A method for the definition of the carbon oxidation number in the gases dissolved in waters and the redox variations using an elemental analyser (FlashEA 1112). Preliminary data from a stratified lake

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Abstract

Obtaining meaningful redox potential determinations is questionable in disequilibrium solutions, like surface waters and groundwater. However, identifying redox processes in a natural ecosystem is a key factor to characterize the chemical quality and, consequently, the sustainability of that environment. To do that, the determination of an exact redox potential is not needed and on many cases it is more important to describe how and if the redox state of a system changes. In particular, the redox status of aqueous systems is assigned based on analysis of samples for redox-sensitive compounds. In general, these analyses do not include measurements of gases dissolved in the waters.

Here, we present an innovative method to underline in an easy way changes in redox state of a system using the elemental composition (C, H, S) measured in the headspace gas after the equilibration with water phase. The results allow defining the empirical formula of the gas, the mean number of oxidation of the carbon and the molar ratio between C:H:S. The method is applied to investigate the redox condition in a stratified lake, San Rocchino (Northern Tuscany, Italy) using an instrument (Thermo Scientific FlashEA 1112) equipped for headspace gas analysis. Samples of waters and dissolved gases were collected at five different depths along a vertical profile. Such analytical system is used to determine the mean oxidation number of the carbon gaseous phase in the headspace, which allows distinguishing relatively oxidizing from relatively reducing condition. Further information on the redox state is reached considering the molar ratio C/H and C/S.

This innovative method is simple and rapid, and the instrument is robust and it needs low maintenance. Moreover, it could be applied to other research and exploratory fields, such as geothermic and volcanic monitoring, polluted aquifer and contaminated sites.

Highlights

► Definition of the empirical formula of a gas ► Determination of the mean oxidation number of carbon gaseous phase in the headspace ► Information on the redox state is reached considering the molar ratio C/H and C/S.

Introduction

The reduction and oxidation (referred as redox) processes that occur in an environment (soil, water and atmospheric system) define the chemical speciation of carbon, hydrogen, nitrogen and sulfur (C, H, N and S, respectively) that affects the chemical quality and the sustainability of these ecosystems, particularly groundwaters and surface waters (lakes, ponds, rivers).

Theoretically, redox half-reaction is defined in terms of the free electron activity, which is, in fact, non-existent in aqueous solutions. The extent of these reactions is expressed by a potential: Eh, which is measurable and directly related to the Nernst equation, or pε, which represents the co-logarithm of the activity of the hypothetical electron in solution. The value of pε is simply a transposition in scale and magnitude of Eh. It can be useful but it is formally wrong from a thermodynamic point of view, because there are no free electrons in solution, and on the basis of the definition of the standard state, the activity of the hypothetical electron is always 1.

Redox potentials of water are measured directly by electrochemical measurements, usually using platinum (or other materials) electrodes. The Eh values obtained by this method became questionable and likely unreliable in many natural systems due to low exchange current over electrode interface, which prevents the system from reaching equilibrium (Lindberg and Runnels, 1984, Stefansson et al., 2005). Meaningful measures are reached in well-buffered acid mine waters (Gezahegne et al., 2007) and iron-, manganese-, or sulfide-rich waters or sediments (Langmuir, 1997 and references herein). However, effective results had not been reached on surface waters or in oxygen-rich environments.

Ioka et al. (2011) described a method, based on long-term in situ potentiometric measurements of Eh, to investigate sulfidic groundwater in unconsolidated sediments. This method may be effective for obtaining Eh values at the equilibrium because the kinetic of reaction between redox species is usually slow.

Values for redox potentials could be calculated indirectly from measured concentrations of redox sensitive species dissolved in waters (Chapelle et al., 1995, Chapelle et al., 2009, Christensen et al., 2000). However, due to the presence of mixed potentials and redox couples, a natural system is generally far from thermodynamic equilibrium, thereby implying that the concentration of redox-sensitive species in surface or wastewaters cannot be used to quantify Eh (Christensen et al., 2000, Stumm and Morgan, 1996).

Nevertheless, identifying redox processes in a natural ecosystem is a key factor to characterize the chemical quality and, consequently, the sustainability of that environment. In situ investigations of Eh of groundwater allow for obtaining qualitative information, important for a proper sampling. For these purposes, the determination of an exact redox potential provides an identification of the dominating redox processes. In particular, the redox status of aqueous systems is assigned based on analysis of groundwater samples for redox-sensitive species (McMahon and Chapelle, 2008) and the content of other redox indicators such as H2S and CH4 gases dissolved in waters, even not routinely measured in monitoring programs.

