Anaerobic sediment arsenate

Brannon and Patrick [129] reported on the transformation and fixation of arsenic V in anaerobic sediment, the long term release of natural and added arsenic, and sediment properties which affected the mobilization of arsenic V, arsenic III and organic arsenic. Arsenic in sediments was determined by extraction with various solvents according to conventional methods. Added arsenic was associated with iron and aluminium compounds. Addition of arsenic V prior to anaerobic incubation resulted in accumulation of arsenic III and organic arsenic in the interstitial water and the exchangeable phases of the anaerobic sediments. Mobilization of... 

Results of Calculation of Conversion of Cacodylic Acid to Arsenate in Anaerobic Sediment... 

Sorption of Phosphate and Arsenic Species by Anaerobic Sediments... 

Demethylation of cacodylic acid to arsenate was chosen as an example of case 1, product adsorbing with reactant not adsorbing, because cacodylic acid is only weakly adsorbed by anaerobic sediments and arsenate is strongly adsorbed. For the same reason, methylation of arsenate to produce cacodylic acid was chosen as an example of case 2, reactant adsorbed and product not adsorbed. Demethylation of methanearsonic acid to arsenate was chosen as an example of case 3, reactant and product both adsorbing. 

Rates of changes in arsenic speciation were studied by spiking anaerobic sediments with arsenic standards, incubating the sediments, and monitoring arsenic speciation over a period of two months. The incubation experiments were modeled using the methods described in the previous section. 

Arsenic speciation in anaerobic sediments is controlled by both microbially mediated transformations of species and by abiotic chemical processes including adsorption. The two... 

There is much evidence for arsenic release into shallow sediment pore waters and overlying surface waters in response to temporal variations in redox conditions. Sullivan and Aller (1996) investigated arsenic cycling in shallow sediments from an unpolluted area of the Amazonian offshore shelf. They found pore-water arsenic concentrations up to 300 p.g in anaerobic sediments with nearly coincident peaks of dissolved arsenic and iron. The peaks for iron concentration were often slightly above those of arsenic (Figure 1). The magnitude of the peaks and their depths varied from place to place and possibly seasonally but were typically between 50 cm and 150 cm beneath the sediment-water interface (Sullivan and Aller, 1996). There was no correlation between pore-water arsenic concentrations and sediment arsenic concentrations (Figure 1). 

The most probable oxidation states of As in soil environments are +3 and +5, although the — 3 and 0 oxidation states are at least possible in strongly reduced soils and sediments. Arsenite (+3), which takes various forms such as As(OH)3, As(OH)7, As02(0H) , and AsOj", is the reduced state of As that is most likely to be found in anaerobic soils. Arsenate ( + 5), AsO ", the oxidized state, is stable in aerobic soils. 

It seems likely that thio-arsenicals result from sulfuration of the oxo analogs, but how this occurs is far more speculative. The conversion occurs rapidly and quantitatively in vitro when the oxo-arsenical is treated with aqueous H2S. Thus, in strongly reducing environmental conditions with concomitant production of H2S, in anaerobic sediments, for example, one might expect to find the thio-arsenical rather than the 0x0 analog, as has been already observed. ... 

These elevated levels are found mainly in anaerobic sediment regions where the chemical has been relatively undisturbed by activity. Low levels of arsenic in the biota of the Wailoa River estuary suggest that arsenic is trapped in the anaerobic sediment layers. 

Following consumption of dissolved O2, the thermodynamically favored electron acceptor is nitrate (N03-). Nitrate reduction can be coupled to anaerobic oxidation of metal sulfides (Appelo and Postma, 1999), which may include arsenic-rich phases. The release of sorbed arsenic may also be coupled to the reduction of Mn(IV) (oxy)(hydr)oxides, such as birnessite CS-MnCb) (Scott and Morgan, 1995). The electrostatic bond between the sorbed arsenic and the host mineral is dramatically weakened by an overall decrease of net positive charge so that surface-complexed arsenic could dissolve. However, arsenic liberated by these redox reactions may reprecipitate as a mixed As(III)-Mn(II) solid phase (Toumassat et al., 2002) or resorb as surface complexes by iron (oxy)(hydr)oxides (McArthur et al., 2004). The most widespread arsenic occurrence in natural waters probably results from reduction of iron (oxy)(hydr)oxides under anoxic conditions, which are commonly associated with rapid sediment accumulation and burial (Smedley and Kinniburgh, 2002). In anoxic alluvial aquifers, iron is commonly the dominant redox-sensitive solute with concentrations as high as 30 mg L-1 (Smedley and Kinniburgh, 2002). However, the reduction of As(V) to As(III) may lag behind Fe(III) reduction (Islam et al., 2004). 

The sorption of phosphate and the arsenic species on the twelve soils used in Wauchope s experiment correlated well with both the clay content and the iron content of the soils. The Menominee River sediments, however, were anaerobic, so iron should have been present as Fe(0H)2 rather than the Fe(0H)3 which was, presumably, present in Wauchope s alluvial soils. 

Sorption of monomethyl arsonic acid (MMAA), dimethyl arsinic acid (DMAA), and arsenate on anaerobic bottom sediments from the Menominee River, Wisconsin are described by Langmuir Isotherms. These results were Incorporated Into a kinetic model of arsenic species transforamtlon which takes sorption Into account. Model predictions were found to be sensitive to the sediment water content and r, the adsorptive capacity of the sediment. Demethylatlon of MMAA and DMAA was observed In sediment Incubation experiments. The predictions of the sorption/kinetic model were In good agreement with the results of the Incubation experiments. 

Laboratory studies have shown that the microorganisms present in natural marine sediments from British Columbia (Canada) and sediments contaminated with mine-tailings were capable of methylating arsenic under either aerobic or anaerobic conditions. Incubation of sediments with culture media produced volatile arsines (including AsHj, MeAsHj and MCjAs) as well as the methylarsenic(V) compounds Me As(0)(0H)j (n= 1,2,3) L... 

The greatest likelihood for As release in soils and sediments typically occurs upon transition from oxidizing to reducing conditions. Under saturated conditions, the rapid consumption of O2 by aerobic microbes combined with the low solubility of O2 induces anaerobic bacteria to utilize alternative electron acceptors. Arsenic may be displaced either through reduction of arsenate to arsenite or through mineralogical transformations (inclusive of dissolution) of the soil matrix. 

Two remaining factors need to be considered in addressing the ultimate fate of arsenic within the sedimentary basins of Southeast Asia. First, arsenic, while cycled in the surface soils, is partially leached into deeper portions of the profile (i.e.. As is not conserved during the anaerobic cycle). Second, these basms are active and contmue to receive sediment loads, leading to continual burial of arsenic deeper in the soil profile. Once deeper in the soil, the cycling of iron, and hence the fate of arsenic, is altered. Dampened fluctuation in anaerobic-aerobic... 

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