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USGS2017

Thermodynamic properties in the Fe(II)-Fe(III)-As(V)-HClO4–H2O and Fe(II)-Fe(III)-As(V)-HCl–H2O systems from 5 to 90 °C

Fe-As mineral solubility and associated aqueous species have been intensively studied because of the environmental need to immobilize arsenic. The thermodynamic data for aqueous iron-arsenic species are inadequately characterized, however. The Gibbs free energy , enthalpy, entropy, and heat capacity and activity coefficients were refined in the Fe(II)-Fe(III)-As(V)-HClO 4 -H 2 O and Fe(II)-Fe(III)-As(V)-HCl-H 2 O systems using redox potential measurements from 5 to 90 °C. The association constants for FeHAsO 4 + and FeH 2 AsO 4 2 + at 25 °C were 10 10.28 and 10 4.02 and the corresponding association reaction enthalpies and heat capacities were 25.74 and 8.73 kJ mol − 1 and 843.1 and − 529.6 J K − 1 mol − 1 , respectively. Activity coefficients for H + , ClO 4 − , Fe 2 + , Fe 3 + , HAsO 4 2 − , and H 2 AsO 4 − at 25 °C in the form of the Hückel equation were derived for ionic strengths up to 1 mol − 1 kg − 1 . Newly derived activity coefficients and thermodynamic data were incorporated into PHREEQCI to calculate the Eh of laboratory solutions. The differences between calculated and measured Eh were all within 10 mV and relative differences were all lower than 1.5%.

Chemical Geology↗

USGS2016

Ionic molal conductivities, activity coefficients, and dissociation constants of HAsO 4 2− and H 2 AsO 4 − from 5 to 90°C and ionic strengths from 0.001 up to 3 mol kg −1 and applications in natural systems

Arsenic is known to be one of the most toxic inorganic elements, causing worldwide environmental contamination. However, many fundamental properties related to aqueous arsenic species are not well known which will inhibit our ability to understand the geochemical behavior of arsenic (e.g. speciation, transport, and solubility). Here, the electrical conductivity of Na 2 HAsO 4 solutions has been measured over the concentration range of 0.001–1 mol kg −1 and the temperature range of 5–90°C. Ionic strength and temperature-dependent equations were derived for the molal conductivity of HAsO 4 2− and H 2 AsO 4 − aqueous ions. Combined with speciation calculations and the approach used by McCleskey et al. (2012b), these equations can be used to calculate the electrical conductivities of arsenic-rich waters having a large range of effective ionic strengths (0.001–3 mol kg −1 ) and temperatures (5–90°C). Individual ion activity coefficients for HAsO 4 2− and H 2 AsO 4 − in the form of the Hückel equation were also derived using the mean salt method and the mean activity coefficients of K 2 HAsO 4 (0.001–1 mol kg −1 ) and KH 2 AsO 4 (0.001–1.3 mol kg −1 ). A check on these activity coefficients was made by calculating mean activity coefficients for Na 2 HAsO 4 and NaH 2 AsO 4 solutions and comparing them to measured values. At the same time Na-arsenate complexes were evaluated . The NaH 2 AsO 4 0 ion pair is negligible in NaH 2 AsO 4 solutions up to 1.3 mol kg −1 . The NaHAsO 4 − ion pair is important in NaHAsO 4 solutions >0.1 mol kg −1 and the formation constant of 10 0.69 was confirmed. The enthalpy, entropy, free energy and heat capacity for the second and third arsenic acid dissociation reactions were calculated from pH measurements. These properties have been incorporated into a widely used geochemical calculation code WATEQ4F and applied to natural arsenic waters. For arsenic spiked water samples from Yellowstone National Park, the mean difference between the calculated and measured conductivities have been improved from −18% to −1.0% with a standard deviation of 2.4% and the mean charge balances have been improved from 28% to 0.6% with a standard deviation of 1.5%.

Chemical Geology↗