Salt Crystallization in Porous Media E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Serie: ISTE Consignment
• Sprache: Englisch

Seitenzahl: 397

Veröffentlichungsjahr: 2024

Beschreibung

The crystallization of one or multiple salts in porous media (rocks, soils, building materials, etc.) is a subject of major interest in connection with numerous issues such as soil salinization, evaporation, CO2 injection into saline aquifers, the durability of building materials and the preservation of our cultural heritage.

Salt Crystallization in Porous Media provides an interdisciplinary review of the key scientific knowledge required to understand this field of research, and illustrates the issues involved through a series of concrete examples.

This book has been written for students completing their Master’s level degree or higher in the field, as well as researchers and engineers interested in this research. It may also be of interest to a wider readership, as certain sections can be used to illustrate basic concepts, reaching beyond the subject of salt crystallization itself.

Leseprobe

SCIENCES

Mechanics, Field Director – Gilles Pijaudier-Cabot

Geomechanics, Subject Head – Cino Viggiani

Salt Crystallization in Porous Media

Coordinated by

Hannelore Derluyn

Marc Prat

First published 2024 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

ISTE Ltd27-37 St George’s RoadLondon SW19 4EUUK

www.iste.co.uk

John Wiley & Sons, Inc.111 River StreetHoboken, NJ 07030USA

www.wiley.com

© ISTE Ltd 2024The rights of Hannelore Derluyn and Marc Prat to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s), contributor(s) or editor(s) and do not necessarily reflect the views of ISTE Group.

Library of Congress Control Number: 2023942766

British Library Cataloguing-in-Publication DataA CIP record for this book is available from the British LibraryISBN 978-1-78945-114-6

ERC code:PE8 Products and Processes Engineering PE8_3 Civil engineering, architecture, maritime/hydraulic engineering, geotechnics, waste treatmentPE10 Earth System Science PE10_11 Geochemistry, crystal chemistry, isotope geochemistry, thermodynamics

Preface

Hannelore DERLUYN1,2 and Marc PRAT3

1 LFCR, CNRS, Université de Pau et des Pays de l’Adour, France

2 DMEX, CNRS, Université de Pau et des Pays de l’Adour, France

3 IMFT, CNRS, Université de Toulouse, France

Salt crystallization phenomena in porous media are found in important applications such as soil salinization, the underground storage of CO2 in saline aquifers or the preservation and conservation of built heritage. In connection with this last application, the crystallization of salts in building materials or porous stones of many components in our architectural or cultural heritage is considered to be one of the most severe threats to their integrity.

The stresses induced by crystallization in pores can indeed lead to fracturing, flaking, or even honeycombing, seriously altering the esthetics of the elements concerned, or even their structural stability. Crystallization in soils can also lead to structural disorders when induced swelling phenomena affect the foundations or surface structures such as roads, tracks and other paved surfaces. With these applications in sight, this book provides an overview of the state of the art of scientific knowledge concerning the crystallization of salt in porous media.

An important feature of this area of research is its interdisciplinary character. For example, when crystallization and the damage induced result from the evaporation of the solution contained in the pores of a material, the analysis involves, in addition to the evaporation phenomenon in a porous medium itself, physicochemical phenomena. These include precipitation or dissolution, solute transport phenomena inside the pore space, capillarity and viscous flow phenomena, thermal, and mechanical phenomena (fracturing, swelling, modification of the pore structure, etc.). Besides the great variety of phenomena, what makes it more complex is that these different phenomena are generally coupled together so that the most complex situations require the consideration of these couplings.

