Arsenic in water

In natural waters, arsenic may exist as one or more dissolved species, whose chemistry would depend on the chemistry of the waters. Over time, arsenic species dissolved in water may (1) interact with biological organisms and possibly methylate or demethylate (Chapter 4), (2) undergo abiotic or biotic oxidation, reduction, or other reactions, (3) sorb onto solids, often through ion exchange, (4) precipitate, or (5) coprecipitate. This section discusses the dissolution of solid arsenic compounds in water, the chemistry of dissolved arsenic species in aqueous solutions, and how the chemistry of the dissolved species varies with water chemistry and, in particular, pH, redox conditions, and the presence of dissolved sulfides. Discussions also include introductions to sorption, ion exchange, precipitation, and coprecipitation, which have important applications with arsenic in natural environments (Chapters 3 and 6) and water treatment technologies (Chapter 7). 

If a reaction in an aqueous solution is at equilibrium, an equilibrium constant (Keq) may be calculated if the activities of all of the products and reactants are known. (However, in practice, Keq is approximated by using molar concentrations instead of activities.) In general, reactions have the following format, where a moles of substance A reacts with b moles of substance B to form c moles of substance C and d moles of substance D  

The equilibrium constant then relates the activities of the products and reactants of Reaction 2.22 through the following equation (Faure, 1998), 112  

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R02019 Method 1631, Revision E Mercury in Water by Oxidation, Purge and Trap, and Cold Vapor Atomic Eluorescence Spectrometry 821R96013 Method 1632 Determination of Inorganic Arsenic in Water by Hydride Generation Elame Atomic Absorption... 

R95028 Method 1632 Determination of Inorganic Arsenic in Water by Hydride Generation Elame Atomic Absorption [Draft]... 

Such analytical measurements are necessary to establish concentrations for most agents in the environment. How much benzene is present in the air at gasoline stations as a result of its evaporation from gasoline What is the concentration of arsenic in water running off the surface of a hazardous waste site where unknown amounts of arsenic were buried over many years What is the polychlorinated biphenyl (PCB) concentration in fish swimming in waters next to a hazardous waste site known to contain this substance How much aflatoxin is in a batch of peanut butter The most reliable answers to these questions are those resulting from chemical analysis. 

The toxicity of chronic exposure to arsenic is well established and the best recommendation is to avoid arsenic exposure. The most common home exposure is from contaminated drinking water and arsenic-treated lumber. Certain areas of the country have higher levels of arsenic in water. The EPA has lowered arsenic drinking water standards, but water providers have until 2006 to meet the new standards. Avoid inhalation of sawdust from arsenic-treated lumber or inhalation of smoke from burning arsenic-treated wood. And of course always wash your hands. This is particularly important if a young child is playing on arsenic-treated wood. 

Site contains a map of the United States showing arsenic in water. 

Most of the studies on the uptake and transformation of arsenic in water have dealt with arsenate it is the predominant arsenic component in seawater, and radiolabeling experiments are most easily carried out with this chemical species. Generally, the results have shown that uptake of arsenate from water is a slow process and is unlikely to be significant in natural environments (160-163). 

In view of the ease with which water attacks an ester of arsenate in water, we wondered if carbon nucleophiles would similarly attack a trialkyl arsenate to form a C—As bond. We (Sparkes and Dixon, unpublished work) therefore treated the sodium salt of diethyl malonate with tripropyl arsenate, and hydrolyzed during workup. Some arsonoacetic acid was formed, but we have not found conditions that give a useful yield. 

The anhydrous salt is obtained1 by heating the crystals to 120° C, If crystallisation takes place at the ordinary temperature, the dodeca-hydrate, Na2HAs04.12H20, is obtained while if the crystals are formed above 36° C. the heptahydrate, Na2HAs04.7H20, is produced. The transition point determined from the solubility curve of sodium monohydrogen arsenate in water 2 is at 22° C. 

Sorption, ion exchange, precipitation, and coprecipitation of arsenic in water 2.7.6.1 Introduction... 

The impacts of arsenic on human health are left to the next chapter, which includes discussions on the toxic effects of arsenic and its biotransformations. Chapter 5 reviews the history of arsenic utilization in human societies, examples of unintentional poisoning events, the role of arsenic in crime, and recent production and market trends. While the discussions in Chapter 3 often have widespread applications in a variety of natural environments, Chapter 6 cites specific field cases, where human activities and natural occurrences of arsenic in water, soils, and sediments have produced exceptionally serious threats to local environments and human populations. 

The solubilities of aluminum, manganese, Fe(II), and Fe(III) arsenates in water are very different than the values for calcium arsenates (Chapter 2). While calcium arsenates are less soluble under alkaline conditions, the solubility of the other arsenates generally increases with increasing pH (Sadiq et al., 2002, 306-307). Under pH 2-12 conditions, Mn(II) and Fe(II) arsenates are also less soluble than aluminum and Fe(III) arsenates (Sadiq et al., 2002, 307). 

Borba, R.P., Figueiredo, B.R. and Matschullat, J. (2003) Geochemical distribution of arsenic in waters, sediments and weathered gold mineralized rocks from Iron Quadrangle, Brazil. Environmental Geology, 44(1), 39-52. 

Glass, R.L. and Frenzel, S.A. (2001) Distribution of Arsenic in Water and Streambed Sediments, Cook Inlet Basin, Alaska, U.S. Geological Survey Fact Sheet FS-083-01. 

Samanta, G Chowdhury, T.R., Mandal, B.K. et al. (1999) Flow injection hydride generation atomic absorption spectrometry for determination of arsenic in water and biological samples from arsenic-affected districts of West Bengal, India, and Bangladesh. Microchemical Journal, 62(1), 174-91. 

Mobilization of arsenic in water 6.4.1 Competitive anion exchange... 

Ferguson, J.F. and Anderson, M.A. (1974) Chemical form of arsenic in water supplies and their removal, in Chemistry of Water Supply, Treatment and Distribution (ed A.J. Rubin), Ann Arbor Science, Ann Arbor, MI. 

Precipitation refers to the process of adding one or more chemical reagents to water so that dissolved contaminants are transformed (precipitated) into insoluble solids (precipitates) (US EPA, 2002b, 17). The precipitates can then be collected and removed from the water by filtration, flotation, centrifugation, or other methods. For arsenic in water, precipitation typically involves reactions between arsenic oxyanions and dissolved cations. A common example is the precipitation of calcium arsenates from the addition of lime to a wastewater containing dissolved As(V). 

Dambies, L. (2004) Existing and prospective sorption technologies for the removal of arsenic in water. Separation Science and Technology, 39(3), 603-27. 

Liang, P. and Liu, R. (2007) Speciation analysis of inorganic arsenic in water samples by immobilized nanometer titanium dioxide separation and graphite furnace atomic absorption spectrometric determination. Analytica Chimica Acta, 602(1), 32-36. 

Ashley, R.P. and Ziarkowski, D.V. (1999) Arsenic in waters affected by mill tailings at the Lava Cap Mine, Nevada County, California. Abstracts with Programs-Geological Society of America, 31(6), 35. 

Juntunen, R., Vartiainen, S. and Pullinen, A. (2004) Arsenic in water from drilled bedrock wells in Pirkanmaa, in Arseeni Suomen Luonnossa Ymparistovaikutukset Ja Riskit(Arsenic in Finland Distribution, environmental impacts and risks) (eds R. K., Loukola and P. Lahermo), Geologian Tutkimuskeskus, Espoo, pp. 111-22. 

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