Magnetic Microparticles for Treatment of Natural Waters and Wastewaters

179

MAGNETIC MICROPARTICLES FOR TREATMENT OF NATURAL WATERS AND WASTEWATERS D. R. DIXON and L. 0. KOLARIK

Division of Chemical and Wood Technology, CSIRO,Bayview Avenue, Clayton, Vic. 31 68, Australia

ABSTRACT This article describes a new approach to the treatment of natural waters and wastewaters. Details of the steps involved and of the progress of the method from laboratory experiments to full-scale commercial plants are combined with the results of fundamental colloid science studies. Also other possible practical applications for this novel process are suggested.

1. INSOLUBLE MICROPARTICLES IN WATER TREATMENT

Clarification of low-turbidity, coloured waters may be difficult. Due to the small number of particles, the rate of floc formation in these systems is slow and the settling characteristics are poor. To improve the process efficiency other solids, are sometimes added [ 13. The role of the added solid-phase is described as twofold: (a) the added solids may influence the rate of floc formation, which is called the "nucleation effect", and (b) they may improve the settling characteristics of the formed aggregates which is called the "weighting effect". In practice this can be achieved by recirculating the settled sludge from the previously treated water or by adding a fresh solid-phase such as insoluble microparticles. Only the latter will be discussed here. The solids frequently used include frnely divided sand, activated silica, bentonite clays and activated carbon [ l , 21. Demeter et al. [3] have proposed the use of sand particles ranging in size from 10-200 pm in conjunction with polyelectrolyte or inorganic coagulants, or both. Generally these materials alone do not promote the destabilization of colloidal dispersions to any appreciable degree. Their handling, recovery and reuse are difficult. Hydrocyclones have been used to separate sand form attached impurities prior to recycling and reuse.

180 2. MAGNETIC MICROPARTICLES

With added magnetic particles it is possible to exploit the rapid kinetics and the weighting action, as well as the magnetic properties of the microparticles themselves, by applying a magnetic field to effect separation of the particles from the liquid phase. A search of the available patent literature over the past forty years shows that in 1941 Urbain and Stemen [4] proposed the use of magnetite in a water clarification process, to aid sedimentation. Magnets placed at the base of a settler were intended to increase the settling rate of the agglomerates. Since then a number of patents have been granted covering the use of magnetite and other magnetic particles for the removal of various waterborne impurities [5-111. The magnetic particles were generally used in conjunction with inorganic coagulants or organic flocculants. A range of magnetic devices was used for the separation of the particles with attached impurities from the water. No activation of the magnetic material was suggested, illustrating that the earlier use of magnetite involved exploitation of its magnetic properties only. Magnetic particles have also found use a few special applications. Magnetic ionexchange resins have been successfully employed in desalination [ 121 and dealkalization [ 131. Magnetic carbon was prepared and used for the removal of unwanted organic substances from a food processing stream [14-161. Processes for controlling surface pollutants, e.g., oil in water, were proposed by Weiss and Battaerd [7]; MitcheU and Chet [18] developed a magnetic separation method for recovering protein from single cell organisms. The latter suggested the use of alkali (2M NaOH) for the recovery of protein, but apparently not for the activation of magnetite. Magnetic particles have also been used as substrates for the immobilization of various inorganic and organic compounds. Nonporous magnetic materials were used as enzyme supports [ 191, e.g., chymotrypsin was successfully attached to precipitated magnetic particles including magnetite. At present there are two promising water treatment processes utilizing magnetic microparticles: high gradient magnetic separation (HGMS) and the technique featured in this paper. Oneexample of a HGMS process is high gradient magnetic filtration (HGMF) which is based on the Kolm type separator [20]. The principles and applications of magnetic fitration are further discussed by Oberteuffer [21]. In HGMF, magnetically susceptible particles are collected on steel wool or on an expanded metal matrix. The background magnetic field magnetises the matrix and produces strong magnetic gradients that converge on the matrix fibrous strands. The method has been used in the mineral benefication of semitaconite [22] and for the removal of sulphur from coal [23]. Bitton et al. [24] showed that phosphate concentration in water could be significantly reduced, using magnetite in conjunction with aluminium sulphate and montmorillonite clay. In 1976, de Latour [25] described the use of the HGMS technique in the water and wastewater field. Magnetite particles were used in conjunction with alum to remove a number of impurities including colour, turbidity, bacteria and phosphate. The magnetite acted as a ”seeding material” in such interactions. But the particles were not reused. The advantages of the HGMS method are its simplicity and very high-fdtration rates.

