Hydrolysis and chemical speciation of dioxouranium(VI) ion in aqueous media simulating the major ion composition of seawater

Marine Chemistry 85 (2004) 103 – 124 www.elsevier.com/locate/marchem

Hydrolysis and chemical speciation of dioxouranium(VI) ion in aqueous media simulating the major ion composition of seawater Antonio Gianguzza a, Demetrio Milea b, Frank J. Millero c, Silvio Sammartano b,* a

c

Dipartimento di Chimica Inorganica e Chimica Analitica ‘‘Stanislao Cannizzaro’’, Universita` di Palermo, Viale delle Scienze, Parco D’Orleans II, I-90128 Palermo, Italy b Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Universita` di Messina, Salita Sperone, 31, I-98166 Messina (Vill. S. Agata), Italy Division of Marine Geology and Geophysics, MAC, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA Received 27 June 2003; received in revised form 29 September 2003; accepted 3 October 2003

Abstract The hydrolysis and chemical speciation of the dioxouranium(VI) ion at 25 jC was studied in a number of binary electrolytes (LiCl, NaCl, MgCl2, CaCl2, Na2SO4) and some mixtures (NaCl – Na2SO4, NaNO3 – Na2SO4, CaCl2 – MgCl2) as well as artificial seawater (SSWE) as a function of ionic strength. The results in LiCl, CaCl2 and MgCl2 solutions confirmed
1 the formation of (UO2)2(OH)22 +, (UO2)3(OH)24 +, (UO2)3(OH)+5 and (UO2)3(OH)
: log Tb22 =
5.76, log 7 species (at I = 0 mol l T b34 =
11.82, log Tb35 =
15.89 and log Tb37 =
29.26). For NaNO3, NaCl and artificial seawater the hydrolysis constant for the formation of the UO2(OH)+ species was also determined (at I = 0 mol l
1: log Tb11 =
5.19). The results in Na2SO4, 0 Na2SO4/NaNO3 and Na2SO4/NaCl required the formation of UO2(SO4)0, UO2(SO4)22
, UO2(OH)SO
4 , (UO2)2(OH)2SO4, 0
(UO2)3(OH)4SO4 and (UO2)3(OH)5SO4 , whose estimated values of each complex formation constant at I = 0 mol (kg H2O)
1 are (log Tbpqr F standard deviation, species in parenthesis): 3.32F 0.02 [UO2SO04], 4.26 F 0.04 [UO2(SO4)22
],
2.30 F 0.01 [UO2(OH)(SO4)
],
2.64 F 0.04 [(UO2)2 (OH)2(SO4)0],
8.45 F 0.04 [(UO2)3(OH)4(SO4)0 ],
13.58 F 0.04 [(UO2)3(OH)5(SO4)
]. All the results were examined using the Pitzer model by considering the interactions of the cation + + 2+ hydrolytic species with Cl
and NO
and Mg2 + and, in addition, the 3 and anion hydrolytic species with Li , Na , Ca ‘‘same sign’’ and ‘‘triple’’ interaction parameters. The resulting Pitzer parameters give an adequate representation of all the hydrolysis constants measured in the binary, ternary and artificial seawater solutions. Alternatively to the interpretation of the dependence of uranyl hydrolysis constants on ionic strength and on ionic medium in terms of variations of activity coefficients of ions, the formation of ion pairs was considered and some complex formation constants among dioxouranium(VI) species and different ions of background salts were calculated. Interactions of uranyl with major components of seawater were taken into account using the ‘‘single salt’’ BA approximation according to which SSWE is considered as a single sea salt (BA) where cation pffiffiffiffiffiffiffiffiffiffiffi B and anion A, having charge z ¼ F =CBA ¼ F1:117, are representative of all major cations (Na+, K+, Ca2 +, Mg2 +) and anions (Cl
and SO24
) of seawater, respectively. Pitzer parameters were also calculated for both the interactions of uranyl with Bz + and Az
ions and for the internal BA interactions. The last ones are: b(0) = 0.1081 and b(1) = 0.4238 for B1.117 + – A1.117


* Corresponding author. Tel.: +39-090-393659; fax: +39-090-392827. E-mail address: [email protected] (S. Sammartano). 0304-4203/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2003.10.002

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A. Gianguzza et al. / Marine Chemistry 85 (2004) 103–124

interactions, and b(0) =
0.3134 and b(1) = 1.5375 for H+ – A1.117
interactions. Literature data were collected carefully, and a critical analysis and accurate comparisons with results presented here were made. D 2003 Elsevier B.V. All rights reserved. Keywords: Hydrolysis; Dioxouranium(VI); Chemical speciation; Seawater; Dependence on ionic strength; Dependence on ionic medium; Pitzer parameters; Ion pair formation

1. Introduction Uranium is present in nature in several forms. Its concentration in open oceans has been estimated to be f14 nM. Most of the uranium (ca. 99%) is present in natural waters in its highest oxidation state, as dioxouranium(VI) ion (UO22 +), best known as uranyl, which is classified, on a Lewis acidity scale, as a hard acid like other ions of actinides series. Indicating its strong acidic behavior, the dioxouranium(VI) ion undergoes strong hydrolysis with the formation of mono- and polynuclear species, that dominate the chemistry of uranyl in aqueous solutions including all the natural waters (Cantaluppi and Degetto, 2000; Swarzenski et al., 1999, Choppin and Wong, 1998). Hydrolysis species of UO22 + have been extensively investigated during last fifty years in several laboratories, by using different experimental conditions and instrumental techniques (for example Ahrland, 1949, 1951; Ahrland et al., 1954; Baes and Meyer, 1962; Ciavatta et al., 2003a,b; Comarmond and Brown, 2000; Dunsmore and Sillen, 1963; Dunsmore et al., 1963; Grenthe et al., 1992; Lemire and Tremaine, 1980; Moll et al., 2000; Nguyen-Trung et al., 1992, 2000; Palmer and Nguyen-Trung, 1995; Peterson, 1961; Plyasunov et al., 1998; Rush et al., 1962; Sylva and Davidson, 1979; Yang and Pitzer, 1989; Wahlgren et al., 1999; Yu et al., 2000; Nguyen-Trung and Hovey, 1990). Many data can be found in some compilation and databases (Baes and Mesmer, 1976; Hogfeldt, 1982; Murray and May, 2000; Pettit and Powell, 1997; Sillen and Martell, 1964, 1971; Smith et al., 1997). The most recent overview has been reported by Grenthe et al. (1992). In spite of this wide number of papers, the results reported by different authors often show large disagreements due to (i) the complexity of the system, (ii) the simultaneous formation of different polynuclear hydrolytic species, (iii) the high percentage of hydrolyzed uranyl that occurs before precipitation of insoluble species and (iv) the

use of a limited range of uranyl concentrations in the experiments. Most of investigations were carried out in perchlorate or nitrate media, whilst few data are reported in ionic media of environmental or biological interest, such as chloride and sulfate salts (Peterson, 1961; Rush et al., 1962; Dunsmore and Sillen, 1963; Dunsmore et al., 1963; Lemire and Tremaine, 1980; Yang and Pitzer, 1989; Nguyen-Trung et al., 1992; Comarmond and Brown, 2000; Moll et al., 2000; Ciavatta et al., 2003a,b; Nguyen-Trung and Hovey, 1990). Moreover, only a few authors report hydrolysis constants of dioxouranium(VI) at different ionic strengths (most of data is reported at a single ionic strength and are not useful for speciation studies). With these complications in mind, we have undertaken a systematic study of the speciation of UO22 + in natural waters and, particularly, in seawater, with the aim of: (a) determining the strength of its interactions with inorganic and organic ligands of environmental and biological interest (including low and high molecular weight species such as polycarboxylates, polyamines, amino acids and nucleotides) (b) modeling the effect of ionic strength and ionic medium on its thermodynamic interaction parameters. These studies require reliable data on the uranyl hydrolysis in chloride and sulfate ionic media at different ionic strengths, as extension of previous investigations carried out in NaNO3 and NaCl aqueous media (De Stefano et al., 2002a). We report here the results of potentiometric measurements ([H+]glass electrode, at t = 25 jC) on the UO22 + system in simple background salts (LiCl, CaCl 2 , MgCl 2 , Na2SO4) and in mixtures (NaCl/Na2SO4, NaNO3/ Na2SO4, CaCl2/MgCl2), in a wide range of ionic strengths (0.1 V I mol l
1 V 3). The interactions of UO22 + and its hydrolytic species with the ionic com-

A. Gianguzza et al. / Marine Chemistry 85 (2004) 103–124

ponents of background salts have been examined by considering all the possible interactions between the components of the background salts. In order to investigate the dioxouranium(VI) hydrolytic processes in marine environments, we also report a study carried out in an ionic medium (Na+, K+, Ca2 +, Mg2 +, Cl
and SO42
) simulating the major components of natural seawater (synthetic seawater for equilibrium studies, SSWE) (De Stefano et al., 1994), at different salinities (5 V S V 45). Interactions of the UO22 + system with major constituents of seawater have been accounted for by using a complex formation model according to which SSWE is considered as a single sea salt (BA) where cation B and anion A are representative of all major cations (Na+, K+, Ca2 +,
Mg2 +) and anions p (Clffiffiffiffiffiffiffiffiffiffiffiffiffi and SO42
), respectively, and have charge z ¼ F I=CBA ¼ F1:117 (De Stefano et al., 1998). This ‘‘single salt approximation’’, which has been tested successfully on several systems, considerably reduces the complexity of the seawater system because only three ‘‘internal’’ ionic interactions (BA self association, Az
protonation and Bz + hydrolysis) have to be considered. It also gives a general picture of the cumulative binding ability of the inorganic components of seawater towards different classes of ligands and metal ions (De Stefano et al., 2002b). Hydrolysis and formation constants obtained have been used to determine interaction parameters of Pitzer equations (Pitzer, 1991; Millero, 1982), in order to improve the database of these parameters already compiled during last years for different acid – base systems, such as sea salts (Pitzer, 1991; Millero, 1982; Millero and Roy, 1997), metal ions (Millero and Hawke, 1992) and organometal cations (De Stefano et al., 1999; Foti et al., 1999, 2000, 2002; De Robertis et al., 1998), and some classes of ligands as, for example, carboxylates (Millero, 1983; Foti et al., 1997, 1998; De Robertis et al., 1999), amines (Millero et al., 1987; Herrero et al., 1992) and amino acids (Herrero et al., 1993; De Stefano et al., 2000).

