Charlotta Tiberg

Modelling lead(II) sorption to ferrihydrite and soil organic matter

Lead(II) adsorption to soil organic matter and iron (hydr)oxides is strong, and may control the geochemical behaviour of this metal. Here, we report the adsorption of Pb(2+) (i) to 2-line ferrihydr ...

Phosphate effects on cadmium(II) sorption to ferrihydrite

HYPOTHESIS Phosphate influences the sorption of metals to iron (hydr)oxides. An enhanced formation of inner-sphere complexes on the (hydr)oxide surface can be attributed to electrostatic interactions and/or to changes in metal coordination on the iron (hydr)oxide surface. Phosphate was expected to increase cadmium(II) sorption on ferrihydrite. It should be possible to identify changes in cadmium(II) coordination upon phosphate addition by Extended X-ray absorption fine structure (EXAFS) spectroscopy and implement the identified complexes in a surface complexation model (SCM). EXPERIMENTS The effect of phosphate addition on cadmium(II) sorption to ferrihydrite was studied by a series of batch experiments covering the pH range from 4 to 8. EXAFS spectroscopy was performed on ferrihydrite from the batch experiments at the cadmium K edge. The identified surface complexes were incorporated in the Charge distribution multisite complexation (CD-MUSIC) model, and new surface complexation constants were optimized. FINDINGS Without phosphate addition cadmium(II) formed inner-sphere bidentate complexes on the ferrihydrite surface. With phosphate there was an increased cadmium(II) sorption that could not be explained by electrostatic interactions alone. The enhancement was best explained by the formation of a ternary complex including cadmium(II), phosphate and ferrihydrite surface groups.

Predicting sulphate adsorption/desorption in forest soils: Evaluation of an extended Freundlich equation

Sulphate adsorption and desorption can delay the response in soil acidity against changes in acid input. Here we evaluate the use of an extended Freundlich equation for predictions of pH-dependent SO4 adsorption and desorption in low-ionic strength soil systems. Five B horizons from Spodosols were subjected to batch equilibrations at low ionic strength at different pHs and dissolved SO4 concentrations. The proton coadsorption stoichiometry (η), i.e. the number of H(+) ions co-adsorbed for every adsorbed SO4(2)(-) ion, was close to 2 in four of five soils. This enabled the use of a Freundlich equation that involved only two adjustable parameters (the Freundlich coefficient KF and the non-ideality parameter m). With this model a satisfactory fit was obtained when only two data points were used for calibration. The root-mean square errors of log adsorbed SO4 ranged from 0.006 to 0.052. The model improves the possibility to consider SO4 adsorption/desorption processes correctly in dynamic soil chemistry models.