- CONTEMPORARY HYDROGEOLOGY: THE GEORGE BURKE MAXEY MEMORIAL VOLUME
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Natural geochemical systems are com- monly interpreted within the framework of equilibrium thermodynamics with pE or Eh and pH as master variables. This approach is exemplified in the texts by Garrels and Christ , and Krauskopf There is little d o u b t that during the 's and 's the use of equilib- rium thermodynamic concepts with emphasis on the use of Eh and pH as master variables fostered considerable progress in the understanding of the geochemistry of natural waters. A severe limiting factor, however, has been the difficulty of acquiring meaningful measurements of the redox conditions of natural waters.
This work has been prompted by recent advances in the capability for analysis of As III and As V in water at concentrations lower than 1.
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Platinum is normally used as the inert electrode. A comprehensive review of the use of electrodes in geochemical redox studies is provided by Langmuir Germanov et al. For electrode measure- ments to yield meaningful data the following conditions are required: 1 the important multivalent species in solution must be electroactive, that is, they must undergo voltage-generating reactions with the metallic electrode; 2 the electrode surface must be inert, that is, it must be free of "poisoning'.
For the data to have exact meaning in the thermodynamic sense the redox species in the solution must be at equilibrium. Providing that the redox measurements are reproducible and reflect important relations with major redox species in the solution, valuable insight can be gained even when measured values are different than predicted, using equilibrium models.https://tiletasdorit.ml/planet-property.php
CONTEMPORARY HYDROGEOLOGY: THE GEORGE BURKE MAXEY MEMORIAL VOLUME
The lack of equilibrium and the need for additional information or more sophisticated theory are then made clear Stumm and Morgan, The conditions governing electrode measurements of redox potential in natural waters c o m m o n l y do not meet the three requirements indicated above. This is reflected in severely drifting electrode potential readings as a f u n c t i o n of time or as readings that arise from potentials generated by minor redox species. Spurious and uninterpretable data are c o m m o n l y acquired.
Whitfield , in a review of the capability of the electrode technique for measurement of Eh, indicates that its usefulness is restricted to a region that is bounded by the Pt-oxide and Pt-sulfide stabil- ity fields, which at a pH of 7 is the Eh interval of approximately mV. A second approach for measurement of the redox status of natural waters involves analysis of water samples for concentrations of two or more dissolved species containing the same element in different oxidation states. The con- centrations are converted to activities using the Debije-Hfickel or mean-salt relations and the pE or Eh values are obtained as indicated below.
The quantities Eh and pE are related by eq. For a system at equilibrium, computed redox levels from each of these analytical pairs have the same value. This approach to studies of pE is described by Stumm and Morgan As a supplement to electrode measurements of redox potential or to the above-mentioned redox pairs, we propose the use of As pairs that arise from the presence of As III and As V in natural waters. The remainder of this paper is directed towards a review of the thermodynamic properties of these As species in the pE--pH domain typical of natural waters and presentation of the results of preliminary laboratory studies of some of the kinetic properties of arsenic behaviour in aqueous solutions.
Half-cell reactions for these species are indicated in Table II. From this information the pE--pH diagram shown in Fig. Each line separating the domains of two species represents points of equal activities for the two species. From each line towards the interior of a domain the proportion of the species indicated for the domain increases, while the total activity of dissolved As remains constant.
In the lowest part of the pE--pH domain in which H20 is stable, concen- trations of As III in solution are limited to microgram per litre levels by the solubility of the As-sulfide minerals orpiment and realgar. There is evidence to indicate, however, that Fe II would limit the sulfide activity and hence As sulfide would not reach saturation Kanamori, The location of this pE--pH zone is shown in Fig.
In all other pE--pH zones, however, concentrations of dissolved As are not limited by the solubility of As com- pounds occurring in nature. The presence of As in water is therefore primarily dependent on the availability of As in the geologic materials in contact with the water or on input from sources of contamination. In some situations concentrations of dissolved As may be limited by adsorption. Levels of total dissolved As in natural waters are generally very low, but nevertheless appreciable relative to the detection limits of modern analytical methods.
The area within the vertical bars represents the c o m m o n p E - - pH domains for natural waters. Log concentration vs. The lettered points refer to those on Fig. For ml water samples these methods yield detection limits of 10 -2 and 5. With either of these analytical capabilities, it is possible with waters of low total As concentration, to obtain detectable concentrations for two of the As species in pE--pH domains that are often of considerable interest.
