Detennination of Organically Bound Sulfur in Swamp and Terrestrial Waters by Continuous Flow Oxidation and lam J O A N CROWTHER,t F R A N C I S B . L O , * MICHAEL W . R A W L I N G S , A N D BERNARD WRIGHT Laboratory Services Branch, Ontario Ministry of Environment and Energy, 125 Resources Road, Etobicoke, Ontario, M9P 3V6 Canada
Aqueous samples were mixed with a dilute solution of hydrogen peroxide and exposed to ultraviolet radiation in a continuous flow system. The organosulfur compounds were oxidized to oxalate, carbon dioxide, and sulfate. Subsequently, the sulfate ion was separated from the other anions including oxalate by automated ion chromatography using anion columns, carbonate eluent, and conductivity detector. By comparing the sulfate concentrations before and after oxidation, the amount of organically bound sulfur could be calculated. This procedure has been successfully applied to the routine analysis of over 2000 swamp and terrestrial waters submitted by the Limnology Section, Water Resources Branch, Ontario Ministry of Environment and Energy.
Introduction Much of the concern of sulfur dioxide deposition involves only inorganic forms of sulfur, and the main form is sulfate. Various sulfur species including sulfate influence the mobility of ions in soils and affect the capacity of the ecosystem to generate alkalinity. Sulfates are reactive and constantly assimilatedby vegetation (1).The organosulfurs are found in humic and fulvic substances, which are complex polymeric substances resulting from the decomposition of natural organic matter, particularly dead plants (2). The brown color is usually a good indication of the content of such organic matter. On subsequent decay, the vegetation releases the organically bound sulfur to the environment as inorganic sulfur. Therefore, the biochemistry of sulfur is a cyclic process between the inorganic and organic forms (Figure 1). The dissolved and particulate organic form of sulfur may constitute up to 20%of the total sulfur in a particular water column (3). Some examples are sulfur-containing amino acids, sulfonates, and organic sulfates ( 4 ) . The study of acid rain has been focused on the hydrogen and sulfate ions of sulfuric acid. Although inorganic forms of sulfur have traditionally been analyzed in water (precipitation, river, lake, and ground), the cyclic nature of the sulfur species demands that any acid rain modeling should include the dynamic aspects of sulfur chemistry (5-8). The content of organosulfurcompounds in water increases with its dissolved organic carbon. Therefore, it is especially important to determine the amounts of organicallybound sulfur in swamp and terrestrial waters. There were two objectives in this study: (1)to determine the presence of organosulfur compounds found in the swamp and terrestrial waters collected for the study of acid precipitation in Ontario, and (2) to develop a laboratory procedure to quantify the organicallybound sulfur portion. Although it has been reported that natural waters with high vegetation contents also contain significant amounts of organosulfur, an instrumental technique was needed to evaluate the extent of its contributions. Since no procedure is available to directly assay the organosulfur compounds, inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was selected for the determination of total sulfur because it is well suited for aqueous matrices and is also selective and sensitive for sulfur. U V irradiation had been used to oxidize dissolved organic carbon (DOC) (91, with subsequent detection of the carbonate by liquid chromatography (10). Hydrogen peroxide was used as an oxidizing agent for aromatic sulfur compounds (11) and hydrogen sulfide (12, 13). The application of porous Teflon tubing, allowing the permeation of carbon dioxide, was reported (14). With modifications, these analytical procedures were adopted by our Ion Chromatography (IC) Laboratory for * Author to whom all correspondence should be addressed; Telephone: (416) 235-5870; FAX: (416) 235-6107. + Retired.
0013-936x/95/0929-0849$09.00/0
Q 1995 American Chemical Society
VOL. 29, NO. 4, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
849
Oxidation
S
Oxidation
SO,
__*
Dissolution
SO,
____+
Dissociation
Assimilation
H,SO,
I
so,-2
Organosulfur 4
(Organically bound _______* (Inorganic form in vegetation) Release through in water) decay
FIGURE 1. Chemistry cycle of sulfur. CONDUCll’VllY DETECTOI?