The elemental analysis of the gas contained in the headspace allows: a) to write an empirical formula of the gas, as a single compound; b) to establish the number of oxidation of carbon, assuming a number of oxidation of sulfur (− II); c) to calculate the molar ratio between C:H:S that classifies the gas in terms of chemical composition. Therefore, the carbon oxidation number and the molar ratios C/H and S/C allow to distinguish relatively oxidizing from relatively reducing conditions. Further information on the redox state is reached considering the molar ratios C/H and C/S. The elemental composition and the determination of the mean oxidation number of C in the gases dissolved in water may be an innovative method to classify relatively oxidizing/reducing state of water, directly related to the content of reduced and oxidized C species.

In order to define the C redox number of the gases dissolved in water, the total carbon content should be determined, as well as the total content of sulfur and hydrogen.

The total content of a chemical in the gaseous phase dissolved in water is analysed in the headspace after the equilibration with the water phase. Usually, the analyses are performed by gas chromatography, which gives the concentration of the gases containing C. Consequently, the sum of all these compounds is considered an estimation of the total content of C in the gaseous phase dissolved in water (Cioni et al., 2003). Direct measurements of total content of C, S and H in the gases dissolved in headspace are not yet commonly used.

The elemental analyser EA (such as CHN and CHNSO) allows for the simultaneous determination of the total content of carbon, hydrogen, nitrogen, sulfur and oxygen composition of many types of matrices, such as polymers (e.g. Lee et al., 2010), petroleum products (e.g. Wahyudiono et al., 2011), pharmaceuticals (e.g. Ensafi et al., 2011), organic products (e.g. Suzuki, 2002), coal (Card and Jones, 1995), feeds (e.g. Anger et al., 2009), soils (Caria et al., 2011), sediments (e.g. Kralovec et al., 2002). The measurements are commonly performed on solids, in particular pure compounds, in order to calculate the compositional formula of organic and inorganic substances (e.g. Moolya and Dharmapraksh, 2007) and the elemental ratio of soils, sediments and atmospheric particulate (e.g. Duan et al., 2009).

The commercial instrumentation is not equipped for headspace gas analysis. In this paper we present an instrumental modification for gas injection on a Thermo Scientific FlashEA 1112, and we describe the application of such analytical system to a lake for the determination of the mean oxidation number of the carbon of dissolved gases, which could provide information on the redox state of the ecosystem.

Section snippets

Apparatus

The elemental characterization of solid and liquid substances in terms of total carbon, hydrogen and sulfur was determined in a single injection using Thermo Scientific FlashEA 1112 analyser, in which a substance undergoes oxidative decomposition and the subsequent reduction of sulfur oxides with the formation of the final products: carbon dioxide, water and sulfur dioxide. The amounts of CO2, H2O and SO2 represent the sum of all the gases containing carbon, hydrogen and sulfur, respectively,

Optimization of FlashEA 1112 conditions

Accurate timing of the sample inlet is very important in this method: the loop was filled with the sample and the delay is adjusted to 8 s, when the valve is switched to position B and He is allowed to flow in the loop and, consequently, in the reactor. The oxygen injection is fixed at a flow of 250 ml/min for 5 s. In this way we secure the total oxidation of the gas and minimize the noise due to pressure flow variation for oxygen injection and for sample introduction.

The setting of the oxygen

Characterizing changes in redox state using elemental composition of headspace gas: the San Rocchino lake

As described in the previous sections, dissolved gases collected at different depths into San Rocchino lake were analysed to test the developed analytical system for the delineation of changes of the redox condition of the waters along a vertical profile.

Changes in the concentration of redox-sensitive constituents are shown in Fig. 6. The figure highlights that oxygen decreases rapidly within the first 4 m to virtually nil values. Once the waters become anoxic, concentrations of ammonia and

Conclusion

The results illustrated in this paper reveal that the determination of the elemental composition of the headspace gas and the number of oxidation of carbon allow to classify the gas in terms of chemical composition and describe relative changes of redox environments, in time and space.

The method described is rapid and easy; the FlashEA 1112 analyser is robust, it needs low maintenance and it can be easily used directly in the field where a power supply is available. This enable the application

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