Anal. Chem. 1985, 57, 1338-1341
1338
organic synthesis (91, but the reaction could be suspected to take place to some extent even in the 0.5 M OH- concentration of the alkaline formaldehyde. Decreases of 2.4 and 6.3% in color intensity were observed when alkaline formaldehyde which had been stored for 12 and 30 days, respectively, was used in parallel with freshly prepared base for analysis of test samples. (Whether this was actually due to a Cannizzaro reaction was not investigated). The alkalide formaldehyde should therefore be freshly prepared every day. No difference was seen in blank magnitude or the color intensity obtained when freshly prepared color reagent and reagent that had been stored 30 days in the laboratory were used in parallel for analysis of a test sample. We are currently working on an adaption of this method for flow injection analysis.
Registry No. Sulfur dioxide, 7446-09-5; pararosaniline, 56961-9; ethandial, 107-22-2; formaldehyde, 50-00-0.
ACKNOWLEDGMENT
RECEIVED for review July 17, 1984. Resubmitted January 22, 1985. Accepted January 22,1985. Grants from the Stiftelsen Bengt Lundqvists Minne are gratefully acknowledged.
Thanks are due to Michael Sharp for linguistic revision of the manuscript.
LITERATURE CITED (1) Irgum, K.; Lindgren, M. Anal. Chem. 1985, 57, 1330-1335. (2) Dasgupta, P. K.; DeCesare, K.; Ullrey, J. C. Anal. Cbem. 1960, 52, 1912-1922. (3) Scaringelli, F. P.; Saltzman, 6.E.; Frey, S.A. Anal. Cbem. 1967, 39, 1709-1719. (4) Irgum, K. Anal. Cbem. 1965, 57, 1496-1498. (5) Saiomaa, P. Acta Cbem. Scand. 1956, 10, 311-319. (6) West, P. W.; Gaeke, G. C. Anal. Cbem. 1956, 28, 1816-1819. (7) Skrabal, A.; Skrabal, R. Monatscb. Cbem. 1936, 69, 11-41. (8) Harned, H. S.;Owen, 6.6. "The Physical Chemistry of Electrolyte Solutions"; Reinhold: New York, 1958; ACS Monograph Series, p 729. (9) March, J. "Advanced Organic Chemistry", 2nd ed.; McGraw-Hill: New York, 1977; pp 1139-1140.
Sulfate Determination in Industrial Wastewater by Liquid Chromatography with Postcolumn Solid-Phase Reaction Detection Kommer Brunt Analytical Department, Potato Processing Research Institute-TNO, Rouaanstraat 27, 9723 CC Groningen, T h e Netherlands
A sulfate selective postcolumn solid-phase reactor (SPR) detectlon system is described for the sulfate determination at parts per million (ppm) levels in wastewaters of the potato starch Industry. The effect of the chromatographic flow rate on the converslon reaction in the SPR Is dlscussed. The linear dynamic range of the detection system In combination with the used anlon-exchange separation is from about 5 to 400 ppm. The repeatability of the system at the 30 ppm sulfate level Is better than 3.5 %. The SPR system can be used for about 13 h without noticeable depletlon effects. Day-to-day repeatablllty Is good. Application of the described detection system for the sulfate determlnatlon in industrlal wastewaters Is dlscussed. Recoveries in spiked wastewater samples were better than 90 %
.
