Thematic Articles

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.

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.

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 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 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.