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Ultrasonics Sonochemistry 10 (2003) 175–179 www.elsevier.com/locate/ultsonch
Sonochemistry and sonoprocessing: the link, the trends and (probably) the future Timothy J. Mason
*
Sonochemistry Centre, School of Science and the Environment, Coventry University, Priory Street, Coventry CV1 5FB, UK Received 26 October 2002; accepted 16 January 2003
Abstract Traditionally the community of scientists involved with ultrasound has been divided broadly into those who use it as a measurement device with no effect on the medium (high frequency low power ultrasound e.g. non-destructive testing) and those who use it to produce physical or chemical effects in a medium (higher power low frequency ultrasound e.g. sonochemistry). Divisions also exist within the broad spectrum of those involved with the latter. In the early days of sonochemistry this did not prove to be a major problem, the subject was new and the field was expanding within the chemistry community. However at a point some years ago Jean-Louis Luche made the very important observation that sonochemistry applications could be subdivided into reactions which were the result of ‘‘true’’ and ‘‘false’’ effects [Synthetic Organic Chemistry by J.-L. Luche, 1998, p. 376]. Essentially these terms referred to real chemical effects induced by cavitation and those effects that could be mainly ascribed to the mechanical impact of bubble collapse. These mechanical effects have not held the interest of synthetic chemists as much as the so-called true ones but nevertheless they are certainly important in areas such as processing. In this paper I will attempt to show that there are links that can be made across many of the ultrasound ‘‘disciplines’’ and that these links can only serve to strengthen research in the general area of power ultrasound. If research on power ultrasound is strong then research into ‘‘pure’’ sonochemistry will also flourish and ‘‘false’’ sonochemistry will be born again as a significant research area. 2003 Elsevier Science B.V. All rights reserved.
1. Introduction Ultrasound has been used for a variety of purposes that includes areas as diverse as communication with animals (dog whistles), the detection of flaws in concrete buildings, the synthesis of fine chemicals and the treatment of disease. Despite its wide-ranging uses and exciting developments the study of ultrasound is a young science. The oldest application, the exploitation of diagnostic ultrasound only dates back to the beginning of the 20th century and ultrasound in processing is even more recent in origin. Developments in the latter began in the 1930s in the years preceding the Second World War when it was being investigated for a range of technologies including emulsification and surface cleaning. By the 1960s the industrial uses of power ultrasound were ac*
Tel./fax: +44-1203-838173. E-mail address: [email protected] (T.J. Mason).
cepted and being used in cleaning and plastic welding which continue to be major applications. Nowadays there are research groups and industries with expertise in a much wider range of activities that involve the uses of ultrasound in electrochemistry, food technology, chemical synthesis, materials extraction, nanotechnology, phase separation, surface cleaning, therapy and water and sewage treatment. Essentially there are three ‘‘strands’’ in ultrasonics research: • Sonochemistry with its origins in chemistry and physics: this includes synthesis, catalysis and fundamental studies of cavitation involving mainly academia. • Power ultrasound with its origins in engineering and processing: this includes cleaning, welding and materials processing involving mainly industry. • Diagnostic ultrasound involving non-destructive testing (NDE) and medical scanning: this attracts major interest in both academia and industry.
1350-4177/03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1350-4177(03)00086-5
176
T.J. Mason / Ultrasonics Sonochemistry 10 (2003) 175–179
It is possible to find links between the strands and from these cross-overs of interests to develop a strengthening and expansion of research. One such area is the use of focussed ultrasound in cancer therapy. Here the development of transducer arrays links with the physical effect of power ultrasound and the sonochemically improved performance of chemotherapeutic agents. A second is in the production and design of nanoparticles linking sonochemistry with ultrasonic precipitation and the surface effects of cavitation. A third example might be in chemical engineering where the design of a sonochemical reactor would not seem to be connected with a study of sonoluminescence. Yet sonoluminescence is an excellent method by which to identify the ‘‘active zones’’ of cavitation and so could provide an accurate picture of cavitation activity within such a reactor. There are many more examples. In this review I will attempt to show that the many and varied links that can be made across different ultrasonic disciplines will strengthen research in the general area of sonochemistry. What is required is that all of those interested in the promotion of the uses of ultrasound, particularly power ultrasound, should become a little more outward looking i.e. looking beyond the perceived but unreal barriers of their own disciplines. In this way a cross-fertilisation of ideas will provide for an unblinkered and freethinking approach to future developments in sonochemistry and related areas over the next five years. This article reflects my personal views on the subject and should not be taken to represent the views of the European Society of Sonochemistry.