2. Experimental 2.1. Chemicals

were standardized against sodium carbonate and potassium hydrogen phthalate, respectively, previously dried in an oven at 110 jC. Stocks of lithium hydroxide solutions were prepared from solid salt (Fluka). In order to minimize the formation of carbonate, titrant solutions were prepared by dilution from high concentrated stocks, stored under nitrogen atmosphere and standardized as described for other hydroxides. Sodium nitrate, lithium chloride, sodium chloride, sodium sulfate, magnesium chloride and calcium chloride were prepared by weight using pure salts (Fluka) previously dried in an oven at 110 jC. Solutions of magnesium and calcium chloride were standardized against EDTA standard solutions. SSWE solutions at different salinities (5 V S V 45) were prepared by mixing different salts, as reported in Table 1. Dioxouranium(VI) was used in form of nitrate salt (Fluka) and was used without further purification. Purity was checked by gravimetric determination of uranium after ignition to the oxide U3O8 and always resulted z 99.5%. All the solutions were prepared with analytical grade water (R = 18 MV cm
1), using grade A glassware. 2.2. Apparatus and procedure Potentiometric titrations (at t = 25.0 F 0.1 jC) were carried out using an apparatus consisting of a Model 713 Metrohm potentiometer, equipped with an half cell glass electrode (Ross type 8101, from Orion), a double junction reference electrode (type 900200, from Orion) and a Model 765 Metrohm motorized

Table 1 Composition of artificial seawater (SSWE) at 35 salinitya and at t = 25 jC Component

NaCl Na2SO4 KCl CaCl2 MgCl2 BAb I a

Acidic and hydroxide solutions were prepared by diluting concentrated ampoules (Riedel-deHae¨n) and

105

Concentration mol l
1

mol (kg H2O)
1

0.4221 0.0288 0.0110 0.0111 0.0548 0.5751 0.717

0.42740 0.02919 0.01112 0.01121 0.05552 0.58240 0.726

Concentrations in the molal scale at different salinities are given by: mS = m3527.56572S/(1000
1.005714S). b Seawater single salt.

106

A. Gianguzza et al. / Marine Chemistry 85 (2004) 103–124

burette. Estimated accuracy was F 0.15 mV and F 0.003 ml for e.m.f. and titrant volume readings, respectively. The apparatus was connected to a PC, and automatic titrations were performed using a suitable computer program to control titrant delivery, data acquisition and to check for e.m.f. stability. All the titrations were carried out under magnetic stirring, and bubbling through the solution purified pre-saturated N2, in order to exclude O2 and CO2 inside. These gases were avoided by using two traps containing a CrCl2 solution on a bed of zinc amalgam and a NaOH solution, respectively. Another ampoule, containing a solution of the same ionic medium of the measurements, was put after the last trap, in order to saturate N2 with the vapors of this new solution. The titrand solution consisted of different amounts of UO22 +, a slight excess of acid and the background salt in order to obtain pre-established ionic strength value (0.1 V I mol l
1 V 3). Potentiometric measurements were carried out by titrating 25 ml of the titrand solution with standard hydroxide solutions until the formation of precipitate was noted. For each experiment, independent titrations of strong acidic solution with standard base were carried out under the same medium and ionic strength conditions as the systems to be investigated, with the aim of determining electrode potential (E0) and acidic junction potential (Ej = ja [H+]). 50– 100 points for each titration were collected. The equilibrium state during the titrations was checked with precautions. These include the study of time necessary for reaching equilibrium and the execution of back titrations. The experimental conditions for all the systems studied are reported in Table 2. 2.3. Calculations BSTAC and STACO computer programs (De Stefano et al., 1993, 1996) were used in the refinement of all the parameters of an acid – base titration (E0, pKw, coefficient of junction potential ja, analytical concentration of reagents) and in the calculation of complex formation constants. The ES4ECI (De Robertis et al., 1986) program was used to draw distribution diagrams and to calculate percentages of species formed. The LIANA (De Stefano et al., 1997) program was used to test the dependence of log K on ionic strength using different equations.

Table 2 Experimental conditions for potentiometric measurements at t = 25 jC Ionic medium

Ionic strength (mol l
1)

CaCl2 MgCl2 LiCl Na2SO4 CaCl2/ MgCla2 NaCl/ Na2SOb4 NaNO3/ Na2SOc4 SSWE

0.15 – 0.60 0.15 – 0.60 0.5 – 3.0 0.5 – 1.5 0.15 – 0.60

UO2(NO3)2 (mmol l
1)

Acid (mmol l
1)

Titrant

1–5 1–5 1–5 1–2 1–5

1–4 1–4 – 4 1–4

NaOH NaOH LiOH NaOH NaOH

0.25 – 1.00

1–5

2–6

NaOH

0.10 – 1.00

1–5

2–6

NaOH

0.5 – 5

1–6

NaOH

5 – 45d

a

CCa/CMg = 3/2, 2/3, 1/4. CSO4 = 25 mmol l
1. c 10 V CSO4 mmol l
1 V 100. d S (salinity, see Table 1). b

Dependence on ionic strength was taken into account by a Debye – Hu¨ckel type equation (Daniele et al., 1997): logK ¼ logT K
z*I 1=2 ð2 þ 3I 1=2 Þ
1 þ CI þ DI 3=2 þ EI 2

ð1Þ

with z* ¼ RðchargesÞ2reactants
RðchargesÞ2products

ð2Þ

where K is the formation constant, TK is the formation constant at infinite dilution and C, D, E are empirical parameters. Results of a series of investigations showed that the E parameter can be neglected when I V 1 mol l
1, whilst, when all interactions in the solution are taken into account, the empirical parameters C and D are given by (Daniele et al., 1997): C ¼ 0:11p* þ 0:20z*

ð3Þ

D ¼
0:075z*

ð4Þ

with p* ¼ RðmolesÞreactants
RðmolesÞproducts

ð5Þ

A. Gianguzza et al. / Marine Chemistry 85 (2004) 103–124

Both STACO and BSTAC computer programs can deal with measurements at different ionic strengths and can refine the empirical parameters for Eq. (1). Dependence on ionic strength was also taken into account by considering the Pitzer equations (Pitzer, 1991; Millero, 1982), according to which, for electrolytes 1 –1, 1 –2 and 2 –1, the activity coefficients of a cation M or an anion X, in the simplest form, can be expressed by: 2 c lncM ¼ ZM f þ2

þ

XX a

þ

X

X

a

2 mc ma ðZM Bca V þ ZM Cca Þ

mc ð2HMc þ

XX a

þ

X

X

f2 ¼
1 þ ð1 þ 2I 1=2 þ 2IÞexpð
2I 1=2 Þ

ð14Þ

ð0Þ

ð1Þ

ð2Þ

BMX ¼ bMX þ bMX ð0:98IÞ
1 f1 þ bMX ð72IÞ
1 f2 ð15Þ ð1Þ

ð6Þ

X

mc ðBXc þ ECXc Þ

ð2Þ

BMX V ¼ bMX ð0:98I 2 Þ
1 f3 þ bMX ð72I 2 Þ
1 f4

ð16Þ

f1 ¼ 1
ð1 þ 1:4I 1=2 Þexpð
1:4I 1=2 Þ

ð17Þ

f2 ¼ 1
ð1 þ 12I 1=2 Þexpð
12I 1=2 Þ

ð18Þ

c

mc ma ðZX2 Bca V þ ZM Cca Þ

ma ð2HXa þ

X

mc WXca Þ

c

mc mcV WXccV

f3 ¼
1 þ ð1 þ 1:4I 1=2 þ 0:98IÞexpð
1:4I 1=2 Þ ð7Þ

ð19Þ

cV

f4 ¼
1 þ ð1 þ 12I 1=2 þ 72IÞexpð
12I 1=2 Þ

and for neutral species: lnc0MX ¼ 2kI

ð20Þ

ð8Þ

where mi is the molality of the cations (c) and anions (a) in the solution, Z the charge, E the equivalent molality (E = 1/2SimiAZiA), I the ionic strength in molal scale and: ð0Þ

