『Abstract
Information on the particle size and reactive surface area of
natural samples and its interaction with natural organic matter
(NOM) is essential for the understanding bioavailability, toxicity,
and transport of elements in the natural environment. In part
I of this series (Hiemstra et al., 2010), a method is presented
that allows the determination of the effective reactive surface
area (A, m2/g soil) of the oxide particles of natural
samples which uses a native probe ion (phosphate) and a model
oxide (goethite) as proxy. In soils, the natural oxide particles
are generally embedded in a matrix of natural organic matter (NOM)
and this will affect the ion binding properties of the oxide fraction.
A remarkably high variation in the natural phosphate loading of
the oxide surfaces (Γ, μmol/m2) is observed in our
soils and the present paper shows that it is due to surface complexation
of NOM, acting as a competitor via site competition and electrostatic
interaction. The competitive interaction of NOM can be described
with the charge distribution (CD) model by defining a 三NOM surface
species. The interfacial charge distribution of this 三NOM surface
species can be rationalized based on calculations done with an
evolved surface complexation model, known as the ligand and charge
distribution (LCD) model. An adequate choice is the presence of
a charge of -1 v.u. at the 1-plane and -0.5 v.u. at the 2-plane
of the electrical double layer used (Extended Stern layer model).
The effective interfacial NOM adsorption can be quantified by
comparing the experimental phosphate concentration, measured under
standardized field conditions (e.g. 0.01M CaCl2),
with a prediction that uses the experimentally derived surface
area (A) and the reversibly bound phosphate loading (Γ, μmol/m2)
of the sample (part I) as input in the CD model. Ignoring the
competitive action of adsorbed NOM leads to a severe under-prediction
of the phosphate concentration by a factor ～10 to 1000. The calculated
effective loading of NOM is low at a high phosphate loading (Γ)
and vice versa, showing the mutual competition of both constituents.
Both constituents in combination usually dominate the surface
loading of natural oxide fraction of samples and form the backbone
in modeling the fate of other (minor) ions in the natural environment.
Empirically, the effective NOM adsorption is found to correlate
well to the organic carbon content (OC) of the samples. The effective
NOM adsorption can also be linked to DOC. For this, a Non-Ideal
Competitive adsorption (NICA) model is used. DOC is found to be
a major explaining factor for the interfacial loading of NOM as
well as phosphate. The empirical NOM-OC relation or the parameterized
NICA model can be used as an alternative for estimating the effective
NOM adsorption to be implemented in the CD model for calculation
of the surface complexation of field samples. The biogeochemical
impact of the NOM-PO4 interaction is discussed.』
1. Introduction
2. Materials and methods
2.1. Soil samples
2.2. Equilibration at standardized field conditions
2.3. Surface complexation modeling
3. Results and discussion
3.1. P-loading and solution composition
3.2. NOM surface species
3.2.1. Classical use of CD
3.2.2. Effective CD using surface components only
3.3. Effective NOM adsorption density
3.3.1. Phosphate NOM competition
3.3.2. Interfacial NOM and DOC
3.3.3. Interfacial carbon loading
3.4. NICA model: linking DOC and 三NOM
3.5. NICA application
3.6. Epilogue
4. Conclusions
Acknowledgments
References