These are topics that are more easily aligned with physics and physical chemistry. In other words our subject is both pure and applied in nature. Both aspects are needed for it to flourish but it is difficult to persuade funding authorities to support ‘‘pure’’ research. At the time of writing the European Union provides a way forward for sonochemistry to obtain funding in applied fields. It fits nicely into several themes that have been identified in Framework 6 (below) and it must be our job to find ways in which these can also be used to support pure research:
2. The trends
2.1.1. Electrochemistry [4,5] The benefits of using ultrasound in electrochemistry, sonoelectrochemistry, have been reported for a number of processes including the electrosynthesis of chemical compounds and conducting polymers, electroanalysis, bioelectrochemistry, electroplating, the preparation of nanomaterials, and electrocatalysis. There is particular interest in the applications of sonoelectrochemistry in processes of environmental importance including waste minimization, the replacement of toxic reagents (including mercury electrodes and other toxic system components), pollutant degradation and water remediation. In addition it has proved possible to improve the efficiencies of sensors for the detection of various species in water and in the atmosphere.
Sonochemistry has reached a time for change [1]. The reported uses of ultrasound in chemistry and processing are increasing and many different conferences (e.g. Forum Acusticum, ICA, UI, WCU) now include sonochemistry as a theme. Those of us who are members of the European Society of Sonochemistry are beginning to ask what would be the best role for ESS in the future. It certainly has a recognisable place in the history of the development of sonochemistry but with so much happening across the field of power ultrasound maybe it is time for it to take a more pro-active role in the planning for the future of our subject. The aim should be to expand the horizons of sonochemistry while maintaining chemistry as one of the core interests [2,3]. In past years many processes involving ultrasound have not been considered to fit under the umbrella of sonochemistry but nevertheless these processes are of interest to the chemical community. At the same time sonochemistry also encompasses cavitation theory, sonoluminescence and free radical production.
(1) (2) (3) (4)
new production processes and devices, genomics and biotechnology for health, nanotechnologies and nanosciences, food quality and safety.
Let us take each of these themes and explore the position of sonochemistry within them. It is to be hoped that by doing this sonochemistry can provide a hub technology for industries of the future. It is my belief that within Europe we have some of the most innovative and influential scientists involved in sonochemistry and related fields. Consequently this is a good route to follow, as long as we can accommodate the basic research that is so vital to underpin applied science. 2.1. New production processes and devices Amongst the possible production processes to which power ultrasound could contribute are Electrochemistry and Green Chemistry together with water and sewage treatment.
2.1.2. Green chemistry [6] The parallels between the aims of the two areas of green chemistry and sonochemistry are striking. Some of the statements that have been used to identify green chemistry are remarkably similar to those that have been used to describe sonochemistry:
T.J. Mason / Ultrasonics Sonochemistry 10 (2003) 175–179
• use of less hazardous chemicals and environmentally friendly solvents, • developing reaction conditions to increase the selectivity of the product, • minimizing energy consumption of chemical transformations, • use of alternative or renewable feedstocks e.g. extracted plant material. There has been a recent upsurge of interest in plant derived chemicals such as drugs, pigments, essential oils, natural polymers. This is perhaps the most obvious definition of green chemistry since plant material provides a renewable resource. This has increased need for efficient extraction methods, amongst which ultrasound has been shown to be of benefit by significantly reducing extraction times and increasing maximum extraction yields [7]. The cavitation processes act by enhancing mechanical fragmentation of biomass (that increases surface areas and the mass transfer rate) and by improving impregnation with solvent via the increasing permeability of vegetal materials. Examples of recent work include the extraction of anti-oxidants from the herb rosemary, of allelopathic materials from Aristolochia clematitis and of polysaccharides from plants and yeast. 2.1.3. Water (and sewage) treatment [8] In water treatment, the destruction/transformation of organic pollutants and the removal of biological contamination (disinfection) are the prime objectives of fundamental and applied investigations involving ultrasound. The degradation of chemical pollutants is possible through the effects of cavitation created by ultrasound. The reaction rate is a function of the physicochemical properties of the target compounds. Volatile and hydrophobic pollutants are degraded by thermal reactions in the ‘‘hot spot’’ of the cavitation bubble. Compounds that are more hydrophilic are decomposed in the bulk liquid by hydroxyl radicals produced in the cavitation bubble. Depending on the pollutants to be eliminated, the combination of advanced oxidation processes such as ozonation with ultrasound or an integrated ultrasonic/biological treatment can significantly improve process efficiency and economy. In sewage sludge treatment, ultrasound is applied as a pre-treatment to improve anaerobic sludge stabilisation. The high shear forces created in the advent of cavitation can be used to improve process efficiency in sludge dewatering and to achieve sludge disintegration. Due to the ultrasonic disruption of putrescible biomass in the sludge, subsequent microbial degradation occurs up to four times faster than in the conventional treatment. Improved water and sewage treatment is of great importance for the general health of the population of Europe and has relevance to the safety of food and drink.
