Modelling phosphate‐sorption kinetics in acid soils

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Summary: A model of P‐sorption kinetics was developed, that accounts for the dependency of reaction rate on concentration, sorbed P content, sorption maximum and time. The sorption phenomena predicted by the model agree with available observations in the literature. The model was tested with batch experiments for 17 acid top– and subsoils. The experiments revealed a significant correlation between the sorption maximum, F m , and the sum of amorphous iron and aluminium in soils. Plotting the dimensionless P–sorption ratio, F(c, t)/F m , which may be interpreted as the fractional saturation of the sorption capacity of a soil, against the natural logarithm of the exposure variable, I , gave S–shaped curves. Different parts of the S–shaped curve can be experimentally assessed, depending on the initial fractional saturation of the P–sorption capacity of soils. Apart from this dependency, one set of parameter values sufficed to describe the sorption kinetics of 10 different sandy top– and subsoils. For non–sandy soils, the parameter values differed and depended on the initial P content. The model enables extrapolation to long times, which is necessary for applications to field conditions.

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