The adsorption of substances by Fuller's earth

The utility of fuller’s earth as a decolourising agent in the industrial prepara­tion of oils has long been recognised. More recently also fuller’s earth, in common with other powerful adsorbents has proved of the greatest use in the isolation of rare substances in the laboratory. Despite, however, the impor­tance of fuller’s earth both from the industrial and from the theoretical point of view, the ideas expressed in chemical literature as to its origin and nature, and the mechanism by which it acts as an adsorbent are diverse in the extreme. It was the object of the investigations here described to throw some light on the mechanism by which fuller’s earth adsorbs organic acids and bases from aqueous solutions. In particular the influence of hydrogen-ion concentration upon the adsorption of these bodies was studied. The results indicate that two types of adsorption take place; that of unionised molecules and that of cations which displace calcium ions from the surface. Geologists seem to be agreed that fuller’s earth consists essentially of alu­minium silicate associated with a little free silica and smaller quantities of iron, calcium and alkalis. It differs from ordinary clays in having a higher percentage of combined water. The exact nature of the silicates actually present in fuller’s earth is necessarily uncertain as a rational analysis of so complex a mixture is obviously extremely difficult to obtain.