Giordano Montegrossi

Chemical composition of fumarolic gases and spring discharges from El Chichòn volcano, Mexico: causes and implications of the changes detected over the period 1998–2000

Abstract Since the March–April 1982 eruption of El Chichon volcano, intense hydrothermal activity has characterised the 1-km-wide summit crater. This mainly consists of mud and boiling pools, fumaroles, which are mainly located in the northwestern bank of the crater lake. During the period 1998–2000, hot springs and fumaroles discharging inside the crater and from the southeastern outer flank (Agua Caliente) were collected for chemical analyses. The observed chemical fluctuations suggest that the physico-chemical boundary conditions regulating the thermodynamic equilibria of the deep rock/fluid interactions have changed with time. The chemical composition of the lake water, characterised in the period 1983–1997 by high Na + , Cl − , Ca 2+ and SO 4 2− contents, experienced a dramatic change in 1998–1999, turning from a Na + –Cl − - to a Ca 2+ –SO 4 2− -rich composition. In June 2000, a relatively sharp increase in Na + and Cl − contents was observed. At the same time, SO 2 /H 2 S ratios and H 2 and CO contents in most gas discharges increased with respect to the previous two years of observations, suggesting either a new input of deep-seated fluids or local variations of the more surficial hydrothermal system. Migration of gas manifestations, enhanced number of emission spots and variations in both gas discharge flux and outlet temperatures of the main fluid manifestations were also recorded. The magmatic-hydrothermal system of El Chichon is probably related to interaction processes between a deep magmatic source and a surficial cold aquifer; an important role may also be played by the interaction of the deep fluids with the volcanic rocks and the sedimentary (limestone and evaporites) basement. The chemical and physical changes recorded in 1998–2000 were possibly due to variations in the permeability of the conduit system feeding the fluid discharges at surface, as testified by the migration of gas and water emanations. Two different scenarios can be put forward for the volcanic evolution of El Chichon: (1) build-up of an infra-crater dome that may imply a future eruption in terms of tens to hundreds of years; (2) minor phreatic–phreatomagmatic events whose prediction and timing is more difficult to constrain. This suggests that, unlike the diminished volcanic activity at El Chichon after the 1982 paroxistic event, the volcano-hydrothermal fluid discharges need to be more constantly monitored with regular and more frequent geochemical sampling and, at the same time, a permanent network of seismic stations should be installed.

Degradation of C2–C15 volatile organic compounds in a landfill cover soil

The composition of non-methane volatile organic compounds (hereafter VOCs) in i) the cover soil, at depths of 30, 50 and 70 cm, and ii) gas recovery wells from Case Passerini landfill site, (Florence, Italy) was determined by GC-MS. The study, based on the analysis of interstitial gases sampled along vertical profiles within the cover soil, was aimed to investigate the VOC behaviour as biogas transits from a reducing to a relatively more oxidizing environment. A total of 48 and 63 different VOCs were identified in the soil and well gases, respectively. Aromatics represent the dominant group (71.5% of total VOC) in soil gases, followed by alkanes (6.8%), ketones (5.7%), organic acids (5.2%), aldehydes (3.0%), esters (2.6%), halogenated compounds (2.1%) and terpenes (1.3%). Cyclics, heterocyclics, S-bearing compounds and phenols are <or=1%. In the wells the VOC composition is characterized by higher concentrations of cyclic (7.6%) and S-bearing compounds (2%) and lower concentrations of O-bearing compounds. The vertical distribution of VOCs in the cover soil shows significant variations: alkanes, aromatics and cyclics decrease at decreasing depth, whereas an inverse trend is displayed by the O-bearing species. Total VOC and CH(4) concentrations at a depth of 30 cm in the soil are comparable, inferring that microbial activity is likely affecting VOCs at a very minor extent with respect to CH(4). According to these considerations, to assess the biogas emission impact, usually carried out on the sole basis of CO(2) and CH(4) emission rates, the physical-chemical behaviour of VOCs in the cover soil, regulating the discharge of these highly contaminant compounds in ambient air, has to be taken into account. The soil vertical distribution of these species can be used to better evaluate the efficiency of oxidative capability of intermediate and final covers.

Determination of Organic Acids in Plants of Silene paradoxa L. by HPLC

According to the general behavior that organic acids steadily bind metals, a specific and highly reproducible HPLC separation method with photodiode array detection has been improved for their determination and quantification in biological materials. The separation was carried out on an Alltima C-18 reverse phase column. The mobile phase was 125 mM KH2PO4, adjusted to pH 2.5 with concentrated H3PO4, and optimum separation efficiency was obtained by using a 2 mL min(-1) flow rate. Detection wavelength for quantitative measurement was 210 nm. The run time of each sample was 20 min, with a spectra collection frequency of 5 spectra s(-1). Organic acids were identified by comparing the retention times of the samples against retention times of the standards and confirmed with spectral (190-700 nm) signature. Because organic acids could steadily bind metals in plant tissues and due to the strong matrix effect observed, the addition method was applied for quantitative analysis and its performance evaluated.

