Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Predominance of Aqueous Tl(I) Species in the River System Downstream from the Abandoned Carnoulès Mine (Southern France)”
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included in The Geochemist’s Workbench11 and in Chess,12 that we have used to calculate the diagram in Figure 2 of our article.2 • The full speciation method uses more CPU but is more precise than the previous one. It consists in calculating a complete geochemical speciation in every significant point of the Eh-pH domain in order to detect the predominant species. With such a method, sodium hydroxide, hydrochloric acid and dioxygen concentrations are used to modify pH and Eh just like a natural reaction path. This method is implemented in Phreeplot13 that uses Phreeqc.14 Although the first method is useful to give a rapid idea of geochemical behavior of simple systems, it is not precise enough for systems containing more than two or three components. Here, the Eh-pH diagram of Tl in Amous DC solution (Figure A1) has been calculated using the second
e thank the Editor for the opportunity to respond to perceptive comments by Smeaton et al.1 regarding our 2 paper. Their comments deal with (1) the calculation of the t h e o r e t i c a l so lu b i l i t y p r o d u c t s f o r d o r a l l c h a r i t e (TlFe3(SO4)2(OH)6), lanmuchangite (TlAl(SO4)2·12H2O) and lorandite (TlAsS2) and (2) the proposed stability field for dorallcharite in the Eh-pH diagram of Tl. Although the conclusions given in the paper are impacted neither by the equilibrium constants, nor by the calculated diagram, this comment gives us the opportunity to give more details on calculations and to correct the Eh-pH diagram. We have used the same methodology than Smeaton et al.1 to obtain the equilibrium constants given in Table 1 in our paper.2 They have an opposite sign relatively to the corresponding solubility products because reactions are written for precipitation whereas solubility products account for dissolution reactions. These equilibrium constants have been obtained using the following values of Gibbs free energy of formation ΔGf0 (25 °C, 1 bar) of aqueous species (in kJ mol−1): −32.40 for Tl+,3,4 −237.14 for H2O,5 −744.00 for SO42‑,5 12.243 for HS−,5 −16.28 for Fe3+,6 −487.64 for Al3+,7 −587.14 for H2AsO3−8 and −639.775 for H3AsO3(aq).9 However, different values of ΔGf0 exist in the literature: −237.18 for H2O,4 −744.53 for SO42−,4 12.05 for HS−,4 −487.628 or −489.404 for Al3+, −587.66 for H2AsO3−10 and −640.03 for H3AsO3(aq).10 The differences in the values of the calculated equilibrium constants probably come from different choices of ΔGf0 values for aqueous species. The reaction corresponding to the formation of lorandite in Table 1 of our paper2 is a combination of the acid base reaction 1 and the dissolution reaction written in the comment (eq 5);1 using the solubility product given by Smeaton et al.,1 the logK value for reaction in Table 1 of our paper2 is logK = −(−28.9)−(−9.17) = 38.07, which is closed to our logK value = 38.25. + H3AsO3(aq) ↔ H2AsO− 3 +H
Figure A1. Eh-pH diagram of Thallium: concentrations used in the PHREEQC simulation are detailed in the text.
method with the same database converted into Phreeqc-format and with mean concentrations (mol kg−1) at station Amous DC (ref 24 of ref 2): 5.33e-3 for C, 2.45e-3 for Ca, 1.00e-3 for Mg, 26e-6 for K, 0.17e-3 for Na, 0.17e-3 for Cl, 4.4e-9 for Tl, 7.9e-4 for S, 1e-6 for Fe, 4e-7 for As, 2.6e-6 for Al, Eh = 0.4 V, pH = 8.0. In this new diagram, the stability domain of dorallcharite evidenced in Figure 2 of our paper2 is absent. Field Eh and pH data are still located in the stability domain of Tl+. Nevertheless, this diagram has to be taken as a first trial to represent the global thallium chemistry of Amous natural water using the upto-date thermodynamic data from Xiong.3,15 Undoubtedly, more studies including the experimental determination of Tl
(1)
As underlined by Smeaton et al.,1 Figure 2 of our paper2 exhibits an abnormal stability field for Thallium minerals. It does not come from thermodynamic constants as suggested by Smeaton et al.1 but from the numerical method used to calculate the diagram. Two methods are available to calculate Eh-pH diagram: • The mosaic diagrams method is fast but imprecise. It consists in an analysis of the thermodynamic database to find, for each element of the studied system, the frontiers of equal activities between two chemical compounds. This defines a mosaic of zones of activity-predominant species for each element, which can be integrated to obtain the final Eh-pH diagram. However, aqueous complexation between elements is not always taken into account, neither any activity correction. This method is © 2012 American Chemical Society
Published: January 5, 2012 2475
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Environmental Science & Technology
Correspondence/Rebuttal
compounds stability constants have to be carried out to better understand Thallium behavior in natural environment.
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Corinne Casiot Marion Egal Odile Bruneel Neelam Verma Marc Parmentier Françoise Elbaz-Poulichet REFERENCES
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