An immediate consequence is that in reality different communities of researchers are interested in this field of research. Each bringing their own methodologies and areas of interest, several lines of research have emerged over the years. One part concerns thermodynamics, with a description of equilibrium situations, which can quickly become very complex when several salts are present; this is frequent in applications and also because of metastable states, which are often found. The description of equilibrium states, phase diagrams and the impact of confinement, especially in pores of submicronic size down to just a few nanometers, can alone occupy a researcher’s life. If thermodynamics is the key to describing these often subtle equilibria, it is generally not sufficient on its own to comprehend a practical situation of crystallization in a porous medium given the importance of transport phenomena. Very often, the growth kinetics of a developing salt structure, whether growing on the surface of the porous material (efflorescence) or within it (subflorescence), can only be described correctly through the sufficient understanding of the phenomena controlling the transport of ions within the porous material as well as within the salt structure itself, the latter often being porous (case of NaCl in particular). Thermodynamics is still very present in describing the local equilibrium at interfaces, but using methods of fluid mechanics and transport phenomena in porous media then becomes essential. Finally, poromechanics comes into play when it comes to analyzing situations leading to fractures and/or deformations. As evidenced by this book, in reality the combined consideration of all these aspects is still only imperfectly carried out. For example, work focusing on the transport and growth laws of efflorescence and subflorescence has mainly focused on the simplest cases where only one salt is present in the solution. Similarly, some aspects highlighted by studies focusing on the transport and growth of salt structures are not taken into account or only in an oversimplified way in the most advanced models of poromechanics.

These elements indicate that the field of salt crystallization remains a very open field of research where much remains to be uncovered. Also, the specificities of this field of research are found in the chapters themselves, which develop in a relatively autonomous way.

In this context, the introduction takes up the previous elements by placing them in relation to the literature. The book is then divided into two main parts. In the first part, the objective is to focus on the basic principles that should be mastered to study problems related to the crystallization of salts in porous media. So, Chapter 1focuses on the thermodynamic properties of saline solutions. The key concept of crystallization pressure is the subject of Chapter 2. The main results concerning the aspects of solute transport in porous media and growth of salt structures are presented in Chapter 3. The state of the art concerning the theories of poromechanics for the prediction of salt-related damage is proposed in Chapter 4. Chapter 5 presents experimental observations in geomaterials, mainly from modern imaging techniques.

Part 2 is more applicative and illustrates not only the operational nature of the fundamental concepts in the previous chapters, but also the developments still to be made to understand and model certain real scenarios. Chapter 6 presents situations where crystallization in the underlying terrain has a considerable impact on engineering structures, bridges or tunnels. It can also be seen as a transition chapter because the coupled thermohydromechanical and chemical modeling of salt precipitation and the deformation of rocks in contact with infrastructures is also presented. This model provides good results compared to field observations. Chapter 7 presents a very important field of application, that of the alteration of porous media due to the crystallization of salts in the context of built heritage, from identifying the origin of salts and the degradation mechanisms to conservation and restoration techniques. Finally, Chapter 8, in continuity with Chapter 7 and the issue of the preservation of built heritage, emphasizes the impact of environmental conditions and the evolution of climatic factors. Like Chapter 7, it illustrates the complexity that can be encountered in real-life scenarios when multiple salts are present.

These chapters have been prepared by specialists in each field concerned. Their name and affiliation are indicated at the beginning of the chapter and at the end of the book. We thank them very warmly for their involvement in the creation of this book.

The latter is aimed primarily at students at master’s degree level and above and at researchers and engineers interested in this field of research and wishing to have an overview of its various facets. However, this book may be of interest to a wider audience, whether those more directly concerned with the applications or those responsible for teaching, certain parts being able to lend themselves to the illustration of basic concepts beyond the problem of salt itself.

Finally, this book would probably not have seen the light of day without a certain structuring of the community of researchers working on the crystallization of salts in porous media, in particular via the series of CRYSPOM workshops “Crystallization in Porous Media”, which, since 2008, at the initiative of Noushine Shahidzadeh and the late Olivier Coussy, allows regular exchanges on this topic.

As with most research the results presented in the various chapters could not have been obtained without the contribution of many people, starting of course with the various doctoral students who developed their thesis on these topics, and the exchanges or collaborations with many colleagues. It is impossible to name them all. May they accept our deepest gratitude.

Introduction

Marc PRAT

IMFT, CNRS, Université de Toulouse, France

The crystallization of a salt in a porous medium is studied in relation to different applications. First, because of its destructive effect. The latter is easily observable when visiting many monuments and historical sites. As noted in the classic work by Goudie and Viles (1997), this is the case, for example, of the Sphinx in Egypt, the stone monuments of Petra in Jordan, several cathedrals in Mediterranean regions, or even the city of Venice. As discussed in Chapters 7 and 8 of this book, the attack by salts results in different forms of degradation such as cracking, flaking, honeycomb weathering (Figure I.1) and can go as far as pulverizing the material. All of the processes linked to the crystallization of salts that lead to the degradation of porous rocks are defined under the term “haloclasty”. Thus, haloclasty is considered to be the main phenomenon causing the alteration of historical monuments in the Mediterranean region.

