First, the bleaching clays must exhibit a certain cation exchange capacity. The other important parameters for natural bleaching clays are a high absorption capacity, particle size distribution, porosity, and large surface area. Natural bleaching clays can be slightly acidic to slightly basic.
Effective adsorption requires a large surface; for practical reasons, the high specific surface (m2/g) of a very porous solid is used. The channels by which molecules reach this surface must be negotiable by the molecules concerned. The nature of the surface must allow acceptably firm bonds, chemical or physical, between it and the adsorbate. In the case of bleaching or purification by adsorption, pigments or other components are selectively retained on the pore surface, and the triglyceride escapes. Gradually, the concentration of pigment on the available surface of the adsorbent and the concentration remaining in the oil come into balance, so further exchange is negligible. The best temperature for the oil/clay contact must be chosen, as must the duration of the contact; an excess of either will encourage undesirable side effects.
A helpful action is to remove from the oil any material like gum or soap which will compete for room on the clay surface. This leaves the surface much freer to work on remaining traces of gum or soaps as well as on an adsorbing pigment.
Bentonite minerals have limited sorptive properties in the natural state and require chemical treatment by acids to create the surface area and porosity needed for bleaching vegetable oils. Bleaching clays of this nature are commonly referred to as “acid” or “acid-activated” clays.