Adsorption by Charcoal

The porous and cellular character which amorphous charcoal frequently assumes, when produced from organic structures of a corresponding form, develops a property which is possessed, indeed, by all substances, but which, in this case, appears with especial distinctness. This is the power possessed by porous charcoal of adsorbing dissolved and gaseous substances from mixtures, and so freeing these gas mixtures or solutions from certain components.

If, for example, wine, litmus solution, or similar coloured solutions are shaken with finely porous charcoal (the most suitable being bone charcoal, obtained by carbonising bones), and then filtered, the liquid passes through the filter either quite colourless or, at least, considerably lighter in colour. Likewise, from turbid, impure, or evil-smelling water there is obtained, by filtration through charcoal, clear water which has lost its smell entirely or to a large extent. For such purposes of purification, charcoal is largely used both in the arts and in the laboratory.

The processes with which we are here dealing are called adsorption, and depend on the fact that at the surface of contact between a solid body and a solution, a different concentration of the dissolved substance is produced from that in the interior of the solution. In many cases, the concentration of the dissolved substance at such bounding surfaces is greater than in the rest of the solution, but the opposite can also occur.

The cause which produces this action is of the same kind as that which effects wetting. The bounding surfaces between different bodies are, generally, the seat of a peculiar kind of energy which is called surface energy. The phenomena of surface tension or the phenomena of capillarity represent only a small portion of the actions of surface energy; indeed, this comes into operation in all cases where different bodies come together, or where surfaces of separation are present.

If, now, certain substances have the property of becoming specially concentrated at a bounding surface, they will be removed from a solution in which they are present when such bounding surfaces are formed in the solution. This is the case with charcoal and the above-mentioned colouring matters. A definite equilibrium is established between the portion in the solution and that adsorbed on the charcoal, the greater part going to the charcoal.

This action depends, in the first place, on the nature of the dissolved substance, but to some extent also on the nature of the solid body. Substances of complex composition generally possess, to a comparatively much greater extent, the property of becoming concentrated at the bounding surfaces, whereas more simple substances remain chiefly in the solution. Since, now, most of the colouring matters which appear as unwelcome by-products in the preparation of organic substances have a very complex nature, they can frequently be removed from the solution by this means. The method is employed with very good results, for example, in the sugar refineries, in order so far to decolorise the dark brown beet juice that white sugar can be obtained from it.

The same holds also for the malodorous products of decomposition of organic bodies, animal excremental matter, etc., which, on account of their complex nature, are also, as a rule, abundantly adsorbed by charcoal.

Finally, what has just been said holds also for gas mixtures. Gases also condense to a more or less considerable extent on the surfaces of solid bodies, and again, the more complex and denser gases do so generally much more than the simple and light ones. The former can, therefore, also be removed more or less completely from mixtures with the other gases.

Since the action takes place at the bounding surface between the solid body and the liquid or the gas, it is proportional to the surface. The amount which 1 sq. cm. of surface can retain in this way is very small; in one special case (that of ammonia on glass) it has been found equal to 1/25000000th gm. Per sq. cm. Even if in the case of other substances the number can become ten or a hundred times as great, still the amounts with which we are here dealing are always exceedingly small. To obtain measurable amounts, therefore, very large surfaces must be employed; for the adsorption of one gram of ammonia a square surface of 50 metre side is necessary. Such large surfaces are found only in the case of very fine powders, or of very finely cellular structures.

This quality is possessed by bone charcoal, because bones contain, brides the organic matter of a gluey nature, large amounts of calcium phosphate. On carbonisation, the cellular structure is very completely preserved by means of this embedded matter, and if the calcium phosphate is removed by solution in hydrochloric acid, a fairly pure charcoal is obtained which for a given amount of substance possesses an exceedingly large surface, and therefore exhibits the phenomena of adsorption with especial distinctness.

If organic substances, e.g. sugar, which do not themselves yield on carbonisation a charcoal with largely developed surface, be mixed Avith calcium phosphate or similar infusible and readily removable salts, a strongly adsorbing charcoal is obtained by the carbonisation of such mixtures, after removal of the admixed substance. In this case the large development of surface has been artificially caused, and with it, also, the corresponding action obtained.

Another action which is connected with the one just described is the catalytic acceleration, especially of gas reactions, which is exerted by substances with largely developed surface. Thus, the oxidation of many substances by free oxygen is greatly accelerated when charcoal is present. Likewise, gases which under given conditions act only slowly on one another, can be made to act more quickly with the help of charcoal. In these cases, however, the actions of charcoal are greatly surpassed by the analogous actions of spongy platinum.