Sediments arsenic methylation

Methyl arsenic, like methyl mercury, is generated from inorganic forms of the element by methylation reactions in soils and sediments. However, the mechanism is evidently different from that for mercury, depending on the attack by a methyl car-bonium ion rather than a methyl carbanion (Craig 1986, Crosby 1998). Methylation... 

Biotic methylation in streams often produces volatile arsenic species, such as alkylarsines (e.g. methy-larsine), which could escape into the air (Mok and Wai, 1994), 113. In stream water and sediments, DMA(V) and MMA(V) may be directly created by microorganisms or degraded from more complex organoarsenicals. Eventually, methylarsenic in streams tends to degrade to inorganic As(V) (Mok and Wai, 1994), 112. 

Demethylation of cacodylic acid to arsenate was chosen as an example of case 1, product adsorbing with reactant not adsorbing, because cacodylic acid is only weakly adsorbed by anaerobic sediments and arsenate is strongly adsorbed. For the same reason, methylation of arsenate to produce cacodylic acid was chosen as an example of case 2, reactant adsorbed and product not adsorbed. Demethylation of methanearsonic acid to arsenate was chosen as an example of case 3, reactant and product both adsorbing. 

The concentrations of the spiked methylated arsenic species decreased with time while the concentrations of inorganic arsenic increased with time. There were no changes in arsenic speciation in the sterilized control sediments. 

Arsenic in aquatic environments is usually more concentrated in sediments and pore water than in the overlying water column (Ahmann et al., 1997 Smedley and Kinniburgh, 2002 Williams, 2001). The most abundant forms of arsenic are arsenate [As(V)] and arsenite [As(III)], but methylated forms can occur in mine-impacted environments (i.e., methylarsenic acid and dimethylarsenic acid) (Smedley and Kinniburgh, 2002) see Chapter 9.02. The principal pathway of arsenic toxicity is through dietary exposure to sediment and suspended particulates by fish, followed by human consumption. Environmental exposure to arsenic is a causal factor in human carcinogenous and other related health issues. Chronic exposure symptoms in humans include hyperkeratosis, hyperpigmentation, skin malignancies, and peripheral arteriosclerosis. Water provides the dominant pathway for arsenic exposure in humans (Williams, 2001). 

Maher used ion-exchange chromatography to separate inorganic arsenic and methylated arsenic species in marine organisms and sediments. This procedure involves the use of solvent extraction to isolate the arsenic species, which are then separated by ion-exchange chromatography and determined by arsine generation. 

The application of these methods to interstitial waters of lake sediments and also to species of lake-weed revealed no methylated arsenic species, only As(III) and As(V) in the 0.5pgP range. 

Laboratory studies have shown that the microorganisms present in natural marine sediments from British Columbia (Canada) and sediments contaminated with mine-tailings were capable of methylating arsenic under either aerobic or anaerobic conditions. Incubation of sediments with culture media produced volatile arsines (including AsHj, MeAsHj and MCjAs) as well as the methylarsenic(V) compounds Me As(0)(0H)j (n= 1,2,3) L... 

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