Although haloclasty has been observed since antiquity (Goudie and Viles 1997), it is only since the 19th century that these phenomena began to be studied seriously. For example, Brard (1828) presented research where the crystallization of salt was studied as analogous to the effect of freezing on porous stones, whereas initial discussions, aiming at explaining that the growth of a crystal in confinement generates constraints, date back to 1853 with the work of Lavalle (1853). Haloclasty has also been identified as an important phenomenon in the evolution of landscapes, contributing to erosion not only on Earth but possibly also on Mars (Malin 1974).

Figure I.1.Examples of damage due to salt crystallization: (a) cracking, (b) flaking (Motomachi Buddha, Oita, Japan) and (c) honeycomb weathering (Santander Cathedral, Spain) (photos (a) and (c), B. Leclère (Leclère 2021); photo (b), H. Derluyn).

As for many other phenomena, the scientific approach to haloclasty and more generally, to the crystallization of salts in porous media and associated phenomena, gained considerable interest in the 20th century. Among the seminal works often cited, we can mention the work of Correns (1949), who proposed an expression of the crystallization pressure, a central concept in the analysis of the generation of stresses due to crystallization (see Chapter 2 of this book). This formulation has since been revisited and we can refer to Steiger (2005) for a more recent presentation according to a purely thermodynamic equilibrium approach. More recent studies (e.g. Gagliardi and Pierre-Louis 2019) indicate, however, that it is important to consider non-equilibrium effects when analyzing the force exerted by a crystal in a pore. The subject of stress generation, due to the crystallization of salt in pores (Scherer 1999; Coussy 2006; Noiriel et al. 2010; Scherer et al. 2014), inducing degradation phenomena in porous stones, as well as the detrimental effects that it can cause on engineering structures (see Chapter 6 of this book), is not the only motivation to study the impact of the presence of one or more salts in a porous medium. Other applications include the following:

Soil salinization

: salinization refers to the accumulation of salts in soils at levels toxic to most plants. It is a main cause of desertification, erosion and land degradation. It makes the soil unsuitable for agriculture. Very likely linked to global warming, the phenomenon is accelerating (Litalien and Zeeb

2020

) and threatens ever larger areas of land. Irrigation is one of the main human causes of salinization. In the event of excessive irrigation, the soil is wet at depth, which establishes hydraulic continuity causing the salt to rise to the surface. A good understanding of the transport of dissolved salt in an unsaturated porous medium is therefore key to understanding and analyzing this phenomenon.

Soil evaporation

: the phenomenon of evaporation in the presence of dissolved salt often leads to the formation of a salt crust on the surface of the soil. Depending on the formation conditions, the formation of the crust can severely reduce evaporation (Fujimaki et al. 2010) and therefore affect soil–atmosphere exchanges. Despite advances (see

Chapter 3

of this book), the detailed understanding and modeling of this phenomenon remain to be fully understood.

Ground heave

: when crystallization occurs at depth, the soil layers above may heave due to stress caused by crystallization. This mechanism is illustrated in

Figure I.2

. We find here a mechanical effect in the context of granular media rather than consolidated media such as porous rocks. This uplift phenomenon can lead to major surface disorders (Feng et al.

2020

; also see

Chapter 6

). Although clearly identified (Hird and Bolton

2016

), it is still insufficiently described with a lack of predictive models.

Underground storage of CO

2

: carbon capture and storage (CCS) in geological formations is a technique aimed at limiting the accumulation of greenhouse gases in the atmosphere. Some of the main issues related to this technique concern the injectivity conditions of the wells used to maintain a high level of injection (Peysson et al.

2014

). When this injection takes place in a saline aquifer, salt precipitation can occur, leading to the clogging of pores in the well, a drop in permeability and ultimately the deterioration of injection conditions. A better understanding of these phenomena is therefore desirable to improve the implementation of this technique.

Impregnation with a saline solution

: with the Covid-19 pandemic, it has been shown that individual masks are essential tools to protect against airborne viral particles. In this context, hypertonic saline solution has been shown to effectively reduce infectious viral load on treated masks (Tatzber et al.