FLOW
1
/-, VAILVE
Tubing
TO WASlE
FIGURE 2. Schematic diagram of equipment.
the oxidation of organosulfur compounds and the subsequent determination of organically bound sulfur by IC.
Experimental Section Apparatus. The schematic diagram of the peristalticpump, the UVlight digester, and the ion chromatographic system is shown in Figure 2. The major components are described below, and the operating parameters of the ion chromatograph are given in Table 1. (1) Sampler: Gilson Model 222 with 8.5-mL tubes. (2)UV-digester: the Technicon Instrument module (Part 188-B097-01) contained a UV lamp (Westinghouse Part G8T5) and a 50-turn 1.5 mm i.d. quartz coil. (3) Microporous tubing: Teflon microporous Gore-Tex
tubing 12 cm x 2 mm i.d., 3.5 pm pore size, W. L. Gore & Associates, Inc., Elkton, MD. (4) Air: cylinder of compressed dry air, regulated at 65 psi, Matheson Gas Products, Canada. (5) Peristaltic pump: Technicon AutoAnalyzer11, pump 111. (6) Interface: a built-by-us electronic unit which contained an infrared relay switch and a solenoid made by Skinner Electrical Valves. It controlled the movement of the sampler and the injector. (7) Ion chromatograph: Dionex Model 10. (8) Integrator: Spectra Physics Model 4270. (9) Computer: IBM compatible. (10) Recorder: Kipp and Zonen dual pen chart recorder. (11) Balances: analytical balance, accurate to 0.1 mg, and top-loading balance, accurate to 0.1 g. Materials. The standards and reagents were prepared in deionized distilled water (DDW) of less than 1 pS/cm conductivity. Ion Chromatography. The eluent, a bicarbonate and carbonate solution, was prepared by dissolving 25 g of sodium bicarbonate (ACS analytical reagent grade, BDH Chemicals) and 25 g of sodium carbonate (ACS anhydrous analytical reagent grade, BDH Chemicals), both weighed on a top-loading balance, in a 1-L volumetric flask and subsequently diluting by a factor of 100. The resulting concentrations were 3 mM NaHC03 and 2.4 mM Na2C03. The spiking solution (0.03 M NaHC03 and 0.024 M NazCO3) was prepared by dissolving 2.5 g each with DDW in a 1-L volumetric flask. The regenerant (12.5 mM H2S04) was obtained by diluting 14 mL of concentrated sulfuric acid (Baker-AnalyzedACS reagent grade, J. T. Baker) to 20 L with DDW. Calibration and Quality Control. Sodium sulfate (ACS anhydrous analytical reagent grade, BDH Chemicals) was dried at 110 ‘C for 3 h and desiccated overnight. An amount of 2.9576 g was accurately weighed on an analytical balance and dissolved in a l - L volumetric flask with DDW to prepare the “stock calibration solution of 2000 mg/L as sulfate. The “intermediate” calibration solution was prepared by diluting 100 mL of the stock to 1 L in a volumetric flask. Subsequent working calibration solutions were prepared from the intermediate solution to produce a series of concentrations: 0.2, 1.0, 2.0,4.0, 6.0, 8.0, and 10.0mglL as sulfate. To assess the sensitivityat lowlevels, a “lowtest” solution containing 0.5 mg/L as sulfate was prepared. The perfor-
TABLE 1
Operating Parameters of Ion Chromatograph instrument injector columns eluent eluent pump pressure gauge spiking solution suppressor regenerant detector integrator mode recorder chart speed
850
Dionex Model 10 Valco Instruments 6-port pneumatic valve with a 0.175-mL sample loop one HPIC-AG1 as guard column; two HPIC-AG3 as separator columns 3 m M NaHC03 and 2.4 m M Na2C03 at 2.5 mL/min Milton Roy single-piston minipump rated at 2000 psi 0.03 M NaHC03 and 0.024 M NazC03 at 0.1 mL/min Anion micromembrane suppressor (AMMS-I), Dionex 12.5 m M HzS04 at 4.0 mL/min conductivity at 10pS/cm full scale Spectra Physics Model 4270 peak height 1.0 V full scale 12 cm/h
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 4, 1995
’ 1
.. -1 -
O L 0
‘ 20
40
80
60
DOC ( m a83 C)
1W
-7
120
FIGURE 3. DOC and total sulfur.
