Spectroscopic Detection of Aqueous Contaminants Using in Situ

Apr 1, 1997 - Some instruments operate on a freely falling water stream giving “windowless” operation, a highly desirable feature for most natural...
1 downloads 0 Views 179KB Size
Download PDF
Anal. Chem. 1997, 69, 1279-1284

Spectroscopic Detection of Aqueous Contaminants Using in Situ Corona Reactions Mark Johnson*

North West Water Ltd., Dawson House, Gt. Sankey, Warrington WA5 3LW, United Kingdom

An apparently novel technique to aid the detection of a variety of inorganic and organic compounds in environmental and drinking water samples is described. Background absorbance due to optical scattering, cell fouling, and a variety of contaminants is suppressed by combining UV spectroscopy with chemical reactions initiated by reactive species generated in a high-voltage corona discharge. Injection of the reactive species takes place through a free water surface from the “corona wind”. Initial measurements on aqueous chlorine in drinking water and BTEX (benzene, toluene, ethylbenzene, and xylene) in unfiltered river water down to parts-per-million concentration are given which show, by comparison with a conventional UV absorption measurement, good background suppression. The experimental arrangement is simpler than that in typical fluorescence detection systems, and the geometrical flexibility means that corona “dosing” can be applied also to Raman and other spectroscopies, to electrochemical detection schemes, and to planar and windowless geometries. Much of the chemical analysis required for water treatment plant protection, process control, and groundwater remediation must currently be carried out in a laboratory. While laboratory analysis offers high performance, the delays incurred in identifying problems are inconvenient and make problem rectification difficult. Many treatment plant operators would, therefore, welcome online instruments that give an earlier warning of imminent problems. Hand-held instrument assays are useful if manpower is available, but ideally these measurements should be performed unattended, on-line and at a high rate. These characteristics would be attractive even with a modest level of detection performance. For an instrument designed to warn of serious problems in source waters being abstracted for potable use, our task is usually to recognize the presence of known contaminants on a large and variable absorbance background due to benign, simultaneously present compounds and high levels of turbidity. In this nonspecific screening application, selectivity of one contaminant species against another is of less importance than suppression of the benign background. The BTEX compounds (benzene, toluene, ethylbenzene, and xylene) form one class of organics of interest which are occasionally present in river waters. BTEX compounds are also important ground contaminants because of their wide use in fuels and as solvents. Hence, ground remediation techniques have been developed to treat solid material * E-mail: [email protected]. S0003-2700(96)01113-4 CCC: $14.00

© 1997 American Chemical Society

contaminated with these compounds.1 An on-line, reagentless monitor for these contaminants would be a worthwhile system addition, allowing process efficiency to be estimated on-site, rather than after time-consuming off-site analysis. In this case, the contaminant species will usually also be known, and so again, selectivity of one organic species against another is not required. The background of natural materials must, however, be adequately suppressed, preferably using only minimal sample preparation. Optical techniques are very attractive for such on-line, reagentless instruments, as they offer the potential for low cost of ownership. Ultraviolet (UV) absorption is widely used on river intakes but has difficulty resolving the necessarily weak absorbance features on a strong background. Fluorescence instruments for the detection of BTEX compounds are commercially available that do avoid much of the background, despite some naturally occurring fluorescent materials. In a typical instrument designed for water screening, fluorescence of the compounds is excited at 254 nm using a mercury vapor lamp, with single-channel (photometric) detection in the 270-300-nm band. Some instruments operate on a freely falling water stream giving “windowless” operation, a highly desirable feature for most natural environment samples. The low-level detection limit is typically 1 ppm, while even for a nonspectrometric instrument cost is high. Much of this cost is due to the difficulty of optically filtering both excitation light (to remove the several source emission lines in the 270300-nm region) and scattered light (to remove the strong 254-nm primary mercury line). Laser excitation eases the optical filtration requirements but increases system costs. It was thought that comparable performance could be obtained with reduced complexity by using the scheme to be described here. The conventional solution for background suppression in chemical instrumentation is the use of chemical reagents which respond selectively to the target species, and a convenient way to use these reagents is in flow injection analysis (FIA). Much of the attraction of this approach is that it allows ac detection. By comparing the detector output with and without the small concentration of reagent, slow drifts in detector sensitivity caused by the inevitable fouling can be compensated. The measurement becomes self-referencing. However, FIA reliability is hard to guarantee under on-site conditions, in part due to the use of liquid reagents. These can dry out, block small-diameter pipework, and have to be replenished periodically. A desirable modification would, therefore, be to use a reagent that can be electrically generated and modulated in situ. Several such reagent systems exist, such as electrochemi(1) Goheen, S. C.; Mong, G. M.; Pillay, G.; Camaioni, D. M. Treatment of organic contaminants in water by a corona discharge reactor, First International Conference on Advanced Oxidation Technology for Water and Air Remediation. London, ON, Canada, June 25-30, 1994.