2021

). The method consists of impregnating the mask with a saline solution of NaCl and then drying it. We can refer to Börnhorst et al. (

2016

) for a discussion on the impregnation of a porous medium by a saline solution in connection with other contexts.

The use of crystallization as a cheap method for the recovery of metals

: a simple method to recover metals from mine tailings is to soak them in a saline solution and subject them to evaporation (Cala-Rivera et al.

2018

). In this situation, as explained in

Chapter 3

, a flow is induced toward the surface in the porous medium. This flow transports the species in solution to the surface where the salt crystallizes. The metals become trapped in the efflorescence, which then only needs to be extracted and dissolved to recover the metals in question.

Conservation and restoration of built heritage

: considering the damage that crystallization can cause, various techniques have been developed to limit the risk of degradation or restore the affected areas, whether for recent constructions or for the old parts of built or cultural heritage (frescoes, paintings, sculptures, etc). These aspects are discussed in

Chapter 7

.

Design of distillation unit evaporators

: desalinating seawater is an important solution to meet the growing demand for drinking water, but also potentially in the context of the energy transition and the production of hydrogen by electrolysis, which also uses water as a resource. In this context, solar-thermal water desalination is an interesting solution, which can generate clean water without significantly depending on fossil energy. In devices designed for this purpose (Wang et al.

2020

), we find (nano)porous media, evaporation and a saline solution, ingredients common to the applications already mentioned.

Figure I.2.Drying a NaCl solution in a Hele–Shaw cell filled with glass beads. The process leads to the formation of subflorescence causing the granular medium to move upwards and lift off the surface (Diouf 2018).

These different applications illustrate the multidisciplinary nature of the field of research that is the subject of this book. Thus, different communities of researchers contribute to its development including specialists in geophysics, geomorphology, underground or surface hydrology, physicochemistry, physics, chemistry, thermodynamics, civil engineering, transfer in porous media, poromechanics, engineering, or even more directly, in the relevant applications.

Even if each specialist can find their own reasons for their interest in this field of research, it is clear that the analysis of the most complex situations requires a certain level of interdisciplinarity. Consider, for example, the very classic scenario where crystallization results from the evaporation of a saline solution contained in a porous medium. In this scenario, evaporation induces an increase in the salt concentration in the solution within the porous medium by the effect of advection and/or volume reduction (see Chapter 3) until a sufficient concentration is reached for crystallization to occur and subflorescence and/or efflorescence to develop. As shown schematically in Figure I.3, the analysis of this type of situation involves focusing on the transport of dissolved salts in a porous medium subjected to evaporation and often unsaturated (case of drying for example), physicochemical aspects, including the often complex thermodynamic equilibria characterizing the solutions (Chapters 1, 7 and 8), precipitation–dissolution phenomena and the dependence of the properties (viscosity, surface tension, wettability, etc.) on the salt concentration, etc. When crystallization has notable mechanical consequences (cracking, movement of grains, etc.), the preceding phenomena become coupled with mechanics, which is often tackled within the framework of macroscopic poromechanics (Chapters 4 and 6), but also with research at the pore scale (Chapter 2).

Figure I.3.Evaporation of a saline solution in a porous medium: simplified diagram of coupled processes.

Indeed, as is conventional for porous media, the crystallization of salt in porous media and the associated phenomena can be studied at different scales, from the pore scale to the scale of underground formations, passing through the pore network scale and the Darcy scale. The current situation is, however, far from having reached the level of development where the model at a given scale, for example the Darcy scale, is deduced in a rigorous way, that is to say by upscaling methods, from the formulation of the model at a lower scale, a pore scale model typically. Even if there is research going in this direction, the present situation is that each study is developing in a fairly independent way. To give just one example, several works have been dedicated in recent years to the crystallization pressure from experiments at the scale of a pore or a channel (Chapter 2). On the other hand, the concept of crystallization pressure is also central in poromechanical models (Chapter 4). However, how a growing subflorescence within the pore network generates stresses has not yet been properly analyzed. In other words, the intermediate link is not understood, which necessarily introduces some uncertainty on the real applicability of the poromechanical models developed so far.

