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Cellular and foamed plastics as separation media
Talanta, Vol. 22, pp. 699-705.PergamonPress, 1975.Printedin Great Britain
TALANTA MINI-REVIEW* CELLULAR AND FOAMED PLASTICS AS SEPARATION MEDIA A NEW GEOMETRICAL FORM OF THE SOLID PHASE IN ANALYTICAL L I Q U I D - S O L I D CONTACT T. BRAUNand A. B. FARAGI" Institute of Inorganic and Analytical Chemistry, L. E/Stv6s University, P.O. Box 123, 1443 Budapest,
Hungary (Received 7 November 1974. Accepted 27 January 1975) Sunmmry--There has been considerable interest during the last few years in using cellular (foamed) plastics (mainly polyurethanes) either unloaded or as a means of immobilizing hydrophobic organic reagents, powdered ion-exchangers or finely divided precipitates, for the collection and separation of inorganic or organic species from aqueous solution. Foamed plastics with anchored (bonded) functional groups have also been used for the same purpose. It has been realized that the application of cellular plastics is often advantageous not only for quantitative work but also for qualitative and semiquantitative analysis. Methods available on the application of cellular and foamed plastics for the collection, separation and recovery of various inorganic and organic components from aqueous solution are reviewed.
Probably the oldest application of a material of using polyurethane foam coated with chromatocellular geometry (in the form of sponge) for chemical graphic-grade greases. Recently, foamed plastics (mainly polyurethanes) purposes was the purification of ethyl alcohol by distillation through a sponge impregnated with olive oil. immobilizing hydrophobic reagents, powdered ionThis method, used by Brunschwig 1 more than four exchangers and finely divided precipitates, or bonding centuries ago, can be considered as partition chroma- certain functional groups, have been recommended tography, the sponge material being the support, olive for several analytical applications. 15-1s This review collects the papers published and gives oil the stationary phase and ethyl alcohol vapour the mobile phase. In 1962 Bayer2 checked this ancient an up-to-date picture of the use of cellular (foamed) plastics as a new geometrical form of the solid phase method and found that it operates well. Lal et al. a have described a method for the extrac- in analytical liquid-solid extraction processes. tion of trace elements from sea-water, by use of ferric hydroxide supported on natural sponges. Several eleDEFINITION, GEOMETRY AND CELL STRUCTURE OF CELLULAR (FOAMED) PLASTICS ments (e.g. Si, Be, A1 and Ti) were extracted by towing ferric hydroxide impregnated sponge through coastal Cellular (foamed) plastics can be defined i9 as plassea-water. tic materials in which a proportion of solid is replaced Recently, however, papers 4-6 have been published by gas in the form of numerous small cells. The gas describing the possibility of using cellular plastics for may be a continuous phase, giving an open cell gas-solid and gas-liquid chromatographic separations. material, or it may be a discontinuous phase, i.e., in Bowen 7 was the first to discover the absorption the form of discrete, non-communicating ceUs. properties of polyurethane foams toward a number From the geometrical point of view, if the gas bubof inorganic and organic species in aqueous solution. bles occupy a volume fraction smaller than 76~ of Since the appearance of this work several investigathe whole, they may be spherical. If they occupy a tors a-ll have used polyurethane foams for the volume fraction larger than 76~, they will be disabsorption and recovery of inorganic and organic torted into polyhedra 2° (mainly pentagonal dodecacompounds from aqueous solution. hedra). In the latter case the polymer is distributed Braun and Farag ~2'~3 suggested in 1972 the anabetween the walls of the bubbles and the lines where lytical application of polyurethane foams loaded with bubbles intersect. The bubbles are called cells, the hydrophobic extractant. Gesser et al} 4 described a lines of intersection strands, and the walls windows method for the collection of pesticides from water by (or membranes). In an open-cell flexible foam, at least two windows (from the total) in each cell must be * For reprints of this Review, see Publisher's announceruptured for fluids to pass freely through the foam. ment near end of this issue. Cell. struotUre (i.e., the presence or absence of wint Present address: National Research Centre, Dokki, Cairo. Egypt. dows in the cells, or the number of windows per cell) ]AL 22/9 A
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is a tunction of the process by which the cellular material is made. It w a s n o t e d 19 that both rigid and flexible foams may be obtained, with open or closed cells. The structure made up of windowless cells (conraining only strands) is called a reticulated foam. 2° It was reported 19 that flexible and rigid materials generally tend to have open and closed cell structures respectively. However, there are many exceptions and, as the type of cell structure is mainly determined by the method of expansion, some materials which can be made by more than one method can exist in both open and closed cell forms. Furthermore, methods are available by which closed cell structures can be converted into the open cell form by rupture of the windows. Rupturing may be caused mechanically by applying pressure, or chemically by hydrolysis or oxidation.
GENERAL M E T H O D S O F P R E P A R I N G CELLULAR PLASTICS
With the highly developed technology for making cellular plastics today, 21 methods exist by which practically every plastic material may be made in cellular form. The general principle for preparing cellular materials is the dispersion of a gas within a liquid to obtain a liquid foam which will then be solidified to a solid cellular plastic. The main methods t9 for uniform dispersion of the gas bubbles are chemical (e.g., thermal decomposition of a chemical blowing agent or blowing by in situ chemical reaction); physical (e.g., low-pressure release of dissolved gas, blowing by vapour from a volatile liquid or temporary filler); and mechanical (e.g., mechanical entrainment of gas or the use of microspheres). Several plastic materials are now commercially available in cellular forms, e.g., poly(vinyl chloride), polyurethane, polymethylmethacrylate, phenolformaldehyde, urea-formaldehyde, polyethylene, polytetrafluoroethylene. In the major part of the published work on the application of cellular plastics in analytical chemistry, polyurethane foams have been employed. Consequently, some information about the preparation of polyurethane foam and its physical and chemical properties is briefly given.
Polyurethane foam preparation Polyurethane foams can be prepared in soft, flexible or rigid forms. 22 They are formed by the reaction of the terminal hydroxyl groups of a polyester or polyether resin and a polyfunctional isocyanate. 23' 24 Foams prepared from a wide variety of hydroxyl compounds (polyethers, polyesters or polyols) and isocyanates are now commercially available. In general, polyols in the molecular weight range 4006000 are employed. 25"26 The most widely used isocyanate compound is toluene di-isocyanate (usually 80/20 and 65/35 mixtures of the 2,4- and 2,6-isomers are used).
Physical and chemical properties of polyurethane foams Generally, the physical properties of polyurethane foams depend on the method by which they are prepared. For example, the windows may or may not be ruptured in the final stage of expansion, depending on the relative rate of molecular growth (gelation) and gas reaction, giving rise to flexible or rigid foam. The. choice of the polyol has a major effect on the foam properties determining its rigidity or flexibility. 22 The cross-link density of the urethane polymer also determines whether the foam will be flexible (low cross-link density) or rigid (high cross-link density). Flexible foams are prepared from polyols of moderately high molecular weight and low degree of branching, while rigid foams are prepared from low molecular weight, highly branched resins. Also, the chemical properties of polyurethane foams are a function of the preparation process. For example, solvent-resistance of the foam material is increased at higher cross-link density, seems to be unaffected by the type of aromatic di-isoeyanate used, and isreduced by the use of a large excess of isocyanate. 2~ Bowen 7 examined the chemical resistance of some batches of flexible polyurethane foam and claimed that they were rather stable and inert. He reported that the foam batches tested were degraded when heated between 180° and 220°, and slowly turned brown in ultraviolet light. They were dissolved by concentrated nitric acid, and reduced alkaline potassium permanganate. They were mostly unaltered, apart from reversible swelling, by water, 6M hydrochloric acid, 4M sulphuric acid, 2M nitric acid, glacial acetic acid, 2M ammonia, 2M sodium hydroxide and the following solvents: light petroleum, benzene, carbon tetrachloride, chloroform, diethyl ether, di-isopropyl ether, acetone, isobutyl methyl ketone, ethyl acetate, isopentyl acetate, and alcohols. Also it was noted that polyurethane foams could be dissolved in hot arsenic(III) chloride solution.
