Environ. Sci. Technol. 1902, 16, 263-268
Liquid Chromatography with Precolumn Sample Preconcentration and Electrochemical Detection: Determination of Aromatic Amines in Environmental Samples John R. Rlce and Peter T. Klsslnger”
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
rn New methodology has been developed for the routine determination of five toxic aromatic amine pollutants in environmental samples a t the low pg/mL level. As an evaluation of this approach, a complete analytical procedure is described for the determination of benzidine and 3,3‘-dimethoxybenzidine, with linearity demonstrated for benzidine added to river water from 25 pg/mL to 5 ng/mL (r = 0.9999) and with an imprecision of 3.83% RSD a t 250 pg/mL. The limit of detection is 5 pg/mL (S/N = 3). More rigorous chromatographic conditions are utilized to determine 4-aminobiphenyl, 3,3’-dichlorobenzidine (DCB), and 4,4’-methylenebis[2-chloroaniline] (MOCA), with linearity of 3,3‘-dichlorobenzidine added to river water evaluated from 500 pg/mL to 10 ng/mL (r = 0.9978). The imprecision a t 5 ng/mL is 1.07% RSD and the limit of detection is 25 pg/mL (S/N = 3). Each procedure utilizes 20-mL sample volumes, and no sample pretreatment is employed. Ground-water samples and soil-core samples known to contain benzidine and 3,3’-dichlorobenzidine were successfully studied via this approach. Introduction In December 1979 the U S . Environmental Protection Agency issued proposed guidelines establishing test procedures for the determination of 113 organic toxic pollutants in municipal and industrial discharges (1). These procedures are intended to meet the monitoring requirements for the filing of applications for National Pollutant Discharge Elimination System permits, for state certifications, and for compliance monitoring under the Clean Water Act. Method 605 of these guidelines, as described further by Riggin and Howard (2),sets forth the approved protocol for the determination of benzidine and 3,3‘-dichlorobenzidine a t levels above 0.5 and 1.0 ng/mL, respectively. These amine-substituted biphenyls are important synthetic intermediates in the manufacture of industrial dyestuffs, although their production has been sharply curtailed since the Occupational Safety and Health Administration declared them to be regulated chemical carcinogens in 1974 (3). Several additional amine-substituted aromatic compounds are similiarly classified, including 4-aminobiphenyl and 4,4’-methylenebis[2-chloroaniline] (MOCA) (3). The strategy adopted by the EPA-approved method, as well as a host of other liquid chromatographic procedures developed in recent years, involves the initial extraction of a very large volume (500 mL) of the original sample with an imiscible organic solvent, back-extraction into an aqueous phase a t low pH, and reextraction into a second organic phase. This is followed by evaporative concentration of the organic extract, dilution of this to several milliters, and injection of a microliter-scale aliquot onto the chromatographic column, with quantitation by means of an amperometric detector. Although affording considerable advantage over alternative methods such as combined gas chromatography-mass spectrometry ( 4 ) , the EPA procedure relies upon a conventional approach to sample preparation that has long been plagued with sig0013-936X/82/0916-0263$0 1.2510
nificant shortcomings. The manual sample manipulations are intended to reduce interferencesvia selective extraction (despite the subsequent use of a highly selective detector) and to increase by a factor of 100 the linear range and limit of detection of the method by decreasing the volume of solution containing the available analyte molecules. Unfortunately, the latter objective is largely negated since only a tiny fraction of the available extract is subsequently injected into the chromatograph. Moreover, considering losses due to incomplete mass transfer in each of the several extractions, it is clear that considerably less than 1% of the analyte molecules present in the original aqueous sample are actually utilized in the final quantitation step. While the conventional approach described above is adequate for many purposes, an alternative technique for preconcentrating and injecting trace amounts of analyte species has recently received attention. Reversed-phase chromatographic packing materials show very strong affinity for hydrophobic molecules in solution containing little or no organic component (5, 6). These stationary phases are capable of adsorbing trace concentrations of such molecules from a flowing aqueous stream. A common approach is to use a small column containing this material as a component of a standard liquid chromatograph, while employing a mobile phase containing a large percent of an organic component to rapidly elute the accumulated analyte from the “sampling column” in a narrow band and to subsequently chromatograph this on a conventional “analytical column”. With this approach, the need to manually extract and/or