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Anal. Chem. 1985, 57,2334-2337
Comparison of Microcolorimetric and Macrotitrimetric Methods for Chemical Oxygen Demand of Oil Shale Wastewaters Bonnie M. Jones’ and Richard H. Sakaji Applied Science Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 Christian G. Daughton* Sanitary Engineering and Environmental Health Research Laboratory, University of California (Berkeley), Richmond, California 94804
Modlficatlons are described for both the mlcrocolorimetricand macrotltrlmetrlc chemical oxygen demand (COD) procedures. For the former, a prolonged coollng period, followed by extensive mixing, minimizes the spectrophotometric Interference from refractive index changes and catalyst-induced preclpltate. For the latter, automated tltration and absorbance monitoring near the chromate-chromic isosbestic point improves accuracy and reproduclbillty. The applicability of these modified methods for quantifying the COD of oil shale wastewaters was demonstrated. For 12 oll shale wastewaters, the microcolorimetric method gave COD values (900-150000 mg/L) that dld not dlffer statlstlcally ( P > 0.10) from those obtained by the macrotltrimetric COD method. The relatlve standard devlatlons for flve repllcates of each wastewater were less than 1.3% for the macrotitrimetric method and generally less than 5.0% for the mlcrocolorimetrlc method; no matrix effects were evident.
The chemical oxygen demand (COD) test was originally used as a rapid estimator of the biochemical oxygen demand (BOD) of wastewater organic material (I). The numerical value obtained from a COD test reflects both the organic carbon concentration and the overall oxidative state of the organic material. The COD of a sample is defined by the absolute amount of hexavalent chromium that is reduced during 2 h of digestion by potassium dichromate in a solution of 50% sulfuric acid. Ideally, organic compounds are completely oxidized to carbon dioxide and water with the simultaneous stoichiometric reduction of the orange, hexavalent dichromate ion (Cr“) to the green, trivalent chromic ion (Cr3+). The degree of reduction is quantified either by colorimetry (i.e., by determination of the remaining dichromate ion or, alternatively, by determination of the newly produced chromic ion) or by titrimetry (i.e., redox titration of the remaining dichromate ion). This value is then related stoichiometrically to equivalents of oxygen. The original method for determining COD (using titrimetric quantitation) has undergone numerous revisions. The major modifications include (i) the addition of silver as a catalyst to enhance oxidation of certain biodegradable compounds (Le., aliphatic hydrocarbons, straight-chain alcohols, and fatty acids) that are not fully susceptible to the chemical oxidant and (ii) the addition of mercuric sulfate to complex chloride ion so that chloride precipitation of the silver catalyst and chloride oxidation by dichromate are minimized. Although these revisions have improved the “accuracy* of the method, the large scale of the macrotitrimetric method (50 mL of sample and a total waste volume of approximately 300 mL/sample) and the concomitant space, energy, and wasteIPresent address: Department of Public Works, City and County of San Francisco, 750 Phelps St., San Francisco, CA 94124-1091.
disposal requirements have prompted a search for alternative means to minimize these costly disadvantages. A micro variation of the COD method, which uses colorimetric detection, has been described (2). The smaller-scale version requires approximately 5% of the normal sample and reagent volumes, which results in a minimal volume of acidic waste. The colorimetric detection step quantifies either the amount of Cr3+ produced or the amount of Cr6+ remaining; with either, the precision, accuracy, and ease of analysis are reportedly improved compared with titrimetry (2). Furthermore, the micro method requires far less space, time, and glassware than the standard method ( 2 ) . Two significant problems have been encountered with the colorimetric determination of COD: (1)the appearance of catalyst-induced precipitates of mercuric and silver salts (3-5) and (2) “stratification” of the acid-water mixture following digestion ( 5 ) . When a temperature gradient exists in the digestate, Schlieren lines form. This severely interferes with colorimetric quantitation because the varying refractive index causes substantial drift and inaccuracy in absorbance readings. The procedure described here was designed to minimize the effect of these phenomena on colorimetric detection. To obviate the problems associated with the microcolorimetric method, yet take advantage of the reduced scale, a semimicrotitrimetric method that uses sealed culture tubes instead of refluxing flasks has been reported (3). It purportedly affords many of the advantages of the microcolorimetric method, while allowing for titrimetric quantitation of Cr“. Manual back titration of Cr6+after sample digestion, while eliminating the problems associated with colorimetric detection, is an insensitive method of detecting unreacted Cr6+ ( 2 ) . Automatic titration near the isosbestic