Monomethyl arsonic acid

As(III), As(V), monomethyl arsonic acid, and dimethylarsenic acid, which gives a quick and reliable estimation of exposure to inorganic arsenic. 

Finally, particular chemical species can be determined in some cases, as when arsenic content is separated into As(III), As(V), monomethyl arsonic acid, and dimethyl arsinic acid using ion-exchange chromatography. Chemical speciation is sometimes possible but is often very difficult. If the metals... 

CH3AsO(OH)2 Methanearsonic Acid Monomethyl Arsonic Acid +3... 

Adsorption processes may be particularly important in influencing species concentrations, since the arsenic present in the pore waters will probably be in equilibrium with arsenic adsorbed on solid surfaces. Arsenic in any species measured in pore waters may be only a fraction of the total amount of that species present in the sediments, the rest being adsorbed to or incorporated into particulate matter. Thus, it is important to study the sorptive characteristics of each of the arsenic species in the sediments. In the Menominee River sediments studied, the four oxygenated arsenic species (arsenate, arsenite, monomethyl arsonic acid and cacodylic acid) are often present together and competing among themselves and with phosphate for the same sorption sites. The competitive adsorptive characteristics of the species could greatly influence... 

The constants given In Table IV show the strong adsorption of phosphate and arsenate relative to cacodyllc acid, with monomethyl arsonic acid Intermediate. Constants for the adsorption of arsenate on amorphous Iron and aluminum hydroxide are also Included In Table IV to compare the sediments with well-characterized adsorbents. It was assumed that loss from... 

Figure 8. Dissolved arsenate and monomethyl arsonic acid (MMAA) concentrations from demethylation of MMAA to arsenate, k = 0.04 day, Too asol = 7.7 mol g KasOji, = 32 mol L = 28 mol HK (a) arsenate (b) MMAA.

Figure 9. Dissolved arsenate and monomethyl arsonic acid (MMAA) concentrations in pore waters of sediments spiked with MMAA and predictions of sorption/ kinetic model. Model parameters k = 0.07 day, T o mmaa = 4.5 fxmol g, Kmmaa = 5.0 fxmol L Tcc,AsOi = 3.5 fxmol g Ka. o, = 5.0 fxmol L k (a) Inorganic arsenic, measured (b) arsenate, predicted (c) MMAA, measured (d) MMAA, predicted.

Sorption of monomethyl arsonic acid (MMAA), dimethyl arsinic acid (DMAA), and arsenate on anaerobic bottom sediments from the Menominee River, Wisconsin are described by Langmuir Isotherms. These results were Incorporated Into a kinetic model of arsenic species transforamtlon which takes sorption Into account. Model predictions were found to be sensitive to the sediment water content and r, the adsorptive capacity of the sediment. Demethylatlon of MMAA and DMAA was observed In sediment Incubation experiments. The predictions of the sorption/kinetic model were In good agreement with the results of the Incubation experiments. 

Before excretion Inorganic arsenic is converted into monomethyl arsonic acid and... 

Cullen et al. (1994) have proposed a possible mechanism of arsenic methylation after the study in which arsenite, arsenate, monomethyl-arsonate or dimethylarsinic acid were added to the growth medium in the presence of the unicellular alga Polyphsa peniculus. Evidence of arsenic biomethylation by the micro-organism Apiotrichum humicola in the presence of L-methionine-methyl-d3 has come from the same laboratory (Cullen et al., 1995). Their findings point to the role of S-adenosylmethionine, or a related sulfonium compound as possible methyl donors. Arsenic biomethylation and biotransformation has also been demonstrated in a freshwater environment (Kuroiwa et al., 1994). 

Mammals, including humans, detoxify inorganic arsenic by methylation, yielding cacodylic acid (dimethylarsinic acid) as the chief urinary excretion product. Disposition by urinary excretion is usually prompt. Elimination of the arsonic acid (monomethyl) compounds has not been extensively studied, but urinary excretion of the unaltered compound and/or a further methylated form would seem likely. 

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