Properties & Reactions of Arsenic
Other Minerals Influencing Adsorption of Arsenic
Dissolved inorganic arsenic species, especially As(III), are strongly adsorbed at the surfaces of ferrous sulfide minerals. Adsorption is predominantly a result of the formation of an inner-sphere complex between surface sulfide atoms and As(III) . Adsorption might be very important at the surfaces of freshly precipitated poorly crystalline ferrous sulfide or the highly reactive sulfides mackinawite (FeS0.9) and greigite (Fe3S4). Also, arsenic can be occluded, probably as As(III), during the precipitation of these minerals.
These minerals are all highly unstable and readily oxidized upon exposure of the soil to air, which also will result in the release of adsorbed arsenic. Because of the relative instability of these minerals, their importance in arsenic retention is dependent on conditions that are favorable for their formation and stability (i.e., highly reduced soils with high sulfur concentrations). Pyrite, especially framboidal pyrite because of its relatively high surface area, can also adsorb appreciable As(III).
Framboidal Pyrite has a higher reactivity because of its higher surface area. Massive Pyrite
As(III) and As(V) species are not retained in interlayer regions of permanently charged 2:1 swelling clays such as montmorillonite (structural diagram | electron micrograph) and vermiculite (microscopic image).
As(III) and As(V) can be retained at edge sites of layer silicates due to specific adsorption at Fe3+ or Al3+ surface sites, by formation of either inner sphere or outer sphere complexes. Edge site bonding might play a more important role with kaolinite (structural diagram | electron micrograph)and chlorite (structural diagram), since these minerals usually have lower net charge and the proportions of edge to interlayer sites are usually greater.
For most soils, the impact of edge site bonding on arsenic retention is relatively minor, due to the importance of iron oxides on arsenic retention. But bonding behavior can differ considerably between soils, depending on mineralogical composition.
Calcium carbonate minerals, especially calcite, can provide sites for arsenic retention. Arsenate can interact specifically with surface structural Ca2+, probably as outer sphere complexes. Arsenic retention at the calcite surface is relatively weak compared to complexes at iron-oxide surfaces. Therefore, in mixed mineral systems, adsorption at calcite surfaces would not be expected to play a dominant role in overall arsenic retention. There are several calcium arsenate and calcium arsenite solid phases, but most of these minerals have relatively high solubility and would not normally be expected to contribute substantially to arsenic retention.