The Characterization of a Phosphate Reaction Product in a Hydrol Humic Latosol in Hawaii

Abstract

Characterization of a phosphate reaction product in a Mydrol Humic Latosol in Hawaii has been carried out. The mineral taranakite ‘N6K3Al5(PO4)8 12H2O’ was obtained in the reactions between Akaka soil and monopotassium phosphate solution ranging from 0.2 to 0.6 molar at pH values of the reacting system of 2.3 to 2.9. The reaction products were characterized in comparison with a synthetic taranakite by use of X-ray, chemical, optical, infra-red absorption and differential thermal techniques. The taranakite was essentially a potassium aluminum phosphate complex. There was no indication of inomorphous substitution of iron for aluminum. The molar ratios of K/PO4 and Al/PO4 of the reaction product were found to be 0.37 and 0.72 respectively. The reaction product, taranakite, was thought to be formed by precipitating a soluble phospho-alumino complex anion with potassium ion. The soluble phospho-alumino anion resulted from the reaction of soluble soil aluminum with free phosphoric acid and K2PO4- ion. Water was not a structural constituent. The crystalline phosphor-reaction products were colorless and pseudohexagonal in form. They were biaxial negative. The refractive indices were nd=1.504, nB=1.507 and n=1.509 respectively, n-nd=0.005. The amount of taranakite produced increased with increasing concentration of phosphate solutions and increasing grams of soil used. The degree of crystallinity of the taranakite, as evaluated by the half-peak breadth of the 15.7 A peak, increased with time. The possibility of the formation of taranakite when soluble potassium and phosphate react with Akaka soil in the field is suggested. During the early stages of reaction, the reaction product is believed to be amorphous to X-ray.