Efectos de fugas de CO2 almacenado en formaciones geológicas marinascinéticas de movilidad de elementos traza de sedimentos marinos

Abstract

There is an urgent need to reduce greenhouse gas (GHG) emissions, especially carbon dioxide (CO2), in order to mitigate climate change. As it is expected that during the next years the primary power source will continue to be based on fossil fuels, the stabilisation of atmospheric CO2 requires a combination of long, medium and short-term strategies. Carbon dioxide Capture and Storage (CCS), which is a promising technology to reduce CO2 emissions to the atmosphere, consists of the capture of this gas from the point source of emission and its deposit in deep underground formations so as to be sequestered up to thousands of years. The CO2 storage can be performed in different geological formations such as depleted hydrocarbon or oil reservoirs, unminable coal seams or deep saline aquifers. These storage sites can be located onshore or offshore, where the largest storage capacity is. Due to the key role that the CO2 sequestration might play while global economy continue to be based on fossil fuel combustion, the legal framework of the CCS is based on different amendments and laws, depending on the country in which the storage is performed. Only five countries in the world (Australia, Canada, Denmark, United States and United Kingdom) have a legal framework for the whole CCS process. In Spain, the European Directive 2009/31/EC and the national law 40/2010 establish the legal framework to be applied across some parts of the CCS project lifecycle. In the case of storage in geological formations under the seabed (offshore), the legal framework also includes the amendments of the London (2006, 2012) and OSPAR (2007) Conventions. Moreover, the potential contamination in the marine environment concerns the Marine Strategy Framework Directive 2008/56/EC. CO2 sequestration involves a high degree of uncertainty and unexpected leakages from the storage site will subject the ecosystem to unprecedented changes. In the case of offshore sequestration, potential CO2 leakages will acidify the marine environment by affecting the ocean carbonate system. CO2 dissolves in seawater and forms carbonic acid (H2CO3), which then disassociates into bicarbonate (HCO3-) and hydrogen ions (H+), increasing the acidity of the medium. The extra hydrogen ions (H+) react with carbonate ions (CO32-) present in the sediment or in the shells of living organisms. Changes in the pH of the marine environment and the subsequent environmental impacts can be also provoked by ocean acidification as well as chemical spills. Sediments are an essential part of aquatic systems and can act as a sink of trace elements. Modifications in the pH trigger geochemical alterations, so trace elements can be released from contaminated sediment to the surrounding environment likely affecting biogeochemical cycles and systems. In order to establish a risk assessment procedure which contributes to the safety of CCS projects, the determination and characterisation of contaminant mobility and availability under different acidification events need to be assessed. Geological formations in which CO2 is naturally present have been studied as natural analogues of storage sites to obtain information about how the gas is confined and how it can release through the surface. Industrial analogues are essential to contextualise the physical, chemical and biological effects of CO2 seepages taking into account the real structure of sediments. Moreover, laboratory experiments, which simulate worst-case scenarios of CO2 leakages, are valuable to assess the mobilisation of trace elements and their impact on marine communities. Leaching tests are useful and flexible tools to assess the release of constituents from solid matrices. Among the wide range of available leaching tests, the pH dependence leaching tests are essential to assess the influence of this key variable on the mobility and availability of trace elements. There are two standardised pH dependence leaching tests: CEN/TS 14429: 2005, superseded by EN 14429: 2015; and CEN/TS 14997: 2006, superseded by EN 14997: 2015. Both Technical Specifications (TS) were superseded by their respective European Standards (EN) with a slight change regarding the pH range of applicability (the Technical Specifications consider a pH range of study from 4 to 12 whereas the European Standards broaden it from 2 to 12). Although both pH dependence leaching tests aim to obtain equilibrium conditions, the CEN/TS 14429: 2005 implies and initial acid/base addition whereas a continuous pH control is mandatory when performing the CEN/TS 14997: 2006. The experimental results obtained from the leaching tests can be modelled to better understand the processes involved in the release behaviour of trace elements. Geochemical modelling gives information about the complexation with organic matter or adsorption reactions. The softwares Visual MINTEQ and PHREEQC have been used to identify, under equilibrium conditions, the distribution of trace elements among aqueous species and mineral solid phases. Reactive transport models performed with the TOUGHREACT or PHREEQC have been applied for the period before the equilibrium is reached. This latter modelling implies the combination of different reactions like mineral dissolution, precipitation or cation exchange. Therefore, specific characteristics of the solid, including the speciation of minor elements, which are usually not possible to be determined in the case of great complex matrices, have to be known. In order to solve this drawback, generalised mathematical models that consider general processes instead of many and complex specific chemical reactions are usually proposed. However, none of the kinetic models proposed until now is able to explain the mobilisation of elements with an initial delay or with adsorption/precipitation processes after their release. The general objective of this thesis is to generate new knowledge about the potential effects of pH changes occurred in the marine environment, especially those provoked by CO2 leakages from storage sites, on trace