Arsenic oxic waters

Se(VI), Te(IV) is the stable form as Te(OH)4. There are few data for this element in the ocean, and profiles of tellurium in the eastern North Pacific (Figures 5D and E) show that its concentrations are 1000 times less than those of selenium and both forms of tellurium show strongly scavenged behavior. It is interesting to note that the most abundant form of tellurium is Te(VI), but it is the least thermodynamically stable. Although there are numerous biological reasons to expect reduced species in oxic waters (i.e., as for arsenic and selenium), this observation is somewhat difficult to explain. The elevated concentrations of Te(VI) at the surface, relative to Te(IV), suggest that the atmospheric or riverine inputs of this element are enriched in this form, but virtually no atmospheric data are available to confirm this speculation. 

Anoxic water samples, because they contain little in the way of particles, are far easier than aquifer materials to develop radioassays for the measurement of arsenate reduction. Arsenic speciation quantitatively changes from arsenate to arsenite with vertical transition from the surface oxic waters to the anoxic bottom depths of stratified lakes and fjords (55,56). This also occurs in Mono Lake, California (57), a transiently meromictic, alkaline (pH = 9.8), and hypersaline (salinity = 70-90 g/L) soda lake located in eastern California (Fig. 11). The combined effects of hydrothermal sources coupled with evaporative concentration have resulted in exceptionally high ( 200 fiM) dissolved arsenate concentrations in its surface waters. Haloalkaliphilic arsenate-respiring bacteria have been isolated from the lake sediments (26), and sulfate reduction, achieved with... 

Stations 23 and 24, located in the LSLE, have been subjected to hypoxic conditions since the 1980s. With depletion of oxygen in the bottom waters, the sediment oxygen penetration depth decreased, and Fe oxides, concentrated in the oxic sediment layer, were reductively dissolved and released adsorbed arsenic. Hence, the low oxygen levels during the last 25 years in the bottom waters is reflected in more reducing conditions in the sediment and increases in both dissolved and HA-extractable Fe and As. 

This chapter discusses the chemical mechanisms influencing the fate of trace elements (arsenic, chromium, and zinc) in a small eutrophic lake with a seasonally anoxic hypolimnion (Lake Greifen). Arsenic and chromium are redox-sensitive trace elements that may be directly involved in redox cycles, whereas zinc is indirectly influenced by the redox conditions. We will illustrate how the seasonal cycles and the variations between oxic and anoxic conditions affect the concentrations and speciation of iron, manganese, arsenic, chromium, and zinc in the water column. The redox processes occurring in the anoxic hypolimnion are discussed in detail. Interactions between major redox species and trace elements are demonstrated. 

The dominant form of arsenic in oxic natural waters is usually dissolved arsenic acid, which includes H3ASO4 under very acidic (pH < 2) conditions and its associated anions (H2As04, HAs042, and/or As043 ) in less acidic, neutral, and alkaline waters (Figure 2.4). In most oxic natural waters that have... 

Abdullah, M.I., Shiyu, Z. and Mosgren, K. (1995) Arsenic and selenium species in the oxic and anoxic waters of the Oslofjord, Norway. Marine Pollution Bulletin, 31(1-3), 116-26. 

Elevated arsenic concentrations in oxic aquifers in Arizona (US) were linked to pH-dependent desorption (Robertson, 1989). Similar results exist for metamorphic aquifers in New England (US), where moderately alkaline waters (pH 7.5-9.3) were found to have elevated concentrations of arsenic (Robinson and Ayotte, 2006). Conversely, (BGS (British Geological Survey), 1989) suggested that arsenic concentrations of <4pgF-1 in water of the Lincolnshire Limestone (UK) cannot be explained by pH values of 7.0-9.5. McArthur et al. (2004) commented that the observations of pH increases with arsenic mobilization by Welch, Lico and Hughes (1988) and Robertson (1989) are not by themselves sufficient to prove that arsenic is mobilized by increasing pH. Arsenic may be mobilized by extended residence times, evaporation, and/or weathering, any of which could lead to both increases in pH and dissolved arsenic concentrations. 

Although purely thermodynamic considerations suggest that arsenic should exist in oxic seawaters almost entirely in the pentavalent state, equilibrium rarely appears to be attained, probably because of the existence of biologically mediated reduction processes and arsenic in most of these waters exists to an appreciable extent in the trivalent state As " As ratios as high as 1 1 have been found in a number of instances. 

Surface waters are exposed to sunlight and the atmosphere, which support the growth of photosynthetic organisms. Nutrient-like behavior of arsenic (i.e., surface depletion in its total concentration) has been observed in surface seawater (23) and in some lakes (24), though, in other cases, surface depletions were not observed (25-28). The occurrence of arsenic in the (thermodynamically unstable) +III oxidation state and of monomethylated and dimethylated arsenic species in oxic surface waters has been attributed to the activity of phytoplankton (25-27,29). 

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