UNLOADED POLYURETHANE FOAMS B o w e n 7 initiated in 1970 the application of foamed polyurethane for the absorption and recovery of a number of inorganic and organic compounds from aqueous solutions in static (batch) experiments. He measured the surface area of various polyurethane foam samPles (of polyether type) and demonstrated that the uptake of different components from aqueous solutions by the foam materials is due to absorption rather than adsorption phenomena. 7'z2 The absorption isotherms of some elements have been measured and the distribution ratios and absorption capacities of the foam materials for these elements have been determined. ~ In some cases [e.g., iodine and gold(III)] the absorption was found to be greater at low temperatures than at higher ones, while in others [e.g., Fe(III)] the reverse was observed. In a subsequent communication Bowen 28 recommended the application of polyurethane foam for the
Cellular and foamed plastics as separation media recovery of gold(III) chloride from mineral wastewaters by the batch technique. The possibility of separating gold(III) chloride from natural waters by polyurethane foams has also been examined by Schiller and Cook. s It was claimed s that gold at ppM (parts per milliard) level can be quantitatively collected from aqueous solutions by shaking the mixture for 90 min. Recently, Braun and Farag 1° investigated the recovery of gold-thiourea complex from aqueous solutions containing perchlorate ion, using polyurethane foams in batch operations. Open-cell polyether and polyester type foams were examined. The uptake of the gold-thiourea complex by different samples of the polyether type foam depended to some extent on the cell dimensions and decreased as the cell size decreased. Further, the absorption capacities of the polyether type foams for the gold complex are generally greater than those of the polyester type. ~° Sukiman 11 described the application of polyurethane foam for the extraction of gold(III) chloride from acidic aqueous solutions and natural waters by the dynamic technique. Gold(III) at trace concentrations (0.02-25 p p M ) c a n be quantitatively collected from aqueous solution by percolating the solution through a short foam column at relatively high flow-rates (1013 ml.cm -2 .min- 1).. Acetone has been used for the recovery of gold from the foam column at a flow-rate of 1 ml.cm-2.min -1. On the other hand, Gesser et al. 9 studied the possibility of using polyurethane foam Columns for the extraction and concentration of organic contaminants from water. The collection of polychlorinated biphenyls at various concentration levels (2-20 ppM) has been successfully achieved by passing the aqueous solution through the foam column at high flow-rates (ca. 80ml.cm -2 .min- 1). Acetone and hexane have been employed for the elution of the biphenyls from the foam column. Further, Gesser et al. 29 have shown that polyurethane foam columns can be Used to monitor organic matter in drinking water. A certain volume of water (1-2 litres) was percolated through the foam column at a flow-rate of 2-4 ml.cm-2.min - t and the extracted organic compounds were then stripped from the foam material with hexane in a Soxhlet extractor. POLYURETHANE FOAMS WITH PHYSICALLY IMMOBILIZED HYDROPHOBIC ORGANIC REAGENTS AND EXTRACTANTS
Although unloaded polyurethane foams have successfully been used for the separation and con,centration of some inorganic and organic components from aqueous solutions, yet the general applications of foamed polyurethanes are reduced by their limited selectivity towards the absorption of various compounds.9,17 This directed attention towards the appli* We consider it necessary to include the cr0ss-sectional area of the column in specification of the flow-rate, which thus has the dimensions of a linear flow-rate, c m . m i n - 1.
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cation of polyurethane foam impregnated with certain organic reagents, to provide selective separation methods. Organic extractants (e.g., tri-n-butyl phosphate) are physically immobilized on the foam matrix by allowing the foam material to swell in solutions of them. 12'13 The hydrophobic character of polyurethane foams together with their high available surface area allowed the immobilization of considerable amounts of a wide variety of organic reagents and extracting agents.11,12,14,3o Polyurethane foam loaded with tri-n-butyl phosphate (TBP) has been prepared 13 by the above-mentioned method. In this case the foam material can be considered as a support for the TBP, which is actually the stationary phase. Separation methods in which these loaded foams are used in chromatographic columns are called reversedphase foam chromatographyJ 3,a° Braun and Farag 13 demonstrated the practical advantages of using TBPloaded foam for the separation of inorganic species from aqueous solution. They made a comparative investigation of the separation of palladium(II), bismuth(III) and nickel(II) in a thiourea-perchloric acid system on the TBP-loaded foam and on TBP-loaded "Voltalef" (polytrifluorochloroethylene) powder. A polyether foam of open-cell type was found 1°' 13 to absorb and retain TBP more efficiently than "Voltalef" powder did. The extraction rate of the palladium-thiourea complex on the loaded foam was proved 17 to be faster than on the loaded "Voltalef" powder. For packing the foam material homogeneously in glass tubes of various lengths and diameters, a vacuum method has been developed. 13 This method was found to produce columns with very good flow characteristics and this vacuum-packing technique proved 31 to be suitable for filling chromatographic columns with granular "Voltalef". A comparative study on the gravity flow-rate attained for columns packed with polyurethane foam and "Voltalef" powder (0.16-0-25 mm grain-size) has shown that the hydrodynamic properties of the foam columns are much superior. The chromatographic behaviour of the palladiumthiourea complex on a TBP-loaded foam column has also been examined. 13 The elution curve (with water as eluent) is symmetrical and the peak relatively sharp. The height equivalent to a theoretical plate (HETP) as calculated 32 from the elution curve of palladium was found 17 to be 1.7 and 2.8 mm for foam and "Voltalef" columns, respectively. The breakthrough and overall capacities of foam and "Voltalef" columns have also been measured 17 by using the palladium-thiourea complex solution. In general, the capacity of the TBP-loaded foam is higher than that of the TBP-loaded "Voltalef" (about double)J 7 Separation of palladium from bismuth and nickel in a thiourea-perchloric acid system can be achieved on the TBP-loaded foam columns33 Furthermore, the separation and concentration of gold(III) from thiourea-perchloric acid systems on
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TBP-loaded polyurethane foam have also been investigated by the batch and column techniques. 33 The gold-thiourea complex is extracted on the loaded foam from 0.1M perchloric acid containing 3~ thiourea and 1~ sodium perchlorate. The uptake of the gold complex by the loaded foam was claimed 33 to be fast and not appreciably affected by the presence of some interfering elements [e.g., zinc(II), iron(III) or bismuth(III)]. Quantitative separation of trace amounts of gold from high concentrations of Zn 2÷, Co 2+, Ni 2+, Fe 3+, Sb a+, Cu 2+, Bi3+ or Pd 2+ is achieved by using short foam eolunms and a flow-rate of 10-12ml.cm-2.min- L The chemical enrichment of gold from dilute aqueous solutions has also been investigated, a3 Gold was completely collected on passing the solution through a short foam column at the flow-rate mentioned above. The gold was then recovered from the foam column by dissolving the foam material in hot concentrated nitric acid. The analytical utility of TBP-loaded polyurethane foam columns for the separation of some metals from hydrochloric acid solution has also been investigated. 3'~ The distribution of cobalt, copper and iron chlorides in a TBP foam-hydrochloric acid system was measured. Using the TBP-loaded foam columns, it was possible to separate iron from nickel, copper or cobalt, and the suitability of using the loaded foam columns for the separation of 58Co and 59Fe isotopes has been demonstrated. Polyurethane foam columns immobilizing finely divided tetrachlorohydroquinone were proved a° to be suitable for the reduction of some metal ions in their higher valency state. Reduction of cerium(W), vanadium(V) and iron(III) on foam-redox columns has been examined. The effect of flow-rate and temperature on the reduction efficiency for each metal ion was investigated. Cerium(W) can be reduced quantitatively on passing its aqueous solution through the foam column at flow-rates of 1-6 m l . c m - 2. minand room temperature. The reduction of vanadium(V) and iron(III) was, however, slower. At 35 ° a relatively high flow-rate could be used without affecting the completeness of the reduction. The application of polyurethane foam immobilizing tetrachlorohydroquinone in a finely divided state or in chlorobenzene solution and packed in a syringe (pulsed column) has been described 3s for the reduction of Ce(W), V(V) and Fe(III). The reduction of the metal ion is simply achieved by pressing and releasing the plunger of the flexible-foam pulsed column several times, with the tip in the test solution. The reduction efficiency of pulsed columns packed with swollen foam materials (i.e., immobilizing the redox reagent in chlorobenzene solution) was found to be better than that of pulsed columns packed with dry foam (i.e., immobilizing the redox reagent in a finely divided state). The reduction of Ce(W), V(V) and Fe(III) was more effective if the aqueous metal ion solution was heated to about 80° before use of the pulsed column. Various amounts (2-20 mg) of the
three metal ions have been determined -by this method. Polyurethane foams immobilizing methyl isobutyl ketone, diethyl ether, isopropyl ether or ethyl acetate have also been examined 1~ for the extraction of gold(III) chloride from aqueous solutions. For the rapid collection of gold at trace concentrations ((~06-25 ppm) the percolation of the aqueous solution through short columns packed with these foams at relatively high flow-rates (10-13 ml. cm -2 . m i n - ' ) was recommended. The gold was then eluted from the foam column with acetone. All the methods mentioned previously describe the possibility of using polyurethane foam immobilizing various organic reagents. However, polyurethane foams were also proved x8'36 to be suitable for the immobilization of inorganic reagents. Immobilized finely divided silver sulphide or metallic copper have been suggested for isotope and redox exchange separations, respectively. The silver sulphide foam was prepared ~8 by loading a heterogeneous cationexchange foam3T with ionic silver and subsequent precipitation of silver sulphide in the foam matrix with sodium sulphide solution. Similarly, foam containing copper was prepared a6 by loading the heterogeneous cation-exchange foam with copper ions and then reducing the ionic copper to the metallic form with sodium hydrosulphite solution. Static and dynamic isotope and redox exchange separations of radiosilver on silver sulphide foam and copper foam, respectively, have been investigated. Columns packed with silver sulphide foam were suitable for the collection of various levels of radiosilver (0-1-100 #g of Ag ÷) from nitric acid solution, at relatively high flow-rates (20 ml.cm-2 .min- 1). Quantitative collection of radiosilver at various concentrations (in 2M nitric acid) by redox exchange reaction on columns packed with copper foam has also been reported, the flow-rate being 1012ml.cm -2 .min- L The possibility of using polyurethane foam immobilizing chromatographic-grade greases (preferably DC-200) for the collection of trace concentrations of organo-chlorine pesticides from water or aqueous suspensions has also been investigated. 14 The grease-loaded foam is prepared by dipping the foam material in a 5~ solution of the grease in a suitable solvent. The extraction efficiencies of different greaseqoaded foam columns for ten different organo-chlorine pesticides have been tested. In the collection of the different pesticides from water, fast flow-rates (ca. 80 ml. cm- 2. min- 1) could be employed. However, in the case of collection from suspensions, low flow-rates (ca. 10 ml. cm - 2. min - 1) were recommended. 14 Generally, foam columns (loaded with DC-200) grease) are able to extract all the ten pesticides from water' almost quantitatively. However in the case of suspensions some pesticides (e.g., p,p'-DDE) are not extracted completely, This was attributed to their ability to adsorb on the suspension.
