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.