ANALYST, JANUARY 1987, VOL. 112 41 Determination of Uranium(V1) in Process Liquors by Ion Chromatography John J. Byerley and Jeno M. Scharer Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario N2L 3G 7, Canada and George F. Atkinson Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G I , Canada Two methods are described for the determination of Uvl in process liquors using ion chromatography based on cation separation and cation - anion separation with ammonium sulphate - sulphuric acid as the eluent. The UV1 species is detected spectrophotometrically at 520 nm after post-column reaction with 4-(2- pyridy1azo)resorcinol. Chromatographic and detector variables, such as eluent composition and concentra- tion, metallochromic indicator concentration and eluent and indicator flow-rates, are discussed.The method is linear for peak heights up to 15 pg ml-1 and has a quantitation limit of 0.04 pg ml-1 using direct injection. Keywords: Uranium(V1) determination; ion chromatography; 4-(2-pyridylazo)resorcinoI; process liquors; mixed-mode column An important requirement of the hydrometallurgical industry is the rapid, inexpensive and reliable determination of metallic species in process liquors and mill effluents. These analytical requirements have generally been met using a number of instrumental spectroscopic techniques, including flame emis- sion spectrometry (FES) , atomic absorption spectrometry (AAS) and atomic emission spectrometry (AES). When these and other related methods are combined with various excita- tion sources and aqueous solution handling procedures, at least 75 elements can be determined.It is to be expected that with this broad range of applicability particular examples will arise where an alternative technique should be considered. Uranium and certain other metals that form refractory oxides in a flame are difficult to determine by atomic absorption spectrometry. This problem is not serious when atomic emission spectrometry is applied in conjunction with direct and inductively coupled plasma excitation sources. Satisfac- tory determination of aqueous uranium in the pg ml-1 range is possible. Lynch et al. 1 have reported a flow injection method involving solvent extraction and adsorptiometric determina- tion of uranium in leachates and effluents which gave a detection limit of 0.1 pg ml-1.Neutron activation analysis is a popular non-spectroscopic method for the determination of uranium in aqueous solution; however, the availability of a neutron source and the need for a preliminary de-watering step are important considerations. The detection limit claimed by commercial suppliers for this method using standard procedures is 0.01 pg ml-1. Our research has been directed towards studying the absorption - desorption characteristics of aqueous metallic species in contact with various microorganisms.2 Of particular interest has been the behaviour of the uranium(V1) present in acid process leach liquor when in contact with biomass. The process liquor contains uranium(V1) at levels of the order of 200 pg ml-1 and substantial levels of other species, particularly iron(II1).Metals such as thorium, zinc, copper, nickel and cobalt are also present in the liquor, which contains about 2.5 g 1-1 of sulphate. In the absorption - desorption studies of uranium(V1) in contact with biomass, both equilibrium and kinetic experi- ments are required. The large number of samples taken in these experiments were initially analysed for uranium(V1) using neutron activation analysis. However, reproducibility problems and the absence of a convenient neutron source prompted a search for an alternative method. Recently a number of reportss5 have indicated that ion chromatography can be applied to the determination of many transition metal species in aqueous solution. The method relies on the use of low-capacity cation- and anion-exchange materials.Cation- exchange separations are obtained using a surface-sulpho- nated microporous polystyrene - divinylbenzene resin. Anion- exchange separations are accomplished using the surface- sulphonated resin coated with aminated latex particles. This exchanger does exhibit a residual cation-exchange capacity. Both exchangers are reported to offer a high efficiency and excellent pH stability. A wide range of selectivity for transition metals can be achieved by a variation in particle size, functional groups, degree of latex cross-linking and, most importantly, by the use of both neutral and anionic complex- ing agents in the eluent. The determination of UVI as U022+ by ion chromatography was reported by Riviello using a sulphate eluent .6,7 This paper describes the application of ion chromatography, post-column derivatisation and spectropho- tometric detections.9 for the determination of UVI in process leach liquors.Experimental All ion chromatographic analyses reported were carried out with a Dionex Series 2010i ion chromatograph equipped with a Dionex Ionpac membrane reactor for post-column derivatisa- tion. Detection was at 520 nm using a Cary 219 UV-visible spectrophotometer fitted with a 10 mm (8 p1) flow