Geochemistry of arsenic is controlled by many factors that include:
• Red-ox potential
• Adsorption/ desorption
• Arsenic speciation
• Biological transformation
In general, most naturally occurring arsenic compounds are insoluble in water.
Redox potential, symbolically termed as Eh is also known as reduction potential, means the tendency of a chemical species to acquire electrons and thereby to reach to a reduced state. Arsenic is a redox-sensitive element. This means that arsenic may gain or lose electrons in redox reactions. As a result, it may be present in a variety of redox states. Arsenate generally predominates under oxidizing conditions, while arsenite predominates when conditions become sufficiently reducing.
Under the pH conditions of most groundwater, arsenate is present as the negatively charged oxyanions H2AsO-4 or HAsO4-2, whereas arsenite is present as the uncharged species H3AsO3. Natural geochemical and biological processes play critical role in controlling the fate and transformation of arsenic in the subsurface.
Arsenite is thermodynamically unstable in aerobic environments and oxidizes to As(V). Presence of other oxides such as FeO, Fe2O3, MnO2 and even clay minerals is capable of oxidizing As(III).
Adsorption/ Desorption of Arsenic
Two categories of processes largely control arsenic mobility in aquifers: (i) adsorption and desorption reactions, and (ii) solid-phase precipitation and dissolution reactions. Attachment of arsenic to an iron oxide surface is an example of an adsorption reaction.
The reverse of this reaction i.e., detachment of arsenic from such a surface, is an example of desorption. Arsenic adsorption and desorption reactions are influenced by changes in pH, occurrence of redox (reduction/oxidation) reactions, presence of competing anions, and solid-phase structural changes at the atomic level. Arsenate and arsenite adsorb to surfaces of a variety of aquifer materials, including iron oxides, aluminum oxides, and clay minerals.
Adsorption and desorption reactions between arsenate and iron-oxide surfaces are important controlling reactions because arsenate adsorbs strongly to iron-oxide surfaces in acidic and near-neutral-pH water. Similarly, redox reactions can control aqueous arsenic concentrations by their effects on arsenic speciation and hence, arsenic adsorption and desorption.. Structural changes in solid phases at the atomic level also affect arsenic adsorption and desorption.
Precipitation and Dissolution of Arsenic
The various solid phases (minerals, amorphous oxides, volcanic gas, and organic carbon) of aquifer material can exist in a variety of thermodynamic states. Solid-phase precipitation is the formation of a solid phase from components present in aqueous solution.
Precipitation of the mineral calcite, from calcium and carbonate present in groundwater, is an example of solid-phase precipitation. Dissolution of volcanic gas within an aquifer is an example of solid-phase dissolution. At any given time, some aquifer solid phases undergo dissolution, whereas others precipitate from solution.
Arsenic contained within solid phases, either as a primary structural component or an impurity in any of a variety of solid phases, is released to groundwater when those solid phases dissolve. Similarly, arsenic is removed from groundwater when solid phases containing arsenic precipitate from aqueous solution.
As an example, arsenic often co-precipitates with iron oxide; iron oxide, in such case, may act as an arsenic source (case of dissolution) or a sink (case of precipitation) for groundwater. Solid-phase dissolution contributes not only arsenic contained within that phase, but also any arsenic adsorbed to the solid-phase surface. The process of release of adsorbed arsenic, as a result of solid-phase dissolution, is distinct from the process of desorption from stable solid phases. Solid-phase precipitation and dissolution reactions are controlled by solution chemistry, including pH, redox state, and chemical composition.
Speciation of an element in water sample means determination of the concentration of different physico-chemical forms of the element which together make up its total concentration in the sample. The speciation of arsenic in environmental material is of interest because of the different levels of toxicity exhibited by various species. Species illustrate the various oxidation states that arsenic commonly exhibits (-3, 0, +3, +5) and the resulting complexities of its chemistry in the environment. Nearly two dozen arsenic species are present in the environmental and biological systems.
Arsenic in groundwater is present in various species like, H3AsO3, H2AsO3, HAsO3, H3AsO4, H2AsO4, and HAsO4. Arsenic species are generally present as arsenate [As(V)] or arsenite [As(III)] for Eh conditions prevalent in most groundwater. Both As(V) and As(III) form protonates oxyanions in aqueous solutions and the degree of protonation depends on pH. Differences in their toxicity, biochemical and environmental behaviors require the determination of these individual arsenic species.
Influence of pH
Alteration in the state of arsenic oxidation is usually influenced by Eh and pH. The most suitable pH for arsenic dissolution is low acidic (pH <2), but it can be dissolved in other pH ranges from 2 to 11 under suitable chemical and physical conditions. Arsenious acids, usually formed at low pH under mildly reduced conditions, are easily replaced by H2O3 when pH increases. HAsO3 is usually formed at very high alkaline pH > 12.
Influence of Competing Ions
Other intrinsic factors of a system which can also exert marked influence on the concentration and speciation of arsenic include: solution composition, competing ions especially, the ratio of Phosphorous to As, and Selenium to As, nature and composition of solid phases present, reaction kinetics and flow regime.
Arsenic undergoes a series of biological transformations in the aquatic environment, yielding a large number of compounds, especially organo-arsenicals. Certain reactions, such as oxidation of As(III) to As(V), may occur both in the presence and absence of micro organisms, whereas other reactions such as methylation, are not thermodynamically favorable in water and can occur only in the presence of organisms, which indicates that many aquatic organisms are capable of accumulating arsenic and may catalyse the oxidation of As(III) to As(V). Biological transformation is significantly important in marine ecology.
GEOCHEMISTRY OF ARSENIC