point (535 nm) in the presence of 1,lO-phenanthroline was investigated in an attempt to improve the precision and sensitivity of this redox titration for the standard macro-COD method. Incorporating the modifications to each technique, the microcolorimetric and macrotitrimetric methods were compared for their abilities to measure COD in oil shale retort wastewaters. This sample matrix is particularly challenging because the process wastewaters presently available from pilot-scale facilities are highly contaminated with a complex spectrum of inorganic and organic solutes (6-8) and some are extremely turbid. Process waters can contain thiosulfate, chloride, and ammonia; up to 20% of the COD for one retort water was reportedly contributed by thiosulfate (9). Much of the characteristic organoleptic properties of these waters are contributed by nitrogen and oxygen heterocycles, compounds whose potential oxygen demands can only be partially accounted for by the COD test because of their resistance to acidic dichromate digestion (1,2, 10-12). EXPERIMENTAL SECTION Apparatus, For the microcolorimetric method, samples were digested in Corning (No. 9826) 16 X 150 mm Teflon-lined
0003-2700/85/0357-2334$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985
screw-cap (cap No. 9998) culture tubes. A fluidized sand bath (Tecam Model SBL-1, Techne, Inc., Princeton, NJ) with OVerflow flange (acc no. 1133) and circular stainless-steel42-place test-tube rack was used to reflux the samples at 150 OC. A vortex mixer was used to mix the digestates, and sample absorbances were determined with a Bausch & Lomb Spectronic 2000 spectrophotometer with micro-flow-through cell (1cm path length) equipped with Viton O-rings and operated with a peristaltic pump. For the macrotitrimetric method, samples were digested in 250-mL Erlenmeyer flasks with 24/40 T outer joints connected to 30-cm West condensers with 24/40 7 innerdrip bottom joints. A 3600-W hot plate was used to heat the samples. The amount of unreduced dichromate was determined by automatic titration using a Sybron/Brinkmann autotitrator (Metrohm Model 655 Dosimat, E 526 titrator, 643 control unit/624 autosampler; Westbury, NY) using a colorimeter with submersible fiberoptic probe (1-cm path length) and 545-nm filter (Brinkmann PC 800). All pipetting was done with air-displacement (Gilson) or positive displacement (SMI) pipets. Reagents. All chemicals were ACS reagent grade, and water was ASTM type I quality. A stock potassium hydrogen phthalate (KHP) solution was prepared by dissolving 850.2 mg of KHP (dried at 105 "C) in 300 mL of water in a 1-L volumetric flask and diluting to volume with water. The solution has a COD of 1000 mg/L (i.e., 1mL = 1mg COD). For the macrotitrimetric procedure, at least one standard was run in parallel to the samples for each series of digestions. To prepare the standard, 10.00 mL of KHP solution and 10.00 mL of water were substituted for 20 mL of sample. Working standards of 50,100,250,500, and 800mg/L COD (required for the microcolorimetric procedure) were prepared by adding the appropriate volume of KHP solution (i.e., 0.50, 1.00, 2.50, 5.00, and 8.00 mL) to a 10-mL volumetric flask and diluting to volume with water. Digestion reagent was prepared by adding 10.216 g of K2Cr207,167 mL of concentrated H2S04,and 33.3 g of HgS04 to 500 mL of water in a 1-L volumetric flask and diluting the cooled solution to volume with water. Sulfuric acid-silver sulfate catalyst solution was prepared by dissolving 22 g of Ag,SO, in a 4.09-kg (9-lb) bottle of concentrated H2SOI. The 0.25 N stock titrant solution was prepared by dissolving 100 g of (NH4)2Fe(S04)2.6H20 (FAS) in 800 mL of water in a 1-L volumetric flask, adding 20 mL of concentrated H2S04, and diluting to volume with water. The 0.10 N titrant solution was prepared by adding 400 mL of 0.25 N stock titrant solution to a l-L volumetric flask and diluting to volume with water. Commercially prepared ferroin indicator (ferrous phenanthroline, A.P.H.A.) (Anderson Laboratories, Inc., Fort Worth, TX) was used in the colorimetric end point detection for the macrotitrimetric method. Wastewater Samples. The origins of nine of the 12 waters and some of the retorting process information have been reported (23);collection and initial storage conditions, however, were often unknown. After receipt, the samples were kept at 4 "C in polyethylene-lined 30- or 50-galion drums or polyethylene reagent bottles. TOSCO HSP retort water was provided directly by The Oil Shale Co. (TOSCO, Golden, CO). LANL retort water was supplied in June 1983 by Los Alamos National Laboratory; it was an equal-volume composite from their bench-scale retort (runs 33,35, 36,41, 46, 51, 64,7479, and 88). The other 10 waters were obtained from the Department of Energy's cold-storage facility in Laramie, WY. The Oxy-6 retort water noncomposite (nc) sample was taken from the same sampling point during burn 6 as the Oxy-6
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2.7
1 .E 0
c
? 10
n
a 0.9
0
Wavelength, nrn
Flgure 1. Absorbance scans of digested potassium hydrogen phthalate standards (mg/L COD) (inset: detail around isosbestic point at 535 nm).