preconcentrate the sample is diminished, and all of the analyte molecules present in the aqueous solution sampled are utilized in the quantitation step. Since an additional consequence is the preconcentration of many other components that are inevitably present in a complex sample, the greatest success has been achieved in the determination of species having an advantageous molecular property, such as fluorescence ( 5 ) or electrochemical activity at modest potential (6),thus permitting the use of a selective chromatographic detector. This report describes the significant advantages that accrue from the application of this new methodology to the determination of hydrophobic, electroactive molecules in various samples of environmental origin. Although aromatic amines are utilized in this evaluation, many hydrophobic phenols and cresols represent other suitable candidate molecules compatible with this method (7). Experimental Section Voltammetric Survey. A survey of the voltammetric characteristics of the aromatic amines utilized in this evaluation was made with cyclic voltammetry and chromatographically assisted hydrodynamic voltammetry. Cyclic voltammograms were obtained with a 0.1 mM solution of analyte and a supporting electrolyte of 595 ethanokO.1 M citrate buffer, pH 5, vs. a Bioanalytical Systems (West Lafayette, IN) Model RE-1 Ag/AgCl reference electrode using a Bioanalytical Systems Model CV-1A in-
0 1982 American Chemical Society
Envlron. Sci. Technol., Vol. 16, No. 5, 1982 263
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Schematic diagram of the rodllkd IIquM chromatograph. text. with moSe required for Compnent designationsare given in large volume precolumn sample preconcemation shaded in gray.
Flgure 1.
strument and plotted on a Hewlett-Packard (San Diego, CA) Model 7015A X-Y recorder. The procedure utilized to obtain a chromatographically assisted hydrodynamic voltammogram (CAHDV) has been described elsewhere (see ref 8 and Supplementary Material). Individual voltammograms were obtained for each analyte with the respective chromatographic conditions employed for the determination of each, as given below. The repetitive injections utilized 10 ng of analyte in a 20-pL injection volume. Liquid Chromatograph. A schematic diagram of a general-purpose liquid chromatograph with capability for column-switching methods (9)and for precolumn sample preconcentration is shown in Figure 1, with the components essential for methods utilizing a large volume (>Z mL) of sample shown shaded in gray. The basic unit is a Bioanalytical Systems Model LC-50, modified with the addition of a second Rhecdyne Perkeley, CA) Model 70-10 high-pressure six-port injection valve (V3) positioned between the analytical column (C2) and the conventional Rheodyne Model 7010 valve 072). The latter is equipped with a standard 20-pL sample loop. Analytical mobilephase selectivity is provided by a low-pressure selection valve (Vl) such that one of several mobile phase compositions (MP1-MP3) may be supplied to the analytical pump (Pl), thus allowing step-change elution of well-retained components from the analytical column. The analytical column is a 15 cm X 4 mm i.d. steel tube slurry-packed with Merck (Darmstadt, DFR) Lichrosorb RPC2 material. A low-pressure two-way valve (V4) permits shunting of the column eluent to waste (W) in order to avoid disruption of detector performance that could be caused by high concentrations of weakly retained, elec264
Envlmn. Sci. Technol., Vol. 16, No. 5, 1982
troactive components or by passage of large, well-retained peaks across the electrode surface of the detector (D). The additional six-port injection valve (V3) has a Rheodyne-Brownlee guard column holder (Cl) in place of the sample loop. A Rheodyne-Brownlee Model C2-GU cartridge (3 cm X 4 mm i.d.) containing l 0 p m C2packing material was employed in C1 as the precolumn/sampling column. This column may be viewed as a method-optional precolumn that may be brought on-line as needed by switching the position of V3. This can be an attractive option when initially examining complex environmental samples by making microliter-scale injections onto the analytical column with V2. When it is switched off-line, the addition of a second pumping and injection valve system employing a second high-pressure pump (PZ)allows C1 to be either flushed free of undesired, well-retained components or permits it to be used to preconcentrate analytes from aqueous samples. Typically, a flushing solution (FS)would contain a large percentage of an organic solvent that would rapidly remove the undesired species and would be followed by a small volume of solution (MP4) similar in composition to that being used as the analytical mobile phase in order to avoid contaminatingthe analytical column and detector. The C1 column may be used as a sampling column by employing one of two sampleintroduction methods. Large volumes of sample (S) are pumped directly through the sampling pump (PZ) at a precise flow rate and are passed