What is more toxic arsenate or arsenite?

Arsenic is a naturally occurring element that was traditionally used in paints, dyes, and pesticides. The World Health Organisation (WHO) listed arsenic as one of the ten most harmful elements. Most attention is focused on the inorganic form of arsenic, which is 100 times more toxic than its organic counterpart. Contaminated water used for drinking, food preparation and irrigation of food crops is the major source of arsenic that can be potentially consumed by humans. The biogeochemistry of arsenic is complex and includes various adsorption and desorption processes. The toxicity of arsenic is connected to its solubility and is affected by pH. Arsenite (As3+) is more soluble than arsenate (As5+) and is more toxic; however, at a lower pH, arsenate becomes more mobile and toxic. It was found that addition of S, P, and FeOx to arsenite soil greatly reduces phytotoxicity of As (Kabata-Pendias, et al 2010).

Arsenate is chemically similar to phosphates, and plants may absorb it instead of this nutrient, which can disrupt cell formation. However, mechanisms of As absorption by plants is still not well understood. Some plants that grow on mine wastes and soils treated with arsenical pesticides develop tolerance to arsenic and can accumulate it in concentrations up to 8000 mg/kg.  

Most countries have strict regulations with respect to testing the presence of arsenic in food, especially seafood and seaweed that can have high concentrations of inorganic arsenic. According to the Food Standards Code of Australia and New Zealand, a limit of 1 mg/kg (ppm) arsenic applies to seaweed and molluscs and 2 mg/kg for fish and crustacea. Rice is another product that has to be tested for arsenic when imported to Australia and New Zealand.

Analysis of arsenic in plant tissue can be valuable in mineral exploration surveys since this element is often a pathfinder for various types of mineral deposits, including gold, base metals, and platinum group elements (Dunn, et al 2007).

During analytical testing, one has to be aware of the volatility of arsenic, which is similar to selenium (Se) and mercury (Hg), and use the appropriate procedures. Most common techniques include wet ashing digestion with nitric-perchloric acids or aqua regia in closed and open vessels, whereas dry ashing is generally not recommended (Kalra, et al 1997). Recovery or loss of the arsenic during the digest can be easily monitored by analysing duplicate samples and reference materials that cover the full range of arsenic concentrations for a specific project.