AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
MIUR-2002
RESEARCH PROJECT
Submitted 30-04-2002
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF
CHROMIUM: ENVIRONMENTAL IMPLICATIONS
Table of contents
Premises ................................................................................................................................... 4
Atomic properties ...................................................................................................................... 5
Toxicology................................................................................................................................ 5
Main aqueous complexes of chromium ....................................................................................... 5
Structure-energy of Cr(III) inorganic species ............................................................................... 9
Structure-energy of Cr(VI) inorganic species..............................................................................12
Structure-energy of Cr organic species .......................................................................................15
Cr geochemical baselines: examples for Liguria Region from the ANPA-CNR project..................17
Distribution and speciation of Cr in natural groundwaters............................................................18
Distribution and speciation of Cr in drainage waters from mining areas hosted in ophiolitic rocks..20
Cr isotopes in nature .................................................................................................................21
Ab-initio Thermochemical properties of Cr complexes................................................................22
Cr isotopic fractionation............................................................................................................24
References ...............................................................................................................................27
1
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
List of figures
Figure 1: Eh-pH diagram of Chromium after Brookins................................................................. 8
Figure 2: structure of the gaseous molecule [Cr(H2 O) 6 ] 3+ . (B3LYP/6-31G).................................. 9
Figure 3 : structure of the bridged dihydroxylate [(H2O) 4 Cr(OH)2Cr(H 2 O) 4 ] 4+ .............................10
Figure 4: aqueous polymeric species of Cr(III) ...........................................................................11
Figure 5: structure of CrO 2 - based on the data of Serebrennikov and Mal tsev[ .............................12
Figure 6: Structure of HCrO 4 - e Cr2O7 2- clusters based on the data of Brito et al. and Martin Zarza et
al.[92] ........................................................................................................................................13
Figure 7: Reaction scheme of Brauer and Wetterhahn .................................................................15
Figure 8: EPR spectrum of Cr(V) species formed by reaction of dichromate with glutathione . ......15
Figure 9: Distribution of total Chromium on Ligurian region in the stream sediments . Extracted
from Archivio Geochimico Nazionale . ......................................................................................17
Figure 10: Distribution of total Chromium in groundwaters of Ligurian region.. Extracted from
Archivio Geochimico Nazionale. ...............................................................................................18
Figure 11: Chromium isotopic fractionation in equilibrium between Cr hexa-aqua ion and chromate
ion...........................................................................................................................................26
2
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
List of tables
Table 1: Gibbs free energy of formation (∆G¡f), enthalpy of formation (∆H¡f), entropy (S¡) and
volume (V¡) of the main aqueous complexes of chromium (hypothetical one molal
solution referred to infinite dilution condition, T=298.15 K and P=1 bar. ...................... 5
Table 2: reaction constants among Cr(VI) aqueous complexes at T=298.15 K and P=1 bar. (*) =
T=293 K..................................................................................................................... 7
Table 3: reaction constants among Cr(III) aqueous complexes at T=298.15 K and P=1 bar. ........... 7
Table 4: Total (electronic + zero point) energy (hartree) and geometrical factors of the [Cr(H2O) 6 ] 3+
- H2O cluster according to Pappalardo et al. (1996) [84] compared with the results of our
UHF computations for the isolated gaseous molecule [Cr(H2O) 6 ] 3+- (B3LYP/6-31G)....... 9
Table 5: Conditional deprotonation constants of Cr(III) oligomers according to Stunzi and Marty
and Beutler (T = 25 ¡C; I = 1 M NaClO4 )....................................................................10
Table 6: Structure-energy of Cr(VI) aqueous complexes..............................................................13
Table 7: Kinetic constants for the catalyzed hydrolysis of dichromate . In the upper part of the table
are listed acidic kinetic constants (KA) and, in the lower part of it, the basic kinetic
constants (KB) p-Br-PhSH = p-bromothyolphenol; n-BuSH = n-buthyilthyole; Et3 N =
diethylheter................................................................................................................17
Table 8: Cr isotopic composition of underground waters of Arizona, North Carolina, Washington
State, Arab Emirates and Giordany after Ball and Bassett. Delta isotope is referred to the
NIST SRM 979 standard.............................................................................................21
Table 9: Coefficients of fractionation equation............................................................................26
3
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
Premises
Aqueous complexes of Cr(VI) have been the object of attentive investigation mainly due to
their toxicity and their role in mutagenesis [27,28].
In spite of the extreme toxicity of Cr (VI), knowledge of the normal abundances and
reactivity of Cr (VI) in the various geological matrixes is still poor. Moreover, to our
knowledge, no systematic investigation has been until now carried out about the isotopic
characterization of the Cr aqueous complexes. As already shown by[100] on pure theoretical
ground, it may be anticipated that reduction of Cr(VI) aqueous complexes by inorganic or
organic donors may result in conspicuous isotopic fractionation at T,P ambient conditions.
Also, oxidative processes which may take place locally in Cr-rich aquifers could sensibly
alter the normal Cr isotopic composition. Recognition of the isotopic imprinting operated by
natural redox agents may be carried out with ab-initio systematization of energy and
vibrational properties of Cr aqueous complexes Application to natural observations may be
carried out either in terms of Eh-pH predominance fields of the various complexes and of
computed isotopic fractionation factors.
Our geographical location is particularly suited for the proposed investigation.
1) In the great urban area of Genova there is the biggest industrial production of hexavalent
chromium in Europe (Stoppani Industry, Cogoleto). The industrial activity resulted in
intensive contamination of geological matrixes. Isotopic characterization of produced
chemicals and exhaust will be of great help in deciphering the potential utilization of
isotopic Cr imprinting in anthropic pollution recognition campaigns.
2) In the La Spezia area there are both a source of Cr(III), represented by the ultramafic
rocks variably affected by serpentinization, and different electron acceptors (Mn oxides,
H2O2 , gaseous O 2, and perhaps Fe(III) oxyhydroxides) potentially able to promote
oxidation of Cr(III) to Cr(VI). Based on this evidence and on the absence of anthropic
Cr sources, the relatively high Cr(VI) concentrations observed in the waters of the study
area was attributed to natural pollution. Fantoni et al. [101] investigated the fate of Cr
during water-rock interaction through reaction path modeling, carried out by means of
the software package EQ3/6 version 7.2b [36,68,102,103]. The adopted model is an extension
of that proposed by Bruni et al. [104,105] for explaining the distribution of major
components in natural waters interacting with serpentines. It turned out that aqueous
Cr(VI) persists stable in Mg-HCO3 and Ca-HCO3 waters, due to the relatively oxidizing
conditions of the shallow environments in which these waters circulate. Only the
electron donors encountered at depth, e.g., Fe(II) and organic matter, determine the
reduction of Cr(VI) to Cr(III), which is chiefly sequestrated by precipitating hydroxides.
Simulations also indicate that the amounts of Cr incorporated in montmorillonites and
saponites are completely negligible under all the investigated PO 2-PCO2-temperature
conditions.
3) Leaching of Cr from ophiolitic rocks is enhanced under acidic conditions, such as those
typically found in abandoned mining areas where pyrite is present. A case history of this
