Arsenic-water system

Szramek, K Walter, L.M. and McCall, P. (2004) Arsenic mobility in groundwater/surface water systems in carbonate-rich Pleistocene glacial drift aquifers (Michigan). Applied Geochemistry, 19(7), 1137-55. 

Since crushed basalt has been recommended as a major backfill component (1), experiments were completed to evaluate the rate of dissolved oxygen consumption and the redox conditions that develop in basalt-water systems under conditions similar to those expected in the near-field environment of a waste package. Two approaches to this problem were used in this study (l)the As(III)/As(V) redox couple as an indirect method of monitoring Eh and (2) the measurement of dissolved oxygen levels in solutions from hydrothermal experiments as a function of time. The first approach involves oxidation state determinations on trace levels of arsenic in solution (4-5) and provides an estimate of redox conditions over restricted intervals of time, depending on reaction rates and sensitivities of the analyses. The arsenic oxidation state approach also provides data at conditions that are more reducing than in solutions with detectable levels of dissolved oxygen. 

Since rates of arsenic redox reactions are slow at room temperature (5), it is assumed that the oxidation state data represent adjustment of arsenic species to the electron activity of the solution at 300°C. A quantitative assessment of the Eh of the basalt-water system at 300°C requires high-temperature thermochemical data for aqueous arsenic species. Such data are not available and, therefore, approximations were used to calculate Eh at 300°C. 

Henry and Thorpe [14] separated monomethylarsonic acid, dimethylarsenic acid, As(III) and As(IV) on an ion exchange column from samples of pond water receiving fly ash from a coal-fired power station. They then determined these substances by differential pulse polarography. The above four arsenic species were present in non saline water systems. Moreover, a dynamic relationship exists whereby oxidation-reduction and biological methylation-dimethylation reactions provide the pathways for the intercoversions of the arsenicals. 

Lipfert, G., Sidle, W. C., Reeve, A. S., Ayuso, R. A., and Boyce, A. (2007). High arsenic concentrations and enriched sulfur and oxygen isotopes in a fractured-bedrock ground-water system. Chem. Geol. 242, 385-399. 

Reductive Dissolution. Many substances in nature contain the same metal or metalloid, but under different oxidation states. For example, the metalloid arsenic may exist as arsenite (AsIII, As03) or arsenate (AsIV, As04) in the forms of ferrous-arsenite or ferric-arsenate, respectively. Ferrous-arsenite is more soluble than ferric-arsenate for this reason, one may be interested in studying the kinetics of arsenate reduction to arsenite. Similar chemistry applies to all elements present in soil-water systems with more than one oxidation state (e.g., iron, manganese, selenium, and chromium). 

This value leads to AfG(H2AsS20"(aq)) = -244.4 kJ mol. Clarke and Helz (20(X)) analyzed phase behavior in the copper + arsenic + sulfur-i- water system and obtained a somewhat different value for the equilibrium constant for eq 11. They obtained p/f = -8.23 0.32. This value leads to AfG(H2AsS20 (aq) = -242.5 kJ-mol. The minimum uncertainty in this value, and presumably the immediately previous value is at least 1.8 kJ-mol. We took the former value that arose solely from the treatment of Eary s solubility determinations over those obtained in the mixed arsenic -i-copper aqueous sulfidic system, because of difficulties in establishing the activity of arsenic sulfide in the latter system. 

The 1996 Amendments further require EPA to establish a mechanism to identify and select new contaminants, as well as specific efforts to establish criteria for arsenic, sulfates, radon, and disinfection by-products. The SDWA required EPA to establish a list of contaminants every five years that are known or anticipated to occur in public water systems and may require further investigation and possible regulation under SDWA. The list is divided into those materials that are candidates for additional research, those that need additional occurrence data, and those that are priorities for consideration in rulemaking. The EPA then must prioritize the critical substances in each category and develop a plan of action for making regulatory decision for the most appropriate candidates. 

As(V)) and arsenite(As(III)) are the most abundant forms of arsenic (Smith et ah, 1998). In soils and water systems, As(V) is dominant under aerobic condition and As(III) under anoxic and anaerobic conditions. But, because the redox reactions between As(V) and As(III) are relatively slow, both oxidation forms are also found in soils regardless of the pH and Eh (Masscheleyn et al., 1992). Reducing soil conditions (Eh < 0 mV) greatly enhances the solubility of arsenic, and the majority of soluble arsenic is present as As(III). 