The pE or Eh range within which both As species can be detected for each specified pH condition is indicated. The width of the pE range for detection of both species, which we will refer to as the redox window for As, depends on the As detection limit and the total concentration of As in solution as indicated on Fig.
The width of the redox window increases with increasing total As values and lower limits of detection. As a further explanation of Figs. The p E - - p H diagram is the same as that in Fig. From Fig. Points B and C represent equi-concentration conditions for pH of 7 and 8, respectively. On Fig. The other two pairs of dashed lines represent the redox windows at the same detection limit 0.
The width of the redox window for each total concentration can be expanded considerably if a much lower detection limit is achieved, such as is feasible if larger water samples are collected for analysis by the method of Shaikh and Tallman or if an analytical technique with a lower detection limit, such as that described by Foreback is used. For illustrative purposes in these diagrams, a detection limit of 0. If only As V species are detected in the water, the redox level is above the As window.
The redox windows for other dissolved species that c o m m o n l y occur in natural waters are displayed in Fig. The detection limits used for calculation of these redox windows are listed in Table III. These correspond to the Winkler method limit for dissolved oxygen and c o m m o n laboratory limits for the N and C species.
The As window partially overlaps with the Fe window and occurs within the redox domain in which electrode measurements can yield useful redox values in some situa- tions. The other is obtained by calculation from measured pH values, with the assumption that it is in equilibrium with a specified solid phase. Ferric hydroxide as a solid phase exists in various forms, from meta-stable amorphous freshly precipitated forms through poorly crystallized material to the crystalline form.
The Gibbs free-energy values for these forms vary considerably, and since the computed redox equilibria of the dissolved species are dependent on the free energy of the solid phase that is assumed to exert the controlling influence on the Fe species, there is considerable uncertainty associated with the value of the pE or Eh computed from measured Fe concentrations. More detailed discussions of the pE--pH relations for Fe solid phases are presented by Hem , The position of the redox window for As is of particular interest in ground- water studies because in m a n y groundwater flow systems, most of the water has progressed sufficiently far along the redox pathway to be devoid of dis- solved oxygen and y e t not sufficiently far to exhibit detectable concentra- tions of H2S, HS-, or CH4.
In much of this domain As species may serve as a redox-level indicator for comparison to or as an alternative to redox values obtained from dissolved Fe or electrode measurements. For As to be most useful as a redox indicator it must occur in the water at concentrations that yield a useful width for the redox window. Ferguson and Anderson also noted that As is the inorganic constituent that more c o m m o n l y than any other con- stituent exceeds the maximum permissable limit for drinking water supplies. Because of the position of its redox window, As should generally be better suited as a redox indicator for use in groundwater than for surface water.
Since As normally occurs in only very small concentrations in natural waters, its speciation involves transfer of only a small number of electrons relative to the transfers that control the dominant redox pairs in the system. Therefore, if the distribution of As species attains equilibrium rapidly, the distribution will be an index of the redox level of the water. A second major requirement for As species behaviour must also be met. The rate of oxidation of As III to As V species must be sufficiently slow to allow for sample storage, preparation and analysis of the species.
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To provide at least a semi-quantitative basis for appraisal of the rates of As oxidation and reduction in aqueous solutions, the laboratory experiments described below were performed. The apparatus and methodology used for As speciation analysis in this study are described by Shaikh and Tallman Distilled deionized water was used to prepare all solutions. Please log in to get access to this content Log in Register for free. To get access to this content you need the following product:.
Springer Professional "Technik" Online-Abonnement. Donovan DJ, Katzer T, Brothers K Review of ground-water recharge estimates in Nevada with an analysis of geologic control on the recharge process. Water Resour Bull no. State of Nevada Office of the State Engineer. J Nevada Water Resour Assoc 5 1. Hardman G Nevada precipitation and acreages of land by rainfall zones.
Hardman G The precipitation of Nevada. Hardman G Nevada precipitation map. Accessed 2 February Lopes TJ, Evetts DM Ground-water pumpage and artificial recharge estimates for calendar year and average annual natural recharge and interbasin flow by hydrographic area, Nevada.
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