Cellular and foamed plastics as separation media PLASI'ICIZED REAGENT FOAMS
Plasticizers can be defined as non-volatile liquids used to modify synthetic resins. 3s Plasticizing of a polymer is a process in which plasticizer molecules neutralize (reduce) the secondary valence forces (van der Waals) between the polymeric chains, thus increasing the mobility of the molecular segments and decreasing the glass-transition temperature of the system. Above the glass-transition temperature the mobility of plasticizer molecules within the polymeric network is apparently quite high. ~9 The preparation of plasticized reagent foams has recently been studied 16'a°'41 by dissolving hydrophobic organic reagents in a plasticizer solution and then immobilizing the solution on an open-cell type polyurethane foam by swelling. Several hydrophobic organic reagents (exd., dithizone, 1,nitroso-2-naphthol and diethylammonium diethyldithiocarbamate) were found to dissolve in various plasticizers (e.g., TBP, ~-di-n-nonyl phthalate, di-n-octyl phthalate or dibutyl adipate). Accordingly, plasticized zinc dithizonate, 16,4° 1-nitroso-2-naphtho141 and diethylammonium diethyldithiocarbamate 41 foams with reasonable capacities and suitable for the preconcentration of metal ions from aqueous solutions have been prepared. The mobilities of metal ions in the plasticized reagent foam were proved to be quite high and consequently their collection proceeded rapidly. This makes the applications of relatively high flow-rates in column operations possible without any appreciable loss in collection efficiency. The collection of traces of silver(I) or mercury(II) on zinc dithizonate foam has been examined. 16,4° It was proved that the collection rates with plasticized zinc dithizonate foams are generally better than with the unplasticized ones. Small amounts of silver or mercury from extremely dilute solutions (e.g., 1 ppM) are collected by percolating the aqueous metal ion solution through the plasticized foam column at a flow-rate of 8-12 ml. cm- 2. min- 1. Quantitative recoveries of silver and mercury from the plasticized zinc dithizonate foam are obtained by elution with sodium thiosulphate solution. 16'4° Complete collection of silver was possible in presence of 106 times as much lead or copper. 16 Traces of cobalt(II) are collected on plasticized 1nitroso-2-naphthol and diethylammonium diethyl-" dithiocarbamate foams. 41 The optimal pH-values of the aqueous solutions for the collection are 6.6-9.0 and 4.5-5.5 for 1-nitroso-2-naphthol and diethylammonium diethyldithiocarbamate foams respectively. Various amounts of cobalt(II) (1-1000 #g) are quantitatively collected from aqueous solutions on foam columns at a flow-rate of 5-6 ml. crn- 2. min- 1. Recently, the preparation of plasticized iodine and silver dithizonate foams suitable for isotope exchange separations of radioactive isotopes has been described. 42 The exchange of radioiodide on the iodine foam was found 42 to be very fast in batch experiments. The highest exchange yield is obtained from aqucous solutions at pH-values lower than 1. It was
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proved 42 that the mobility of the iodide ion in the plasticized foam material is quite high and consequently equilibrium is attained rapidly, Complete separation (by exchange) of radioiodide from a large excess of sodium, potassium, chloride and bromide ions, which are known to interfere seriously in the determination of iodine in biological materials by activation analysis, 43 was achieved. Quantitative collection of radiosilver ((~01-1 #g) from 0.1M nitric acid was realized on columns packed with plasticized polyurethane foam immobilizing primary silver dithizonate. 42 On the other hand, plasticized polyurethane foams immobilizing chromogenic hydrophobic organic reagents have been suggested 44 for rapid detection and semiquantitative determination of very low concentrations of metal ions in aqueous solution. The name "chromofoam" was proposed for these reagent foams. It was claimed 44 that the organic reagent solution, whic h is homogeneously distributed on the large available surface area of the plasticized reagent foam (chromofoam)~ can function as an effective collector for metal ion traces from relatively high volumes of aqueous solution. This together with the greater possibility of observing the reaction products on the surface of the foam material allows the detection of traces of metal ions with a chromofoam by shaking one small cube of it with one or more ml of the aqueous solution in a test-tube. Detection and semiquantitative determination of zinc(II) or lead(II) with plasticized dithizone foam and of copper(II) and cobalt(II) with rubeanic acid and Amberlite LA-I foams, respectively, have been investigated. 44 Generally, the sensitivity of the foam test is better than or equal to that of the usual spot-tests on a spot-plate or filter paper. Also, the detection of cohalt(II) with Amberlite LA-1 foam in the presence of thiocyanate ions was proved 44 to be more sensitive than the resin spot-tests. 45 The selectivity of the chromofoam test has been examined in the case of cobalt by studying the detection of 1 #g of cobalt in the presence of up to 10 nag of more than 40 elements. The chromofoam test was found to be quite selective. A further advantage of chromofoams is that columns packed with them can be used for the detection and semiquantitative determination of metal ions at the ppM level. This is simply achieved by passing large volumes of the aqueous solution through the reagent foam column at a flow-rate of 1015 ml .cn1-2 .min- 1 and measuring (in comparison with a standard) the length of the coloured zone.
POLYURETHANE FOAMS WITH ANCHORED (BONDED) FUNCI'IONAL GROUPS
The preparation of cellular (foamed) plastics to which specific functional groups are chemically bonded has also been attempted.15.37 Gesser e t al.~ 5 described a method for the preparation of SH-polyur-
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ethane foam. Columns packed with the SH-foam were evaluated for the adsorption of mercuric chloride and methylmercuric chloride from extremely dilute aqueous solution. It was found that mercury at concentrations ranging between 0.4 and (~0004 ppm can be quantitatively collected on the foam columns when 100ml of the aqueous solution are allowed to pass through them at a flow-rate of about 13 m l . c m - 2 . m i n - ~. At higher mercury concentrations (e.g., 4 ppM) the retention efficiency of the foam columns is decreased because of oversaturation. In general, the effective capacity of the foam columns for mercury was much lower for methylmercuric chloride than for mercuric chloride. This was attributed to the probable steric hindrance effect of the methyl group. The fraction of mercury adsorbed on the foam column was generally increased as the concentration of mercury decreased. The recovery of mercury from the SH-foam has also been investigated, the foam material being extracted with 2M hydrochloric acid in a Soxhlet extractor. On the other hand, Braun et al., a7 during their studies on the preparation of ion-exchange foams, described various methods suitable for the direct and indirect introduction of functional groups (ionogenic groups) in the foamed skeleton structure. They prepared phenol-formaldehyde resin foam to which sulphonic acid groups were chemically bonded by direct sulphonation of a commercially available phenol-formaldehyde foam. It was repOrted that the mechanical properties of the original phenol-formaldehyde foam were not much changed by sulphonation. Also, the ion-exchange capacity of the foams was reasonable (1.85 meq/g). Indirect introduction of the ionogenic groups into the foam material has also been described. Two different methods are used. The first method is based on carrying out a polymer analogue reaction after joining the foam to an easily transformable polymer. Styrene-polyurethane interpolymer foam was prepared by this method and the anion-exchange groups then introduced by chloromethylation and amination. The mechanical properties of the foams depended on the polymerization conditions and the quality of the initiator used. The second method for the indirect introduction of ionogenic groups into the foam matrix was based on the radiation grafting of a monomer with ionogenic groups. The radiation grafting of open-cell polyurethane and closed-cell polyethylene foams with methacrylic acid has been investigated. 37 Foams with excellent properties and good ion-exchange capacities (4 meq/g) have been prepared. In our opinion, the analytical use of foamed plastics with bonded functional groups is a very promising field. Foam materials with very selective properties should be obtained by anchoring functional groups to the foam skeleton, and could become very important if suitable methods could be developed for the preparation of such foams.