kind is that of Libiola. This is an abandoned mine district located on the Ligurian coast
not far from Sestri Levante, extensively investigated for alteration mineralogy[106,107] and
water chemistry[106-112]. The speciation and the isotopic composition of chromium were
not taken into account yet, although Cr concentrations up to 1,600 ppb were measured in
acid mine waters.
4
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
Atomic properties
II-transition element, Cr-subgroup. Atomic number = 24; atomic weight = 51.996 ; density
=7.19 g/cm 3. Electronic configuration = [Ar]3d54s1. Cosmic abundance (normalized to 106
Si atoms)= 1.24×10 4. Stable isotopes [1,2]: 50Cr (4.352±0.024); 52Cr (83.764±0.036); 53Cr
(9.509±0.027); 54Cr (2.375±0.018). Radiogenic isotopes [1,3,4]: 51Cr (t1/2= 27.8 d; observed at
ultra-trace level in meteorites). Ionization potentials (eV) [5]: I1=6.764, I2=19.49, I3=31,
I4=(51), I 5=73, I6=90.6. Pauling electronegativity=1.6 eV [5]. Sanderson elettronegativity
=1.88 [6]. Free ion polarizability[6]: Cr¡=6.8—16.6 3; Cr+=1.141—1.16 3; Cr2+=(0.50) 3;
Cr3+ =(0.33) 3 ; Cr 6+ =0.084 3 . Pauling s univalent radii in VI-fold coordination [7]:
Cr3+ =0.63 ; Cr6+ =0.52 . Non-polar covalent radius [8]: Cr =1.17 .
Toxicology
Cr(VI) compounds, whenever adsorbed, are eliminated by kidney, producing degeneration
of renal tubule and, in some cases, nephrosis[113-115]. A single ingestion of 5 g K2Cr2O7 is
lethal[15,16] as 1-2 g of CrO3 and 6-8 g of soluble chromates[17].
It is well known that thioester and glutathion complexes of hexavalent chromium play a
fundamental role in mutagenesis and associated pathologies[27,28].
According to experimental evidences Cr(VI) does not appear to react in vitro with DNA,
while some light reactivity is observed for Cr(III) with the formation of DNA-Cr complexes.
Based on the above observations Connett and Wetterhahn [39] proposed the capture-reduction
model to explain the carcirogenecity of Cr(VI). Cr(III) aqueous complexes with solvent
molecules H2 O (octahedral symmetry) are not readily transported through the cellular
membrane, while transportation is much easier for the smaller Cr(VI)- O 2- complexes. Once
inside cells the organic Cr(VI) complexes are progressively reduced by intracellular
components. The generation of reduced Cr(III) (which forms stable complexes with
DNA[40]), takes place in a stepwise fashion, through species at intermediate redox state (i.e.
Cr(V) — Cr(IV) and finally (Cr(III)) capable of damaging the cellular DNA.
Main aqueous complexes of chromium
Table 1 resumes standard state (T=298.15, P=1 bar; hypothetical one molal solution referred
to infinite dilution condition) Gibbs free energy of formation from the elements (∆G¡f),
enthalpy of formation from the elements (∆H¡f), entropy (S¡) and volume (V¡) of the main
aqueous complexes of chromium. These data will be compared with thermodynamic
properties obtained by ab-initio computation in this study. Data in brackets are estimated; i.s.
and o.s. specify the coordination field: i.s.=inner sphere; o.s.=outer sphere.
Table 1: Gibbs free energy of formation (∆G¡f ), enthalpy of formation (∆H¡f ), entropy (S¡) and volume
(V¡) of the main aqueous complexes of chromium (hypothetical one molal solution referred to infinite
dilution condition , T=298.15 K and P=1 bar.
______________________________________________________________________________________
∆ G¡ f(kJ/mole)
∆ H¡ f(kJ/mole) S¡(J/mole_K) __ V¡(cm 3/mole) Ref.
Species
______________________________________________________________________________________
CrO43-737.339
[11]
CrO42-727.765
-881.150
50.208
20.4
[9]
-727.76
[10]
HCrO4-764.835
-878.222
184.096
44.0
[9]
-764.71
[10]
Cr2O72-1301.224
-1490.341
261.918
73.0
[9]
-1301.10
[10]
H2CrO4
-760.6
-841
293
[11]
5
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
CrO3Cl HCrPO72H2CrPO 7Cr2+
Cr(H2O)63+
Cr3+
Cr(H2O)5(OH)2+
Cr(OH)2+ (i.s.)
Cr(H2O)4(OH)2+
Cr(OH)2+ (i.s.)
Cr(OH)2+
Cr(OH)4Cr2(OH)24+
Cr3(OH)45+
Cr(H2O)5Cl2+
CrO3Cl CrCl2+ (i.s.)
CrCl2+ (o.s.)
Cr(H2O)4Cl2+
CrCl2+ (i.s.)
Cr(H2O)5Br2+
CrBr2+ (i.s.)
Cr(H2O)5SCN2+
CrSCN2+ (i.s.)
CrO2CrC4H4O4¡
CrC5H3O4¡
CrC6H8O4¡
Cr(COO)2¡
Cr(COO)2+
CrCH2(COO)2¡
-664.8
-1660.6
-1675.7
-146
~-1617.5
~-194.5
- 215.48
-1595.8
-410.0
-409.425
-430.95
-1556
-607
-632.62
-803.876
~-987
-987.404
-835.632
-1487.433
-1507.5
-664.8
-321.7
-325.5
-1407.1
-458.6
-1469.0
-283.3
-1305.4
-119.7
-535.55
-868.151
-861.557
-844.101
-865.552
-917.823
-877.083
-736
-754.8
205
-143.5
~-1954
~-238
(-100)
~100
~-317
-1912
-481
~176
~-176
-1879
~-134
~146
-1247
-109
-1816
-754.8
-385
146
205
-205
-1678
-531
-1768.6
-339.3
-1600.4
-171.1
~217
~-63
173.6
-175.7
~205
~-146
-1058.209
-1083.932
-1098.794
-984.989
-1062.569
-1034.824
80.7
138.5
136.8
44.8
-105.4
52.7
[11]
[11]
[11]
[11]
[11]
[11]
[12]
[11]
[11]
[66]
[13]
[11]
[11]
[14]
[66]
[11]
[66]
[66]
[66]
[11]
[11]
[11]
[11]
[11]
[11]
[11]
[11]
[11]
[11]
[14]
[26]
[26]
[26]
[26]
[26]
[26]
Equilibrium between chromate CrO42 —and dichromate Cr2O72- ion has been the object of
intensive studies in the last 20 years[57-59] . Moreover, the existence of species H2CrO4 ,
HCrO4-, HCr2O7- although postulated since long time [58,60,61] is still contradictory[62]. Michel
et al. [57,59] based on Raman investigations of chromate, dichromate and Cl-chromate
deduced that HCrO4- does not exist in aqueous solutions of Cr(VI). Actually, by examining
the Raman spectra of Cr(VI) solutions at different molal concentration the Authors showed
that neither HCrO 4-, nor H2CrO4 or HCr2O7- should be present. We may note to this purpose
that the equilibrium between chromate and dichromate ion is almost invariantly described in
terms of the acid-base partial reactions
H + + CrO42 − ⇔ HCrO4−
(1)
2 HCrO4− ⇔ Cr2O72 − + H2O
(2)
Neglecting the existence of HCrO4-, the equilibrium between chromate and dichromate
species may be written as
2CrO42 − + 2 H + ⇔ Cr2O72 − + H2O
(3)
6
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
which is simply the summation of the two partial equilibria 1-2. Based on equilibrium 3 the
relative predominance of the two species are dictated by the acidity of the system. At pH=11
all chromium would thus be present in solution as CrO42 — while at pH=1.2 Cr2O72 — would
be virtually the unique species. Based on experiments conducted at T = 293 K and KNO3 =
0.8M [57,59] the conditional equilibrium constant logK3 = 13.77±0.02 may be estimated.
More recently, Raman spectroscopic evidences and other experimental methods (EMF
galvanic cell measurements, endothermic calorimetry, electron spectroscopy, nuclear
magnetic resonance, etc.) have been critically evaluated by Brito et al. (1997) [36] which also
performed ab-initio computations of Cr(VI) gaseous species.
Table 2 shows the various reaction constants among Cr(VI) aqueous complexes according to
various sources and Table 3 those relative to Cr(III) species. The listed values will be
compared with the results of this study, obtained from ab-initio computation of gaseous
molecules.
Table 2: reaction constants among Cr(VI) aqueous complexes at T=298.15 K and P=1 bar. (*) = T=293 K
_____________________________________________________________________
∆G¡ reaction (kJ/mole) lnK
Reaction
Ref.
_____________________________________________________________________________
H + + CrO42 − ⇔ HCrO4−
H + + HCrO4− ⇔ H2 CrO40
H + + Cr2O72 − ⇔ HCr2O7−
2 H + + 2CrO42 − ⇔ Cr2O72 − + H2O
2 H + + CrO42 − ⇔ H2 CrO40
3 H + + 2CrO42 − ⇔ HCr2O7− + H2O
2 HCrO4− ⇔ Cr2O72 − + H2O
-33.723
-37.071
13.603
14.954
[36]
[9]
-9.414
4.235
3.797
-1.708
[36]
[9,11]
-10.460
4.219
[36]
-79.161
-82.872
-78.615*
31.932
33.429
31.71*
[36]
[9]
[57,59]
-43.137
-32.835
17.401
13.245
[36]
[9,11]
88.282
-36.118
[36]
-11.715
4.726
[36]
-8.732
3.522
[9]
_____________________________________________________________________________
Table 3: reaction constants among Cr(III) aqueous complexes at T=298.15 K and P=1 bar.
____________________________________________________________________________
∆ G¡ reaction (kJ/mole)
Reaction
lnK
Ref.
____________________________________________________________________________
Cr 3 + + H2O ⇔ Cr(OH )
2+
+ H+
Cr( H2O)6 ⇔ Cr( H2O)5 (OH )
+ H+
20.382
21.678
21.708
21.700
-3.57
-8.744
-8.757
-8.753
[64]
[11]
[12,13]
[11]
56.178
61.856
57.216
61.500
-22.662
-24.952
-23.080
-24.808
[64]
[11]
[12,14]
[11]
Cr 3 + + 3 H2O ⇔ Cr(OH )3 + 3 H +
92.431
-37.285
[64]
Cr 3 + + 4 H2O ⇔ Cr(OH )4 + 4 H +
157.859
-63.678
[64]
3+
2+
Cr 3 + + 2 H2O ⇔ Cr(OH )2 + 2 H +
+
Cr( H2O)6 ⇔ Cr( H2O)4 (OH )2 + 2 H +
3+
+
0
−
7
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
156.212
-63.013
[11]
2Cr 3 + + 2 H2O ⇔ Cr2 (OH )2 + 2 H +
-131.597
53.084
[64]
Cr 3 + + 3 H2O ⇔ Cr(OH )3( solido ) + 3 H +
53.381
-21.533
[65]
4+
______________________________________________________________________
We may note in Table 3 a fairly good agreement for Cr(III) solute species. We may note also
how reactions may be explicated either with the metal in its ionic form or (more correctly) as
a solvated aqua-ion.
Values in bold in Table 1 have been adopted by Brookins [56] to depict the Eh-pH diagram of
Figure 1.
Figure 1: Eh-pH diagram of Chromium after Brookins[56]
The relative predominance fields of Figure 1 deserve some cautionary comment.