ð13Þ

ma WMca Þ

ma maV WMaaV

c

XX c

f1 ¼ 1
ð1 þ 2I 1=2 Þexpð
2I 1=2 Þ

a

a

þ

ð12Þ

aV

lncX ¼ ZX2 f c þ 2 þ

þ 1:667lnð1 þ 1:2I 1=2 Þ

a

c

þ

f c ¼
0:392½I 1=2 ð1 þ 1:2I 1=2 Þ
1

For 2 – 2 electrolytes BMX and BMX V terms in Eqs. (6) and (7) are given by:

ma ðBMa þ ECMa Þ

c

XX

107

ð1Þ

BMX ¼ bMX þ bMX ð2IÞ
1 f1

ð9Þ

BMX V ¼ bMX ð2I 2 Þ
1 f2

ð1Þ

ð10Þ

ð/Þ

ð11Þ

CMX ¼ CMX ð2AZM ZX A1=2 Þ
1

b(0), b(1), b(2) and C(/) represent interaction parameters between two ions of opposite signs, H interaction parameters between two ions (+ + or

) of the same sign, W triple interaction parameters (+
+,
+
) and k the interaction parameter for neutral species with the ions in the solution. At I < 3 mol (kg H2O)
1, H and W parameters can generally be neglected. Values of Pitzer parameters for the major salts are given in Table 3. Dioxouranium(VI) hydrolysis constants, bpq are given according to the equilibrium ð2p
qÞ þ qHþ pUO2þ 2 þ qH2 O ¼ ðUO2 Þp ðOHÞq

bpq

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A. Gianguzza et al. / Marine Chemistry 85 (2004) 103–124

Table 3 Pitzer parameters for the most common salts and mixtures (Pitzer, 1991) b(0)

Electrolyte HCl HNO3 LiCl NaCl NaNO3 KCl H2SOa4 MgCl2 CaCl2 UO2Cl2 UO2(NO3)2 Na2SO4 K2SO4 MgSO4 CaSO4 UO2SO4 HCl/LiCl HCl/NaCl HCl/KCl a

0.1775 0.1168 0.1494 0.0765 0.0068 0.04835
0.0084 0.35235 0.3159 0.4274 0.4607 0.01869 0.04995 0.2210 0.20 0.322 – – –

b(1)

b(2)

C (/)

H

W

0.2945 0.3546 0.3074 0.2664 0.1783 0.2122 0.3147 1.6815 1.614 1.644 1.6133 1.27575 0.77925 3.343 3.1973 1.827 – – –

– – – – – – – – – – – – –
37.23
54.24 – – – –

0.00080
0.00539 0.00359 0.00127
0.00072
0.00084 0.01019 0.00519
0.00034
0.03686
0.03154 0.005549 – 0.0250 –
0.0176 – – –

– – – – – – – – – – – – – – – – 0.015 0.036 0.005

– – – – – – – – – – – – – – – – 0.000
0.004
0.007

H+ – SO24
interaction (Clegg et al., 1994).

Overall formation constants bpqr for uranyl – hydroxide – ligand complexes, where ligands could be anions or cations, are given according to equilibrium z pUO2þ 2 þ qH2 O þ rX

¼ ðUO2 Þp ðOHÞq Xð2p
qþrzÞ þ qHþ r

bpqr

The values of the formation constants are expressed in the molar concentration scale (M, mol l
1). Some of values, when specified, are in the molal concentration scale [m, mol (kg H2O)
1].

3. Results and discussion 3.1. Hydrolysis of UO22+ in aqueous LiCl, CaCl2, MgCl2 and in CaCl2/MgCl2 mixtures The experimental conditions used in studying the hydrolysis of UO22 + are reported in Table 2. According to a previous paper (De Stefano et al., 2002a), we performed several trials on all systems investigated to examine the results in terms of the formation of uranyl hydrolytic species suggested in the literature. The following species ( p,q): (1,1), (2,1), (1,2), (2,2),

(3,4), (3,5), (3,7), (4,6), (4,7), (5,8) were considered (other species having higher p/q ratio were not considered since they are formed at pHH6). These preliminary calculations indicated that the formation of UO2(OH)+, (UO2)2(OH)22 + and three trinuclear [(UO2)3(OH)42 +, (UO2)3(OH)5+ and (UO2)3(OH)7
] species gave the best fit, confirming our previous studies of the hydrolysis in sodium nitrate and chloride media (De Stefano et al., 2002a), and are in good agreement with some literature data (Lemire and Tremaine, 1980; Palmer and Nguyen-Trung, 1995; Nguyen-Trung et al., 2000). Dioxouranium(VI) hydrolysis was first studied in aqueous lithium chloride solutions with the aim of solving the open question on the speciation immediately before the pH at which the precipitation of insoluble uranium(VI) oxide species occurs (pH f 6). Many authors include in their speciation models tetranuclear species as (UO2)4(OH)62 + and/or (UO2)4(OH)7+ (Ahrland et al., 1954; Peterson, 1961; Dunsmore and Sillen, 1963; Dunsmore et al., 1963; Sylva and Davidson, 1979; Comarmond and Brown, 2000), whilst other groups affirm the existence of (UO2)3(OH)7
species (Lemire and Tremaine, 1980; Palmer and Nguyen-Trung, 1995; Nguyen-Trung et al., 2000; De Stefano et al., 2002a) or, at least, refuse

A. Gianguzza et al. / Marine Chemistry 85 (2004) 103–124

to believe the existence of the cited tetranuclear complexes (Rush et al., 1962; Baes and Meyer, 1962). A critical overview on these arguments is given by Grenthe et al. (1992). Measurements performed in lithium chloride as background salt showed that the pH at which precipitation occurs was slightly higher (pH = 6.3) than that which can be studied in other ionic media as, for example, in NaCl under the same experimental conditions. These results allowed us to better identify the (UO2)3(OH)7
species whose formation occurs in the pH range 5.7 –6.0 and was only hypothesized in our previous investigations carried out in NaCl medium (De Stefano et al., 2002a). By refining our potentiometric data by both STACO and BSTAC computer programs, we tried to include or substitute in our speciation model different hydrolytic species previously mentioned. These produced worse fits and, in some cases, did not converge. This work confirmed that the species proposed for dioxouranium hydrolysis in NaNO3 and NaCl media (De Stefano et al., 2002a) [i.e. (UO 2 ) 2 (OH) 22 + , (UO 2 ) 3 (OH) 42 + , (UO2)3(OH)5+ and (UO2)3(OH)7
, with the exception for UO2(OH)+, which did not form under our experimental conditions] also exist in LiCl media. Experimental values of hydrolysis constants in LiCl are reported in Table 4. As one can see, the conditional hydrolysis constants obtained in lithium chloride are systematically smaller than the corresponding values in sodium chloride (see Table 5). This supports the hypothesis that acid – base behavior of UO22 + depends not only on the presence of the interacting anion in the background salt, but also on the type of cation component of ionic medium. Hydrolysis of uranyl was also studied in calcium and magnesium chloride aqueous media and in their mixtures under the experimental conditions reported in Table 2. Hydrolysis constants at different ionic

109

Table 5 Hydrolysis constants of UO22 + in NaNO3 and NaCl, at different ionic strengths (mol l
1) (De Stefano et al., 2002a) I

log b11

log b22

log b34

log b35

log b37

NaNO3 0.10 0.25 0.50 0.75 1.00


5.38
5.42
5.41
5.37
5.33


5.98
6.04
6.09
6.12
6.13


12.29
12.50
12.73
12.90
13.06


16.52
16.70
16.80
16.83
16.83


29.57
29.62
29.58
29.48
29.39

NaCl 0.10 0.25 0.50 0.75 1.00 2.00 3.00 4.50


5.45
5.57
5.72
5.84
5.96
6.43
6.92
7.73


5.98
6.06
6.14
6.19
6.24
6.41
6.60
6.98


12.22
12.33
12.38
12.40
12.41
12.44
12.57
13.01


16.55
16.76
16.93
17.04
17.13
17.44
17.81
18.57


29.68
29.77
29.79
29.79
29.80
29.93
30.35
31.58

strengths for pure salts and for their mixtures of different composition are shown in Table 6. As can be seen from this table, the UO2(OH)+ species is not included in the model, and the constants for other species are smaller than the corresponding ones in sodium chloride. Another interesting aspect of the results (Table 6) is that hydrolysis constants in mixtures of calcium and magnesium chlorides do not seem to have, as expected, an ‘‘intermediate’’ value between those in the pure salts. This can be explained by considering the formation of weak mixed species, as proposed later in this paper. The values of hydrolytic constants reported in Tables 4 –6, clearly show that the hydrolysis of dixouranium(VI) ion is fairly dependent on medium and ionic strength. This is better shown in Figs. 1– 4, where the speciation diagrams of dioxouranium(VI) system are reported in pure alkali and alkaline earth metal chlorides.

Table 4 Hydrolysis constantsa of UO22 + in LiCl, at different ionic strengths (mol l
1) I

log b22

0.5 1.0 2.0 3.0


6.18 F 0.02
6.32 F 0.02
6.55 F 0.01
6.77 F 0.01 a

F 3 standard deviation.

log b34
12.44 F 0.01
12.53 F 0.03
12.66 F 0.02
12.88 F 0.02

log b35
17.09 F 0.03
17.45 F 0.03
18.07 F 0.01
18.72 F 0.05

log b37
29.93 F 0.04
29.99 F 0.09
30.16 F 0.05
30.28 F 0.02

110

A. Gianguzza et al. / Marine Chemistry 85 (2004) 103–124

Table 6 Hydrolysis constantsa of UO22 + in CaCl2, MgCl2 and in their mixtures of different composition (mol l
1) CCa

CMg

log b22 (F 0.05)a

log b34 (F 0.07)a

log b35 (F 0.04)a

log b37 (F 0.10)a

0.05 0.10 0.20 – – – 0.03 0.06 0.12 0.02 0.04 0.08 0.01 0.02 0.04

– – – 0.05 0.10 0.20 0.02 0.04 0.08 0.03 0.06 0.12 0.04 0.08 0.16


6.02
6.10
6.19
6.05
6.14
6.27
6.03
6.10
6.23
6.01
6.06
6.12
6.05
6.15
6.28


12.23
12.24
12.21
12.24
12.28
12.24
12.24
12.27
12.24
12.26
12.32
12.33
12.22
12.22
12.17


16.64
16.68
17.00
16.67
16.85
17.08
16.61
16.75
16.91
16.63
16.78
16.95
16.63
16.81
18.96


29.75
29.86
29.97
29.99
30.33
30.85
29.47
29.30
28.85
29.63
39.65
29.49
29.66
29.67
29.62

a

F 3 standard deviation.