HOMOGENEOUS AND HETEROGENEOUS ION-EXCHANGE FOAMS
Homogeneous ion-exchange foams are prepared by 37'~ (/) physical immobilization of liquid ionexchangers on or in flexible polyether-type polyurethane foam, (ii) direct anchoring of ion-exchange groups on previously prepared phenol-formaldehyde foam, (iii) indirect introduction of the ionogenic groups into polyurethane or polyethylene foams. The direct and indirect introduction of the ionogenic groups into the different foams has been described in the previous section. Physical immobilization of a benzene solution of tri-n-octylamine (TNOA), liquid anion-exchanger on polyurethane foam was found 46 to be possible by allowing the foam material to swell in a TNOA-benzene solution. The analytical utility of the foam was tested by investigating the separation of cobalt(II) and nickel(II) in hydrochloric acid media. Columns packed with foam materials containing 11.4 and 17.7% w/w of TNOA in benzene were found46 to be the most suitable for the quantitative retention (and subsequent elution) of cobalt from hydrochloric acid solution. Separation of nickel and cobalt at different relative concentrations was achieved by using 8M and 1M hydrochloric acid for the elution of nickel and cobalt respectively. Plasticized polyurethane foam immobilizing Amberlite LA-1 (liquid anion-exchanger) has also been prepared: 44 The application of this foam material for rapid and selective detection and semiquantitative determination of cobalt(II) in batch and column operations was described above. A method for the preparation of a heterogeneous cation-exchange foam has also been reported. 37 This method is based on foaming a very finely ground powder of a commercially available cation-exchanger (Varion KS) with the precursors of open-cell polyether-type polyurethane foam. The possibility of using this cation-exchange foam for rapid separations in aqueous and alcoholic solutions has been investigated. 4 7 Sorption of metal ions (e.g., Cu 2+) on the cation-exchange foam took place in one rapid step, i.e., gel diffusion was not the rate-controlling step as in common ion-exchange beads. The t~/2 value for equilibrium .8 sorption on the cation-exchange foam, as calculated from the rate-curve for copper(II), was found47 to be 0"6 min. However, the capacity of the cation-exchange foam is much higher than that of surface-sulphonated resin beads. A comparison between the efficiency of cationexchange beads (Varion KS) and the cation-exchange foam for the elution of copper(II) with hydroxylammonium chloride solution shows that flow-rates as high as 3 m l . c m - 2. min- ~ could be applied in the case of foam columns without any considerable loss in column performance, while in the case of bead columns quantitative elution is only obtained at a flow-rate of 1 ml. cm-2. min- 1. The selectivity of the cation-exchange foam was proved to be more or less the same as that of the original cation-exchange beads.
Cellular and foamed plasticsas separation media SPECIALLY TREATED POLYURETHANE FOAMS Bauman et al. 19' so reported that open-cell polyurethane foam can be used as a support for starch gel containing enzymes. They described a method for the preparation of immobilized horse-serum cholinesterase products in which the enzyme in the starch gel is physically entrapped on the surface of open-cell polyurethane foam pads. This immobilized enzyme pad is used to monitor water and air continuously for atmospheric pollutants which are enzymic inhibitors of cholinesterase. Goodson et al. 51 have recently described an improved method for the preparation of polyurethane foam coated with horse-serum cholinesterase. They suggested the adsorption of the horse-serum cholinesterase on aluminium hydroxide gel before the starchgel preparation. Portions of this foam were found 51 to be suitable for the detection of low concentrations of anticholinesterase substances in air, by use of a special cell in which the enzyme activity of the foam pad is observed electrochemically. O n the other hand, Evans et al. 52 examined the possibility of using open-ceU polyurethane foam in the immunoadsorption of cells. Reticulated polyurethane foam of the polyester-type to which the antibody is coupled was found 52 to serve as a matrix for the immunological binding of erythrocytes. REFERENCES
1. H. Brunschwig, Liber de Arte Distillandi, 1512 (see also A. Bittel, Dissertation. Tiibingen, 1957). 2. E. Bayer, Gaschromatographie, 2nd Ed., p. 4. SpringerVerlag, Heidelberg, 1962. 3. D. Lal, J. R. Arnold and B. L. K. Somayajulu, Geochim. Cosmochim. Acta, 1964, 28, 1111. 4. W. D. Ross, R. T. Jefferson, J. Chromatog. Sci., 1971, 8, 386. 5. H. Schnecko and O. Bieber, Chromatographia, 1971, 4. 109. 6. F. D. Hileman, R. E. Sievers, G. G?. Hess and W. D. Ross, Anal. Chem., 1973, 45, 1126. 7. H. J. M. Bowen, J. Chem. Soc. (A), 1970, 1082. 8. P. Schiller and G. B. Cook, Anal. Chim. Acta, 1971, 54, 364. 9. H. D. Gesser, A. Chow, F. C. Davis, J. F. Uthe and J. Reinke, Anal. Letters, 1971, 4, 883. 10. T. Braun and A. B. Farag, Anal. Chim. Acta, 1973, 66, 419. 11. S. Sukiman, Radiochem. Radioanal. Letters, 1974, 18, 129. 12. T. Braun and A. B. Farag, Talanta, 1972, 19, 828. 13. ldem, Anal. Chim. Acta, 1972, 61, 265. 14. J. F. Uthe, J. Reinke and H. D. Gesser, Environmental Letters, 1972, 3, 117. 15. M. A. J. Mazurski, A. Chow and H. D. Gesser, Anal. Chim. Acta, 1973, 65, 99.
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16. T. Braun and A. B. Farag, ibid., 1974, 69, 85. 17. Idem, in Extraction Chromatography, T. Braun and G. Ghersini, eds. Akad6mia Kiad6, Budapest, 1975. 18. ldem, Radiochem. Radioanal. Letters, 1974, 19, 275. 19. C. R. Thomas, Brit. Plastics, 1965, 552. 20. Proceedings of a Conference on Cellular Plastics, Natick, Massachusetts, 1966, NAS-NRC, Washington, D.C., 1967. 21. T. H. Ferringo, Rigid Plastic Foams. Reinhold, New York, 1963. 22. Modern Plastics Encyclopedia, ed. S. Gross, Vol. 46, p. 248. McGraw-Hill, New York, 1969. 23. C. R. Thomas and H. Briscall, Brit. Plastics, 1967, 79. 24. M. E. Bailey, J. Chem. Educ., 1971, 48, 809. 25. C. J. Benning, Plastic Foams: The Physics and Chemistry of Product Performance and Process Technology, Vol. l, Chemistry and Physics of Foam Formation. Wiley, New York, 1969. 26. H. J. David and H. B. Staley, Analytical Chemistry of the Polyurethanes, Vol. XVI, Part III. Wiley, New York, 1969. 27. H. J. M. Bowen, Radiochem. Radioanal. Letters, 1969, 2, 169. 28. Idem, ibid., 1971, 7, 71. 29. H. D. Gesser, A. B. Spading, A. Chow and C. W. Turner, J. Am. Water Works Assoc., 1973, 65, 220. 30. T. Braun, A. B. Farag and A. Klimes-Szmik, Anal. Chim. Acta, 1973, 64, 71. 31. T. Braun and A. B. Farag, ibid., 1972, 62, 476. 32. E. Glueckauf, Trans. Faraday Soc., 1955, 51, 34. 33. T. Braun and A. B. Farag, Anal. Chim. Acta, 1973, 65, 115. 34. T. Braun, L. Bakos and Zs. Szab6, ibid., 1973, 66, 57. 35. T. Braun and A. B. Farag, ibid., 1973, 65, 139. 36. Idem, Radiochem. Radioanal. Letters, 1974, 19, 377. 37. T. Braun, O. B6keffy, I. Haklits, K. Kfidfir and G. Majoros, Anal. Chim. Acta, 1973, 64, 45. 38. Modern Plastics Encylcopedia, J. Frados and S. Gross, eds., Vol. 45, p. 428. McGraw-Hill, New York, 1968. 39. R. Kosfeld, Plasticization and Plasticized Processes, R. F. Gould, ed., p. 43. American Chemical Society, Washington, D.C., 1956. 40. T. Braun and A. B. Farag, Anal. Chim. Acta, 1974, 71, 133. 41. ldem, ibid., in the press. 42. Idem, J. Radioanal. Chem., 1975, 25, 5. 43. M. Heurtebise and W. J. Ross, Anal. Chem., 1971, 43, 1438. 44. T. Braun and A. B. Farag, Anal. Chim. Acta, 1974, 73, 301. 45. M. Fujimoto and Y. Nakatsukasa, ibid., 1962, 27, 373. 46. T. Braun, E. Husz/tr and L. Bakos, ibid., 1973, 64, 77. 47. T. Braun and A. B. Farag, ibid., 1974, 68, 119. 48~ M. Skafi and K. H. Lieser, Z. Anal. Chem., 1970, 249, 182. 49. E. K. Bauman, L. H. Goodson, G. G. Guilbault and D. N. Kramer, Anal. Chem., 1965, 37, 1378. 50. E. K. Bauman, L. H. Goodson and J. R. Thomson, Anal. Biochem., 1967, 19, 587. 51. L. H. Goodson, W. B. Jacobs and A. W. Davis, ibid., 1973, 51, 362. 52. W. H. Evans, M. G. Mage and E. A. Peterson, J. Immunol., 1969, 102, 899.