through C1 for a specific length of time. Cross-sample contamination is prevented by thoroughly flushing this portion of the system with sample while the C1 column is on-line with the analytical column. The execution of this sequence is controlled via a Bioanalytical Systems Model LC-24 digital programmer, which automatically begins operation of the sampling pump and switches the sampling column to receive the sample stream for the desired length of time by means of a pneumatically driven Rheodyne Model 70-01 valve actuator, which switches the position of V3. A t the completion of the sampling cycle, the programmer switches C1 on-line with the analytical column and begins operation of the detector recorder (R), which is subsequently turned off a t the end of the chromatographic elution. Sample volumes between 10 pL and 2 mL may be introduced into the sampling column via a third high-pressure injection valve (V5) equipped with a sample loop of the desired size. In this case, the sample is carried to the sampling column by an aqueous carrier solution (MP4), which is essentially a sampling system "mobile phase" and which is continuously pumped through the injection valve V5 and the sampling pump P2. So that effective retention of the desired sample components on C1 is ensured, the carrier solution does not contain an organic solvent component. A n advantage of this mode is that weakly retained sample components may be flushed out of the sampling column completely by the carrier solution and hence not be subsequently introduced onto the analytical column or passed through the detector. An analytical procedure based upon this approach has been described in detail by Koch (6). Selection of sample, flushing solution, or carrier solution options is provided by valve V6. A flow restrictor (PR) is an additional option that maintains pressure within the sampling column pumping system equivalent to the back pressure of the analytical column, thus preventing possible degassing of the mobile phase, which may occur when C1 is switched on-line. Reagents and Samples. Standards of the aromatic amines determined in this study were obtained in milli-
gram quantities from RFR Corp. (Hope, RI) and were used as received. Reagent grade chemicals were used in the preparation of all solutions, as was deionized, distilled water. Technical grade methanol was distilled prior to use as a mobile-phase component. 1.0 M ammonium acetate was prepared from concentrated acetic acid and concentrated ammonium hydroxide and was then diluted 1:lO for use as a mobile-phase component. River water was obtained from the Wabash River ca. 1 mile upstream from West Lafayette, IN, refrigerated at 5 OC, and used within 1 week. Soil-core samples and ground-water samples were collected near a clay dumping pit, which collected the waste-acid process stream from an industrial site where benzidine and 3,3’-dichlorobenzidine have been manufactured. These were tightly sealed in opaque glass containers and refrigerated.
Procedure Sample Preparation. The river-water samples that were used to evaluate this methodology were prepared by diluting 1 mL of a standard aqueous solution of each aromatic amine of interest to 100 mL with freshly collected, untreated river water. The ground-water samples were filtered and analyzed. The soil-core samples were prepared for analysis by slurrying 1-10 g of the well-blended sample in 100-1000 mL of distilled water, with the amounts of each used dependent upon the amount of sample available and upon its amine content. Prior to the column preconcentration step, all of the above aqueous sample solutions were filtered through a Millipore (Bedford, MA) Type GS 0.22-pm filter disk. For each determination, 20 mL of this filtered solution was pumped through the sampling column a t 1.0 mL/min. A representative sampling of the residue trapped by the filtration process should be routinely evaluated for adsorbed analytes by slurrying the residue in 5-10 mL of distilled water, which is then subjected to the analytical procedure. The influence of variations in the stated sample collection and storage procedure on accuracy and imprecision should also be routinely evaluated. ChromatographicConditions. For the determination of benzidine and 3,3‘-dimethoxybenzidine,the analytical mobile phase consisted of methanol/O.l M ammonium acetate in a 3565 ratio, pH 6.2, a t a flow rate of 1.0 mL/min. The detector electrode was carbon paste-oil a t an applied potential of +550 mV vs. Ag/AgCl. The procedure for the determination of 4-aminobiphenyl, 3,3’dichlorobenzidine, and MOCA utilized as the mobile phase 455.5 methanol/O.l M ammonium acetate, pH 6.2, a t a flow rate of 1.0 mL/min, with a glassy carbon electrode a t an applied potential of +950 mV vs. Ag/AgCl. Quantitation of each analyte was accomplished by measurement of the resultant peak height and comparison with a linear calibration plot obtained for the compound added to distilled water or, if available, interference-free river water or ground water.