Analytical Chemistry, Vol. 69, No. 7, April 1, 1997 1279

Figure 1. Modifications to an HP8452A diode array spectrometer to allow in situ corona dosing.

cally generated species, electrochemically produced pH and redox changes, and electromagnetic fields such as light. We have previously used2 a UV light flux as such a photolytic “electroreagent”. In an attempt to combine the convenience of an absorption measurement, the background suppression and AC modulation characteristic of reagent-based FIA, and more convenient reagents, I have investigated an apparently novel approach that uses UV absorption measurement together with a chemical reaction step induced in situ via highly reactive species generated in a highvoltage, point-to-liquid corona discharge.3 By processing the double signature of absorption spectra measured before and after the contactless, electroreagent corona reaction, suppression of the background of nonreacting species is obtained. This work can be seen as a logical development of the use of high-voltage plasmas for the large-scale destruction of organic contaminants.1 While measurements of spectra will be shown, the advantage of the technique is greatest in the design of simple, single-wavelength band photometers. EXPERIMENTAL SECTION To investigate the application of point-to-liquid coronas to chemical analysis, two experimental configurations were used. The first was used for quantitative UV absorption investigations. For this, a Hewlett Packard HP8452A diode array spectrophotometer was used, unmodified except in its sample handling (Figure 1). The 40-mm-path length and 10- × 30-mm cross-section cuvette, of UV-grade silica, was fitted with an internal electrical connection consisting of a 50-mm long, 150-µm-diameter platinum wire. This was fixed through a drilled entry point to the cuvette base using high-temperature epoxy adhesive. In the open top of the cuvette, a close-fitting machined fluoropolymer lid was placed which supported four tungsten probe tips of the type used for semiconductor chip probing. Their tip radius was 12 µm. The probe tips were carefully arranged to be collinear and were separated from the undisturbed sample-water meniscus by approximately 4 mm. This was achieved by accurately defining the sample volume using a digital pipet. A 4 mL water sample pipetted into the cuvette filled it to a depth of 10 mm, with a curved meniscus. In the second configuration, a single tungsten probe electrode was placed 5 mm above the surface of a water sample contained (2) Johnson, M.; Melbourne, P. Analyst 1996, 121, 1075-1078. (3) Chang, J.-S.; Lawless, P. A.; Yamamoto, T. IEEE Trans. Plasma Sci. 1991, 19, 1152-1166.

1280 Analytical Chemistry, Vol. 69, No. 7, April 1, 1997

Figure 2. Current/voltage characteristic of the point-to-liquid corona discharge at 5-mm separation. The insert shows the inner “dimpling” of the water surface by the corona wind.

