Editing Arsenic (section)

=== Redox transformation of arsenic in natural waters ===

Arsenic is unique among the trace [[metalloids]] and oxyanion-forming trace metals (e.g. As, Se, Sb, Mo, V, Cr, U, Re). It is sensitive to mobilization at pH values typical of natural waters (pH 6.5–8.5) under both oxidizing and reducing conditions. Arsenic can occur in the environment in several oxidation states (−3, 0, +3 and +5), but in natural waters it is mostly found in inorganic forms as oxyanions of trivalent arsenite [As(III)] or pentavalent arsenate [As(V)]. Organic forms of arsenic are produced by biological activity, mostly in surface waters, but are rarely quantitatively important. Organic arsenic compounds may, however, occur where waters are significantly impacted by industrial pollution.<ref>{{cite journal |last1=Smedley |first1=P.L |last2=Kinniburgh |first2=D.G |title=A review of the source, behaviour and distribution of arsenic in natural waters |journal=Applied Geochemistry |date=May 2002 |volume=17 |issue=5 |pages=517–568 |doi=10.1016/S0883-2927(02)00018-5 |bibcode=2002ApGC...17..517S |url=http://nora.nerc.ac.uk/id/eprint/12311/1/Abstract.pdf }}</ref>

Arsenic may be solubilized by various processes. When pH is high, arsenic may be released from surface binding sites that lose their positive charge. When water level drops and [[sulfide]] minerals are exposed to air, arsenic trapped in sulfide minerals can be released into water. When organic carbon is present in water, bacteria are fed by directly reducing As(V) to As(III) or by reducing the element at the binding site, releasing inorganic arsenic.<ref>[https://web.archive.org/web/20110308104034/http://www.civil.umaine.edu/macrae/arsenic_gw.htm How Does Arsenic Get into the Groundwater]. Civil and Environmental Engineering. [[University of Maine]]</ref>

The aquatic transformations of arsenic are affected by pH, reduction-oxidation potential, organic matter concentration and the concentrations and forms of other elements, especially iron and manganese. The main factors are pH and the redox potential. Generally, the main forms of arsenic under oxic conditions are {{chem2|H3AsO4}}, {{chem2|H2AsO4-}}, {{chem2|HAsO4(2-)}}, and {{chem2|AsO4(3-)}} at pH 2, 2–7, 7–11 and 11, respectively. Under reducing conditions, {{chem2|H3AsO4}} is predominant at pH 2–9.

Oxidation and reduction affects the migration of arsenic in subsurface environments. Arsenite is the most stable soluble form of arsenic in reducing environments and arsenate, which is less mobile than arsenite, is dominant in oxidizing environments at neutral [[pH]]. Therefore, arsenic may be more mobile under reducing conditions. The reducing environment is also rich in organic matter which may enhance the solubility of arsenic compounds. As a result, the [[adsorption]] of arsenic is reduced and dissolved arsenic accumulates in groundwater. That is why the arsenic content is higher in reducing environments than in oxidizing environments.<ref>Zeng Zhaohua, Zhang Zhiliang (2002). "The formation of As element in groundwater and the controlling factor". Shanghai Geology 87 (3): 11–15.</ref>

The presence of sulfur is another factor that affects the transformation of arsenic in natural water. Arsenic can [[precipitate]] when metal sulfides form. In this way, arsenic is removed from the water and its mobility decreases. When oxygen is present, bacteria oxidize reduced sulfur to generate energy, potentially releasing bound arsenic.

Redox reactions involving Fe also appear to be essential factors in the fate of arsenic in aquatic systems. The reduction of iron oxyhydroxides plays a key role in the release of arsenic to water. So arsenic can be enriched in water with elevated Fe concentrations.<ref>{{cite journal |title=Redox control of arsenic mobilization in Bangladesh groundwater |doi=10.1016/j.apgeochem.2003.09.007 |volume=19 |issue=2 |journal=Applied Geochemistry|pages=201–214|bibcode=2004ApGC...19..201Z |year=2004 |last1=Zheng |first1=Y. |last2=Stute |first2=M. |last3=van Geen |first3=A. |last4=Gavrieli |first4=I. |last5=Dhar |first5=R. |last6=Simpson |first6=H.J. |last7=Schlosser |first7=P. |last8=Ahmed |first8=K.M.  |display-authors=6 }}</ref> Under oxidizing conditions, arsenic can be mobilized from [[pyrite]] or iron oxides especially at elevated pH. Under reducing conditions, arsenic can be mobilized by reductive desorption or dissolution when associated with iron oxides. The reductive desorption occurs under two circumstances. One is when arsenate is reduced to arsenite which adsorbs to iron oxides less strongly. The other results from a change in the charge on the mineral surface which leads to the desorption of bound arsenic.<ref>Thomas, Mary Ann (2007). [http://pubs.usgs.gov/sir/2007/5036/pdf/sir20075036_web.pdf "The Association of Arsenic With Redox Conditions, Depth, and Ground-Water Age in the Glacial Aquifer System of the Northern United States"]. U.S. Geological Survey, Virginia. pp. 1–18.</ref>

