Groundwater arsenic in the Red River delta, Vietnam ... - Fiva

Jessen et al., manuscript to be submitted to Geochim. Cosmochim. Acta
4.4. Surface modelling
The high As mobility in the reduced aquifer suggested by the data in Fig. 7 could be an
artefact, if a large decrease in the concentration of competitive solutes releases sorption
sites and in that way strengthens the sorption of As. The observed behaviour in the field
can be compared to predictions of surface complexation models. Stachowicz et al.
(2008) successfully modelled the competitive sorption of PO4 3- , Ca 2+ , CO3 2- and As(III)
on goethite using the CD-MUSIC model. In the CD-MUSIC model, the surface density
is calculated from the surface geometry of the mineral of interest (in this case goethite)
and the primary charge of the surface groups is calculated from bond valence analysis.
The net change in surface charge due to a surface complexation reaction is distributed
over the 0 and the 1 plane, according to the molecular structural location and orientation
of the surface complex. Recently, the CD-MUSIC model was implemented into the
reactive transport modelling program PHREEQC. For comparison to the CD-MUSIC
model, the generalized two layer model for ferrihydrite by Dzombak and Morel (1990)
was employed (D&M model), used also by Swartz et al. (2004) and Postma et al.
(2007).
The number of surface sites for both models was adjusted to produce the amount
of As adsorbed in the natural groundwater environment according to the
adsorption/desorption experiment (Fig. 3), which corresponded to roughly 50 μmoles
As per liter of contacting groundwater. For the CD-MUSIC model, with a uni- and tri-
coordinated surface oxygen density of 3.45 sites/nm 2 and 2.7 sites/nm 2 , respectively,
and a surface area of 50 m 2 /g goethite, this corresponded to 6.5 g goethite/L yielding in
total 3.2 mM sites. Specific anion sorption is defined only for the uni-coordinated
surface groups (1.86 mM sites); the tri-coordinated surface groups (1.46 mM sites) are
27