in a shallow glass dish. A wire electrode in contact with the water surface was used as a ground electrode. This apparatus was used to observe the water surface during corona irradiation. In each configuration, the point electrodes were connected to individual 10-MΩ current-limiting, high-voltage resistors and thence to a common, variable-voltage power supply. Positive and negative voltages up to 7 kV were available at up to 1 mA. The wire in contact with the bulk of the sample served as ground connection, through which the corona current was measured both with a 1-kΩ resistor and oscilloscope and with a dc current meter. The high-frequency response of the meter was reduced to 1 Hz with a simple RC low-pass filter to allow averaging of fast current fluctuations. RESULTS AND DISCUSSION Physical Effects. When an electrical potential is applied to the sharp electrode above a grounded, weakly conductive liquid surface, the high-field gradient near the tip causes current to flow. With a negative potential, electrons are emitted, which flow away from the tip. The electron stream carries with it a stream of gas molecules, which impinge on the water surface. This is the “corona wind”. In our single-electrode configuration, upon increasing the (negative) probe tip voltage from zero, a current began to flow at a threshold potential of approximately 1.2 kV. Above this threshold, two structures were visible in the water surface. Directly under the electrode, a dimple formed which was easily visible in the reflection of a small light source. At 2 kV, the diameter of the dimple was of the order of 5 mm, and 1 mm deep. A photograph of this is shown in the inset of Figure 2. Additionally, a second, very sharp interface could be seen on the surface with careful adjustment of the illumination. This was circular and exhibited a size that varied with the applied voltage, moving from the inner dimple periphery out to a diameter of at least 80 mm and back again under voltage control. A collimated HeNe laser beam shone onto the circular interface split into two reflected spots, suggesting that the interface is a real, topological interface between surface regions of different slope, rather than just a reflectivity variation. Small dust particles floating on the surface were repelled from the interface. The diameter of the circular region was also a sensitive function of the surface tension of the water. Its origin is unknown. At the first sight of the inner dimpling, the current was made up of a series of sharp pulses at an average repetition rate of ∼1

Figure 3. Bleaching of aqueous methylene blue dye with successive negative voltage corona wind irradiations.

MHz. These are known as Trichel pulses3 and characterize one of the regimes of corona operation. This regime could also be recognised by a faint blue glow from the probe tip, easily visible in subdued lighting. Above the threshold voltage, the current increased approximately with the square of the applied voltage (Figure 2). With increasing voltage, the rate of current pulsing increased, a faint hiss was audible, and narrow-band resonances in the electrical frequency spectrum could be measured, extending out to 50 MHz. At 6 kV, the total current was 480 µA, which increased the depth of surface dimpling, and caused rapid agitation of the water surface. The individually resolved current pulses had merged at this point into a uniform continuum. Investigation with a tiny dye droplet injected into the water showed that efficient mixing was taking place, driven by rapid convection currents. At even higher voltages, the current increased beyond 500 µA and became much less stable, and breakdown of the air between the probe tip and the water surface was visible as bright purple feathery sparks. Operation with positive tip voltages was also possible but was much less controllable, with a difficulty in starting the corona conduction. This is attributed to the lower mobility compared with electrons of positive ions generated near the probe tip. Positive and negative coronas produced similar but not identical optical effects in our samples; the results described here are restricted to negative tip voltage operation. Chemical Effects. To confirm the reactive chemical nature of the corona wind, we measured in the UV/visible spectrometer of Figure 1 the absorbance of a small quantity of methylene blue dye, dosed by the corona wind in the single-electrode configuration. Figure 3 shows the dye absorption before corona dosing and after subsequent 60-s-long doses with a negative corona wind of 3 kV potential at a current of 55 µA. Each dose reduced the strength of the two main peaks at 660 and 290 nm. It seemed that the dye had been simply degraded to materials that did not absorb in this part of the spectrum. Methylene blue (methylthionine chloride) is an organic dye often used to check the effectiveness of remediation processes such as photolytic fluidized beds. However, it is also a redox indicator, which can be converted between a blue (oxidized) and a colorless (reduced) state. In order to resolve the ambiguous apparent bleaching to a colorless state, which can also be caused by reduction, color changes were also investigated in the redox indicator ferroin (1,10-phenanthroline ferrous complex). While this material exhibits a rapid, reversible color change from pale blue to red on reduction, no such color changes were measured

Figure 4. Increase in absorption near 200 nm in deionized water caused by the corona irradiation. Labels indicate the irradiation time. The 256-nm absorption was measured in the atmosphere above the sample and is indicative of ozone presence.

