The determination of acid-reactive sulfide

Environment International, Vol. 9, pp. 129-133, 1983

0160-4120/83 $3.00 + .00 Copyright © 1983 Pergamon Press Ltd.

Printed in the USA. All rights reserved.

THE DETERMINATION OF ACID-REACTIVE SULFIDE W u n c h e n g Wang Water Quality Section, Illinois State Water Survey, Box 697, Peoria, Illinois 61652, USA

Michael J. Barcelona Aquatic Chemistry Section, Illinois State Water Survey, Box 5050, Station A, Champaign, Illinois 61820, USA (Received 5 October 1982; Accepted 23 November 1982)

Acid-reactive sulfide minerals make up a large fraction of total sulfur in freshwater sediments, waste sludges, and waterlogged soils. Quantitative determinations of acid-reactive sulfide (ARS) by hydrogen sulfide evolution methods have been hampered by sample handling losses, standardization problems, and poor overall recoveries. A systematic study has been made of the optimal conditions for the determination of ARS. The modified method employs a zinc sulfide suspension as a standard in contrast with the current standard of sodium sulfide solution. This change resulted in a great improvement in precision; the addition of sodium hydroxide to the hydrogen sulfide absorption solution improved recoveries to better than 95°7o in the range of interest, 0.4--4 nag sulfide. ARS in freshly collected lake sediment samples was determined with an average of less than ± 14% relative standard deviation. Storage of sediment samples for periods exceeding6 months was found to increase variability only slightly. The method should prove applicable to various sulfide-containing samples and, with prescribed handling precautions, samples may be stored for extended periods.

Introduction

t e t r a g o n a l FeS (Mackinawite), spinel FesS, (Greigite), and the acid-reactive sulfides of Mn, Zn, Cd, and P b (Berner, 1970; Sweeney and Kaplan, 1973). The distribution o f ARS in marine sediments with depth and location has been linked to the presence of elemental sulfur, organic matter (pyritization reactions), and the degree of sulfur diagenesis (Berner 1964a, 1970). In freshwater sediments from Lake Mendota, WI, Nriagu (1968) observed that 58°7o-85°7o of the total sulfur resides in the acid-reactive fraction. Studies of sediment oxygen demand have identified reduced sulfur species and associated solids as important contributions to observed oxygen sags resultant from sediment disturbance or dredging operations (Schubel et al., 1978; Wang, 1980). Despite the ongoing interest in the aquatic chemistry of sulfur, limited improvements have been reported in analytical methodologies for ARS. Accuracy and precision data are sparse, and frequent cautions have been included as to the recovery efficiencies of reported methods ( A P H A , 1980; U.S. E P A , 1979, ASTM, 1978). Marine geochemical procedures call for the acidification

Sulfur precipitation and mineral f o r m a t i o n are important processes in waste stabilization and aquatic sediment systems. Besides its usefulness as an indicator of redox conditions, sulfur has significant effects on trace metal solubility and speciation, as well as the biogeochemistry of sedimentary environments (Nriagu and H e m , 1978). Much of what is known a b o u t the reactivity of sulfur species and stable forms of sulfur under reducing conditions has been drawn f r o m studies of marine environments, where sulfur is a m a j o r elemental constituent. Knowledge o f the principal forms of sulfur and their reactivity in sediments, waste-treatment sludges, and soils is of interest to workers in diverse fields, who are involved in studies o f the ultimate fates of chemicals in solid/water systems, Over the years, a m a j o r proportion o f total sulfur in these systems has been identified as the acid-reactive sulfide (ARS) fraction. This fraction is generally considered to contain soluble sulfides in interstitial waters, a m o r p h o u s ferrous sulfide, FeS, • H~O (hydrotroilite), 129