cell. Two chromatographic columns were employed for the Um deter- minations, an HPIC-CS2 cation separator and HPIC-CSS cation - anion separator (Dionex). Both separators were operated with the appropriate guard columns. Eluent reagents, ammonium sulphate and sulphuric acid were of analytical-reagent grade and were obtained from J.T.Baker Chemical Company. The metallochromic indicator, 4-(2- pyridy1azo)resorcinol (PAR) , was supplied as the mono- sodium salt monohydrate by Aldrich Chemical, Milwaukee, WI, USA. The indicator was supplied to the membrane reactor from a nitrogen pressurised reservoir. Other com- ponents of the indicator solution, ammonia solution and acetic acid were of analytical-reagent grade (J. T. Baker Chemical Company). Synthetic uranium solutions used for this study were prepared from analytical-reagent grade uranyl nitrate (BDH, Canada). A large carboy of biologically produced uranium process liquor was obtained from Dennison Mines, Elliott Lake, Ontario, Canada. The analysis of this bulk sample for UVI was carried out by Dennison Mines.The reported Uw concentration was confirmed by a commercial laboratory using neutron activation. The addition of other42 ANALYST, JANUARY 1987, VOL. 112 metal species to synthetic uranium solutions was carried out using atomic absorption standards. The metal content of process liquors and synthetic solutions was determined using atomic absorption spectrometry. Results and Discussion Typical chromatograms of synthetic solutions containing 2.0 and 1.0 pg ml-1 of UVI as U022+ are shown in Fig. 1. These chromatograms were obtained using an anion - cation mixed- mode separator (Dionex HPIC-CS5) and a cation separator (Dionex HPIC-CS2) followed by post-column derivatisation and spectrophotometric detection.The chromatogram for the CS5 column exhibits an early peak (above and below the absorption base line), which results from water in the sample and unretained ionic species eluting in the void volume. This early dip has been eliminated in the chromatogram for the CS2 column by matching the sample matrix with the eluent. This was found to be good practice whenever possible, but is especially significant when separating ions at low levels. In comparing the two columns, it was observed that the cation separator produced a UO22+ response that was 30% higher than the anion-cation separator under the same chromatographic conditions. Table 1 summarises the con- ditions used for operating both columns and the post-column detection system. These conditions are similar to those reported by Riviello6.7 for the cation separator and were routinely used for U V I determination.Alternative conditions were often used for certain analytical requirements; the effects of these variations on column performance serve to charac- terise the columns. 300 200 100 0300 200 100 0 Timeis I 1 I I I I I I I 200 100 0200 100 0 Time/s Fig. 1. Typical chromatograms for synthetic UVI solutions, 1.0 and 2.0 pg ml-I. (a) CS5 column and ( b ) CS2 column. Conditions as in Table 1 Table 1. Operating conditions for cation (Dionex HPIC-CS2) and cation - anion (Dionex HPIC-CS5) columns Eluent . . . . . . . . Eluentflow-rate . . . . Column pressure drop . . Metallochromic indicator Indicatorflow-rate . . . . Samplematrix . . . . Samplevolume . . . . Absorptionscale .. . . Wavelength . . . . . . 0.02 M (NH4)2S04 and 0.20 M H2SO4 1 .O ml min-1 720-740 lb in-2 = 49&5100 kPa (CS5) 48&510 lb in-2 = 3310-3520 kPa (CS2) 4 x 10-4 M 4-(2-pyridylazo)resorcinol 3.0 M NH3 solution 1 .O M CH,COOH 0.4 ml min-1 Variable 50 p1 0.20 absorbance units full scale (a.u. f.s.) 520 nm Table 2 summarises the column performance for the determination of UvI employing both cation and cation - anion separation modes. Peak heights were normally used as an indication of UVI concentration. Fig. 2 shows chromatograms obtained by direct sample injections of 5.0, 2.0 and 1.0 pg ml-1 UVI standards using the cation - anion separator column (CS5) under standard operating conditions. Similar performance was obtained using the cation separator column (CS2).Both columns produced chromatograms that verify the linearity of the chromato- graphy up to 10 pg ml-1. An extension of the linear range was possible with an adjustment in operating conditions. An increase in the metallochromic indicator flow-rate was effective in increasing the range, but a much higher base-line variation was observed. A decrease in the eluent flow-rate at constant indicator flow-rate extended the linear range to 15 pg ml-1 with no adverse effect on the base-line stability, but the peak heights were slightly reduced. As noted in Table 2 the retention times observed for the two separation modes were somewhat different. For the cation column separation, which shows a shorter retention time (125 s), it is possible that the complex formed with U022+ in the eluent stream dissociates on the column and moves through as a simple aquated species.If the cation - anion column is used, it is possible that the anionic uranyl sulphate species present undergo anion exchange and that the corre- sponding aquated