Dixon reports on a fresh solution to the smelly problem of 'rotten-egg' gas

Now that the safety conscious have deleted everything but the most insipid materials from chemistry sets and party gimmicks, we rarely encounter the stink bombs of yesteryear with their stench of 'rotten-egg' gas (hydrogen sulphide).

Nevertheless, hydrogen sulphide remains with us. Indeed, it is a nuisance in many parts of industry. Generated by manufacturing processes, this foul-smelling substance has to be removed from waste gases and liquid effluents in which it is dissolved, before they can be released into the environment. And, because hydrogen sulphide causes quite a pong even when it escapes in tiny concentrations, the task of removing it is both technically demanding and costly.

Now a research team at the Russian Academy of Sciences' Institute of Biochemistry and Physiology of Micro-oganisms, based at Pushchino, has announced a solution to the problem that is not only significantly cheaper than existing methods but has economic benefits as well.

At the heart of the process is a member of a group of microbes known as purple sulphur bacteria, which release sulphur from sulphur-containing substances. The bacteria contain a purple form of chlorophyll, related to the green chlorophyll in plants, which uses light to convert carbon dioxide from the air into carbohydrates.

The bacterial centrepiece of the Russian work is especially versatile. Rejoicing in the name of Ectothiorhodospira shaposhnikovii, it turns hydrogen sulphide into sulphur, a valuable commodity. Its cells also carry large quantities of animal nutrients and, in certain circumstances, other substances with potential applications in industry.

If the results of pilot plant tests conducted recently at Delitzsch, near Leipzig in Germany, are sustained on a larger scale, the Russian researchers could have discovered a near-perfect biotechnological process. Small wonder that they are so proud of E shaposhnikovii, which can transform stinking effluent into marketable products, using little more than sunlight, water and carbon dioxide.

Each of the present industrial methods of extracting hydrogen sulphide has drawbacks, and all are expensive. One technique is to wash the gas out with a solvent, methanol, but this has to be done under very high pressure. Another is to pass the effluent over iron filings. The gas and iron combine, though relatively slowly, to produce iron sulphide.

Several years ago a major advance came with the use of bacteria called thiobacilli, which convert the gas into sulphuric acid. While this process has many applications throughout industry, it is also highly corrosive - and poses problems in designing and operating the equipment.

The first steps in harnessing photosynthetic bacteria for the removal of hydrogen sulphide were taken in the early Eighties using green bacteria, which photosynthesise using chlorophyll similar to that found in plants.

Like E shaposhnikovii, these microbes also yield elemental sulphur, but they have at least one serious disadvantage in that they do not function when oxygen is present.

Unlike plants, neither green nor purple bacteria produce oxygen when they photosynthesise. Indeed, the textbooks describe both as anaerobes (organisms that grow in the absence of oxygen).

However, the green variety is much more fastidious in this respect and does not perform well in the presence of air, which is difficult to exclude from the type of equipment used.

Writing in the current issue of the World Journal of Microbiology and Biotechnology, M B Vainshtein and colleagues in Pushchino highlight another benefit of the purple bacterium. It contains large quantities of the precursor of vitamin A, together with several other nutrients, making it an excellent ingredient for animal fodder.

Above all, as demonstrated in both laboratory experiments and large scale work at the Delitzsch pilot plant, E shaposhnikovii is highly industrious in obliterating the stench of 'rotten-egg' gas.

The hydrogen sulphide is simply bubbled through a 'bioreactor' containing the bacterium suspended in water with various dissolved salts.

The reactor is lit by sunlight during the day and artificially overnight and when the weather is cloudy. E shaposhnikovii thrives under these conditions, generating sulphur (but no corrosive sulphuric acid) - plus more of its own cells.

It was a Russian bacteriologist, Sergei Winogradsky, who established a tradition of applied microbiology in his country by pioneering research into the sulphur bacteria a century ago. More recently, scientists there have been facing severe financial problems following the disintegration of the Soviet Union. But here is evidence that Russian microbiologists are once again harnessing the biotechnological skills of microbes to solve a major industrial problem.

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