Properties & Reactions of Arsenic

Arsenic Containing Solid Phases
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As(III) and As(V) interact strongly with many cat ions, including aluminum (Al3+), calcium (Ca2+), iron (Fe2+, Fe3+), magnesium (Mg2+), to form a variety of minerals in geologic systems. Arsenic is frequently a component of sulfide ores, in which it occurs as metal arsenides.

Arsenates of aluminum, barium, bismuth, calcium, cobalt, copper, iron, lead, magnesium, uranium and zinc occur naturally, along with arsenic trioxide, which are formed as the weathering products of arsenides. None of these minerals occurs very commonly in soils. The exception is As(III) in Fe(II) sulfide minerals, which can exist under reduced conditions.

The arsenic oxides, As2O3 and As2O5, are important since they are often used as primary  standards for As(III) and As(V), respectively.

Arsenic Sulfides

Arsenic sulfide minerals include:

  • As2S3 --- orpiment; arsenic exists As(III)
  • As4S4 --- realgar; arsenic exists as As(II)
  • FeAsS --- arsenopyrite; arsenic exists in -1 oxidation state
  • FeS2 --- arsenian pyrite (with arsenic from a trace to atom percent)
  • Amorphous arsenic sulfide

Each of these minerals is only stable under reduced conditions and is readily oxidized under oxidizing conditions. None of the minerals above is commonly identified in soils, because of their relatively low concentrations. Arsenic is readily incorporated into precipitating ferrous sulfide minerals. Therefore, soluble arsenic is likely to be coprecipitated with ferrous sulfide under conditions that are favorable for the formation of these phases, namely highly reduced conditions, the presence of dissolved Fe2+, and relatively high dissolved sulfur concentrations.

Metal Arsenates

Metal arsenates, including the Fe3+, Al3+, Mn4+ and Ca2+ arsenates, have rarely been identified in soils. Fe3+ arsenates (e.g., scorodite, FeAsO4.2H2O) and Al3+ arsenates are more stable at low pH values. Their relative stabilities decrease with increasing pH. Ca2+ arsenates are more stable at high pH conditions and are unlikely to occur at low pH. It is conceivable that metal arsenate phases could form if conditions are locally favorable, including proper pH and redox conditions, and locally high dissolved arsenic and metal cat ion concentrations. The very low substantiated incidence of these minerals in soils might be at least partially attributable to their localized occurrence, very low concentration, and difficulties in determination. In addition, the stabilities of the metal arsenates are very highly dependent on environmental conditions.