Environ. Sci. Technol. 1991, 25, 1722-1727
Ito, S.; Okuda, H.; Katayama, R.; Kunai, A.; Sasaki, K. J . Electrochem. SOC.1988,135, 2996-3000. Czuczwa, J.; Leuenberger, C.; Giger, W. Atrnos. Environ. 1988,22, 907-916. Capel, P. D.; Gunde, R.; Zurcher, F.; Giger, W. Environ. Sci. Technol. 1990, 24, 722-727. Tremp, J.; McDow, S. R.; Leuenberger, C.; Czuczwa, J.; Giger, W. Mitt. Naturforsch. Ges. Luzern 1988,30,111-133.
(34) Leuenberger, C.; Czuczwa, J.; Tremp, J.; Giger, W. Chemosphere 1988, 17, 511-515. (35) Sundarajan, S.; Wehry, E. L. J. Chem. SOC.,Chern. Commun. 1970, 38, 267-268.
Received for review August 7,1990. Revised manuscript received April 15, 1991. Accepted manuscript May 31, 1991.
Fluidized-Bed Solid-Phase Extraction: A Novel Approach to Time-Integrated Sampling of Trace Metals in Surface Watert Johannes W. Hofstraat" Tidal Waters Division, Ministry of Transport and Public Works, P.O. Box 20907, NL-2500 EX The Hague, The Netherlands
John A. Tlelroolj and Hajo Compaan Department of Analytical Chemistry, Division of Technology for Society, Netherlands Organization of Applied Scientific Research TNO, P.O. Box 217, NL-2600 AE Delft, The Netherlands
Wlm H. Mulder Institute for Inland Water Management and Wastewater Treatment, Ministry of Transport and Public Works, P.O. Box 17, NL-8200 AA Lelystad, The Netherlands
A new technique for solid-phase preconcentration of contaminants dissolved in surface waters is presented. The method is based on application of extraction material that is not packed, as in conventional solid-phase extraction systems, but instead is present in a freely floating form, as in a fluidized-bed reactor. The feasibility of the fluidized-bed extraction approach is demonstrated for the determination of heavy metals in surface waters using 8hydroxyquinoline attached to solid supports as complexing agent. Recoveries, repeatibility, and sensitivity appear satisfactory for this application, even when no filtration of the sample is done. As fluidized-bed extraction is based on free-floating, unpacked, extraction material, the pressure drop over the column is minimal and filtration is not required. Hence the technique seems eminently suited for deployment as an in situ, long-term, sampling method. As such it will provide time-integrated contamination levels that are not biased by biological variability or filtration artifacts, disadvantages of the commonly used methods for monitoring of contaminants in surface water. Introduction
Determination of dissolved contaminants is an important requirement for the assessment of the quality of both salty and fresh surface waters. The dissolved contaminants form the major part of the contaminants that are available for uptake by many aquatic biota. Even though the dissolved concentrations of environmental pollutants are extremely low (in particular for apolar organic compounds that have very low aqueous solubility), they can be harmful as a result of bioaccumulation. Data on concentrations of dissolved contaminants also provide information that is vital for the assessment of the effectiveness of regulations aimed at the reduction of the input of pollutants into the aqueous environment. 'Presented at the XVIIth FACSS Meeting, Cleveland, OH, October 1990. *Author to whom correspondence should be addressed: AKZO Research Laboratories, CRL, P.O. Box 9300, NL-6800 SB Arnhem, The Netherlands. 1722
Environ. Sci. Technol., Voi. 25, No. 10, 1991
The very low concentration of dissolved trace materials in surface waters, for heavy metals in the ppb range (I), and for organic micropollutants even down to the sub-ppt region ( 1 , 2 )requires the application of lengthy and time consuming analytical procedures. Vital steps in these procedures are preconcentration of the components of interest and the removal of sample constituents that interfere with the determination. For the determination of heavy metals in seawater in general the-timeconsuming-method described by Danielsson et al. is applied (3). The procedure consists of extraction of the metals from the seawater into an organic phase by application of dithiocarbamate as complexing agent. In this way the very abundant alkali and alkaline earth elements, which are not complexed and interfere with the subsequent determination, are effectively removed. Subsequently, the organic phase is acidified and the heavy metal ions are extracted back into an aqueous phase. Apart from cleanup this procedure also results in a preconcentration of the complexed ions by a factor of -50. For determination of trace organics even more rigorous extraction procedures have to be applied ( 2 , 4 , 5 ) .Apart from being laborious, such concentration methods have a significant risk of loss of compounds and of possible contamination. Alternatively, use can be made of solid-phase extraction of the analytes from the aqueous solution. In this case, a column filled with a specific adsorbent that only retains the materials of interest is used to