Even if there are modeling studies where all the phenomena mentioned in Figure I.3 are taken into account (Koniorczyk and Gawin 2012; Derluyn et al. 2014), many phenomena are in fact still poorly understood or poorly described. To cite just one, we can mention the effect of the formation of salt crusts on evaporation, which can, depending on the conditions, severely reduce evaporation or, on the contrary, have no significant impact. A purely phenomenological model has, for example, been proposed to take into account the impact of the crust on evaporation when this greatly reduces it (Grementieri et al. 2017). However, the detailed physical analysis of the mechanisms leading to this impact remains a very largely open question. A recent study suggests that it is due to the formation of submicron pores on the surface of the crust and to the Kelvin effect (Licsandru et al. 2022).

This also explains, beyond the applications, that research still continues, not only by trying to integrate all the phenomena, but also in a specific way. This book, by the structuring of its topics, nicely reflects the different lines of current research.

From a more methodological point of view, research on the study of porous media follows a fairly general trend with the increasingly frequent use of modern imaging techniques (Chapter 5). Access to these techniques (X-ray microtomography, neutron radiography, etc.) renews the questions with the possibility of considering all the complexity, in particular geometric, of real environments at the scale of their microstructure. Nevertheless, given the complexity and diversity of the phenomena associated with crystallization in porous media, the implementation of very diverse approaches, experimental and numerical, including on model systems that can go as far as a simple capillary tube, remains completely of topical interest.

Finally, it should be noted that simple saline solutions, most often made up of a single salt, are considered in the majority of academic studies. This is natural in a logic of increasing complexity, where the simplest situations must first be understood. As illustrated in Chapters 7 and 8, the situation in the field can be much more complex with the presence of many salts. Even in the case of simple solutions, the salt considered leads to specificities. For example, sodium chloride has only one crystalline form; it is anhydrous, under typical temperature conditions in applications, whereas sodium sulfate, for example, can have several crystalline forms. These specificities have important consequences, which imply clearly distinguishing what can be generic, that is to say common to all salts, from what is specific to certain salts. For example, from an application point of view, it is well established that sodium sulfate presents a greater threat than sodium chloride with regard to the destructive effect of salts.

In summary, the study of the crystallization of salts in porous media interests specialists from very diverse disciplines, whether in connection with applications or because of the many facets that this field of research presents. It is rich in open questions and although many advances have been made over the past decades, as evidenced by this book, much remains to be done, particularly with regard to predictive models, in connection with field situations. Nevertheless, this book bears witness to the degree of maturity reached, which, although it does not always provide answers to practical questions, offers a very coherent body of knowledge that also makes it possible to analyze very concrete situations in a convincing manner, as evidenced in Chapter 6.

References

Börnhorst, M., Walzel, P., Rahimi, A., Kharaghani, A., Tsotsas, E., Nestle, N., Besser, A., Kleine Jäger, F., Metzger, T. (2016). Influence of pore structure and impregnation–drying conditions on the solid distribution in porous support materials.

Drying Technology

, 34(16), 1964–1978.

Brard, C.-P. (1828). Sur le Procédé pour reconnaître, immédiatement, les pierres qui ne peuvent pas résister à la gelée et que l’on désigne ordinairement par les noms de pierres gelives ou pierres gelisses.

Annales de chimie et de physique

, 38, 160–192.

Cala-Rivera, V., Arranz-González, J.C., Rodríguez-Gómez, V., Fernández-Naranjo, F.J., Vadillo-Fernández, L. (2018). A preliminary study of the formation of efflorescent sulfate salts in abandoned mining areas with a view to their harvesting and subsequent recovery of copper.

Minerals Engineering

, 129, 37–40.

Correns, C.W. (1949). Growth and dissolution of crystals under linear pressure.

Discussions of the Faraday Society

, 5, 267–271.

Coussy, O. (2006). Deformation and stress from in-pore drying-induced crystallization of salt.

Journal of the Mechanics and Physics of Solids

, 54, 1517–1547.

Derluyn, H., Moonen, P., Carmeliet, J. (2014). Deformation and damage due to drying-induced salt crystallization in porous limestone.

Journal of the Mechanics and Physics of Solids

, 63, 242–255.

Diouf, B. (2018). Évaporation avec cristallisation de sel en milieu granulaire : localisation des cristaux et déplacements mécaniques induits. PhD Thesis, Université de Toulouse, Toulouse.

