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Environmental fate & pathways

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Arsenic acid (AA) is an inorganic compound of arsenic (As). There are no proprietary data investigating the stability of AA unless a range of publications dealing with the abiotic fate of As are available in the literature. This section aims to provide a concise overview of the abiotic fate of As in water and atmosphere compartments. As is a naturally occurring element but As also enters into environment through releases from human activities.

Stability in contact with water

There are no existing studies investigating the hydrolysis of AA in aqueous media. The hydrolysis information requirement is waived since the information requirement is not applicable to AA which does not exhibit hydrolysable moieties. As (+5) is a thermodynamically stable form of As. In water and under aerobic conditions, AA exists as a mixture of arsenate ions based on its pKa:

H3AsO4 <-> H+ + H2AsO4 - (pKa1=2.22)

H2AsO4 - <-> H+ + HAsO42 - (pKa2=6.98)

HAsO42 - <-> H+ + AsO43- (pKa3=11.53)

Considering the influence of pH alone, the protonated form H3AsO4 is the dominant form of arsenate at very low pH. Under most environmental conditions (pH 5 - 9), H2AsO4 - and HAsO42 - are the dominant forms of arsenate ions.

As (+3) as arsine is usually present and sometimes dominates in low oxygen waters (e.g. bottom water). Some arsenate and arsenite species can interchange oxidation states depending on Eh, pH and biotransformation. The relevant abundance of oxidised and reduced species of As is significantly influenced by iron and manganese oxides.

As (-3) and As (0) occur only under strong reducing conditions (low Eh and anaerobic conditions).

Stability in contact with light

As enters into the atmosphere mainly through the release of As2O3 or less frequently as volatile organic compounds (arsine). Trivalent As species and arsine undergo oxidation into As (+5) forms. Photolysis in atmosphere is not considered as a significant breakdown process or As compounds.

The rates of photochemical decomposition in water of arsenite, DMA and MMA have been studied in both distilled water and seawater. All species were found to degrade rapidly in aerated distilled water. Half-life values for the degradation of DMA, MMA and arsenite were 9.2, 11.5 and 0.9 min for aerated distilled water and 25, 19 and 8 min for de-aerated distilled water. In seawater, the photochemical decomposition rate is slower with 20% of DMA converted to MMA after 300 min.

Photolysis

It has no influence on the behaviour and transformation of arsenate in waters.