130 of a fresh sediment sample in a closed reaction-H,S absorption system, operated under negative pressure (Berner, 1964b; Zhabina and Volkov, 1978). The reaction flask is then heated to boiling to encourage H~S removal; after capture in zinc acetate solution, an iodometric or colorimetric procedure completes the ARS determination. This method has been reported to be reproducible to 4- 5% and to be sufficiently accurate that other forms of sulfur may be converted to H2S for quantitative determination. The need for fresh samples and the boiling step place significant limitations on analytical input, and also pose potential hazards when applied to solid samples, which may contain reactive or hazardous constituents. Early investigations of ARS in freshwater sediment samples (Nriagu, 1968) have relied on soil analysis procedures for sulfide (Black, 1965). Standardization techniques, reported accuracy, and precision vary considerably. Furthermore, it is difficult to assess the proportion of ARS in situ, since sample handling may promote oxidation of reduced sulfur and introduce bias to the analytical result. Our interest in sediment geochemistry and chemical contributions to sediment oxygen demand prompted an assessment of ARS methods. The primary criteria for the study included ease and reliability of standardization, precautions for sample handling or storage, and the precision obtainable from the procedure, Methods, Materials, and S a m p l e s The determination of ARS by a variety of reported methods has several common steps. A weighed solid sample is first added to an inert gas swept reaction/absorbed flask assembly. Then the stirred sample suspension is acidified. The evolution of hydrogen sulfide is then promoted by slight vacuum or inert gas (CO2 or N2) purging, and then absorbed in a zinc or cadmium solution. The absorption of H2S is continued until presumed completion. Finally, the contents of the absorber flask are treated with an excess of acidified standard I~-KI and the excess iodine is back-titrated with a standard thiosulfate solution. The most problematic steps of these procedures are calibration and quantitative recovery of the H2S. Both sodium sulfide solution and zinc sulfide crystal suspension were evaluated as procedural standards using the apparatus shown in Fig. 1. It was constructed of readily available laboratory materials and allowed for the addition of acid to the reaction flask without exposure to the air. Two absorption flasks were used unless otherwise noted. Concentrated sulfuric acid, taken into the 10-mL Mohr pipette, was forced into the reaction flask by using the rubber bulb, due to the airtightness of the flask. APHA procedures were used for reagent preparation (unless otherwise specified) and as a starting point for improvement of the ARS method. All chemicals were reagent grade with the exception of the Na2S • 9H=O, which was

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99.90/0 (Research Organic/Inorganic Chemical Corporation, Sun Valley, CA), and prepurified grade nitrogen, which was certified to contain less than 1 ppm (/zL/L) oxygen. Two sulfide standards were prepared. Fresh sodium sulfide solution was prepared every day. The solution was made by selecting clean crystals which were rinsed with absolute methanol, patted dry with lint-free towels, rapidly weighed and added to a volumetric flask under slight N2 overpressure, and made to volume with nitrogen-purged double-distilled water. The other standard is zinc sulfide suspension, which was prepared in 0.1 r~ sodium hydroxide directly from the reagent grade compounds. The suspension was stirred while aliquots were taken to insure homogeneity. These suspensions were found to be stable for several months. Both sets of standards were restandardized by iodometric titration before each experimental run. Preliminary trials disclosed that stirring rate in the reaction and absorber flasks had little effect on results, so it was fixed at - 2 0 0 rpm for all experiments. Samples of lake sediments were taken from fresh gravity cores contained in predrilled 5-cm dia. polycarbonate core liners. All handling was done in a nitrogenfilled glove box; 5 mL cutoff polyethylene syringes were used and sealed following the method described by Schubeletal. (1978). This arrangement yielded approximately 4-mL subsamples at 2-cm intervals in each core without exposure to the atmosphere. For the storage experiment, the septum-sealed syringes were frozen in Whirl-pac ® bags which had been purged with nitrogen. Approximately 1 mL of wet sediment was taken each time for sulfide determination. Before and after subsampling, the sealed syringe was weighed with an analytical balance to 0.1 rag. The sediment samples were taken from Lake Paradise and Lake Eureka, two public water supply im-

Determination of acid-reactive sulfide poundments located in central Illinois. Lake Paradise is a shallow (7 m m a x i m u m depth) 63-ha lake with a watershed area exceeding 4000 ha. L a k e Eureka is a 9-ha i m p o u n d m e n t whose watershed is approximately 700 ha. Its m a x i m u m depth is about 6 m. In both cases, watershed use is over 8 0 0 agricultural or pastureland predominantly planted with row crops. These lakes exhibit taste, odor, and sediment oxygen demand problems due to runoff-related inputs of nutrients and sediment. Lake Eureka has been treated with copper sulfate on a seasonal basis for at least 30 yr for the control of nuisance algae. Lake Paradise has not received such treatment. It was anticipated that overall lower levels of total sulfide would be found in Lake Paradise than in Lake Eureka. Moisture determinations were made at the same depth interval as ARS analyses by drying at 105 °C. The depth intervals are nominal and have not been corrected for compaction due to coring,

