Arsenic
David J. Vaughan – Guest Editors
Table of Contents
Overview
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2006 Topics
Overview
Arsenic is an element known throughout history as a classic poison. Currently, very small but highly significant concentrations of this element in drinking water supplies are causing massive health problems to many millions of people in some of the world’s poorest nations, and more localized sources related to mining and processing are also a concern. This issue of Elements will present background information on arsenic chemistry, occurrence in the Earth, production and uses, and its toxic properties.
- Arsenic
- Chemistry and Mineralogy of Arsenic
- Microbial Transformations of Arsenic in the Environment: From Soda Lakes to Aquifers
- Arsenic in Shallow, Reducing Groundwaters in Southern Asia: An Environmental Health Disaster
- Arsenic in Soils, Mine Tailings, and Former Industrial Sites
- Arsenic in Drinking Water: Impact on Human Health
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Next Issue
v2n3 Water on Mars
Guest editor: Harry Y. McSween Jr (University of Tennessee),
During the past several decades, spacecraft data have transformed the planets from astronomical objects into geologic worlds. Mars is the cur- rent focus of planetary exploration, and NASA’s objectives for this effort are based on the theme, “follow the water.” This issue will address new discoveries from spacecraft and from Martian meteorites about where water or ice was (or is) located and about the role of water in determin- ing the mineralogy, petrology, and geochemistry of the Martian surface.
- Water on Mars Harry Y. McSween Jr. (University of Tennessee)
- Geomorphological Evidence for Water on Mars Victor R. Baker (University of Arizona)
- The Orbital Search for Altered Materials on Mars Michael B. Wyatt (Arizona State University) and Harry Y. McSween Jr. (University of Tennessee)
- Water at the Poles and in Permafrost Regions of Mars Philip R. Christensen (Arizona State University)
- Aqueous Processes Recorded by Martian Meteorites Analyzing Martian Water on Earth Laurie A. Leshin NASA Goddard Space Flight Center) and Edward Vicenzi (Smithsonian Institution)
- Evidence for Water at Meridiani Bradley L. Jolliff (Washington University), Scott M. McLennan (Stony Brook University), and the Athena Science Team
2006 Topics
- User Research Facilities in the Earth Sciences (February 2006 )
- Arsenic (April 2006)
- Water on Mars (June 2006)
- Early Earth (August 2006)
- Glasses and Melts: Linking Geochemistry and Materials Science (October 2006 )
- The Nuclear Fuel Cycle: Environmental Aspects (December 2006)
Thematic Articles
Arsenic is an element known throughout history as a classic poison. Currently, very small but highly significant concentrations of this element in drinking water supplies are causing massive health problems to many millions of people in some of the world’s poorest nations. More localised sources related to mining and processing are also a concern. A review of background information on arsenic chemistry, occurrence in the Earth, production and uses, as well as its toxic properties, leads in to the other articles in this issue of Elements.
Arsenic has diverse chemical behavior in the natural environment. It has the ability to readily change oxidation state and bonding configura- tion, which creates rich inorganic and organic chemistry. This behav- ior is a consequence of the electronic configuration of its valence orbitals, with partially filled states capable of both electron donation and overlap in covalent bonds. In natural compounds, arsenic bonds primarily to oxygen and sulfur, generating a variety of aqueous species and minerals. The affinity of arsenic for these two elements, along with its stable bonding to methyl groups, constitutes the structural basis for most organic and biosynthetic compounds. The agile chemistry of arsenic helps to explain its contradictory action as both a toxin and a curative, and its sometimes-elusive behavior in the environment.
Arsenic is a highly toxic element that supports a surprising range of biogeochemical transformations. The biochemical basis of these microbial interactions is described, with an emphasis on energy- yielding redox biotransformations that cycle between the As5+ and As3+ oxidation states. The subsequent impact of As3+-oxidising and As5+-reducing prokaryotes on the chemistry of selected environments is also described, focusing on soda lakes with naturally high concentrations of the metalloid and on Southeast Asian aquifer sediments, where the microbial reduction of sorbed As5+ and subsequent mobilisation of As3+ into water abstracted for drinking and irrigation threaten the lives of millions.
Arsenic concentrations in shallow, reducing groundwaters in Bengal, Southeast Asia, and elsewhere constitute a major hazard to the health of people using these waters for drinking, cooking, or irrigation. A comparison of occurrences in the Ganges–Brahmaputra, Mekong, and Red River basins shows that common geological characteristics include (1) river drainage from the rapidly weathering Himalayas, (2) rapidly buried organic- bearing and relatively young (ca. Holocene) sediments, and (3) very low, basin-wide hydraulic gradients. Anaerobic microbial respiration, utilizing either sedimentary or surface-derived organic carbon, is one important process contributing to the mobilization of arsenic from host minerals, notably hydrous iron oxides. In spite of the paucity of data from before the extensive develop- ment of groundwater pumping in these areas, there is sufficient evidence to make a prima facie case that human activity might exacerbate arsenic release into these groundwaters. The difficulties in implementing comprehensive groundwater remediation suggest serious attention should be given to developing treatment technologies for alternative surface-water supplies.
Much progress has recently been made on the relation between the crystal chemistry of arsenic and its speciation and distribution at the Earth’s surface. The investigation of As-impacted soils and acid mine drainages, using synchrotron-based techniques, shows the importance of As adsorption on, or coprecipitation with, hydrous ferric oxides in delaying the long-term impact of As on the biosphere. Arsenic mobility often depends on bacterial activity, with accompanying major seasonal modifications of As speciation, even at extreme As concentrations. Remediation technologies use geochemical affinities between arsenic and specific low-temperature phases to reduce the bioavailability of arsenic.
In terms of its impact on human health, arsenic is unique in that most of the evidence linking it to diseases comes from epidemiological work; animal studies have not provided good models. It is also unique in causing a large number of different damaging effects and, as more studies are conducted, more such effects are found. To date, we know that arsenic from drinking water can cause severe skin diseases including skin cancer; lung, bladder, and kidney cancers, and perhaps other internal tumors; peripheral vascular disease; hypertension; and diabetes. It also seems to have a negative impact on repro- ductive processes (infant mortality and weight of newborn babies). The toxi- cology of arsenic involves mechanisms that are still not completely understood, but it is clear that a number of factors can affect both individual and popula- tion-level susceptibility to the toxic effects of arsenic-contaminated drinking water. Current research is addressing some of these, including genetic suscep- tibility and lifestyle factors that may increase arsenic’s toxic effects, such as smoking, diet, and concurrent exposure to other substances. The reversibility of some effects upon cessation of exposure is also being investigated.