element mobilisation. The achievement of this goal is accomplished by means of the analysis and modelling of arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb) and zinc (Zn) release from contaminated estuarine sediment through modified pH dependence leaching tests. The pH dependence leaching test with initial acid/base addition, based on the CEN/TS 14429: 2005, is performed with deionised water and seawater as leaching liquids in order to assess the influence of the acidification through the whole pH range (0 - 14). Moreover, the experimental results are geochemically modelled with the Visual MINTEQ software. The modified CEN/TS 14997: 2006 leaching test is performed by using seawater as the leaching liquid and nitric acid (HNO3) (pH range: 4 - 7) or CO2 (pH range: 6 - 7) to acidify the sediment-seawater mixture. Hence, the effect of CO2 leakages from storage sites under total mixed conditions at a constant pH value is assessed. In addition, a generalised mathematical kinetic model, also useful in areas highly contaminated by iron (Fe), is proposed to explain trace element mobilisation over time. With regard to the results of the pH dependence leaching test with initial acid/base addition, the estuarine sediment used in this thesis shows a low base neutralisation capacity and a high acid neutralisation capacity. The buffering capacity of the leaching liquid also influences the test, so a slightly higher amount of acid or base is needed when the seawater is used. Therefore, more difficulties were found to reach equilibrium conditions in the set of experiments carried out with seawater. Although the trace element release shows similar concentrations and tendencies, there is a slightly greater mobilisation when deionised water is used, especially at highly alkaline pH values. The geochemical model used takes into consideration the surface adsorption to iron and aluminium (Fe- and Al-) (oxy)hydroxides for all the studied elements and the complexation with organic matter only in the cases of Cu and Pb. Modelled curves fit well with the experimental release, with values in the same or one higher order of magnitude, except in the case of Cd at acidic pH values. The trace element release obtained from the modified pH dependence leaching test with continuous pH control when HNO3 is used to acidify the sediment-seawater mixture, shows different leaching patterns over time that are not explained by the previous kinetic models. The release of Zn, Pb, Cd (pH ≤ 6.0) and Cu (pH = 4.0) shows an initial delay likely caused by the slow oxidation kinetics of their sulphides, whereas the oxyanion As is adsorbed or precipitated after an initial release. To explain these leaching behaviours, a three in-series reactions mathematical model is proposed. This kinetic scheme considers (i) the speciation change of the element over time through an oxidation reaction of the reduced fraction of the sediment; (ii) the release reaction in which the element mobilises from the oxidised fraction to the bulk dissolution and (iii) the potential adsorption or precipitation of the element after an initial release. The parameters of the model, which consist of the maximum concentrations of each element that can be leached from the reduced and oxidised fractions of the sediment and the kinetic coefficients, are estimated using the Aspen Custom Modeler software. The modelled data explain well (R2 ≥ 92.8) the experimental mobilisation of each studied element, even when it is applied to the experimental release from the sediment with different levels of oxidation. The evolutions of the redox potential and Fe release in the pH dependence leaching assay with continuous pH control (CEN/TS 14997: 2006) carried out with CO2 at pH = 6.0 are different with respect to assays performed at higher pH values (6.5 and 7.0). In this way, the influence of the initial Fe content of the seawater on trace element release is proposed to be assessed in this research. Therefore, assays that use seawater with different Fe concentrations are performed at pH = 6.0. The previously proposed generalised model is applied to explain trace element leaching behaviour over time. However, it has to be modified in order to obtain a good fit between the experimental results and modelled data at pH = 6.0 when the Fe content of the seawater is high. Hence, a Fe/multi-ion-dependent mechanism for trace element release is proposed, extending the scope and the applicability of the previous model because this modified generalised model is flexible enough to work with different sediments and in highly contaminated areas by Fe. As previously, the maximum concentrations that can be released from the oxidised and reduced fractions of the sediment and the kinetic coefficients are estimated. Although this modified model can be applied to all the experimental results, when it is used to fit experimental results from assays at pH values of 7.0 and 6.5, the parameters that are not included in the initial proposed model take the value of zero. Therefore, a trade-off between estimation and accuracy should be made by the modeller. In contrast to the assays acidified by HNO3, the kinetic coefficients do not present a clear trend as a consequence of the impact that CO2 has on the ionic competition which enhances the influence of the displacement reactions on element release. Finally, a global comparison that considers the modelled kinetic parameters between both types of acidification (HNO3 and CO2) and different redox levels of the sediment in assays performed at pH = 6.5 is addressed. Moreover, the proposed kinetic model is successfully validated to predict contaminant release under resuspension conditions, in which the pH of the medium varies over time. The results shown in this thesis complement previous studies in which other leaching tests were performed to address the trace element mobilisation from contaminated sediment that was sampled in the same area. The current experimental and modelled results can contribute to the development of the CCS technology, especially of the injection step, being useful as a line of evidence input for the risk assessment of the above mentioned technology.