Arsenic volatilization is a dissipation mechanism to remove arsenic from soil and water systems. Many soil and water microorganisms are capable of mediating arsenic volatilization, largely in the form of methylated arsines. Under highly reduced conditions, arsenate can be transformed to trimethylarsine as shown in Figure 2. 

Anions have also been determined using conventional IMS with an FSI ion source and included arsenate, phosphate, sulfate, nitrate, nitrite, chloride, formate, and acetate. Distinct peak patterns and reduced mobility constants were observed for respective anions. Application to authentic water samples for the determination of nitrate and nitrite demonstrated the feasibility of using FSI-IMS as a rapid analytical method for monitoring nitrate and nitrite in water systems. The method was used for on-site measurement by exchanging air for nitrogen as the drift gas without complications. The linear dynamic range was 1,000, and detection limits were 10 ppb for nitrate and 40 ppb for nitrite. 

Al, Mn, Ag, and Zn. It should be noted that in January 2001, the MCL goal for arsenic (As) in drinking water was set at zero. This was a health-based initiative and was not actually enforceable. However, in February 2002, an enforceable MCL of 10 ppb was applied to community and noncommunity water systans, which are not presently subject to arsenic standards. In addition, the EPA Office of Water (www.epa.gov/ow) has stated that all water systems, nationwide, had to be fully compliant by January 2006. This extremely low level means that only ICP-MS or GFAA (under Method 200.9) methods can be used to determine arsenic, because the ICP-OES methodology (inc. Method 200.7) cannot meet the required limits of quantitation. 

As a result of soil erosion, industrial and agricultural application, arsenic and it s compounds are widely distributed through the aquatic environment [16]. Arsenic can be removed from industrial waste by several methods before the waste is discharged into the water system. Some of the methods include precipitation by calcium oxide and ferric chloride, basic anion exchange resins, passage through lime and ashes, and flocculation with chlorine saturated water and ferrous sulfate [10]. 

A major advantage of this hydride approach lies in the separation of the remaining elements of the analyte solution from the element to be determined. Because the volatile hydrides are swept out of the analyte solution, the latter can be simply diverted to waste and not sent through the plasma flame Itself. Consequently potential interference from. sample-preparation constituents and by-products is reduced to very low levels. For example, a major interference for arsenic analysis arises from ions ArCE having m/z 75,77, which have the same integral m/z value as that of As+ ions themselves. Thus, any chlorides in the analyte solution (for example, from sea water) could produce serious interference in the accurate analysis of arsenic. The option of diverting the used analyte solution away from the plasma flame facilitates accurate, sensitive analysis of isotope concentrations. Inlet systems for generation of volatile hydrides can operate continuously or batchwise. 

Metafile arsenic can be obtained by the direct smelting of the minerals arsenopyrite or loeUingite. The arsenic vapor is sublimed when these minerals are heated to about 650—700°C in the absence of air. The metal can also be prepared commercially by the reduction of arsenic trioxide with charcoal. The oxide and charcoal are mixed and placed into a horizontal steel retort jacketed with fire-brick which is then gas-fired. The reduced arsenic vapor is collected in a water-cooled condenser (5). In a process used by Bofiden Aktiebolag (6), the steel retort, heated to 700—800°C in an electric furnace, is equipped with a demountable air-cooled condenser. The off-gases are cleaned in a sembber system. The yield of metallic arsenic from the reduction of arsenic trioxide with carbon and carbon monoxide has been studied (7) and a process has been patented describing the gaseous reduction of arsenic trioxide to metal (8). 

The cmde oxide is pressure-leached in a steam-heated autoclave using water or circulating mother hquor. The arsenic trioxide dissolves, leaving behind a residue containing a high concentration of heavy metal impurities and sihca. The solution is vacuum-cooled and the crystallisation is controUed so that a coarse oxide is obtained which is removed by centrifuging. The mother hquor is recycled. The oxide (at least 99% purity) is dried and packaged in a closed system. 

---