Results The electrochemical behavior of the five aromatic amines of interest in this study is presented in Figure 2. Diagrams such as this are finding increasing utility for the efficient presentation of voltammetric information (8). In Figure 2, the data for each compound are summarized by two adjacent bars. The ends of each upper bar correspond to the E3/4and Ell4potential values, and the center line corresponds to the EIlzvalue, taken from the chromatographically assisted hydrodynamic voltammogram. These are the potentials when the current is one-fourth, one-half,
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and three-fourths that of the maximum current value of the current-limited region of the voltammogram. The lower bar is positioned between the cyclic voltammetric peak potential (E,) and the potential at one-half the peak current (E,,J. The shading of the lower bars indicates the degree of chemical reversibility of the electron-transfer process (see ref 8 and Supplementary Material). This figure illustrates the close agreement between the information gained from the milligram quantities of compound usually required by an established electroanalytical technique (cyclic voltammetry) and the nanogram of material injected into the instrument with the CAHDV technique. Since the total amount of analyte is frequently limited in environmental work, the information available from the CAHDV technique may have great utility as semiqualitative confirmation of analyte identity. In addition, Figure 2 compares the relative ease of oxidation of the amines studied. From this, the minimum applied potential necessary for maximum sensitivity of the amperometric detector for any one or a combination of these compounds may be closely estimated to be 50 mV beyond the highest E, value of the compound(s) of interest. The detector wifl then be operating in the current-limiting region of the current-potential curve for all of the compounds. Applied detector potentials substantially higher than this should be avoided, since additional components may be oxidized, with the resultant current contributing to the chromatogram as an increase in the background level. If present in sufficient concentration, these may appear as discrete interferences. The effectiveness of the procedures developed was evaluated via the analysis of freshly collected river water to which the compounds of interest were added and by the examination of several soil-core samples and ground-water samples collected near an industrial facility that has, in the past, manufactured benzidine and 3,3’-dichlorobenzidine (DCB). A typical chromatogram of river water to which was added 50 pg/mL of benzidine and 25 pg/mL of 3,3‘-dimethoxybenzidine is shown in Figure 3. The linearity and imprecision of this method as determined for benzidine added to river water is given in Table I. Statistically similar values, reflecting slight differences due to chromatographic band broadening and a variation in the detector response factor, would also be expected for 3,3’-dimethoxybenzidine. Figure 4 shows the chromatogram for the determination of 15 ng/g (15 ppb) of benzidine in a 1.0-g soil sample. This sample was also found to contain a 100-fold higher concentration of DCB (data not shown), which eluted much later with the chromatographic conditions used in Figure 4 and which was not detected a t an applied potential of 550 mV. Environ. Sci. Technol., Vol. 16, No. 5, 1982 265
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Flgurb 3. Example chromatogram of river water wlth 50 pg/mL of benzidine (BD) and 25 pg/mL of 3,3'dlmethoxybenzldIne (DMBD) added, uslng a 20-mL sample volume preconcentratedon a 3 cm X 4 mm precolumn and separated on a 15 cm X 4 mm analytical column, both packed wlth Merck Lichrosorb RP-C2 material. Mobile phase consists of methanok0.1 M ammonium acetate In a 35:65 ratio, pH 6.2, with the amperometric detector (carbon paste) operated at 4-550 mV vs. Ag/AgCI.