In basic solutions with pH>6 , the main aqueous species is the tetrahedral ion CrO42- which
causes a yellowish color to the solution. Though not shown in Figure 1, in the pH range
6>pH>2, HCrO4- and dichromate ion Cr 2O72- (orange-reddish color in solution) coexist [36]
and at pH<1 the main solute species is H2CrO4. Equilibria among protonated species may
be represented in the form:
H + + CrO42 − ⇔ HCrO4−
(4)
+
−
H + HCrO4 ⇔ H2 CrO4
(5)
[36]
with pK values respectively 5.9 and 0.6 according to . Dichromate formation at the
expenses of HCrO4- :
2 HCrO4− ⇔ Cr2O72 − + H2O
(6)
has a dimerization constant K6 = 159 at standard conditions[36].
8
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
Structure-energy of Cr(III) inorganic species
The structure of hexa-aqua-chromium [Cr(H2O) 6 ]3+ has been optimized by Pappalardo et al.
(1996)[84] with the self-consistent field procedure (SCRF). This procedure is a quantummechanical extension of the Kirkwood - Onsager[85] approach devoted to quantify the effect
of the solvent (seen as a dielectric continuum) on the charge distribution (hence on energy)
of the aqueous ion, conceived as immersed in a spherical cavity surrounded by solvent
molecules. This computation, carried out with the automated procedure GAUSSIAN-92[86],
lead to the energies and geometries of Table 4. On the same table are listed energy and
geometries obtained in this study by Unrestricted Hartree-Fock calculations conducted with
the GAUSSIAN-98 release[116]. Figure 2 shows the conformation of the cluster, on the basis
of the optimized geometry.
Table 4: Total (electronic + zero point) energy (hartree) and geometrical factors of the [Cr(H 2O)6]3+ - H2O
cluster according to Pappalardo et al. (1996) [84] compared with the results of our UHF computations for the
isolated gaseous molecule [Cr(H 2O)6]3+- (B3LYP-6.31G).
_______________________________________________
3+
2
6
2
_________________________________________________________________
ET (a.u.)
d
O-H
()
H-O-H (¡)
d
Cr-O
()
[Cr(H O) ]
HO
Ref.
-1498.320
-1501.549
0.967
0.980
107.54
≈107.
2.050
1.945
-76.010
[84]
PESTO, unpubl.
[84]
PESTO, unpubl.
[84]
PESTO, unpubl.
[84]
PESTO, unpubl.
0.957
104.5
_______________________________________________
Figure 2: structure of the gaseous molecule [Cr(H2O)6]3+ . (B3LYP-6.31G).
9
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
Table 5 lists some conditional deprotonation constants of Cr(III) polymers according to
various sources [65,83] . The constants are expressed in terms of molal concentration, i.e.
(H ) × ( X )
= − log
( HX )
ν+
+
pK
I
a
(7)
ν +1
terms in brackets denoting the molality obtained by alkalimetric titration in one-molal
NaClO4 solutions at T =298.15 K:
pKaI →
Cr( H2O)6 ⇔ Cr( H2O)5 (OH )
pKaII →
Cr( H2O)5 (OH )
3+
2+
2+
+ H + (8)
⇔ Cr( H2O)4 (OH )2 + H + (9)
+
Table 5: Conditional deprotonation constants of Cr(III) oligomers according to Stunzi and Marty[65] and
Beutler[83] (T = 25 ¡C; I = 1 M NaClO 4)
____________________________________________________
monomer
Cr3+
dimer
Cr2(OH)24+
trimer
Cr3(OH)45+
tetramer
Cr4(OH)66+
Ref.
4.29
4.30
6.1 ± 0.1
6.08
6.0 ± 0.1
3.68
3.80
6.04
4.35
2.55 ± 0.06
[65]
5.63
5.75
5.08
[65]
____________________________________________________________________________________________
pKaI
pKa
I
pKa
I
[83]
[83]
[65]
___________________________________________________
As anticipated by Niels Bjerrum (1908) in his masterpiece [74] , Cr(III) is seldom present in
monomeric form and a vast number of polynuclear species form in partially neutralized
Cr(III) solutions. The best characterized species are represented by the bridged
monohydroxylate [ (H2O)5Cr(OH)Cr(H2O)5]5+ and the bridged dihydroxylate
[(H2O)4Cr(OH)2Cr(H2O)4]4+ complex whose structure is shown in Figure 3.
Figure 3 : structure of the bridged dihydroxylate [(H2O)4Cr(OH)2Cr(H2O)4]4+ according to ref. [65]
Structure and reactive properties of dimeric Cr(III) species have been the object of attentive
studies. Some data concerning polynuclear species are also available in literature, although
information is less detailed [82], (Figure 4)
10
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
Figure 4: aqueous polymeric species of Cr(III) according to ref. [65]
For the energy of the trimeric species from monomers Finholt et al.[87] assign a conditional
constant at T=298.15 K, P= 1 ranging from 2.9×10-8 and 76×10-8. The corresponding Gibbs
free energy of reaction ∆G¡ is comprised between +34.9 e +43.0 kJ/mole. This last figure is
fairly good agreement with what obtainable from the thermodynamic data compilation of
Table 1 [11,66] (+44.8 kJ/mole).
The tetrameric isomers 3a, 3b of Figure 4 have been shown to convert with extremely fast
kinetics at environmental conditions (less than 1 s [81] ). In acidic solution tetrameric species
decompose by detachment of bridging bonds with a stepward release of [Cr(H 2O)6]3+
monomers according to the following kinetics ( [H + ]=1 M and T=298.15 K [65] ):
= 3h
1/ 2
Cr4 (OH )6  
→ Cr 3 + + Cr3 (OH )4
6+
t
5+
= 21g
1/ 2
Cr3 (OH )4  
→ Cr 3 + + Cr2 (OH )
5+
t
= 7g
1/ 2
→ 2Cr 3 +
Cr2 (OH )  
5+
t
5+
(10.1)
(10.2)
(10.3)
According to St nzi and Marty[81] the individual polymerization constants:
are K11.1≅10
3..3
Cr(OH )
2+
+ Cr(OH )
⇔ Cr2 (OH )2
(11.1)
Cr(OH )
2+
+ Cr2 (OH ) ⇔ Cr3 (OH )
(11.2)
Cr(OH )
2+
+ Cr3 (OH )
2+
3+
3
4+
5
4+
5+
4
6+
6
⇔ Cr4 (OH )
(11.3)
, K11.2≅10 4.5, K11.3≅10 3.5. Based on the suggested values the Gibbs free
11
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
energy of reaction of equilibrium 11.1 should be -18.3 kJ/mole, not far from the value
obtainable from the thermodynamic data compilation of Table 1 (-15.6 kJ/mole[11,66]).
Chromite ion CrO2- has a relatively simple (although poorly defined) structure, with a
dihedral angle variable between 105¡ [88] ,112¡ [89] and 128±5¡ [90] according to the various
sources. Interatomic distances are not known, but are known vibrational frequencies (also
variable from Author to Author; i.e. ν1=895-960; ν2=220±20; ν3=965-978; see Table 1 in
Wenthold et al. [91] for more details). Figure 5 shows the cluster CrO2- with a dihedral angle
105¡.
Figure 5: structure of CrO2- based on the data of Serebrennikov and Mal tsev[88]
Structure-energy of Cr(VI) inorganic species
Aqueous complexes of Cr(VI) have been the object of attentive investigation mainly due to
their toxicity and their role in mutagenesis and associated diseases[27,28]. Raman and Infrared
spectroscopic investigations indicate that aqueous CrO42- monomer ha tetrahedral symmetry
T d with four vibrational modes: ν 1(A 1) = 847cm-1, ν2(E) = 348cm-1, ν 3 (T2) = 884cm-1,
ν4(T2) = 368cm-1[97]. Dimeric complexes maintain tetrahedral symmetry (although slightly
distorted) in the monomeric units, which are linked by a bridging oxygen (Figure 6).
12
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
Figure 6: Structure of HCrO4- e Cr2O72- clusters based on the data of Brito et al.[36] and Martin Zarza et al.[92]
Table 6 shows the details of bond distances and dihedral angles of the various complexes
according to various authors. The total electronic energy of the gaseous molecule is also
listed. Most data refers to the results of ab-initio computations.
Table 6: Structure-energy of Cr/VI) aqueous complexes. * Cr-bridging O distances.
___________________________________________________________________________________________
Species
d Cr-O
d Cr-OH
dihedral angles
Energy (a.u.)
Ref.
______________________________
O-Cr-O
Cr-O-H
Cr-O-Cr
___________________________________________________________________________________________
CrO421.591
tetrahedral
-309.0614
[36]
1.604
tetrahedral
[93]
tetrahedral
-1327.9192
[94]
tetrahedral
-1329.9148
[94-experim]
1.66
tetrahedral
[93]
HCrO4-
1.513
1.537
1.853
1.794
106.6-111.9
H2CrO40
1.434
1.745
108.7-110.7
Cr2O72-
1.56-1.64 1.76-1.81
122-139
119.5
133.51
-309.81
[36]
[92]
-310.3292
[36]
[95-experim]
HCr2O7-
1.485
1.795
106.5-112
123.1
162.6
-544.1636
[36]
1.628-1.912*
[36]
___________________________________________________________________________________________
Concerning the optimized energies, the comparison with existing reaction constants (Table
2) indicate some problems for the chromate complex H2CrO40 while energies are consistent
for the deprotonated species. The lack of data does not allow to establish the consistency for
the dichromate species HCr2O7-.
In extremely acidic media and at high bulk Cr(VI) concentration, polychromates are formed
according to the equilibria[62]
13
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
3Cr2O72 − + 2 H + ⇔ 2Cr3O102− + H2O
(12)
4Cr3O102− + 2 H + ⇔ 3Cr4O132− + H2O
(13)
A comparison of the various interatomic Cr-O distances in Cr(VI) complexes has lead to the
following generalizations [96]:
a) the bond length of terminal bonds and of the inner bonds of tetrahedral groups do
increase with the polymerization of the complex
b) the non-bridging bonds shorten in the same sequence.
The formation of polymeric Cr(VI) species has been quantified by Brito et al.[36] in the
following terms:
( p−2q)
pH + + qCrO4− ⇔ H p (CrO4 )q
(14)
with the following mass balances:
B = b + ∑ qCpq
BZ = ∑ pCpq = H − h + Kw h
−1
(15)
(16)
