Role of Kaolin in Anion Sorption and Exchange

'-T~»HE process or mechanism by which soluble phos1 phates are "fixed" in the soil and the factors influencing the process have been the subjects of many investigations. It has been rather widely accepted that the components of the clay fraction and the iron and aluminum of the soil solution are chiefly responsible for "fixation" in acid soils. Ravikovitch (13, I4) concluded that phosphate was sorbed as a result of equivalent ionic exchange with other anions of the soil complex and could be released from the sorbed condition by anionic exchange. Scarseth (15) pro-posed that phosphate ions replaced hydroxyl ions from the exposed alumina groups of the aluminasilicate complex of electrodialized bentonite at pH 6.0. Murphy (11) observed that ballmilled kaolin was very effective in sorbing phosphate from the solutions containing H2PO4ions and that sorption increased progressively as the pH of the solution decreased to a maximum at pH 3.0 and at pH's below 3.0 decomposition of the mineral prevented further fixation. By measuring the water formed by the interaction of phosphates with ballmilled kaolin and halloysite and from X-ray data obtained from these phosphated minerals, Stout (17) concluded that "fixation" constituted a simple ionic exchange of phosphate for hydroxyl ions in the crystal lattice of these minerals. He also concluded that the process of exchange was reversible. Subsequent investigations by Black (i) and by Kelly and Midgley (5) largely confirmed the findings of Stout; however, each of these investigators emphasized the importance of free aluminum and iron oxides in the "fixation" process. Coleman (2) attributed the sorption of phosphate by both kaolinitic and montmorillonitic clays to the free hydrous oxides of iron and aluminum, which had been incompletely removed in the isolation of these minerals, by a process in which one OH ion was replaced by one H^PCV ion. That the hydrated iron and aluminum oxides can act effectively in the sorption of phosphate had been established earlier by Lichtenwalner, Flenner, and Gordon (8), by Heck (4), and more recently by Kelley and Midgley (5). If anionic exchange is largely responsible for phosphate fixation and if the mineral kaolin can be considered as an anion exchange complex, then certain fundamental concepts concerning phosphate fixation and exchange in soils might be established by studying the factors influencing this process using kaolin as a representative soil mineral. The advantages are that this mineral may be obtained in a relatively "pure" state and that the procedure is analogous to that which has been used rather widely for establishing facts concerning base exchange reactions with such materials as bentonites, kaolinites, etc. If anionic exchange is a property of kaolin then a lyotropic series for anions, similar to that which has been fairly well established for cations, should exist. Such a lyotropic series would show the relative affinity of the various anions for kaolin and other soil colloids. Kurtz, DeTurk, and Bray (7) reported a vast difference in the replacing abilities of the various anions for sorbed phosphate in Illinois soils. The fluoride ion was the only ion that quantitatively replaced the phosphate. Dean and Rubins (3), in a very recent report, show that citrate and hydroxyl ions removed all of the sorbed phosphate from eight soils having a rather wide range of phosphate sorption capacities; however, fluoride removed the sorbed phosphate from all soils except those having a very high capacity for phosphate sorption. Arsenate and tartrate were less effective than fluoride, and acetate was the least effective of any of the ions tested. These investigators report that there is always more phosphate than arsenate retained by soils as exchangeable anions when comparable methods of saturation are employed. Arsenates and phosphates are so much alike chemically it was thought that they would react in a very similar manner in anionic sorption and exchange. Data concerning the relative reactivities of theseanions in the exchange reaction with soil colloids would throw much light on the agronomic practices to be employed under conditions where large amounts of arsenates are used as insecticides and herbicides. If arsenates are fixed in a manner similar to phosphate and w,ith about the same energy of fixation, then their addition to the soil would result in a liberation of fixed phosphate with subsequent temporary advantage for the growing crop; however, an accumulation of fixed arsenate in the soil as a result of continual dusting and spraying programs, might lead to a very toxic condition whenever soluble phosphates were added as fertilizing materials with the resulting release of the fixed arsenates into the soil solution. The replacing ability of any ion for another in ionic exchange has been pointed out by Mattson (10) as depending upon their relative activities or concentrations and their affinities or energies of combination for the sorbing complex. To establish an ion's relative replacing ability, it would be necessary to contact a wide variation of concentrations of the ion in solution with a constant quantity of sorption complex containing a definite amount of the ion to be replaced and then to determine the relative concentration of the two ions in the equilibrium Solution. This has been the method most commonly employed for determining the relative replacing abilities of the yarious cations in base exchange reactions.