Properties & Reactions of Arsenic

Adsorption of Arsenic by Oxide Minerals

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Arsenic is strongly adsorbed by Fe, Al, and Mn oxides. These adsorption reactions are the predominant processes that control the solubility and transport of arsenic in oxidized or moderately reduced soils and sediments.

Fe oxides

The predominant Fe oxides likely to be encountered in soils are hematite, goethite, and ferrihydrite. Other Fe oxide minerals that are also commonly observed, but in more restricted environments, include lepidocrocite, magnetite, maghemite, green rust, and Schwertmannite. The Fe oxides are the weathering products of other Fe-containing primary and secondary minerals and are present in virtually all oxidized or moderately reduced soils.

Each of the Fe oxide minerals usually occur in the clay (< 2 micrometer) particle-size fraction, except for magnetite which is found in silt and sand particle size fractions. It is because of their small particle size and strong arsenic retention characteristics that the Fe oxides strongly impact arsenic dynamics in most soils.

The Adsorption Reaction

When arsenic in solution comes in contact with an Fe oxide, the arsenic is adsorbed, as is shown in the following equation.

Fe oxide + As(V) >> Fe oxide–As(V)

Therefore, as a result of adsorption, the concentration of dissolved soluble arsenic will decrease. This reaction is summarized in the adsorption isotherm, in which the quantity of arsenic adsorbed is plotted against the equilibrium concentration of arsenic in solution.

Adsorption of Arsenic on Ferrihydrite

With low arsenic additions, the As(V) is adsorbed almost quantitatively by the Fe oxide. The adsorption reaches a maximum as surface adsorption sites are filled. Click here for an explanation of how the adsorption isotherm is determined.