bring about preconcentration and cleanup of the sample prior to analysis. For preconcentration of organics mostly apolar adsorbents, such as resins or silica spheres coated with alkyl chains, are used (2, 5 , 6). For heavy metals complex-forming agents attached to a variety of solid supports have been applied (7,8). As the extraction procedures, solid-phase techniques are most suitable for use in the laboratory. Furthermore, large sample volumes have to be used to obtain sufficiently high concentrations of the analytes in the final extract. The large volumes, which can be in the order of 10 L for determination of organic compounds, are unfavorable from a logistic point of view. An elegant alternative approach is to use biota for preconcentration in situ. In particular, mussels have been
0013-936X/91/0925-1722$02.50/0
0 1991 American Chemical Society
propagated for this purpose (9). The “mussel watchn is applied by the Tidal Waters Division to monitor concentrations of contaminants in the Dutch coastal waters and seawaters (10). Mussels are transplanted from a relatively uncontaminated location to various stations, where they are exposed for 6 weeks. After this period the mussels are collected, pooled, and homogenized. Subsequently, the homogenates are freeze-dried and extracted. Organic and inorganic pollutants in the extracts are determined, after extensive cleanup. An important advantage of the mussel watch approach is that concentrations of contaminants are enlarged via bioaccumulation, so that the accuracy and the precision of the determination are improved. Furthermore, in contrast with conventional manual sampling methods, a time-averaged contamination level is determined, where all short-term concentration fluctuations have been removed. On the negative side, the sample pretreatment and cleanup are even more time consuming than described above for aqueous samples, and use is made of biota with inherent biological variability. A more detailed evaluation of the mussel watch in contrast with the novel approach introduced in this paper is given below. The sampling method put forward in this paper presents time-integrated sampling via preconcentration and thus provides all the advantages of the mussel watch. On the other hand, it does not have the inherent disadvantages of the latter approach: use is made of physical principles (so no biological variability), and sample pretreatment and cleanup are very simple. An instrument for in situ sampling of trace metals was described by Johnson et al., who introduced a complete solid-phase extraction system into the water (11). The system consisted of a column filled with the complexing agent 8-hydroxyquinoline attached to silica particles. Seawater was filtered prior to extraction, to prevent blockage of the column. Due to the large pressure drop over the sampler and the limited capacity of the filter, application for long time intervals with this instrument is not feasible. Moreover, filtration prior to the extraction column is unwanted as it can result in a significant change in the sample composition. In particular, for trace organics a major part of the analytes is lost due to adsorpt,ion to the filter (2, 12). In the fluidized-bed-type extractor presented here no packed column is used: the complexing agent is attached to particles with a higher specific mass than water, which are allowed to float freely. When water is pumped through the extractor, the particles will behave in a manner similar to that in a fluidized bed, an approach often used in industry for heterogenic processes. As the extraction material is not packed, the pressure drop over the extractor is small, so that a low-power pump can be employed to lead the water through the adsorbent. In addition, no filtration of the sample is required: suspended materials will pass through the extractor with the water, without causing blockage of the extraction column. Hence, the sampling approach proposed in this paper offers the possibility of long-term and autonomous in situ sampling and preconcentration of contaminants. It opens a gate to highly accurate and useful data on dissolved contaminants in surface waters, data that are vital for environmental monitoring programs. Also, this approach may offer new insights into the speciation of contaminants in the aqueous environment and their availability to biotic species. The principles of this novel solid-phase extraction method were investigated by studying the example of the determination of metals in seawater and in freshwater using 8-hydroxyquinoline as complexing agent. This material is well-characterized and forms complexes with more
Table I. Column Materials
material
abbrev
supplier
controlled pore glass 8-HQ silica-immobilized 8-HQ
CPG-8HQ Pierce, IL Si-8HQ Serva, Heidelberg (FRG) Spheron oxine 1000 SO-1000 Lachema, Brno (CS) Kelex 100, adsorbed on XAD-2 XAD-8HQ prepared as in ref 15
waste
4/ 1
joining
‘\
Flgure 1. Schematic of the fluidized-bed extractor.