The speciation diagram of UO22 + system in LiCl (Fig. 1) is obtained under the same experimental conditions of NaCl diagram (Fig. 2), and both are calculated till the pH value at which precipitation of insoluble oxide species occurs. Fig. 1 shows that this pH value is ~0.3 units higher than in NaCl. This allows us to see that the formation of the (UO2)3 (OH)7
species is up to 40%. This confirms the hypothesis of the existence of this species in the pH range just before the formation of insoluble species.

Moreover, it can be noted that, under the experimental conditions used for all the UO22 + system speciation diagrams, the UO2(OH)+ species is present only in NaCl medium, and the (UO2)3(OH)7
species is not formed in MgCl2. Therefore, on the basis of the above considerations, it can be concluded that in the chloride ionic media considered here the main species are (UO2)2(OH)22 +, (UO2)3(OH)42 + and (UO2)3(OH)5+, the last one achieving a formation of about 80%, although UO2(OH)+ and (UO2)3(OH)7
species are formed in significant amounts under conditions other than those used for calculating the speciation diagrams and must be considered for a more correct and complete speciation model. Uranyl behavior in different chloride media can be interpreted both in terms of (a) formation of ion pairs between uranyl species and one or more components of background salts and (b) changes in activity coefficients of these species in each medium. 3.2. Hydrolysis of UO22+ in Na2SO4 aqueous medium and in NaNO3/Na2SO4 and NaCl/Na2SO4 mixtures After characterizing the hydrolytic species in chloride media, the investigation on the UO22 + hydrolysis in aqueous solution containing the major constituents of seawater has been extended to studies in Na2SO4, Na2SO4/NaCl and Na2SO4/NaNO3 ionic media. Analysis of potentiometric data, obtained at different ionic strengths, gave evidence of very strong interactions

Fig. 1. Distribution diagram of dioxouranium(VI) species vs. pH in LiCl at t = 25 jC. Species: 1: UO2, 2: (UO2)2(OH)2, 3: (UO2)3(OH)4, 4: (UO2)3(OH)5, 5: (UO2)3(OH)7; [charges omitted for simplicity]. Experimental conditions: CUO2 = 0.001 mol l
1; I = 0.5 mol l
1.

A. Gianguzza et al. / Marine Chemistry 85 (2004) 103–124

111

Fig. 2. Distribution diagram of dioxouranium(VI) species vs. pH in NaCl at t = 25 jC. Species: 1: UO2, 2: (UO2)2(OH)2, 3: UO2(OH), 4: (UO2)3(OH)4, 5: (UO2)3(OH)5, 6: (UO2)3(OH)7; [charges omitted for simplicity]. Experimental conditions: CUO2 = 0.001 mol l
1; I = 0.5 mol l
1.

between simple and/or hydrolytic UO22 + species and sulfate ion. For this reason, we preferred to treat the measurements in sulfate media by considering the formation of ion pairs between dioxouranium(VI) species and the sulfate ion. Although the literature records many speciation models including different uranyl – hydroxide – sulfate species (Ahrland, 1951; Peterson, 1961; Dunsmore et al., 1963; Lemire and Tremaine, 1980; Comarmond and Brown, 2000; Moll et al., 2000; Ciavatta et al., 2003b), the best fit of our experimental data was obtained considering a model that includes the following complexes: UO2SO40,


0 UO 2 (SO 4 ) 2
2 , UO 2 (OH)SO 4 , (UO 2 ) 2 (OH) 2 SO 4 , 0
(UO2)3(OH)4SO4 , (UO2)3(OH)5SO4 . Complex formation constants for these species, obtained in pure sodium sulfate and in different mixtures with sodium nitrate or sodium chloride, are reported in Table 7. As can be seen, there are small differences between complex formation constants for the same species in pure sulfate medium and in mixtures with nitrate or chloride. This effect is not only due to the variation in activity coefficients of complexes in different electrolytes, but it is also a consequence of the probable formation of very weak mixed ternary complexes

Fig. 3. Distribution diagram of dioxouranium(VI) species vs. pH in CaCl2 at t = 25 jC. Species: 1: UO2, 2: (UO2)2(OH)2, 3: (UO2)3(OH)4, 4: (UO2)3(OH)5, 5: (UO2)3(OH)7; [charges omitted for simplicity]. Experimental conditions: CUO2 = 0.001 mol l
1; I = 0.6 mol l
1.

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A. Gianguzza et al. / Marine Chemistry 85 (2004) 103–124

Fig. 4. Distribution diagram of dioxouranium(VI) species vs. pH in MgCl2 at t = 25 jC. Species: 1: UO2, 2: (UO2)2(OH)2, 3: (UO2)3(OH)4, 4: (UO2)3(OH)5, [charges omitted for simplicity]. Experimental conditions: CUO2 = 0.001 mol l
1; I = 0.6 mol l
1.

between UO22 + (and/or its hydrolytic species) and two or more different ions of the background media. 3.3. Hydrolysis of UO22+ in synthetic seawater In order to give a complete picture of dioxouranium(VI) hydrolysis in seawater, we also carried out some measurements in a multicomponent ionic medi-

Table 7 Complex formation constantsa of (UO2)p(OH)q(SO4)r(2p
q
2r) species in Na2SO4 and in its mixtures with NaCl or NaNO3 (mol l
1) log b101 log b102 log b111 log b221 log b341 log b351 (F 0.1)a (F 0.15)a (F 0.2)a (F 0.1)a (F 0.2)a (F 0.2)a

I

Na2SO4 0.50 2.0 1.00 1.8 1.50 1.7

2.8 2.6 2.5


3.6
3.9
4.1


4.4
4.8
5.0


10.8
11.4
11.9


13.6
15.3
15.9

Na2SO4/NaNO3 0.10 2.5 0.25 2.3 0.75 2.0 1.00 2.1

3.6 3.5 3.1 3.0


2.9
3.2
3.4
3.4


3.8
4.0
4.2
4.1


9.6
9.8
9.9
9.7


14.6
14.9
15.0
14.9

Na2SO4/NaCl 0.25 2.2 0.50 2.1 1.00 1.9

3.5 3.1 2.7


3.0
3.2
3.6


3.7
4.3
4.3


9.7
10.0
10.2


14.6
15.1
15.4

a

F 3 standard deviation.

um (Na+, K+, Mg2 +, Ca2 +, Cl
and SO42
) simulating the major composition of natural seawater (SSWE, De Stefano et al., 1994). Hydrolysis constants of UO22 + in SSWE at different salinities (5 V S V 45) are reported in Table 8. The distribution of hydrolytic species in SSWE at S = 35 is shown in Fig. 5. Although under experimental conditions of our measurements, the formation of insoluble species starts at pH lower than natural seawater (pH~8.1), the precipitation does not occur in real systems, owing to (a) the very low uranyl total concentration and (b) the presence of other organic and inorganic ligands (humics, carbonate, bicarbonate, fluoride and carboxylates) that strongly interact with UO22 + and its hydrolytic species. Investigations performed by Palmer and Nguyen-Trung (1995), on uranyl hydrolysis in the pH range 3– 12, by using tetramethylammonium trifluoromethanesulfonate as supporting electrolyte, show that, at pH~8, the main species is (UO2)3(OH)7
, with a percentage higher than 90%. Therefore, results obtained here for UO22 + hydrolysis can be easily extrapolated to higher pH, typical of seawater (pH = 8.1), without large errors. When SSWE multicomponent ionic medium is used as background salt in the speciation studies, the internal ionic interactions between the components cannot be neglected because they lower the concentration of free ions. If we take into account these interactions, along with the other hydrolytic and mixed species formed in the UO22 + system, a considerable number of species needs to be considered. To

A. Gianguzza et al. / Marine Chemistry 85 (2004) 103–124

113

Table 8 Hydrolysis constantsa (mol l
1) of UO22 + in SSWE at different salinities log b11

S


5.38 F 0.03
5.50 F 0.03
5.47 F 0.06
5.52 F 0.03
5.54 F 0.03
5.58 F 0.03
5.58 F 0.03
5.62 F 0.06
5.48 F 0.09

5 10 15 20 25 30 35 40 45 a

log b22
6.41 F 0.09
6.49 F 0.03
6.72 F 0.09
6.71 F 0.02
6.69 F 0.09
6.74 F 0.06
6.76 F 0.09
6.94 F 0.09
6.90 F 0.03

log b34
12.58 F 0.03
12.97 F 0.12
12.98 F 0.12
13.07 F 0.21
13.44 F 0.06
13.53 F 0.03
13.66 F 0.03
13.66 F 0.09
13.92 F 0.15

log b35
17.17 F 0.09
17.53 F 0.06
17.84 F 0.09
17.86 F 0.09
18.00 F 0.09
18.12 F 0.06
18.24 F 0.03
18.45 F 0.12
18.36 F 0.02

log b37
30.70 F 0.42
30.54 F 0.12
30.85 F 0.06
30.67 F 0.21
30.94 F 0.02
30.95 F 0.03
30.89 F 0.03
30.89 F 0.02
30.93 F 0.12

