PHOSPHATE AND ACCOMPANYING CATION-TRANSPORT IN A CALCAREOUS CATION-EXCHANGE RESIN SYSTEM

Cation-exchange interactions are known to modify the movement of the individual cations involved. It is uncertain, however, how such interactions affect the movement and immobilization of an anionic species such as phosphate. An experiment was conducted to assess the influence of cation-exchange reactions on the transport and immobilization of phosphate ions in a system of cation-exchange resin with CaCO3 as the buffer. A 0.2-g amount of KH2PO4 salt was uniformly applied to the surface of columns packed with either a mixture of CaCO3 and quartz sand or a mixture of CaCO3, quartz sand, and Ca2+-saturated exchange resin. Phosphate ions diffused to a distance of 45, 60, and > 60 mm from the surface after 3, 7, and 14 d of incubation, respectively, in columns packed with the CaCO3-sand mixture. In the columns containing both CaCO3 and Ca2+-saturated exchange resin, phosphate ions diffused about 35, 45, and 50 mm down the column during the same periods of incubation. The reduced movement of phosphate in the presence of Ca2+-saturated resin is explained by the magnitude of precipitated phosphate obtained. In the columns containing CaCO3 and quartz sand, the amount of precipitated phosphate in the upper 6-mm portion ranged from 20 to 180 mmol L-1. The corresponding value for columns containing Ca2+-saturated exchange resin ranged from 250 to 550 mmol L-1. The movement of K+ was similarly retarded by the presence of Ca2+-saturated resin. The results showed that the influence of cation-exchange interactions on ion mobility is not limited to the cations involved. The mobility of an anionic species can also be affected where the exchangeable cation, when liberated into solution, can form a precipitate with the anion. In soils containing exchangeable cations capable of reacting with phosphate to form a precipitate, there is a need to take cation exchange into consideration when describing the fate of applied P.