Fixation Kinetics in Potassium–Iron—Orthophosphate Systems

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Abstract
The rate of removal of phosphate and potassium from solution was studied by contacting an iron‐impregnated, cation exchange resin with solutions of monopotassium orthophosphate in concentrations of 0.05 to 0.25M and with atomic ratios Fe:P from 0.25 to 4.0. The removal of both P and K from solution followed first‐order reaction models for all experimental conditions, the K being removed faster than the P. The rate constants, k1 and k2, for P and K removal, respectively, increased with the ratio Fe:P and were affected to a lesser extent by the initial concentration of KH2PO4. The concentration of Fe with time agreed with a consecutive first‐order reaction mechanism involving liberation, k3, and precipitation, k4, of Fe. The rate constants were evaluated by nonlinear regression analysis, and the reaction model represents the data well. The rate constants, k3 and k4, increased with the ratio Fe:P, whereas increasing the initial concentration of KH2PO4 caused k3 to increase and k4 to decrease. The precipitate was prepared in 1M KH2PO4 solution and identified as potassium iron phosphate, K[Fe(OH)1.33]3·(H2PO4)6·2H2O, by chemical and x‐ray analysis.