20
0
40
w
( m a= C )
80
100
120
FIGURE 4. Correlation between DOC and difference in sulfate.
mance of the ion chromatographic system was evaluated with two quality control (QC) solutions, QCA (8.0 mg/L) and QCB (2.0 mglL). The DDW used for the QC solutions was retained as the long-term blank (LTB). An “in-run’’ solution of 6.0 mg/L as sulfate was used to monitor the stability of the analytical conditions. Oxidation. The oxidizing agent, a 10% (wlw)hydrogen peroxide solution, was prepared by diluting 20 mL of 50% (wlw)reagent grade, stabilizedH202(Fisher Certified, Fisher Scientific Ltd.) with DDW. The pH was 4.3 and increased to 5.1 after mixingwith the samples. A fresh solution was prepared as required. Assessing the Effectiveness of Oxidation Procedure. Solutions of potassium hydrogen phthalate (KOOC-CsH4COOH, analytical grade, BDH Chemicals) containing 25, 50, 100, 150, and 200 mglL as carbon and sodium sulfide (Na2S.9H20,analytical grade, BDH Chemicals) containing 2.234, 4.468, and 6.770 mg/L as Na2Swere used to assess the effectiveness of the oxidation procedure. Water Samples. The swamp and terrestrial water samples were taken from Dickie Lake and Harp Lake watersheds near Dorset, Ontario, and were selectedbecause
Y = 35.0 t 55.6xldX + 350p 500
0
1
2
3
4 5 6 7 8 Conc. of Sulfate ( m aas S04)
9
10
11
FIGURE 5. Calibration curve of sulfate.
of their high DOC content and the familiar yellow-brown color which was indicative of the presence of “organic” substances. In addition,other river and soil extract samples from the Dorset area, suspected to contain organosulfur compounds, were also used.
TABLE 2
Comparison of Total Sulfur and Inorganic Sulfate sample no.
DOC (mg/L as C)
total S by ICP-AES ( m a as S)
sulfate by ICP-AESa
sulfate by ICb
sulfate differenceC
sulfate ratiod
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
2.9 5.7 9.1 10.6 15.8 21.4 25.0 28.0 34.0 40.5 46.5 53.0 54.5 55.0 67.5 73.0 76.0 89.0 100.0 102.0
2.645 2.376 1.621 3.361 2.619 3.957 1.384 4.201 1.876 3.194 2.053 0.971 3.190 4.008 1.852 1.goo 2.252 5.091 4.917 2.346
7.92 7.12 4.86 10.07 7.85 11.85 4.15 12.59 5.62 9.57 6.15 2.91 9.56 12.01 5.55 5.69 6.75 15.25 16.06 7.03
7.02 6.94 4.58 9.13 7.68 10.90 3.20 11.40 4.28 8.85 5.95 1.41 9.15 10.75 4.27 3.90 4.60 11.54 13.87 4.94
0.90 0.18 0.28 0.94 0.17 0.95 0.95 1.19 1.34 0.72 0.20 1.50 0.41 1.26 1.28
0.886 0.975 0.942 0.907 0.978 0.920 0.771 0.905 0.762 0.925 0.967 0.485 0.957 0.895 0.769 0.685 0.681 0.757 0.864 0.703
1.79
2.15 3.71 2.19 2.09
aThe sulfate results are obtained by multiplying the sulfur results by 2.996 and expressed as mg/L as SO4. bThe sulfate results are expressed as mg/L as SO4. The difference between the ICP-AES and IC results is an estimation of the amount of organically bound sulfur. dThe ratio of sulfate results between IC and ICP-AES.