177
2.1.4. Devices––the challenge of scale-up [9] The design of sonochemical reactors and the rationale for the scale up of successful laboratory ultrasonic experiments are clearly goals in sonochemistry and sonoprocessing. The majority of the advantages of the uses of ultrasound in the processing of liquids can be directly related to the physical effects of acoustic cavitation. It is for this reason that an understanding of the phenomenon of acoustic cavitation and its physical and chemical effects are crucial to developments in ultrasound leading to process optimisation. To a large extent this involves the reduction in energy consumption through the design of reactors incorporating appropriate ultrasonic emitters and this is mainly the domain of the engineers and physicists. The modelling of pressure and cavitation bubble fields are not easy especially in heterogeneous systems since these give very complex acoustic fields. Such studies must be underpinned with fundamental research into cavitation. Several methods for modelling can be compared, from very fundamental models of cavitation bubble dynamics, through an understanding of sonochemical activity in reactors via measurements of parameters such as chemi- and sonoluminescence, radical formation and mass transfer. 2.2. Genomics and biotechnology for health The use of high frequency ultrasound (around 5 MHz) at low powers in medical imaging can now be regarded as a routine procedure in diagnostic medicine. It is also possible however to use ultrasound in therapy but for such applications it is necessary to generate enough energy to cause temporary or permanent changes in the tissue. There are a number of applications of such higher power ultrasound currently in use and these include physiotherapy, dental descaling and the use of ultrasonic scalpels. One of the most exciting recent developments is in the field of cancer treatment where therapeutic ultrasound shows great promise on three fronts: • High-intensity focused ultrasound (HIFU). This involves the use of an array of transducers that are located outside of the body that provide a strong, small, focus inside targeted at the cancerous tissue. The concentrated energy can then directly kill the tumour. Clinical trials are well advanced in the use of HIFU for the treatment of patients with liver and other soft tissue cancers. It is important to develop an understanding of the mechanisms of interaction of ultrasound with tissue––thermal effects and cavitation––in order to optimise HIFU treatments and to expand the range of cancers that can be treated. • Sonodynamic therapy (SDT). This involves the use of ultrasound to increase the uptake of drugs in cells.
178
T.J. Mason / Ultrasonics Sonochemistry 10 (2003) 175–179
SDT is potentially important for the enhancement of chemotherapy. • Sonoporation. Involves the temporary increase in membrane permeability during exposure to ultrasound. This is seen as an important method for the improvement of transdermal delivery of drugs. There is already some use of the technique in physiotherapy for delivery of pain relieving drugs. 2.3. Nanotechnologies and nanosciences [10] There are close to 20 different methods for the fabrication of nanomaterials, these are regarded as the chemical and engineering materials of the future. What makes the use of power ultrasound effective and different from the other methods of synthesis are properties such as: • The ability to produce nanomaterials in the amorphous state. This is of particular importance in catalysis, magnetism, coatings etc. • The shorter reaction times involved e.g. mesoporous materials (MSPM) can be prepared in hours (it normally takes days by the sol–gel method). • The insertion of nanoparticles into the pores of MSPM without blockage of the pores. • The syntheses of inorganic fullerenes at room temperature. Other methods normally require high temperatures.