TALANTA MINI-REVIEW* CELLULAR AND FOAMED PLASTICS AS SEPARATION MEDIA A NEW GEOMETRICAL FORM OF THE SOLID PHASE IN ANALYTICAL L I Q U I D - S O L I D CONTACT T. BRAUNand A. B. FARAGI" Institute of Inorganic and Analytical Chemistry, L. E/Stv6s University, P.O. Box 123, 1443 Budapest,
Hungary (Received 7 November 1974. Accepted 27 January 1975) Sunmmry--There has been considerable interest during the last few years in using cellular (foamed) plastics (mainly polyurethanes) either unloaded or as a means of immobilizing hydrophobic organic reagents, powdered ion-exchangers or finely divided precipitates, for the collection and separation of inorganic or organic species from aqueous solution. Foamed plastics with anchored (bonded) functional groups have also been used for the same purpose. It has been realized that the application of cellular plastics is often advantageous not only for quantitative work but also for qualitative and semiquantitative analysis. Methods available on the application of cellular and foamed plastics for the collection, separation and recovery of various inorganic and organic components from aqueous solution are reviewed.
Probably the oldest application of a material of using polyurethane foam coated with chromatocellular geometry (in the form of sponge) for chemical graphic-grade greases. Recently, foamed plastics (mainly polyurethanes) purposes was the purification of ethyl alcohol by distillation through a sponge impregnated with olive oil. immobilizing hydrophobic reagents, powdered ionThis method, used by Brunschwig 1 more than four exchangers and finely divided precipitates, or bonding centuries ago, can be considered as partition chroma- certain functional groups, have been recommended tography, the sponge material being the support, olive for several analytical applications. 15-1s This review collects the papers published and gives oil the stationary phase and ethyl alcohol vapour the mobile phase. In 1962 Bayer2 checked this ancient an up-to-date picture of the use of cellular (foamed) plastics as a new geometrical form of the solid phase method and found that it operates well. Lal et al. a have described a method for the extrac- in analytical liquid-solid extraction processes. tion of trace elements from sea-water, by use of ferric hydroxide supported on natural sponges. Several eleDEFINITION, GEOMETRY AND CELL STRUCTURE OF CELLULAR (FOAMED) PLASTICS ments (e.g. Si, Be, A1 and Ti) were extracted by towing ferric hydroxide impregnated sponge through coastal Cellular (foamed) plastics can be defined i9 as plassea-water. tic materials in which a proportion of solid is replaced Recently, however, papers 4-6 have been published by gas in the form of numerous small cells. The gas describing the possibility of using cellular plastics for may be a continuous phase, giving an open cell gas-solid and gas-liquid chromatographic separations. material, or it may be a discontinuous phase, i.e., in Bowen 7 was the first to discover the absorption the form of discrete, non-communicating ceUs. properties of polyurethane foams toward a number From the geometrical point of view, if the gas bubof inorganic and organic species in aqueous solution. bles occupy a volume fraction smaller than 76~ of Since the appearance of this work several investigathe whole, they may be spherical. If they occupy a tors a-ll have used polyurethane foams for the volume fraction larger than 76~, they will be disabsorption and recovery of inorganic and organic torted into polyhedra 2° (mainly pentagonal dodecacompounds from aqueous solution. hedra). In the latter case the polymer is distributed Braun and Farag ~2'~3 suggested in 1972 the anabetween the walls of the bubbles and the lines where lytical application of polyurethane foams loaded with bubbles intersect. The bubbles are called cells, the hydrophobic extractant. Gesser et al} 4 described a lines of intersection strands, and the walls windows method for the collection of pesticides from water by (or membranes). In an open-cell flexible foam, at least two windows (from the total) in each cell must be * For reprints of this Review, see Publisher's announceruptured for fluids to pass freely through the foam. ment near end of this issue. Cell. struotUre (i.e., the presence or absence of wint Present address: National Research Centre, Dokki, Cairo. Egypt. dows in the cells, or the number of windows per cell) ]AL 22/9 A
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is a tunction of the process by which the cellular material is made. It w a s n o t e d 19 that both rigid and flexible foams may be obtained, with open or closed cells. The structure made up of windowless cells (conraining only strands) is called a reticulated foam. 2° It was reported 19 that flexible and rigid materials generally tend to have open and closed cell structures respectively. However, there are many exceptions and, as the type of cell structure is mainly determined by the method of expansion, some materials which can be made by more than one method can exist in both open and closed cell forms. Furthermore, methods are available by which closed cell structures can be converted into the open cell form by rupture of the windows. Rupturing may be caused mechanically by applying pressure, or chemically by hydrolysis or oxidation.
GENERAL M E T H O D S O F P R E P A R I N G CELLULAR PLASTICS
With the highly developed technology for making cellular plastics today, 21 methods exist by which practically every plastic material may be made in cellular form. The general principle for preparing cellular materials is the dispersion of a gas within a liquid to obtain a liquid foam which will then be solidified to a solid cellular plastic. The main methods t9 for uniform dispersion of the gas bubbles are chemical (e.g., thermal decomposition of a chemical blowing agent or blowing by in situ chemical reaction); physical (e.g., low-pressure release of dissolved gas, blowing by vapour from a volatile liquid or temporary filler); and mechanical (e.g., mechanical entrainment of gas or the use of microspheres). Several plastic materials are now commercially available in cellular forms, e.g., poly(vinyl chloride), polyurethane, polymethylmethacrylate, phenolformaldehyde, urea-formaldehyde, polyethylene, polytetrafluoroethylene. In the major part of the published work on the application of cellular plastics in analytical chemistry, polyurethane foams have been employed. Consequently, some information about the preparation of polyurethane foam and its physical and chemical properties is briefly given.
Polyurethane foam preparation Polyurethane foams can be prepared in soft, flexible or rigid forms. 22 They are formed by the reaction of the terminal hydroxyl groups of a polyester or polyether resin and a polyfunctional isocyanate. 23' 24 Foams prepared from a wide variety of hydroxyl compounds (polyethers, polyesters or polyols) and isocyanates are now commercially available. In general, polyols in the molecular weight range 4006000 are employed. 25"26 The most widely used isocyanate compound is toluene di-isocyanate (usually 80/20 and 65/35 mixtures of the 2,4- and 2,6-isomers are used).
Physical and chemical properties of polyurethane foams Generally, the physical properties of polyurethane foams depend on the method by which they are prepared. For example, the windows may or may not be ruptured in the final stage of expansion, depending on the relative rate of molecular growth (gelation) and gas reaction, giving rise to flexible or rigid foam. The. choice of the polyol has a major effect on the foam properties determining its rigidity or flexibility. 22 The cross-link density of the urethane polymer also determines whether the foam will be flexible (low cross-link density) or rigid (high cross-link density). Flexible foams are prepared from polyols of moderately high molecular weight and low degree of branching, while rigid foams are prepared from low molecular weight, highly branched resins. Also, the chemical properties of polyurethane foams are a function of the preparation process. For example, solvent-resistance of the foam material is increased at higher cross-link density, seems to be unaffected by the type of aromatic di-isoeyanate used, and isreduced by the use of a large excess of isocyanate. 2~ Bowen 7 examined the chemical resistance of some batches of flexible polyurethane foam and claimed that they were rather stable and inert. He reported that the foam batches tested were degraded when heated between 180° and 220°, and slowly turned brown in ultraviolet light. They were dissolved by concentrated nitric acid, and reduced alkaline potassium permanganate. They were mostly unaltered, apart from reversible swelling, by water, 6M hydrochloric acid, 4M sulphuric acid, 2M nitric acid, glacial acetic acid, 2M ammonia, 2M sodium hydroxide and the following solvents: light petroleum, benzene, carbon tetrachloride, chloroform, diethyl ether, di-isopropyl ether, acetone, isobutyl methyl ketone, ethyl acetate, isopentyl acetate, and alcohols. Also it was noted that polyurethane foams could be dissolved in hot arsenic(III) chloride solution.
