Brines ionic strength

Use of equation 1.5 for the relation between activity and concentration makes it possible to express these activities as concentrations. Except for high ionic strength solutions such as brines (ionic strength > 1), it is usually sufficient to approximate the activity of water as unity, thus eliminating the an2o term. The resulting equations for concentrations are ... 

Figure 7 Variation of interfacial tension between Athabasca bitumen and D20 containing Sun Tech IV (2 g/L) as a function of pH and temperature at constant ionic strength of 10 M. The dashed line represents data from reference T211 at 50 C in the absence of added surfactant or brine.

Can the species activity coefficients be calculated accurately An activity coefficient relates each dissolved species concentration to its activity. Most commonly, a modeler uses an extended form of the Debye-Hiickel equation to estimate values for the coefficients. Helgeson (1969) correlated the activity coefficients to this equation for dominantly NaCl solutions having concentrations up to 3 molal. The resulting equations are probably reliable for electrolyte solutions of general composition (i.e., those dominated by salts other than NaCl) where ionic strength is less than about 1 molal (Wolery, 1983 see Chapter 8). Calculated activity coefficients are less reliable in more concentrated solutions. As an alternative to the Debye-Hiickel method, the modeler can use virial equations (the Pitzer equations ) designed to predict activity coefficients for electrolyte brines. These equations have their own limitations, however, as discussed in Chapter 8. 

In the calculation results (Fig. 24.1), amorphous silica, calcite (CaCCF), and sepiolite precipitate as water is removed from the system. The fluid s pH and ionic strength increase with evaporation as the water evolves toward an Na-C03 brine (Fig. 24.2). The concentrations of the components Na+, K+, Cl-, and SO4- rise monotonically (Fig. 24.2), since they are not consumed by mineral precipitation. The HCO3 and Si02(aq) concentrations increase sharply but less regularly, since they are taken up in forming the minerals. The components Ca++ and Mg++ are largely consumed by the precipitation of calcite and sepiolite. Their concentrations, after a small initial rise, decrease with evaporation. 

Although the PO WW ER system can treat concentrated and dilnte aqueous wastes, treatment of dilute aqueous waste may require increased energy (however, brine disposal costs will be lower). Also, the PO WW ER system can treat a broad range of mixed aqneons waste streams, but the specific characteristics of the wastewater to be treated can affect the performance of the system. In addition, the pH and ionic strength of the waste stream, contaminant loading, nature of the contaminants, foaming, and catalytic poisons can all affect system performance. 

Previous studies of the adsorption of ions on montmorillonite have emphasized low salt concentration regions (see, e.g. ref. 1-11). Our main interest is in higher salt concentrations, because many of the brines in oil reservoirs are in this range, and because some favored locations for nuclear waste disposal, bedded salt deposits and salt domes, may result in high ionic strength environments. 

Numerous studies on the thermodynamics of calcium chloride solutions were published in the 1980s. Many of these were oriented toward verifying and expanding the Pitzer equations for determination of activity coefficients and other parameters in electrolyte solutions of high ionic strength. A review article covering much of this work is available (7). Application of Pitzer equations to the modeling of brine density as a function of composition, temperature, and pressure has been successfully carried out (8). 

Estimating the lifetimes of MSA in the aerosol particles themselves is considerably more difficult due to the problem of estimating an OH concentration in what is essentially an add brine. A measurement of k(OH + MSA) as a function of ionic strength, possibly using NaC104 as an ionic strength adjuster, could be carried out using the procedure used in this work. 

In 1988, a version of PHREEQE was written including PITZER equations for ionic strengths greater 1 mol/L thus applicable for brines or highly concentrated electrolytic solutions (PHRQPITZ, Plummer et al. 1988). PHREEQM (Appelo Postma 1994) included all options of PHREEQE and additionally a one-... 

Although naturally occurring brines and some high ionic strength contaminated waters may require the more complicated expressions developed in the Davies, SIT, or Pitzer models, the use of Equations (3.3)-(3.5) is justified for the ionic strengths of many freshwaters. 

Osmotic and Activity Coefficient Interpolation From Published Tables. Glueckauf (17) has developed equations for the osmotic and activity coefficients of single salt solutions which, when "tuned to measured data, will accurately interpolate between measured values and predict values to ionic strengths well above those usually encountered in natural brines. For brines of ionic strengths above 2 molal, the following equations for the osmotic and activity coefficients were developed (17)i... 

A comparison of mean activity coefficient splitting models and their results in the ionic strength range of interest to a brine study (1 ) quickly shows that, perhaps contrary to expectation, the second assumption is the more questionable. 

Limitations of the Model. The internal supporting routines for the otherwise completely general model restrict the apppli-cabillty of this model to sodium chloride dominated brines of ionic strengths from 2 to 6 molal. Temperature compensations were included to make the model applicable to solutions in the 10 to 35°C temperature range, approximately. 

In saline soils and soils contaminated with geothermal brines, the ionic strengths of the soil solution may exceed 0.5 M. This fact poses the necessity of using equations which have been developed to describe the activity coefficients of ions in concentrated, multicomponent electrolyte solutions. As part of a study on the chemistry of ore-forming fluids, Helgeson (50) has proposed that the true individual ion activity coefficients for ions present in small concentrations in multicomponent electrolyte solution having sodium chloride as the dominant component be approximated by a modified form of the Stokes-Robinson equation. The equation proposed is ... 

According to Helgeson (50), equation 8 can be used to estimate the individual ion activity coefficient for ions present in small concentrations in sodium chloride solutions of true ionic strength up to 3.0 M. Since saline soils and geothermal brines are often dominated by sodium chloride, it will be appropriate to use the equation proposed by Helgeson (50). Therefore, in GEOCHEM, ionic activity coefficient calculations for such systems are performed by equation 8. 

With the arrival of the Pitzer method for calculating activity coefficients at high ionic strengths (si m), research by Harvie and Weare (1980) led to computations of equilibrium mineral solubilities for brines. They could calculate... 

Introduction actinide solubilities in reference waters. In this section, the environmental chemistry of the actinides is examined in more detail by considering three different geochemical environments. Compositions of groundwater from these environments are described in Tables 5 and 6. These include (i) low-ionic-strength reducing waters from crystalline rocks at nuclear waste research sites in Sweden (ii) oxic water from the J-13 well at Yucca Mountain, Nevada, the site of a proposed repository for high-level nuclear waste in tuffaceous rocks and (iii) reference brines associated with the WIPP, a repository for TRU in... 

Because of the high ionic strength of the brines, the calculations were carried out using a Pitzer ion interaction model (US DOE, 1996) for the activity coefficients of the aqueous species (Pitzer, 1987, 2000). Pitzer parameters for the dominant non-radioactive species present in WIPP brines are summarized in Harvie and Weare (1980), Harvie et al. (1984), Felmy and Weare (1986), and Pitzer (1987, 2000). For the actinide species, the Pitzer parameters that were used are summarized in the WIPP Compliance Certification Application (CCA) (US DOE, 1996). Actinide interactions with the inorganic ions H, Na, K, Mg, CU, and HCO /COa were considered. 

Luckscheiter and Kienzler (2001) examined uranyl sorption onto corroded HEW glass simulant in deionized water, 5.5 M NaCl and 5.0 M MgCl2, and found that sorption was greatly inhibited by the magnesium-rich brine, while the NaCl brine had little effect. Uranyl sorption at high ionic strength was also studied by Vodrias and Means (1993), who examined uranyl sorption... 

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