Table I. Linearity, Imprecision, and Limit of Detection: Determination of Benzidine and 3,3'-Dichlorobenzidine in River Water by Precolumn Sample Preconcentration benzidine linear range evaluated correlation coeff. imprecision (% RSD, n = 6 ) 25 pg/mL 250 pg/mL 500 pg/mL 5000 pg/mL detection limit sample volume
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25 pg/mL to 500 pg/mL to 5 ng/mL 10 ng/mL 0.9999 0.9978 9.13 3.83 5pg/mL 20 mL
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The sampling column was apparently very effective in removing all of those analyte molecules examined in this study from the sample solutions. This was established by collection of the effluent from the sampling column during a sample analysis and subsequent retesting of this in an identical manner. In all cases, the residual analyte concentrations were below the stated limit of detection. Since significantly different chromatographic conditions and a much higher detector potential are necessary for the simultaneous determination of 4-aminobiphenyl, DCB, and MOCA, the effectivenes of column preconcentration when employing these instrumental parameters was evaluated in a similar way, with DCB as the representative analyte. These values are also given in Table I, and statistically similar values are expected for 4-aminobiphenyl and MOCA. The concentration range of linear detector response for DCB is somewhat higher than that found for 266
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Figure 4. Example chromatograph of the determination of 15 ng/g of benzidine (BD) in a 1.O-g soil-core sample collected near a facility that has manufactured benzidine. The sample was slurried In 200 mL of distilled water, followed by precolumn preconcentration of 20 mL of the filtered solution. Chromatographicconditions are as specified in Figure 3.
benzidine, which reflects the lower sensitivity of glassy carbon as the detector electrode used for the DCB determination relative to the carbon paste used for benzidine, as well as a higher level of background noise seen a t the higher applied detector potential. Figure 5 shows a chromatogram of river water with 1 ng/mL each of 4aminobiphenyl, DCB, and MOCA added. Although these conditions were selected to optimize the determination of this combination of pollutants, they could be altered if the determination of any one of the individual compounds was the analytical goal. The conditions used in Figure 5 were, nonetheless, found to be excellent for the quantitation of 1ng/mL of DCB in a filtered ground-water sample in the presence of several unidentified components, as shown in Figure 6. A detectable concentration of benzidine was also found in this sample (data not shown), but in Figure 6 it has eluted shortly after the void volume and is not resolved. Discussion The analytical methodology under consideration in this report represents a distinct improvement in the technology available for the determination of a significant number of priority organic pollutants. Several variations of a complete procedure based on this technology are described and evaluated for five toxic aromatic amines of industrial importance. Each involves simple filtration of a raw aqueous sample, utilization of a large volume of this for column preconcentration of the analytes of interest, separation of these components by reversed-phase liquid chromatography, and quantitation by electrochemical detection. This approach allows quantitation of representation analytes a t the low pg/mL level in river water and ground water,
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Flgure Example chromatogram of river water with ng/mL each of 4-a ioblphenyl (ABP), 3,3’dlchiorobenzidlne (I: ), and 4,4’methylenebis [2-chloroaniline] (MOCA) added. Columns and sample volume used are ldentlcal with those of Figure 1. The mobile phase consists of methanok0.1 M ammonium acetate In a 4555 ratio, pH 6.2, with the amperometric detector (glassy carbon) operated at 4-950 mV vs. Ag/AgCI.
