ELIMINATING THE SALT

Te Awamutu Courier, Volume 52, Issue 3750, 1 May 1936, Page 3

ELIMINATING THE SALT

PURIFICATION OF SEAWATER. The presence of salt in sea water renders the huge oceans of the world quite useless for domestic and agricultural purposes. To have such large quantities at our very doors and to be unable to utilise them for ordinary purposes is, to say the least, exasperating. Innumerable attempts have been made throughout the ages to develop a cheap and efficient method of converting salt water into fresh water. All that is required is the removal of the dissolved salt. It sounds simple enough, but even with our modern knowledge the problem is as far from solution as ever. One recent development, however, has prompted the writing of this article. As everyone knows, the usual method of purifying sea water is by distillation. The water is boiled, and the steam is condensed and collected as fresh water, the salt being left behind. The method is extremely efficient, but it is not cheap, and on occasion it is far from convenient.

Now, it has occurred to people times without number that it should not be such an extraordinarily difficult matter to devise some means of removing the salt, either by precipitation or by filtration. Do we not in ordinary chemical routine continually precipitate undesirable substances from solution, or remove them by filtration through charcoal or zeolites, or even through sand ? Why should it not be possible to adapt such methods to the case in hand ? ADDITION OF FOREIGN SUBSTANCES. It is true that precipitation methods are extremely common, indeed, both in the laboratory and in industry. But the method depends upon the addition of some foreign substance whose business it is to throw the dissolved salt out of solution. In all but a few exceptional cases the added substance in effect takes the place of that which is thrown out, so that anything in the nature of a complete purification is not realised at all. It is obviously useless for our purpose to replace the salt of sea water by an equal amount of some other substance, particularly when that substance, as events show, is equally harmful. Ordinary filtration methods are even less hopeful. Attention has often been called to the fresh-water wells which are to be found on beaches throughout Australia just above hightide mark, and it has been suggested that filtration through large amounts of sand has changed the sea water into fresh. This, of course, is an entirely erroneous idea. No amount of sand filtration will freshen sea water, for the simple reason that the salt molecules in solution are about the same size as the water molecules . themselves, and wherever the water molecules can penetrate—between the grains of sand, for example there also will the salt molecules manage to find their way. Only very large molecules, or clusters of molecules, can be filtered by porous bodies. AN EXCEPTION. There is, however, one exception to this rule. It is known that certain

surfaces have a particular attraction for special salts, or rather for the ions of which those salts are composed. These ions can be removed from solution when they are made to pass over the given surface. If a filtration material be found with a particular attraction for the ions of sodium chloride one could imagine it being used for the purification of sea water. There is more than a hint that this has actually been achieved, for a scientific publication just to hand states, all too briefly, that “ by filtering sea water through a specially prepared series of acid and basic synthetic resin, most of the salt content of the sea water can be tomoved.”

This statement is sufficiently important to deserve analysis. During the lapt year or so chemical research in England has revealed that some of the synthetic resins their general characteristics have already been reviewed in these columns possess base-exchange properties comparable with those of the best zeolites—that is to say, they can be used as filtration material for the filtering of hard water, when they will remove the calcium and magnesium ions from the hard water and replace them by an equivalent amount of sodium ions. Whenever this is done the water is always “softened.”

The utilisation of synthetic resins as water softeners is in itself an extremely important development, but as the net result is a replacement, and hot a purification, we seem no nearer the solution of our problem. What we really require is the complete removal of the troublesome salt, with nothing put in its place. ( (USE OF SYNTHETIC RESINS. The claim is now made that this can be brought about by the alternate use of two different kinds of synthetic resins. When certain phenols arid tannins are used in the production of one kind of resin, the product resembles a zeolite; not only has it base-exchange properties, but in the pure state it can selectively remove basic radicles, such as sodium ions, from solution.

Ori the other hand, if resins are prepared containing aromatic bases, small. The same applies to the removal of chlorine ions by “basic ” resin. These small operations, alternately superimposed on one another, would entail innumerable filtrations before anything like a measurable purification could be achieved.

It would appear, therefore, that the process in its present form is more of academic interest than utility. Taking it by and large, we are as far from the economic Solution of the problem as ever. resins have undoubtedly been prepared, and their structures indicate that they will act as recorded, but one m|ay be allowed to question the efficiency of the operation and the rate at which it works.

From electrostatic considerations the removal of sodium ions by “ acid ” resins Can proceed only to a very slight extent; chemically, it involves the enforced hydrolysis of the salt,

durable circumstances, is invariably such as aniline, they are found to possess acid-exchange properties, and in the pure state can remove acid radicles, such as chlorine ions, from solution. Accordingly, if sea water is passed through these resins alternately, the sodium ion is removed by one, and the Chlorine ion by the other, with the result that the salt as a whole is completely removed from the water.

Although there is no reason to doubt thtf truth of this developriient,