The last of these––surface cleaning––is applicable to a range of disciplines and applications including sensors, filters, substrates, reactors, catalysers and heat exchangers. Ultrasonic irradiation has been shown to be particularly effective for in situ cleaning in conjunction with chemical treatment and offers the following advantages: (a) reduced chemical consumption, (b) reduction of direct worker contact with hazardous cleaning chemicals/substances, (c) enhanced cleaning speed, (d) cleaning consistency––the ultrasonic activity is micro in nature and reaches all areas of complex configurations for uniform cleaning, (e) automatic operation and control savings in energy costs, labour and floor space.
3. The future It seems quite clear that within Europe at this time research funding is targeted at applied topics. It is a general complaint throughout most of the world that research funding is decreasing and that funding for fundamental research is decreasing fastest of all. So we, as scientists, must find ways in which our applied studies can also support fundamental work. We are fortunate in sonochemistry that our individual core interest in cavitation can find so many outlets in chemistry and processing. There are many overlaps of interests as I have tried to illustrate below.
Power ultrasound provides one of the most exciting ways to synthesise pure and supported nanomaterials for research and industry. 2.4. Food quality and safety [11] Within food technology we can find almost all of the examples of processing to which ultrasound can be applied. The mechanical effects of ultrasound can initiate and control crystallisation in fats and sugars. Foams, which cause general difficulties in process control e.g. in fermentation, can be destroyed. Food dehydration, a method of preserving food, is enhanced without effecting the product quality. Among other applications are improvements in the extraction of flavourings, filtration, freezing and heat exchange, mixing and homogenisation and the precipitation of airborne powders. 2.5. Chemical and biochemical effects These have been employed for sterilisation (in conjunction with heat), effluent treatment, alteration of enzyme activity and the removal of deposits and biofilms from the surfaces of equipment.
Such cross-disciplinary links require a good grasp of the fundamentals as well as the underlying theory and so it is my contention that sonochemistry cannot reach its full potential without input from both theorists and applied scientists of many different backgrounds. We have a broad and exciting field of studies that is certain to lead to substantial advances. We must remember however that our own research, whether it be specialised or broad based is part of a whole that should form a team. That team is the European Society of Sonochemistry.
T.J. Mason / Ultrasonics Sonochemistry 10 (2003) 175–179
References [1] M. Chanon, J.-L. Luche, Sonochemistry: Quo Vadis, in: J.-L. Luche (Ed.), Synthetic Organic Chemistry, Plenum, 1998, pp. 376–392. [2] K.S. Suslick, G.J. Price, Applications of ultrasound to materials chemistry, Annu. Rev. Mater. Sci. 29 (1999) 295–326. [3] M. Ashokkumar, F. Grieser, Ultrasound assisted chemical processes, Rev. Chem. Eng. 15 (1999) 41–83. [4] S.S. Phull, D.J. Walton, Sonoelectrochemistry, in: T.J. Mason (Ed.), Advances in Sonochemistry, vol. 4, JAI Press, 1996, pp. 205–284. [5] J.C. Ball, R.G. Compton, Application of ultrasound to electrochemical measurements and analyses, Electrochemistry 67 (1999) 912–919. [6] T.J. Mason, P. Cintas, Sonochemistry, in: Handbook of Green Chemistry and Technology, Blackwell, 2002, pp. 372–396.
179
[7] M. Vinatoru, M. Toma, T.J. Mason, Ultrasonically assisted extraction of bioactive principles from plants and their constituents, in: T.J. Mason (Ed.), Advances in Sonochemistry, vol. 5, JAI Press, 1999, pp. 209–248. [8] T.J. Mason, A. Tiehm (Eds.), Ultrasound in Environmental Protection (Theme Issue), Advances in Sonochemistry, vol. 6, Elsevier, 2001. [9] F.J. Keil, K.M. Swamy, Reactors for sonochemical engineering–– present status, Rev. Chem. Eng. 15 (1999) 85–155. [10] V. Kesavan, D. Dhar, Y. Koltypin, N. Perkas, O. Palchik, A. Gedanken, S. Chandrasekaran, Nanostructured amorphous metals, alloys, and metal oxides as new catalysts for oxidation, Pure Appl. Chem. 73 (2001) 85–91. [11] M. Povey, T.J. Mason (Eds.), Ultrasound in Food Processing, Blackie Academic and Professional, 1998.