species are retained on the substrate resin by cation exchange. The dual functionality of this column would be expected to provide a superior selectivity for transition metals and may be the reason for the longer retention times. The performance of the system was investigated using routine techniques and minor alterations in the column and detection conditions. As noted in Table 2, the quantitation limit is estimated to be 0.090-0.120 pg ml-1 using modest spectrophotometric sensitivity. The limit of detection is lower than the quantitation limit.An increase in injection volume from 50 to 100 pl reduced the detection limit proportionally. Larger injection volumes may be used in some instances, but when process samples containing high concentrations of other ionic species that are readily retained by the column are injected, the intervals between sample injection must be greatly increased to avoid interference from these slow eluting species. An improvement in the quantitation limit is more readily achieved by increasing the sensitivity of the detecting instru- ment and reducing the base-line variation. In the case of the cation separator, using a full-scale absorbance range of 0.02 and a reduced metallochromic indicator concentration (1 x 10-4 M), with all other conditions remaining the same, the direct injection of a standard containing 0.10 pg ml-1 of UVI yielded a peak height of about 11.5% of the full scale and a base-line variation of 2%.Fig. 3 shows the chromatograms obtained for three standard UVI solutions containing 0.20, ~ ~ Table 2. Column performance. Operating conditions as in Table 1 c s 2 Retentiontime* . . . . 125s Linearrange . . . . 610pgml 1 Peak height . . . . 13% full scale at 1 .O pg ml- (0.20 a.u.f.s.) Quantitation limit (direct injection) . . 0.090 pg ml-1 Base-line variation . . 0.4% of full scale (0.20 a.u.f.s.) Signaltonoiseratio . . 3 : 1 c s 5 225 s <10pgml-l 1Ooh full scale at 1 .O pg ml-1 (0.20 a.u.f.s.) 0.120 pg ml- 0.4% of full scale (0.20 a.u.f.s.) 3 : 1 * Retention time reported as the time from sample injection to elution peak.ANALYST, JANUARY 1987, VOL.112 43 300 200 100 0300200 100 0 200 100 0 Timeis Fig. 2. Linearity verification, CS5 column. Synthetic Uvl solutions, 5.0, 2.0 and 1.0 pg ml-l; conditions as in Table 1 200 100 0 100 0 100 0 200 100 0 Time/s Fig. 4. Evidence of reproducibility of CS2 column. Synthetic UVI solution, 1.10 pg ml-l; conditions as in Table 1 I To.01 AU I I I I Y I Timeis Fig. 5. conditions as in Table 1 Uranium process liquor chromatogram using CS5 column; 200 100 0200 100 0 200 100 0 Ti m e/s Fig. 3. Low-level chromatograms using CS2 column. Synthetic Um solutions, 0.20, 0.10 and 0.05 pg ml-1; reagent, 1 x 10-4 M; other conditions as in Table 1 0.10 and 0.05 pg ml-1 of UVI using the above conditions. The quantitation limit if a signal to noise ratio of 3 : 1 is assumed is about 0.05 pg ml-1.It was observed that if the (NH&S04 concentration in the eluent was increased to 0.08 M the peak height was enhanced by about 25% with no effect on the base-line variation but with the retention time reduced to about 100 s. Under these conditions the quantitation limit may be decreased to 0.4 pg ml-1. The performance of the system using the cation - anion separator was found to be similar to this, but the reduced peak height response for this column resulted in a proportionately higher quantitation limit. The quantitation and detection limits of both columns could be greatly improved by the substitution of a concentrator column in place of the direct injection sample loop.When analysing samples containing UVI concentrations below about 0.50 pg ml-1 it was necessary to be particularly careful in flushing the entire system, otherwise sample to sample contamination resulted in memory effects. It was generally observed that the optimum chromatographic perfor- mance was achieved when the sample matrix was matched to the eluent. This was particularly true in the determination of low level samples. If the sample matrix is perfectly matched (which is not always possible) to the eluent, the water dip is eliminated. Both the cation and cation - anion separator modes showed excellent reproducibility. Fig. 4 shows the chromatograms of four successive injections of a standard 1.1 pg ml-1 Uvl sample using the cation separator column. Taking the peak heights of the four peaks shown and a further four injections, the digitally recorded data gave a mean peak height absor- bance above base line of 0.0283 and a standard deviation of 0.0005. The matrix of the sample used in this series was matched to the eluent, although minor mismatch was found to have little influence on reproducibility.The delivery of a sufficient and constant supply of derivati- sation reagent (PAR) to the post-column reactor was an important factor in achieving a linear response, acceptable reproducibility and minimum base-line variation. This did not normally present a problem. Some consideration was given to the question of the kinetics of colour development. Experi- ments conducted independently of the flow system showed that