F 3 standard deviation.

simplify equilibrium calculations, we recently proposed a more simple approach (De Stefano et al., 1998) designed to take into account all the interactions among the major components of seawater, by considering the components of synthetic seawater (SSWE) as a single 1:1 salt (BA), whose concentration is CBA = 1/2SCi (Ci = concentrationp offfiffiffiffiffiffiffiffiffiffiffiffiffi all the ions) and z ion charge calculated as z ¼ F I=CBA ¼ F1:117. The use of the single salt approximation (valid in the range 5 V S V 45) considerably reduces the complexity of the systems to be investigated. In fact, only three species deriving from internal ionic medium interactions must be considered: BA0 (with log K =
0.03, at I = 0 mol l
1 and t = 25 jC), HA(1
z) (with log K = 0.24, at I = 0 mol l
1 and t = 25 jC) and B(OH)(z
1) (log K =
12.75, relative to the reaction:

Bz + + H2O = B(OH)(z
1) + H+, at I = 0 mol l
1 and t = 25 jC). For this reason, the speciation of dioxouranium(VI) in SSWE was studied using the single salt (BA) approximation, by including the above equilibrium constants for the BA system in the calculations carried out to quantitatively determine the interactions between uranyl species and the seawater salt. Calculations performed by STACO and BSTAC computer programs gave evidence of the formation of the six following species: UO2A0.883 +, UO2(OH)A0.117
, (UO2)2(OH)2A0.883 +, (UO2)3(OH)4A0.883 +, (UO2)3(OH)5A0.117
, (UO2)3(OH)7B0.117 +. Overall formation constants at I = 0 mol l
1 found were, respectively: 1.63 F 0.02,
3.73 F 0.02,
4.15 F 0.04,
9.78 F 0.05,
14.44 F 0.05 and
27.60 F 0.05 ( F 3 standard deviation). Extrapolation of these

Fig. 5. Distribution diagram of dioxouranium(VI) species vs. pH in SSWE at t = 25 jC. Species: 1: UO2, 2: (UO2)2(OH)2, 3: UO2(OH), 4: (UO2)3(OH)4, 5: (UO2)3(OH)5, 6: (UO2)3(OH)7; [charges omitted for simplicity]. Experimental conditions: CUO2 = 0.001 mol l
1; S = 35.

114

A. Gianguzza et al. / Marine Chemistry 85 (2004) 103–124

constants to infinite dilution was performed in two different ways, in order to independently confirm the values. In the first treatment, we examined the entire experimental dataset obtained in SSWE by BSTAC and STACO computer programs, using Eq. (1) to account for the effect of ionic strength on the constants. In the second one, we examined the equilibrium constants for cited complexes at single ionic strengths, and calculated the ‘‘pure water’’ values by extrapolation, again using Eq. (1). These latter results, assumed that empirical parameters of Eq. (1) were the same for all the formation constants (i.e. species have the same dependence on ionic strength). This assumption is justified by the fact that, in the ‘‘single salt approximation’’, all internal interactions are taken into account (considering autoassociation of ‘‘BA’’, protonation of ‘‘A’’ and hydrolysis of ‘‘B’’, see above) and, in these conditions, C and D parameters of Eq. (1) become constant (see Eqs. (3) and (4)). The distribution diagram of the UO22 + cations in SSWE as single salt is shown in Fig. 6. In this Figure we also show the predicted values of the dioxouranium(VI) speciation at higher pH. In seawater at pH = 8.1 and salinity S = 35 at the natural concentration of uranyl, the main species is (UO2)3(OH)7B0.117 + and the other species present, in all pH range, are UO22 + – AB complexes with formation percentages always over 60%.

3.4. Pitzer interaction parameters for dioxouranium(VI) hydrolysis constants in NaNO3, NaCl, LiCl, alkaline earth chlorides and their mixtures Pitzer equations (Pitzer, 1991; Millero, 1982) have been widely used in speciation studies, because the model can account for the activity coefficients of all the species as a function of ionic strength and composition. Although somewhat complex in its mathematical formulation, the main advantage is that it considers all possible interactions occurring in the solutions, taking also into account interactions between ions of the same sign (+ +,

) and triple interactions (+
+,
+
). These aspects become more important when calculations need to be extended to ionic strengths up to 3 molal and result in improved fits of the experimental data. The extensive database of interaction parameters have been set up to include Pitzer interaction parameters for sea salts (Pitzer, 1991; Millero, 1982; Millero and Roy, 1997), metals (Millero and Hawke, 1992) and organometal cations (De Stefano et al., 1999; Foti et al., 1999, 2000, 2002; De Robertis et al., 1998), some ligand classes as carboxylates (Millero, 1983; Foti et al., 1997, 1998; De Robertis et al., 1999), amines (Millero et al., 1987; Herrero et al., 1992) and amino acids (Herrero et al., 1993; De Stefano et al., 2000). In order to contribute to further extension of the database, we determined the interaction parameters of Pitzer equations (Eqs. (6) – (20)) for dioxouranium(VI) and

Fig. 6. Distribution diagram of dioxouranium(VI) species vs. pH in SSWE as single salt AB at t = 25 jC. Dashed lines mean extrapolated values. Species: 1: UO2, 2: UO2A, 3: UO2(OH)A, 4: UO2(OH), 5: (UO2)3(OH)5A, 6: (UO2)3(OH)7B, 7: (UO2)3(OH)7; [charges omitted for simplicity]. Experimental conditions: CUO2 = 310
6 mol l
1; S = 35.

A. Gianguzza et al. / Marine Chemistry 85 (2004) 103–124

115

Table 9 Pitzer interaction parameters for dioxouranium(VI) hydrolytic species Species

Interacting ion

UO2(OH)+ UO2(OH)+ UO2(OH)+ (UO2)2(OH)22 + (UO2)2(OH)22 + (UO2)2(OH)22 + (UO2)3(OH)24 + (UO2)3(OH)24 + (UO2)3(OH)+5 (UO2)3(OH)+5 (UO2)3(OH)
7

NO
3 Cl
Cl
c NO
3 Cl
Cl
c NO
3 Cl

NO3 Cl
Na+

a log Tbpq


5.182
5.199
5.199
5.782
5.750
5.752
11.795
11.815
15.870
15.901
29.25

b(0)

b(1)

C(/)

rb


0.0939 0.5689 0.5387 0.9519 0.4291 0.3711 1.0896 0.06649
0.08552 0.07369
0.5798

0.7574 0.5785 0.5798 0.9213 2.1173 2.1095 2.3995 2.2839 1.1719 1.2522 1.0034

– – –
0.1759
0.00365
0.00398 – 0.02615 0.07810 0.01642 0.08821

0.004 0.003 0.003 0.001 0.005 0.005 0.003 0.006 0.001 0.003 0.015

I = 0 mol (kg H2O)
1. Standard deviation of the fit. c Calculations performed considering the ‘‘high order terms’’. a

b

its species using hydrolysis and complex formation constants (converted to the molal scale) in all the previous studies including NaCl and NaNO3 (from De Stefano et al., 2002a), over a wide range of ionic strengths and salinities. Table 9 gives the Pitzer interaction parameters from the hydrolysis constants in NaCl and NaNO3 ionic media. The extrapolated constants to I = 0 mol (kg H2O)
1 are also reported in this table. These constants estimated with Pitzer model can be compared with those previously determined by De Stefano et al. (2002a) using Eqs. (1) – (5) and give values of log T bpq of
5 .189 [UO 2 (OH) + ],
5.75 9 [(UO2)2(OH)22 +],
11.818 [(UO2)3(OH)42 +],
15.886 [(UO2)3(OH)5+] and
29.254 [(UO2)3(OH)7
]. The two models yield constants that agree within the experimental error. Since the measurements for NaCl were performed at elevated ionic strengths, we decided to treat our data using ‘‘higher order terms’’ (Pitzer, 1991; Millero, 1982). These results are reported in Table 9 for first two hydrolysis constants. The addition of these higher order terms did not improve our fits and, therefore, we opted for the simpler model (i.e. without higher order terms). These Pitzer parameters for interactions between cationic hydrolytic species and chloride were used to calculate parameters in lithium, calcium and magnesium chloride solutions. Slight differences between experimental and calculated constants were observed. These discrepancies have been attributed to ‘‘same sign’’ and ‘‘triple’’ interactions. This choice is justified by the fact that, of the three cited media, the first is composed of a small monovalent ion

with an high ‘‘nuclear effective charge’’, whilst the last two are divalent cations. The values of the higher order terms (H and W) for uranyl species in the three chloride media are given in Table 10. Reliability of these parameters given in Tables 9 and 10 was tested by comparing the measured and calculated dioxouranium(VI) hydrolysis constants in CaCl2/MgCl2 mixtures. These results are given in Table 11 along with the mean deviations for each species. All of the estimates are within experimental errors and therefore, demonstrate the consistency of our assumptions. Looking carefully at the data shown in Table 11, another interesting aspect emerges. The highest deviations occur at higher ionic strengths, and are probably due to further (weaker) interactions. As regards to (UO2)3(OH)7
, the Pitzer parameters have been calculated not only for interactions between this species and Table 10 (+ +) and (+ +
) Pitzer interaction parameters for dioxouranium(VI) cationic hydrolytic species Species (UO2)2(OH)22 + (UO2)2(OH)22 + (UO2)2(OH)22 + (UO2)3(OH)24 + (UO2)3(OH)24 + (UO2)3(OH)24 + (UO2)3(OH)+5 (UO2)3(OH)+5 (UO2)3(OH)+5 a b

H

M +

Li Ca2 + Mg2 + Li+ Ca2 + Mg2 + Li+ Ca2 + Mg2 +

0.0777
0.4059 0.0601 0.1139
2.6649 2.1238 0.3660
1.4412
0.8430

Triple interactions with Cl
. Standard deviation of the fit.