where:
H= total analytical concentration of H+ (mole/L)
B =total analytical concentration of metal (mole/L)
h = equilibrium concentration of H+
b = equilibrium concentration of CrO42Cpq= equilibrium concentration of pq complex
Kw= water dissociation constant
Z= mean H+ bond number per monomer CrO42- .
14
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
Structure-energy of Cr organic species
Brauer and Wetterhahn [41] showed that Cr(VI) (solubilized in the initial form K2Cr2O7)
forms a thiolate complex with γ-Glutathion ( γ-Glutamylcysteinglycine) according to the
reaction scheme of Figure 7
Cr2O72 − + GSH ⇔ GSCrO3− + HCrO4−
(17)
Figure 7: Reaction scheme of Brauer and Wetterhahn [41]
Figure 8: EPR spectrum of Cr(V) species formed by reaction of dichromate with glutathione. (from Brauer and Wetterhahn
(1991)[41] .
The data of Brauer and Wetterhahn [41] in EPR spectroscopy show the existence of two
distinct glutathione complexes, one with g=1.973 and the second with g=1.987 .
The first paramagnetic complex of Cr(V) has a [Cr=O]3+ square pyramidal unit with three
15
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
oxygens coordinated by oxo, aquo, carboxilate or carboxyl ligands and a group thiolated by
a glutathione molecule[41]. The second complex is composed of a [Cr=O]3+ unit with squarepyramidal structure formed by two thiole molecules bonded in a bidentate fashion through
cystenil-sulfohydroxilate and amidic groups of glutathione [42,43]
According to the Authors the initial form of Cr(VI) is important in determining the bulk
reactivity. In fact there is no experimental evidence of thioesters formation stemming from
CrO42- or HCrO4-. The reason of this fact is not perfectly understood but, according to the
Authors it has to do with the lability (sic.) of the HCrO 4- complex, which should
decompose rapidly according to the scheme:
2 HCrO4− ⇔ Cr2O72 − + H2O
(18)
−
2−
+
(19)
HCrO4 ⇔ CrO4 + H
HCrO4- according to the above equilibria should favor equilibrium 1 by mass action effect.
According to Mazurek et al. [44] interaction of thioles RSH (R = n-butil, t-butil, pbromophenyl, p-nitrophenyl and phenyl) with dichromate results in the formation of
thioester anionic groups of Cr(VI) ([RSCrO3]-) which are slowly but progressively
decomposed by reducing processes with final formation of Cr(III) and disulphydes.
Reactivity tests with dymethylformamide (DMF) show that the initial reaction between
thioles and Cr2O7- is a hydrolytic process catalyzed by bases (B) or acids (A) followed by
substitution of HCrO4- with thiole. Arylthioles catalyze hydrolysis more efficaciously (and
substitute HCrO4- more rapidly) with respect to alkylthioles. Arylthioesters of Cr(VI)
moreover, decompose more rapidly than alkylthiol derivatives, with formation of Cr(V) in
the intermediate steps of the process. The overall reduction reaction of dichromate from
thioles results in formation of H2O according to the reaction scheme:
Cr2O72 − + 2 RSH ⇔ 2 RSCrO3− + H2O
(20)
Reaction kinetics are represented as coupled first-order kinetic equilibria:
A ," RSH "
(21)
→ 2 HCrO4−
H2O + Cr2O72 − K
K4
−
−
(22)
RSH + HCrO4 → RSCrO3 + H2O
" RSH "
−
(23)
RSCrO3  → final products
The first step is actually composed of three distinct partial reactions with different kinetics,
with the first one rate-determining. Denoting A- the acidic group:
A , slow
→ HCrO4− + CrO3 + A−
(24)
HA + Cr2O72 − K
rfast
−
+
(25)
CrO3 + H2O  → HCrO4 + H
fast
+
−
(26)
H + A → HA
[44]
Eventual base-catalyzed reactions may be represented as :
B , slow
B + Cr2O72 − K
→ CrO42 − + BCrO3
(27)
fast
−
+
(28)
BCrO3 + H2O → HCrO4 + BH
fast
−
2−
+
(29)
HCrO4 + B → CrO4 + BH
16
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
Table 7 resumes reaction kinetics in H 2 O and DMF media (second order constants)
according to Mazurek et al. (1990) [44] (KA in Table 7 is in practice the value obtained
dividing the pseudo-first-order constant by the catalyst concentration).
Based on the existing data the acidic catalysis is a generalized phenomenon whose kinetics
increase with acid strength [45] (see also Table 7).
Table 7: Kinetic constants for the catalyzed hydrolysis of dichromate [44,45] . In the upper part of the table
are listed acidic kinetic constants (K A) and, in the lower part of it, the basic kinetic constants (KB) p-BrPhSH = p-bromothyolphenol; n-BuSH = n-buthyilthyole; Et 3N = diethylheter
______________________________________________
Solvent
Additive
_____________________________
DMF [44]
H2O[45]
______________________________________________
NH3
OH-
17
1250
670
3,5-Lutidine
0.019
2,6-Lutidine
0.6
Et3 N
<10-4
H2 O
1.6×10-5
1.2×10-3
______________________________________________
HCOOH
26
p-Br-PhSH
2.0
n-BuSH
0.16
______________________________________________
Cr geochemical baselines: examples for Liguria Region from the ANPA-CNR project
Figure 9: Distribution of total Chromium on Ligurian region in the stream sediments- Extracted from Archivio
Geochimico Nazionale .
17
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
Figure 10: Distribution of total Chromium in groundwaters of Ligurian region. - Extracted from Archivio Geochimico
Nazionale.
Distribution and speciation of Cr in natural groundwaters
The distribution of total Cr in natural groundwaters was investigated by Barnes and
Langmuir[117]. They found total Cr thresholds, corresponding to the 97.7 percentile, of 10-19
ppb for 647 groundwaters associated with carbonate rocks, sandstones (including quartzites,
arkoses, greywackes, and conglomerates), shales (comprising clays, siltstones, and slates),
and felsic to intermediate igneous and meta-igneous rocks. A total Cr threshold of 32 ppb
was found, instead, for 35 groundwaters coming from mafic and ultramafic igneous and
meta-igneous rocks. These distinct thresholds in the aqueous phase reflect the different
concentrations in rocks. In fact, the worldwide average Cr contents of peridotites and
basalts are, with 1800 and 185 ppm, respectively, much higher than those of limestones, 11
ppm, granites, 22 ppm, sandstones, 35 ppm, and shales, 90 ppm [118]. However, these data
are insufficient to define the natural (background) concentrations of Cr(VI) and Cr(III) in
waters, a fact of utmost importance due to their different toxicity. Concentrations of Cr(VI)
up to 12 ppb and Cr(III) up to 11 ppb were recently found in groundwaters interacting with
ultramafites[119], indicating that these lithotypes may cause natural pollution of waters not
only in Cr(III) but also in Cr(VI).
Ultramafic rocks, usually affected by extensive serpentinization, are present in the La
Spezia province and high Cr concentrations in some groundwaters interacting with these
lithotypes, and used as drinking waters, were detected by the local Environmental
Protection Agency. A systematic study of these waters, aimed at defining the distribution
18
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
and fate of both Cr(III) and Cr(VI) in these hydrogeological circuits was recently carried out
by the PESTO Group[101].
It was found that 30 of the 58 sampled groundwaters have Mg-HCO3 to Ca-HCO3
composition, undetectable Cr(III) contents, and virtually equal concentrations of total
dissolved Cr and Cr(VI). Therefore, dissolved Cr is present in toto as Cr(VI), with
concentrations of 5 - 73 ppb. These values are above the maximum permissible level for
drinking waters, 5 ppb.
According to Robles-Camacho and Armienta[119] the chromite edges, mostly made up of
magnetite, are the first to dissolve. Chromium release could also derive from dissolution of
the magnetite which is typically present at the edges of olivine and pyroxenes, as well as
between serpentine fibers. These Cr-rich solid phases are abundant in ophiolites, especially
serpentinites and ultramafites, present in the La Spezia area and their dissolution evidently
releases comparatively high amounts of Cr to waters. However, since Cr is present as
Cr(III) in rock-forming minerals, its release to the aqueous solution requires oxidation of
Cr(III) to Cr(VI).
In principle, the possible electron acceptors include Mn oxides, Fe(III) oxyhydroxides,
H2O2, dissolved O2, and gaseous O 2. Mn oxides are able to oxidize Cr(III), as expressed by
the following reaction:
Cr(OH)2+ + 1.5 MnO2 = HCrO4- +1.5 Mn2+,
(30)
which proceeds relatively fast[120-124]. According to Fendorf[125], the Mn oxides are the only
possible electron acceptors involved in Cr(III) oxidation, but the picture might be more
complicated. The possible role of Fe(III) oxyhydroxides, which are produced through
magnetite decomposition, deserves further investigation. Hydrogen peroxide is a strong
oxidant and the rate of the H2O 2-driven Cr(III) oxidation to Cr(VI) is fast[126]. For a H2O2
concentration of 26 × 10-6 moles, which is a reasonable value for rain waters of oceanic
regions[127], a pH of 6, a common value for rain waters not affected by strongly acid gases
and suspended solid particles, and a temperature 15¡C, the half-life of the Cr(III) oxidation
to Cr(VI) is approx. 4.6 days, based on the relationship by Pettine and Millero[126]. The role
of dissolved O2 is probably subordinate. In fact, studies on the oxidation kinetics of Cr(III)
