Environmental arsenic compounds arsenite

The arsenic compounds most commonly found in environmental and biological materials, and in working places, are arsenite and arsenate ions [As(III) and As(V)], monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), arsine, di- and tri-... 

Most arsenic compounds are highly toxic, causing dermatitis, acute and chronic poisoning, and possibly cancer. Arsenic is found in virtually all soil and other environmental matrices [16]. Arsenic is present in coal, pesticides, preservatives, etc. Arsenite, a commercial form of arsenic, is one of the most toxic forms of arsenic. 

For many years, arsenic has been regarded as an important environmental pollutant. It can enter the environment in many ways, for example use as an agrochemical, in smelting operations, and from coal fired power plants. Buchet and Lauwerys [70] list the relative toxicities for several arsenic compounds of high concern. In descending order they give arsine > arsenite > arsenate > methanearsonic acid (MMA) > dimethylarsinic acid (cacodylic acid, DMA). Methods which can accurately assess environmental and occupational exposure to these toxic arsenic compounds are necessary. 

Another intensively studied element in speciation analysis is arsenic. The biological and environmental effects of arsenic species and their transformation pathways have been studied in numerous papers.40- 42 Both arsenite and arsenate accumulate in living tissues because of their affinity for proteins, lipids and other cellular compounds.43 Arsenic species can undergo transformation via... 

All ars operons encode an ArsC arsenate reductase enzyme. However, as mentioned above, there are two sequence-unrelated families of arsenate reductases whose role is to reduce less toxic As(V) to more toxic As(lll) (21,34,36). It is only As(lll) and not As(V) that is pumped out from the cells by the ArsB transport protein. It seems counterintuitive from an environmental biology or metabolic chemistry point of view to convert a less toxic compound to a more toxic form. We have speculated that arsenite pumping activity evolved prior to the existence of oxygen in the atmosphere, and ArsC evolved only after an oxidizing atmosphere developed (8). At that point arsenite would spontaneously oxidize to arsenate, presenting a selective pressure for evolution of a reductase. The two types of arsC genes are positioned last in all ars operons shown in Fig. 2. The... 

Arsenic may occur as different oxidation states, principally as the -1-5 arsenate and the more toxic -I- 3 arsenite. Arsenite is usually determined in acidic (lmoll HCl) conditions by DPP, two reduction waves appearing at —0.4 and —0.85 V versus SCE, respectively. However, under such conditions the arsenate is not electrochemically active, therefore, arsenate must be reduced to arsenite and then measured polarographically as arsenite (thereby, giving the total arsenic concentration). The detection limit for this approach is of the order of the 3 X 10 mollDue to the toxic nature of As compounds, detection is principally carried out in biological and environmental samples. 

Many inorganic and organic chemicals can undergo oxidation or reduction reactions in the soil. An indicator of a compound s abihty to be oxidized or reduced is provided by its oxidation potential (EO), which is the voltage at which it is transformed to its reduced state. A similar measure of a soil s ability to reduce a compound is provided by the redox potential (pE), which is a measure of electron activity. Redox potentials are relatively high and positive in oxidized environments (e.g., surface waters), and low and negative in reduced environments (e.g., aquatic sediments and the subsurface soil layers). These environmental conditions are especially important for inorganic chemicals that are rarely present in their elemental form in the environment. Arsenic, for example, exists primarily in its oxidized form (arsenate) in the atmosphere and in surface waters and in its reduced form (arsenite) in sediments. 

---