Geochemical modelling of natural contaminants in groundwaters and their removal by membrane processes
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Fuoco, Ilaria
Critelli, Salvatore
Apollaro, Carmine
Marini, Luigi
Bartolo, Gabriele
Figoli, Alberto
De Rosa, Rosanna
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Dottorato di Ricerca in
Scienze ed Ingegneria per l'Ambiente, le Costruzioni e l'Energia. Ciclo XXXIII; The presence of harmful elements dissolved in groundwaters represents one of the
main environmental issues of present times. The aim of my PhD Thesis was to study the
rock-to-water release and the fate of some inorganic pollutants dissolved in the
groundwaters of the Calabria Region, and sometimes in other Italian areas, as well as to
select suitable membrane technologies for their removal based on the acquired geochemical
knowledge. Taking into account the peculiar characteristics of the groundwaters of the
Calabria Region, three elements were considered: arsenic (As), fluorine (F) and chromium
(Cr). The salient results of my PhD studies are presented in this Thesis, which is organized in
four self-consistent Chapters structured as follows.
Chapter 1. Geochemical modelling of As and F release into the crystalline aquifers of
the Calabria Region. This section has been devoted to understanding the water-rock
interaction processes occurring in the crystalline aquifers of the Calabria Region. Three
different reaction path modeling exercises of granite dissolution were performed,
reconstructing the water-rock interaction processes which occur: (i) in shallow and relatively
shallow crystalline aquifers in which no As and F anomalies were observed; (ii) in As-rich
areas, coupling the reaction path modeling of granite dissolution with the simulation of the
adsorption of dissolved As onto precipitating crystalline and amorphous hydrous ferric oxide
(HFO); (iii) in deep crystalline aquifers where high F concentrations were detected. A total of
160 water samples discharging from the crystalline aquifers of the Calabria region were used
to fix the boundary conditions as well as to validate the outcomes of geochemical modeling.
The results of the three geochemical modeling exercises of granite dissolution are in
agreement with the analytical data and, therefore, it is reasonable to assume that they
reproduce satisfactorily the water-rock interaction processes occurring during the travel of
meteoric waters from shallow to deep crystalline aquifers, hosted both in granite rocks
without mineralizations and in mineralized granites. Moreover, based on the results of the
geochemical survey, some As- and F- rich groundwaters were selected and used as feeds for
the treatment tests. Some relevant results have been already published by Fuoco et al.
(2021a).
Chapter 2. Geochemical modelling of Cr(VI) release into the ophiolite aquifers of Italy.
This section has been focused on the water-rock interaction processes occurring in the main
ophiolite aquifers of Italy. The obtained results were already published by Apollaro et al.
(2019a). Reaction path modelling of serpentinite dissolution was performed varying the
Fe2O3/(FeO +Fe2O3) weight ratio of serpentine and reproducing the analytical concentrations
of relevant solutes, including Cr(VI), in the Mg-HCO3 groundwaters hosted in the ophiolite
aquifers of Italy. The occurrence of geogenic Cr(VI) in these groundwaters appears to be
potentially controlled by the oxidation of trivalent Cr to the hexavalent redox state, driven
by the reduction of trivalent Fe to the divalent redox state. In fact, trivalent Fe is the only
oxidant present in suitable amounts in serpentinite rocks, and even serpentine contains high
contents of trivalent Fe as proven by recent studies. In contrast, the generally accepted
hypothesis that geogenic Cr(VI) in waters interacting with serpentinites is driven by the
2
reduction of trivalent and tetravalent Mn is questionable. To validate the outcomes of the
geochemical modeling of serpentinite dissolution and rock-to-water release of Cr(VI), the
redox state of Fe in serpentine minerals of different Italian areas was measured (see next
section). Moreover, the water sample characterized by the highest concentration of Cr(VI)
was selected for the treatment tests.
Chapter 3. Determination of the iron redox state in serpentine minerals by using TEMEELS
analysis and its environmental implications. This section has been addressed to
determine the Fe3+/FeT ratio of the serpentine minerals hosted in five serpentinite samples
coming from the main ophiolite areas of Italy, in order to validate the results of the
geochemical modeling of serpentinite dissolution and rock-to-water release of hexavalent
chromium (see previous section). The electron energy-loss spectroscopy (EELS) combined
with transmission electron microscopy (TEM) was selected as most suitable analytical
technique because it provides the highest spatial and energy resolution allowing to obtain a
good qualitative and quantitative information on iron redox speciation compared to other
analytical techniques. Ten site-specific TEM foils were prepared and analyzed in the
laboratories of the GFZ German Research Centre for Geosciences, in Potsdam. It has not
been the simple application of a well-established analytical technique, but rather a
challenging study, because it has been necessary to develop a new spectra processing
method and prove its validity. The performed analyses provided in-depth knowledge on
nanoscale structures of the studied samples and showed that Fe3+ represents from 75 to 85
% (median values) of total iron. The obtained results provide further support to the
hypothesis concerning the oxidation Cr(III) to Cr(VI) driven by the concurrent reduction of
Fe(III) to Fe(II), as suggested by geochemical modeling (see previous section).
Chapter 4. Application of membrane processes to remove As, F and Cr(VI) from
contaminated groundwaters. This section has been devoted to the removal of As, F and Cr
from polluted groundwaters by means of membrane processes. Relevant results have been
already published by Figoli et al. (2020), Fuoco et al. (2020) and Fuoco et al. (2021b). The
geochemical approach was used as strong-scientific tool for pre-selection of suitable
remediation systems and the contaminated groundwaters were chosen from the previous
data collections. Several type of commercial nanofiltration/reverse osmosis membranes, not
already tested in the pertinent literature, were selected depending on the type of
application. Their efficiency was evaluated in terms of arsenic, fluoride and chromium
rejection, water production and feed composition. The other main ions that contribute to
water chemistry were also taken into account to evaluate the possible intended use of the
considered waters after the treatments. Satisfactory results were obtained for each specific
case of contamination. Moreover, an innovative membrane with absorbent capacities
against As(III) and As (V) species was developed and the preliminary results are quite
promising. Summing up, the findings obtained in this work are useful for the understanding
of the rock-to-water release of the chemical elements of interest and their fate in natural
waters. Moreover, the treatment of natural As, F, and Cr-contaminated groundwaters
improved the knowledge and the data availability for future scientific and application
developments in similar geological settings worldwide.Soggetto
Arsenic; Chromium; Fluoride; Geochemical modelling; Water remediation
Relazione
GEO/08;