to Cr(VI) driven by dissolved O 2 in sea water have shown that this is a rather sluggish
process, which cannot explain the high Cr(VI) concentrations in sea water [128-131]. Gaseous
O2 of the atmosphere could oxidize the trivalent Cr of Cr-rich spinels upon forest burning, a
rather frequent event in the study area. This natural process mimics the high-temperature,
O2-driven oxidation of chromite to sodium chromate:
2 FeCr2O4 + 4 Na2CO3 + 7/2 O2 = 4 Na2CrO4 + Fe2O3 + 4 CO2,
(31)
which is carried out in industrial chemical plants for chromate production.
In the La Spezia area there are both a source of Cr(III), represented by the ultramafic rocks
variably affected by serpentinization, and different electron acceptors (Mn oxides, H2O2,
gaseous O 2, and perhaps Fe(III) oxyhydroxides) potentially able to promote oxidation of
Cr(III) to Cr(VI). Based on this evidence and on the absence of anthropic Cr sources, the
relatively high Cr(VI) concentrations observed in the waters of the study area was attributed
to natural pollution.
Fantoni et al. [101] investigated also the fate of Cr during water-rock interaction through
reaction path modeling, carried out by means of the software package EQ3/6 version
7.2b[102-103]. The adopted model is an extension of that proposed by Bruni et al. [104-105] for
19
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
explaining the distribution of major components in natural waters interacting with
serpentinites. It turned out that aqueous Cr(VI) persists stable in Mg-HCO3 and Ca-HCO3
waters, due to the relatively oxidizing conditions of the shallow environments in which
these waters circulate. Only the electron donors encountered at depth, e.g., Fe(II) and
organic matter, determine the reduction of Cr(VI) to Cr(III), which is chiefly sequestrated
by precipitating hydroxides. Simulations also indicate that the amounts of Cr incorporated
in montmorillonites and saponites are completely negligible under all the investigated PO2PCO2-temperature conditions.
Distribution and speciation of Cr in drainage waters from mining areas hosted in
ophiolitic rocks
Leaching of Cr from ophiolitic rocks is enhanced under acidic conditions, such as those
typically found in abandoned mining areas where pyrite is present. A case history of this
kind is that of Libiola, which was extensively investigated for alteration mineralogy [106,107]
and water chemistry[106-112]. The speciation and the isotopic composition of chromium were
not taken into account yet, although Cr concentrations up to 1,600 ppb were measured in
acid mine waters.
The sulfide ore deposit of Libiola is located into the Gromolo basin, ∼8 km NE of Sestri
Levante. It is one of the numerous mineralizations associated with the Apennine
ophiolites[132,133]. These ore deposits were originally generated by convective circulation of
seawater through hot rocks at spreading ridges and underwent subsequent metamorphic and
tectonic processes during the Apennine orogenesis[134]. The Libiola ore deposits was made
up of massive lens-shaped bodies concordant with pillow basalts and disseminated
mineralizations, constituted by small aggregates of sulfides, either filling vesicular cavities
or concordant with the textures of pillows. Also the serpentinites tectonically overlying the
basalts host small irregular mineralized veins[133]. Due to a long-lasting mining activity a
considerable amount of wastes were produced. These constitute heaps without vegetation
and covering a total area of ∼500,000 m2. Mine wastes are made up of poorly sorted, either
sterile or mineralized rock fragments, chiefly serpentinites and basalts, and secondary
mineral phases produced by weathering, mainly Fe oxyhydroxides brown-reddish-orange in
color. Locally these Fe oxyhydroxides form a hard surface layer which favors high-angle
stratification of waste piles.
Magnesium and SO4 are the prevailing cation and anion, respectively, not only in mine
waters but also in river waters downstream of the mine discharges. The abundance of SO 4 is
evidently due to oxidative dissolution of pyrite, whereas the dominance of Mg reflects
dissolution of serpentinites and other ophiolitic rocks.
Two well distinct groups of waters are found at Libiola, the red waters and the blue waters,
whose names reflect the color of the solid phases deposited. Red waters have low pH (2.42.8) and high Eh (∼600 mV) whereas blue waters have close to neutral pH (7.0-7.5) and
lower Eh values (150-200 mV). Red waters have very high concentrations of sulfate (3500 9600 ppm) and dissolved metals, especially Fe (100-1000 ppm), Al (20-200 ppm), Cu (20180 ppm), and Zn (20-50 ppm). Blue waters are poorer in metals than red waters, and
sulfate attains 1400 - 2200 ppm. Red waters are discharged by the two longest and lowest
tunnels, called Ida and Castagna, whereas blue waters emerge from the third longest gallery,
named Margherita. All these sites have rather constant flow rate throughout the year[109],
20
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
consistent with their location close to the intersection of the piezometric surface with the
topographic surface.
Most water discharges located at higher altitudes into the mine area are, instead, strongly
affected by fluctuations of the water table and some of them are active after the rainy
periods only. They exhibit remarkable changes in physical and chemical characteristics with
time[109]. For example, a pH variation from 3.8 to 6.2, accompanied by a marked decrease in
dissolved Fe, Cu, and Al and by a change in color of the solid precipitates, from red to blue,
was observed for one of these water emissions[107].
The irreversible mass exchanges occurring during both interaction of meteoric waters with
mine-spoil materials and mixing between acid mine waters and local groundwaters were
simulated by means of the software package EQ3/6 by Marini et al. [112], to understand the
role of these processes. Comparison of theoretical results with analytical data suggests that
the main process governing the chemistry of all the mine waters of Libiola is variable
interaction of meteoric waters with mine tailings, while addition of local groundwaters to
acid mine waters produces colloidal precipitates which remain in suspension in river waters.
Although precipitation of these minerals may scavenge some metals, hexavalent chromium,
if present, is expected to remain in the aqueous phase and may thus determine further
environmental damages to the aquatic ecosystems.
Cr isotopes in nature
Contrarily to what commonly assumed until few years ago, the Cr isotopic composition of
natural waters varies from site to site. Table 8 shows for instance some analytical results
reported by Ball and Bassett[135]. We believe that good part of the delta per mil deviation
could be attributed to anthropic pollution. In fact, because the last step in the industrial
process for the production of chromic acid takes place at sufficiently low T (i.e. below
400¡C) and the conversion from the initial form Cr(III) is not complete, one could expect for
the industrial reagents some peculiar isotopic imprinting. The initially altered Cr isotopic
composition could be further modified during complexation and transport in aqueous
environment. Finally, after assimilation by living organisms of Cr(VI), formation of organic
complexes and their following reduction could lead to further modifications of the Cr
isotopic composition.
Table 8: Cr isotopic composition of underground waters of Arizona, North Carolina, Washington State,
Arab Emirates and Giordany after Ball and Bassett[135]. Delta isotope is referred to the NIST SRM 979
standard
__________________________________________________________________________
53
Sample location
Cr/ 52Cr
standard
δ53Cr
Mean value
deviation
(per mil)
__________________________________________________________________________
Abu Dhabi, well LWS-1
0.11314
0.00007
+0.6
Abu Dhabi, well LWS-1
0.11308
0.00005
+0.1
Western Arizona, well RAN-10
0.11283
0.00007
-2.1
Maqarun, Giordania, spring M15
0.11341
0.00005
+3.0
Elizabeth City, NC, well
0.11350
0.00004
+3.8
Hughes, Tucson, AZ, well E-12
0.11296
0.00003
-1.0
Hanford, WA, wells K22+K37
0.11308
0.00004
+0.1
Pantex, TX, well 0794-4
0.11341
0.00006
+3.0
Elizabeth City, NC, well ML-21-5
0.11328
0.00005
+1.9
Elizabeth City, NC, well ML-21-7
0.11396
0.00005
+7.9
Elizabeth City, NC, well ML-21-8
0.11368
0.00004
+5.4
Elizabeth City, NC, well ML-31
0.11368
0.00006
+5.4
Elizabeth City, NC, well ML-34
0.11374
0.00010
+5.9
21
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
___________________________________________________________________________
Ab-initio Thermochemical properties of Cr complexes
Ab-initio procedures allow to determine (besides zero point electronic energy and structure)
the thermodynamic properties from vibrational analysis and the kinetic reactivity of the
various complexes of interest through transition state theory. From the partition function that
we indicate here as Q(V,T) (to render explicit that this is a function of variables P and V one
may in fact compute entropy S, internal energy U and isochoric heat capacity CV by
applying[98] :
 Q(V , T ) 
 ∂ ln Q 
S = nk + nk ln