Wa
0.0254 1.1767 0.0497 0.0144 3.0568 1.2293
0.0263 2.5663 1.9152

rb 0.01 0.007 < 0.001 0.01 0.008 0.005 0.01 < 0.001 0.005

116

A. Gianguzza et al. / Marine Chemistry 85 (2004) 103–124

Table 11 Experimental and calculated values of cationic hydrolytic species of dioxouranium(VI) in CaCl2/MgCl2 mixtures exp log b22

mCa

mMg

Im

0.030 0.060 0.121 0.020 0.040 0.081 0.010 0.020 0.040

0.020 0.040 0.081 0.030 0.060 0.121 0.040 0.080 0.161

0.151
6.03 0.301
6.10 0.604
6.23 0.151
6.01 0.301
6.06 0.604
6.12 0.151
6.05 0.301
6.15 0.604
6.28 Mean deviation

a

cal log b22


6.03
6.11
6.22
6.03
6.11
6.22
6.04
6.12
6.22

Da 0.00 0.01
0.01 0.02 0.05 0.10
0.01
0.03
0.06 0.03

exp log b34

cal log b34


12.24
12.27
12.23
12.26
12.32
12.32
12.22
12.22
12.16


12.24
12.28
12.32
12.24
12.28
12.30
12.24
12.28
12.28

0.00 0.01 0.09
0.02
0.04
0.02 0.02 0.06 0.12 0.04

exp log b35

cal log b35

Da


16.61
16.74
16.90
16.63
16.77
16.94
16.63
16.80
16.95


16.64
16.82
17.07
16.65
16.83
17.08
16.65
16.84
17.09

0.03 0.08 0.17 0.02 0.06 0.14 0.02 0.04 0.14 0.05

D = log bpqcal
log bpqexp.

sodium (see Table 9), but also with cations of other investigated ionic media (i.e. Li+, Ca2 + and Mg2 +). The results given in Table 12 show some other interesting features. First, the interaction parameters involving (UO2)3(OH)7
and alkali metal cations are similar (see Table 9), whilst the parameters for calcium and magnesium are quite different. In fact, although the best fits for both alkaline earth cations were obtained without considering the values of b(1), the value of b(0) for Mg2 + is unexpectedly higher than the value for Ca2 +. These divergences represent a real index of a different behavior of uranyl in the two alkaline earth chloride media, which is shown in differences in the hydrolysis constants. 3.5. Pitzer interaction parameters for (UO2)p(OH)q (SO4)(2p
q
2r) species r As previously mentioned, the experimental data indicated that very strong interactions occur between UO22 + and its hydrolytic species with the sulfate anion, which we expressed in terms of complex formation constants. In spite of this, the Pitzer model usually does not take into account the explicit concept of ‘‘complex formation’’ between divalent metals with sulfate, but utilizes ‘‘supplementary’’ parameters (like,

for example, b(2) for 2 –2 interactions) to improve the fits. This procedure fails, especially when the formation of ion pairs is thermodynamically very favored. This is the case for dioxouranium(VI)/sulfate aqueous systems, where not only simple uranyl cation strongly interacts with one or more sulfate anions, but also its hydrolytic species do. For this reason, we have considered the formation of the ion paired species
q
2r) (UO 2) p (OH) q (SO 4)(2p in the solutions of r Na2SO4, Na2SO4/NaNO3 and Na2SO4/NaCl mixtures, using the complex formation constants reported in

Table 13
q
2r) Pitzer interaction parameters for (UO2)p(OH)q(SO4)(2p r species Species

Interacting ion b(0)

b(1)

UO22 +

SO24


UO2SO04 UO2SO04

Na2SOb4 Na2SO4/ NaNO3b Na2SO4/ NaClb Na+ Na+ Na2SO4b Na2SO4/ NaNO3b Na2SO4/ NaClb Na2SO4b Na2SO4/ NaNO3 b Na2SO4/ NaClb Na+

6.12 – –
0.256 0.09 –
0.165

UO2SO04 UO2(SO4)22
UO2(OH)(SO4)
(UO2)2(OH)2(SO4)0 (UO2)2(OH)2(SO4)0 (UO2)2(OH)2(SO4)0

Table 12 Pitzer interaction parameters for dioxouranium(VI) anionic hydrolytic species Species (UO2)3(OH)
7 (UO2)3(OH)
7 (UO2)3(OH)
7 a

Da

b(0)

M +

Li Ca2 + Mg2 +


0.2803 0.1433 5.5802

Standard deviation of the fit.

b(1) 1.235 – –

C(/)
0.0367
2.0228
2.7560

ra 0.02 0.001 0.001

(UO2)3(OH)4(SO4)0 (UO2)3(OH)4(SO4)0 (UO2)3(OH)4(SO4)0 (UO2)3(OH)5(SO4)
a b

Standard deviation of the fit. Background electrolytes.

– – –

k

ra

–

–


0.111

0.203 0.370 – –

– – – –

– 0.15 – 0.16 0.109 0.25
0.183

–

–


0.183

– –

– –

0.766 0.18
0.795

–

–


0.692

0.015

–

–

0.25

A. Gianguzza et al. / Marine Chemistry 85 (2004) 103–124

Table 7 for all sulfate media. In Table 13, Pitzer
q
2r) parameters for (UO2)p(OH)q(SO4)(2p species r (1) are reported together with b parameter for the UO22 + –SO42
interaction. This value was obtained together with other interaction parameters in all sulfate media, after several tests with other models (e.g. including b(0), b(2), C(/) and all combinations). It can be related to the interaction of the ‘‘free’’ uranyl cation with sulfate when the formation of sulfate complexes is considered, and is strictly valid only with these assumptions. The high value of b(1) comes out from the fact that this parameter, on its own, is needed to explain the dependence on ionic strength of the activity coefficient of the cited ‘‘free’’ uranyl cation. As can be observed from analysis of data in Table 13, for other interactions, only b(0) parameter was necessary to obtain acceptable fits. During calculations of Pitzer interaction parameters for (UO 2 ) p (OH) q
q
2r) (SO4)(2p species in the three sulfate media, we r simultaneously estimated corresponding values of each complex formation constant at I = 0 mol (kg H2O)
1. This resulted in the values of log T bpqr to be ( F standard deviation, species in parenthesis): 3.32 F 0.02 [UO2SO40], 4.26 F 0.04 [UO2(SO4)22
],
2.30 F 0.01 [UO2(OH)(SO4)
],
2.64 F 0.04 [(UO2)2(OH)2 (SO 4 ) 0 ],
8.45 F 0.04 [(UO 2 ) 3 (OH) 4 (SO 4 ) 0 ],
13.58 F 0.04 [(UO2)3(OH)5(SO4)
]. 3.6. Dependence on ionic strength of dioxouranium(VI) hydrolysis constants in synthetic seawater for equilibrium studies In order to account for the effect of ionic strength (i.e., salinity) on the hydrolysis constants obtained in SSWE, we handled the data reported in Table 8 by two different methods. First, we used the Pitzer model, and then we fitted the constants with a polynomial function of salinity. As concerns Pitzer equations, we preferred to calculate interaction parameters with seawater considered as single sea salt. This choice is a logical consequence of what we did when we obtained complex formation constants for uranyl species—single salt ion pairs and, furthermore, allows us to reduce the complexity of mathematical calculations. Because, to follow this scheme, Pitzer interaction parameters for the electrolyte ‘‘single sea salt’’ (BA) were needed, we calculated them from activity coefficients of single sea salt reported by ‘‘Robinson (1954)’’ and ‘‘Millero and

117

Leung (1976)’’. We obtained a very good fit (r < 0.001) calculating, for B1.117 + – A1.117
interactions, b(0) and b(1) only, that resulted to be, respectively: 0.1081 and 0.4238. As concerns Pitzer interaction parameter for H+ –A1.117
interactions, we obtained them from values of protonation constants of A1.117
tabled in ‘‘De Stefano et al. (1998)’’ (without taking into account autoassociation of BA). Values of b(0) and b(1) are, respectively,
0.3134 and 1.5375, with a standard deviation for the whole fit of 0.007. Once we calculated these Pitzer parameters for internal interactions (i.e. B1.117 + – A1.117
and H+ – A1.117
), we proceeded with simultaneous determination of parameters related to interactions of uranyl ion and its hydrolytic species with the cation or the anion of the single sea salt. As can be observed from the analysis of Table 14, where these parameters are reported, we obtained a good fit calculating only b(0) , except for the uranyl and (UO2)2(OH)22 + species, that required the calculation of b(1) in addition. In second approach, we handled our experimental hydrolysis constants in SSWE fitting them with a polynomial function of the square root of salinity logb ¼ logT b þ aS 1=2 þ bS