Some species of bacteria catalyze redox transformations of arsenic. Dissimilatory arsenate-respiring prokaryotes (DARP) speed up the reduction of As(V) to As(III). DARP use As(V) as the electron acceptor of anaerobic respiration and obtain energy to survive. Other organic and inorganic substances can be oxidized in this process. [[Chemoautotrophic]] arsenite oxidizers (CAO) and [[heterotrophic]] arsenite oxidizers (HAO) convert As(III) into As(V). CAO combine the oxidation of As(III) with the reduction of oxygen or nitrate. They use obtained energy to fix produce organic carbon from CO<sub>2</sub>. HAO cannot obtain energy from As(III) oxidation. This process may be an arsenic [[detoxification]] mechanism for the bacteria.<ref>{{cite journal|author=Bin, Hong |year=2006|title=Influence of microbes on biogeochemistry of arsenic mechanism of arsenic mobilization in groundwater|journal= Advances in Earth Science |volume=21 |issue=1|pages= 77–82|url=http://www.adearth.ac.cn/EN/abstract/abstract3466.shtml}}</ref>

Equilibrium thermodynamic calculations predict that As(V) concentrations should be greater than As(III) concentrations in all but strongly reducing conditions, i.e. where [[sulfate]] reduction is occurring. However, abiotic redox reactions of arsenic are slow. Oxidation of As(III) by dissolved O<sub>2</sub> is a particularly slow reaction. For example, Johnson and Pilson (1975) gave [[Half-life|half-lives]] for the oxygenation of As(III) in seawater ranging from several months to a year.<ref>{{cite journal|doi=10.1080/00139307509437429|pmid=236901 |title=The oxidation of arsenite in seawater |volume=8 |issue=2 |journal=Environmental Letters|pages=157–171|year=1975 |last1=Johnson |first1=D. L. |last2=Pilson |first2=M. E. Q. }}</ref> In other studies, As(V)/As(III) ratios were stable over periods of days or weeks during water sampling when no particular care was taken to prevent oxidation, again suggesting relatively slow oxidation rates. Cherry found from experimental studies that the As(V)/As(III) ratios were stable in anoxic solutions for up to 3 weeks but that gradual changes occurred over longer timescales.<ref>{{cite book|author=Cherry, J. A.|title=Contemporary Hydrogeology – the George Burke Maxey Memorial Volume|chapter=Arsenic species as an indicator of redox conditions in groundwater|doi=10.1016/S0167-5648(09)70027-9 |volume=12|pages=373–392|series=Developments in Water Science|year=1979|isbn=978-0-444-41848-7}}</ref> Sterile water samples have been observed to be less susceptible to speciation changes than non-sterile samples.<ref>{{cite journal|doi=10.1021/cr00094a002 |title=Arsenic speciation in the environment |volume=89 |issue=4 |journal=Chemical Reviews |pages=713–764|year=1989 |last1=Cullen |first1=William R |last2=Reimer |first2=Kenneth J |hdl=10214/2162 |hdl-access=free }}</ref> Oremland found that the reduction of As(V) to As(III) in Mono Lake was rapidly catalyzed by bacteria with rate constants ranging from 0.02 to 0.3-day<sup>−1</sup>.<ref>{{cite journal|author1-link=Ronald Oremland|author=Oremland, Ronald S.|title=Bacterial dissimilatory reduction of arsenate and sulfate in meromictic Mono Lake, California|doi=10.1016/S0016-7037(00)00422-1 |bibcode=2000GeCoA..64.3073O|volume=64|issue=18|journal=Geochimica et Cosmochimica Acta|pages=3073–3084|year=2000|url=https://zenodo.org/record/1259591}}</ref>