181 2.1. Reusable Magnetic Particles

The concept of reusable magnetic particles was first applied in ionexchange to overcome the difficulties associated with the handling of very fine particles [ 121. This approach also led to the development of a dealkalisation process for non-clarified wastewater [13] and to the use of non-functional magnetic polymers as filter aids [26]. In 1975-1976 Bolto et al. summarized the development and use of magnetic polymer-coated particles in various water treatment processes [27, 281. The potential of magnetic ionexchange particles for the removal of colour and turbidity from water was first studied using a magnetic cation exchange resin [29]. A further development was the use of positively charged magnetic ”whisker resins” [30]. Deposition of an amphoteric, amorphous Fe(OH)3 gel coating onto magnetite particles was also investigated [3 1]. Although such coated magnetite particles showed excellent coagulationadsorption properties, economic or technical difficulties (i.e., mechanical stability, and fouling) have prevented their commercial applications. 3. THE”SIROFL0C” PROCESS*

The use of alkali-treated magnetite in water purification processes was described for the first time in 1977 [32]. It was shown that treatment of fine particles (1-10 pm) of magnetite with a dilute (0.1 M) sodium hydroxide solution imparted excellent coagulation-adsorption characteristics to the particles which can be described as a solid reusable coagulant - adsorbent. The properties of these materials derive from the amphoteric behaviour of the hydroxyl groups present at the oxide surface [33,34]. This process has been patented [35,36]. Alkali-treated magnetite interacts strongly with turbidity particles and the colour substances present in natural waters. Depending on the initial colour and turbidity values, such particles can be used alone as a solid coagulant-adsorbent or in conjunction with another primary coagulant. In the latter case significant savings of the coagulant can be achieved. Both inorganic coagulants (alum or ferric chloride) and organic synthetic polyelectrolytes may be used successfully. Water is treated in a fraction of the time normally needed. The magnetite particles used in the coagulation-adsorption steps are demagnetised. To separate magnetite from the water the particles are magnetized. The material is reused after reactivation with alkali which releases the attached impurities. A relatively small volume of a fmal alkaline effluent is produced. The block diagram of the process is shown in Fig. 1. There are two main stages in the process; (i) removal of impurities from the feedwater, which consists of three steps: PREMIX, COAGULANT/FLOCCULANT ADDITION AND AFTERMIX and SEPARATION, and (ii) regeneration prior to reuse of the particles, which includes ALKALI REACTIVATION and WASHING.

* SIROFLOC is an AUSTEP Pty Ltd registered trademark for the clarification and decolourization of water with regenerable magnetic particles.

182

Fig. 1. Steps in the Sirofloc process.

3.1. Premix

In the first step of the process, activated magnetite and feedwater are contacted at pH values 4-6 for ten minutes. This time is sufficient for a significant reduction in colour and turbidity, and for waters containing low colour and turbidity this step is often effective enough to produce water of acceptable quality. It was also shown that colour and turbidity removal is strongly pH dependent and usually increases with decreasing pH. 3.2. Coagulant/Flocculant Addition -Aftermix

If the amount of impurities exceeds certain levels, more magnetite may be added to complete the treatment. However, the addition of a coagulant or flocculant is a preferable alternative. After the addition of the coagulant, further mixing (4 to 5 minutes) completes the effective attachment of the remaining colloidal particles to the magnetite. 3.3. Separation

The magnetite particles with attached impurities and coaqulant are magnetiz'ed. The magnetically-flocculated agglomerates then settle out rapidly, leaving clarified water. 3.4. Alkali Regeneration

The magnetite slurry is then treated with a dilute solution of sodium hydroxide.

183

Fig. 2. Full size plant a t Mirrabooka, Western Australia.

