The Geochemical Numerical Model of Liguria: a stochastic tool to evaluate the uncertainty of elemental concentration estimates.

Stream sediments have intrinsically the capability to describe the geochemistry of the sample's upstream basin, averaging each elemental contribution of the different outcropping lithotypes. Finer fraction (<150 um) is used as indicator of landscape status respect to pollutant, because of high reactivity with surficial water, groundwater and biota in a long-term environmental monitoring. Furthermore a sampling point on a stream network shows a nearly stable composition under the assumption of watershed time/space stationarity. Aside it is useful to underline that hot topic in environmental geochemistry is the definition of wide-area elemental background concentration value in geological matrixes. Moreover it is often mandatory for regulatory activities to define local elemental reference values over a discrete area to be compared to point concentration. This need is strictly related to a change of support passing from point measurement to evaluation over a representative area unit. Ultimately those reference values in whatever manner are defined over heterogenous geological settings can reach limit of applicability or worst overused if not defined in a probabilistic frame. The heterogeneity of the Ligurian Alps and Appennines invokes a change of strategy for what concerns the description of the spatial distributed geochemistry capturing the local variability of the regolith composition. With the aim to develop a flexible and highly adaptive tool based on geostatistical approach, the Geochemical Numerical Model of Liguria (GNM-L) of stream sediments is created. That digital product is the result of a joint project of the Regione Liguria and DISTAV to generate a tool able to fix the geochemistry of the environment based on stream sediments and to become a reference layer for future environmental intervention. To manage lithotypes heterogeneities computations are led on a-priori defined irregularly shaped discrete mosaic of the Liguria with the development of a geostatistical knowledge of regional variability. The database used to compute the spatial patterns of 32 elements by means of 8 statistical moments and probabilistic cut points of distribution functions is composed by 1830 sampling stations along the stream network. Each of them are sampled with a quasi-random sampling schema based on a subset of the grid GRID generated as a reference layer during the creation of the Archivio Geochimico Nazionale in the frame of the ANPA-CNR-UNIGE joint project. Through the application of a stochastic method based on gaussian sequential simulations over a multi resolution grid computation each chemical element is deeply investigated as a random function on a final support of 200m x 200m. The enhancement of the information density on each single element of the GNM allows the end-user to understand more profoundly the environment with the help of a sufficiently flexible tool that more adherently can represent the point uncertain reality

The iron-isotope fractionation dictated by the carboxylic functional: An ab-initio investigation

The ground-state geometries, electronic energies and vibrational properties of carboxylic complexes of iron were investigated both in vacuo and under the effect of a reaction field, to determine thermodynamic properties of iron\u2013acetates and the role of the carboxylic functional on the isotopic imprinting of this metal in metalorganic complexation. The electronic energy, zero-point corrections and thermal corrections of these substances at variational state were investigated at the DFT/B3LYP level of theory with different basis set expansions and the effect of the reaction field on the variational structures was investigated through the Polarized Continuun Model. Thermochemical cycle calculations, combined with solvation energy calculations and appropriate scaling from absolute to conventional properties allowed to compute the Gibbs free energy of formation from the elements of the investigated aqueous species and to select the best procedure to be applied in the successive vibrational analysis. The best compliance with the few existing thermodynamic data for these substances was obtained by coupling the gas phase calculations at DFT/B3LYP level with the [6-31G(d,p)]\u2013[6-31G+(d,p)] (for cations and neutral molecules \u2013 anions; respectively) with solvation calculations adopting atomic radii optimized for the HF/6-31G(d) level of theory (UAHF). A vibrational analysis conducted on 54Fe, 56Fe, 57Fe and 58Fe gaseous isotopomers yielded reduced partition function ratios which increased not only with the nominal valence of the central cation, as expected, but, more importantly, with the extent of the complexation operated by the organic functional. Coupling thermodynamic data with separative effects it was shown that this last is controlled, as expected, by the relative bond strength of the complex in both aggregation states. Through the Integral Equation Formalism of the Polarized Continuum Model (IEFPCM) the effect of the ionic strength of the solution and of a T-dependent permittivity on the energy and separative effects of the solvated metalorganic complexes were analyzed in detail. The solvent effect in the standard state (hypothetical one-molal solution referred to infinite dilution; T = 298.15 K, P = 1 bar) is a limited reduction of the separative effects of all the isotopomeric couples. With an increase in T (and the concomitant decrease in the dielectric constant of the solvent) this effect diminishes progressively

Ab-initio structure, energy and stable Fe isotope equilibrium fractionation of some geochemically relevant H\u2013O\u2013Fe complexes

The hexa-aqua complexes [Fe(H2O)6mn(OH)n](2n)+ n = 0 > 3, m = 0 > 6 n; [Fe(H2O)6mn(OH)n](3n)+ n = 0 > 4, m = 0 > 6 n were investigated by ab-initio methods with the aim of determining their ground-state geometries, total energies and vibrational properties by treating their inner solvation shell as part of their gaseous precursor1 (or \u201chybrid approach\u201d). After a gas-phase energy optimization within the Density Functional Theory (DFT), the molecules were surrounded by a dielectric representing the Reaction Field through an implicit Polarized Continuum Model (PCM). The exploration of several structural ligand arrangements allowed us to quantify the relative stabilities of the various ionic species and the role of the various forms of energy (solute\u2013solvent electronic interaction, cavitation, dispersion, repulsion, liberation free energy) that contribute to stabilize the aqueous complexes. A comparison with experimental thermochemistries showed that ab-initio gas-phase + solvation energies are quite consistent with experimental evidence and allow the depiction of the most stable form in solution and the eventual configurational disorder of water/hydroxyl species around central cations. A vibrational analysis performed on the 54Fe, 56Fe, 57Fe and 58Fe isotopomers indicated important separative effects systematically affected by the extent of deprotonation. The role of the system\u2019s redox state (fO2) and acidity (pH) on the isotopic imprinting of the aqueous species in solution was investigated by coupling the separative effects with speciation calculations. The observed systematics provided a tool of general utility in the interpretation of the iron isotopic signature of natural waters. Applications to the interpretation of isotopic fractionation in solution dictated by redox equilibria and to the significance of the Fe-isotopic imprinting of Banded Iron Formations are given

A Theoretical Model Chemistry Algorithm for the ab-initio computation of reactive properties of aqueous molecules of geochemical interest

metodi per il calcolo di propriet\ue0 termodinamiche con procedure ab-initi