UNLOADED POLYURETHANE FOAMS B o w e n 7 initiated in 1970 the application of foamed polyurethane for the absorption and recovery of a number of inorganic and organic compounds from aqueous solutions in static (batch) experiments. He measured the surface area of various polyurethane foam samPles (of polyether type) and demonstrated that the uptake of different components from aqueous solutions by the foam materials is due to absorption rather than adsorption phenomena. 7'z2 The absorption isotherms of some elements have been measured and the distribution ratios and absorption capacities of the foam materials for these elements have been determined. ~ In some cases [e.g., iodine and gold(III)] the absorption was found to be greater at low temperatures than at higher ones, while in others [e.g., Fe(III)] the reverse was observed. In a subsequent communication Bowen 28 recommended the application of polyurethane foam for the
Cellular and foamed plastics as separation media recovery of gold(III) chloride from mineral wastewaters by the batch technique. The possibility of separating gold(III) chloride from natural waters by polyurethane foams has also been examined by Schiller and Cook. s It was claimed s that gold at ppM (parts per milliard) level can be quantitatively collected from aqueous solutions by shaking the mixture for 90 min. Recently, Braun and Farag 1° investigated the recovery of gold-thiourea complex from aqueous solutions containing perchlorate ion, using polyurethane foams in batch operations. Open-cell polyether and polyester type foams were examined. The uptake of the gold-thiourea complex by different samples of the polyether type foam depended to some extent on the cell dimensions and decreased as the cell size decreased. Further, the absorption capacities of the polyether type foams for the gold complex are generally greater than those of the polyester type. ~° Sukiman 11 described the application of polyurethane foam for the extraction of gold(III) chloride from acidic aqueous solutions and natural waters by the dynamic technique. Gold(III) at trace concentrations (0.02-25 p p M ) c a n be quantitatively collected from aqueous solution by percolating the solution through a short foam column at relatively high flow-rates (1013 ml.cm -2 .min- 1).. Acetone has been used for the recovery of gold from the foam column at a flow-rate of 1 ml.cm-2.min -1. On the other hand, Gesser et al. 9 studied the possibility of using polyurethane foam Columns for the extraction and concentration of organic contaminants from water. The collection of polychlorinated biphenyls at various concentration levels (2-20 ppM) has been successfully achieved by passing the aqueous solution through the foam column at high flow-rates (ca. 80ml.cm -2 .min- 1). Acetone and hexane have been employed for the elution of the biphenyls from the foam column. Further, Gesser et al. 29 have shown that polyurethane foam columns can be Used to monitor organic matter in drinking water. A certain volume of water (1-2 litres) was percolated through the foam column at a flow-rate of 2-4 ml.cm-2.min - t and the extracted organic compounds were then stripped from the foam material with hexane in a Soxhlet extractor. POLYURETHANE FOAMS WITH PHYSICALLY IMMOBILIZED HYDROPHOBIC ORGANIC REAGENTS AND EXTRACTANTS
Although unloaded polyurethane foams have successfully been used for the separation and con,centration of some inorganic and organic components from aqueous solutions, yet the general applications of foamed polyurethanes are reduced by their limited selectivity towards the absorption of various compounds.9,17 This directed attention towards the appli* We consider it necessary to include the cr0ss-sectional area of the column in specification of the flow-rate, which thus has the dimensions of a linear flow-rate, c m . m i n - 1.
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cation of polyurethane foam impregnated with certain organic reagents, to provide selective separation methods. Organic extractants (e.g., tri-n-butyl phosphate) are physically immobilized on the foam matrix by allowing the foam material to swell in solutions of them. 12'13 The hydrophobic character of polyurethane foams together with their high available surface area allowed the immobilization of considerable amounts of a wide variety of organic reagents and extracting agents.11,12,14,3o Polyurethane foam loaded with tri-n-butyl phosphate (TBP) has been prepared 13 by the above-mentioned method. In this case the foam material can be considered as a support for the TBP, which is actually the stationary phase. Separation methods in which these loaded foams are used in chromatographic columns are called reversedphase foam chromatographyJ 3,a° Braun and Farag 13 demonstrated the practical advantages of using TBPloaded foam for the separation of inorganic species from aqueous solution. They made a comparative investigation of the separation of palladium(II), bismuth(III) and nickel(II) in a thiourea-perchloric acid system on the TBP-loaded foam and on TBP-loaded "Voltalef" (polytrifluorochloroethylene) powder. A polyether foam of open-cell type was found 1°' 13 to absorb and retain TBP more efficiently than "Voltalef" powder did. The extraction rate of the palladium-thiourea complex on the loaded foam was proved 17 to be faster than on the loaded "Voltalef" powder. For packing the foam material homogeneously in glass tubes of various lengths and diameters, a vacuum method has been developed. 13 This method was found to produce columns with very good flow characteristics and this vacuum-packing technique proved 31 to be suitable for filling chromatographic columns with granular "Voltalef". A comparative study on the gravity flow-rate attained for columns packed with polyurethane foam and "Voltalef" powder (0.16-0-25 mm grain-size) has shown that the hydrodynamic properties of the foam columns are much superior. The chromatographic behaviour of the palladiumthiourea complex on a TBP-loaded foam column has also been examined. 13 The elution curve (with water as eluent) is symmetrical and the peak relatively sharp. The height equivalent to a theoretical plate (HETP) as calculated 32 from the elution curve of palladium was found 17 to be 1.7 and 2.8 mm for foam and "Voltalef" columns, respectively. The breakthrough and overall capacities of foam and "Voltalef" columns have also been measured 17 by using the palladium-thiourea complex solution. In general, the capacity of the TBP-loaded foam is higher than that of the TBP-loaded "Voltalef" (about double)J 7 Separation of palladium from bismuth and nickel in a thiourea-perchloric acid system can be achieved on the TBP-loaded foam columns33 Furthermore, the separation and concentration of gold(III) from thiourea-perchloric acid systems on
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TBP-loaded polyurethane foam have also been investigated by the batch and column techniques. 33 The gold-thiourea complex is extracted on the loaded foam from 0.1M perchloric acid containing 3~ thiourea and 1~ sodium perchlorate. The uptake of the gold complex by the loaded foam was claimed 33 to be fast and not appreciably affected by the presence of some interfering elements [e.g., zinc(II), iron(III) or bismuth(III)]. Quantitative separation of trace amounts of gold from high concentrations of Zn 2÷, Co 2+, Ni 2+, Fe 3+, Sb a+, Cu 2+, Bi3+ or Pd 2+ is achieved by using short foam eolunms and a flow-rate of 10-12ml.cm-2.min- L The chemical enrichment of gold from dilute aqueous solutions has also been investigated, a3 Gold was completely collected on passing the solution through a short foam column at the flow-rate mentioned above. The gold was then recovered from the foam column by dissolving the foam material in hot concentrated nitric acid. The analytical utility of TBP-loaded polyurethane foam columns for the separation of some metals from hydrochloric acid solution has also been investigated. 3'~ The distribution of cobalt, copper and iron chlorides in a TBP foam-hydrochloric acid system was measured. Using the TBP-loaded foam columns, it was possible to separate iron from nickel, copper or cobalt, and the suitability of using the loaded foam columns for the separation of 58Co and 59Fe isotopes has been demonstrated. Polyurethane foam columns immobilizing finely divided tetrachlorohydroquinone were proved a° to be suitable for the reduction of some metal ions in their higher valency state. Reduction of cerium(W), vanadium(V) and iron(III) on foam-redox columns has been examined. The effect of flow-rate and temperature on the reduction efficiency for each metal ion was investigated. Cerium(W) can be reduced quantitatively on passing its aqueous solution through the foam column at flow-rates of 1-6 m l . c m - 2. minand room temperature. The reduction of vanadium(V) and iron(III) was, however, slower. At 35 ° a relatively high flow-rate could be used without affecting the completeness of the reduction. The application of polyurethane foam immobilizing tetrachlorohydroquinone in a finely divided state or in chlorobenzene solution and packed in a syringe (pulsed column) has been described 3s for the reduction of Ce(W), V(V) and Fe(III). The reduction of the metal ion is simply achieved by pressing and releasing the plunger of the flexible-foam pulsed column several times, with the tip in the test solution. The reduction efficiency of pulsed columns packed with swollen foam materials (i.e., immobilizing the redox reagent in chlorobenzene solution) was found to be better than that of pulsed columns packed with dry foam (i.e., immobilizing the redox reagent in a finely divided state). The reduction of Ce(W), V(V) and Fe(III) was more effective if the aqueous metal ion solution was heated to about 80° before use of the pulsed column. Various amounts (2-20 mg) of the
three metal ions have been determined -by this method. Polyurethane foams immobilizing methyl isobutyl ketone, diethyl ether, isopropyl ether or ethyl acetate have also been examined 1~ for the extraction of gold(III) chloride from aqueous solutions. For the rapid collection of gold at trace concentrations ((~06-25 ppm) the percolation of the aqueous solution through short columns packed with these foams at relatively high flow-rates (10-13 ml. cm -2 . m i n - ' ) was recommended. The gold was then eluted from the foam column with acetone. All the methods mentioned previously describe the possibility of using polyurethane foam immobilizing various organic reagents. However, polyurethane foams were also proved x8'36 to be suitable for the immobilization of inorganic reagents. Immobilized finely divided silver sulphide or metallic copper have been suggested for isotope and redox exchange separations, respectively. The silver sulphide foam was prepared ~8 by loading a heterogeneous cationexchange foam3T with ionic silver and subsequent precipitation of silver sulphide in the foam matrix with sodium sulphide solution. Similarly, foam containing copper was prepared a6 by loading the heterogeneous cation-exchange foam with copper ions and then reducing the ionic copper to the metallic form with sodium hydrosulphite solution. Static and dynamic isotope and redox exchange separations of radiosilver on silver sulphide foam and copper foam, respectively, have been investigated. Columns packed with silver sulphide foam were suitable for the collection of various levels of radiosilver (0-1-100 #g of Ag ÷) from nitric acid solution, at relatively high flow-rates (20 ml.cm-2 .min- 1). Quantitative collection of radiosilver at various concentrations (in 2M nitric acid) by redox exchange reaction on columns packed with copper foam has also been reported, the flow-rate being 1012ml.cm -2 .min- L The possibility of using polyurethane foam immobilizing chromatographic-grade greases (preferably DC-200) for the collection of trace concentrations of organo-chlorine pesticides from water or aqueous suspensions has also been investigated. 14 The grease-loaded foam is prepared by dipping the foam material in a 5~ solution of the grease in a suitable solvent. The extraction efficiencies of different greaseqoaded foam columns for ten different organo-chlorine pesticides have been tested. In the collection of the different pesticides from water, fast flow-rates (ca. 80 ml. cm- 2. min- 1) could be employed. However, in the case of collection from suspensions, low flow-rates (ca. 10 ml. cm - 2. min - 1) were recommended. 14 Generally, foam columns (loaded with DC-200) grease) are able to extract all the ten pesticides from water' almost quantitatively. However in the case of suspensions some pesticides (e.g., p,p'-DDE) are not extracted completely, This was attributed to their ability to adsorb on the suspension.