composite sample, but at a different time. Oxy-7&8 gas condensate was collected during the simultaneous modified in situ burns 7 and 8 at Occidental Oil Shale Company's Logan Wash site in 1982. The 12 waters ranged from a relatively dilute sour water (COD of about 920 mg/L) to an extremely concentrated direct-mode retort water (COD of over 150000 mg/L). Each water was pressure-filtered through 0.4-pm pore-diameter polycarbonate membranes before analysis. Procedures. All culture tubes and Erlenmeyer flasks were soaked overnight in a 35% nitric acid bath, rinsed with water, and either air-dried or oven-dried at 100 "C. The screw caps were rinsed thoroughly with water and allowed to air-dry. For the microcolorimetric methods, 1.50 mL of digestion reagent and 2.50 mL of sample were pipetted into a digestion tube. Catalyst solution (3.5 mL) was then carefully added down the side of the tube so that the acid formed a layer beneath the mixture of digestion solution and sample. The tube was capped tightly and mixed. Three reagent blanks and a set of standards (50-800 mg/L COD) were prepared in the same manner and were analyzed with each set of samples. Samples, blanks, and standards were digested in a sand bath at 150 "C (the reflux temperature of 50% sulfuric acid). After 2 h, the tubes were removed and cooled to room temperature. To minimize the interference from Schlieren lines, the digestates were again thoroughly mixed, and particulates were allowed to settle before absorbance measurement. Increase in Cr3+absorbance was measured against a water reference at 600 nm for samples with COD concentrations exceeding 250 mg/L COD (Figure 1,inset). Alternatively, the decrease in Cr6+was measured against the 250 mg/L COD standard at 440 nm for samples with relatively low COD concentrations ( 0.10) between the two methods: F, < F,,,, (0.29 < 2.77), although there was a significant interaction effect between methods and wastewaters, Fs> F,,, (6.33 > 2.46). The results of Tukey’s test for nonadditivity indicated that an insignificant portion (P > 0.10) of the interaction was nonadditive; therefore the assumptions of the ANOVA were not violated, Although it has been hypothesized in the literature that sealed-tube digestion methods have improved COD recovery because of the capture of volatile compounds that would be lost during refluxing ( 2 , 5 ) ,the comparison study data did not reflect any statistically significant difference between the two methods for oil shale process waters. The accuracy of the two COD procedures was assessed for high (192.7 mg/L COD) and low (10.4 mg/L COD) EPA quality control standards obtained from the Environmental
ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985
Table I. Comparison of Macrotitrimetric and Microcolorimetric Methods for Determination of COD in Filtered Oil Shale Process Waters'
process water Paraho LANL composite* 150-Ton s-55 TOSCO HSP Oxy-6 retort water Geokinetics Oxy-7&8 gas condensate Omega-9 Oxy6 retort water (nc) Oxy-6 gas condensate Rio Blanc0 sour
macrotitrimetric mean RSD 151600 46710 22110 11050 9414 9193 8967 7191 3602 3596 3 408 2308 912.4
0.79 0.74 0.59 1.1 0.35 0.47 0.52 0.81 1.3 1.2 0.50 1.0 0.33
microcolorimetric mean RSD 141400 50500 23280 11660 9781 9459 8990 7578 2 940 3729 3 170 2074 924.8
4.5 13 2.5 2.4 2.4 2.3 0.86 1.5 1.6 1.2 8.5 6.4 0.74
mg/L COD; n = 5 for each sample. *Equal volumes of each water, except Oxy-7&8, Oxy-6 (nc), and LANL. (I
Monitoring and Support Laboratory, US.EPA, Cincinnati, OH. For both standards, the recoveries using the macrotitrimetric method were within 4% of the theoretical COD and within 1% of the empirical value reported by EPA; the relative standard deviation values for five replicates were 2.4% and 5.7% for the high and low samples, respectively. For the microcolorimetric method, the recovery from the high-range standard was within 5% of the theoretical COD and within 1% of the empirical value reported by EPA (relative standard deviation = 13.9%). The COD concentration of the low-range standard (10.4 mg/L COD) was below the lowest standard (50 mg/L COD) and therefore could not be accurately analyzed without modification of the existing method. The "accuracy" of COD values is impossible to validate for a complex matrix such as oil shale process water; each wastewater is an undefined mixture of hundreds of organic compounds each of which may be oxidized by a COD method to various degrees. The incomplete recovery from a complex mixture of a spike of an easily mineralized organic compound, such as KHP, gives an indication of matrix effects. A composite oil shale process water (comprising equal volumes of each retort water, except Oxy-7&8, Oxy-6 (nc), and LANL) was diluted 80-fold with water and various volumes of KHP standard solution so that the final KHP spike concentrations of duplicate samples were 100,200,300, and 400 mg/L COD; the COD values were determined by the microcolorimetricand macrotitrimetric methods. The linear regression equations for COD recovered vs. spike added were nearly identical (microcolorimetric, y = 0.949~+ 307; macrotitrimetric, y = 0.948~ 295). The recovery of KHP spike from diluted
+
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composite samples ranged from 83.0% to 95.7% for the microcolorimetric method and from 94.0% to 96.3% for the macrotitrimetric method. The extrapolated x intercept values for amount recovered vs. amount added were within 6% of the respective zero-spike values indicating that matrix effects were minimal-324 mg/L COD vs. 312 mg/L COD, for the microcolorimetric, and 312 mg/L COD vs. 294 mg/L COD, for the macrotitrimetric method. The coefficient of determination (?) values for both methods exceeded 0.995 (microcolorimetric = 0.996; macrotitrimetric = 1.000).