VOL. 29. NO. 4, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY e 851
A
B
W i t h o u t O u t i on
After Oxidatia
TABLE 3
Precision Data concn range (mg/L as SOP) 0-1.0 1.0-5.0 5.0-10.0 10.0-20.0
standard dev. (mg/L as SO4)
precision (YoRSD)
0.012 0.120 0.148 0.245
7.2 3.9 2.1 2.1
Procedures Analysis for Total Sulfur by ICP-AES.
Surface water samples were analyzed for total sulfur using the AtomScan I1 spectrometer (Therm0 Jarrell-Ash, Franklin, MA). The prominent emission line at 181.977 nm was selected with spectral background correction. Oxidation by Continuous Flow Injection. A continuous flow system for mixing the sample with hydrogen peroxide solution was constructed on the platform of the peristaltic Pump. Hydrogen peroxide at 0.06 M was used to oxidize hydrogen sulfidein natural waters (13). In this investigation, 25 mL of sample was mixedwith 2.5 mL of lO%(w/w)H202, resulting in 1% (wlw) H202 (0.3 M) in the final solution prior to introduction into the W-digester. A higher concentration was selected because of the expected higher levels of organic substances in the samples. The peroxide would be needed to ensure the complete oxidation of the various sulfur moieties and all the organic substances present. The W-digester module, installed after the pump, provided the radiation energy for oxidation. The flow rate of the peroxide-treated sample was regulated at 2 mL/min, which allowed 3.5 min of exposure time through the quartz coil. A 12 cm long section of porous Teflon tubing, placed between the digester and the ion chromatograph, permitted the release of carbon dioxide gas built up from the photochemical oxidation of organic matter. Quantificationof Sulfate by IC. The ion chromatograph was set up according to the parameters given in Table 1 (15). The stability of the system and the retention time of sulfate were established by injecting the in-run solution. The low test solution was analyzed to ensure detection at low levels. Calibrationwas based on the peak height counts of sulfate recorded by the integrator and via a secondorder calibration curve generated by the computer subroutine. The QC solutions, A and B, were analyzed to evaluate the accuracy of the calibration. The LTB was analyzed to check for contamination and carryover.
FIGURE 6. Chromatograms of before and after oxidation.
Every 10 samples were bracketed with the in-run solution. Using the sulfate responses of the in-run along the entire run, any minor and gradual changes in sample peak responses would be adjusted by an algorithm. Sudden or drastic changes were carefully evaluated. If necessary, the affected samples were repeated. If the sample results were over the upper limit of 10.0 mg/L as sulfate, the analyses were repeated after appropriate dilutions with DDW. Oxidation of Organic Carbon and Sulflde. Potassium hydrogen phthalate was chosen to establish the optimum concentration of hydrogen peroxide because it is both an alkyl and an aryl compound. Furthermore, it is the chemical used in the preparation of standards in the analysis for DOC (9). Sulfide has the lowest oxidation state of sulfur at -2. Oxidation to sulfate at +6 is the largest step, requiring the most vigorous conditions. The sodium sulfide solutions were treated with oxidation and analyzed for sulfate. Separation and Identification of Ionic Species after Oxidation. Since the oxidation products included oxalic acid as well as sulfate, a 10 mg/L oxalate standard solution was prepared from sodium oxalate and introduced into
TABLE 4
Oxidation of Potassium Hydrogen Phthalate H202 concn in sample solution
1.0 (% w/w)
0.5 (% w/W)
2.5 (% W/W)
original DOC concn (mg/L as C)
DOC found (mg/L as C)
Yo of DOC
DOC found (mg/L as C)
Yo Of DOC oxidized
DOC found (mg/L as C)
% of DOC
oxidized
25 50 75 100 150 200
0.3 0.7 1.3 1.9 3.0 4.8
98.8 98.6 98.3 98.1 98.0 97.6
0 0.1 1.2 0.9 2.0 3.3
100.0 99.8 98.4 99.1 98.7 98.4
0 0.3 0.9 1.6 2.6 3.8
100.0 99.4 98.8 98.4 98.3 98.1
852
1
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4, 1995
oxidized
TABLE 5
Analysis of Samples for DOC, Total Sulfur, Sulfate, and Oxalate DOC (mg/L as C)
sulfate by ICP-AES (maR as Sod
sulfate by IC (mgR as Sod
organosulfur mgA as S)
oxalate peak height (in mm)
sample no.
