In the course of some studies on the effects of anions on the viscosities of clay suspensions, it was observed that anions such as pyro- and metaphosphate, oxalate and fluoride were much more effective in reducing the viscosity of kaolih suspensions than were chloride and nitrate. There is relatively little information available regarding the specific adsorption of anions by clay minerals; it is known, however, that soils take up considerable quantities of orthophosphate from acid solutions. Whether the active component of the soil, in this respect, is the clay mineral or hydrated iron and aluminium oxides, the adsorbed phosphate may be readily released by shaking the soil with neutral or acid solutions containing fluoride, oxalate, citrate or malonate.
In addition, a soil which is normally capable of taking up appreciable quantities of phosphate will not do so if fluoride is present in the phosphate solution.
It has been recognized that the ability of this group of anions to form soluble complexes with iron and aluminium is responsible for their effectiveness in this respect.