Using both a laboratory investigation and a mathematical model, we simultaneously described two-step sorption-desorp-tion and movement of phosphate displaced through water-saturated columns of Astatula sand. Removal of P from the soil solution was assumed to proceed initially as a rapid physical adsorption step on sorption sites and to be followed by a slower chemisorption step. Soil columns were packed with surface soil materials from a pastured land area that had received periodic irrigation with municipal wastewater over a previous 7-yr period. Phosphate solution with 10 g mminus;3was displaced through the water-saturated columns during steady liquid flow. We compared experimental P breakthrough curves and sorption isotherms with those given by the model. The model was observed to provide general but less than adequate description of the P breakthrough curves for the relatively large pore water velocities. As expected, reaction rate coefficients obtained using only stirred batch sorption experiments were shown to provide less than desirable simulation of aqueous flow through soil columns.
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