Under laboratory conditions, contacting the magnetite slurry with 0.1 M NaOH solution for 10 minutes results in separation of attached impurities. 3.5. Washing

The particles must be washed prior to their reuse to remove excess alkali. Vigorous mixing and magnetization - demagnetization during regeneration and washing, exhances the separation of the impurities from magnetite. 4. DEVELOPMENT OF THE PROCESS

Since 1977, the Sirofloc process has undergone considerable engineering development. The advances in design and operation of a pilot plant have been discussed [38, 391, as well as important commercial considerations [40]. Further engineering developments of the process and commissioning of the first demonstration full-size plant have recently been reported [41]. The demonstration plant at Mirrabooka in Western Australia (as shown in Fig. 2 ) is designed to treat 35 ML/day of underground water. This plant was officially operated in July 1981. More recently (February 1983) another large plant has been

184 Tab. 1. Species commonly found in or added t o natural waters

Component

1. Soluble Species - hardness cation Caz+,Mg2+

Mechanism of Interaction with magnetite

Application

Treatment of hard feedwaters Metal ion recovery from effluents - anions PO:-, SO:Adsorption Removal of PO:- from agricutural effluents - organic acids, humic, fulvic acids Adsorption Removal of colour from feedwaters and on occasions pesticides, surfactants and other man-made pollutants. 2. Suspended Solids - clays, silica Heterocoagulation Removal of turbidity - biocolloids, algae, viruses, bacteria Heterocoagulation Algal harvesting, removal of pathogens 3. Additives - AP+, Fe3+ salts Adsorption As secondary coagulants or surface coatings - polyelectrolytes Adsorption Improved clarification and regeneration - oxidants Cl, , H,O,, 0, Redox reactions Treatment of anaerobic waters removal of trihalomethanes -

heavy metal ions PbZ+,Co2+,Zn2+,Mn2+

Adsorption Adsorption

established at Bell Bay in Tasmania which treats 20 ML/day of highly coloured, lowturbidity surface water. 5. COLLOID AND SURFACE CHEMISTRY

While the practical aspects of the process have been well established, the study of the fundamental aspects is in its early stages. It has long been recognized that the phenomena of coagulation and flocculation occur at solid-liquid interfaces [42]. They involve particles of colloidal dimensions whose existence in natural systems depend on the presence of an electrical charge at the surface. The magnitude of this surface charge depends not only on the nature of the colloid but also on the composition of the aqueous phase. Adsorption or binding of solutes to the colloid surface may increase, decrease or even reverse the effective charge on the solid [43]. Both coagulation and flocculation rely upon neutralization of the surface charge to achieve destabilization. In natural waters the suspended solids are usually negatively charged and thus alum and/or cationic polyelectrolytes are added to bring about coagulation and/or flocculation. To obtain a detailed understanding of these phenomena one must examine the colloid and surface chemistry of the system. This is no less true for the Sirofloc process of water treatment in which magnetite is used to replace some or all of the alum or polyelectrolytes commonly used. Much of the research carried out in t h i s study has been directed towards an understanding of the process occurring on the magnetite surface in an attempt to improve the efficiency of the process in various plant situations.

185

I0

W

N

-20-

-30-

-GO-507

4

5

6

?,,

8

b

(0

11

Fig. 3. Surface charge development and isoelectric point of magnetite.

At this point it may be instructive to list the compounds and species likely to be found in a natural water. These are included in Tab. 1. 6. MAGNETITE

The starting point for any study of the Sirofloc process is magnetite and attention has been focussed on the magnetite-water interface [44,45]. The surface characteristics of magnerite in contact with water are determined by the dissociation of the surface hydroxyl groups as shown in the following equations. Development of a positive charge is by reaction with protons Fe-OH+H**Fe-OHl and of a negative charge by reaction with hydroxyl ions Fe - OH +OH-+ Fe - 0 - +H,O

(2)

The surface charge of magnetite is therefore pH dependent. Experimentally the surface charge of a mineral can be measured directly by potentiometric titration methods. More frequently microelectrophoresis is used to determine the zeta potential of the particles which can be related to surface charge. At a certain pH, the

