Environ. Sci. Technol. 1997, 31, 1794-1800
Applicability of Capillary Electrophoresis with Amperometric Detection to Study Degradation of Chlorophenols in Contaminated Soil ABDELKADER HILMI, JOHN H. T. LUONG,* AND AN-LAC NGUYEN Biotechnology Research Institute, National Research Council Canada, Montreal, Quebec, Canada H4P 2R2
Details are presented for assembling an amperometric detector compatible with commercial capillary electrophoresis (CE) instruments. The detector assembled with a gold electrode achieves a sufficient sensitivity in analyzing liquid extracts to enable determination of pentachlorophenol (PCP) in soils at 35 mg/kg (PCP/soil). The CE system equipped with amperometric detection is used to study the oxidation of chlorophenols with ceric sulfate. The selected chlorophenols are completely oxidized by ceric sulfate in 5 min and the ceric sulfate can be recovered and reused. The same CE system is also applied for monitoring the photodegradation of PCP in contaminated soil, in the presence of titanium dioxide. Complete degradation of PCP, with no trace of electroactive products, is achieved in 7 h. This study demonstrates that capillary electrophoresis can be an important technique for monitoring processes of degradation and bioremediation.
Introduction Chlorophenols, being widely used as herbicides, pesticides, and wood preservatives, constitute a major class of pollutants that contaminate the ecosystem extensively as well as accumulate in the food chain (1, 2). Since air stripping and carbon adsorption, as recommended by the U.S. Environmental Protection Agency (U.S. EPA), only remove chlorophenols from aqueous solutions but do not destroy them, mineralization procedures have been extensively investigated in recent years (3). Photodegradation catalyzed by titanium oxide is very effective in the mineralization of chlorophenols as well as many other organic pollutants, and a large-scale photocatalytic plant using solar light has been operated for several years (4). In parallel, the simplicity of photodecomposition aided by potent oxidizing agents such as hydrogen peroxide, perchlorate, etc. has also sustained a significant interest (5, 6). High-performance liquid chromatography (HPLC) has been the standard assay for chlorophenols (4-6), although gas chromatography (GC) was also used (7). For pentachlorophenol (PCP) monitoring, GC is the usual choice (8, 9), but this analyte exhibits poor chromatographic characteristics due to its acidity and requires derivatization due to its low vapor pressure. Recently, capillary electrophoresis (CE) was established as one of the most efficient methods for separation of charged components in mixtures, with resolution better than conventional GC or HPLC (10). Generally it does not use organic solvents and only requires * Corresponding autho: phone 514-496-6174; fax: 514-496-6265; e-mail:
[email protected].
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 6, 1997
very a small sample volume (20 µL), CE has been proven very effective for analyzing a multitude of analytes (11), and each analysis only consumes about 5 nL of the sample. CE systems are usually equipped with an UV/VIS detector, and a concentration detection limit of 10-8-10-5 M is often cited for this type of detector. However, the lower detection limit is only achievable for highly chromophoric analytes with high molecular mass (6-10 kDa). In practice, the detection limit for small molecules is about 1 ppm. Sensitivity could be improved with sample stacking and an extended light path (bubble cell). Both of these techniques, in addition to a (very complex) matrix removal manipulation, have enabled a PCP detection at 5-10 ppb in the injected sample. Coupled with off-line sample concentration, such a procedure permitted detection in parts per trillion (12). In another direction, electrochemical detectors (promising high sensitivity, simplicity, and low cost) have been devised and coupled with CE to detect carbohydrates, amino acids, and electroactive neurotransmitters such as dopamine and catecholamine (13, 14). This paper describes the components of an amperometric detector that has been demonstrated to achieve 10 ppb detection of chlorophenols in our laboratory (15). The CE system with amperometric detection was used to analyze extracts from chlorophenol-contaminated soils and to follow the performance of some treatment processes. Various aspects of a degradation procedure using ceric sulfate as an oxidizer were monitored in detail, and samples from a typical photodegradation process catalyzed by titanium dioxide were also investigated to demonstrate the applicability of CE equipped with amperometric detection.
Experimental Section Materials. Gold wire (0.127 mm diameter), silver wire (0.5 mm diameter), ceric sulfate solution (in 1 N sulfuric acid), TiO2, and all chemicals were products of Aldrich (Milwaukee, WI). Preparation of Detecting Electrodes. The amperometric detection cell required a working electrode prepared with gold wire. A 3-cm gold wire (0.127 mm diameter) was inserted into a glass capillary (1.2 mm i.d., commonly used for melting point determination, 1 cm long) with the wire recessing about 2 mm from one end of the capillary. This capillary end was heated with a Bunsen burner flame to seal the glass around the wire. The capillary end was then polished on fine silicon carbide abrasive paper aided by an alumina slurry (CF-1050, Bioanalytical System (BAS), West Lafayette, IN). Inspection was done with a microscope to retain only those having a circular gold cross section well sealed with the glass capillary. A piece of 0.5-mm silver wire was attached to the other end of the small wire so that the 0.5-mm wire almost touched the end of the glass capillary. Epoxy glue was used to secure a rigid connection between the 0.5-mm wire and the glass capillary (Figure 1). Amperometric Detection Cell and Capillary Electrophoresis. Polyimide-coated fused silica capillaries of 20 µm i.d.(internal diameter) were purchased from Polymicro Tech (Phoenix, AZ). Electropherograms were obtained with the Applied Biosystems CE instrument (ABI-270A, Perkin Elmer, Foster City, CA). Pressure injection (10 s) and 20 µm i.d. capillaries (80 cm long) were used in all cases. The capillary was installed in the normal fashion at the anodic end but was inserted to the cathodic reservoir through a septum opening, created at the bottom, instead of the one provided on top (Figure 1). Three other septa were installed on the side of the reservoir to allow insertion of the electrodes. The reference electrode was a silver wire with AgCl formed at the tip by
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1997 American Chemical Society
slurry was taken and filtered with a 0.1 µm syringe-end filter (Gelman Sciences) to provide a sample for CE analysis.