Sonochemistry and sonoprocessing: the link, the trends and (probably) the future Timothy J. Mason
*
Sonochemistry Centre, School of Science and the Environment, Coventry University, Priory Street, Coventry CV1 5FB, UK Received 26 October 2002; accepted 16 January 2003
Abstract Traditionally the community of scientists involved with ultrasound has been divided broadly into those who use it as a measurement device with no effect on the medium (high frequency low power ultrasound e.g. non-destructive testing) and those who use it to produce physical or chemical effects in a medium (higher power low frequency ultrasound e.g. sonochemistry). Divisions also exist within the broad spectrum of those involved with the latter. In the early days of sonochemistry this did not prove to be a major problem, the subject was new and the field was expanding within the chemistry community. However at a point some years ago Jean-Louis Luche made the very important observation that sonochemistry applications could be subdivided into reactions which were the result of ‘‘true’’ and ‘‘false’’ effects [Synthetic Organic Chemistry by J.-L. Luche, 1998, p. 376]. Essentially these terms referred to real chemical effects induced by cavitation and those effects that could be mainly ascribed to the mechanical impact of bubble collapse. These mechanical effects have not held the interest of synthetic chemists as much as the so-called true ones but nevertheless they are certainly important in areas such as processing. In this paper I will attempt to show that there are links that can be made across many of the ultrasound ‘‘disciplines’’ and that these links can only serve to strengthen research in the general area of power ultrasound. If research on power ultrasound is strong then research into ‘‘pure’’ sonochemistry will also flourish and ‘‘false’’ sonochemistry will be born again as a significant research area. 2003 Elsevier Science B.V. All rights reserved.
1. Introduction Ultrasound has been used for a variety of purposes that includes areas as diverse as communication with animals (dog whistles), the detection of flaws in concrete buildings, the synthesis of fine chemicals and the treatment of disease. Despite its wide-ranging uses and exciting developments the study of ultrasound is a young science. The oldest application, the exploitation of diagnostic ultrasound only dates back to the beginning of the 20th century and ultrasound in processing is even more recent in origin. Developments in the latter began in the 1930s in the years preceding the Second World War when it was being investigated for a range of technologies including emulsification and surface cleaning. By the 1960s the industrial uses of power ultrasound were ac*
Tel./fax: +44-1203-838173. E-mail address: [email protected] (T.J. Mason).
cepted and being used in cleaning and plastic welding which continue to be major applications. Nowadays there are research groups and industries with expertise in a much wider range of activities that involve the uses of ultrasound in electrochemistry, food technology, chemical synthesis, materials extraction, nanotechnology, phase separation, surface cleaning, therapy and water and sewage treatment. Essentially there are three ‘‘strands’’ in ultrasonics research: • Sonochemistry with its origins in chemistry and physics: this includes synthesis, catalysis and fundamental studies of cavitation involving mainly academia. • Power ultrasound with its origins in engineering and processing: this includes cleaning, welding and materials processing involving mainly industry. • Diagnostic ultrasound involving non-destructive testing (NDE) and medical scanning: this attracts major interest in both academia and industry.
1350-4177/03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1350-4177(03)00086-5
176
T.J. Mason / Ultrasonics Sonochemistry 10 (2003) 175–179
It is possible to find links between the strands and from these cross-overs of interests to develop a strengthening and expansion of research. One such area is the use of focussed ultrasound in cancer therapy. Here the development of transducer arrays links with the physical effect of power ultrasound and the sonochemically improved performance of chemotherapeutic agents. A second is in the production and design of nanoparticles linking sonochemistry with ultrasonic precipitation and the surface effects of cavitation. A third example might be in chemical engineering where the design of a sonochemical reactor would not seem to be connected with a study of sonoluminescence. Yet sonoluminescence is an excellent method by which to identify the ‘‘active zones’’ of cavitation and so could provide an accurate picture of cavitation activity within such a reactor. There are many more examples. In this review I will attempt to show that the many and varied links that can be made across different ultrasonic disciplines will strengthen research in the general area of sonochemistry. What is required is that all of those interested in the promotion of the uses of ultrasound, particularly power ultrasound, should become a little more outward looking i.e. looking beyond the perceived but unreal barriers of their own disciplines. In this way a cross-fertilisation of ideas will provide for an unblinkered and freethinking approach to future developments in sonochemistry and related areas over the next five years. This article reflects my personal views on the subject and should not be taken to represent the views of the European Society of Sonochemistry.
