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Informal Progress Report: Acid-Base Behavior in Hydrothermal Processing of
Wastes DE-FG07-96ER14687
A major obstacle to the development of hydrothermal technology for treating
DOE wastes has been a lack of scientific knowledge of solution chemistry,
thermodynamics and transport phenomena. Our progress over the last year is highlighted
in the following four abstracts from manuscripts which have been submitted to journals.
We also have made considerable progress on a spectroscopic study of the acid-base
equilibria of Cr(VI).
Experimental studies
We have utilized novel spectroscopic indicators to study acid-base equilibria up to
380°C. Until now, very few systems have been studied at such high temperatures,
although this information is vital for hydrothermal processing of wastes.
1.
Boric Acid Equilbria in Near-Critical and Supercritical Water
The pH values of aqueous solutions of boric acid and KOH were measured with
the optical indicator 2-naphthol at temperatures from 300 to 380°C. The equilibrium
constant Kb"
1
for the reaction B(OH)3 + OH- = B(OH)4- was determined from the pH
measurements and correlated with a modified Born model. The titration curve for the
addition of HCl to sodium borate exhibits strong acid-strong base behavior even at 350°C
and 24.1 MPa. At these conditions, aqueous solutions of sodium borate buffer the pH at
9.6 ± 0.25. submitted to hid. Eng. Chem. Res.
2.
Neutralization of Acids and Bases in Subcritical and Supercritical Water: Acetic Acid
and HCl
Acid-base titrations for the KOH-acetic acid or NH
3
-acetic acid systems were
monitored with the optical indicator 2-naphthoic acid at 350°C and 34 MPa, and those for
the
HC1
T
C1-
system with acridine at 380 °C and up to 34 MPa (5000 psia). KOH remains
a much stronger base than NH
4
OH at high temperature. From 298 K to the critical
temperature of water, the dissociation constant for HCl decreases by 13 orders of
magnitude, and thus, the basicity of Cl- becomes significant. Consequently, the addition
of NaCl to HCl raises the pH. The pH titration curves may be predicted with reasonable
accuracy from the relevant equilibrium constants and Pitzer's formulation of the Debye-
Hiickel equation for the activity coefficients.
submitted to J. Supercritical Fluids
Theoretical studies
To understand solvation in supercritical water, computer simulation studies of solution
structure and solvation free energies of ions are being carried out. The simulation results