Cellular and foamed plastics as separation media PLASI'ICIZED REAGENT FOAMS
Plasticizers can be defined as non-volatile liquids used to modify synthetic resins. 3s Plasticizing of a polymer is a process in which plasticizer molecules neutralize (reduce) the secondary valence forces (van der Waals) between the polymeric chains, thus increasing the mobility of the molecular segments and decreasing the glass-transition temperature of the system. Above the glass-transition temperature the mobility of plasticizer molecules within the polymeric network is apparently quite high. ~9 The preparation of plasticized reagent foams has recently been studied 16'a°'41 by dissolving hydrophobic organic reagents in a plasticizer solution and then immobilizing the solution on an open-cell type polyurethane foam by swelling. Several hydrophobic organic reagents (exd., dithizone, 1,nitroso-2-naphthol and diethylammonium diethyldithiocarbamate) were found to dissolve in various plasticizers (e.g., TBP, ~-di-n-nonyl phthalate, di-n-octyl phthalate or dibutyl adipate). Accordingly, plasticized zinc dithizonate, 16,4° 1-nitroso-2-naphtho141 and diethylammonium diethyldithiocarbamate 41 foams with reasonable capacities and suitable for the preconcentration of metal ions from aqueous solutions have been prepared. The mobilities of metal ions in the plasticized reagent foam were proved to be quite high and consequently their collection proceeded rapidly. This makes the applications of relatively high flow-rates in column operations possible without any appreciable loss in collection efficiency. The collection of traces of silver(I) or mercury(II) on zinc dithizonate foam has been examined. 16,4° It was proved that the collection rates with plasticized zinc dithizonate foams are generally better than with the unplasticized ones. Small amounts of silver or mercury from extremely dilute solutions (e.g., 1 ppM) are collected by percolating the aqueous metal ion solution through the plasticized foam column at a flow-rate of 8-12 ml. cm- 2. min- 1. Quantitative recoveries of silver and mercury from the plasticized zinc dithizonate foam are obtained by elution with sodium thiosulphate solution. 16'4° Complete collection of silver was possible in presence of 106 times as much lead or copper. 16 Traces of cobalt(II) are collected on plasticized 1nitroso-2-naphthol and diethylammonium diethyl-" dithiocarbamate foams. 41 The optimal pH-values of the aqueous solutions for the collection are 6.6-9.0 and 4.5-5.5 for 1-nitroso-2-naphthol and diethylammonium diethyldithiocarbamate foams respectively. Various amounts of cobalt(II) (1-1000 #g) are quantitatively collected from aqueous solutions on foam columns at a flow-rate of 5-6 ml. crn- 2. min- 1. Recently, the preparation of plasticized iodine and silver dithizonate foams suitable for isotope exchange separations of radioactive isotopes has been described. 42 The exchange of radioiodide on the iodine foam was found 42 to be very fast in batch experiments. The highest exchange yield is obtained from aqucous solutions at pH-values lower than 1. It was
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proved 42 that the mobility of the iodide ion in the plasticized foam material is quite high and consequently equilibrium is attained rapidly, Complete separation (by exchange) of radioiodide from a large excess of sodium, potassium, chloride and bromide ions, which are known to interfere seriously in the determination of iodine in biological materials by activation analysis, 43 was achieved. Quantitative collection of radiosilver ((~01-1 #g) from 0.1M nitric acid was realized on columns packed with plasticized polyurethane foam immobilizing primary silver dithizonate. 42 On the other hand, plasticized polyurethane foams immobilizing chromogenic hydrophobic organic reagents have been suggested 44 for rapid detection and semiquantitative determination of very low concentrations of metal ions in aqueous solution. The name "chromofoam" was proposed for these reagent foams. It was claimed 44 that the organic reagent solution, whic h is homogeneously distributed on the large available surface area of the plasticized reagent foam (chromofoam)~ can function as an effective collector for metal ion traces from relatively high volumes of aqueous solution. This together with the greater possibility of observing the reaction products on the surface of the foam material allows the detection of traces of metal ions with a chromofoam by shaking one small cube of it with one or more ml of the aqueous solution in a test-tube. Detection and semiquantitative determination of zinc(II) or lead(II) with plasticized dithizone foam and of copper(II) and cobalt(II) with rubeanic acid and Amberlite LA-I foams, respectively, have been investigated. 44 Generally, the sensitivity of the foam test is better than or equal to that of the usual spot-tests on a spot-plate or filter paper. Also, the detection of cohalt(II) with Amberlite LA-1 foam in the presence of thiocyanate ions was proved 44 to be more sensitive than the resin spot-tests. 45 The selectivity of the chromofoam test has been examined in the case of cobalt by studying the detection of 1 #g of cobalt in the presence of up to 10 nag of more than 40 elements. The chromofoam test was found to be quite selective. A further advantage of chromofoams is that columns packed with them can be used for the detection and semiquantitative determination of metal ions at the ppM level. This is simply achieved by passing large volumes of the aqueous solution through the reagent foam column at a flow-rate of 1015 ml .cn1-2 .min- 1 and measuring (in comparison with a standard) the length of the coloured zone.
POLYURETHANE FOAMS WITH ANCHORED (BONDED) FUNCI'IONAL GROUPS
The preparation of cellular (foamed) plastics to which specific functional groups are chemically bonded has also been attempted.15.37 Gesser e t al.~ 5 described a method for the preparation of SH-polyur-
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T. BRAUNand A. B. FAV.AG
ethane foam. Columns packed with the SH-foam were evaluated for the adsorption of mercuric chloride and methylmercuric chloride from extremely dilute aqueous solution. It was found that mercury at concentrations ranging between 0.4 and (~0004 ppm can be quantitatively collected on the foam columns when 100ml of the aqueous solution are allowed to pass through them at a flow-rate of about 13 m l . c m - 2 . m i n - ~. At higher mercury concentrations (e.g., 4 ppM) the retention efficiency of the foam columns is decreased because of oversaturation. In general, the effective capacity of the foam columns for mercury was much lower for methylmercuric chloride than for mercuric chloride. This was attributed to the probable steric hindrance effect of the methyl group. The fraction of mercury adsorbed on the foam column was generally increased as the concentration of mercury decreased. The recovery of mercury from the SH-foam has also been investigated, the foam material being extracted with 2M hydrochloric acid in a Soxhlet extractor. On the other hand, Braun et al., a7 during their studies on the preparation of ion-exchange foams, described various methods suitable for the direct and indirect introduction of functional groups (ionogenic groups) in the foamed skeleton structure. They prepared phenol-formaldehyde resin foam to which sulphonic acid groups were chemically bonded by direct sulphonation of a commercially available phenol-formaldehyde foam. It was repOrted that the mechanical properties of the original phenol-formaldehyde foam were not much changed by sulphonation. Also, the ion-exchange capacity of the foams was reasonable (1.85 meq/g). Indirect introduction of the ionogenic groups into the foam material has also been described. Two different methods are used. The first method is based on carrying out a polymer analogue reaction after joining the foam to an easily transformable polymer. Styrene-polyurethane interpolymer foam was prepared by this method and the anion-exchange groups then introduced by chloromethylation and amination. The mechanical properties of the foams depended on the polymerization conditions and the quality of the initiator used. The second method for the indirect introduction of ionogenic groups into the foam matrix was based on the radiation grafting of a monomer with ionogenic groups. The radiation grafting of open-cell polyurethane and closed-cell polyethylene foams with methacrylic acid has been investigated. 37 Foams with excellent properties and good ion-exchange capacities (4 meq/g) have been prepared. In our opinion, the analytical use of foamed plastics with bonded functional groups is a very promising field. Foam materials with very selective properties should be obtained by anchoring functional groups to the foam skeleton, and could become very important if suitable methods could be developed for the preparation of such foams.
