Redox potential groundwater

Fig. 3. Simulations calculated with the PHREEQC geochemical code (Parkhust Appelo 1999) (a) time-dependent diagram for the pH evolution of the Aspo ground water/bentonite interaction (b) time-dependent diagram for the pe evolution of the Aspo groundwater/bentonite interaction. Curves correspond to different initial partial oxygen pressures. Initial calcite and pyrite contents are 0.3 wt% and 0.01 wt% respectively, except for the curve of log/02 = —0.22 where calcite and pyrite contents are 1.4 wt% and 0.3 wt%, respectively, pe calculated stands for the cases where the oxygen fugacity is obtained from the groundwater redox potential (Bruno et at. 1999).

Other factors affecting performance include the presence of toxic material, the redox potential, salinity of the groundwater, light intensity, hydraulic conductivity of the soil, and osmotic potential. The rate of biological treatment is higher for more permeable soils or aquifers. Bioremediation is not applicable to soils with very low permeability, because it would take a long time for the cleanup process unless many more wells were installed, thus raising the cost. 

Grenthe, I., W. Stumm, M. Laaksuharju, A. C. Nilsson, and P. Wikberg (1992), "Redox Potentials and Redox Reactions in Deep Groundwater Systems", submitted to Chemical Geology. 

Soil Boesten et al. (1992) investigated the transformation of [ C]l,2-dichloropropane under laboratory conditions of three subsoils collected from the Netherlands (Wassenaar low-humic sand, Kibbelveen peat, Noord-Sleen humic sand podsoil). The groundwater saturated soils were incubated in the dark at 9.5-10.5 °C. In the Wassenaar soil, no transformation of 1,2-dichloropropane was observed after 156 d of incubation. After 608 and 712 d, however, >90% degraded to nonhalogenated volatile compounds, which were detected in the headspace above the soil. These investigators postulated that these compounds can be propylene and propane in a ratio of 8 1. Degradation of 1,2-dichloropropane in the Kibbelveen peat and Noord-Sleen humic sand podsoil was not observed, possibly because the soil redox potentials in both soils (50-180 and 650-670 mV, respectively) were higher than the redox potential in the Wassenaar soil (10-20 mV). 

In situ redox manipulation (ISRM) is an in situ, groundwater remediation technology for manipulating the oxidation-reduction (redox) potential of an unconfined aquifer to immobilize inorganic contaminants (metals, inorganic ions, and radionuclides) and to destroy organic contaminants (primarily chlorinated hydrocarbons). 

In this chapter we will pay most attention to the isolation function of the innermost of the barriers, the waste matrix, and its potential interactions with the contacting water. In addition, and because of the similarities in the processes involved, we will also discuss the key processes that control the mobility of some of the critical components of waste in ground-waters. These key processes are bentonite/ groundwater interactions, which can exert a large influence on the processes controlling the master pH/pe variables, iron corrosion processes responsible for poising the redox potential of the system and the interactions between the waste matrix itself and the contacting fluids, which produce radiolysis reaction processes. 

The Redox Potential. The groundwaters at great depths in igneous rocks are essentially free from dissolved oxygen ( ). The redox potential is determined and buffered by the presence of redox couples, mainly Fe(111)/Fe(11). For the reaction... 

Considerably higher when contaminated with salt water Related to the redox potential 0 in aerated groundwater Ionic strength at equilibrium... 

The groundwater transport of radionuclides through waterbearing interbed layers in the Columbia River basalt formation will be controlled by reactions of the radionuclides with groundwater and interbed solids. These interactions must be understood to predict possible migration of radionuclides from a proposed radioactive waste repository in basalt. Precipitation and sorption on interbed solids are the principle reactions that retard radionuclide movement in the interbeds. The objective of the work described herein was to determine the sorption and desorption behavior of radionuclides important to safety assessment of a high-level radioactive waste repository in Columbia River basalt. The effects of groundwater composition, redox potential, radionuclide concentration, and temperature on these reactions were determined. 

Effects of Groundwater Composition and Eh. Radionuclide sorption on geologic solids is dependent on the chemical composition of the groundwater solution and the redox potential (Eh) of the solid-groundwater system. Aquifers at various depths in the Columbia Plateau formation have -been observed to have significant differences in composition. To accurately model radionuclide migration, it is necessary to understand the effects of chemical components and Eh on sorption and solubility of key radionuclides. An additional benefit of this work is to better understand the mechanisms of sorption and desorption of the radionuclides. 

Oxidation-reduction reactions (redox reactions) determine the chemical fate of many contaminants in groundwater and process water. Measuring the ORP (redox potential) enables us to evaluate the mobility and reactivity of non-metallic elements (sulfur, nitrogen, carbon) and metals in process water and to assess the types of redox reactions that take place in groundwater. We also use the ORP measurement as a well stabilization indicator in groundwater sampling. 

In groundwater, the range between lines a and b is interesting. At typical pH values of groundwaters (pH = 6-8), oxidizing (aerobic +400-500 mV), anoxic, and reducing (anaerobic below -100 mV) atmospheres are usually separated on the basis of redox potential. 

Plants affect the water balance of a site they change the redox potential and pH, and stimulate microbial activity of the soil (Trapp and Karlson, 2001). These indirect influences may accelerate degradation in the root zone or reduce leaching of compounds to groundwater. Compounds taken up into plants may be metabolized, accumulated, or volatilized into air. 

From this a regression equation can be derived which hnks log [C/N] to the minimum redox potential required to sustain the corresponding metabolic oxidation as the oxidant must still persist in soil or groundwater, namely ... 

Table 2.21 Sequestering ligands and minimal (external, ambient) redox potentials required to produce them from ambient reactants still present in soil or groundwater...

Grenthe I., Stumm W., Laaksuhaiju M., Nilsson A. C., and Wikberg P. (1992) Redox potentials and redox reactions in deep groundwater systems. Chem. Geol. 98, 131-150. 

Example 8.15. Solubility of Fe(OH)3 from Redox Potential Data in Deep Groundwaters Figure 8.18 gives measured redox potentials and Fe data (from Grenthe et al., 1992). These latter data were obtained from analysis of dissolved [Fe(II)] and corrected for complex formation with carbonate (Fe -I- C03 = FeC03(aq) log K = 5.56, I = 0). Assuming that the measured redox potential refers to the Fe(II)/Fe(III) system, calculate the solubility constant K for the reaction... 

Figure 8.18. Measured redox potentials in a deep groundwater. Experimental values of the measured redox potentials (recalculated to the standard hydrogen electrode scale) versus (3pH + log[Fe ]). The concentration of [Fe J has been obtained from the analytical determinations by correction for the complex formation with carbonate. The notation refers to the different test sites. The full-drawn line has been calculated using the selected value of the standard potential E. The straight line has the theoretical Nemstian slope of +0.056 V, at the temperature of measurements. (Adapted from Grenthe et al., 1992.)...

In the absence of oxygen, with extreme care, a reasonably reliable E can be measured, for example, in groundwater (see Example 8.15 and Figure 8.18). But even in the case mentioned (Grenthe et al., 1992) it took more than 20 days to establish a redox potential of the Fe(OH)3(s)-Fe(II) system of that groundwater. For a recent review on redox potential measurements in the laboratory and in the field see Grenthe et al. (1992) and Lindberg and Runnels (1984). 

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