These are topics that are more easily aligned with physics and physical chemistry. In other words our subject is both pure and applied in nature. Both aspects are needed for it to flourish but it is difficult to persuade funding authorities to support ‘‘pure’’ research. At the time of writing the European Union provides a way forward for sonochemistry to obtain funding in applied fields. It fits nicely into several themes that have been identified in Framework 6 (below) and it must be our job to find ways in which these can also be used to support pure research:
2. The trends
2.1.1. Electrochemistry [4,5] The benefits of using ultrasound in electrochemistry, sonoelectrochemistry, have been reported for a number of processes including the electrosynthesis of chemical compounds and conducting polymers, electroanalysis, bioelectrochemistry, electroplating, the preparation of nanomaterials, and electrocatalysis. There is particular interest in the applications of sonoelectrochemistry in processes of environmental importance including waste minimization, the replacement of toxic reagents (including mercury electrodes and other toxic system components), pollutant degradation and water remediation. In addition it has proved possible to improve the efficiencies of sensors for the detection of various species in water and in the atmosphere.
Sonochemistry has reached a time for change [1]. The reported uses of ultrasound in chemistry and processing are increasing and many different conferences (e.g. Forum Acusticum, ICA, UI, WCU) now include sonochemistry as a theme. Those of us who are members of the European Society of Sonochemistry are beginning to ask what would be the best role for ESS in the future. It certainly has a recognisable place in the history of the development of sonochemistry but with so much happening across the field of power ultrasound maybe it is time for it to take a more pro-active role in the planning for the future of our subject. The aim should be to expand the horizons of sonochemistry while maintaining chemistry as one of the core interests [2,3]. In past years many processes involving ultrasound have not been considered to fit under the umbrella of sonochemistry but nevertheless these processes are of interest to the chemical community. At the same time sonochemistry also encompasses cavitation theory, sonoluminescence and free radical production.
(1) (2) (3) (4)
new production processes and devices, genomics and biotechnology for health, nanotechnologies and nanosciences, food quality and safety.
Let us take each of these themes and explore the position of sonochemistry within them. It is to be hoped that by doing this sonochemistry can provide a hub technology for industries of the future. It is my belief that within Europe we have some of the most innovative and influential scientists involved in sonochemistry and related fields. Consequently this is a good route to follow, as long as we can accommodate the basic research that is so vital to underpin applied science. 2.1. New production processes and devices Amongst the possible production processes to which power ultrasound could contribute are Electrochemistry and Green Chemistry together with water and sewage treatment.
2.1.2. Green chemistry [6] The parallels between the aims of the two areas of green chemistry and sonochemistry are striking. Some of the statements that have been used to identify green chemistry are remarkably similar to those that have been used to describe sonochemistry:
T.J. Mason / Ultrasonics Sonochemistry 10 (2003) 175–179
• use of less hazardous chemicals and environmentally friendly solvents, • developing reaction conditions to increase the selectivity of the product, • minimizing energy consumption of chemical transformations, • use of alternative or renewable feedstocks e.g. extracted plant material. There has been a recent upsurge of interest in plant derived chemicals such as drugs, pigments, essential oils, natural polymers. This is perhaps the most obvious definition of green chemistry since plant material provides a renewable resource. This has increased need for efficient extraction methods, amongst which ultrasound has been shown to be of benefit by significantly reducing extraction times and increasing maximum extraction yields [7]. The cavitation processes act by enhancing mechanical fragmentation of biomass (that increases surface areas and the mass transfer rate) and by improving impregnation with solvent via the increasing permeability of vegetal materials. Examples of recent work include the extraction of anti-oxidants from the herb rosemary, of allelopathic materials from Aristolochia clematitis and of polysaccharides from plants and yeast. 2.1.3. Water (and sewage) treatment [8] In water treatment, the destruction/transformation of organic pollutants and the removal of biological contamination (disinfection) are the prime objectives of fundamental and applied investigations involving ultrasound. The degradation of chemical pollutants is possible through the effects of cavitation created by ultrasound. The reaction rate is a function of the physicochemical properties of the target compounds. Volatile and hydrophobic pollutants are degraded by thermal reactions in the ‘‘hot spot’’ of the cavitation bubble. Compounds that are more hydrophilic are decomposed in the bulk liquid by hydroxyl radicals produced in the cavitation bubble. Depending on the pollutants to be eliminated, the combination of advanced oxidation processes such as ozonation with ultrasound or an integrated ultrasonic/biological treatment can significantly improve process efficiency and economy. In sewage sludge treatment, ultrasound is applied as a pre-treatment to improve anaerobic sludge stabilisation. The high shear forces created in the advent of cavitation can be used to improve process efficiency in sludge dewatering and to achieve sludge disintegration. Due to the ultrasonic disruption of putrescible biomass in the sludge, subsequent microbial degradation occurs up to four times faster than in the conventional treatment. Improved water and sewage treatment is of great importance for the general health of the population of Europe and has relevance to the safety of food and drink.