HOMOGENEOUS AND HETEROGENEOUS ION-EXCHANGE FOAMS
Homogeneous ion-exchange foams are prepared by 37'~ (/) physical immobilization of liquid ionexchangers on or in flexible polyether-type polyurethane foam, (ii) direct anchoring of ion-exchange groups on previously prepared phenol-formaldehyde foam, (iii) indirect introduction of the ionogenic groups into polyurethane or polyethylene foams. The direct and indirect introduction of the ionogenic groups into the different foams has been described in the previous section. Physical immobilization of a benzene solution of tri-n-octylamine (TNOA), liquid anion-exchanger on polyurethane foam was found 46 to be possible by allowing the foam material to swell in a TNOA-benzene solution. The analytical utility of the foam was tested by investigating the separation of cobalt(II) and nickel(II) in hydrochloric acid media. Columns packed with foam materials containing 11.4 and 17.7% w/w of TNOA in benzene were found46 to be the most suitable for the quantitative retention (and subsequent elution) of cobalt from hydrochloric acid solution. Separation of nickel and cobalt at different relative concentrations was achieved by using 8M and 1M hydrochloric acid for the elution of nickel and cobalt respectively. Plasticized polyurethane foam immobilizing Amberlite LA-1 (liquid anion-exchanger) has also been prepared: 44 The application of this foam material for rapid and selective detection and semiquantitative determination of cobalt(II) in batch and column operations was described above. A method for the preparation of a heterogeneous cation-exchange foam has also been reported. 37 This method is based on foaming a very finely ground powder of a commercially available cation-exchanger (Varion KS) with the precursors of open-cell polyether-type polyurethane foam. The possibility of using this cation-exchange foam for rapid separations in aqueous and alcoholic solutions has been investigated. 4 7 Sorption of metal ions (e.g., Cu 2+) on the cation-exchange foam took place in one rapid step, i.e., gel diffusion was not the rate-controlling step as in common ion-exchange beads. The t~/2 value for equilibrium .8 sorption on the cation-exchange foam, as calculated from the rate-curve for copper(II), was found47 to be 0"6 min. However, the capacity of the cation-exchange foam is much higher than that of surface-sulphonated resin beads. A comparison between the efficiency of cationexchange beads (Varion KS) and the cation-exchange foam for the elution of copper(II) with hydroxylammonium chloride solution shows that flow-rates as high as 3 m l . c m - 2. min- ~ could be applied in the case of foam columns without any considerable loss in column performance, while in the case of bead columns quantitative elution is only obtained at a flow-rate of 1 ml. cm-2. min- 1. The selectivity of the cation-exchange foam was proved to be more or less the same as that of the original cation-exchange beads.
Cellular and foamed plasticsas separation media SPECIALLY TREATED POLYURETHANE FOAMS Bauman et al. 19' so reported that open-cell polyurethane foam can be used as a support for starch gel containing enzymes. They described a method for the preparation of immobilized horse-serum cholinesterase products in which the enzyme in the starch gel is physically entrapped on the surface of open-cell polyurethane foam pads. This immobilized enzyme pad is used to monitor water and air continuously for atmospheric pollutants which are enzymic inhibitors of cholinesterase. Goodson et al. 51 have recently described an improved method for the preparation of polyurethane foam coated with horse-serum cholinesterase. They suggested the adsorption of the horse-serum cholinesterase on aluminium hydroxide gel before the starchgel preparation. Portions of this foam were found 51 to be suitable for the detection of low concentrations of anticholinesterase substances in air, by use of a special cell in which the enzyme activity of the foam pad is observed electrochemically. O n the other hand, Evans et al. 52 examined the possibility of using open-ceU polyurethane foam in the immunoadsorption of cells. Reticulated polyurethane foam of the polyester-type to which the antibody is coupled was found 52 to serve as a matrix for the immunological binding of erythrocytes. REFERENCES
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16. T. Braun and A. B. Farag, ibid., 1974, 69, 85. 17. Idem, in Extraction Chromatography, T. Braun and G. Ghersini, eds. Akad6mia Kiad6, Budapest, 1975. 18. ldem, Radiochem. Radioanal. Letters, 1974, 19, 275. 19. C. R. Thomas, Brit. Plastics, 1965, 552. 20. Proceedings of a Conference on Cellular Plastics, Natick, Massachusetts, 1966, NAS-NRC, Washington, D.C., 1967. 21. T. H. Ferringo, Rigid Plastic Foams. Reinhold, New York, 1963. 22. Modern Plastics Encyclopedia, ed. S. Gross, Vol. 46, p. 248. McGraw-Hill, New York, 1969. 23. C. R. Thomas and H. Briscall, Brit. Plastics, 1967, 79. 24. M. E. Bailey, J. Chem. Educ., 1971, 48, 809. 25. C. J. Benning, Plastic Foams: The Physics and Chemistry of Product Performance and Process Technology, Vol. l, Chemistry and Physics of Foam Formation. Wiley, New York, 1969. 26. H. J. David and H. B. Staley, Analytical Chemistry of the Polyurethanes, Vol. XVI, Part III. Wiley, New York, 1969. 27. H. J. M. Bowen, Radiochem. Radioanal. Letters, 1969, 2, 169. 28. Idem, ibid., 1971, 7, 71. 29. H. D. Gesser, A. B. Spading, A. Chow and C. W. Turner, J. Am. Water Works Assoc., 1973, 65, 220. 30. T. Braun, A. B. Farag and A. Klimes-Szmik, Anal. Chim. Acta, 1973, 64, 71. 31. T. Braun and A. B. Farag, ibid., 1972, 62, 476. 32. E. Glueckauf, Trans. Faraday Soc., 1955, 51, 34. 33. T. Braun and A. B. Farag, Anal. Chim. Acta, 1973, 65, 115. 34. T. Braun, L. Bakos and Zs. Szab6, ibid., 1973, 66, 57. 35. T. Braun and A. B. Farag, ibid., 1973, 65, 139. 36. Idem, Radiochem. Radioanal. Letters, 1974, 19, 377. 37. T. Braun, O. B6keffy, I. Haklits, K. Kfidfir and G. Majoros, Anal. Chim. Acta, 1973, 64, 45. 38. Modern Plastics Encylcopedia, J. Frados and S. Gross, eds., Vol. 45, p. 428. McGraw-Hill, New York, 1968. 39. R. Kosfeld, Plasticization and Plasticized Processes, R. F. Gould, ed., p. 43. American Chemical Society, Washington, D.C., 1956. 40. T. Braun and A. B. Farag, Anal. Chim. Acta, 1974, 71, 133. 41. ldem, ibid., in the press. 42. Idem, J. Radioanal. Chem., 1975, 25, 5. 43. M. Heurtebise and W. J. Ross, Anal. Chem., 1971, 43, 1438. 44. T. Braun and A. B. Farag, Anal. Chim. Acta, 1974, 73, 301. 45. M. Fujimoto and Y. Nakatsukasa, ibid., 1962, 27, 373. 46. T. Braun, E. Husz/tr and L. Bakos, ibid., 1973, 64, 77. 47. T. Braun and A. B. Farag, ibid., 1974, 68, 119. 48~ M. Skafi and K. H. Lieser, Z. Anal. Chem., 1970, 249, 182. 49. E. K. Bauman, L. H. Goodson, G. G. Guilbault and D. N. Kramer, Anal. Chem., 1965, 37, 1378. 50. E. K. Bauman, L. H. Goodson and J. R. Thomson, Anal. Biochem., 1967, 19, 587. 51. L. H. Goodson, W. B. Jacobs and A. W. Davis, ibid., 1973, 51, 362. 52. W. H. Evans, M. G. Mage and E. A. Peterson, J. Immunol., 1969, 102, 899.