The Bacteria Legion in a Battle against Water Pollution

The Problem of Water Pollution Affects All

Water pollution is a global problem, and is particularly serious in crowded cities such as Hong Kong. The sorry state of the Shing Mun River near the University, for example, is known to many. In general, water quality is threatened by the disposal of untreated human and animal waste into rivers and seas; where there is rapid industrialization, industrial wastes also pose serious problems. Effluents from the dyeing industry carry residual dyes that are not easily degradable; effluents from the electroplating industry carry large quantities of toxic heavy metals.

Now a research project in the Department of Biology offers the prospect of helping to reduce water pollution. The project on the removal and recovery of nickel ions in effluent by bacterial cells immobilized on magnetites is carried out by Dr. P.K. Wong of the Department of Biology. It follows an earlier investigation on metal removal funded by an internal direct grant in 1987, and won competitive funding ($540,000) from the Research Grants Council in 1992.

In electroplating, metals are first dissolved in acid, and plated onto the host object by the passage of an electric current. In most electroplating processes, a layer of nickel is first deposited as a base on which the ultimate surface layer (which may be copper, silver or gold) is then deposited. As a result, nickel is one of the most commonly occurring metals in the electroplating process, and also in the discharged effluent. It is estimated that 800kg of nickel sulphate is discharged every day by the electroplating industry in Hong Kong. Nickel is discharged as ions in the effluent liquid, an ion being a nickel atom which has lost two electrons, and hence carries two units of positive electric charge. Nickel ion, in contrast to the metal itself, is easily absorbed if ingested and is toxic in two significant ways: in posing hazards to the neural system (neuro-toxic) and also in possibly causing gene mutations (mutagenic).

So the removal of nickel from electroplating effluents is a matter of top priority. Incidentally, nickel is also expensive, so its recovery should pay, in part or all, for the cost of the abatement technology.

The conventional technology for nickel recovery is chemical. Caustic soda is added to the effluent which turns the nickel ions into nickel hydroxide, which is then precipitated as a slurry sludge. The sludge is removed, and is often buried in landfills; but it may seep from the landfill, creating further problems. However, such methods also tend to be expensive, and reaction conditions have to be controlled rather precisely. Alternatives have to be explored, and a clever biological scheme that Dr. Wong has developed appears to offer very good promise.

The idea is a three-step process. In short, one tries to let the nickel attach itself to bacteria, let the bacteria attach themselves to magnetite, and finally let the magnetite attach itself to a strong magnet.

A prototype bioreactor to remove heavy metals from polluted water

Nickel ions carry positive charges. It turns out that certain types of bacteria carry negative charges on their cell walls, so very naturally the nickel will adhere to the bacteria. But the bacteria, which are less than 10^_9 of a cm³ in size, float about in the waste liquid and are difficult to collect. So the second step is to let these bacteria attach themselves to magnetite. Magnetite is a magnetic species of iron oxide. Under certain acidity conditions, magnetite exhibits a slight positive charge, so naturally the bacteria, which are still negatively charged, will adhere to the magnetite. In fact, it is speculated that the magnetite may have a highly porous structure, and once the bacteria burrow their way into these pores, they may become trapped and are unable to come out, quite apart from the electrical attraction. The bacteria are said to be immobilized. Finally, the magnetite, because of its magnetic property, can be separated from the effluent using a strong magnet — in much the same way as a toy magnet picks up metal paper clips.

To recover the nickel that has been attached, one need only apply an acidic solution onto the magnetite. The nickel will be separated and washed away with the solution. The remaining bacteria that have been immobilized by the magnetite can be regenerated by applying a neutral buffer.

This separation scheme was proposed in a primitive form some eight years ago. Dr. Wong has investigated the basic science, and made several very significant improvements. A class of bacteria has been identified that has a special affinity for nickel. The research team has also established the best conditions for the removal and recovery processes: temperature, acidity, dissolved oxygen concentration etc. It has also been found that five to ten minutes under moderate stirring is sufficient to remove most of the nickel — this is very fortunate, and is likely to lead to a very efficient removal process.

Once the basic science has been established, the next step is to make a bioreactor. A prototype laboratory scale reactor with five litre capacity has been built and tested. Design studies are in hand for scaling up to 1,000 litres, which will be large enough to be tested in a realistic small factory situation.