Prediction of phosphate transport in small columns with an approximate sorption kinetics model

Abstract

The reaction of phosphate (P) with soil is described with two processes: adsorption and diffusion precipitation. In case of small pH variations in the acid pH range (pH 4.5–6.5) P adsorption may be described with the Langmuir equilibrium or Langmuir kinetics equation. The diffusion‐precipitation process is described with an expression taking the effects of P concentration and reaction time into account. This description is based on the unreacted shrinking core model, where the metal oxides are converted into metal phosphates, and these two phases are separated by a sharp interface inside the particles. Assuming that the adsorption process is reversible and the diffusion‐precipitation process is irreversible, methods are discussed for parameter assessment of the sorption model. These methods are illustrated for a soil. The description of the experimental data is good. For an evaluation of the predictive capabilities of the sorption model, P transport in small columns is predicted, using independently assessed sorption and transport parameters. These predictions are compared to experimental data. The predicted P breakthrough curves are overall well in agreement with the experimental results obtained by percolation of a pulse of a P solution. The main difference between predicted and experimental breakthrough is observed in the sorption stage of the pulse with a feed concentration (c0) of 0.2 mol P m−3. After initial breakthrough the correspondence of the prediction and the data is good for these triplicated experiments at c0=0.2 mol/m3. At high feed concentrations (c0=3 mol/m3) only small differences between prediction and experiment are observed in the desorption stage of the concentration pulse. For experiments involving a first pulse at c0=3 mol/m3 followed by two pulses at c0=0.2 mol/m3 the same agreement of numerical and experimental breakthrough curves is found as for the experiments involving only one pulse. Because P transport involving both sorption and desorption stages was modelled reasonably for two different concentrations we conclude that the P sorption model developed describes the reactions of P with soil accurately.