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Research article2018Peer reviewed

Modelling heterogeneous phosphate sorption kinetics on iron oxyhydroxides and soil with a continuous distribution approach

Warrinnier, R.; Goossens, T.; Braun, S.; Gustafsson, J. P.; Smolders, E.

Abstract

The kinetics of orthophosphate (PO4) sorption in soil has far reaching consequences on its long-term fate. Traditionally, a distinction is made between fast, reversible adsorption and slow fixation. The kinetics are commonly described by compartmental models (CMs) assuming kinetically distinct homogeneous pools (e.g. a rapid- and slow-sorbing pool), with phenomenological equations or with complex diffusion-based models. Alternatively, this process can be described by assuming frequency distributions of both adsorption and desorption rate constants and, thereby, enabling better description of experimental data with fewer adjustable parameters. Here, we developed such a rate constant distribution (RCD) model and compared it with CMs and other commonly used rate equations. Batch (PO4)-P-33 sorption was measured in agitated suspensions between 2minutes and 20days after spiking in 13 contrasting types of soil and two iron oxyhydroxides. Overall, the RCD model, with three adjustable parameters, describes the data better than the other models tested. The so-called slow reactions, denoted as the factor change in soluble (PO4)-P-33 between 1 and 20days after spiking, were described better by the RCD model and ranged from 1.0 (i.e. no change) to 6.9. The extent of slow reactions increased with the increase in the fraction of poorly crystalline iron in the soil (r=0.69; P=0.0088). Equilibrium was elusive up to 20days for PO4 sorption on ferrihydrite and on soil samples with a large fraction of poorly crystalline iron oxyhydroxides. The RCD model code is available as freeware from the first author.

Published in

European Journal of Soil Science
2018, Volume: 69, number: 3, pages: 475-487
Publisher: WILEY