Competitive Arsenate and Phosphate Adsorption on Ferrihydrite as Described by the CD-MUSIC Model

The solubility and
bioavailability of arsenic in the environment
are to a large extent governed by adsorption reactions with iron (hydr)­oxides,
the extent of which is affected by competitive interactions with other
ions, for example, phosphate. Here, batch experiments were performed
with ferrihydrite suspensions to determine the adsorption of arsenate
[As­(V)] and phosphate (PO4) at different As­(V)–PO4 ratios. A surface complexation model based on the Charge
Distribution MUltisite Ion Complexation (CD-MUSIC) concept (the “Ferrihydrite
CD-MUSIC model”) was developed to describe these interactions
in a way consistent with results from spectroscopic studies. For this
purpose, several previously published data sets on As­(V) and PO4 adsorption in ferrihydrite suspensions were reviewed, including
a number of systems containing other major ions (CO32– and Ca2+), and new surface complexation
constants were derived. During model development, it was found that
the inclusion of ternary complexes was not needed to describe the
observed Ca2+–PO4 interactions. For both
As­(V) and PO4, the resulting model predicts the presence
of corner-sharing bidentate complexes as well as monodentate complexes,
with the latter being important particularly at low pH. The experimental
results showed that As­(V) and PO4 displayed similar adsorption
patterns in the single-ion systems studied, which were conducted using
a constant anion-to-Fe ratio of 0.2. Even so, As­(V) was preferentially
adsorbed over PO4 in competitive systems, particularly
at low As­(V)-to-PO4 ratios when the Kd values for As­(V) were up to 2.1 times as high as those for
PO4. The model, which described these patterns very well,
suggests that adsorbed As­(V) consists of a larger fraction of bidentate
complexes than in the case of PO4. This causes a flatter
adsorption isotherm for As­(V), which leads to a stronger As­(V) adsorption
as the As­(V)-to-Fe ratio decreases, compared to that for PO4.