The biogeochemistry and dynamics of As and other metalloids vary between soil and aquatic environments. In the case of soil environment, a substantial proportion of the metalloids is associated with the solid phase and their fate is strongly influenced by physicochemical interactions
(e.g., adsorption–desorption) with the solid phase. Whereas in the case of aquatic environment, depending on the sediment content, a substantial proportion of metalloids remains in solution and their fate is controlled largely by biological transformation.
BIOGEOCHEMISTRY OF ARSENIC IN THE SOIL
As can exist in soil in different oxidation states but mostly as inorganic species, As(V) or As(III). In addition to inorganic species, microbial methylation of As in soil results in the release of organic methyl-arsenic compounds, such as MMA and
DMA, and ultimately arsine gas. Both inorganic and organic species of As undergo various biological and chemical transformations in soils, including adsorption, desorption, precipitation, complexation, volatilization, and methylation (Fig. 2). The most thermodynamically stable species of As(III) (i.e., H3AsO3 and H2AsO-4 ) and As(V) (i.e., HAsO2-4 ) occur over the normal soil pH range of 4 to 8.
Adsorption and Surface Complexation
The adsorption and retention of As by soils determine its persistence, reactions, movement, transformation, and ecological effects (toxicity). As in the case of most other metalloids and nonmetals, one of the most commonly reported, and perhaps the first reaction to occur in soils, is As adsorption onto soil particles. Numerous studies have dealt with As adsorption on to specific minerals and uncontaminated soils.
Ferrous oxides/hydroxides are involved most commonly in the adsorption of As in both acidic and alkaline soils. Carbonate minerals adsorb As in calcareous soils. In acidic soils, Mn oxides and biogenic particles play a dominant role in the adsorption of As.
Arsenic is known to have high affinity for oxide surfaces, and several biogeochemical factors are found to play a major role in adsorption. Soil particle size, organic matter, type and nature of constituent minerals, pH, redox potential, and competing ions have all been shown to influence As adsorption.
The type and quantity of silicate clay minerals present in soil also influence the retention of As. Soils having higher clay content retain more As than sandy soils with low clay content. The degree of As sorption onto silicate clay minerals decreases in the order of kaolinite >illite>montmorillonite.
The silicate clay minerals also generally adsorb more As(V) than As(III), and adsorption by clay minerals is affected by pH. Soil organic matter content also affects the adsorption of As and thus its bioavailability as organic molecules compete with As for sorption to surface sites .The maximum adsorption of As(V) on humic acids occur around pH 5.5, whereas adsorption of As(III) increased up to pH 8.
In soil and aquatic environments, redox reactions not only determine the nature of chemical species, but also the solubility and mobility of As and thus its environmental significance. Arsenic in soils is subject to both abiotic and biotic redox reactions. The Fe( III) oxides, M n(III) oxides, and organic compounds in soils play a major role in catalyzing the abiotic oxidation of As(III) through an electron transfer mechanism.
Similarly, abiotic redox reactions are also responsible for the release of As from arsenopyrite through oxidation by Fe(III), considered to be a predominating process inducing the release of As into the groundwater in areas where well waters are highly contaminated with As. This was attributed to the release of As(V) during reductive dissolution of Fe oxyhydroxide minerals that have a strong affinity for As(V) and the subsequent reduction of As(V) to As(III).
Due to its strong adsorption onto organic and clay colloids, As(V) is likely to persist in soils for a long time, especially in fine-textured soils with high iron (Fe) contents. In these soils, leaching of As(V) is low and therefore As contamination of groundwater is considered unlikely. However, under certain environmental conditions (i.e., low pH and low Eh), As would leach in the soil profile, thereby contaminating the surface and groundwater.
Considerable amounts of solubilized As could move downward in the soil profile with leaching water, especially in coarse-textured soils. It is for this reason that abandoned wood preservative sites may threaten groundwater quality. For example, in examining the leaching of Cu, Cr, and As from CCA solution through free-draining, coarse-textured surface and subsurface soils using undisturbed soil lysimeters.
BIOGEOCHEMISTRY OF ARSENIC IN AQUATIC ENVIRONMENT
Arsenic (As) is stable in four oxidation states (+5, +3, 0, -3) under the Eh conditions that occur in aquatic systems. At high Eh values (mostly exist in oxygenated waters), arsenic acid species (i.e., H3AsO4, H2AsO-4, HAsO24,and AsO3-4 ) are stable. At mildly reducing conditions, arsenious acid species (i.e., H3AsO3, H2AsO-3, and HAsO2-3) become stable.
The speciation of As in aquatic environment is critical in controlling the adsorption/desorption reactions with sediments. Adsorption to sediment particles may remove As(V) from contaminated water, as well as inhibiting the precipitation of As minerals such as scorodite (FeAsO4.2H2O) that control the equilibrium aqueous concentration. Under the aerobic and acidic to near-neutral conditions (typical of many aquatic environments), As(V) is adsorbed very strongly by oxide minerals in sediments.
The highly nonlinear nature of the adsorption isotherm for As(V) in oxide minerals ensures that the amount of As adsorbed is relatively large, even when dissolved aqueous concentrations of As are low. Such adsorption occurring in natural environments protects water bodies from widespread As toxicity problems.
Arsenic undergoes a series of biological transformations in the aquatic environment, yielding a large number of compounds, especially organ arsenicals (Organic Arsenic). Certain reactions, such as oxidation of As(III) to As(V), may occur both in the presence and in the absence of microorganisms, whereas other reactions, such as methylation, are not thermodynamically favorable in water and can occur only in the presence of organisms. Oxygenated waters, As(V) is the thermodynamically favored form, whereas As(III) is stable under reducing conditions (Ferguson and Gavis, 1972). Some bacteria and marine phytoplankton are capable of reducing As(V) to As(III) or oxidizing As(III) to As(V) (Andrea, 1977). Biological reduction of As(V) to As(III) reportedly occurs most easily at a pH between 6 and 6.7.
Benthic microbes are capable of methylating As under both aerobic and anaerobic conditions to produce methylarsines and methyl-arsenic compounds with a generic formula (CH3)nAs(O)(OH)3n where n may be 1, 2,or 3.