186 zeta potential is zero and this point is called the isoelectric point - i.e.p. as shown in Fig. 3. Standard colloid chemistry techniques such as microelectrophoresis and streaming potential measurement have been used to determine the characteristics of the magnetite surface [45]. These studies have shown that the raw mineral obtained from Savage River, Tasmania, contains anionic impurities which can be removed by treatment with acid, alkali ot both. There is an accompanying increase in the i.e.p. of the oxide, which is reflected in clarification perfomance (i.e., jar tests). The correlation between the degree of pretreatment, the i.e.p. and the efficiency in jar tests is now well established. It has also been demonstrated that inefficient regeneration, i.e., the failure to release adsorbed colloids, lowers the i.e.p. and eventually leads to poor clarification. The importance of surface chemistry to the process is illustrated by the routine use of electrophoretic data to compare magnetite samples from different sources, from different stages of the pilot plant or after different procedures have been used in the pilot plant. In keeping with the declared objective of bringing the laboratory system as near as possible to the real situation, the effect of introducing other soluble components on the surface properties of magnetite has been examined in a number of projects. The results from these studies will be briefly reviewed. 6.1. Cation Adsorption

The ability of oxides to adsorb heavy metal ions is well known [46]. Magnetite is also capable of removing ions such as Cu2+,Pb2+, Zn2', Mn" from solution. The rate and extent of the adsorption and the corresponding effects on the magnetite depend upon pH, metal ion concentration and the solid-liquid ratio [47]. Desorption of these ions is best achieved at low pH and thus in a practical situation, a two-stage regeneration scheme would be needed - acid treatment to recover the metal ions and alkali treatment to reactivate the magnetite surface. Likely applications for the recovery of metal ions include treatment of metal refinery effluents, electroplating effluents, sewage effluents and sludges. A preliminary study has shown that addition of Alp and Fe3+ions has a similar effect on the surface properties of magnetite, i.e., increasing the positive charge to an extent dependent on parameters such as pH and the cation concentration. Thus these metal ions may be used as secondary coagulants to increase the capacity of magnetite to treat any turbid or highly coloured feedwaters. Alternatively by incorporation of these ions into the magnetite surface it may be possible to modify the surface properties and increase clarification efficiency. Such surface modifications are only possible when the product is of a higher market value than that currently given to drinking water, More recently the effect of Caz+and Mg2+ions (hardness), which are common to many feedwaters, on the magnetite surface properties has been studied [48]. It was found that these ions also adsorb strongly, markedly affecting the surface properties of magnetite. In many waters, there is sufficient hardness present to reverse the surface charge of magnetite, thereby aiding clarification but hindering regeneration. This has now been verified both in laboratory experiments (e.g. microelectrophoresis) and in jar tests on

187

a number of feedwaters. For a pilot plant treating such hard feedwaters, this effect has necessitated changes in regeneration and in some cases, the introduction of an acid desorption stage prior to regeneration. 6.2. Anion Adsorption

The interactions between iron oxides such as goethite and hematite and anions such as phosphate, sulphate and chloride have been examined and are reported in the literature [49]. However, it has been assumed that for the Sirofloc process, the adsorption of these simple ions would be secondary; the larger organic anions which exist in higher concentrations were thought likely to be adsorbed preferentially. While this assumption remains untested, recent results [47] have indicated that the presence of phosphate ions lower the i.e.p. of magnetite due to strong adsorption. This occurs even in the presence of competing solutes such as humic acid. Thus magnetite could be used to remove unwanted phosphate ions that are present in waters or wastewaters. Another possible application is the use of magnetite for removal of chromate ions from electroplating effluents [50]. 6.3. Organics

The colour bodies present in most waters are generally defined as organic acids, humic and fulvic acids. The higher molecular weight, humic compounds which are of colloidal dimensions may be regarded as organic colloids and treated in a way similar to that for the inorganic clays and silicates present. The smaller fulvic acids can be viewed as complex anions and their interaction with magnetite treated not as heterocoagulation, but rather as anion adsorption, similar to phosphate adsorption. The qualitative evidence that exists at present favours the latter mechanism, emphasizing the specific nature of the interaction between Fe ions on the magnetite surface and these complex anions which suggests the forniation of a metal complex. The removal of these colour bodies has assumed additional importance since the discovery that some are precursors of the trihalomethanes produced upon disinfection of the product water with chlorine. 6.4. Polymer Adsorption

The adsorption of polyelectrolytes by inorganic substrates has been neglected as an area of research and it is only in the last decade that efforts have been made to unravel the mechanism by which polymers affect the stability of colloidal materials. The concept of polymer bridging between particles and the more recent idea of charge neutralization as the dominant factor are the two main theories to have evolved from thistesearch [51, 521. One of the main difficulties encountered with research in this area is the lack of suitable experimental techniques to provide information about polymer configurations and the particle size of the floc during adsorption. There is hope that modern instruments may overcome these problems.

