Arsenic Speciation

Arsenic (As) is a naturally occurring contaminant found in predominantly one of two forms (valences or oxidation states): trivalent arsenic (also known as As(III), As(+3), or arsenite) and pentavalent arsenic (also known as As(V), As(+5), or arsenate). In natural groundwater or surface water, arsenic may exist as arsenite, arsenate, or a combination of both that changes in as a function of the natural chemistry, temperature, pH, etc. (see the below pourbaix diagram as an example). In addition, various organoarsenic forms are also possible depending on the system chemistry and biological processes. 

Although both inorganic forms of arsenic are potentially harmful to human health, trivalent arsenic is considered more harmful than pentavalent arsenic and the organoarsenics are generally considered low toxicity. This means that measuring total arsenic alone, although helpful for both screening and general result comparison to regulatory limits, offers limited information when trying to learn what percentage of the arsenic is the more toxic form of concern. Therefore, arsenic speciation analysis can be helpful and at times necessary for environmental and human health risk assessment.

Secondary to risk assessment, arsenic speciation is also needed for proper optimization of water treatment systems.  For drinking water, the US EPA originally proposed maximum contaminant level (MCL) of 0.005 mg/L or 5 μg/L (micrograms per liter or parts per billion, ppb) and took comment on options as low as 3 μg/L. The agency elected to set the standard at 10 μg/L, higher than the technically feasible level of 3 μg/L. It believed that an MCL of 10 μg/L established an enforceable regulation for arsenic in public water supplies, based on the best available science to prevent potential health problems in consideration of treatment costs, benefits and the ability of public water systems to detect and affordably remove contaminants using suitable treatment technologies. The maximum contaminant level goal (MCLG) a non-enforceable public health goal was set at zero for arsenic.  Since the form of arsenic species in natural waters changes with water chemistry and these species may require different technologies to facilitate their removal, understanding the arsenic species distribution can be critical to the development of effective treatment processes.

Eurofins has over two decades of experience measuring and developing/optimizing methods and solution for arsenic speciation in waters, tissues, and soils within our specialty metals group. Please do not hesitate to contact us for more information on arsenic speciation.