Phosphate transport in calcium-saturated systems. II. Experimental results in a model system
1991
Akinremi, O.O. | Cho, C.M.
This study investigated the diffusive transport of phosphate in a cation-exchange-resin model system. The objective was to demonstrate the effect of cation-exchange reactions on the movement of the phosphate ion. To a cylindrical wax column of Ca-saturated resin, 0.2 g of KH2PO4 was surface applied to simulate one-dimensional diffusive transport. Other ions of interest were Ca2+, K+, and H+. Extraction and analysis of 2-mm increments of the column after the desired incubation period (3, 7, and 14 d) revealed the following results. The pH of the system became low as a result of the interaction between added phosphate and Ca2+ from the exchange resin. The higher the cation-exchange capacity (CEC) the lower the pH became. Calcium ions, present on the exchange sites, were displaced into solution by cation exchange with K+. The higher the CEC of the column, the higher was the amount of Ca2+ displaced into soil solution. The movement of K+ and phosphate ions was retarded in the system, and the degree of retardation increased with increasing CEC of the soil column. The retardation of K+ movement was due to ion-exchange interaction with exchangeable Ca2+, while P movement was retarded as a result of chemical interaction with the displaced Ca2+. The experimental results showed the effect that the K- Ca exchange reaction has on the diffusive movement of P. There is a need to take this into consideration in any model simulating P transport in the soil.
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