Results and Discussion
FIGURE 1. Amperometric detection with details of the detecting electrode. Components were not drawn to scale. Electrodes: (1) detecting, (2) counter, (3) reference. electroformation while a platinum wire served as the counter electrode. The gold electrode, as described above, was used as the working (detecting) electrode and positioned directly at the capillary outlet. The separation capillary was positioned so that its outlet was as close to the tubing center as possible. Due to the large diameter of the detecting electrode (0.127 mm) as compared to the internal diameter of the capillary (20 µm), good oxidation efficiency was achieved without precise microalignment. During electrophoresis, a CV-1B voltammograph (Bioanalytical System) was used to apply + 800 mV to the detecting electrode. The response current was displayed on the monitor of a PC 486 computer equipped with an A/D board (DP500-AD supplied with an interface box, Labtronics Inc, Guelph, ON, Canada). The time response data were stored in ASCII files and translated to PRN files for treatment by a graphic program. In most cases, the running buffer was mixed phosphate/borate, 25 mM each, pH 8. Soil Extraction. A soil sample (0.1 g) was placed in a 15-mL vial that contained 5 mL of an extracting medium (water, buffer, or buffer/methanol). Magnetic agitation was effected for the desired time and then the slurry was centrifuged and filtered with a 0.1 µm filter (Gelman Sciences, Montreal, Canada) to obtain the sample for electrophoresis Conversion with Ceric Sulfate. Ceric sulfate solution (0.25 N in 1 N sulfuric acid, 0.1 mL) was added to 1 mL of chlorophenol containing sample (prepared solution or soil extract) and magnetically stirred for 5 min and then 1 mL of phosphate (350 mM, pH 7) was added to precipitate the cerium ions (Ce3+ and Ce4+). Water was added to attain 2.5 mL, and the solution was centrifuged and then filtered with a 0.1-µm filter (Gelman Sciences) before CE analysis. Photodegradation with TiO2. A pentachlorophenol (PCP)contaminated soil sample (0.2 g) was placed in 10 mL of water, and 20 mg of TiO2 was added. After 24 h of stirring, 3.5 mL of the slurry (containing soil particles as well as TiO2) was taken (while the slurry was being stirred) and placed in a quartz cuvette. Air was gently bubbled into the slurry for 5 min and then the cuvette was positioned in a holder (Model 30750) that was attached to a lamp housing (Model 66006, Oriel) equipped with an F/1 condenser for beam focusing. The cuvette holder allowed magnetic stirring of the slurry during illumination by a xenon arc lamp (150 W, Model 6253) operated with a power supply (Model 68805). All components of the illumination setup were products of Oriel (Stratford, CT). After an appropriate illumination time, 0.1 mL of the
Development of CE Equipped with an Amperometric Detector. The most common mode of CE applies a high potential (10-30 kV) across a small and long capillary (10-50 µm i.d., 50-100 cm long) to effect separation of charged molecules. A high potential coupled with the capillary length provides good separation while the small diameter ensures efficient dissipation of the generated heat. The principle of CE operation is very simple, and a CE system can be assembled with readily available components although several commercial instruments are now available. Standard ultraviolet/ visible (UV/VIS) detectors, for which the capillary diameter is the detection path length, offer a detection limit of about 10 ppm for most compounds, although 1 ppm detection has been reported in certain cases, and in the optimal conditions 50-100 ppb could be achieved for highly chromophoric analytes. Some commercial systems are also equipped with a laser-induced fluorescence detector to reach detection limits of about 10-100 ppb, provided that analytes are fluorescent. Since the pioneering study by Wallingford and Ewing (13), amperometric detection has been improved significantly to attain a routine detection limit of 100 ppb to 1 ppm, although an extremely low limit of 0.055 ppb has been reported for hydroquinone (16, 17). There was also one short note describing the separation of chlorophenols using a 25 µm i.d. capillary with detection by a 10-µm carbon fiber (18). However, it was not clear whether a porous glass joint was used, and a current response of 50-200 pA was reported for a standard mixture containing analytes at 5-8 ng/L. It should be noted that the concentrations mentioned were unusually small and that the specific current response was far out-ofline from those reported by other studies. For example, a recent study dealing with samples of catecholamines (19), using 50 µm capillary, 33 µm carbon fiber, reported response currents about 25 pA for analytes at 100 nM (about 10 µg/L), and this was in agreement with the results previously reported in the literature. Although the cited note (18) dealt with a different class of analytes, the large response current reported for such small concentrations raised some doubt, particularly when other discrepancies can also be spotted in the note. In general, fragility is a drawback of most CE systems equipped with amperometric detectors, although more sturdy devices have recently been reported (20, 21). The fragility is a consequence of the need to isolate the detecting electrode from the high voltage applied across the separating capillary since the electrophoretic current produced in the capillary is often 5-6 orders of magnitude (up to 200 µA) higher than the electrochemical currents measured at the amperometric detector (