Sulfate is widely distributed in nature. It is present in natural waters such as surface water and rain water in concentrations ranging from almost zero to rather high levels of hundreds of parts per million. Industrial wastes often contain considerable amounts of sulfate. It is possible to remove sulfate by anaerobic wastewater treatment, as is done by the Dutch potato starch industry ( I ) . Knowledge of sulfate concentrations in the different steps may be an important parameter for the process control. Moreover these data are also important for a better understanding of the chemical and microbiological processes which occur in the anaerobic purification reactors (1). Different analytical procedures are described for the determination of sulfate (2, 3). It is beyond doubt that gravimetric and turbidimetric methods are cumbersome and
time-consuming, especially when the chemical oxygen demand (COD) of the wastewater is high (10000 mg of 0 2 / L and higher). Colorimetric ( 4 , 5 ) ,continuous flow (6, 7 ) ,and flow injection methods ( 5 , 6) are not satisfactory because too many interfering compounds are present in a lot of industrial wastes. The sulfate determination by modern ion chromatography with silica-based columns and with the two-column system marketed by Dionex Corp. have been described extensively (e.g., ref 8-10). However, the described applications concern mostly analyses in surface, rain, and ground waters, plating baths, and brine solutions. Unfortunately silica-based columns will deteriorate rapidly due to the high COD levels in the wastewater samples if no (expensive) guard columns are used. The more classical polystyrene-divinylbenzene cross-linked ionexchange resins are much less susceptible to deterioration by the organic compounds present in the wastewater. In contrast, with silica-based stationary phases, ion-exchange resins are easy to regenerate in case the column efficiency decreases due to contamination. However, application of these resins is attended by the use of solutions of relatively highly ionic strength as a mobile phase. Hence, it is not possible to detect sulfate in the parts per million range with a conductivity detector. Therefore, we have applied a sulfate-selective postcolumn solid phase reaction detection system. Such heterogeneous derivatization systems are usually applied to enzymatic conversions by immobilized enzymes in flow systems (11). Recently, Ruter and Neidhardt described a solid-phase reactor (SPR) for the detection of manganese species (12). The use of SPR systems for derivatization in liquid chromatography was reviewed in depth by Xie et al. (13). According to these papers, SPR systems do not introduce any additional
0003-2700/85/0357-1338$01.50/0 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
extra-column dead volume other than that normally introduced with any conventional HPLC guard column. Detailed theoretical discussions and experimental investigations concerning peak-broadening effects by homogeneous derivatization in postcolumn packed bed reactors in liquid chromatography have been published amongst others by Huber et al. (14) and Deelder et al. (15). The band broadening in an SPR packed with catalyst has been discussed recently by Nondek et al. (16). An advantage of an SPR system for derivatization is that it does not require additional (expensive) instrumentation (e.g., pumps, mixing chamber, and reaction chamber) as is needed for the more conventional homogeneous method of postcolumn derivatization. This paper describes the use of an SPR system combined with an anion-exchange separation for the detection of sulfate in anaerobic wastewaters of the potato starch industry, with very high COD values. THEORY The postcolumn reaction detection is based on the indirect colorimetric determination of sulfate by barium chloranilate as was described by Bertolacini and Barney in 1957 (5). They added the rather insoluble solid barium chloranilate to the sulfate-containing solution. The solubility of barium sulfate (pK,, = 10.01) is less than that of barium chloranilate (pK,, = 7.32). Hence the sulfate present will precipitate as barium sulfate, and an equivalent amount of the highly colored acid chloranilate ions is released (eq 1). S042- BaC,Cl,O4(1) H+ BaS04(1) HCBCl2O4: (1) It was reported that the barium sulfate precipitation was completed within 10 min. Later on, Gales et al. (6) adapted the method to a Technicon AutoAnalyzer system for the sulfate determination in surface water in the range of 4-400 ppm. Unfortunately, many cations and