Ind. Eng. Chem. Res. 1996,34,2841-2848
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“The Waters Were Made Sweet”. Advances in Ion Exchange Technology? Michael Streat* Department of Chemical Engineering, Loughborough University of Technology, Loughborough, Leicestershire, England
The history of ion exchange is marked by many important milestones, notably the development of novel polymeric materials and considerable advances in our understanding of the underlying fundamental principles. Separately, there have been major advances in the design and development of the apparatus and equipment required to perform industrial ion exchange processes in a wide range of modern applications. This paper reviews the progress that has been made during the latter part of the twentieth century.
Advances in the Fundamentals of Ion Exchange Ion exchange is concerned with the interaction of ionic species in an aqueous electrolyte solution with adsorbent solid materials. It is distinguished from conventional adsorption by the nature and morphology of the adsorbent material which in most cases is either a functional or “dynamic”polymer matrix or an inorganic structure containing exchangeable functional groups. Ion exchange is strictly stoichiometric and is governed by the laws of electroneutrality. In its simplest form, ion exchange is often encountered in the home. The domestic water softener uses conventional cationic polymeric ion exchange materials to exchange the hardness in tap water, i.e., calcium and magnesium ions with sodium ions. This application, and the demineralization of water are probably the best known applications of ion exchange. However, there have been enormous advances in the field of adsorption and ion exchange in the past 30 years and many important industrial processes have been facilitated by major innovations in the chemistry and chemical engineering of the technique. Ion exchange is not a twentieth century phenomenon. Several notable early textbooks refer us to the Holy Bible and draw our attention to the passage in the book of Exodus, Chapter 15, verses 22-25, which recites the passage describing Moses leading the children of Israel from bondage into the wilderness. The reference to Moses was first mentioned by Kunin and Myers (1950) in the introduction to their book on ion exchange resins, and later it was repeated in the book on ion exchange published by Helfferich (1959). The passage reads as follows: And Moses led Israel onward from the Red Sea, and they went out into the wilderness of Shur; and they went three days in the wilderness and found no water. And they came to Marah, they could not drink of the waters of Marah, for they were bitter. Therefore the name of it was called Marah. And the people murmured against Moses, saying “What shall we drink?” And he cried unto the Lord; and the Lord showed him a tree and he cast it into the waters and THE WATERS WERE MADE SWEET. This paper is based in part on the Inaugural Lecture given by Professor M. Streat on Nov 10, 1993, a t Loughborough University of Technology, England. 8 E-mail:
[email protected]. FAX: ( f 4 4 ) 1509 222500. +
The holy scriptures record the fact that Moses used a tree to sweeten the waters; a clear reference t o the fact that water was rendered potable by the removal of some salt-bearing minerals containing sodium, calcium, and magnesium. The scriptures give no indication as to whether it was a flourishing branch, a withered, decaying branch, or the dead parched branch of a native tree of the region. We can only speculate on the event and draw the conclusion that it was probably the fact that the rotted cellulose of the tree was able to exchange cations in the “bitter”brackish waters at Marah. Most vegetable matter contains cellulose, and today we know that processed cellulose is a most effective cation exchange material and for this reason has found many modern day process applications, e.g., Grubhofer (1991). Another possibility that springs to mind is that the tree contained an accumulation of fungal growth which we now know is also capable of biosorption of cationic species in water (see Gadd (1990)). The clear reference to the concept of ion exchange in the Old Testament challenges our intellectual interpretation of the event. Ion exchange, as we know it today, is defined as the counterexchange of dissociated ions in solution with fixed functional groups of opposite charge within a porous solid matrix. The phenomenon of ion exchange is universally attributed to two English chemists, Thompson (1850) and Way (18501, who studied the exchange of ions on cultivated soils. Despite this early breakthrough, the phenomenon remained very much a curiosity for nearly 100 years. It was Adams and Holmes (19351, chemists also working in England, who discovered that it was possible to produce synthetic organic ion exchange resins, and this is probably the most significant milestone in the history of the evolution of ion exchange as an industrial process. The advent of polymeric ion exchange resins has fascinated the physical chemist, the analyst, the chromatographer, and the separation scientist alike. When the work performed in the Manhattan Project was finally released in 1947, it was evident that ion exchange provided the most important separation steps leading to the recovery and purification of the lanthanide and the transuranic elements. The discovery of element 61, i.e., promethium, is attributed t o ion exchange. This element is not found in nature, and its isotope was first separated by ion exchange in the fission products of uranium by Marinsky et al. (1947). The 1940s and 1950s were halcyon days for physical chemists, who embarked on comprehensive studies of the phenomenon of ion exchange using newly discovered
