Benny K.G. Theng, “The Chemistry of Clay-Organic Reactions”, Second Edition, CRC Press, Boca Raton, 2024, 316pp
Book review by Jock Churchman, Adelaide, Australia
The publication of this book is a remarkable achievement by its author, Benny K.G. Theng. This 2024 book has been published a neat 50 years after the first edition of the same title in 1974,1 for which Dr Theng won the Adam Hilger Prize for a technical book published by a young (under 35) author. The author, who began the first edition while working in Australia for a short while, has had a distinguished career in New Zealand since then. Among his many awards, he is a Fellow of the Royal Society Te Apārangi. The most useful achievement of the first edition was that it was the only comprehensive book on the topic worldwide at that time and has remained so until now, when this second edition supplants it as its unique successor.
The author set out to offer a critical assessment rather than a neutral review of the literature and succeeds very well in this aim. It is up to date and widely sourced over the whole relevant literature, not just that specifically devoted to clays, clay minerals and soils.
The book is readable and logical. It has an appealing conversational style, revealing what the author is thinking and bringing together associated and contrasting viewpoints and interpretations.
While the book is not a historical treatise on the development of a field, the fact that the same author has been gathering and analysing the literature for well over 50 years means that it not only gives a wide and thoughtful summary of this one field, but it also enable us to see, alongside the author, how and why activities in this field have changed over that quite considerable time, indeed, since the first of such studies.
The first chapter of the book gives a complete description of clay minerals, their structures and properties and is a useful handbook on these by itself. The advances in techniques and therefore in knowledge since 1974 is immense. It is noteworthy that a major quantum leap in the knowledge and application of many new mainly spectroscopic techniques in the clay community occurred in the late 1970s through an international course sponsored by NATO that was held over two weeks in 1979 in the US. It led to a book2 which is cited, along with subsequent books, that trace the further application of these and also more recently developed and applied techniques.
Also notable is that not all curiosities have been solved or even advanced in the 50 years since the first book. For example, there is considerable discussion about the relationship between basal spacing of complexes and the orientation of the intercalated complexes of primary n-alcohols where questions remain unsolved but yet no apparent new contributions have been made since the early 1970s. Research possibilities abound here and elsewhere in the book.
Gaps in our understanding like these indicate that there is much to be done in fundamental understanding. And the author indicates that there may be a good reason why this is the case. He shows that there has been much recent work on the uptake of phenols by clays. This has clearly been initiated by the occurrence of phenols as highly toxic compounds in industrial effluents and wastewaters. Hardly a paper on this aspect is cited in the 1970s but many are from the 2000s. It is thereby shown how much research in this field, as in others, is owed to the need to solve practical problems. And clays prove useful for this kind of task.
Nonetheless, sorption of organic compounds by clays have always proved useful for characterising the clays, both by type (e.g. montmorillonite versus vermiculite) and also by the generally useful and often characteristic property of the extent of their surfaces. Work on the sorption of polyhydric alcohols like ethylene glycol and glycerol was carried out long ago but there was a burst of quite considerable refinement of this process as an identification and characterisation technique around the 1990s.
As for the alcohols, so for amines and amides, when it comes to understanding their interactions with clays. We are fortunate that there was much early work on both these molecular types that had a fundamental approach. Their attraction for studies as groups is that they represent clear stepwise changes in their properties so that their interactions with clays can be compared with changes in their ranges of properties. For amines and perhaps even more so for amides, practical applications in industrial and environmental fields involving clays have meant considerable recent work on their complexes with clays. Thus, some pesticides are based on amines as also are surfactants used in water-based drilling fluids. Amides find a particular use as urea used as a soil nitrogen fertiliser and in herbicides. The great advantage of applying these in association with clays is that the strength of these associations confers a restraint on their release, hence maximising their productive use and minimising their wasteful and often toxic release into the environment.
Studies of other uncharged organic compounds like ketones, aldehydes, ethers, nitriles and hydrocarbons are also described. Many of these continue to be carried out. There was a particular enthusiasm, largely in the 1980s and 1990s, for the use of clay-organic complexes, especially montmorillonites into which quaternary ammonium ions had been intercalated, for the uptake from water and even from the vapour phase of non-polar aromatic and often toxic pollutants such as benzene and toluene and their derivatives. The pollutants can be removed by adsorption or by their partitioning into the hydrophobic/organophilic interlayers of the clay-organic complex, bringing discussions in the book into physico-chemical as well as structural realms.
Chapter 3 covers practical applications of clay minerals in association with molecules of environmental importance such as pesticides (including herbicides), whether positively or negatively charged or uncharged. The literature covered is mostly quite recent, indicating an active field of study and use. This chapter also deals with clay mineral associations with antibiotics, pharmaceuticals and toxins, clearly also a currently active field. A considerable part of this chapter goes on to deal with the interaction of clays with biologically important molecules, namely amino acids, peptides, purines, pyrimidines, nucleosides, nucleotides, fatty acids, fats and saccharides. Among other applications, these are relevant to understanding soils and the formation of petroleum hydrocarbons, with work on them continuing in those contexts. Work on a wide range of dyes which clay minerals are used to extract from industrial wastewaters is also covered and helpfully tabulated, showing an active field of study.