on repeated 60-s corona doses under the same conditions of Figure 3. The visible wavelength absorption bands were simply reduced in strength. These measurements do not prove that a redox change is not taking place but strongly suggest that a more violent reaction is dominant. To investigate any changes due to pH effects of the corona wind,4 a variety of pH indicators was irradiated. In all cases (BDH universal indicator, chlorophenol red, m-cresyl purple), no color changes indicating pH variation could be seen. Rather, the absorption strengths across the spectrum of the indicator dyes were again simply reduced by corona dosing. While Figure 3 shows a bleaching of the absorption at most wavelengths, at wavelengths below 240 nm the absorbance increased with dose. This was suspected to be caused by the degradation products of the dye. However, dosing pure, deionized water with the negative corona produced a similar effect, as can be seen in Figure 4. The originally very low absorption (B(λ)) of deionized water in the range 200-400 nm was modified by the appearance of a strong absorption feature (A(λ)) centered on 202 nm. It is not known what causes this absorption, but it is suspected to be due to nitric acid or nitrates formed by the injection with the corona wind of oxides of nitrogen.5 Diluting pure nitric acid to the same peak absorbance value gave a spectrum indistinguishable from that of the generated species of Figure 4. Another contender for degradation of the dyes is the ozone formed by the high-voltage discharge in the humid atmosphere above the liquid sample. Figure 4 also shows a weak absorption centered on 256 nm in the atmosphere above our sample. This is characteristic of ozone, and the familiar ozone smell was present in the cuvette. A similar but very weak absorption was just detectable in the absorption data of the water itself, although this is not visible at the resolution of this plot. A further candidate for the active degrading species, and the one that is normally associated with groundwater remediation and pathogen disinfection by high-voltage plasmas,1 is the OH radical. We have not yet answered this question of the active species. Whatever the cause, the corona wind performs chemical modifica(4) Brisset, J. L.; Lelie`vre, J.; Doubla, A.; Amouroux, J. Rev. Phys. Appl. 1990, 25, 535-543. (5) Brandvolt, D. K.; Martinez, P.; Dogruel, D. Atmos. Environ. 1989, 23, 18811883.

Analytical Chemistry, Vol. 69, No. 7, April 1, 1997

1281

Figure 5. Absolute absorption spectrum of a high-quality raw river water sample containing 350 ppm benzene (lower curve). The upper curve shows the same sample absorption after 60 s dosing with a negative corona wind at 55-µA current.

tion of certain compounds and with it modulation of their absorption spectra. BTEX Detection in Surface Waters. The UV absorption spectrum of a typical, high-quality river water from our region is shown in Figure 5. At wavelengths below 225 nm, there is a strong absorption feature which is attributed to the wide variety of inorganic species present in these waters. The lack of fine structure in these spectra does not allow the separation of individual ions. However, this feature is retained in processed drinking water obtained from this source, and chemical analysis of this allows the bulk of the feature to be attributed to the nitrate ion, at concentrations in this sample of approximately 10 ppm. The background absorption region around 250 nm is attributed partially to humic acids,6 materials associated with vegetable matter; this region is often used as a measure of so-called disinfection byproduct precursors, which on disinfection with chlorine may produce a range of undesirable halogenated organic compounds. At longer wavelengths, a gradually reducing apparent absorption is visible, caused by scattering from small particles. To this sample has been added 350 ppm benzene. This is visible as the barely resolved peaks around 250 nm in the lower of the two curves. The small amplitude of these features, even at this high concentration, demonstrates the difficulty of performing UV photometric determination of these materials in real samples. For a low-cost screening instrument, we would like to quantify absorbance changes integrated over the range 240-270 nm at least 2 orders weaker than these, on a background absorbance that can vary grossly with time depending on the environmental conditions. The upper curve of Figure 5 shows the absorbance of the same, benzene-containing raw water sample after irradiation for 60s with a 3-kV negative corona wind. The absorption has been increased slightly at wavelengths near the benzene absorption features, with a larger change at longer and shorter wavelengths. This change in the spectrum vanishes with no benzene present. Figure 6 shows before-after difference spectra, B(λ) - A(λ), obtained from a range of concentrations of benzene in the same raw river water. Several features are visible in these difference spectra. The gross effect shown is a corona-induced increase in absorption. In the region of the known absorption features of benzene, which are still visible, there is a small negative change in the difference absorption. At shorter wavelengths, a larger (6) Edzwald, J. K.; Becker, W. C.; Wattier, K. L. J. Am. Water Works Assoc. 1985, 77, 122-132.