0888-588519512634-2841$09.00/00 1995 American Chemical Society
2842 Ind. Eng. Chem. Res., Vol. 34, No. 8, 1995
ion exchange resins synthesized from functionalized copolymers of styrene and divinylbenzene. Ion exchange equilibria, selectivity, and diffusion kinetics were studied copiously in laboratories around the world, and this paved the way for the immediate application of these materials to a variety of industrial applications. Foremost was industrial boiler feed water treatment, but a simultaneous strategic need was identified in the nuclear arms race, and there was a frantic rush t o develop process technology for the recovery and purification of uranium from its naturally occurring ore bodies. Water treatment and hydrometallurgy prompted some of the most important advances in ion exchange technology, and this thrust continues until this day, though the emphasis has changed considerably. It is particularly interesting t o review the development of ion exchange from the early 1960s and trace progress up to the present day. At that time, ion exchange resins based on styrene/divinylbenzene copolymers were widely available from a range of industrial suppliers and large-scale applications were well established in the water industry, in sugar refining, and in the recovery of natural uranium from its mineral ore bodies. The demand for industrial water as well as drinking water has always been great, and this was evident 30 years ago. Some of the major advances in ion exchange were related to the production of treated water for high-pressure boilers. Most conventional coalfired power stations needed large volumes of makeup water of low conductivity, and the availability of polymeric cation and anion exchange resins in bulk satisfied this requirement efficiently and relatively cheaply. Organic impurities in feed water, especially the derivatives of naturally occurring substances such as humic acid and fulvic acid, caused fouling problems with anion exchange resins, and the 1960s saw considerable effort in the development of more amenable polymeric structures capable of avoiding this problem. The emergence of new polymer networks and entirely new matrix structures based on a poly(methy1methacrylate) backbone seems to have been the answer (see Kressman (1969) and Kunin (1969)). I can well remember the excitement and controversy that surrounded the introduction and commercialization of these novel “macroreticular” polymers. The delegates at the international ion exchange conference organized by the Society of Chemical Industry in London in 1969 were riveted by the animated discussion of the protagonists from the various ion exchange resin manufacturers. Macroreticular ion exchange resins differed from polyelectrolyte gel type materials by possessing a distinct porous structure. These resins had a bidisperse structure, and consisted of a network of micropolymer spheres interconnected into a macrospherical bead shape. The open pore structure so formed was capable of allowing diffusion of long chain aliphatic acids, e.g., humic acid and fulvic acid, into and, more importantly, out of the internal matrix of anion exchange resins. This was an important breakthrough in ion exchange, and this innovation was quickly followed by the resin manufacturers’ developing alternative polymerization techniques to produce “open pore” matrix structures to facilitate the mobility of long chain molecular species within the resin matrix. These advances in resin development, coupled with improved process technology, such as the introduction of counterflow regeneration and improved mixed bed operation, have continued relentlessly over a 30 year
period so that ion exchange is now standard practice for the production of high-quality water for the power, chemical, food, and beverage industries. Today, there is a need to produce ultrapure water for applications in the nuclear, semiconductor, and pharmaceutical and drug industries. Water containing less than 1ppb impurities is now demanded in the coolant circuit of the present generation of pressurized water reactors operating mainly in the USA. This stringent requirement is satisfied by modern mixed bed ion exchange plants operating at nuclear power utilities throughout the world. Similarly, the need to provide large quantities of ultrapure water for the semiconductor industries has been successfully met by ion exchange. This task is even more exacting: the levels of impurities are quoted in parts per trillion (i.e,, 1part in It is difficult t o comprehend these trace concentrations, and the major companies in the field are inevitably pushed to the limits of detection in their endeavor to meet these strict demands. Ion exchange also satisfies the needs of the biochemical industries by producing ultrapure quality water which in conjunction with UV sterilization satisfies all the criteria for the Food and Drug Administration in the USA and similar organizations worldwide. It is fair to say that about 90% of all ion exchange resins manufactured today find application in water treatment. Most of the important developments during this century can be traced to the need to produce high-quality water for our