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

Summary 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 distin...

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Bibliographic Details
Published in:European journal of soil science 2018-05, Vol.69 (3), p.475-487
Main Authors: Warrinnier, R., Goossens, T., Braun, S., Gustafsson, J. P., Smolders, E.
Format: Article
Language:English
Subjects:
Adsorption
Constants
Crystal structure
Crystallinity
Diffusion rate
Distribution
Dye dispersion
Freeware
Iron
Kinetics
Markvetenskap
Mathematical models
Modelling
Orthophosphate
Parameters
Phosphates
Rate constants
Reaction kinetics
Soil
Soil Science
Soils
Sorption
Spiking
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Summary:Summary 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 33PO4 sorption was measured in agitated suspensions between 2 minutes and 20 days 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 33PO4 between 1 and 20 days 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 20 days 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. Highlights A new rate constant distribution (RCD) model for PO4 sorption kinetics was developed. The RCD model offers a simple approach to take surface heterogeneity into account. The RCD model fitted PO4 sorption kinetics better than commonly used equations. Equilibrium was elusive for PO4 sorption on ferrihydrite and soil with poorly crystalline iron.
ISSN:1351-0754
1365-2389
1365-2389
DOI:10.1111/ejss.12549