Properties & Reactions of Arsenic
Mechanism of Arsenic Retention
As(V) and As(III) are bound to the Fe oxide minerals by an inner-sphere ligand-exchange mechanism, in which the arsenic oxyanion competes with and exchanges with surface-OH or -OH2 groups that are directly coordinated to structural Fe3+ at the Fe oxide surface. An inner sphere complex is one in which the adsorbed ligand coordinates directly with the surface structural cation.
In the case of As(V), the absorbed arsenate anion is usually coordinated to two adjacent structural Fe3+ cations (as shown below), in a mechanism similar to the adsorption of phosphate by Fe oxide. This surface complex is called a bidentate binuclear-bridging complex. This is a very tight bond, and once the bond is formed, removal of the arsenic is difficult. This adsorption mechanism has been verified by infrared and x-ray absorption spectroscopic experiments with goethite and ferrihydrite, as well as with freshly precipitated Al(OH)3.
In the case of As(III) adsorption by Fe oxides, both bidentate binuclear-bridging complexes and monodentate complexes have been observed. A monodentate complex is one in which a single oxygen atom from the arsenite oxyanion coordinates a single structural Fe3+ at the Fe-oxide surface, as shown below.
As(III) is considerably less tightly bound to Al(OH)3 than to Fe oxides. The predominant bonding mechanism might involve either a monodentate bond or the formation of an outer-sphere complex. An outer sphere complex is one in which the ligand is not coordinated directly to the structural cation but instead is bound to the surface OH or OH2, probably by means of a hydrogen bond.
Influence of pH on Adsorption
The adsorption of As(III) and As(V) by Fe oxides is influenced by pH. For example, the adsorption of As(III) and As(V) by goethite is summarized in the figure below. This type of plot is called an “adsorption envelop” and is used to summarize the influence of pH on adsorption.
The adsorption of As(V) by most Fe oxides, including goethite, lepidocrocite and ferrihydrite, is greatest at low pH. However, the adsorption of As(III) is usually greatest at high pH. In most cases the maximum adsorption of As(III) occurs between pH 8 and pH 10.
The Fe oxides also have a pH-dependent charge. The point of zero charge (PZC) of Fe oxide occurs at approximately pH 8, which means that As(III) and As(V) are adsorbed to both positively and negatively charged surfaces. That is, the negatively charged As(V) species can be adsorbed to the negatively charged Fe oxide surface. This behavior is a characteristic of many inner-sphere adsorption complexes. Click here for an explanation of PZC.