industrial base. The requirement for water is ever increasing, and the specifications for purity and quality are ever more demanding. Ion exchange was viewed with great interest by the UK nuclear industry in the early 1960s. The first gascooled nuclear power station had been commissioned at Calder Hall, and nuclear fuel reprocessing was in full swing at Windscale (now Sellafield). At that time, there was considerable interest in the primary separation of irradiated nuclear fuel. Solvent extraction, using trin-butyl phosphate (TBP), was the established technology, having been developed rapidly in the UK over the preceding 15 or so years. Nevertheless, there was scope to research alternative processes capable of separating uranium, plutonium, and the fission products. This is an extremely arduous chemical separation, performed in an ambient environment of high energy ionizing radiation. The process reagents must be able to withstand the severe effects of high-intensity y-radiation associated with the fission products as well as short range a-particle attack by the transuranics. My own Ph.D. research was a study of the radiation stability of ion exchange resins, encouraged at that time by the support of the nuclear industry and UKAEA at Harwell (see Hall and Streat (1963)). I look back on that period with nostalgia, since I felt that this work was likely t o break new ground in nuclear fuel reprocessing. As it turned out, the established technology based on TBP was impregnable and has dominated nuclear fuel processing to this day. Nevertheless, the early work prompted by the nuclear industry in the UK and elsewhere did not go in vain and lead to many novel and important advances in the field of ion exchange technology.
Advances in the Applications of Ion Exchange Technology Ion exchange and solvent extraction are extensively used in hydrometallurgy, which is the wet chemical recovery and purification of metals from naturally
Ind. Eng. Chem. Res., Vol. 34, No. 8, 1995 2843 occurring minerals, e.g., base metals such as copper, nickel and zinc, as well as precious metals such as gold, platinum, rhodium, etc. The recovery and purification of natural uranium is possibly the most important hydrometallurgical development over the past 50 years, and this is discussed by Streat and Naden (1987). This is particularly evident at the Rossing Uranium Mine in Namibia, the largest uranium recovery project in the world. Uranium is normally recovered by leaching the crushed mineral ore body with sulfuric acid and thereby liberating the soluble uranyl sulfate anions into solution. Separation of uranium from the entire range of associated metal ions in solution is achieved by a selective anion exchange process involving either a polymeric ion exchange resin or alternatively a “liquid”ion exchanger based on long chain organic tertiary amines, such as tri-n-octylamine. The immediate postwar rush to produce uranium for strategic and peaceful applications saw the development of new ion exchange plants in uranium-rich regions of the world, notably USA, Canada, Australia, and South Africa. Large ion exchange recovery plants were erected in haste in the late 1940s and early 1950s to produce uranium concentrate, known as yellow cake, and almost all of them were based on fxed bed anion exchange resin installations. Nuclear grade uranium was obtained in a subsequent process using conventional TBP solvent extraction. Later, solvent extraction with tertiary amines, socalled “liquid ion exchange”,was introduced at the mine site, since it was found possible to produce almost nuclear grade material in one process operation. Solvent extraction also had the major advantage of operating as a continuous countercurrent process with all the improvements in efficiency that this provided. Ion exchange was a t best only a semicontinuous batch process involving a cascade of large ion exchange columns operating on a “merry-go-round”principle (see Merritt (197 1)). There had been many attempts to develop continuous countercurrent ion exchange during the 1950s. The earliest attempts involved crude apparatus based on the idea of using agitated baskets of ion exchange resins suspended in pregnant uranium solutions. The techniques were mechanically unsophisticated but facilitated the application of ion exchange to the treatment of unclarified leach liquors. Several so-called “resinin-pulp” plants were installed on mines in the USA, and these successfully recovered uranium a t a time of great strategic need. In the 1950s, Higgins (1957), who was then employed at the Oak Ridge National Laboratory, USA, developed and published his work on a truly countercurrent ion exchange contactor. This was a milestone in the advance of ion exchange technology since he demonstrated the practical and process advantages of continuous countercurrent ion exchange at a time of undoubted scepticism. The “Higgins”contactor was imaginative and based on the concept of a moving fixed bed of ion exchange resin in a closed loop comprising separate sorption, washing, and regeneration sections. Hydraulic transfer of a sliding fxed bed of resin could be achieved between the various sections of the loop provided adequate valves could be developed. Control valves acting on the resin bed were the major problem of this equipment, and much further development was necessary over the ensuing years to overcome resin attrition. An early Higgins column and ancillary equipment is shown in Figure 1.
