Arsenic, a chalcophilic element, is widespread in the environment. Although arsenic may possibly be an essential element for life (Cox 1995) and some microorganisms are known to use arsenic for energy generation (Oremland and Stolz 2003), no firm data are available on its essentiality for biological systems (Francesconi 2005). In contrast to its possible essentiality in life, many studies have focused on its high toxicity, which has been well known from various cases of poisoning throughout the ages (Nriagu 2002). The toxicity is especially high for inorganic arsenic; trivalent inorganic arsenic [arsenite; As(III)] is known to bind readily to sulfhydryl groups of enzymes leading to enzyme inhibition, whereas pentavalent inorganic arsenic [arsenate; As(V)], which is structurally similar to phosphate, may disrupt metabolic reactions that require phosphorylation (Cox 1995). Symptoms of acute intoxication in humans by inorganic arsenic include severe gastrointestinal disorders, hepatic and renal failure, and cardiovascular disturbances, whereas chronic exposure causes skin pigmentation, hyperkeratosis, and cancers in the lung, bladder, liver, and kidney as well as skin (Gorby 1994; WHO 2001). At present, arsenic contamination of groundwater is a worldwide problem (Mandai and Suzuki 2002), particularly in the Bengal Delta where chronic ingestion of arsenic in groundwater poses a significant health risk to about 36 million people (Nordstrom 2002). Thus, the development and use of techniques to remove arsenic from polluted groundwater is an urgent necessity (Chowdhury 2004). In contrast to the hazards of arsenic, it is useful in medicine. For example, arsenic trioxide (As₂O₃) has recently attracted considerable attention as a therapeutic agent for treatment of acute promyelocytic leukemia and other cancers, although the precise mechanisms by which it produces results are not fully understood (Zhu et al. 2002).