188 Preliminary data indicate that cationic polyelectrolytes behave similarly to inorganic cations, raising the positive zeta potential of magnetite and increasing its i.e.p. Part of the Division’s work has been to correlate such data with polyelectrolyte structure and ultimately with jar-test and pilot-plant results. A number of commercial polyelectrolytes were used. It must be emphasized that in the plant situation, the polymers perform more than one task; not only do they assist in turbidity removal during clarification and influence turbidity release during regeneration, but they also affect turbidity shearing and magnetite carryover during separation of the loaded magnetite from the product water. 6.5. Two-component Systems -Heterocoagulation

The object of some of the Division’s work has been to investigate the interaction of magnetic particles with other colloids likely to be present in both natural waters and domestic and industrial effluents. Examples of such colloids include silica, clay, organic colloids (e.g. humic acids), bacteria, virus and algae. Some of these systems have been investigated and a brief summary of the results will be presented. 6.5.1. Inorganic colloids

The clay used in this study was bentonite, which was found to be negatively charged over the pH range 2-10. Experimentally, the phenomenon ofheterocoagulation was examined by the use of a light scattering technique to d e r e d n e the extent of coagulation of the non-magnetic colloid, immediately following the removal of magnetite by the application of a magnetic field. Preliminary results indicate that heterocoagulation does occur when the two colloids are oppositely charged, and is dependent upon pH, particle size and sofids ratio. It remains to be seen what effect the addition of other components has in this system, and how the results correlate with data on the removal of turbidity obtained from jar test experiments. 6.5.2. Biocolloids

Although the Sirofloc process employing treated magnetite, was designed for the removal of undesirable colour and turbidity from water, the conditions of this process are such that a satisfactory proportion of some viruses likely to be present b contaminated water. will also be removed [53]. Furthermore there is evidence that some viruses are disrupted at the pH values employed in magnetite regeneration, so that infectious particles are not released from magnetite during regeneration. The efficiency of virus adsorption by magnetite is affected by ions, suspended matter such as clay and the components of sewage effluent. These variables represent those likely to be encountered. in the purification treatment of some natural waters or in sewage effluent ”renewal”. It has been shown that the addition of a low concentration of polyelectrolyte will largely counter the interfering effect of the water components listed.

189

Hence, it appears that the Sirofloc process can be used to treat otherwise unusable water containing a small number of viruses. Further, "renewal" of even heavily COIItaminated water may be achieved in a multistage process involving the use of polyelectrolyte. In the latter case it is likely that process conditions will need to be achieved to suit the particular water under treatment. In order to provide a further safety margin, terminal disinfection by some conventional means, such as chlorination, would be desirable when Sirofloc is used to adsorb viruses from clarified sewage effluents. It has also been established that magnetite is efficient in adsorbing bacteria from aqueous suspensions [54]. The process of adsorption is chiefly electrostatic. The bacteria and algae tested have a negatively charged surface above their i.e.p. values (approximately pH 3.0) and adsorb readily to the positively charged magnetite at neutral or mildly acidic pH values. The capacity of the magnetite to adsorb bacterial cells is large. Suspended bacterial cells of concentrations less than 200 pg/ml dry weight of cell material can be removed in one step with 10 g/l magnetite. Bacterial suspensions of higher concentration must be subjected to several adsorption steps to produce a clear supernatant. Certain green algae can also be removed with magnetite. This may enable algal harvesting to be carried out more efficiently. The capacity of magnetite to remove suspended microbes may have applications other than for the treatment of effluent waters. It may be of value in microbial processes where the separation of the cells from the liquid phase by centrifugation or filtration is either difficult or expensive. Recent results have indicated that microbes adsorbed by magnetite can be gainfully employed in processes such as denitrification and dehalogenation. It has been demonstrated that numerous species of microbes adsorbed to magnetitecan accumulate the insecticides lindane and DDT from an aqueous solution. The same occurs to a lesser extent with the herbicide 2,4D. The nature of the process is not known but it appears to a partitioning of the halo-organic compounds into lipid portions of the cells. It may be practical to remove the lindanecontaining cells by desorption at pH 10. Evidence obtained suggests that adsorbed bacterial cells could be used several times before desorption.