 + nkT 
 ∂T  V
 n 
 ∂ ln Q 
U = nkT 2 

 ∂T  V
 ∂U 
CV = 

 ∂T  n, V
(31)
(32)
(33)
with n = number of molecules.
Restricting the computation to a single molecule and splitting the partition function in its
N
; N A k = R , with R= gas constant):
various contributions one has ( n =
NA
 ∂ ln Q 
S = R + R ln(Q(V , T )) + RT 
 =
 ∂T  V
 ∂ ln Q 
= R ln[Q(V , T ) ⋅ e] + RT 
 =
 ∂T  V
(34)

 ∂ ln Q  
= R ln(Qtrans × Qrot × Qvib × Qe × e) + T 

 ∂T  V 

(when adding the first two terms the identity ln e = 1 → R = R ln e has been utilized[99]
For the translational term the partition function assumes the value [98] :
3
2πkT  2

Qtrans =
V
(35)
 h2 
with h = Planck constant and k = Boltzmann constant. The partial derivative of Qtrans on T
is
3
 ∂ ln Qtrans 
(36)

 =
 ∂T  V 2T
Because in the computational approach gaseous molecules are imagined as isolated entities
(perfect gas) one poses [99]:
 N
PV = nRT =   × N A kT
(37)
 NA 
kT
that, with N=1 bring to V =
(38)
P
22
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
Equation 35 may be now converted to:
3
2πkT  2 kT

Qtrans =
 h2  P
(39)
For the general case of non-linear molecules the rotational term has form[99] :
3
1

T 2
π 2
Qrot =
(40)

1 
σ rot  (Θ rot , x × Θ rot , y × Θ rot , z ) 2 
h2
with Θ rot = 2 ; I = moment of inertia and σ rot= symmetry factor. His partial derivative
8π Ik
on T is:
3
 ∂ ln Qrot 
(41)

 =
 ∂T  V 2T
In the harmonic approximation the vibrational partition function has form [71]
exp( − Xi 2)
(42)
Qvib = ∏
i 1 − exp( − Xi )
with:
hν
Xi = i
(43)
kT
being νi the ith vibrational frequency
The contribution of electronic motion is [99] :
Qe = ω 0
(44)
 ∂ ln Qe 
(45)

 =0
 ∂T  V
with ω0 = electron spin multiplicity.
By combining opportunely the various contributions to the partition function and their
partial derivatives on T [99] one obtains entropy, internal energy and isochoric heat capacity:
3
S = Strans + Srot + Svib + Se = R ln Qtrans + 1 +  +

2
(46)
3
X
R ln Qrot +  + R∑  Xi i − ln 1 − e − Xi  + R ln Qe




2
e −1
i
3
1
1 
U = Utrans + Urot + Uvib = RT + RT + R∑ Xi T  + − Xi

2
2 e − 1
i
2
3
Xi 
Xi 
CV = CV , trans + CV , rot + CV , vib = R + R + R∑ e
(48)
 e − X i − 1
2
i
Once U is known, enthalpy is readily obtained by applying:
(
H = U + PV = U + kT
)
(47)
(49)
and the Gibbs free energy is also obtained applying:
G = H − TS
(50)
23
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
Cr isotopic fractionation
Isotopic fractionations among the various Cr aqueous species may be obtained from
vibrational calculations conducted stemming from the optimized structures.
At high T (low Xi), Qvib reduces to ∏ (1 Xi ) . Because the potential energy of isotopomers
i
is similar, one may define a separative effect based on the ratio of the partition functions of
heavy and light isotopomers ( Q• , Qo respectively) in such a way that the translational and
rotational contributions cancel out:
 s• 
 s •   Q•   m o 
=
f
 o
 o  o  • 
s 
 s  Q  m 
32
[
[
]
]
•
•
Xi• exp( − Xi 2) 1 − exp( − Xi )
=∏ o
o
o
i Xi exp( − Xi 2 ) 1 − exp( − Xi )
(51)
 s• 
The symmetry numbers ratio  o  in (51) reflects the probability of forming symmetric or
s 
 mo 
asymmetric molecules and  •  are the masses of molecules undergoing isotopic
m 
exchange. Denoting ∆Xi the frequency shift observed passing from heavy to light molecules
(i.e. ∆Xi = Xio − Xi• ), because ∆Xi is intrinsically positive the above equation may be
rewritten as:
{
[
]}
1 − exp −( Xi• + ∆Xi )
 s• 
Xi•
exp( ∆Xi 2)
 o f = ∏ •
s 
1 − exp( − Xi• )
i Xi + ∆Xi
[
]
(52)
The Helmoltz free energy of each component in reaction is related to the partition function
through:
F = kT ln Q
(53)
As indicated by Urey (1947)[72], each isotopic exchange may be expressed by an equilibrium
of type:
aAo + bB• ⇔ aA• + bBo
(54)
where Ao, A• and Bo, B• are respectively isotopomers of the same molecules. In the light of
equation 6.10, the equilibrium constant of reaction 54 reduces to:
K6.11
 Q• 
=  Ao 
 QA 
a
 QB• 
 o
 QB 
b
(55)
24
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
The fractionation factor a represents the relative distribution of heavy and light isotopes
between two phases (or, as in our case, two complexes) at equilibrium.
(X
α=
(X
•
•
X o) A
(X
•
X o) B
(56)
X o) A in equation 56 is the ratio of isotopic abundances of isotope A. Assuming a
random distribution for isotopes under exchange, the fractionation factor is related to the
isotopic equilibrium constant [73]:
α = K1 r
(57)
with r= number of exchanging isotopes per unit formula.
Since the modifications of isotopic composition in natural materials are extremely restricted
the delta per mil notation is usually adopted:

 ( X • X o)
∆ AB =  • o A − 1 × 10 3 = (α − 1) × 10 3
(58)

 ( X X ) B
Isotopic measurements are conducted usually by comparison with standard material of
reference expressing the per mil difference in terms of delta parameter. Denoting st the
standard material of reference one has:
 ( X • X o)

δ A =  • o A − 1 × 10 3
(59)
 ( X X )st

 ( X • X o)

=
δ B  • o B − 1 × 10 3
(60)
 ( X X )st

Based on what above stated the relationships between fractionation factor α and the
parameters δ and ∆ are the following:
α AB =
∆ AB
δ A 10 3 + 1 δ A + 10 3
=
δ B 10 3 + 1 δ B + 10 3
 δ A + 10 3 
=
− 1 × 10 3
3
 δ B + 10