r
AI
&
I
8 k;:r / A
Pregnonl feed solution
BOckwoshan
Barren solution
Rinse out Rinse out
+&
! I
I I
Figure 1. Schematic layout of the Higgins continuous countercurrent ion exchange contactor (Higgins, 1957).Valve positions during cycle: Run cycle: valves A, E, F, G , I, J, K open; valves B, C, D, H closed. Pulse cycle: valves B, C, D, H open; valves A, E, F, G , I, K closed.
Equipment of this type found commercial applications in water treatment, chemicals processing, and uranium recovery. Scale-up was limited to vessel diameters of about 2 m, and this therefore restricted the application to relatively low throughput operations; otherwise there was duplication and proliferation of contactors. Nevertheless, the Higgins ion exchange contactor was the forerunner of most continuous ion exchange equipment development in the succeeding 30 years. There was great reluctance t o abandon the concept of a packed bed for ion exchange applications, even for very favorable reactions, despite the fact that fluidization was already an established technology in other branches of chemical engineering. The idea of applying fluidization to ion exchange was contrary t o all belief and resisted by many practitioners in the field. However, the pioneering work of Don Weiss and his collaborators working in Australia in the 1950s cleared the way for subsequent developments and innovations. Swinton and Weiss (1953), working at CSIRO, were responsible for the earliest innovations in continuous countercurrent ion exchange using suspendedagitated beds of resin. They first proposed the idea of a multistage countercurrent ion exchange contactor for mineral processing, and an illustration of the technique is given in Figure 2. The idea incorporated the chemical engineering concept of sieve plates and downcomers, presumably deduced from conventional vaporAiquid columns used for distillation. Though logical in concept, the idea is impractical since hydraulic instabilities preclude the use of downcomers in liquidsolid contactors. This was evident to the workers in Australia in the 1950s, and they resorted to other fluidized bed techniques in order to recover uranium from unclarified pulps. Arden et al. (1958) developed and tested an agitated bed contactor in an attempt to provide an efficient semicontinuous countercurrent ion exchange process. Had it not been for operational difficulties, notably the blockage of screens handling suspensions containing wood pulp, the first truly commercial countercurrent ion exchange process would have occurred in the 1950s. Unfortunately, these purely practical problems were not easily overcome and the project lapsed. The development of continuous countercurrent fluidized bed ion exchange contactors was resumed in the
2844 Ind. Eng. Chem. Res., Vol. 34,No. 8, 1995
.1Fin
liquid feed
W Figure 2. Schematic diagram of the Swinton and Weiss multistage fluidized bed column.