anions interfered. Many divalent cations form insoluble chloranilates, resulting in incorrectly low sulfate values. On the other hand, anions may interfere by forming insoluble barium salts which cause incorrectly high sulfate values. These interferences can be eliminated by first applying an anion-exchange separation followed by the above-mentioned indhect colorimetric detection of sulfate. When an anion-exchange column is used, the cations elute in the solvent front and sulfate is separated of most anions. The conversion of sulfate anions into acid chloranilate ions can be performed in an SPR loaded with solid barium chloranilate. It is beyond doubt that a reaction time of 10 min is unacceptable for an SPR detection system. However, it is well-known from chemical engineering that in a continuous-flow packed bed reactor, the reaction rate of an heterogeneous ionic reaction can be increased considerably, due to the very thorough mixing of the dissolved reactants with the solid reactants and the removal of the reaction products by the forced convective diffusion. Moreover, as discussed by Krull and Lankmayr (11),100% conversion is not necessary in a postcolumn reaction system on the condition that the conversion factor does not depend on the sample concentration. In case a buffered sulfate sample is pumped through the SPR packed with barium chloranilate, the system can be considered as a continuous heterogeneous packed bed reactor producing the acid chloranilate ions. In principle the heterogeneous conversion reaction in the SPR is pseudo first order in sulfate. The amount of acid chloranilate ions released ([HC6C12o4-lrel) equals the amount of sulfate precipitated ([S042-],,ec) in the SPR. From first-order reaction kinetics, the following expression can be derived
+
+
-
+
[HC&1204-Irel = [S0d2-]jnit(1 - e-kt) (2) in which [S0d2-]init is the initial sulfate concentration, It a constant, and t the residence time in the SPR. The conversion
ANION EXCHANGE COLUMN
COLUMN
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uv
VALVE MOBILE PHASE
Flgure 1. Liquid chromatographic system with the postcolumn solid-
phase reactor detection system. factor (7) is given by the quotient of [S042-Iprec and [S0d2-Ibit (eq 3) and is independent of the initial sulfate concentration.
7 depends on the residence time t of the sulfate sample in the
SPR, indicating that the sensitivity of the detection system may be flow-dependent. EXPERIMENTAL SECTION Liquid Chromatographic System. All separations were performed on a Hewlett-Packard 1084 B liquid chromatograph equipped with an automatic sample introduction system. A 10 cm x 0.62 cm i.d. column was home-packed with a size fraction (particle size 40-60 pm) of an AG1-X8 anion-exchangeresin (Bio Rad Labs.) in the nitrate form. The particle size distribution of this fraction, obtained by a sequence of sedimentations, was determined by microphotography. The packing procedure was essentially the same as we have described before for a cationexchange resin (17). The postcolumn reaction column (5 cm X 0.46 cm id.) was dry-packed by the tap-and-fill method with a 1:l mixture of silica gel G (10-40 wm; E. Merck) and barium chloranilate (p.a. quality; E. Merck). In front of the outlet frit (porosity 0.5 pm) of the reaction column, an extra 0.5-cm glass wool filter was applied in order to avoid plugging of the detection system by the freshly formed barium sulfate precipitate. Both the anion-exchange column and the SPR column were kept at 35 "C in the column oven. A mixture of one part ethanol and four parts aqueous solution of 0.2 M KNOB,0.01 M phthalate buffer (pH 4.0), and 0.001 M Ca-EDTA was used as the mobile phase. The flow rate was 0.8 mL.min-l. The released acid chloranilate was monitored at a wavelength of 530 nm with an LC-UV Pye Unicam variable wavelength spectrophotometer detector. In Figure 1a scheme is given of the used liquid chromatographic system with the postcolumn solid-phase reaction detection. Procedure. The wastewater samples were taken in 300-mL sample bottles, already filled with 5.0 mL of a preservation solution containing 1%formaldehyde, 2 M zinc nitrate (or zinc acetate), and 1M calcium chloride. About 25 mL of the sample was heated for 15 min in boiling water. The coagulated protein was removed by centrifugation with a simple bench-top centrifuge. A 10.0-mL portion of this centrifugated solution was acidified with concentrated formic acid to pH 1. The solution was then sucked through an activated quaternary amine disposable extraction column of the Baker 10 SPE