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and no interferences with the amines of interest were observed. Soil samples are very conveniently analyzed by extraction with a large volume of water, followed by column preconcentration of a portion of the filtered solution. In this situation, additional sensitivity may be gained by
increasing the ratio of solid sample weight per volume of solution utilized. A relatively small ratio was employed in the example shown in Figure 4. The apparently very low concentrations of suitably hydrophobic, electroactive molecules that may be determined in this manner arise from the strong adsorption of such species from a flowing aqueous sample solution onto the reversed-phase packing material of the sampling column and the subsequent rapid desorption of the accumulated material in a narrow band by the chromatographic mobile phase. The most significant advantage that results is that all of the molecules present in the sample volume utilized for the analysis participate in the quantitation step, since none are lost or wasted as a consequence of sample pretreatment steps. Although time is required to pass the filtered sample through the sampling column, the extraction is now performed in situ, hence the time and effort previously required by the solvent extraction and evaporative steps of existing methods has been eliminated. The height of the chromatograph peak resulting from the preconcentration step is dependent upon the amount of material adsorbed onto the column and subsequently detected and not upon the volume from which it was derived. Consequently,since the values given in Table I are derived from a selected sample volume of 20 mL, they could be improved or extended if larger volumes of sample are employed. An inherent difficulty with many analytical methods based on liquid chromatography is an unambiguous confirmation of the chemical identity of the species being detected. Consequently, the greatest merit of precolumn sample preconcentration as an alternative means of sample injection lies in the extention of the range of analyte concentrations that becomes accessible to accurate quantitation without extensive sample manipulation. Concentrations that lie above this “extended range” are probably more effectively determined by procedures utilizing conventional sample-injection techniques. Since no other methods are readily available that achieve comparable qualitative or quantative performance with equivalently simple requirements for sample preparation, this approach is most suitable for the quantitation of analytes known or suspected of being present in the samples of interest. This must be based on a knowledge of the history of the sampling site or from other samples collected nearby (or upstream) containing higher analyte levels from which the presence of the compound(s) of interest may be established by other analytical means. With the present methodology, however, the information presented in Figure 2 makes possible partial confirmation of the suspected identity. For quantitation purposes, the amperometric detector is operated on the plateau of the current-potential curve, where peak height is independent of applied potential. This peak will be reduced by one-fourth, one-half, or three-fourths if the potential is reduced to the E, Elf2, or El/, value of the corresponding compound. In this way, a unique property (current-potential response) may be used to support the identity of an analyte in a sample by comparing the variation in peak height with applied detector potential to that for a standard. Studies are currently underway to define the reliability of this approach.
Supplementary Material Available A description of the procedure utilized to obtain a chromatographically assisted hydrodynamic voltammogram and a description of the construction and interpretation of diagrams which display voltammetric information (3 pages) will appear following these pages in the microfilm edition of the journal. Photocopies of the supplementary material from this paper or microfiche (105 Environ. Sci. Technol., Vol. 16, No. 5, 1982
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Environ. Sci. Technol. 1982, 16, 268-273 X 148 mm, 24X reduction, negatives) may be obtained from Distribution Office, Books and Journals Division, American Chemical Society, 1155 16th St., N.W., Washington, D.C. 20036. Full bibliographic citation (journal, title of article, author) and prepayment, check or money order for $3.00 for photocopy ($4.50 foreign) or $4.00 for microfiche ($5.00 foreign), are required.
Literature Cited Fed. Regist. 1979, 44, 69 464-69 575. Riggin, R. M.; Howard, C. C. Anal. Chem. 1979,51,210-214. Fed. Regist. 1974. 39. 3756-3797. Perry, 6. L.; Chuang; C. C.; Jungclaus, G. A.; Warner, J.
S. Battelle Columbus Labs, Columbus, OH, February 1979, NITS PB-294 794, p 3.
(5) Ogan, K.; Katz, E.; Slavin, W. J . Chromatogr. Sei. 1978, 16, 517-522. (6) Koch, D. D.; Kissinger, P. T. Life Sci. 1980,26,1099-1107. (7) Bratin, K.; King, W. P.; Kissinger, P. T.; Rice, J. R. In
“RecentAdvances in Pesticide Analytical Methodology”; Harvey, J. C., Ed.; American Chemical Society: Washington, D.C., 1980, ACS Symp. Ser. No. 136, Chapter 5. (8) Miner, D. J.;Rice, J. R.; Riggin, R. M.; Kissinger, P. T. Anal. Chem. 1981,53, 2258-2263. (9) Davis, G. C.; Kissinger, P. T. Anal. Chem. 1979, 51, 1960-1965. Received for review June 8,1981. Revised manuscript received October 26, 1981. Accepted January 12, 1982.