early 1960s. Cloete and Streat (1963) collaborated at Imperial College London during the period 1960-1969 on what is today generally referred to as the “CloeteStreat” countercurrent contactor. This work started in 1961 with a benchtop concept in the laboratories of Imperial College. Cloete and Streat achieved a breakthrough in the operation of multistage liquid fluidized beds by devising a simple means of handling solid particles and liquids in countercurrent flow. The method adopted was to impose a periodicity or cyclic flow
pattern to the liquid phase in order to induce countercurrent flow of particles. Particles do not normally flow easily, except in the form of a fluidized bed, and this concept was used in conjunction with periodic flow reversal of liquid to obtain countercurrent flow in either a horizontal or a vertical arrangement of discrete stages. A typical operating cycle is depicted in Figure 3. Event 1shows the completion of resin transfer to the top stage, events 2 and 3 illustrate upward flow of feed solution fluidizing resin in each stage of the contactor, event 4 is a settling period (relatively short) to enable the resin to settle on the distributor plates, and event 5 shows reverse flow of feed solution t o facilitate downflow and net contercurrent flow of resin. A similar technique was also independently proposed a t about the same time by George et al. (1968) at the US Bureau of Mines Research Laboratory, Salt Lake City, and their work was applied to the recovery of uranium from mine waters and ore pulps. The early work of Cloete and Streat (1962) led to many publications and several patents and ultimately flourished into large scale exploitation of the technique, often referred to as “NIMCIX”columns, on the major goldluranium mines in South Africa during the late 1970s. A schematic diagram of a uranium plant is shown in Figure 4 illustrating both the sorption and elution columns. Most of these plants still exist today, but the worldwide slump in the uranium industry has caused most of them to be mothballed awaiting an upturn in the fortunes of the nuclear power industry around the world. It is probably fair to say that fluidized bed technology has revolutionized uranium production in South Africa in the period from about 1975 t o 1985. In North America, Himsley (1975) developed a slightly different multistage fluidized bed continuous countercurrent ion exchange contactor. The so-called “Himsley” contactor avoids the periodicity of the Cloete-Streat technique by the use of sequential transfer of ion exchange resin from stage t o stage in a vertical column (see Figure 5). The fluidized stages are separated by a distributor plate comprising only one inlet, and transfer of resin is facilitated by an array of ancillary valves and a pump to convey the resin in counterflow to the liquid feed
Operating cycle Completion of resin transfer
I
Forward flow period .Resin fluidized
Pregnant feed solution
Resin settling
Pregnant feed solution
Figure 3. Typical operating cycle of the Cloete-Streat fluidized bed contactor.
Resin transfer
I
Ind. Eng. Chem. Res., Vol. 34, No. 8, 1995 2846
Concentrated eluate
Pregnant feed soiuti
Figure 4. Schematic layout of a continuous countercurrent ion exchange plant for the recovery of uranium. Uranium extractian, resin loading
Eluted resin transfer to
Loop (b)
Figure 5. Schematic diagram of the Himsley column. which is maintained in continuous flow. The Himsley concept has been installed on a number of uranium sites in the USA and Canada. The largest fluidized bed ion exchange recovery plant is installed at the Rossing uranium mine in Namibia. The plant comprises an horizontal arrangement of fluidized stages and was devised by Bob Porter, and details are published by Vernon and Sylvester (1980). The “Porter” contactor uses air-lift devices to transfer resin countercurrently from stage to stage in a horizontal arrangement of rectangular rubber-lined concrete vessels. Today, there is less demand for uranium recovery, but this has been replaced by a vigorous development
program in the recovery of gold from very dilute cyanide solutions often arising in mine tailings. Adsorption using activated carbon is a major innovation in the gold industry, and this has been facilitated by advances in process technology involving the use of fluidized agitated beds of carbon particles. The technology associated with the gold industry follows the pattern set in the uranium industry, adopting large countercurrent multistage “carbon-in-pulp”plants operating as elegant, highly profitable solid-liquid recovery processes (see McDougall and Fleming (1987)). Activated carbon is highly selective for gold in cyanide solution, but the high temperature regeneration stage is rather more complex and expensive t o run. This has led to the development
2846 Ind. Eng. Chem. Res., Vol. 34,No. 8, 1995 Table 1. Approximate Industrial Water Utilization -4 L of water to brew 1pint of beer -450 L of water to produce 1ton of ready mixed concrete -4 500 L of water to produce 1 ton of steel -30 000 L of water to build an average car -230 000 L of water to produce 1ton of polypropylene (includes 225 000 L of cooling water)
600 1 crj
.-2” 500 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C ,g 400 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -.-
Table 2. Maximum Admissible Concentrations (MAC) of Toxic Substances in Drinking WateP parameter cadmium mercury lead pesticides and related productsb substances considered separately total polycyclic aromatic hydrocarbons
MAC (Ugm 5 1
50 (in running water) 0.1 0.5 0.2
Extract from EC Directive relating to the Quality of Water intended for Human Consumption (80/778/EEC) 1992. Pesticides and related products means insecticides, herbicides, fungicides, PCB’s, and PCT’s. a
of new anion exchange resins, as reported by Schwellnus and Green (19901, capable of selective recovery of gold from cyanide solution and offering a simple and straightforward chemical regeneration step.