(61)
(62)
Because:
10 3 ln( X ) ≅ X
(63)
the following approximation if often utilized in geochemistry:
δ A − δ B ≅ ∆ AB ≅ 10 3 ln α AB
(64)
25
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
The result of our preliminary calculations in terms of α = K
reaction
1
r
fractionation factors for the
H oCrO4− + •Cr( H2O)6 ⇔ H •CrO4− + oCr( H2O)6
3+
3+
(65)
are listed in Table 9 and graphically resumed in Figure 9.
Based on the computed factors conspicuous fractionations may be predicted at ambient P,T
conditions during natural redox processes.
Table 9: coefficients of fractionation equation [100] :
1000 ln α = a + b × 10 6 / T 2 + c × 1012 / T 4
_____________________________________________________________
couple
a
b
c
R2
_________________________________________________________________________________________________
Cr/52Cr
0.3814
1.5382
-0.0303
0.9999
Cr/53Cr
0.5663
2.2720
-0.0447
0.9999
50
Cr/54Cr
0.7403
2.9752
-0.0585
0.9999
52
Cr/53Cr
0.1846
0.7337
-0.0144
0.9999
52
Cr/54Cr
0.3572
1.4375
-0.0282
0.9999
53
Cr/54Cr
0.1728
0.7035
-0.0138
0.9999
_____________________________________________________________
50
50
35
30
Cr isotopic fractionation
(reaction 65)
25
1000ln
50-52
50-53
50-54
20
52-53
15
52-54
53-54
10
5
0
0
5
10
1/T
15
2
Figure 11: Chromium isotopic fractionation in equilibrium between Cr hexa-aqua ion and chromate ion[100]
26
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
References
[1]
Shiraki K. (1978) Handbook of Geochemistry II-3 (scheda Cr). Springer-Verlag Berlin,
Heidelberg, New York.
[2]
Flesch G. D., Svec H. J., Staley H. G. (1960) Geochim. Cosmochim. Acta, 20, 300.
[3]
Honda M. and Arnold J. R. (1964) Science 143, 203.
[4]
Cressy P. J. Jr. (1970) Geochim. Cosmochim. Acta 34, 771.
[5]
Samsonov G. V.˚(1968) Handbook of the physicochemical properties of the elements. IFI/Plenum,
New York — Washington.
[6]
Viellard P. (1982) Modele de calcul des energies de formation des mineraux bati sur la
connaisance affinee des structures cristallines. C.N.R.S. Memoirs, 69, Universit Louis Pasteur,
Strasbourg.
[7]
Huheey J. E.: (1975) Inorganic Chemistry. Principles of structure and reactivity, Harper and Row,
New York.
[8]
Sanderson R. T. (1966) J. Inorg. Nucl. Chem., 28, 1553..
[9]
Shock E. L. and Helgeson H. C. (1988) Geochim. Cosmochim. Acta, 53, 2157.
[10]
Wagman D.D., Evans W. H., Parker V. B., Shumm R. H., Halow I., Bailey S. M., Churney K. L.
and Nuttall R. L. (1982) J. Phys. Ref. Data, 11, supp. 2, 392.
[11]
Dellien I., Hall F. M. and Hepler G. L. (1976) Chemical Reviews, 76, 283.
[12]
Barner H. E. and Scheuerman R. V. (1978) Handbook of thermochemical data for compounds and
aqueous species. Wiley: New York.
[13]
Latimer W. M. (1952) Oxidation states of the elements and their potentials in aqueous solutions,
2nd ed. Prentice-hall, New York.
[14]
Garrels R. M. and Christ C. L. (1965) Solutions, Minerals, and Equilibria. Freeman Cooper, San
Francisco.
[15]
Enciclopedia Medica Italiana (1988). USES Edizioni Scientifiche, Firenze.
[16]
Valente R. e Malesani M. G. (1992) Dizionario Medico Larousse. Editrice SAIE.
[17]
Marrubini M. B., Laurenzi R. G., Uccelli P. (1998) Intossicazioni acute-Meccanismi, diagnosi,
terapia. OEMF International s.r.l., Milano.
[18]
Derache R. (1988): Tossicologia e sicurezza degli alimenti. Tecniche Nuove, Milano.
[19]
Freeze R. A. and Cherry J. A. (1979) Groundwater. Prentice hall, Englewood Cliff, NJ.
[20]
Appelo C. A. S. and Postma D. (1999) Geochemistry, groundwater and pollution. A.A. Balkema,
Rotterdam.
[21]
Amdur M.O., Doull J., Klaassen C. D. (1993) Tossicologia-I fondamenti dell azione delle
sostanze tossiche. E.M.S.I. Edizioni Mediche Scientifiche Internazionali, Roma.
[22]
Maglia S., Santoloci M. (1999) Il codice dell ambiente, X ed., Editrice La Tribuna, Piacenza.
[23]
Pasqualini Salsa C. (2000) Diritto ambientale. Principi, norme, giurisprudenza. Maggioli Editore.
[24]
Supp. Ord. n. 218 della Gazzetta Ufficiale n. 293 del 15/12/1999 (DM 25/10/1999 n. 471).
[25]
Supp. Ord. n. 146/L della Gazzetta Ufficiale n. 177 del 30/07/1999 (d.lgs. 11/5/1999 n. 152).
[26]
Prapaipong P., Shock E. L., Koretsky C. M. (1999) Geochim. Cosmochim. Acta, 63, 2547-2577.
[27]
Brauer S.L. and Wetterhahn K.E. (1991) J. Am. Chem. Soc. , 113, 3001
[28]
Mazurek W., Nichols P.J. and West B.O. (1991) Polyhedron, 10, 753
[29]
Michel G. and Machiroux R. (1983) J. Raman. Spect., 14, 22
[30]
Michel G. and Cahay R. (1986) J. Raman. Spect., 17, 79
[31]
Cieslak-Golonka M. (1991) Coord. Chem. Rev., 109, 223
[32]
Cieslak-Golonka M. (1996) Polyhedron, 15, 3667
[33]
Poupopoulou V.G., Vrachnou E., Koinis S. and Katakis D. (1997) Polyhedron, 16, 521
[34]
Palmer D.A., Begun G.M., and Ward F.H. (1993) Rev. Sci. Instrum., 64, 1994
[35]
Brasch N.E., Buckingham D.A. and Clark C. R. (1994) Inorg. Chem., 33, 2683
[36]
Brito F., Ascanio J., Mateo S., Hernandez C., Araujo L., Gili P., Martin-Zarza P., Dominguez S.
and Mederos A. (1997) Polyhedron, 16,3835
[37]
Wolery T.J. (1983) EQ3NR. A computer program for Geochemical aqueous speciation solubility calculations: User’s guide and documentation. Lawrence Livermore Laboratory,
Livermore, CA, UCRL-94221
[38]
Tsapakos M.J. and Wetterhahn K.E. (1983) Chem. Biol. Interactions, 46, 265
[39]
Connett P.H. and Wetterhahn K.E. (1985) J. Am. Chem. Soc., 107, 4282
[40]
Standeven A.S. and Wetterhahn K.E. (1989) J. Am. Coll. Tossicol., 8, 1275
[41]
Brauer S.L. and Wetterhahn K.E. (1990) J. Am. Chem. Soc., 113,3001
27
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
[42]
O Brien P., Pratt J., Swanson F.J., Thornton P. and Wang G. (1990) Inorg. Chim. Acta, 169, 265
Aiyar J., Borges K.M., Floyd R.A. and Wetterhahn K.E. (1989) Toxicol. Environ. Chem., 22, 135
[44]
Mazurek W., Nichols P.J. and West B.O. (1991) Polyhedron., 10, 753
[45]
Baharad R., Perlmutter-Hayman B. and Wolff M.A. (1969) J. Phys. Chem., 73, 4391
[46]
Javandel I., Doughty C. and Tsang C.F. (1984) Groundwater transport: handbook of
mathematical models. American Geophysical Union, Water Resources Monogram 10, 228 pp.
[47]
Grove D.B. and Stollenwerk K.G. (1984) Computer model of one-dimensional equilibriumcontrolled sorption processes. U.S. Geological Survey, Water-Resources Investigations Report 844059
[48]
Freeze R. A. and Cherry J. A. (1979) "Groundwater". Prentice hall, Englewood Cliff, NJ
[49]
Ottonello G. (2000) Convenzione ANPA-CNR finalizzata alla realizzazione di prototipi di carte
tematiche di parte del territorio nazionale alla scala 1:250.000 riportanti i tenori naturali di alcuni
elementi chimici di significativo impatto ambientale. Rapporto Semestrale di Attivit , DIPTERIS,
Universit di Genova
[50]
Marini L., Ottonello G., Canepa M., Cipolli M., Vetuschi Zuccolini M. e Distefano D. (1999)
Atlante degli Acquiferi del Comune di Genova, Volume 2, Pacini Editore, Pisa.
[51]
Ottonello G., (2000) R.G.N.R. 18089/98 Accertamenti tecnici non ripetibili (ART. 360 C.P.P)
Rilievi del C.T. Parti Offese.
[52]
Vetuschi Zuccolini M. (2000) In: Atti della Scuola di Geochimica dei Fluidi, Arezzo, Societ
Geochimica Italiana.
[53]
Matheron F. (1965) Les variables regionale et leur estimation, Masson, Paris
[54]
Marcotte D. (1995) Math. Geol., 27, 749
[55]
Convenzione ANPA-CNR finalizzata alla realizzazione di prototipi di carte tematiche di parte del
territorio nazionale alla scala 1:250.000 riportanti i tenori naturali di alcuni elementi chimici di
significativo impatto ambientale — Carte Tematiche — Acque di Falda,DIPTERIS, Universit di
Genova
[56]
Brookins D.G. (1988) Eh-pH diagrams for Geochemistry, Springer-Verlag, Berlin-HeidelbergNew York.
[57]
Michel G. and Machiroux R. (1983) J. Raman Spectrosc., 14, 22
[58]
Tandon R.K., Crisp P.C., Ellis J. and Baker R.S. (1984) Talanta, 31, 227
[59]
Michel G. and Cahay R. (1986) J. Raman Spectrosc., 17, 79
[60]
Beattie J.K. and Haight G.P.Jr. (1972) Prog. Inorg. Chem., 17, 98
[61]
Kondratenko N.A. and Sherkyuk V.P. (1986) Teor. Eksp. Khim., 22, 686
[62]