Reactions of Chlorine Dioxide with Hydrocarbons: Effects of Activated Carbon Abraham S. C. Chen, Rlchard A. Larson, and Vernon L. Snoeylnk”
Department of Civil Engineering, University of Illinois, Urbana, Illinois 6 1801 Chlorine dioxide was shown to react rapidly with one group of easily oxidized hydrocarbons in dilute (ca. 0.5 mg/L, 5 X lo4 M) aqueous solution. Hydrocarbons with benzylic hydrogen atoms (ethylbenzene, indan, Tetralin, diphenylmethane, and fluorene) reacted, probably by radical pathways, to give oxidized derivatives such as ketones and (sometimes) alcohols a t the benzylic positions. In the presence of activated-carbon columns, additional products were formed under some conditions. When a hydrocarbon was allowed to react with chlorine dioxide in aqueous solution for 2.9 min a t pH 3.5 and the reaction mixture was passed over a bed of HD3000 granular activated carbon, monochloro and/or dichloro derivatives were produced in addition to the oxygenated compounds observed in the absence of the carbon. W
Introduction Chlorination, as normally practiced in water and waste-water treatment, results in the formation of trihalomethanes and other chlorinated organic compounds which may be undesirable from the viewpoint of waterpollution control and human health. Accordingly, alternatives to chlorine for the disinfection of water and waste water are being sought; chlorine dioxide (C102) is one disinfectant that has received considerable attention as an alternative disinfectant to chlorine. Activated carbon has been used effectively for the removal of trace organic contaminants in water supply. Should chlorine dioxide be used as a disinfectant, it would react with some organic compounds in water and come into contact with activated carbon in the treatment processes (1). It is important to ascertain that potentially toxic organic compounds will not be formed in chlorine dioxide treated water or on the activated-carbon surface and eventually pass into the effluent from the activated-carbon bed. The end products from the treatment processes therefore need to be carefully examined. The reactions of chlorine dioxide with most of the organic compound types frequently found in water have not been thoroughly studied. Most of the work reported so far has been done with tertiary amines ( 2 , 3 )and phenols (4-6); there are a few reports on its reactions with olefins (7-9). Its oxidative reactivity toward alcohols and aldeh268
Envlron. Sci. Technol., Vol. 16, No. 5, 1982
ydes (to form the corresponding carboxylic acids) appears to be greater than that of aqueous chlorine (10). Chlorine dioxide does not react with saturated aliphatic hydrocarbons and aliphatic side chains (11),but the latter may be split from aromatic rings or other functional groups (12). The chlorination of aromatic and unsaturated aliphatic hydrocarbons by chlorine dioxide has been reported by some researchers (7,10,12,14,15). No trihalomethanes have been detected as reaction products of chlorine dioxide with organic materials (11,12, 16-18). There is no information available to date on the reaction of chlorine dioxide with activated carbon, although granular activated carbon (GAC) has been known to react readily with free chlorine (HOC1and OCl-) to produce C1(19) and to react much more slowly with monochloramine (20). During the reaction with free chlorine, suface changes have been reported (21-23). In addition, a high molecular weight colored product as well as some smaller chlorinated organic molecules and aromatic hydrocarbons are obtained under harsh conditions (2.5 g as C12/g of GAC) of reaction of free chlorine with bituminous-based activated carbon (1). It is not known, however, if chlorine dioxide will produce similar materials from activated carbon. Most investigations of organic compounds in the presence of activated carbons have been determinations of their adsorptive properties. Very few studies have been done on the role of activated carbon in the reaction of sorbed organic compounds. (A few reports concerning activated-carbon-catalyzed reactions include oxidation of oxalic acid to COz (24) and conversion of n-butylmercaptan to dibutyl disulfide (25).) Recently, McCreary and Snoeyink (26)and McCreary (27) observed that GAC, in the presence of free chlorine, promoted the formation of new products that were not observed in solution. The mechanistic role of GAC in these reactions was not determined; oxygen-containing functional groups or metals on the surface may have played roles (24,25,28). This study was therefore designed to characterize the products of the reactions between aqueous chlorine dioxide and some selected hydrocarbons either contained in aqueous solution or adsorbed on an activated-carbon surface. Chlorine dioxide tends to disproportionate into chlorite (C102-)and chlorate (C103-) in alkaline solution (24-31), but no appropriate analytical techniques are
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