than 60 metal ions with high aqueous-phase formation constants (8). For concentration of heavy metal ions from seawater it is advantageous, as the complex formation constants for the alkali and alkaline earth elements are very much lower than for the transition metals one wants to determine. The experiments described in this paper were done under laboratory conditions. At a later stage a field trial will be undertaken. Also, attention will be paid to the development of the fluidized-bed extraction approach for sampling of trace organics.
Experimental Section Several column materials, all using 8-hydroxyquinoline (8-HQ) as complexing agent, have been tested for their suitability as sorbent in a fluidized-bed extractor (see Table I). Polypropylene fluidized-bed extractors were loaded with 0.4-2.4 g of sorbent, which is amply sufficient to concentrate heavy metals from 1L of surface water. The extractor columns were extensively cleaned by prolonged rinsing with an acidic solution prior to their use; an extractor could be used several times for preconcentration of metals without reduction of performance. A diagrammatic representation of the fluidized-bed extraction setup is shown in Figure 1. Coastal seawater and freshwater (from the river Rhine) were pumped through the extractor with a Technicon Proportioning I1 peristaltic pump, placed after the extractor. Pumping rates varied between 0.05 and 0.42 mL/min. Volumes of water varying from 0.2 to 1L were preconcentrated, resulting in expositions of the extraction columns during periods of time from a few hours up to a maximum of 2 weeks. Experiments were done with filtered and unfiltered surface water from the river Rhine and from a coastal location near the eastern Scheldt. No acidification or buffering of the samples was applied. Complexed metals were eluted with -25 mL of a solution containing Suprapur (Merck, Darmstadt, FRG), 2 M HC1, and 0.1 M Environ. Sci. Technol., Vol. 25, No. 10, 1991
1723
"03. Hence, a preconcentration factor of 8-40 was realized. This factor can be enlarged by an increase of the volume of water that is preconcentrated or by reduction of the eluent volume. The latter requires a change of the present column design, which has a relatively large dead volume. For the next deployment, the adsorbent is cleaned by washing the column with 25 mL of Suprapur, 3 M HC1, Then the extractor is brought to pH and 0.2 M "0,. 8 by washing with 25 mL of 0.1 M ammonium acetate. For comparison, samples were also analyzed with the liquid-liquid extraction method described by Danielsson (3). For extraction, a mixture of ammonium pyrrolidinedithiocarbamate (APDC) and diethyldithiocarbamate (DDC) in Freon TF was applied. Analyses were done on a Perkin-Elmer 5100 (Norwalk, CT) graphite furnace atomic absorption spectrometer with Zeeman background correction.
