The Mechanism of Phosphate Fixation by Montmorillonitic and Kaolinitic Clays

RUSSELL CoLEMAN TTHAT the clay fraction of the soil is responsible for phosphate fixation has been definitely established by numerous investigations, but the exact mechanism by which clay is able to fix phosphate is not known. Considerable controversy has arisen over themanner in which phosphate is fixed, and many investigations have been conducted in an effort to determine how phosphate is fixed by the clay. A brief review of the literature will show some of the conflicting opinions oh phosphate fixation. For a more comprehensive review, the reader is-referred to Midgley (5). Investigators who have studied the phosphate fixation problem disagree as to whether phosphate is fixed by chemical precipitation, adsorption by simple addition, or by anion .exchange. It is commonly accepted that in neutral or alkaline soils with an excess of divalent cations, phosphate may be fixed by being precipitated as calcium and magnesium phosphate, but there is no agreement as to how phosphate-is fixed in acid soils. Earlier workers believed that phosphate . was precipitated in acid soils by soluble iron and aluminum, but since only small amounts of iron and aluminum are soluble within the reaction range of most soils, it was impossible to account for large amounts of phosphate fixation in this manner. Heck (2) found that it was not necessary for iron and aluminum to be in the soil solution in order for them to fix phosphate. He believed that the reaction between soluble PO4 and iron and aluminum was one of simple addition, in which the PC>4 was attracted and held by the insoluble iron and aluminum on the clay. Practically all earlier workers agreed that phosphate was fixed in some manner by the iron and aluminum oxides in' the clay. Later, investigators found that certain clay minerals fix phosphate. Scarseth (7) believed that the alumino-silicate minerals were able to hold phosphate on their colloidal surfaces. Murphy (6) found that finely ground kaolinite was able to fix large amounts of phosphate, and Stout (8) concluded that phosphate fixation by kaolinite is an anion exchange reaction, the PC>4 replacing the OH ions on the surface of the kaolinitic crystal lattice. Although some have contended that the clay minerals are largely responsible for phosphate fixation in acid soils, others have doubted that the clay minerals which occur in soils are able to fix phosphate in a manner similar to fixation by finely ground pure clay minerals. Midgley (5) has found that kaolinite fixes only small amounts of phosphate unless it'is ground finer than it ordinarily exists in soils. The writer (i) found that kaolinitic and montmorillonitic clay minerals fix only small amounts of phosphate after their free iron and aluminum oxides are removed. Metzger (4) concluded that for acid soils, chemical precipitation may largely account for phosphorus fixation, and that adsorption by clay minerals is probably of small practical importance in soils. Recently, Kelly and Midgley (3) found that the hydrated iron oxide which coats the clay mineral fixes phosphate by exchanging its OH ions for PO4 ions. A review of the literature shows that progress has been made in studying PO4 fixation by clay, but that there is still confusion as to the exact mechanism by which phosphate is fixed. The purpose of the present investigation is to study some of the factors influencing phosphate fixation by montmorillonitic and kaolinitic clays, and to throw more light on how phosphate is fixed by clay.