Phosphate Fixation by Soil Minerals: V. Time of Reaction

V T first four papers of this series include a general review of the phosphate fixation problem (8), studies of phosphate fixation by minerals of the xmice and related groups (5), studies of phosphate fixation by the oxide minerals as quartz and hematite (6), and the effect of particle size of the minerals on phosphate fixation (7). The present paper deals with the effect of time of reaction between mineral and phosphate solution at several pH values on phosphate fixation. The minerals reported in the present paper are montmorillonite and kaolin of the clay group; muscovite of the mica group; and hematite, limonite, and quartz of the oxide group. The minerals were ground to pass .a zoo-mesh screen, suspended in water, the pH adjusted, orthophosphoric acid added, and then shaken for periods of time varying from i minute to 4 months. Phosphate fixation was determined by analyzing for phosphate left in solution. The general procedure of the earlier investigations was followed in the work reported in the present paper. Massive crystals of the minerals were obtained. These were crushed, and then picked over by hand to obtain a material of high degree of purity. The minerals, with the exception of muscovite, were ground with "a mullite mortar and'pestil to pass a loo-mesh screen. The muscovite was ground in a porcelain ball mill as this mineral is very difficult to handle with a mortar and pestil. All of the minerals were sieved frequently during grinding in order to obtain a relatively small amount of fine material. The largest particles passing a loo-mesh screen are approximately 150 microns in diameter. Five-gram portions of the powdered minerals were mixed with sufficient water to bring the final volume to 25 mis, normal fifth HC1 or NaOH was added to give the desired range of pH values, and the indicated quantity of phosphoric acid was then added. The amount of phosphate added was approximately 50% of the maximum amount shown by previous studies (5, 6) to be fixed in 16 hours. Duplicate tubes were shaken for periods of I minute, 2 hours, 16 hours, i week, and 4 months. The I-minute tubes were shaken by hand. The 2-hour, 16-hour, and i-week tubes were shaken in an endover-end shaker, turning at 6 r. p. m. for the designated time. The 4-month tubes were placed in the shaker for i week and then removed and shaken vigorously by hand four times per week for the remainder of the 4-month period. All shaking and storage took place in a constant temperature room maintained at 25° C. After shaking for the designated time, the samples were centrifuged and the supernatant liquid tested for phosphate remaining in solution and for pH. The phosphate was determined by the coeruleo-molybdate method of Deninges as improved by Atkins ( i) and the pH value by the quinhydrone electrode. The results obtained are shown in Figs. I to 12. -!_', ' „ • • ' • The minerals selected for these studies are common soil minerals "Montmorillonite and kaolin were selected as representatives of the clay group, muscovite of the mica group, and hematite, limonite, and quartz of the oxide group. Work on other clay minerals, such as halloysite, nontronite, beidellite, dickite, etc., which are generally classified as either of the montmorillonite or kaolinite group, would be highly desirable but inability to obtain such minerals in massive crystals of a high degree of purity has discouraged such efforts. It is considered that all of the soil particles in the clay fraction belong either to the montmorillonite or kaolinite group (4). These two typical clay minerals differ somewhat in'chemical composition, montmorillonite having a somewhat higher silica alumina ratio than kaolin (2). The X-ray patterns of the two minerals show somewhat similar structure, but montmorillonite has a much higher base exchange capacity (4) and is less stable than kaolin, especially under acid conditions. Montmorillonite is more frequently found in the soils of regions of lesser rainfall