complete derivatisation occurred in less than 2 s.In our flow system, the time interval from the post-column contact of reagent and eluted UVI to spectrophotometric detection was about 2 s. Increasing this time interval by 50% had no detectable effect on the analytical results if the tendency for peak broadening was ignored. It should be noted that this behaviour cannot be assumed for all metal species where post-column derivatisation and downstream spectrophoto- metric detection is employed. Work being carried out by the authors indicates that the detection of ThIV by spectro- photometry using PAR requires a residence time of more than 40 s for full colour development.10 Synthetic samples containing UVI (2.0 pg ml-1) and Ni", Co", CuII, ZnII, FeII (10.0 pg ml-I), FeIII (20.0 pg ml-1) and ThIV (2.0 pg ml-I), both individually and in combination, were prepared in sulphate solution.Passing through the cation anion separator the bivalent metals appeared as one peak with a retention time of about 550 s. The elution of FeI" did not begin until 800 s and resulted in a very broad peak with an estimated retention time of 1130 s. Chromatograms of samples containing UVI and each metal in turn verified the makeup of the bivalent metal peak. Retention times varied from 530 s for Zn" to 565 s for CoII. As expected, FeIII was strongly retained and was eluted at the same time as observed when present in the composite sample. The presence of ThIV was not observed on any of the chromatograms. This is probably because it was not suf- ficiently retained under the chromatographic conditions, or because the conditions for derivatisation were not appropriate for its detection.The reproducibility and linearity of the Uvl peak heights after many injections of composite samples was comparable to the performance observed for pure UVI samples, indicating minimal impurity accumulation and column capacity loss. An acid leach uranium process liquor (pH 2.3) was reported by Dennison Mines to contain 160 pg ml-1 of UVI, 1120 pg ml-1 of FeIII, 9.0 pg ml-1 of Zn", 2.6 pg ml-1 of Cu", 2.3 pg ml-1 of CO" and 2.0 pg ml-1 of Ni", together with unreported amounts of aluminium, calcium, magnesium and thorium. The UVI concentration was confirmed using neutron activation analysis by NAS, Hamilton, Ontario. This liquor was diluted 100-fold with the eluent and injected repeatedly into the system. Fig.5 shows the chromatogram obtained for the sample of process liquor. This chromatogram was similar to that obtained for the synthetic composite sample. All the peaks were reproducible and there was no indication of ionic accumulation. The presence of large amounts of Fe"1 in uranium process liquors requires an interval at least 25 min between injections44 ANALYST, JANUARY 1987, VOL. 112 700 600 500 400 300 200 I 0 Ti meis Fig. 6. Uranium process liquor chromatogram using CS5 column. Eluent, 0.08 M (NH4)+S04, 0.2 M H,SO,; other conditions as in Table 1 to ensure the complete elution of Fe"'. The Fe"' could be rapidly eluted by using oxalate, although re-establishing equilibrium with the original eluent would then need to precede a further determination.As a reasonable compro- mise, if the (NH4)*S04 level was increased to 0.08 M, the elution of Fe"1 was complete within 12 min. Fig. 6 shows the chromatogram obtained. The use of ion chromatography for the determination of Uvl in a wide range of aqueous solutions, both synthetic and industrial, is now routine practice in our laboratory. Over 500 samples have been analysed using the system described with no apparent loss of column response. Biological processes related to the sample create no observable problems in the procedure. The method deserves consideration for the deter- mination of Uvl in the low pg ml-1 range and could be used successfully in the ng ml-1 range with pre-concentration. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. The support of the National Sciences and Engineering Research Council of Canada and the Department of Supply and Services of Canada (Contract OSQ84-00277) is gratefully acknowledged. References Lynch, T. P., Taylor, A. F., and Wilson, J. N., Analyst, 1983, 108,470. Byerley, J. J., Scharer, J. M., and Charles, A. M., "Evaluation of Biomass for UVI Recovery from Process Streams," Govern- ment of Canada Department of Supply and Services, Ottawa, No. OSQ84-00277, May 1985 pp. 1-178. Pohl, C. A., and Riviello, J. M., Paper No. 108, 24th Rocky Mountain Conference, Denver, CO, August 1982. Herberling, S. S., and Riviello, J . M., 27th Rocky Mountain Conference, Denver, CO, July 1985. Riviello, J. M., in Naden, D., and Streat, M., Editors, "Ion Exchange Technology," Society for Chemical Industry, Lon- don; Ellis Horwood, Chichester, 1984, pp. 584-594. Riviello, J. M., personal communication, 1985. Application Note No. 48, Dionex, Sunnyvale, CA, August 1983. Fritz, J . S., and Story, J. N., Anal. Chem., 1974,46, 825. Fritz, J. S . , and Story, J . N., Talanta, 1974, 21, 892. Byerley, J. J., Atkinson, G. F., andTrang, C. V., unpublished work. Paper A6190 Received March 17th, 1986 Accepted July 31st, 1986