Naturally dissolved arsenic concentrations in the Alpine/Mediterranean Var River watershed (France).

A detailed study on arsenic (As) in rocks and water from the Var River watershed was undertaken aiming at identifying (i) the origin and the distribution of As in this typical Alpine/Mediterranean basin, and (ii) As input into the Mediterranean Sea. Dissolved As concentrations in the Var River range from 0.1 to 4.5 µg⋅L(-1), due to high hydrological variability and the draining through different geological formations. In the upper part of the Var drainage basin, in the Tinée and the Vésubie valleys, high levels of dissolved As concentrations occur (up to 263 µg⋅L(-1)). The two main sources of As in rocks are the Hercynian metamorphic rocks and the Permian argilites. Highly heterogeneous distribution of As in waters draining through metamorphic rocks is probably related to ore deposits containing arsenopyrite. As, U, W and Mo concentrations in water and rocks correspond to the formation of As-rich ore deposits around Argentera granite by hydrothermal fluids deposited at the end of the Hercynian chain formation, which occurred about 300 My ago. In 2009, weekly monitoring was performed on the Var River (15 km upstream of the mouth), highlighting an average dissolved As concentration (<0.45 µm) of 2.7 ± 0.9 µg⋅L(-1), which is significantly higher than the world-average baseline for river water (0.83 µg⋅L(-1)). Taking the average annual discharge (49.4 m(3)⋅s(-1)) into account and the As levels in the dissolved phase and in deposits of the Var River, dissolved As input into the Mediterranean Sea would be 4. 2± 1.4 tons⋅year(-1) which represents 59% of the total As flux. This study also reveals a probable non-conservative As behaviour, i.e., possible transfer between aqueous and solid phases, during the mixing of the Var River with a tributary.

Formation of MSeO(4)(aq) complexes (M(2+) = Mg(2+), Co(2+), Ni(2+), Cu(2+), Cd(2+)) studied as a function of temperature by affinity capillary electrophoresis.

Complexation of divalent cations (Mg(2+), Co(2+), Ni(2+), Cu(2+), Cd(2+)) by selenate ligand was studied by ACE (UV indirect detection) in 0.1 mol/L NaNO(3) ionic strength solutions at various temperatures (15, 25, 35, 45 and 55°C). For each solution, a unique peak was observed as a result of a fast equilibrium between the free ion and the complex (labile systems). The migration time corresponding to this peak changed as a function of the solution composition, namely the free and complexed metal concentrations, according to the complexation reactions. The results confirmed the formation of a unique 1:1 complex for each cation. The thermodynamic parameters were fitted to the experimental data at 0.1 mol/L ionic strength: (25°C) = -(6.5 ± 0.3), -(7.5 ± 0.3), -(7.7 ± 0.3), -(7.7 ± 0.3), and -(8.1 ± 0.3) kJ/mol and = 2.5 ± 0.2, 4.7 ± 0.4, 4.5 ± 0.6, 8.4 ± 1.1, and 7.2 ± 0.6 kJ/mol for M(2+) = Mg(2+), Co(2+), Ni(2+), Cu(2+), and Cd(2+), respectively. Complexes with alkaline earth and transition metal cations could be distinguished by their relative stabilities. The effect of the ionic medium was treated using the specific ion interaction theory and the thermodynamic parameters at infinite dilution were compared to previously published data on metal-selenate, metal-sulfate, and metal-chromate complexes.

Formation of CaSO4(aq) and CaSeO4(aq) studied as a function of ionic strength and temperature by CE.

Ca(2+) complexation by both sulfate and selenate ligands was studied by CE. The species were observed to give a unique retention peak as a result of a fast equilibrium between the free ions and the complexes. The change in the corresponding retention time was interpreted with respect to the equilibrium constant of the complexation reaction. The results confirmed the formation of CaSO(4)(aq) and CaSeO(4)(aq) under our experimental conditions. The formation data were derived from the series of measurements carried out at about 15, 25, 35, 45 and 55 degrees C in 0.1 mol/L NaNO(3) ionic strength solutions, and in 0.5 and 1.0 mol/L NaNO(3) ionic strength solutions at 25 degrees C. Using a constant enthalpy of reaction enabled to fit all the experimental data in a 0.1 mol/L medium, leading to the thermodynamic parameters: Delta(r)G(0.1M)(25 degrees C)=-(7.59+/-0.23) kJ/mol, Delta(r)H(0.1 M)=5.57+/-0.80 kJ/mol, and Delta(r)S(0.1 M)(25 degrees C)=44.0+/-3.0 J mol(-1) K(-1) for CaSO(4)(aq) and Delta(r)G(0.1 M)(25 degrees C)=-(6.66+/-0.23) kJ/mol, Delta(r)H(0.1 M)=6.45+/-0.73 kJ/mol, and Delta(r)S(0.1 M)(25 degrees C)=44.0+/-3.0 J mol(-1) K(-1) for CaSeO(4)(aq). Both formation reactions were found to be endothermic and entropy driven. CaSO(4)(aq) appears to be more stable than CaSeO(4)(aq) by 0.93 kJ/mol under these experimental conditions, which correlates with the difference of acidity of the anions as expected for interactions between hard acids and hard bases according to the hard and soft acids and bases theory. The effect of the ionic medium on the formation constants was successfully treated using the Specific ion Interaction Theory, leading to significantly different binary coefficients epsilon(NA+,SO(2-)(4)) = -(0.15 +/- 0.06) mol/kg-1 and epsilon(NA+,SO(2-)(4)) = -(0.26 +/- 0.10) mol/kg-1.

Evidence of different stoichiometries for the limiting carbonate complexes across the lanthanide(III) series: a capillary electrophoresis-mass spectrometry study.

The electrophoretic mobilities (mu ep,Ln) of twelve lanthanides (not Ce, Pr and Yb) were measured by CE-ICP-MS in 0.15 and 0.5 mol L(-1) Alk2 CO3 aqueous solutions for Alk+ = Li+, Na+, K+ and Cs+. In 0.5 mol L(-1) solutions, two different mu ep,Ln values were found for the light (La to Nd) and the heavy (Dy to Tm) lanthanides, which suggests two different stoichiometries for the carbonate limiting complexes. These results are consistent with a solubility study that attests the Ln(CO3)3(3-) and Ln(CO3)4(5-) stoichiometries for the heavy (small) and the light (big) lanthanides, respectively. The Alk+ counterions influence the mu ep,Ln Alk2 CO3 values, but not the overall shape of the mu ep,Ln Alk2 CO3 plots as a function of the lanthanide atomic numbers: the counterions do not modify the stoichiometries of the inner sphere complexes. The influence of the Alk+ counterions decreases in the Li+ > Na+ >> K+ > Cs+ series. The K3,Ln stepwise formation constants of the Ln(CO3)3(3-) complexes slightly increase with the atomic numbers of the lanthanides while K4,Ln, the stepwise formation constants of Ln(CO3)4(5-) complexes, slightly decrease from La to Tb, and is no longer measurable for heavier lanthanides.