Results and Discussion The application of the extraction material in a fluidized-bed-like manner requires that the particles are dispersed and freely floating in the water. Development of channels in the extraction agent, e.g., as a result of coagulation or insufficient dispersion, results in suboptimal contact with the water phase and hence in low recoveries and unreproducible results. The requirement of freely floating particles, of course, not only imposes restrictions on the column material, but also requires sufficient linear velocity of the water in the sample container. The dimensions of the column were kept small to provide the linear velocities required for a fluidized bed and at the same time allow for the relatively low flow rates that are needed for prolonged sampling in a laboratory situation. For the feasibility study described in this paper, the quality of the fluidized bed was evaluated mainly by visual observation. For the fluidized-bed material that was found to be the most suitable on the basis of the comparative study described in the first section below, the performance has been studied more thoroughly. After further optimization of the fluidized-bed column, its application has been demonstrated for both seawater and freshwater (Rhine). Of course, for application of the technique in the field, attention has to be paid to the design and construction of an upscaled version of the fluidized-bed extractor used for the laboratory research. Selection of Extraction Material. Several extraction materials have been investigated in order to select the most optimal packing for the fluidized-bed column. Criteria that have been considered are the suitability of the material for application as a fluidized-bed extraction agent, the capacity of the adsorbent, the efficiency of the extraction, and the long-term stability of the extractor. These factors were studied for the four materials summarized in Table I, using the heavy metals Cd, Cu, and P b as model substances. In the first experiments, it appeared that it was not possible to get a properly working fluidized-bed-type extractor using SO-1000 as adsorbent. The very apolar particles with low specific mass were not dispersed in the water, but floated in the extractor, so that not very efficient contact could be achieved. The capacities of the three remaining materials were determined for a solution of Cd in seawater; they were 28,18, and 5 ,umol/g for XAD-8HQ, Si-8HQ and CPG-8HQ, respectively. Dissolved heavy metals are present in seawater at concentrations in the order of 0.1 pmol/L, so that sufficient capacity can be achieved by using 0.5-1 g of adsorbent. Next, elution curves were determined to ensure quantitative removal of the metals from the extractor. Figure 1724
Environ. Sci. Technol., Vol. 25, No. 10, 1991
600 500 -
400 300-
zoo 100-
0
1
2
4
8
6
12
10
14
16
Flgure 2. Elution profile of Cd from a fluidized-bed-type extractor.
Table 11. Cd, Cu, and Pb Content" of Filtered Seawater as Determined by Fluidized-Bed Extraction
cu XAD-8HQ Si-8HQ CPG-8HQ conventional method
1.05 ( f O . l ) 0.52 (f0.21) 1.5 (f0.13) 1.55
Cd
Pb
0.12 (3tO.02) 0.19 (f0.04) 0.06 (f0.03) 0.09 (f0.04) 0.12 (3tO.008) 0.20 (f0.04)
0.13
0.22
In micrograms per liter.
2 shows an elution curve obtained for an extractor fully loaded with Cd. The Cd is found to be quantitatively eluted when 12 mL of an acidic solution ( 2 M HCl and 0.1 M "0,) is applied. The efficiency of the preconcentration with a fluidized-bed extractor was determined with 600 mL of coastal seawater, filtered over an 0.45-pm Millipore filter. The flow was 0.2 mL/min. The heavy metal content of the seawater was also determined in batch by the conventional extraction method of Danielsson et al. (3). The results are presented in Table 11. Si-8HQ clearly is the least suitable material of the three: the efficiency (vs the conventional extraction procedure) is less than 50%. Visual inspection of the fluidized bed shows that after some time in the Si-8HQ material small channels are formed, so that the extraction becomes less efficient. The other two materials show high efficiencies. The CPG-8HQ appears the most optimal substance on the basis of its better repeatibility (see Table 11). Finally, the long-term stability of the extraction materials has been investigated. After 6 weeks of intensive use, the capacity was determined for all three materials. No significant loss of extraction capacity was found. This prestudy indicated that efficient concentration of dissolved heavy metals from aqueous solution could be achieved by application of a fluidized-bed extractor. Further studies were performed using CPG-8HQ as extraction material. Applications on seawater and freshwater were studied. Further Investigation of CPG-8HQ. The application of fluidized-bed extraction was further investigated for CPGBHQ. Cd, Cu, Fe, Ni, Mn, Pb, Zn, and Cr have been reported to form highly stable complexes with 8-HQ. For
Table 111. Blanks and Limits of Determination (L0D)O for Preconcentratiun of a Volume of 1 L
blank LOD
Cd
Cu
Fe
Ni
Mn
Pb
Zn
0.005 0.002