ð21Þ

where b is the hydrolysis constant, Tb is the hydrolysis constant at infinite dilution, and a and b are empirical parameters, shown in Table 15 together with extrapolated log Tb for each species. As can be observed, these constants are, within the experimental error, equal to the corresponding ones calculated from other systems and already published (De Stefano et al., 2002a). The consistency of the two different approaches (i.e. Pitzer and polynomial) is proved by the analysis of Table 16, where hydrolysis constants in SSWE at different salinities, calculated by the two models, are reported Table 14 Pitzer interaction parameters for dioxouranium(VI) and its hydrolytic species in SSWE (as single sea salt BA)a Species UO22 + UO2(OH)+ (UO2)2(OH)22 + (UO2)3(OH)24 + (UO2)3(OH)+5 (UO2)3(OH)
7 a

Interacting ion 1.117


A A1.117
A1.117
A1.117
A1.117
B1.117 +

F 0.1 standard deviation of the fit.

b(0)

b(1)

0.246
0.415
0.110 0.850 0.250
0.610


0.581 – 1.796 – – –

118

A. Gianguzza et al. / Marine Chemistry 85 (2004) 103–124

Table 15 Empirical parameters of Eq. (21) for the dependence of dioxouranium(VI) hydrolysis constants on salinity Species UO2(OH)+ (UO2)2(OH)22 + (UO2)3(OH)24 + (UO2)3(OH)+5 (UO2)3(OH)
7 a

log Tb

a

b


5.19 F 0.12a
5.77 F 0.15
11.82 F 0.15
15.90 F 0.15
29.28 F 0.24


0.11 F 0.06a
0.30 F 0.09
0.78 F 0.12
0.64 F 0.09
0.61 F 0.12

0.009 F 0.012a 0.020 F 0.012 0.005 F 0.015 0.041 F 0.012 0.057 F 0.015

F 3 standard deviation.

together with their differences with corresponding experimental values. In fact, as can be noted, deltas are always within the errors of measurements, except for some values that, however, show high discrepancies in both models: these differences could be attributed to rough experimental data. The satisfactory predictive power of these two models in synthetic seawater, which emerged from the calculations shown in Table 16, allows us to figure out the hydrolytic behavior of dioxouranium(VI) cation in natural aqueous systems (like seawater), where hydrolysis studies are of fundamental importance for the understanding of the binding ability of cations towards several organic and inorganic ligands and, therefore, for the comprehension of their environmental and biological activity.

3.7. Alternative interpretation of medium effects As pointed out by other authors (Daniele et al., 1985, 1994), the effect of media can often be interpreted in terms of the formation of weak complexes. By considering the simple model represented by Eq. (1), with the medium independent coefficients (Eqs. (3) and (4)), valid at I V 1 mol l
1, we were able to calculate the formation constants of some weak species. The lowering of hydrolysis constants in chloride media can be ascribed to the formation of UO2Cl+ ion pair, and the increasing effect of Cl
on b34 can be interpreted (and it was already interpreted in this way, see Baes and Mesmer, 1976) in terms of the formation of (UO2)3(OH)4Cl+ species (errors given as 3 standard deviation):
þ UO2þ 2 þ Cl ¼ UO2 Cl

logK ¼ 0:4F0:3


þ ðUO2 Þ3 ðOHÞ2þ 4 þ Cl ¼ ðUO2 Þ3 ðOHÞ4 Cl logK ¼ 1:6F0:2

The dependence on medium of hydrolysis constant for the anionic species (UO2)3(OH)7
can be inter-

Table 16 Dioxouranium(VI) hydrolysis constants in SSWE at different salinities, calculated by Eq. (21) and Pitzer equations log b11

S

Da

log b22

Da

log b34

Da

log b35

Da

log b37

Da

Eq. (21) 5
5.40 10
5.46 15
5.50 20
5.52 25
5.54 30
5.55 35
5.56 40
5.56 45
5.56

0.02
0.04 0.03 0.01 0.00
0.02
0.02
0.05 0.09


6.34
6.52
6.63
6.71
6.76
6.80
6.83
6.85
6.86


0.07 0.03
0.09 0.00 0.08 0.07 0.08
0.08
0.03


12.55
12.84
13.05
13.23
13.38
13.52
13.64
13.75
13.85


0.02
0.12 0.08 0.17
0.05
0.01
0.01 0.10
0.06


17.12
17.51
17.75
17.93
18.06
18.16
18.24
18.29
18.33


0.04
0.02
0.08 0.08 0.08 0.06 0.01
0.14
0.01


30.37
30.65
30.80
30.88
30.91
30.92
30.90
30.86
30.81


0.32 0.12
0.04 0.23
0.01
0.01 0.04 0.00
0.09

Pitzer 5 10 15 20 25 30 35 40 45

0.02
0.01 0.07 0.04 0.03
0.01
0.02
0.08 0.04


6.29
6.49
6.62
6.71
6.77
6.81
6.84
6.86
6.86


0.12 0.00
0.10 0.00 0.08 0.08 0.09
0.08
0.03


12.54
12.88
13.12
13.30
13.43
13.54
13.62
13.68
13.73


0.03
0.08 0.15 0.24 0.00 0.02
0.03 0.03
0.18


17.05
17.46
17.74
17.94
18.08
18.19
18.26
18.31
18.33


0.11
0.06
0.09 0.09 0.10 0.08 0.04
0.13
0.01


30.24
30.56
30.76
30.87
30.92
30.94
30.91
30.86
30.78


0.45 0.04
0.08 0.22 0.01 0.01 0.05 0.00
0.12

a


5.40
5.49
5.53
5.56
5.57
5.56
5.55
5.53
5.51

Experimental values of Table 8 minus calculated in corresponding left column.

A. Gianguzza et al. / Marine Chemistry 85 (2004) 103–124

preted in terms of weak interaction with the cation of the supporting electrolyte. By performing appropriate calculations, we found the following species with the relative formation constants: þ 0 ðUO2 Þ3 ðOHÞ
7 þ Na ¼ ðUO2 Þ3 ðOHÞ7 Na

logK ¼ 1:2F0:2 2þ ðUO2 Þ3 ðOHÞ
¼ ðUO2 Þ3 ðOHÞ7 Caþ 7 þ Ca

logK ¼ 2:74F0:08 2þ ¼ ðUO2 Þ3 ðOHÞ7 Mgþ ðUO2 Þ3 ðOHÞ
7 þ Mg logK ¼ 1:7F0:3

Moreover, potentiometric data for Ca2 + – Mg2 + mixtures give evidence for the formation of a mixed metal species (UO 2 ) 3 (OH) 7 CaMg 3 + (log b =
25.38 F 0.10): ðUO2 Þ3 ðOHÞ7 Mgþ þ Ca2þ ¼ ðUO2 Þ3 ðOHÞ7 MgCa3þ logK ¼ 2:2F0:3 Formation constants reported in this section are given at I = 0 mol l
1 and can be calculated at other ionic strengths using Eqs. (1) – (4). 3.8. Comparisons with literature In spite of the wide number of literature reports on the hydrolysis and complex formation of uranyl ion (Grenthe et al., 1992; Ahrland, 1949, 1951; Ahrland et al., 1954; Peterson, 1961; Rush et al., 1962; Baes and Meyer, 1962; Dunsmore et al., 1963; Dunsmore and Sillen, 1963; Sylva and Davidson, 1979; Lemire and Tremaine, 1980; Yang and Pitzer, 1989; NguyenTrung et al., 1992, 2000; Palmer and Nguyen-Trung, 1995; Plyasunov et al., 1998; Wahlgren et al., 1999; Comarmond and Brown, 2000; Moll et al., 2000; Yu et al., 2000; Ciavatta et al., 2003a,b; Nguyen-Trung and Hovey, 1990), few papers give results on measurements carried out in ionic media that are of environmental or biological interest (i.e. NaCl, KCl, Na2SO4 and so on) (Peterson, 1961; Rush et al., 1962; Dunsmore et al., 1963; Dunsmore and Sillen, 1963; Lemire and Tremaine, 1980; Yang and Pitzer, 1989; Nguyen-Trung et al., 1992; Comarmond and Brown,

119

2000; Moll et al., 2000; Ciavatta et al., 2003a,b; Nguyen-Trung and Hovey, 1990). Moreover, few authors report hydrolysis constants of dioxouranium(VI) at different ionic strengths. No data are available on UO22 + hydrolysis in lithium, calcium and/or magnesium chlorides, although results in Ca(ClO4)2, Mg(ClO4)2 and Mg(NO3)2 are (Dunsmore et al., 1963; Hietanen et al., 1963; Schedin and Frydman, 1968). Measurements in real or synthetic seawater are also not available in the literature. The present paper for the first time presents quantitative data on dioxouranium(VI) speciation in natural systems, especially as regards hydrolysis and complex formation in seawater systems. Many earlier papers characterized hydrolytic species at pH values close to those where the formation of insoluble species starts. Most authors divide into two groups with different theories. One hypothesize the existence of the (UO2)4(OH)62 + and/or (UO2)4(OH)7+ species. The formation of these species is mainly supported by several Scandinavian authors (Ahrland et al., 1954; Peterson, 1961; Dunsmore and Sillen, 1963; Dunsmore et al., 1963). In particular, Sillen first proposed the ‘‘core-plus-link’’ theory, where the formation of just [(UO2)n(OH)2n
2]2 + species was stated, or, eventually, some of their deprotonated species as, for example, (UO2)3(OH)5+ and (UO2)4(OH)7+ (Ahrland et al., 1954). Probably due to the importance of Sillen’s work, other research groups postulated the presence of these species (Sylva and Davidson, 1979; Comarmond and Brown, 2000). Very often, however, different hypothesis were not considered as, for example, the article by Sylva and Davidson (1979). In their paper, they consider a number of models consisting of various groups of the core-plus-link species [(UO2)n(OH)2n
2]2 + and the two further deprotonated species [(UO2)3(OH)5]+ and/or [(UO2)4(OH)7]+. Palmer and Nguyen-Trung (1995) demonstrated that, on the same experimental data, (UO2)4(OH)62 + could be replaced by (UO2)3(OH)7
species without significantly altering the fit. These authors belong to the second group of researchers that, after testing several different models [including (4,6) and (4,7) species], proposed the existence of the negative species (UO2)3(OH)7
or, at least, refused the core-pluslink theory (Rush et al., 1962; Baes and Meyer, 1962; Lemire and Tremaine, 1980; Palmer and NguyenTrung, 1995; Nguyen-Trung et al., 2000; De Stefano

