Role of Bacteria in Arsenic Removal from an Aqueous Environment

Arsenic occurs naturally in rocks, where it can reach concentrations of up to 6,500 mg/ kg, and in soils, where its average concentration is 7.5 mg/ kg. Arsenic may be released to an aqueous environment through oxidative weathering and dissolution of arsenic-containing minerals. It exists in the environment in the forms of inorganic arsenic trioxide, sodium arsenate, and sodium arsenite, as well as organic methylarsenic acid and dimethylarsenic acid. When dissolved, these compounds exist in water in two common oxidation states: arsenite, As(III), and arsenate, As(V). Arsenic is also widely used in a variety of industries, including metal refining, agriculture, high technology laser processes, and the automobile industry. Arsenic is notorious as a lethal poison and has been linked to liver, lung, kidney, skin, and bladder cancers. Chronic exposure to low levels of arsenic may lead to basal and squamous cell carcinomas, anemia and leukopenia, and peripheral neuropathy. Ingestion of drinking water is the most important source of arsenic exposure. It is estimated that over 56 million people in 25 states consumed water from systems containing arsenic at levels posing a potentially fatal cancer risk above the level that is the United States Environmental Protection Agency (EPA) highest acceptable cancer risk (1 in 10,000). In order to reduce such risk, EPA recently promulgated a final rule to lower the arsenic maximum contaminant level ( MCL) to 0.010 mg/ L from the existing MCL of 0.050 mg/ L. Arsenic concentrations in groundwater in the United States are generally highest in the west and parts of the Midwest and Northeast. The United States Geological Survey reported that, in 24% of the U.S. counties surveyed, more than 10% of tested samples had arsenic concentrations exceeding 0.010 mg/ L. The prospect of lowering arsenic MCL in drinking water will undoubtedly intensify our effort to improve arsenic removal efficiency. The existing arsenic removal processes include precipitation, adsorption, ion exchange, membrane filtration, greensand filtration, and point-of-use devices. In groundwater, arsenic is mostly found as trivalent arsenite, As(III), due to prevailing reduced conditions. The trivalent arsenite is far more potent and genotoxic and is more soluble and mobile than the pentavalent arsenate. Furthermore, the oxidized As(V) is more effectively removed by the conventional treatment processes than As(III). Chemicals such as free chlorine, hypochlorite, ozone, permanganate, and Fenton’s reagent sH2O2 +F e 3+ d are commonly used for arsenite oxidation. Oxidation of As(III) may also be accomplished through UV irradiation. However, the conventional oxidation methods may be costly and/or possess the possibility of overdosing and formation of harmful byproducts and sludge. The fact that arsenite can be oxidized to arsenate through microbial activity in natural environments was realized a long time ago. Several arsenite-oxidizing bacteria have been isolated from various environments—cattle-dipping fluids, soil, arseniteenriched sewage, a mineral leaching site, a gold mine, and arsenic-contaminated water. These bacteria belong to the genera of Agrobacterium, Alcaligenes, Pseudomonas, Sulfolobus, Ochrobactrum, and Zoogloea, with few isolates remaining to be classified. Arsenite oxidation was also observed in mixed cultures of bacteria. As with all enzymatic reactions, microbial oxidation of arsenite is temperature dependent. Most known arsenite-oxidizing bacteria isolated are mesophilic, which grow and oxidize arsenite at 25‐ 37 ° C. However, growth was also reported at 3 ° C and as high as 44 ° C. Biological oxidation of arsenite may occur over a wide pH range. The optimum pH range for arsenite oxidation was, for the most part, near neutral. An acidophilic arseniteoxidizing bacterium found in acid mine water grew at optimum pH between 3 and 4. The arsenite oxidation activity of bacteria is observed to depend upon their growth stages. The effects of adaptation to arsenic on microbial growth and arsenite oxidation have been well documented. Laboratory observations suggest that arsenite-oxidizing enzymes are acquired by growth-dependent induction. The physiological role of arsenite oxidation in bacterial cells is, as yet, not well understood. The observation that electrons pass from arsenite to oxygen via an electron transport system that involves cytochrome c and cytochrome oxidase suggested that arsenite-oxidizing organisms may be able to couple arsenite oxidation to ATP synthesis, and thus derive energy from this process. Despite thermodynamic considerations that arsenite oxidation reaction is exergonic, and there is a sufficient amount of energy available, the utilization of this energy for growth has not yet been observed with heterotrophic arsenite-oxidizing bacteria. However, there has been indirect evidence to suggest that maintenance energy may be derived from arsenite oxidation by such bacteria. Rather than an energy production process, As(III) oxidation was also considered as a detoxification mechanism in bacterial cells. Limited evidence obtained with a few chemolithotrophic arsenite-oxidizing species, however, suggests the use of energy released during arsenite oxidation for growth. In order to utilize bacteria for arsenic removal in engineered processes, we must have a better understanding of both the kinetics and the physiological role of biological oxidation of arsenite. This information is needed for assessing the effectiveness of biological processes for arsenite oxidation and as a possible alternative control approach to the conventional physicochemical processes.