7. CONCLUSION

The potential of the Sirofloc processes for the treatment of natural waters has been demonstrated for a range of feedwaters on laboratory and pilot plant scales. The commercial viabihty of the process has now been proven by two large-scale plants operating on vastly different raw waters. In this article we have attempted to extrapolate from fundamential colloid studies of the mechanism by which the process Sperates, to suggestions for other practical applications. Some of the more promising of these include recovery of metal ions from effluents and wastes, removal of phosphate from domestic and agricultural effluents, algal harvesting, removal of pathogens from drinking water supplies and the use of microbes attached to magnetite. It will be of interest to follow the progress of these applications in future years.

190 ACKNOWLEDGEMENTS

The authors readily acknowledge the support and assistance of all members of the water group within the CSIRO Division of Chemical and Wood Technology, Clayton, Victoria. In particular the technical expertise of Mr T. C. Ha and Ms P. A. Freeman are gratefully recorded.

REFERENCES

1 American Waters Works Association, Inc. Water Quality and Treatment, 3rd Ed., 1971,p. 94. 2 K. J. Ives, Effluent and Water Treatment Jour., November 1974,636. 3 L. Demeter, and D. Galgaczi, U.S. Patent 3350302, 1967, Hungarian Patent, Ser. No. 396.895, 1964. 4 0.M. Urbain, and W. R. Stemen, U.S.Patent No. 2,232,294,Feb. 18, 1941. 5 M. Mamula, et aL, Czechoslovakian Patent No. 132624,June 15,1969. 6 J. A. Bartnik, and A. F. San Miguel, U.S. Patent No. 3,536,198,Oct. 27, 1970. 7 D. F. Peck, et al., U.S. Patent No. 3,549,527,Dec. 22, 1970. 8 D. S. Blaisdell, and R. E. B. Klaas, U.S. Patent No. 3,697,420,Oct. 10, 1972. 9 Ch. de Latour, U.S. Patent No. 3,983,033,Sept. 28, 1976. 10 Mitugi Miura et al., U.S. Patent No. 4,039,447,Aug. 2, 1977. 11 A. B. Boliden, Belgium Patent No. 871,458,Apr. 23, 1979. 12 N. V. Blesing, B. A. Bolto, D. L. Ford, R. McNeill, A. S. Macpherson, J. D. Melbourne, F. Mort, R. Siudak, E. A. Swinton, D. E. Weiss and D. Willis, Ion Exchange in the Process Industries, SOC. Chem Ind., London 371,1970. 13 B. A. Bolto, D. R. Dixon, A. J..Priestley, and E. A. Swlnton, Prog. Water Techn., 9,833,1977, Pergamon Press, London.

14 G. Sutherland, U.S. Patent No. 3,803,033,Apr. 9, 1974. 15 D. E. Weiss, S. M. West, and D. R. Dixon, Australian Patent Appl. 20760/76. 16 D. R. Dixon, J. Lydiate, and J. Lubbock, Australian Pat. AppL 39951/78. 17 D. E. Weiss, and H. A. J. Battaerd,U.S. Patent No. 3,890,224,June 17, 1975. 18 R. Mitchell, and I. Chet, U.S. Patent No. 4,001,197,Jan. 4, 1977. 19 P. A. Munro, P. Dunnill, and M. D. Lilly, Biotech. Bioengineering XIX, 101,1977. 20 H.H. Kolm, Magnetic Monopoles, Science Jour. 4,9,60,1968,and H. H.Kolm, E. Maxwell, J. A.

Oberteuffer, D. R. Kelland, C. de Latour, and E. P. Marston, 17th Ann. Conf. on Magnetism and Magnetic Materials, API, IEEE, AIME, ONR and ASTM, Chicago, Nov. 1971. 21 J. A. Oberteuffer, IEEE Transactions on Magnetics, Vol. Mag. 9,No. 3, 303, 1973. 22 D. R. Kelland, IEEE Transactions on Magnetics, Vol. Mag. 9,No. 3, 307, 1973. 23 S. C. Trindade, and H. H. Kolm, IEEE Transactions on Magnetics, Vol. Mag. 9,No. 3, 310, 1073. 24 G.Bitton, R. Mitchell, C. de Latour, and E. Maxwell, Water Research, 8, 107, 1974. 25 C. de Latour, J.A.W.W.A., 68,325,443,498,1976. 26 B. A. Bolto, K. W. V. Cross, R. J. Eldridge, E. A. Swinton, and D. E. Weiss, Chem. Eng. Prog., 71,

47,1975. 27 B. A. Bolto, D. R. Dixon, R. J. Eldridge, E. A. Swinton, D. E. Weiss, D. Willis, H. A. J. Battaerd, and P. H. Young, J. Polymer Sci., Symp. No. 49, 211, 1975. 28 B. A. Bolto, D. R. Dixon, R. J. Eldridge, L. 0. Kolarik, A. J. Priestley, W. G. C. Raper, J. E.