Environmental Pollution Control Environmental pollution is the price the industrialised world must pay for its technological achievements, and this is considered unacceptable by much of modernday society. Government intervention in the UK has not always been swift, but lately there has been an avalanche of new environmental legislation and regulation which is bearing down heavily on the process industries. Adsorption and ion exchange have an important role to play in the elimination of pollution as it relates to water, wastewater, and industrial effluents. Water is a major industrial commodity, and modern industry requires very large volumes of process and cooling water. This is depicted in Table 1 by approximate figures for water utilization in some typical UK industrial sectors. There has been much discussion in the media about the quality of UK drinking water, and there are fears of contamination by toxic substances. Table 2 gives an indication of the maximum admissible concentrations of some specific toxic substances in drinking water. The supply of water for industrial and domestic use is the province of the privatized water companies. Public water supply in 199111992stood at about 20 000 Muday in the UK. Household use of water is about 140 m e a d per day, and about 3% of this is drinking water, i.e., about 10 Uday per household. It is interesting t o note the systematic rise in the consumption of bottled drinking water in the UK as indicated in Figure 6. The reason for the upsurge in consumption of bottled water is that it tastes better than tap water, which is often very heavily chlorinated in an attempt to destroy bacteria and microorganisms. Bottled water is not without trace impurities, and ’there is little quality control as compared to the tight specifications laid down in national and EC directives on the supply of tap water. Looking to the future, there is considerable scope in applying the principles of adsorption and ion exchange to water pollution control. Ion exchange is already
cl
1 ?!1983
19841985198619871988198919901991
Year.
Figure 6. UK bottled water consumption.
extensively used for the removal of ionic species from effluents, e.g., traces of chromate from electroplating wastes, zinc from acid pickle liquors, phenols and substituted phenols from water, etc. There is, however, a need to regenerate and recycle ion exchange resins, and this inevitably leads t o a secondary concentrated waste solution which must be h r t h e r treated to render it harmless for disposal. Of course, it may be possible to recover a trace metal from the regenerant solution by precipitation or electrolysis, though this is not usually economically viable. There is a need to research alternative approaches. Activated carbon is widely applied for the decontamination of water and waste streams. Recent work by Streat and Nair (1992) has confirmed that activated carbon is not only capable of adsorbing organic molecules but possesses a significant weakly acidic ion exchange character capable of removing trace metal contamination from water. Typical surface functionalities are shown in Figure 7 and comprise carboxylic, phenolic, quinone, lactone, fluorescein, and cyclic peroxide type groups. The former finding is already well-known but the latter is less well-known, and there has been little attempt t o develop this feature for the purpose of water pollution control. The prospect of developing novel activated carbons based on cheap starting materials capable of treating drinking water and wastewater is therefore likely. It is possible to improve the properties of commercially available activated carbons based on coal, coconut shell, and wood by oxidizing the surface (see Streat and Nair (1992)). Simple chemical oxidation of the surface of activated carbon enhances the weakly acidic functional groups thereby significantlyraising the ion exchange capacity for trace metals. It is possible to decontaminate water from trace metals, i.e., transition metals and heavy metals, at neutral or near-neutral pH values. Also, it is possible to adsorb trace organics, e.g., chlorinated hydrocarbons and aromatics, often found in surface water as pesticides and herbicides. New work has shown that it is possible to synthesize effective activated carbons from waste vegetable matter, and our success is demonstrated with carbonized straw. Straw is overproduced in abundance in the TJK, and with the current restrictions on burning, about 7 million metric tons per annum goes to waste. Straw has been converted into a useful adsorbent material by carbonization under controlled conditions, and Streat et al. (1995) describe how this could form the basis of a cheap disposable source of activated carbon. More work is needed to optimize the synthesis procedures and to
Ind. Eng. Chem. Res., Vol. 34, No. 8, 1995 2847 ion exchange contactor, though it presents us with many more operating problems which still require much study. However, the concept could prove to be a major breakthrough in the handling of finely divided adsorbents for the purpose of large scale decontamination processes in the field of environmental liquid pollution control.