Origin and Distribution of Thiophenes and Furans in Gas Discharges from Active Volcanoes and Geothermal Systems

The composition of non-methane organic volatile compounds (VOCs) determined in 139 thermal gas discharges from 18 different geothermal and volcanic systems in Italy and Latin America, consists of C2–C20 species pertaining to the alkanes, alkenes, aromatics and O-, S- and N-bearing classes of compounds. Thiophenes and mono-aromatics, especially the methylated species, are strongly enriched in fluids emissions related to hydrothermal systems. Addition of hydrogen sulphide to dienes and electrophilic methylation involving halogenated radicals may be invoked for the formation of these species. On the contrary, the formation of furans, with the only exception of C4H8O, seems to be favoured at oxidizing conditions and relatively high temperatures, although mechanisms similar to those hypothesized for the production of thiophenes can be suggested. Such thermodynamic features are typical of fluid reservoirs feeding high-temperature thermal discharges of volcanoes characterised by strong degassing activity, which are likely affected by conspicuous contribution from a magmatic source. The composition of heteroaromatics in fluids naturally discharged from active volcanoes and geothermal areas can then be considered largely dependent on the interplay between hydrothermal vs. magmatic contributions. This implies that they can be used as useful geochemical tools to be successfully applied in both volcanic monitoring and geothermal prospection.

Fumarole migration and fluid geochemistry at Poás Volcano (Costa Rica) from 1998 to 2001

Abstract We report the results of a geochemical survey of fumaroles, thermal springs, and gas discharges from areas in and around the active crater lake of Poás volcano (Costa Rica) from February 1998 to February 2001. The springs are highly acidic-sulphate waters with temperatures approaching boiling point, whereas gas chemistry is characterized by typical magmatic species, such as SO2, HF, HCl, H2, and CO. From February 1998 new fumarolic fields formed inside the southern part of the crater. They moved anticlockwise from the S to the NE inner walls of the crater, while those located in the southern part of the crater and close to the pyroclastic cone south of the crater lake diminished or disappeared altogether, during 1999 and 2000. This shift was also characterized by chemical variation of the magmatic gas species. In spite of the chemical changes of fumaroles, the composition of the lake changed little during this time. This fact, together with the chemical profile with depth of the lake, suggests that the lake is a very efficient condenser of magmatic fluids. An apparent chemical stratification of the lake suggests that dilution with meteoric water is not complete, due to the presence of liquid sulphur at the lake bottom and/or due to the continuous influx of new magmatic components.

Phase composition of CuxS thin films: spectroscopic evidence of covellite formation

Transition metal sulphides, grown in alternate layers by Electrochemical Atomic Layer Deposition (ECALD) to form thin films, are relevant candidates as photovoltaic materials for solar-cell production. However, the knowledge of the phase composition of the deposited sulphide, which is a fundamental information to assign and predict its effective semiconducting properties, is often hindered by the limited amount of material and by its morphological constraints. In the present study, an X-ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM) investigation of a copper sulphide thin film, synthesised according to the ECALD method, is proposed as an alternative and efficient way to identify the deposited semiconductor. The detailed XPS analysis of the Cu and S contents in the film points to a 1:1 stoichiometric ratio, corresponding to the CuS compound. Moreover, Cu is observed only as monovalent, and the possible co-existence of sulphide ions and of disulphide units is inferred. Accordingly, the attribution of the structure of the thin film to covellite, CuS, is proposed, as being the only one able to account for all experimental evidences. We suggest that the covellite structure can be set up by spontaneous rearrangement starting from a wurtzite-type CuS deposit, driven by the negative Gibbs energy involved in the formation of the compound from the elements. Finally the morphological characterisation of the film confirms its spatial homogeneity, thus making the covellite film a potential candidate for solar-cell applications.

ESEEM of industrial quartz powders: insights into crystal chemistry of Al defects

A set of raw industrial materials, that is, pure quartz and quartz-rich mixtures, were investigated through electron paramagnetic resonance and electron spin echo-envelope modulation spectroscopies, with the aim of evaluating the effective role played by defect centres and of assessing whether they can be used to monitor changes in the physical properties of quartz powders with reference to their health effects. The obtained results point to two interactions of the Al defect centres with H+, hosted in sites within the channels parallel and perpendicular to the c axis of quartz, respectively. These two Al/H+ (hAl) centres exhibit a weak chemical bond, and their relative amounts appear to be modified/controlled by the thermo-mechanical processes underwent by powders. Indeed, a mechanically promoted inter-conversion between the two kinds of site is suggested. As a consequence, the hAl centres are effective in monitoring even modest activations of powders, through thermal or mechanical processes, and they are also supposed to play a specific, relevant role in quartz reactivity during the considered industrial processes.

Operando SXRD study of the structure and growth process of Cu2S ultra-thin films

Electrochemical Atomic Layer Deposition (E-ALD) technique has demonstrated to be a suitable process for growing compound semiconductors, by alternating the under-potential deposition (UPD) of the metallic element with the UPD of the non-metallic element. The cycle can be repeated several times to build up films with sub-micrometric thickness. We show that it is possible to grow, by E-ALD, Cu2S ultra-thin films on Ag(111) with high structural quality. They show a well ordered layered crystal structure made on alternating pseudohexagonal layers in lower coordination. As reported in literature for minerals in the Cu-S compositional field, these are based on CuS3 triangular groups, with layers occupied by highly mobile Cu ions. This structural model is closely related to the one of the low chalcocite. The domain size of such films is more than 1000 Å in lateral size and extends with a high crystallinity in the vertical growth direction up to more than 10 nm. E-ALD process results in the growth of highly ordered and almost unstrained ultra-thin films. This growth can lead to the design of semiconductors with optimal transport proprieties by an appropriate doping of the intra metallic layer. The present study enables E-ALD as an efficient synthetic route for the growth of semiconducting heterostructures with tailored properties.

Geochemical Barriers in \hbox {CO}_{2} Capture and Storage Feasibility Studies

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