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Received for review March 6,1992. Revised manuscript received June 15,1992. Accepted June 25,1992. This project is supported by National Science Foundation Grant ATM-8913723 and National Park Service Cooperative Agreements CA-0424-6-8002 and CA-0424-1-9005. Preliminary results of the surrogate surface experiments reported in this paper were first presented at The Fifth International Conference on Precipitation Scavenghg and Atmosphere-Surface Exchange Processes,Richland, W A (46).
Chloride Interference in the Analysis of Dissolved Organic Carbon by the Wet Oxidation Method George R. Alken US. Geological Survey, Water Resources Division, Box 25046, M.S. 458, Denver Federal Center, Denver, Colorado 80225-0046
The presence of Cl- in concentrations greater than 0.02 M is shown to interfere with the analysis of aqueous DOC concentrations by the wet oxidation method of analysis when a reaction time of 5 min is employed. Chloride competes with DOC for S2082-, lowering the overall oxidation efficiency. The resulting HOC1 from the oxidation of C1- reacts with DOC, producing significant amounts of chlorinated intermediate compounds in addition to COP. These compounds were found in the waste effluent from the reaction chamber and in the gas stream transporting C02to the detector. While a possible C1- effect has been noted for DOC measurements in the past, it has not previously been demonstrated to be a source of error a t the concentrations reported in this paper. The interference can be overcome either by increasing the digestion time or by diluting samples to contain less than 0.02 M C1-.
Introduction Since the early 1960s,dissolved organic carbon (DOC) concentration has become a common and important parameter measured in all types of water samples from saline to fresh waters. With the introduction of wet oxidation
methods for the analysis of DOC in seawater (I, 2), a number of papers have been published focusing on different technologies and problems inherent in the measurement of DOC (3-5). Presently, the most commonly employed methods for determining DOC concentrations involve the oxidation of organic matter to C02. These methods include high-temperature combustion (5),persulfate oxidation (2,4),and ultraviolet photooxidation (6). Difficulties arise in the analysis of DOC, in part, because of the nature of the samples themselves (3). The quantities of DOC present in a given sample may be low, as is the case in seawater and groundwater samples. In addition, DOC comprises a complex mixture of organic compounds that have a range of molecular sizes, weights, and reactivities. Other problems related to the method of analysis include the efficiency of oxidation, the possibility of sample contamination, and the difficulty of obtaining reasonable values for blanks (4). The method using wet oxidation with persulfate, for instance, is dependent on the efficiency of oxidation, which may not be the same for all compound classes or in all sample matrices. Other factors, such as the thermal degradation of persulfate at elevated temperature, also affect the reaction (7).
Not subject to U S . Copyright. Publlshed 1992 by the American Chemical Soclety
Environ. Scl. Technol., Vol. 26, No. 12, 1992 2435
This paper describes the effects of chloride ions on the determination of DOC by a commonly used wet oxidation method. This method is a version of the original method of Menzel and Vaccaro (2),differing only in the mechanics of the oxidation step. In the original method, the sample was sealed in an ampule along with persulfate and oxidized to C02 at 130 OC for 0.5 h. The efficiency of the oxidation step of this method (2) was demonstrated for a suite of organic compounds in the presence of [Cl-] 0.5 M. The method used in this study involves the oxidation of organic matter by persulfate in a gas-tight reaction chamber at a temperature of 100 OC for 5 min.
