Phosphate sorption from seawater solutions: Particle concentration effect

Phosphate sorption experiments were performed with beach sand artificially coated with iron(III) (hydr)oxides and with a natural intertidal sandy sediment. Initial fast sorption was followed by a much slower exchange of phosphate between sorbent and seawater solution. A two-step kinetic model was developed to reproduce the time series sorption data. Because in many cases equilibrium was not reached at the end of the two-week experiments, the model was extrapolated to a nominal duration of one year to construct (near-) equilibrium isotherms. The results for both the iron(III) coated sand and the intertidal sediment showed a clear particle concentration effect, with higher sorption densities and steeper sorption isotherms at lower solid–solution ratios. Thus, while the commonly used Langmuir and Freundlich isotherms successfully reproduced individual experimental data series, the corresponding parameter values were dependent on the particle concentration. The particle concentration effect was accounted for by modifying the Freundlich isotherm according to the Metastable Equilibrium Adsorption (MEA) theory of Pan and Liss (1998a). The MEA-modified isotherm accounted well for the mixed suspensions of iron(III) coated sand, however, over-predicted phosphate sorption to the iron(III)-coated sand obtained under stagnant conditions. Although a mechanistic interpretation of the sorption process cannot be inferred from a fit to the MEA-modified Freundlich isotherm, the resulting parameter values facilitate the comparison of sorption studies carried out at variable solid–solution ratios.

► P sorption experiments were performed using Fe coated sand and intertidal sediment.
► A numerical model was developed to mimic and extrapolate the experiment results.
► An effect of particle concentration was identified.
► Higher sorption densities were found for the lower concentrations.
► A modified Freundlich equation fitted the data at different particle concentrations.