1282 Analytical Chemistry, Vol. 69, No. 7, April 1, 1997

Figure 6. Differential (before-after) absorption spectra of raw river water containing concentrations of 350, 250, 75, 30, and 10 ppm benzene on 60-s corona irradiation.

Figure 7. Differential (before-after) absorption spectra of benzene in deionized water at the same concentrations as in Figure 6.

negative feature is seen. This could be an increase in absorption caused by the reaction products of the benzene, but it is more likely just the increase in absorption below 225 nm already seen in Figure 4. At even shorter wavelengths, the change reduces to zero. This is an artifact of our measurement process, in which the already high absorption due to source water nitrate has been augmented by the corona-generated short-wavelength species to such a high value that the spectrometer dynamic range has been exceeded. Much more significant and useful is the increase in absorption in a broad feature to longer wavelengths reaching as far as 300 nm. This is coincident with the wavelengths of emission of benzene fluorescence, but without the mirror-image spectral features typical of fluorescence. By the production of a new absorption feature in a region with reduced particulate scattering and absorption from other materials, our detection task has been significantly eased. The other organic and inorganic compounds present in this natural water are only negligibly affected by the corona irradiation. Some scatter and nonlinearity is visible in the curves, especially near to 250 nm. It is not known what causes this. Figure 7 shows the same benzene dosing and corona wind irradiation experiments carried out in a deionized water sample. Although the magnitude of the unassigned short-wavelength absorption feature has been greatly increased by the change to pure water, the overall character of the 240-300-nm benzeneeffected changes is similar. Hence, the benzene difference spectra from 240 to 300 nm seem to be largely unaffected by the presence of other absorbing and scattering material in of this natural sample. The background absorbance of 0.7 at 250 nm has been completely suppressed. Note that no sample preparation whatsoever was performed on the river water samples. The samples went from

Figure 8. Differential absorption spectra of phenol in river water. Labels indicate the concentrations.

river to spectrometer. It is one aim of this work to develop measurement techniques that can operate successfully on treatment plant and source abstraction sites, with little operator training, and especially with only minimal levels of sample preparation. In practice, micrometer-scale filtration would be acceptable, which process removes the greater fraction of the long wavelength apparent absorption caused by particulate scattering. This also reduces calibration scale errors caused by the masking effects of high levels of particles. Toluene, ethylbenzene, and xylene in our surface waters exhibit similar characteristics, with the production of new absorption features to the long wavelength side of their direct absorption. They differ in detail. Figure 8 shows equivalent data obtained for phenol. The absorption change at the main UV absorption at 265 nm was very small, with strong new absorption bands being formed at 245 and 285 nm. Aqueous Chlorine Detection. We have assumed above that the inorganic species present in the water are not affected by the reactive corona wind. This is largely true, but useful exceptions exist. It is well known that chlorine-based disinfectants in water are degraded by natural sunlight, and we have shown in other work2 that aqueous chlorine can be usefully degraded in situ in a spectrometer using a xenon flash lamp or low-pressure mercury vapor lamp. Very similar degradation effects occur using the corona wind. Whether obtained from chlorine gas or from a hypochlorite compound, chlorine disinfectants in water exist either as hypochlorous acid (HOCl) or as the hypochlorite ion (OCl-), depending on the ambient water’s acid or alkaline pH, respectively. Both species absorb in the UV, in broad peaks centered on 234 and 292 nm, and both offer disinfectant power, although the HOCl form is more effective. To measure the disinfection efficiency in a potable water stream, we need either to know the total chlorine concentration together with an accurate pH value, which is difficult in practice, or to measure separately the concentration of each chlorine species. Even potable waters exhibit high and variable apparent absorption due largely to nitrate ions (wavelengths < 220 nm) and weak particulate scattering (throughout the UV and visible). As above, direct detection of chlorine at low concentrations is, therefore, difficult. Figures 9 and 10 show difference absorption spectra of a 2.5 ppm chlorine concentration in tap water (7.5 ppm nitrate) at pH values of 5.0 and 8.7, after successive 30 s applications of a 55-µA negative corona wind. The large feature at 210 nm is the unknown generated species seen in Figure 4. The increase in (before-

Figure 9. Differential absorption spectra of chlorine at pH 5.0 with successive corona dosing (30/s per irradiation step), showing degradation of the weak HOCl absorption at 234 nm.

