THE SELF-DIFFUSION OF STRONGLY ADSORBED ANIONS IN SOIL - A 2-PATH MODEL TO SIMULATE RESTRICTED ACCESS TO EXCHANGE SITES

The consequences of slow diffusion of strongly adsorbed solutes into soil aggregates are not fully understood. The distribution of P-32 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 Cl-36. A model was developed that considers the soil to provide linked parallel intra-aggregate and interaggregate 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 die experimental one. The intra-aggregate impedance factor for P-32 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.