Arsenic is a naturally occurring trace element that poses a threat to human and ecosystem
health, particularly when incorporated into food or water supplies. The greatest risk imposed by
arsenic to human health results from contamination of drinking water, for which the World
Health Organization recommends a maximum limit of 10 g L-1. Continued ingestion of
drinking water having hazardous levels of arsenic can lead to arsenicosis and cancers of the
bladder, skin, lungs and kidneys . Unfortunately, arsenic tainted drinking waters are a global
threat and presently having a devastating impact on human health within Asia. Nearly 100
million people, for example, are presently consuming drinking water having arsenic
concentrations exceeding the World Health Organization’s recommended limit (Ahmed et al.,
2006).
Arsenic contamination of the environment often results from human activities such as
mining or pesticide application, but recently natural sources of arsenic have demonstrated a
devastating impact on water quality. Arsenic becomes problematic from a health perspective
principally when it partitions into the aqueous rather than the solid phase. Dissolved
concentrations, and the resulting mobility, of arsenic within soils and sediments are the
combined result of biogeochemical processes linked to hydrologic factors. Processes favoring
the partitioning of As into the aqueous phase, potentially leading to hazardous concentrations,
vary extensively but can broadly be grouped into four categories: (1) ion displacement, (2)
desorption (or limited sorption) at pH values > 8.5, (3) reduction of arsenate to arsenite, and (4)
mineral dissolution, particularly reductive dissolution of Fe and Mn (hydr)oxides . Although
various processes may liberate arsenic from solids, a transition from aerobic to anaerobic
conditions, and commensurate arsenic and iron/manganese reduction, appears to be a dominant,
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but not exclusive, means by which high concentrations of dissolved arsenic are generated.
Within the subsequent sections of this chapter, we explore and describe the biological and
chemical processes that control the partitioning of arsenic between the solid and aqueous phase.