177
2.1.4. Devices––the challenge of scale-up [9] The design of sonochemical reactors and the rationale for the scale up of successful laboratory ultrasonic experiments are clearly goals in sonochemistry and sonoprocessing. The majority of the advantages of the uses of ultrasound in the processing of liquids can be directly related to the physical effects of acoustic cavitation. It is for this reason that an understanding of the phenomenon of acoustic cavitation and its physical and chemical effects are crucial to developments in ultrasound leading to process optimisation. To a large extent this involves the reduction in energy consumption through the design of reactors incorporating appropriate ultrasonic emitters and this is mainly the domain of the engineers and physicists. The modelling of pressure and cavitation bubble fields are not easy especially in heterogeneous systems since these give very complex acoustic fields. Such studies must be underpinned with fundamental research into cavitation. Several methods for modelling can be compared, from very fundamental models of cavitation bubble dynamics, through an understanding of sonochemical activity in reactors via measurements of parameters such as chemi- and sonoluminescence, radical formation and mass transfer. 2.2. Genomics and biotechnology for health The use of high frequency ultrasound (around 5 MHz) at low powers in medical imaging can now be regarded as a routine procedure in diagnostic medicine. It is also possible however to use ultrasound in therapy but for such applications it is necessary to generate enough energy to cause temporary or permanent changes in the tissue. There are a number of applications of such higher power ultrasound currently in use and these include physiotherapy, dental descaling and the use of ultrasonic scalpels. One of the most exciting recent developments is in the field of cancer treatment where therapeutic ultrasound shows great promise on three fronts: • High-intensity focused ultrasound (HIFU). This involves the use of an array of transducers that are located outside of the body that provide a strong, small, focus inside targeted at the cancerous tissue. The concentrated energy can then directly kill the tumour. Clinical trials are well advanced in the use of HIFU for the treatment of patients with liver and other soft tissue cancers. It is important to develop an understanding of the mechanisms of interaction of ultrasound with tissue––thermal effects and cavitation––in order to optimise HIFU treatments and to expand the range of cancers that can be treated. • Sonodynamic therapy (SDT). This involves the use of ultrasound to increase the uptake of drugs in cells.
178
T.J. Mason / Ultrasonics Sonochemistry 10 (2003) 175–179
SDT is potentially important for the enhancement of chemotherapy. • Sonoporation. Involves the temporary increase in membrane permeability during exposure to ultrasound. This is seen as an important method for the improvement of transdermal delivery of drugs. There is already some use of the technique in physiotherapy for delivery of pain relieving drugs. 2.3. Nanotechnologies and nanosciences [10] There are close to 20 different methods for the fabrication of nanomaterials, these are regarded as the chemical and engineering materials of the future. What makes the use of power ultrasound effective and different from the other methods of synthesis are properties such as: • The ability to produce nanomaterials in the amorphous state. This is of particular importance in catalysis, magnetism, coatings etc. • The shorter reaction times involved e.g. mesoporous materials (MSPM) can be prepared in hours (it normally takes days by the sol–gel method). • The insertion of nanoparticles into the pores of MSPM without blockage of the pores. • The syntheses of inorganic fullerenes at room temperature. Other methods normally require high temperatures.