before OX’
after OXb
before OXc
before OXd
after OX’
original‘
after 0x9
1
48.5 49.5 51.0 19.5 36.5 33.0 23.0 29.5 22.5 25.5 38.5 49.0 35.0 38.0 37.0 34.5 68.0 53.5 31 .O 25.5 20.0 15.0 31.5 37.5 34.5 19.0 37.5 38.5 35.0 32.5 33.5 36.0 31.0 25.5 24.5 30.5 19.5 38.5 37.5 45.5 30.0 34.5 38.5 37.5 23.5 22.5 26.5
1.3 2.1 2.2 0.6 0.8 1.1 0.3 0.9 0.5 1.2 1.2 3.7 2.4 2.7 2.6 2.1 5.4 3.8 2.0 1.8 1.1 0.8 2.0 2.0 1.4 0.7 0.9 1.7 1.2 2.5 2.2 2.4 1.9 1.4 1.2 1.3 0.6 1.2 2.6 3.3 1.6 1.4 1.3 2.2 0.6 0.6 1.2
10.16 9.98 10.07 4.70 15.64 44.64 18.61 42.39 49.40 4.92 16.03 14.11 14.02 10.91 1 1.89 8.84 9.71 6.44 5.51 4.52 3.64 3.42 6.11 4.28 2.19 1.86 2.43 3.51 2.10 8.81 9.41 7.76 9.41 8.15 7.88 42.45 5.00 15.49 7.37 8.36 6.41 44.82 16.72 12.40 17.65 46.32 4.85
9.88 9.42 5.26 4.26 14.66 43.90 16.36 42.55 46.50 3.60 14.60 11.88 11.58 8.14 11.56 7.89 8.79 5.07 4.91 4.41 3.07 3.18 5.51 3.26 0.35 0.68 0.40 2.24 0.36 8.50 8.15 6.83 7.76 6.78 6.50 35.20 3.43 11.26 5.12 5.89 4.34 32.15 10.44 7.98 16.60 28.95 2.38
11.54 10.08 10.74 5.39 15.74 45.00 17.58 42.30 48.40 3.60 15.80 13.28 12.42 10.50 11.85 8.63 9.87 6.31 5.73 4.63 3.46 3.28 6.39 4.77 2.06 1.75 2.47 3.69 1.67 8.72 8.90 8.25 9.39 8.48 8.03 40.64 5.33 15.84 7.39 8.92 6.83 43.71 16.06 11.80 17.71 45.83 5.35
0.55 0.22 1.83 0.38 0.36 0.37 0.41 -0.08 0.63 0 0.40 0.47 0.28 0.79 0.10 0.25 0.36 0.41 0.27 0.07 0.13 0.03 0.29 0.50 0.57 0.36 0.69 0.48 0.44 0.07 0.25 0.47 0.54 0.57 0.51 1.82 0.63 1.53 0.76 1.01 0.83 3.86 1.88 1.28 0.37 5.63 0.99
16.0 39.5 19.0 5.5 7.5 6.5 3.5 5.0 3.0 1.5 3.5 20.0 20.0 15.0 20.5 37.5