Figure 10. Differential absorption spectra of the same sample of Figure 9, with the pH raised to 8.7, converting the aqueous chlorine to OCl-, which absorbs at 292 nm. Successive corona doses of 30/s per irradiation step.

after) difference absorption at 234 and 292 nm shows a simple degradation of the HOCl and OCl- features, without the formation of other absorbing species visible in this wavelength range. The OCl- feature is well resolved, while the HOCl feature is partially obscured at short wavelengths by the generated species’ absorption. Nevertheless, calculation of the difference spectrum has completely suppressed the background scatter and absorption of the single measured spectrum, which amounted to approximately 0.2 at 250 nm and 2.2 at 210 nm. Spectral Analysis. Between pairs of spectra taken in these measurements, there was considerable baseline drift seen due to the imperfect stability of the spectrometer, variation in cuvette temperature, small movements of the optical path, etc. These are of the order of 0.005 AU in our configuration. They have been removed in our analyses by normalizing the absorbance changes to zero at a wavelength greater than 350 nm. As these drifts are experienced as simple absorbance shifts which are independent of wavelength, they do not greatly disturb any data analysis which uses the whole measured spectrum. Calibration of the technique for BTEX detection using spectral information was performed using multiple linear regression on absorbance values at wavelengths from 225 to 325 nm, programmed in Mathcad. The two spectral signatures, i.e., both absolute (B(λ) + A(λ)) and difference (B(λ) - A(λ)) spectra, were combined into a single data set for processing. The calibration so obtained was linear in the range up to 200 ppm for phenol and benzene, with a 2σ repeatability of 50 ppb. This allows reliable detection to well below 1ppm concentration. Analytical Chemistry, Vol. 69, No. 7, April 1, 1997

1283

In designing a low-cost instrument, we would prefer to perform, instead of spectral measurements, photometric measurements in one or two wavelength bands, using narrow-band sources or interference filters. Making a single direct measurement of absorption in the short-wavelength band (S ) 240-260 nm) would, however, only be useful where the BTEX absorption dominates background absorption and scatter. This limits the minimum detectable BTEX concentration in the water of Figure 5 to >1000 ppm. Alternatively, making a corona-dosed difference (double) measurement in the long-wavelength band (L ) 260-290 nm) completely suppresses the background, allowing reliable detection down to 3 ppm, below which it becomes limited by instrumental drift during the 60-s dosing period. This drift error can be reduced if absorption in both bands S and L is measured, with concentration being determined from (beforeS - afterS) - (beforeL - afterL). In our configuration, this approach, calculated here by integrating the full spectrum over the two bands rather than by building the photometer, improved the detection limit by a factor of 3. This equates to the advantages of ac modulation typically obtained from conventional FIA. Further improvement in drift suppression is expected by performing more sequential measure/dose operations as rapidly as possible, with averaging.

1284

Analytical Chemistry, Vol. 69, No. 7, April 1, 1997

CONCLUSIONS These preliminary experiments have shown that direct, UV spectrophotometric detection plus an in situ corona reaction give additional information to aid the quantitative detection of certain organic and inorganic compounds. For the BTEX compounds, a single-wavelength band measurement based on the technique offers performance comparable to those of mercury lamp-excited fluorescence techniques, although we consider that the method described here is simpler. The corona dosing capability can easily be added to a variety of commercially available spectrometers and other optical and nonoptical detection schemes. Only a minimal level of sample preparation of environmental samples is required. ACKNOWLEDGMENT I would like to thank Paul Melbourne for his help with the sample preparation and spectral measurements.

Received for review October 31, 1996. Accepted January 17, 1997.X AC961113X X

Abstract published in Advance ACS Abstracts, February 15, 1997.