Mechanisms of uptake of organic molecules by clays are highlighted in Chapters 4 (positively charged organic molecules) and 5 (negatively changed organic molecules). Since clay minerals are net negatively charged in most natural situations, reactions with positively charged entities make intuitive sense but Chapter 4 indicates the unique and flexible nature of the interlayer space in expandable clays as sites for reactions. It also shows that clays which are naturally hydrophilic can be made hydrophobic and capable of adsorbing organic molecules as a result of their like-on-like organophilic association with organic cations incorporated in clay mineral interlayer regions. These give rise to their applications as surfactants, in gelation and as thickening agents. Albeit that negatively charged molecules would appear to be repelled by generally negatively charged clay minerals, the clays are variably charged and, at acid pHs typical of many soils, clearly form complexes with humic substances (HS), the major organic component of soils. Other components, especially polyvalent inorganic cations, can play an important bridging role joining organic anions in HS to clay minerals. Although important, these interactions have been studied less often, leaving Chapter 5 the shortest in the book.
Albeit that smectites such as montmorillonite with a 2:1 Si:Al ratio which swell easily in water are generally good choices of clay minerals for the uptake of organic molecules, clays with a 1:1 ratio, especially platy kaolinite and halloysite, which occurs in a variety of crystal shapes from platy to tubular, can also interact strongly with organic compounds. These interactions are covered comprehensively in Chapter 6. Since kaolinites especially occur widely in soils, these interactions can be important. Furthermore, ancient practices such as the formation of porcelain in China are based on complexation of kaolinite with urea, applied from urine, prior to firing. These and other applications of complexes of 1:1 clay minerals, both ancient and modern, add much interest to the book. Complexation with kaolinite and halloysites is shown to often involve various surface modifications of the clays before their addition to organic compounds to increase the amount and strength of organic uptake. There has been particular interest lately in the applications and reactions of tubular halloysites, known as halloysite nanotubes (HNTs). The coverage of these has a particular relevance to New Zealand, which has extensive halloysite, tubular and non-tubular, in soils, as well as in several commercial deposits which include the world’s largest of this type (at Matauri Bay, in Northland). On the contrary, New Zealand lacks much kaolinite, in either soils or in deposits. This is unusual world-wide.
In terms of common clay minerals and their organic reactions, the book is complete. It covers chrysotile, only lightly researched, the layer-ribbon minerals, palygorskite and sepiolite, and short-range order minerals, allophane and imogolite. Palygorskite and sepiolite are non-swelling but their channels can take up organic molecules, depending largely on steric factors. As a result, they have environmental and some industrial uses. A colour photograph is enclosed to illustrate the ancient use of these layer-ribbon minerals to complex with indigo dye, used as ‘Maya blue’ by the early Mayas as a distinctive paint in Mesoamerica. Allophane and imogolite are highly influential in many soils due particularly to their high specific surface areas resulting from their nanoparticles. Imogolite can also be prepared synthetically and recent work on its organic complexes is described.
The 1974 predecessor of this book has been both relevant and useful for my career as for others with a similar scientific trajectory to mine. I began as an inorganic and physical chemistry graduate whose doctoral studies took me into clays, initially for industry but soon into soil science. Necessary course choices took me away from organic chemistry in my BSc (Hons) studies. However, it wasn’t to be long in studying soils that it became clear that soils are essentially (clay) mineral-organic complexes. Even when applied to agriculture, but especially when applied to environmental problems and challenges, it is necessary to understand how minerals, from the breakdown of rocks, on the one hand, and organic compounds, from the breakdown of plants and animals, on the other, come together and stay together. With the urgent problems involved in climate change, hardly anything is more important in soil studies than being able to understand how molecules containing carbon, i.e. organic molecules, can be taken up into, and retained within, soils.
Despite several earlier authors having suggested it,3 soil organic matter does not have a formula or known structure. Indeed, it has been proposed by more than one worker that no two instances of soil organic matter are alike.4 Even so, it was concluded from a review of the biological nature of soil productivity that “the interaction between clays and organic matter in soils is as vital to the continuation of life on earth as, and less understood than, photosynthesis”5. This latest book by Theng contains the essentials for understanding interactions between clays and defined organic compounds that must make up the various melanges that comprise the infinite variety of soil organic matter.
Furthermore, a relatively recent (2019) book of Theng’s6 shows the vast scope that there is for clay minerals to be used as catalysts for reactions involving organic molecules in industrial and environmental applications and even for understanding the prebiotic synthesis of molecules from the past to the beginnings of life on earth.
This timely book is written by a true master of knowledge in his field. Its first edition in 1974 was followed quite soon after by a book by Theng on the interactions between clays and polymers7 and preceded quite recently by the author’s book on clay mineral catalysts for organic reactions6 and a second edition of his book on clay-polymers.3 He has written or edited several other books on clays and soils, sometimes with others. He has also made many important contributions to the field by way of published research. This new book is well-written, hence easily read, reflecting Dr Theng’s international award for its 1974 first edition. I am very pleased to recommend this book as the most up-to-date and comprehensive description of a fascinating and increasingly useful field of study.
References
- Theng, B.K.G. The Chemistry of Clay-Organic Reactions, Adam Hilger: London, 1974.
- Stucki, J.W.; Banwart, W.L. (eds.) Advanced Chemical Methods for Soil and Clay Mineral Research, D. Reidel: Dordrecht, the Netherlands, 1980.
- Theng, B.K.G. Formation and Properties of Clay-Polymer Complexes, 2nd ed. Elsevier: Amsterdam, 2012.
- Churchman, G.J.; Velde, B. Soil clays. CRC Press: Boca Raton, Florida, 2019.
- Jacks, G.V. Soils and Fertilisers 1963, 26, 85-88.
- Theng, B.K.G. Clay Mineral Catalysis of Organic Reactions, CRC Press: Boca Raton, Florida, 2019.
- Theng, B.K.G. Formation and Properties of Clay-Polymer Complexes, Elsevier: Amsterdam, 1979.
