The self‐diffusion of strongly adsorbed anions in soil: a two‐path model to simulate restricted access to exchange sites

Summary The consequences of slow diffusion of strongly adsorbed solutes into soil aggregates are not fully understood. The distribution of 32 P after diffusing down a soil column cannot be explained from a consideration of liquid‐phase impedance factor and isotopic exchange alone, as can that of 36 CI. A model was developed that considers the soil to provide linked parallel intra‐aggregate and inter‐aggregate pathways. With this geometry, simulations agreed fairly well with experimental data when the intra‐aggregate impedance factor was 0.001, using other model parameters that had been determined in independent equilibration studies. With this intra‐aggregate impedance factor, the model yielded straight‐line plots for chloride and the overall impedance factor derived agreed closely with the experimental one. The intra‐aggregate impedance factor for 32 P also agreed with the rate of reaction of the rapidly‐exchanging P fraction previously determined. The slowly‐exchanging P fraction has little influence on the P concentration profiles up to 10 d, but the small exchanged amount does have an effect at run times of 57 d. The slow intra‐aggregate diffusion of strongly adsorbed solutes decreases the amount adsorbed from a surface source of supply, and it also decreases the amount that is taken up by a surface sink. However, the interaggregate solution concentration remote from a source of supply is considerably increased, which may be important if the solute is a pollutant or a nutrient.