|Title||Quantifying the specific surface area of tropical soils|
|Group||Soil Chemistry and Chemical Soil Quality|
|Supervisor(s)||Supervisor(s) Elise Van Eynde, Juan Carlos Mendez Fernandez|
|Description||A successful application of surface complexation models (SCM�s) to describe and predict the adsorption of ions to the soil�s natural oxide fraction requires an accurate determination of the reactive surface area (A= m2/g soil) of such soil samples. Recently, an approach to estimate A was proposed, which is based on the adsorption competition to the oxide fraction between phosphate and (bi)carbonate ions. The values of A and the total reversible adsorbed PO4 (Rev= �mol/m2) were estimated from the change in solution concentration of PO4 in soil extraction with 0.5 M NaHCO3 at different SSR, and by interpreting the experimental data with the CD-model (Hiemstra et al. 2010). Well-crystalline goethite and the amorphous ferrihydrite were used as model oxide, with the latter giving more realistic and consistent estimations of A and Rev for Dutch soil samples.
So far, the NaHCO3 dilution method has been tested only in soils with relative high P-loadings but not in highly weathered soils (e.g. soils from tropical regions) with lower P-loadings and large fraction of Fe and Al oxides. In many tropical soils, the intense leaching of Si and basic cations enhances the formation crystalline Fe- and Al-(hydr)oxides (e.g. goethite and gibbsite). In such a case, the of the fraction containing nano-sized oxides (poorly-crystalline) (e.g. ferrihydrite), in a mass base, may be relatively small. Nonetheless, due to their high reactive surface area and high density of reactive groups, ferrihydrite nanoparticles may still greatly contribute to the total PO4 adsorption even at very low concentrations. Therefore, it would be interesting to test which Fe oxide (Fh or goethite) is a good proxy for natural oxide fraction in weathered tropical soil samples. Additionally, it needs to be tested whether the values of A, as estimated with the NaHCO3 method, can be related to any specific fraction of Fe and Al oxides that is dissolved with commonly extraction methods.
1) Is the dilution NaHCO3 method, as proposed by Hiemstra et al. (2010), suitable to derive the A (m2/g soil) of the natural oxide fraction in oxide-rich tropical soils? If not, what would be a proper experimental set-up for estimating A for this type of soils?
2) Is the value of A, as derived with the NaHCO3 method, related to the amount of Fe and Al extracted by ascorbic acid and/or DCB?
3) Are the ferrihydrite nanoparticles, rather than well-crystallized goethite, a good proxy for describing the adsorption interaction between PO4 and CO3 in oxide-rich tropical soils?
4) How does the modelling prediction for the ion partitioning over the solid-solution interphase would change for a particular ion if either the NaHCO3 dilution method or selective dissolution extractions (e.g. ascorbic acid or DCB) are used to estimate A?
Literature: Hiemstra, Tjisse, Juan Antelo, Rasoul Rahnemaie, and Willem H van Riemsdijk. 2010. �Nanoparticles in Natural Systems I: The Effective Reactive Surface Area of the Natural Oxide Fraction in Field Samples.� Geochimica et Cosmochimica Acta 74 (1): 41�58.
|Used skills||- Geochemical modelling
- Laboratory work
- Data processing and writing