0
Carboxyl groups
Phenolic hydroxyl groups
Quinone-type carbonyl groups
0
Conclusions Let me conclude with a quotation from the nineteenth century scholar and writer Mark Pattison, who wrote the following passage in 1875: “In research the horizon recedes as we advance and is no nearer a t sixty than it was at twenty. As the power of endurance weakens with age, the urgency of the pursuit grows more intense ... and research is always incomplete”. How right he was!
Normal lactones
Fluorescein-type lactones
0
/I
Carboxylic acid anhydrides
Cyclic peroxides
Figure 7. Surface functional groups on activated carbon.
evaluate the products further for application in water, wastewater, and effluent treatment. Work on new process technology is also in progress. At Loughborough, we have identified the need to develop a continuous technique for handling finely divided activated carbon and other adsorbents on a large scale. Powdered activated carbon is already used in some applications but is bedeviled by the difficulties of contacting fine powders in the size range of about 5-20 pm with a flowing water or effluent stream. There are many logistic problems, not least the wettability of the carbon particles and the ultimate problem of filtering the treated water. We are developing a novel concept to overcome these difficulties by supporting the finely divided adsorbent particles on the surface of an air bubble and thereby creating a so-called “stabilized bubble” reactor capable of operating as a countercurrent fluidized bed. It is possible to stabilize a submerged air bubble in the presence of finely divided particles by the use of a trace amount of spreading oil provided that the contact angle allows partial wetting of the particle by oil and water a t the oiVwater interface. The concept is not dissimilar to the stabilization of an emulsion in the presence of fine particles. Stabilizing air bubbles is not quite that easy and can only be achieved in a specially designed bubble generator producing a vigorous vortex shear field to ensure the creation of fully coated stabilized bubbles. Our work is based on the pioneering studies carried out over 40 years ago by Moir et al. (1953) in Australia. We have shown that these coated macrobubbles, in the diameter range of 0.5-2 mm, can then be transferred to a conventional fluidized bed reactor where they can be applied as if they were discrete particulates for the treatment of a contaminated liquid stream. There is close resemblance of this technique to an upturned Cloete-Streat fluidized bed
Literature Cited Adams, B. A.; Holmes, E. L. Adsorptive properties of synthetic resins. J. SOC. Chem. Znd. 1935a, 54, 1. Adams, B. A.; Holmes, E. L. Base exchange resins. Chem Age 1935b, 38, 117. Arden, T. V.; Davis, J. B.; Herwig, G. L.; Seward, R. M.; Swinton, E. A.; Weiss, D. E. Extraction of uranium from acid leach pulps by jigged bed ion exchange. Second UN International Conference on The Peaceful Uses of Atomic Energy, Geneva; September 1958. Cloete, F. L. D.; Streat, M. A new continuous solid-fluid contacting technique. Nature 1963,200, 1199-1200. Cloete, F. L. D.; Streat, M. Improvements in and relating to solid fluid contacting. UKPatent 1,070,251, Oct 19,1962;USA Patent 3,511,118, Dec 29, 1970; USA Patent 3,738,814, June 12, 1973. Gadd, G. M. Biosorption. Chem. Znd. 1990, 13, 421-426. Gadd, G. M. Interaction of fungi with toxic metals. New Phytol. 