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1
SATURATION OF SEAWATER WITH RESPECT TO THE
OTAVITE-CALCITE SOLID SOLUTION
J. Carneiro, M. Prieto, and H. Stoll
Departamento de Geología, Universidad de Oviedo,
C/ Jesús Arias de Velasco, s/n, 33005 Oviedo,
Spain
1. Introduction
4. Results
In the oceans, the vertical distribution of
dissolved cadmium (Cd) generally exhibits a
nutrient-type profile (Chester, 2003). Therefore,
it has been considered that its depletion in the
surface waters is regulated by biogeochemical
processes (Chester, 2003 Abe, 2001). An example
of this profile is shown in Fig. 1. On the other
hand, experiments on the sorption of Cd2 to
calcite and/or aragonite surfaces (Tesoriero and
Pankow, 1996 Prieto et al., 2003 Cubillas et
al., 2005) have shown the efficiency of these
minerals in the removal of Cd from freshwater
solutions. It has been demonstrated that if one
of these phases is present, partition of Cd2 to
the solid phase will always occur, even if Cd is
only present at trace levels in solution
(Tesoriero and Pankow, 1996).
As can be observed in Fig. 2, the calculated O
indicate that, for this range of depths, seawater
is supersaturated with respect to pure calcite
(Ogt1) and very subsaturated with respect to pure
otavite (Olt1). Computing the O(x) function,
with x varying from 0 to 1, we can observe that
the values of maximum supersaturation (Omax)
obtained for the different depths do not exactly
coincide with pure calcite (Fig. 4(a)), but to a
solid composition that includes a very small
fraction of Cd2 ions substituting for Ca2 in
the mineral structure. With depth, this value is
enhanced as the content of Cd2 in solution
increases. As an example, in Fig. 3 is shown the
supersaturation of seawater at 20 m of depth, in
relation to the entire compositional range of the
solid solution. In addition, we have calculated
values the distribution coefficients of Cd2
between the solid and the aqueous phase,
corresponding to the Omax at each depth. These
values varied from 3981, at 20 m, to 4677, at 175
m.
Fig. 2. Comparison between the saturation state
of seawater with respect to calcite (O Calcite)
and to otavite (O Otavite), for the range of
depths.
Fig. 1. Vertical distribution of dissolved Cd in
the western equatorial Pacific (lat0º,
long145ºE). Samples collected in January 1999
(Source Abe, 2001).
Fig. 3. Supersaturation of seawater, at 20 m, in
relation to the entire compositional range of the
(Cd,Ca)CO3 solid solution. Inset graph shows the
supersaturation maximum, close to the calcite
endmember.
2. Objective
The aim of this research is to study the sorption
of Cd onto calcite, as a process of inorganic
removal of Cd from solution, considering the
formation of (Cd,Ca)CO3 solid solutions in
seawater. In this study, prior to any
experiment, we have calculated the saturation
state of seawater with respect to otavite-calcite
solid solutions.
3. Methods
To perform our calculations, we have used data
collected in the Equatorial Pacific (lat0º,
long-140º), at depths ranging from 20 to 175
meters. These data include the parameters
temperature, pressure and salinity (Murray and
Wheeler, 1998), total dissolved inorganic carbon
and total alkalinity (Goyet, 1998), and the
concentration of Cd (Martin, 1994). Such values
have been input into the computer code PHREEQC
(Parkhurst and Appelo, 2003), with the purpose of
calculating the activities of the ions Cd2, Ca2
and CO32-, as well as the saturation state of
seawater with respect to pure calcite and pure
otavite. In order to calculate the saturation
state (O) of the an aqueous solution with respect
to all the possible solid solution compositions,
we used the stoichiometric supersaturation
function (Prieto et al., 1993). Considering the
(Cdx,Ca1-x)CO3 solid solution a(Cd2),
a(Ca2) and a(CO32-) ? activities of the ions in
the aqueous phase KCdCO3 and KCaCO3 ?
equilibrium solubility products of otavite and
calcite, respectively ?CdCO3 and ?CaCO3 ?
activity coefficients of these endmembers in the
solid phase x XCdCO3 ? composition of the
solid solution given by the mole fraction of
otavite in the solid phase.
6. References
Abe, Kazuo (2001). Mar. Chem. 74,
197-211. Chester, R. (2003). Marine Geochemistry.
Blackwell Publishing, London. pp. 506. Cubillas,
P. Kohler, S. Prieto, M. Causserand, C.
Oelker E. H. (2005). Geochim. Cosmochim. Acta
69, 5459-5476. Goyet, Catherine. Total Carbon
Dioxide and Total Alkalinity. United States
JGOFS Data Server. Woods Hole Institution, USA
U.S. JGOFS Data Management Office, iPub 23
December 1998. Accessed 22 March 2006.
http//usjgofs.whoi.edu/jg/serv/jgofs/eqpac/tt008/
tco2.html1 Martin, John. Dissolved Trace Metals
filtered at 0.4um. United Sates JGOFS Data
Server. Woods Hole Institution, USA U.S. JGOFS
Data Management Office, iPub 13 September 1994.
Accessed 22 March 2006. http//usjgofs.whoi.
edu/jg/serv/jgofs/eqpac/tt008/trace_metals_d.html0
Murray, Jim Wheeler, Pat. Merged bottle and
Nutrient Data. United States JGOFS Data Server.
Woods Hole Oceanographic Institution, USA U.S.
JGOFS Data Management Office, iPub 23 December
1998. Accessed 22 March 2006.
http//usjgofs.whoi.edu/jg/serv/jgofs/eqpac/tt008/
bottle_merged.html0 Parkhurst, D. L. Appelo, C.
A. J. (2003) Users Guide to PHREEQC (Version 2)
? A Computer Program for Speciation,
Batch-Reaction, One-Dimensional Transport, and
Inverse Geochemical Calculations. U.S. Geological
Survey Water-Resources Investigations Report
99-4259, 310 p. Prieto M., Putnis A. and
Fernández-Díaz L. (1993). Geol. Mag. 130,
289-299. Prieto, M. Cubillas, P.
Fernández-Gonzalez, A. (2003). Geochim.
Cosmochim. Acta 67, 3859-3869. Sherrell, R. M.
Boyle, E. A. (1992). Earth Planet. Sci. Lett.
111, 155-174. Tesoriero A. J. and Pankow J. F.
(1996). Geochim. Cosmochim. Acta 60, 1053-1063.