Modeling Waite et al. 1994 in Phreeqc

["Frank-Andreas Weber" <Frank-Andreas.Weber@xxxxxxxxxxxx>]

Dear Mr. Parkhurst,

I am having problems in modeling Uranyl sorption to ferrihydrite as it is
laid out in Waite et al. (1994) Geochimica et Cosmochimica Acta. In this
paper they define the mass-action law for the formation of a bidentate
(=FeO)2-UO2 complex with an exponent of 1 for the =FeOH, that is

[(=Fe-O)2UO2] * {H+}² / [=FeOH] (power1!!) * {UO2+2} = K * exp(2*F*Psi/RT)

but in the mass-balance equation 2 mole =FeOH are consumed per mole of
(=FeO)2-UO2 formed.

In the Phreeqc user guide I have found that you have already considered this
case. Under the topic SURFACE_SPIECES it says:
"In PHREEQC version 1, the -no_check option was included to permit the
stoichiometry of a species to be defined separately from the mass-action
equation. Specifically, the sorption of uranium on iron oxides as described
by Waite and others (1994) provides an example, where they use different
coefficients in the mass-action equation than the mole-balance equations.
However, activity of a surface species is defined as mole fraction of sites
occupied by the species in PHREEQC version 2 which is inconsistent with
activity that is defined as molality by Waite and others (1994) and PHREEQC
version 1. It is noted that formulas with coefficients of only 1 in the
mass-action-equation will give identical results for PHREEQC version 1 and
2."

Am I correct that as in the mass action equation only the H+ activity has an
exponent of 2, Phreeqc should provide similar results to Waite et al.? I set
it up as follows but I run into problems that the total H balance of the
system is distorted. I would be glad if you have any suggestions.

SURFACE_SPECIES
Hfo_wOH + UO2+2 = (Hfo_wO)2UO2 + 2H+
    log_k     -6.06
    -no_check
    -mole_balance (Hfo_wO)2UO2

Best regards,
Frank-Andreas Weber
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  Dipl.-Ing. Frank-Andreas Weber
  Environmental Fluid Mechanics Lab
  Department of Civil & Environmental Engineering
  Stanford University
  Stanford, CA  94305-4020

  work phone (650) 723-1825
  home phone (650) 566-1379
  Frank-Andreas.Weber@xxxxxxxxxxxx
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Attachment: U Sorption to Ferrihydrite Waite et al 1994.pqi
Description: Binary data