1993,124, 25-60. George, D. R.; Ross, J. R.; Prater, J. D. Byproduct uranium recovery with new ion exchange technique. Min. Eng. 1968,20, 1-5. Grubhofer, N. Cellulose ion exchangers. In Ion Exchangers; Dorfner, K., Ed.; Walter de Gruyter: Berlin, 1991; pp 443-460. Hall, G. R.; Streat, M. Radiation-induced decomposition of ion exchange resins; Part 1: Anion exchange resins. J. Chem. SOC. 1963, 5205-5211. Helfferich, F. G. Zon Exchange; McGraw Hill: New York, 1962; first published in German under the title Zonenaustauscher; Verlag Chemie GmbH: Weinheim, 1959. Higgins, I. R. US Patent, 2,815,322, 1957. Higgins, I. R.; Chopra, R. C. Chem-Seps continuous ion exchange contactor and its applications to demineralization processes. In Zon Exchange in the Process Industries; Society of Chemical Industry: London, 1969; pp 121-126. Himsley, A. Fluid solid contact process and apparatus. Canadian Patent 980,467, 1975. Kressman, T. R. E. Properties of some modified polymer networks and derived ion exchangers. Zon Exchange in the Process Industries; Society of Chemical Industry: London, 1969; pp 3-9. Kunin, R. Pore structure of macroreticular ion exchange resins. Ion Exchange in the Process Industries; Society of Chemical Industry: London, 1969; pp 10-15. Kunin, R.; Myers, R. J. Zon Exchange Resins; John Wiley and Sons: New York, 1950. Marinsky, J. A.; Glendenin, L. E.; Coryell, C. D. The chemical identification of radioisotopes of neodymium and of element 61. J . Am. Chem. SOC.1947,69, 2781-2785. McDougall, G. J.; Fleming, C. A. Extraction of precious metals on activated carbon. In Zon exchange and sorption processes in hydrometallurgy; Streat, M., Naden, D., Eds.; John Wiley and Sons: Chichester, UK, 1987; pp 56-126. Merritt, R. C. The extractive metallurgy of uranium; Colorado School of Mines Research Institute: Golden, CO, 1971;pp 167182. Moir, G.; Ross, M.; Weiss, D. E. Counter-current adsorption separation processes V. froth flotation in columns. Aust. J . Appl. Sci. 1953, 4, 543-551.
2848 Ind.Eng.Chem. Res., Vol. 34, No. 8, 1995 Schwellnus, A. H.; Green, B. R. The chemical stability, under alkaline conditions, of substituted imidazoline resins and their model compounds. React. Polym. 1990,12,167-176. Streat, M.; Naden, D. Ion exchange in uranium extraction. In Zon exchange and sorption processes in hydrometallurgy; Streat, M., Naden, D., Eds.; John Wiley and Sons: Chichester, UK, 1987; pp 1-55. Streat, M.; Nair J. K. Adsorption of trace metals on modified activated carbons. In Zon Exchange Advances; Slater, M. J., Ed.; Society of Chemical Industry: London, 1992; pp 264-271. Streat, M.; Patrick, J. W.; Camporro Perez, M. J. Sorption of phenol and p-chlorophenol from water using conventional and novel activated carbons. Water Res. 1995,29, 467-472. Swinton, E. A.; Weiss, D. E. Countercurrent adsorption separation processes 1: equipment. Aust. J. Appl. Sci. 1953,4,316-328. Thompson, H.S.On the adsorbent power of soils. J . R. Agric. SOC. Engl. 1850,11, 68.
Vernon, P. N.; Sylvester, C. W. The Rossing continuous ion exchange plant. National Institute for Metallurgy Symposium on Ion Exchange and Solvent Extraction in Mineral Processing; NIM: Randburg, 1980. Way, J. T. On the power of soils to absorb manure. J. R. Agric. SOC. Engl. 1850,11, 313; Ibid. 1852,13, 123; Ibid. 1855,15, 491. Received for review November 29, 1994 Revised manuscript received February 3, 1995 Accepted March 16, 1995@ IE940702V ~
Abstract published in Advance ACS Abstracts, J u n e 15, 1995. @