ACKNOWLEDGMENT The authors express their gratitude to Gregg W. Langlois and Karen Yu for their attention to detail in performing many of the COD analyses. LITERATURE CITED (1) Moore, W. A.; Kroner, R. C.; Ruchhoft, C. C. Anal. Chem. 1949, 21, 953-957. (2) Jirka, A. M.; Carter, M. J. Anal. Chem. 1975, 47, 1397-1402. (3) Himebaugh, R. R.; Smith, M. J. Anal. Chem. 1979, 51, 1085-1087. (4) Gibbs, C. R . "Introduction to Chemical Oxygen Demand"; Hach Chemical Co.: Loveland, C O 1979; Technical Information Series-Booklet No. 8. (5) Messenger, A. L. J.--WaterPo/lut. ControlFed. 1981, 53,232-238. (6) Hawthorne, S. B.; Slevers, R. E. Environ. Sci. Techno/. 1984, 18, 483-490. (7) Leenheer, J. A.; Noyes, T. I.; Stuber, H. A. Environ. Sci. Techno/. 1982, 16, 714-723. (8) Raphaelian, L. A.; Harrison, W. "Organic Constituents in Process Water from the In-Sltu Retorting of Oil from Oil-Shale Kerogen"; ANL/PAG5, Argonne National Laboratory: Argonne, IL, 1981. (9) Wong, A. L.; Mercer, B. W. "Contrlbutlon of Thiosulfate to COD and BOD in Oii Shale Process Wastewater"; PNL-SA-7795, Battelle Pacific Northwest Laboratories: Richland, WA, 1979. (10) APHA "Standard Methods for the Examination of Water and Wastewater", 15th ed.; American Public Health Association, American Water Works Association, Water Pollution Control Federation: Washington, DC, 1981; pp 489-493. (11) ASTM "Annual Book of ASTM Standards, Part 31, Water"; American Society for Testing and Materials: Philadelphia, PA, 1980, pp 665-669. (12) Medalia, A. I . Anal. Chem. 1951, 23, 1318-1320. (13) Langlois, G. W.; Jones, B. M.; Sakaji, R. H.; Daughton, C. G. J. rest. Eva/. 1984, 12, 227-237. (14) Molof, A. H.; Zaleiko, N. S. "Proceedings of 19th Industrial Waste Conference"; Purdue University: West Lafayette, IN, 1964; pp 540-55 1. (15) Jones, B. M.; SakaJI,R. H.; Daughton, C. G. I n "A Manual of Analytical Methods for Wastewaters", 2nd ed.; Daughton, C. G., Ed.; Lawrence Berkeley Laboratory: Berkeley, CA, 1984; Chapter V I ; LBL-17421 (NTIS DE84015967). (16) Rohlf, J. F.; Sokal, R. R. "Statistical Tables"; W. H. Freeman and Co.: San Francisco, CA, 1969. (17) Sokal, R. R.; Rohlf, J. F. "Biometry"; W. H. Freeman and Co.: San Francisco, CA, 1969.
RECEIVED for review January 16,1985. Resubmitted February 21,1985. Accepted June 4,1985. This work was supported by the Assistant Secretary for Fossil Energy, Office of Oil Shale, Division of Oil, Gas, and Shale Technology of the US. Department of Energy under Contract No. DE-AC0376SF00098.