The last of these––surface cleaning––is applicable to a range of disciplines and applications including sensors, filters, substrates, reactors, catalysers and heat exchangers. Ultrasonic irradiation has been shown to be particularly effective for in situ cleaning in conjunction with chemical treatment and offers the following advantages: (a) reduced chemical consumption, (b) reduction of direct worker contact with hazardous cleaning chemicals/substances, (c) enhanced cleaning speed, (d) cleaning consistency––the ultrasonic activity is micro in nature and reaches all areas of complex configurations for uniform cleaning, (e) automatic operation and control savings in energy costs, labour and floor space.
3. The future It seems quite clear that within Europe at this time research funding is targeted at applied topics. It is a general complaint throughout most of the world that research funding is decreasing and that funding for fundamental research is decreasing fastest of all. So we, as scientists, must find ways in which our applied studies can also support fundamental work. We are fortunate in sonochemistry that our individual core interest in cavitation can find so many outlets in chemistry and processing. There are many overlaps of interests as I have tried to illustrate below.
Power ultrasound provides one of the most exciting ways to synthesise pure and supported nanomaterials for research and industry. 2.4. Food quality and safety [11] Within food technology we can find almost all of the examples of processing to which ultrasound can be applied. The mechanical effects of ultrasound can initiate and control crystallisation in fats and sugars. Foams, which cause general difficulties in process control e.g. in fermentation, can be destroyed. Food dehydration, a method of preserving food, is enhanced without effecting the product quality. Among other applications are improvements in the extraction of flavourings, filtration, freezing and heat exchange, mixing and homogenisation and the precipitation of airborne powders. 2.5. Chemical and biochemical effects These have been employed for sterilisation (in conjunction with heat), effluent treatment, alteration of enzyme activity and the removal of deposits and biofilms from the surfaces of equipment.
Such cross-disciplinary links require a good grasp of the fundamentals as well as the underlying theory and so it is my contention that sonochemistry cannot reach its full potential without input from both theorists and applied scientists of many different backgrounds. We have a broad and exciting field of studies that is certain to lead to substantial advances. We must remember however that our own research, whether it be specialised or broad based is part of a whole that should form a team. That team is the European Society of Sonochemistry.
T.J. Mason / Ultrasonics Sonochemistry 10 (2003) 175–179
References [1] M. Chanon, J.-L. Luche, Sonochemistry: Quo Vadis, in: J.-L. Luche (Ed.), Synthetic Organic Chemistry, Plenum, 1998, pp. 376–392. [2] K.S. Suslick, G.J. Price, Applications of ultrasound to materials chemistry, Annu. Rev. Mater. Sci. 29 (1999) 295–326. [3] M. Ashokkumar, F. Grieser, Ultrasound assisted chemical processes, Rev. Chem. Eng. 15 (1999) 41–83. [4] S.S. Phull, D.J. Walton, Sonoelectrochemistry, in: T.J. Mason (Ed.), Advances in Sonochemistry, vol. 4, JAI Press, 1996, pp. 205–284. [5] J.C. Ball, R.G. Compton, Application of ultrasound to electrochemical measurements and analyses, Electrochemistry 67 (1999) 912–919. [6] T.J. Mason, P. Cintas, Sonochemistry, in: Handbook of Green Chemistry and Technology, Blackwell, 2002, pp. 372–396.
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[7] M. Vinatoru, M. Toma, T.J. Mason, Ultrasonically assisted extraction of bioactive principles from plants and their constituents, in: T.J. Mason (Ed.), Advances in Sonochemistry, vol. 5, JAI Press, 1999, pp. 209–248. [8] T.J. Mason, A. Tiehm (Eds.), Ultrasound in Environmental Protection (Theme Issue), Advances in Sonochemistry, vol. 6, Elsevier, 2001. [9] F.J. Keil, K.M. Swamy, Reactors for sonochemical engineering–– present status, Rev. Chem. Eng. 15 (1999) 85–155. [10] V. Kesavan, D. Dhar, Y. Koltypin, N. Perkas, O. Palchik, A. Gedanken, S. Chandrasekaran, Nanostructured amorphous metals, alloys, and metal oxides as new catalysts for oxidation, Pure Appl. Chem. 73 (2001) 85–91. [11] M. Povey, T.J. Mason (Eds.), Ultrasound in Food Processing, Blackie Academic and Professional, 1998.
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