=
Methods Reagents. Aquatic fulvic acids from the Suwannee River (Georgia) and Lake Fryxell (Antarctica) were used as standards in this study. These samples were isolated on Amberlite XAD-8 resin according to the method of Thurman and Malcolm (8). The use of trade names is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey. All reagents used were reagent grade. DOC Measurements. DOC measurements using the persulfate wet oxidation method with the reaction taking place in a gas-tight reaction vessel were made on an Oceanography International (01)Model 700 carbon analyzer. Samples were introduced into the reaction vessel by means of a fixed-volumesample loop. Sample volume was constrained by the desire to maintain linear instrument response (W50pg of C). The standard, automated analytical conditions called for 0.5 mL of 5% by volume H3P04to be added to the sample. The sample was then purged with N2 for 2.0 min to remove inorganic carbon, and 0.5 mL of 0.42 M sodium persulfate solution was added. The standard reaction time of 5 min for the persulfate oxidation step was employed for routine measurements. DOC measurements by the sealed-ampule, wet oxidation method with persulfate were made using an 01Model 524 carbon analyzer according to method 5310D in ref 9. Ampules were maintained at 130 "C for 4 h. DOC measurements were also made by wet oxidation using the persulfate-UV oxidation method according to the procedure outlined in method 5310C in ref 9 on a Dohrmann DC-180 total organic carbon analyzer. TOX Measurements. Samples were analyzed for total organic halogen (TOX) concentration by an adsorptionpyrolysis-titrimetric method as outlined in method 5320B in ref 9. Samples for analysis were collected directly from the reaction chamber of the 01 Model 700 carbon analyzer immediately after the digestion sequence was completed. All remaining oxidant was then reacted with an excess of Na2S03. Complete consumption of the oxidant was ensured by measuring for residual oxidizing potential with a Hach Residual Chlorine kit, Model CN-21P. Samples were then acidified to pH 2 with concentrated "OB. Analysis of Volatile Organic Compounds. Volatile organic compounds generated during the digestion of organic matter in the reaction chamber of the 01 Model 700 carbon analyzer were adsorbed onto a '/8 in. diameter X 12 in. long column containing OV-1, Tenax, silica gel, and charcoal sorbents that was inserted in the same transfer line of the carbon analyzer immediately downstream of the reaction chamber. After a single run, the trap was removed from the carbon analyzer and installed in a Tekmar Model LSC 2000 purge and trap concentrator. Analytes were desorbed from the trap for 1min at 180 "C with helium at a flow rate of 40 mL/min and transferred to a gas chromatograph containing a 30 m X 0.53 mm i.d. DB-624 2436
Envlron. Scl. Technol., Vol. 26, No. 12, 1992
o
a 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
-log Concentration Flgure 1. Percent of total DOC obtalned for DOC measurements on solutbns of potassium hybogen phthalate (KHP), Suwannee Rlver fuhk add (SRFA), and 2,4didJorophend (DW) In the presence of lncreashg concentrations of chlorMe and bromide (KHP-Br).
megabore column (J&W Scientific, Inc.). The following temperature program was used: -20 OC (hold 1 min), to 20 "C at 20 "C/min, to 100 "C at 5 OC/min, to 160 "C at 20 OC/min (hold 2 min), to 200 "C at 40 OC/min (hold 3 rnin). The column was coupled directly to a Finnigan Model INCOS 50 mass spectrometer, which was set to scan from 45 to 310 Da with a scan time of 0.75 s. Compound identifications were based on comparison of the GC retention time and the mass spectrum of an unknown compound with the retention times and mass spectra of standard compounds.
Results and Discussion Evidence that C1- has a significant effect on DOC measurements by the wet oxidation method employed by the 01 Model 700 carbon analyzer is presented in Figure 1. The data presented for potassium hydrogen phthalate (KHP), Suwannee River fulvic acid (SRFA), and 2,4 dichlorophenol (DCP) at concentrations of approximately 5 mg of C/L show that the effect is noticeable at concentrations as low as 0.01 M C1- and increases with increasing C1- concentration. Past work assessing the efficiency of the oxidation step in seawater using the ampule method (2) indicates efficient oxidation for a suite of organic compounds in the presence of [Cl-] r 0.5 M. The concentrations of Cl- used in the measurements reported here are substantially lower than those found in brines and are representative of waters approximately 16 times less salty than seawater. The effect, therefore, is not limited to brine samples, but has a significant bearing on analyses of waters of medium salinity. The efficiency of the method for the determination of DOC in the presence of chloride depends, in part, on the amount of persulfate present in the reaction chamber. Data presented in Figure 2 show the effect of varying the concentration of NaS20son the measurement of the DOC concentration of a solution of KHP (5.2 mg of C/L) in the presence of 0.1 M Cl-. Even though the percent converted to C02 increases with increasing [s2082-],it was not possible to attain 100% conversion, even with 10 times the amount of S2OS2-normally used to make measurements on DOC samples in the absence of C1-. While the effect appears to be strongly dependent on both the Cl- and S20:- concentrations, it does not appear to be dependent on DOC concentration over the concen-
Reaction Time (min) 0
100
l
10
20
30
p
40
50
60
Table 11. Percent of Total DOC Obtained for DOC Measurements for Various Solutes in the Presence of 0.1 M Chloride’
70
d
KHP benzoic acid sucrose starch
Reaction Time 50
Suwannee River fulvic acid
1
2,4-dichlorophenol trichloroacetic acid
0 0.5 1 1.5 2 2.5 Amount of Persulfate Injected (millimoles)
Figure 2. Effect of increasing persulfate concentration and reaction tlme on the percent of total DOC obtained for DOC measurementson solutions of potassium hydrogen phthalate (5.2 mg of CIL) in the presence of 0.1 M chloride.