Rowney, E. A. Swinton, and D. E. Weiss, The Theory and Practice of Ion Exchange, SOC.Chem. Ind., London, 271, 1976. 29 N. J. Anderson, R. J. Eldridge, L. 0. Kolarik, E. A. Swinton, and D. E. Weiss, Water Research 14,

959,1980. 30 N. J. Anderson, B. A. Bolto, R. J. Eldridge, L. 0. Kolarik, and E. A. Swinton, Water Research 14, 967,1980. 31 N. J. Anderson, L. 0. Kolarik, E. A. Swinton, and D. E. Weiss, Water Research 16, 1327, 1982.

191 32 L. 0. Kolarik, A. J. Priestley, and D. E. Weiss, Proc. 7th Federal Convention of Australian Water and Wastewater Association, Canberra 21-24 Sept., 1977. 33 G. A. Parks, Equilibrium Concepts in Natural Water Systems, Ch. 6, Advances in Chem. Series, No. 67, 1967. 34 G. A. Parks, Chem Rev., 65,177,1965. 35 D. E. Weiss, L. 0. Kolarik, and A. J. Priestley, Aust. Patent No. 512,553, U.K. Patent No. 1,583,881. 36 L. 0. Kolarik, N. J. Anderson, D. E. Weiss, and A. J. Priestley, Aust. Patent No. 518,159, U.S. Patent No. 4,279,756 and 4,363,749. 37 L. 0. Kolarik, Water Research, 17,141,1983. 38 N. J. Anderson, N. V. Blesing, B. A. Bolto, L. 0. Kolarik, Proc. of 8th Federal Conv. of Australian Water and Wastewater Association 12-16 Nov. 1979. 39 A. J. Priestley, and P. R. Nadebaum, Water, Jour. of Australian Water and Wastewater Association, 7, No. 3, 19, 1980. 40 P. R. Nadebaum, and H. V. Nguyen, Proc. of 8th Federal Conv. of Australian Water and Waste water Association, Gold Coast, 12-16 Nov. 1979. 4 1 P. R. Nadebaum, N. V. Blesing, A. Easton, and R. Vaughan, Proc. of 9th Federal Conv. of Australian Water and Wastewater Association, Perth, April 1981. 42 J. Th. Overbeek, Colloid Science, ed H. R. Kruyt, Elsevier, Amsterdam, Vol. 1, 1952. 43 D.C.Grahame, Chem Rev. 41,441,1947. 44 L. 0. Kolarik, D. R. Dixon, P. A. Freeman, D. N. Furlong and T. W. Healy, Presented at the AIME Conference on Fine Particles Processing, Las Vegas, Febr. 1980, P; Somasundaran, ed. Ch. 34, 652. 45 L. 0. Kolarik, Ph. D. Thesis, University of Melbourne, 1983. 46 E. Matijevii, Principles and Applications of Water Chemistry, ed Faust, S. D. and Hunter J. V., John Wiley, New York 1962. 47 D. R. Dixon, Water Research, accepted for publication 1983. 48 D. R. Dixon, Colloids and surfaces, submitted for publication 1982. 49 H.P. Boehm, Disc. Faraday SOC.,82,264,1971. 50 N. J. Anderson, L. Pawlowski, and B. A. Bolto, Nuclear and Chemical Waste Management, submitted for publication 1983. 5 1 V. K. La Mer, and T.W. Healy, J. Phys. Chem, 67,2417,1963. 52 D. R. Kasper, Ph. D. Thesis, Californian Inst. of Techn. 1971. 53 J. G. Atherton, and S. S. Bell, Water Research, 17, 943, 949, 1983. 54 I. C. MacRae, and S. Evans, Water Research, 17, 271, 1983.