system. The sulfate was retained, and the (undissociated)organic acids passed this clean-up column. Subsequently the quaternary amine column was washed with 5 mL of ethanol/water (1:4). With 4.0 mL of a mixture of ethanol/aqueous 0.4 M KNOB(1:4), followed by 4.0 mL of ethanol/water (1:4), the sulfate was eluted from the clean-up column. One hundred microliters of the homogenized eluate was injected in the liquid chromatogrphic system. RESULTS AND DISCUSSION Flow Rate Dependence. At different flow rates, a linear relationship was found between the response of the detection system (peak height) and the sulfate concentration of the standard samples injected on the anion-exchange column. The calibration graphs were calculated by linear regression curve fitting (Table I). At flow rates of 0.8 ml-min-' and less, no significant differences were found in the slopes of the calibration graphs. At higher flow rates, a decrease in slope was
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
Table I. Sulfate Calibration Graphs at Different Flow Rates, Calculated by Linear Regression Curve Fitting of the Peak Height ( h ) vs. the Concentration (C in SO& flow rate, ml-rnin-'
concn range, ppm
0.40 0.60 0.80 1.00 1.20
10-200 5-300 5-300 5-300 10-200
calibration graph h h h h h
corr. coeff (r2)
no. of determinations
0.9993 0.9993 0.9996 0.9988 0.9875
5 7 7 7 5
= 0.23C - 0.6 = 0.23C - 0.8 = 0.23C - 0.6 = 0.20c - 0.0 = 0.19C - 0.8
n
5
IO
I5
20
25
5
10
15
20
25
5
IO
15
MINUTES
MINUTES
MINUTES
A
B
C
20
25
IO
5
15
20
MINUTES
D
Flgure 2. Effect of the Zn2+ and Ca" addltion on the oxalate interference. A: 47 ppm SO4'- sample not treated with Zn" and Ca". B: 47 ppm SO4'- sample treated with Zn2+ and Ca'+. C: sample Containing 47 ppm SO4'- and 66 ppm oxalate not treated with Zn'+ and Ca". D: sample containing 47 ppm S O : and 66 pprn oxalate treated with Zn2+ and Ca2+.
observed. From the linearity of the respective calibration graphs can be deduced that the conversion factor 7 is independent of the sample concentration as predicted in eq 3. At flow rates less than 0.8 ml-min-l, 7 probably equals 100%. At higher flow rates, the slope and thus 7 decrease, indicating that the residence time t of sulfate in the SPR is too short to complete the conversion of sulfate into the acid chloranilate. Interfering Compounds. The major interfering compounds present in the wastewater were phosphate, oxalate, sulfite, and sulfide. Phosphate and oxalate anions also form insoluble compounds with Ba2+cations. Hence, these anions can be detected by the barium chloranilate based detection system. By optimizing the chromatographic conditions, it was possible to separate phosphate and sulfate. However, oxalate and sulfate were not completely separated. Oxalate already interfered at a concentration of less than 100 ppm whereas the phosphate peak was only detectable at concentrations of about 500 ppm and more. By addition of Zn2+and Ca2+to the sample prior to the protein coagulation and centrifugation, the interfering anions were precipitated as their insoluble salts. The addition of Zn2+ and Ca2+ does not effect the sulfate concentration in the sample (Figure 2). ZnS04 is highly soluble, and the solubility of CaS04 is sufficiently high with respect to the Ca2+and 502- concentrations involved. With the quaternary amine column of the Baker 10 SPE system, the excess of the added cations is removed. The interferences of sulfite and sulfide are basically different. Sulfate, sulfite, and sulfide are separated on the anion-exchange column. However, the sulfite and sulfide may interfere strongly in the sulfate determination because they are easily oxidized to sulfate. During sampling of the anaerobic digesters, introduction of some atmospheric oxygen into the sample cannot be avoided. Hence, oxidation of sulfite and sulfide starts in principle immediately after the sampling. Moreover, it is reported that during the chromatographic separation, considerable amounts of sulfite can be oxidized to sulfate on the column (18). Hence, before sampling, the sample bottles were filled with 5 mL of sample preservation solution, as described in the Experimental Section. Due to
Table 11. Effect of Formaldehyde Addition during the Wastewater Sampling on the Measured Concentration
wastewater stream (Figure 3)
measd sulfate concn, ppm without with formformaldehyde aldehyde
from potato starch factory from protein refinery cooling (surface) water influent digestion reactor effluent digestion reactor influent methane reactor effluent methane reactor
715 230 105 230 105 55