Table I. Percent of Total DOC Obtained for DOC Measurements on Increasing Concentrations of Potassium Hydrogen Phthalate (KHP), Suwannee River Fulvic Acid, and 2,4 Dichlorophenol in the Presence of 0.1 M Chloride’ solute
KHP
Suwannee River fulvic acid
2,4-dichlorophenol
5.0 10.0 14.8 20.1 4.6 9.1 13.9 18.5 4.6 8.6 12.9 17.2
4.3 8.6 13.0 17.4 3.3 6.9 10.2 14.1 3.2 6.0 8.7 11.4
86 86 88 87 72 76 73 76 70 70 67 66
‘All measurements were made using a l-mL aample loop. tration range measured (Table I). Relative to the variation in the concentrations of both C1- and Sz082-,the range of DOC concentrations measured is small. The extent to which the presence of C1- influences the efficient oxidation of DOC, however, depends on the type of organic matter in the DOC. Data presented in Table I1 for a variety of organic compounds clearly show that compounds such as sucrose and starch are more efficiently oxidized than materials such as fulvic acid. Of the compounds that were analyzed, the fulvic acid samples are more representative of a large fraction of the types of compounds expected to compose DOC than either of the standards. Given the differences in response for different organic compounds, it must be concluded that it is inappropriate to correct for the presence of C1- simply by running standards in the same chemical matrix as that of the actual sample. Nor is a method of standard additions using a specific chemical compound useful in correcting for the chloride effect. In the strict sense, the method of standard additions requires the addition of the analyte of interest, but in DOC analysis, this is impossible since DOC actually represents a complex mixture of organic compounds. In a study of the kinetics of oxidation of organic compounds in fresh water by persulfate, Goulden and Anthony (7) found that while different organic molecules had different oxidation efficiencies, the kinetics of the reactions
a
0.0 0.1 0.0 0.1 0.0 0.1 0.0 0.1 0.0 0.1 0.0 0.1 0.0 0.1
DOC(mg of C/L)
%DOC recovered
4.9 3.1 5.4 3.3 2.7 2.0 4.5 3.5 5.0 2.6 4.8 2.1 5.0 0.9
100 63 100 61 100 74 100 78 100 52 100 44 100 18
All measurementa were made using a 5-mL sample loop.
could be expressed by simple rate equations. In the absence of chemical interferences, the autoxidation of the S202- and the oxidation of the DOC to C02were the only reactions of importance. Goulden and Anthony (7) concluded that organic materials in fresh water can be completely oxidized at 90-100 OC in a few minutes if a large excess of persulfate is used. In the case of the wet oxidation method employed here, the reaction is carried out at 100 OC and there is approximately lo2 times more persulfate than C present on a molar basis. Under these conditions it appears that all the organic matter is completely oxidized in the absence of C1-. The efficiency of the persulfate oxidation of organic matter, however, can be affected by the presence of any oxidizable chemical species that will consume persulfate. In the case of C1-, the oxidation reaction can be written as 2C1- Clz 2e- with the most likely form of the C12 being HOC1. At sufficiently high concentrations of C1-, therefore, lower recoveries could be due to the decreased concentration of persulfate available to oxidize the organic matter. Under the reaction conditions used for the determination of DOC, the concentration of 0.1 M C1- is approximately twice that of the persulfate, and much greater than the DOC concentrations. Chlorine itself is a powerful chemical oxidant for organic compounds (IO, II), and it is likely that the nature of the chloride interference in the analysis of DOC k more complicated than a simple competition for the persulfate. In studies of the effects of chlorination on the chemistry of organic compounds, such as aquatic fulvic acid in water treatment processes,it has been found that a large number of chlorinated organic molecules are generated, predominantly nonpurgeable chlorinated species such as trichloroacetic acid (11). With this in mind, two chlorinated species representative of the products of chlorination reactions were included in the above suite of compounds (Table 11). The expectation was that the amount of C02 produced with these compounds would be lower than for other organic species, if chlorination were occurring in the reaction vessel. The data presented in Table I1 clearly show that the efficiencies of oxidation for dichlorophenol and trichloroacetic acid in the presence of chloride were the lowest measured. The results of total organic halogen (TOX) measurements on the effluent from the reaction vessel provide direct evidence for the reaction of chlorine with the DOC. Very low amounts of TOX (