Grementieri, L., Molari, L., Derluyn, H., Desarnaud, J., Cnudde, V., Shahidzadeh, N., de Miranda, S. (2017). Numerical simulation of salt transport and crystallization in drying Prague sandstone using an experimentally consistent multiphase model.

Building and Environment

, 123, 289–298.

Feng, R., Wu, L., Wang, B. (2020). Numerical simulation for temperature field and salt heave influential depth estimation in sulfate saline soil highway foundations.

International Journal of Geomechanics

, 20(10), Article ID 04020196.

Fujimaki, H., Shimano, T., Inoue, M., Nakane, K. (2006). Effect of a salt crust on evaporation from a bare saline soil.

Vadose Zone Journal

, 5(4), 1246–1256.

Gagliardi, L. and Pierre-Louis, O. (2019). The nonequilibrium crystallization force.

Europhysics Letters

, 127(5), 59002.

Goudie, A. and Viles, H. (1997).

Salt Weathering Hazards

. Wiley, New York.

Hird, R. and Bolton, M.D. (2016). Migration of sodium chloride in dry porous materials.

Proceedings of the Royal Society A

, 472, 20150710.

Koniorczyk, M. and Gawin, D. (2012). Modelling of salt crystallization in building materials with microstructure – Poromechanical approach.

Construction and Building Materials

, 36, 860–873.

Lavalle, J. (1853). Recherche sur la formation lente des cristaux.

Comptes rendus de l’Académie des Sciences

, 36, 493–495.

Leclère, B. (2021). Damage induced by crystallisation in confined reservoir conditions. PhD Thesis, Université de Pau et des Pays de l’Adour, Pau.

Licsandru, G., Noiriel, C., Geoffroy, S., Abou Chakra, A., Duru, P., Prat, M. (2022). Detachment mechanism and reduced evaporation of an evaporative NaCl salt crust.

Scientific Reports

, 12(7473).

Litalien, A. and Zeeb, B. (2020). Curing the earth: A review of anthropogenic soil salinization and plant-based strategies for sustainable mitigation.

Science of the Total Environment

, 698(1), 134235.

Malin, M.C. (1974). Salt weathering on Mars

. Journal of Geophysical Research

, 79(26), 3888-3894.

Noiriel, C., Renard, F., Doan, M.L., Gratier, J.P. (2010). Intense fracturing and fracture sealing induced by mineral growth in porous rocks.

Chemical Geology

, 269, 197-209.

Peysson, Y., André, L., Azaroual, M. (2014). Well injectivity during CO

2

storage operations in deep saline aquifers. Part 1: Experimental investigation of drying effects, salt precipitation and capillary forces.

International Journal of Greenhouse Gas Control

, 22, 291-300.

Scherer, G.W. (1999). Crystallization in pores.

Cement and Concrete Research

, 29, 1347–1358.

Scherer, G.W., Flatt, R.J., Caruso, F., Aguilar Sanchez, A.M. (2014). Chemomechanics of salt damage in stone.

Nature Communications

, 5, 1-16.

Steiger, M. (2005). Crystal growth in porous materials I: The crystallization pressure of large crystals.

Journal of Crystal Growth

, 282(3/4), 455–469.

Tatzber, F., Wonisch, W., Balka, G., Marosi, A., Rusvai, M., Resch, U., Lindschinger, M., Moerkl, S., Cvirn, G. (2021). Coating with hypertonic saline improves virus protection of filtering facepiece manifold - Benefit of salt impregnation in times of pandemic.

International Journal of Environmental Research and Public Health

, 18(14), 7406.

Wang, X., Hsieh, M.L., Bur, J.A., Lin, S.Y., Narayanan, S. (2020). Capillary-driven solar-thermal water desalination using a porous selective absorber.

Materials Today Energy

, 17, 100453.

PART 1Fundamental Aspects

1Thermodynamics and Salt Crystallization Kinetics

Lionel MERCURY

ISTO, CNRS, BRGM, Université d’Orléans, France

1.1. Introduction

The crystallization of salts from aqueous solutions is a common, even daily event, the appearance of which does not seem to cause much difficulty. The increase in temperature or the evaporation of water are two typical causes, sometimes concurrent, which cause this phase transition, while the initial composition of the solution makes it favorable to the generation of salts.