120

A. Gianguzza et al. / Marine Chemistry 85 (2004) 103–124

et al., 2002a). Palmer and Nguyen-Trung (1995) and other scientists also demonstrated the presence of this species at pH>5 by Raman spectroscopic analysis (Nguyen-Trung et al., 2000). In this study they tested the existence of a wide number of uranyl hydrolytic species [tetranuclear ones too] and they obtained, as results, substantial evidence for the presence, at high pH values, of four trinuclear species [(UO2)3(OH)7
, (UO2)3(OH)82
, (UO2)3(OH)104
, (UO2)3(OH)115
] and one mononuclear [UO2(OH)42
] at 5.63 V pH V 14.96. They also affirmed that ‘‘[. . .] (UO2)3(OH)7
is the most dominant species over most of the pH range (4.53 – 12.78) [. . .]’’, whilst the monomeric one, UO2(OH)42
, dominates in highly basic solution (12.48 V pH V 14.96). These assertions are in reasonable agreement with what was successively proposed by Wahlgren et al. (1999) (Scandinavian school), about the formation of the UO2(OH)42
monomer in strong alkaline solutions. On the other hand, these authors, in the same paper, refute the existence of trinuclear species postulated by Palmer and NguyenTrung. Our results are in excellent agreement with the latter authors, showing that Raman and potentiometric studies are in accord (Palmer and NguyenTrung, 1995, Nguyen-Trung et al., 2000). Our measurements performed in LiCl, reaching higher pH values, demonstrated that high (~40%) percentages of (UO2)3(OH)7
were present before onset of precipitation (Fig. 1). These results confirm the existence and the significance of this anionic species. All the measurements in the literature (Ahrland, 1951; Peterson, 1961; Dunsmore et al., 1963; Nguyen-Trung and Hovey, 1990; Nguyen-Trung et al., 1992; Moll et al., 2000; Comarmond and Brown, 2000; Ciavatta et al., 2003a,b) agree with our postulation that uranyl cation and its hydrolytic species strongly interact with sulfate. The review by Grenthe et al. (1992) also supports the formation of two UO22 + – SO42
complexes, UO2SO40 and UO2(SO4)22
. Values of the corresponding formation constants at I = 0 mol l
1 are also in good agreement, within experimental errors, with our recommended values (we found 3.32 F 0.02 and 4.26 F 0.04 against 3.15 F 0.02 and 4.14 F 0.07, respectively, for UO2SO40 and UO2(SO4)22
species). Our formation constants are also in agreement with the results of the more recent work (log Tb are 3.08 and 4.28 F 0.15) of Ciavatta et al. (2003a). Few authors, however, pro-

pose the existence of UO2(SO4)34
, which is suggested by Grenthe et al. (1992). This complex is very weak and, if formed at all, only very low or negligible percentages (Grenthe et al., 1992). Therefore, its absence does not affect uranyl speciation. In spite of the common conclusions on free dioxouranium(VI) – sulfate ion pairs, some divergences emerge on the nature (and, therefore, on the strength) of uranyl –hydroxide –sulfate ternary systems. In fact, most authors suggest the formation of species with more than two sulfate ligands bonded to uranyl hydrolytic species (Ahrland, 1951; Peterson, 1961; Dunsmore et al., 1963; Lemire and Tremaine, 1980; Comarmond and Brown, 2000; Moll et al., 2000; Ciavatta et al., 2003b). With the assumption in this work that non-covalent interactions involve only one sulfate anion per uranyl species, good fits were obtained for the data for all our experimental conditions. In support, we note that identical results were obtained by treating separately the experimental data obtained in pure Na2SO4 media and in its mixtures with NaNO3 and NaCl. The more recent work by Ciavatta et al. (2003b) suggests that only ternary complexes with one bonded sulfate ion exist (3,4), (3,5) and (2,1). There are slight differences between values of overall complex formation constants at infinite dilution for (UO2)3(OH)4(SO4)0 and (UO2)3 (OH) 5 (SO 4 )
and their calculated values: we found
8.45 F 0.04 and
13.58 F 0.04 ( F standard deviation) against
7.0 and
12.6 ( F 0.4 standard deviation), respectively. These divergences can be attributed to differences in the speciation models proposed. It is important to note that these authors emphasize the fact that one of the most important species in sulfate media is the monosulfate complex (UO2)3(OH)4(SO4)0. As discussed earlier the Pitzer interaction parameters calculated in the present paper for UO22 + –SO42
interaction are in dispute. Little information is in the literature except for the work by Plyasunov et al. (1998). These authors used literature data to compare the SIT and Pitzer models for interactions of several uranium species with a wide number of counter ions. They considered interactions directly of some strong uranium complexes with other ions. For example, they reported b(0) and b(1) parameters for UO2(SO4)22
Na+ interaction (0.30 and 1.9, respectively); these are in very good agreement with the b(0) value we reported

A. Gianguzza et al. / Marine Chemistry 85 (2004) 103–124

here (0.203), if we take into account the fact that our calculations did not consider the b(1) parameter. Excellent agreement is also evident when b(0) and b(1) for interactions of uranyl hydrolytic species and chloride or nitrate anions are compared (always remembering that in some case we calculated C(/) in addiction). For interaction among (UO2)3(OH)5+ and Cl
or NO3
, they report: b(0) =
0.02, b(1) = 1.4 (Cl
) and b(0) =
0.15, b(1) = 1.1 (NO3
). We found: b(0) = 0.07, b(1) = 1.3 (Cl
) and b(0) =
0.09, b(1) = 1.2 (NO3
). Identical considerations for (UO2)3(OH)42 + and Cl
or NO3
, they report: b(0) =
0.15, b(1) = 2.4 (Cl
) and b(0) = 0.50, b(1) = 2.1 (NO3
). We found: b(0) = 0.07, b(1) = 2.3 (Cl
) and b(0) = 1.09, b(1) = 2.4 (NO3
). Last, for (UO2)2(OH)22 + and Cl
or NO3
, they have: b(0) = 0.40, b(1) = 2.0 (Cl
) and b(0) = 0.42, b(1) = 1.9 (NO3
). We found: b(0) = 0.43, b(1) = 2.1 (Cl
) and b(0) = 1.0, b(1) = 0.9 (NO3
). Finally, we feel that our results provide a good representation of uranyl speciation in many ionic media and in reasonable agreement with already published results. In addition, our studies on the dependence on ionic strength and on ionic medium of uranyl hydrolysis and complex formation by different models, in particular by Pitzer equations, allow predictions on the dioxouranium(VI) chemical behavior in experimental conditions different from those studied in the present paper as, for example, in hyper saline or waste waters. 3.9. Final remarks Since the biological and chemical behavior of the dioxouranium(VI) cation is strictly related to the form in which it is present in the environment, speciation in natural waters represents the basis to understand its biochemistry. Main conclusions on the hydrolysis and chemical speciation of uranyl cation in aqueous media simulating the major composition of seawater can be summarized as follows: (a) we reported results on UO22 + chemical speciation in a wide number of aqueous ionic media, the most of them of biological and environmental interest; (b) in particular, hydrolysis carried out in synthetic seawater, complex formation studies using the single salt approximation and evidences on the possibility of the extrapolation of our results to

121

higher pH values make it possible to characterize the speciation of dioxouranium(VI) in this ionic medium; (c) this characterization allows further considerations of uranyl speciation in real seawater, where the comprehension of the binding ability of this cation towards several organic and inorganic ligands present in this system need to be considered; (d) our results obtained, in several media, on dioxouranium(VI) hydrolysis in the pH region close to the onset of precipitation, together with a careful analysis of literature data, give strong support to the existence of the anionic (UO2)3(OH)7
species, instead of (UO2)4(OH)62 + and/or (UO2)4(OH)7+; (e) our studies on the dependence on ionic strength and on ionic medium of uranyl hydrolysis and complex formation by different models, in particular by Pitzer equations, allow predictions on the dioxouranium(VI) physical chemical behavior in experimental conditions different from those studied in the present paper; this is mainly important for uranyl speciation studies in hyper saline or waste waters; (f) further development of this study on the speciation of UO22 + in natural fluids must certainly include the analysis, both by new experimental data and by literature critical review (see, e.g., Ciavatta et al., 1979, 1981; Ferri et al., 1981, 1983; Grenthe et al., 1984; and references therein), of the interactions with carbonate and fluoride together with the relative dependence on medium.

Acknowledgements A.G., D.M. and S.S. thank the Italian Ministero dell’Istruzione, dell’Universita` e della Ricerca (MIUR) for financial support. F.J.M. thanks the Oceanographic Section of the National Science Foundation for supporting his studies. Associate editor: Dr. Keith Hunter

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