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The effects of cleaning treatments on the surface composition of porous materials Stanislav Jelavi#, A.R. Nielsen, M Blazanovic, N. Bovet, K Bechgaard, and Susan Louise Svane Stipp Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03586 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018
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The effects of cleaning treatments on the surface
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composition of porous materials
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∞ S. Jelavić *, A.R. Nielsen, M. Blažanović, N. Bovet, K. Bechgaard , S.L.S. Stipp
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Nano-Science Center, Department of Chemistry, University of Copenhagen, Universitetsparken 5,
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Copenhagen Ø 2100, Denmark ∞
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deceased
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Keywords: organic contamination, petroleum, core plug, clay mineral, recalcitrant organic matter
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ABSTRACT
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Removal of organic compounds is a common first step in surface characterisation of inorganic materials.
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To explore the surface reactivity of environmental samples such as rocks, soil and sediments, it is
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essential to know if they are pure or covered with organic material. Our purpose was to evaluate the
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influence of some common cleaning procedures on the surface composition of inorganic solids and to
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investigate if such treatments result in completely clean mineral surfaces. We used Soxhlet solvent
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extraction and chemical oxidation to remove crude oil from samples that we synthesised to represent oil
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bearing sandstone. After each cleaning step, we tracked surface composition using X-ray photoelectron
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spectroscopy. We demonstrated that: i) it was impossible to completely remove the organic material with
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any of the methods, ii) the oxidation treatment dissolved soluble minerals and iii) surface composition
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was dominated by clay particles adhering to the larger grains, thus was responsible for a significant
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fraction of the residual adsorbed organic compounds. The most important conclusion is that the pore
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surface composition, i.e. the clay fraction not the bulk matrix composition, ought to be used for
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thermodynamic and kinetic calculations to predict pore fluid composition and evolution during transport
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through pore networks.
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INTRODUCTION
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Removing organic compounds from the mineral surfaces in rocks, soils and sediments (inorganic
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matrices) is essential for valid nanoscale characterisation and interpretation of mineral or matrix
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properties1,2. We rely on nanoscale characterisation in studies of surface reactivity because this approach
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offers molecular insight into the processes that control the interaction between minerals and
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environmental fluids, allowing them to be described from fundamental principles. However, even a small
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amount of organic material is enough to dramatically change the properties of inorganic surfaces and
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render studies of the inorganic substrate challenging or impossible3–5. The cleanliness of investigated
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surfaces is important in a range of disciplines6,7 but it is essential for studies of the reactivity of minerals
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with small particle size and high adsorption capacity because organic contamination can change their
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properties from essentially hydrophilic to hydrophobic8–12. Thus, if the results of experiments are to have
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meaning and conclusions are to be translated to other systems, it is necessary to know if the surfaces of
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natural materials are pure, clean terminations of the bulk mineral or if adsorbed organic compounds
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dominate.
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There are many methods for removing organic compounds from inorganic substrates. Techniques of
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physical separation are based on a difference in solubility between the organic and inorganic phases and
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methods of chemical separation are based on differences in reactivity towards oxidation. The classical
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methods include Soxhlet, shake-flask and sonication extractions. Some more recently developed, more
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technically demanding methods include supercritical fluid extraction, pressurised liquid extraction,
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microwave assisted extraction and matrix solid phase dispersion13,14. These are less common because of
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equipment costs, method development time and higher risk of damage to the inorganic material because
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they all use a combination of high temperature and pressure.
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Among the traditional separation techniques, the Soxhlet extraction has been used most. It gives good
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reproducibility and high recovery, i.e. effective separation of the organic phase15,16. Soxhlet extraction is
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very good for removing complex organic mixtures from a porous medium, such as oil from core plugs or
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humus from soil, because the extraction efficiency increases with molecular weight of the extract17. The
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overall efficiency of the Soxhlet extraction however, depends on the properties of the organic compounds
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and the solvent used for extraction. The possibilities for organic solvent are many13 and the choice is
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usually based on reactivity and polarity. For successful extraction, solvents should cause minimal change
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in the composition of the extract and the solid and ought to be of similar polarity as the extract.
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Most previously reported studies about removing organic material from an inorganic matrix have
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concentrated either on the analysis of the organic compounds after solvent extraction13,15–19 or on changes
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in bulk composition of the porous medium20–22. We could find no reports in the literature about the
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influence of solvent extraction, such as the Soxhlet treatment, on the pore surfaces. If the matrix materials
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are to be used for further studies, such as for determining the extent of their reactivity, it is essential to
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choose an extraction method that is effective at removing the organic material but that has minimal effect
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on the surface composition and structure.
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Chemical oxidation degrades organic compounds to CO2 and H2O with various by-products that depend
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on the initial composition of the organic material. Oxidation is quick and relatively simple but there is a
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risk of violent reaction. Unlike the Soxhlet method, which is suitable for handling large fractions of
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organic material within an inorganic matrix (e.g. oil saturated reservoir core plugs or point contamination
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in soil), chemical oxidation is preferred where the fraction of organic material is small. However,
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chemical oxidation risks changes in the composition of the inorganic matrix, dissolving some phases,
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precipitating undesirable secondary phases or changing the structural properties of the minerals or the
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matrix22. Often, the rate or the extent of the oxidising reaction is controlled by the pH of the solution but
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changes in acidity can detrimentally affect mineral solubility. The effects of the treatment then raise
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questions about the nature of the investigated surfaces: How successful are the common cleaning
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methods? Do cleaned surfaces make the inorganic solid surface accessible? Or is organic contamination
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always present?
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The purpose of this study was to compare the effectiveness of Soxhlet solvent extraction and chemical
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oxidation in recovering the pure, inorganic surfaces and to determine what changes in composition the
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cleaning procedures promote in the inorganic matrix. We compared the surface composition after each
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cleaning step, so we could follow the sequential removal of the various carbon species and monitor for
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dissolution of less stable minerals. We synthesised a sample by mixing known proportions of known
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minerals to produce an unconsolidated sand sample with realistic mineral composition. We then exposed
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our model matrix to crude oil for 30 days. Using our model sand instead of a real rock or sediment
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provided us with a sample where we were sure of the initial composition, both bulk and surface, where
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we could test each treatment in sequence and accurately monitor changes in the surface composition,
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avoiding uncertainties arising from amorphous or highly refractive organic compounds that could be
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associated with real, consolidated samples.
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We used X-ray photoelectron spectroscopy (XPS) to track changes in surface composition before it was
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mixed with oil and then after the oil was removed, by physical separation using: i) a mixture of
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dichloromethane and methanol (DCMM) and ii) toluene. We chose DCMM for extracting the polar and
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nonpolar organic compounds and toluene for extracting the very resistant, nonpolar molecules. Later, the
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residual oil compounds, that were resistant to physical separation, were chemically removed by oxidation
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using iii) hydrogen peroxide. We chose H2O2 as the oxidant because, among the many possible
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compounds, it affects the least the original surface composition of the minerals. Other oxidants dissociate during the reaction and promote adsorption or exchange of surface cations for Na+ ions (e.g. NaOCl and Na2S2O8).
MATERIALS AND METHODS Preparation of the model sand. We mixed pure or purified mineral standards to make the model sample. Phyllosilicate minerals were obtained from the Source Clays Repository.23 Kaolinite (KGa1b) was used as received and was not further purified because the standard purification by size separation does not decrease the amount of TiO2 impurities. Clinochlore (CCa2) is a rock. To decrease it to the µm particle size readily encountered in sands, but to avoid overgrinding it and damaging its layered structure, CCa2 was only hand crushed in an agate mortar and we used the