Yet it is not trivial to accurately predict when and in what form a salt will appear in water. Also, it is not so easy to know where the salt will form, especially when the solution is moving through a network of channels and/or pores. The calcification of heating pipes, very heterogeneous and often atypical, is a good example of these complexities.

The purpose of this chapter is to look precisely at what thermodynamics, the science behind the driving forces, and kinetics, the science of transformation paths, say about these phase transitions. Far from an overview of commonly reviewed concepts, this chapter aims to highlight some subtle concepts that establish unsuspected connections between the water of the solution, the species which are dissolved therein, the pore walls enclosing it and local physicochemical conditions.

1.2. Thermodynamic driving force

1.2.1. Equilibrium, spontaneity, irreversibility

Energy is always difficult to define in a strict sense, but a fairly natural understanding of this concept can be made by pointing out that energy is a product of a force by an extensive term (quantity dependent); typically the elastic energy of a spring is written as: E = F.l, with F being the force of the spring and l its length. The force characterizes the intensity under which the considered energy is present in the system, for example, the force of a table lamp spring is not comparable to that of a car shock absorber. The term extensive translates the extent under which the force can be expressed: here, the elongation of the spring allows a “stronger” reservoir, more “intense”, to communicate elastic energy to the spring studied. Similarly, during its shortening, the spring makes energy available in proportion to the variation in its length.

According to the first law, the law of conservation of energy, two reservoirs in contact will exchange energy until their energy forces equalize, when all exchange stops. The state of equilibrium is thus the attractor of any system that is not in equilibrium: the evolution of natural systems is neither arbitrary nor random. The thermal energy goes from the hot source to the cold source, and the opposite is never observed: the energy therefore travels in the direction of decreasing temperature, the temperature here measuring the intensity, the force under which the thermal energy is present in the two reservoirs. The first law therefore makes it possible to state that the energy is conserved during the exchanges, which take place from the most intense reservoirs to the less intense reservoirs, by an exchange of extensity that decreases in the strongest reservoir and increases in the weakest reservoir. One can give as an example the flow of electric charges, electric extensity, which moves in the direction of decreasing electric voltage.

The second law adds precision to these possible developments. This law of directionality fixes the direction in which the transformations become spontaneous. This point is often misunderstood, in the sense that the first law already states that the transformations are not arbitrary, since they occur in the direction of decreasing intensity. The point that the second law clarifies is the fact that there are transformations that take place without exchanging extensive terms between two reservoirs. When a hot source heats a cold source, there is a flow of thermal extensity (entropy, immaterial extensity) from the first to the second. It is not a spontaneous transformation, the contact between the two sources must be ensured so that the extensity flows from one to the other. Spontaneity occurs when a transformation takes place without the need for an entropy source: subjected to an intensity gradient, the system begins to create entropy in proportion to this gradient, as if it were in contact with an entropy source. The immaterial aspect of entropy makes it possible to create it without contravening the law of conservation of matter, and since it is an extensity and not an energy, this creation does not contravene the first law either. The system therefore evolves spontaneously, since the necessary entropy is created on site, and irreversible because it created entropy that can no longer be destroyed. To conclude, it is therefore clear that a system subjected to an energy gradient can transform itself even in the absence of any extensive reservoir, if the creation of entropy makes it possible to establish an energy exchange chain that respects the first law.

With these rather abstract considerations having been made, now let us see how a spontaneous transformation can be quantified. For this, it is necessary to know if the creation of entropy is possible for the transformation of interest. One might assume that it suffices to measure the change in entropy in the system. The difficulty is that the entropy exchanged and the entropy created are indistinguishable, while spontaneity is only expressed by the second. The solution lies in monitoring the thermodynamic potential of the system, which is a property whose value changes in direct relation to the creation of entropy. It is easy to understand that an isothermal, isochoric system (constant T, V) can only create entropy in the same way as an isothermal, isobaric system (constant T, P); in fact, several thermodynamic properties can become a thermodynamic potential depending on the state variables of the system. In the case that concerns us with nucleation and crystal growth in an aqueous solution, the adequate thermodynamic potential is known as the free enthalpy or Gibbs free energy.

Gibbs free energy, as thermodynamic potential, is written as follows:

[1.1]