Properties & Reactions of Arsenic

Factors Contributing to Desorption of Arsenic
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Four primary reactions contribute to desorption and mobility of arsenic in soils: (i) reductive dissolution, (ii) oxidative dissolution, (iii) ligand exchange, and (iv) ligand enhanced dissolution.

Reductive Dissolution

Reductive dissolution involves the dissolution of a mineral as a result of a reduction process. This process actually involves the consumption of electrons by the mineral. Reduction of soils, which occurs as a consequence of flooding or submersion of soils, impacts arsenic release primarily as a result of reductive dissolution of soil iron oxides (such as ferrihydrite or goethite) as is summarized in the following equation.

Fe(III) oxide–As(V) + e- >> Fe2+ + As(V), As(III)s

In soils, this process is largely biotic and results in the reduction of structural Fe3+ to Fe2+, with resulting dissolution of the iron oxide minerals and release of arsenic. The Fe oxide minerals differ considerably in their relative ease of dissolution, which decreases in the following order:

ferrihydrite >>> lepidocrocite > goethite > hematite

The relative ease of dissolution of ferrihydrite, with resulting release of adsorbed arsenic, is an important characteristic of this mineral under flooded conditions, as with flooded rice culture.

The reduction of As(V) to As(III) is another factor that can impact the release and mobility of arsenic.

Oxidative Dissolution

Oxidative dissolution involves the dissolution of a mineral as a result of an oxidation process. Oxidation of reduced soils, which occurs as a consequence of exposure of a soil to air or to increased dissolved O2 concentration, can impact arsenic release as a result of oxidative dissolution of ferrous sulfide minerals. This reaction is summarized in the following equation.

FeS-As(III) + 2 O2 >> Fe2+ + SO42- + As(III), As(V)

In soils, this process is largely abiotic and results in the oxidation of structural S2- or S- in sulfide minerals to SO42-, with resulting dissolution of the mineral and release of arsenic. The ferrous sulfide minerals differ considerably in their relative ease of oxidation. Ease of oxidation decreases in the following approximate order:

poorly crystalline ferrous sulfide >> mackinawite, greigite >> framboidal pyrite > massive pyrite

The relative ease of oxidation of poorly crystalline ferrous sulfide, mackinawite and greigite, with the resulting release of adsorbed or occluded arsenic, is an important characteristic of these minerals under conditions of increasing oxidation.

Ligand Exchange

Ligand exchange involves the exchange of a specifically adsorbed anion by another anion at a mineral surface. An example is the exchange of adsorbed As(V) by phosphate at an iron-oxide surface.

Fe oxide–As(V) + phosphate >> Fe oxide–PO4 + As(V)

Possible exchange ligands might include inorganic anions, such as phosphate and sulfate, or organic anions, such as oxalate, malate and citrate.

The ease of ligand exchange is dependent on the properties of the adsorbing surface and the competing ligand, as well as environmental parameters such as pH. Many ligand exchange reactions are time dependent reactions that could take hours or days to reach equilibrium.

Inorganic arsenic species, especially As(V) species, are very tightly bound to iron oxides. Of the common inorganic anions (including phosphate, sulfate, carbonate and silicate) and organic ions (including oxalate and citrate), only phosphate is highly competitive for adsorption sites. This behavior is due to the similar chemical characteristics between phosphate and arsenate, including symmetry, ion size and pKa’s. Yet once arsenate is adsorbed, only a portion of the arsenate is readily desorbed, and quantitative exchange by phosphate is difficult. This behavior might be partially attributable to the fact that phosphate could be bound to Fe oxide at several different types of surface sites with different bonding strengths and by possibly different mechanisms (e.g., monodentate versus bidentate bonding).

Ligand-enhanced Dissolution

Ligand-enhanced dissolution involves the complexation and dissolution of surface structural cations of a mineral, resulting in dissolution of the mineral. The example below shows the dissolution of ferrihydrite by oxalate, with the resulting release of surface adsorbed arsenic.

Fe oxide–As(V) + oxalate >> Fe3+-oxalate + As(V)

The rate of this reaction with organic ligands, such as oxalate, malate and citrate, varies substantially with mineral phase. The reaction rates decrease in the following trend.

ferrihydrite >>> lepidocrocite > goethite

In the absence of light, the dissolution of goethite is negligible. However, in the presence of light, the dissolution of goethite is substantial due to the photo-induced reduction of Fe3+ to Fe2+.

Implications to Arsenic Assessment in Soils and Sediments

Each of the reactions discussed above is used by soil scientists and geologists for the assessment of arsenic in soils and sediments. The application of these reactions to assessing arsenic and understanding arsenic behavior in soils will be addressed in a section currently in preparation.

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