CieslaK-Golonka M. (1991) Coord. Chem. Rev., 109, 223
[63]
Arnek R. (1970) Arkiv Kemi, 32, 81
[64]
Rai D., Sass B.M. and Moore D.A. (1987) Inorg. Chem., 26, 345
[65]
St nzi H. and Marty W. (1983) Inorg. Chem., 33, 2145
[66]
Baes C.F.Jr. and Mesmer R.E. (1976) The hydrolysis of cations, John Wiley, New York
[67]
Fendorf S.E. (1995) Geoderma, 67, 55
[68]
Wolery T.J. (1983) EQ3NR. A Computer Program for Geochemical Aqueous SpeciationSolubility Calculations: User’s Guide and Documentation Lawrence Livermore Laboratory,
Livermore, CA, UCRL-53414
[69]
Ball J.W. and Bassett R.L. (2000) Chem. Geol., 168, 123
[70]
Ottonello G. (1997) Principles of Geochemistry, Columbia University Press, New York
[71]
Bigeleisen J. and Mayer M.G. (1947) J. Chem. Phys., 13, 261
[72]
Urey H.C. (1947) J. Chem. Soc. (London), 1, 562
[73]
Epstein S. (1959) In: Researches in Geochemistry, P.H. Abelson (ed.), John Wiley, New York
[74]
Bjerrum N. (1908) Studies on basic chromic compounds. A contribution to the theory of
hydrolysis. Dissertation, Copenhagen
[75]
Ardon M. and Limenberg A. (1961) J. Phys. Chem., 65, 1443
[76]
Thompson M. and Connick R.E. (1981) Inorg. Chem., 20, 2279
[77]
Laswick J.A. and Plane R.A. (1959) J. Am. Chem. Soc., 81, 3565
[78]
Ardon M. and Plane R.A. (1959) J. Am. Chem. Soc., 81, 3197
[79]
Kolaczkowski R.W. and Plane R.A. (1964) Inorg. Chem., 3, 222
[80]
Meyenburg U., S roky O. and Schawarzenbach G. (1973) Helv. Chim. Acta, 56, 1099
[81]
St nzi H. Ritringer F.P. and Marty W. (1984) Inorg. Chem ., 23, 2160
[82]
Springborg J. (19??) Adv. Inorg. Chem., 32, 55
[43]
28
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
[83]
Beutler P. (1976) Tesi di Ph.D., Universit di Berna
Pappalardo R.R., Martinez J.M. and Marcos E.S. (1996) J. Phys. Chem., 100, 11748
[85]
Boettcher C.J.F. (1973) Theory of electric polarization. 2nd ed. Elsevier, Amsterdam
[86]
Gaussian 92 . Revision C.4. Frisch M.J. et al., Gaussian Inc., Pittsburg, PA.
[87]
Finholt J.E., Thompson M.E. and Connick R.E. (1981) Inorg. Chem., 20,4151
[88]
Serebrennikov L.V. and Mal tsev A.A. (1975) Vestn. Mosk. Univ. Sr., 16, 251
[89]
Almond M.A. and Hahna M. (1988) J. Chem. Soc. Dal ton Trans., 809
[90]
Chertihin G.V., Bare W.D. and Andrews L. (unpublished)
[91]
Wenthold P.G., Jonas K.L. and Lineberger W.C. (1997) J. Chem. Phys., 106, 9961
[92]
Mart n-Zarza P., Gili P., Ru z-P rez C., Rodr guez Romero F.V., Lotter G., Arrieta J.M., Torrent
M., Mestres J., Sola M. and Dur n M. (1997 ) Inorg. Chim. Acta, 258, 53
[93]
Dobbs K. and Hehre W. (1987) J. Comp. Chem., 8, 861
[94]
Condor J., Hillier J., Saunders V., Wood M. and Barber M. (1972) Mol. Phys., 24, 497
[95]
Mart n-Zarza P., Gili P., Rodr guez Romero F.V., Ru z-P rez C. and Solans X. (1995)
Polyhedron, 14, 2907
[96]
Pressprich M.R., Willett R.D., Poshusta R.D., Saundars S.C., Davis H.B. and Gard G.L. (1988)
Inorg. Chem., 27, 260
[97]
Nakamoto K. (1987) Infrared and Raman spectra of Inorganic and Coordination Compounds,
Wiley, New York
[98]
Mc Quarrie D.A. and Simon J.D. (1999) Physical Chemistry: A molecular approach, McGraw
Hill, New York
[99]
Ochterski J.W. (2000) Thermochemistry in Gaussian, Gaussian Inc., Pittsburg, PA
[100]
Ottonello G. And Vetuschi Zuccolini M. (2001) Calcoli ab-initio di frazionamento isotopico per
complessi acquosi del cromo: evidenze preliminari. Atti del Convegno Geoitalia 2001, FIST, Chieti
[101]
Fantoni D., Brozzo G., Canepa M., Cipolli F., Marini L., Ottonello G., Vetuschi Zuccolini M.
(2002) Environmental Geology (in press)
[102]
Wolery T (1992) EQ3NR, A computer program for geochemical aqueous speciation-solubility
calculations: Theoretical manual, user’s guide and related documentation (version 7.0). Report
UCRL-MA-110662 PT III, Lawrence Livermore National Laboratory, Livermore
[103]
Wolery T, Daveler SA (1992) EQ6, A computer program for reaction path modeling of aqueous
geochemical systems: Theoretical manual, user’s guide, and related documentation (version 7.0).
Report UCRL-MA-110662 PT IV, Lawrence Livermore National Laboratory, Livermore
[104]
Bruni J, Canepa M, Cipolli F, Marini L, Ottonello G, Vetuschi Zuccolini M, Chiodini G, Cioni
R, Longinelli A (2001) Proc. X WRI Conf. Balkema, Rotterdam
[105]
Bruni J., Canepa M., Cipolli F., Marini L., Ottonello G., Vetuschi Zuccolini M., Chiodini G.,
Cioni R., Longinelli A. (2002) Applied Geochemistry, 17, 455-474
[106]
Derron M.-H. (1999) Interaction eau-roche de basse temp rature: g ochimie des m taux dans
l alt ration m t orique des roches mafiques alpine. Th se de doctorat. Facult des Sciences de
l Universit de Lausanne.
[107]
Dinelli E. and Tateo F. (2002) Appl. Geochem. (in press).
[108]
Dubuis R., Moulin C., Pfanzelter A., Roch K. (1998) Etude geochimique et isotopique des eaux
des bassins versants Petronio & Gromolo. Facult des Sciences de l Universit de Lausanne.
[109]
Dinelli E., Lucchini F., Fabbri M., Cortecci G. (2001) J. Geochem. Explor. 74, 141-152.
[110]
Cortecci G., Dinelli E., Lucchini F., Vaselli O. (2001) Proc. X WRI Conf. Balkema, Rotterdam
[111]
Saldi G. (2001) Mappatura geochimica dell entroterra di Chiavari — Sestri Levante, mediante la
caratterizzazione delle acque di falda e delle acque acide della miniera di Libiola. Tesi di Laurea in
Sciene Geologiche, Universit di Genova
[112]
Marini L., Saldi G., Cipolli F., Ottonello G., Vetuschi Zuccolini M. (2002) Geochemical Journal
(in press)
[113]
Enciclopedia Medica Italiana (1988). USES Edizioni Scientifiche, Firenze.
[114]
Valente R. e Malesani M. G. (1992) Dizionario Medico Larousse. Editrice SAIE.
[115]
Marrubini M. B., Laurenzi R. G., Uccelli P. (1998) Intossicazioni acute-Meccanismi, diagnosi,
terapia. OEMF International s.r.l., Milano.
[116]
Gaussian98 (revision A.1), M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb,
J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery, R.E. Stratmann, J.C. Burant, S.Dapprich, J.M.
Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O.Farkas, J. Tomasi, V. Barone, M. Cossi, R.
Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala,
[84]
29
AQUEOUS SPECIATION AND ISOTOPIC FRACTIONATION OF CHROMIUM: ENVIRONMENTAL IMPLICATIONS
MIUR-2002 - RESEARCH PROJECT - Coordinator: Giulio Ottonello e-mail: [email protected]
Q. Cui, K. Morrocuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski,
J.V. Ortiz, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin,
D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe,
P.M.W. Gill, B.G. Johnson, W. Chen, M.W. Wong, J.L. Andres, M. Head-Gordon, E.S. Reploghe
and J.A. Pople, Gaussian Inc., Pittsburg PA, (1998)
[117]
Barnes HL, Langmuir D (1978) Geochemical prospecting handbook for metals and associated
elements. Natl Science Foundation Grant No AER77-06511 AO2, Annual Report
[118]
Faure G (1992) Principles and applications of inorganic geochemistry. Maxwell-Macmillan,
New York
[119]
Robles-Camacho J, Armienta MA (2000 J Geochem Explor., 68, 167-181
[120]
Eary LE, Rai D (1987) Environ Sci Technol., 21: 1187-1193
[121]
Saleh FY, Parketon TF, Lewis RV, Huang JH, Dickson KL (1989) Sci. Tot. Environ., 86, 25-41
[122]
Fendorf SE, Zasoski RJ (1992) Environ Sci Technol., 26, 79-85
[123]
Johnson CA and Xyla AG (1991) Geochim Cosmochim Acta, 55, 2861-2866
[124]
Raphael MW, Boulis SN (1982) Surf Technol., 16, 243-248
[125]
Fendorf SE (1995 Geoderma, 67, 55-71
[126]
Pettine M, Millero FJ (1990 Limnol. Oceanogr., 35, 730-736
[127]
Yuan J. and Shiller AM (2000) Atmospheric Environ., 34, 3973-3980
[128]
Cranston RE, Murray MJ (1978) Anal. Chim. Acta, 99, 275-282
[129]
Fukai RM, Vas D (1969) J. Oceanogr. Soc. Jpn., 25, 47-49
[130]
Schroeder DC, Lee GF (1975) Water Air Soil Pollut., 4, 355-365
[131]
Emerson S Cranston RE, Liss PS (1979 Deep-Sea Res., 26, 859-878
[132]
Ferrario A. (1973) Rend. S.I.M.P., 29, 485-495.
[133]
Brigo L. and Ferrario A., (1974) Rend. S.I.M.P., 30, 305-316
[134]
Bonatti E., Zerbi M., Kay R., Rydell H. (1976) Geol. Soc. Amer. Bull., 87, 83-94.
[135]
Ball J.W. and Bassett R.L. (2000) Chem. Geol., 168, 123
30