Results and Discussion ARS calibration runs with fresh sodium sulfide standard solutions were made in duplicate on two separate dates. Nitrogen flow rates were maintained at 200 m L min-' and experiments were run at the 8-mg ARS level for periods ranging from 0.5 to 1.3 h. Recoveries averaged 67°70-102070 of theoretical values and deviations, 17070-29070 f r o m mean values. The low reproducibility was rather disturbing. Similar problems were also encountered with sodium sulfide standards for spectrophotometric determinations o f reactive sulfide in lake sediment pore waters. Although standard sodium sulfide solutions can be preserved for a period of weeks if they are made up in 1 r~ N a O H with 5070 hydrazine hydrate (N2H4 • H20) added as an antioxidant (Jackson, 1979), no further experiment was performed in this series, The remaining experiments were conducted by using the standard aqueous suspensions of zinc sulfide, since they are chemically stable and closely meet the requirements of an overall procedural standard. Successive restandardizations over a 6-month period varied no more than 4-5°70 from the initial titer. This standard was used to conduct a series o f experiments for developing a standard procedure. The aim was to achieve at least 95070 recovery with a single absorption flask. Initial runs on zinc sulfide standard aliquots were made utilizing a 0.05 N zinc acetate absorption solution, as suggested by A P H A (1980) and U.S. E P A (1979). The percent recoveries are shown in Table 1 as a function of purge conditions and ARS level, The observed recoveries were poor, averaging 52070 ( N = 23). For purging times of 2 h or more at 200 m L min-', the average recovery was 64070 with a relative standard deviation of 18070. The recoveries generally decreased with increasing sulfur content. These larger samples also showed significant carryover of sulfide

131 Table 1. Percent recovery for zinc sulfide standards with zinc acetate absorption solutions. Stripping time (h) 1 2 3 N2 Flow R a t e ( m E • rain-~) ARS (mg) 100 200 100 200 200 1.6 4.0 8.0 16.0

19

49; 48

30

20 14

20 30

29; 45

71; 68; 80 74; 58; 57; 54 49; 55 57; 54

74; 84

into the second absorption flask. The succeeding series o f experiments were performed with limited sample size ( < 8 mg ARS) in an effort to improve the recovery of hydrogen sulfide, using only a single absorption flask. Several trials using extra-fine fritted gas diffusers and several surfactants to disperse the zinc sulfide precipitate in the absorption flask resulted in m i n i m u m improvement of recovery with a single absorption flask. The best overall reproducibility and recovery was achieved by making the zinc acetate absorption solution alkaline (0.0015 N N a O H ) . The results are shown in Table 2. The recoveries were much improved over previous trials using zinc acetate alone in the absorption solution (Table 1). They averaged 8 4 0 for 15 determinations. The data further show that 2 h of purging at 200 m L rain-' results in nearly complete recovery at or below the 4.0 mg ARS level. The average for this range was 95 +7.6070 in seven trials. At 4.0 mg sulfide, stripping 1 h with 200 m L min-' is obviously insufficient (57070recovery), as is 8.0 mg sulfide, stripping 2 h with 200 m L min-' (74070). In general, in the range of 0.4 to 4.0 mg sulfide, nitrogen gas purging at 200 m L min-' for 2 h yielded near quantitative recoveries. The experiments with two lake sediment samples showed that 1 m L wet sediment subsamples contained sulfide approximating this range. For unknown samples, it is advisable to determine the approximate sulfide content in a first run and then to adjust the sample size accordingly. The next step was to apply the modified ARS method to sediment cores f r o m two central Illinois lakes. The results f r o m the five cores are shown in Fig. 2. Acidreactive sulfide levels are reported in mg of ARS per gram of dry sediment. The observed variability among multiple determinations at selected intervals have been repre-

Table 2. Percent recovery for zinc sulfide standards with alkaline zinc acetate absorption solution. N2 flow rate 200 mL min-L Stripping Time (h) ARS (mg)

1

2

0.8 4.0 8.0

83; 90; 100 54; 56; 60

89; 98; 108 88; 89; 98; 97 60; 78

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Fig. 2. Acid-reactivesulfide (ARS) in freshwatersediments. sented by horizontal bars. Cores from Lake Paradise show ARS distributions similar to those observed by Nriagu (1968) for shallow areas of Lake Mendota. These levels are clearly lower than those in Lake Eureka by a factor of 2. Both lakes are shallow and the sediments are subject to resuspension, bioturbation, and increased oxygen tension which regulate the periods of strong reducing conditions. Lake Eureka sediments, which received the additional sulfur input resultant from copper sulfate addition, contain much higher levels of ARS. The subsurface maxima at 8-15 cm depth show ARS levels between 0.1070 and 0.6070 of the sediment's dry weight. Of the three cores, the maximum was most pronounced in the deepest water core. Cores LE-2 and LE-3 were taken 6 months apart. The coherence between ARS profiles reflects the seasonal changes in redox conditions which affect the surface sediments, The deeper core, LE-9, shows two distinct subsurface maxima that correspond roughly with the added algicide inputs of sulfur to the lake system in the past 30 yr. Multiple ARS determinations at depth intervals within these cores show average relative standard deviations of ± 14°70 from mean values. Since the modified method yields precision better than ± 8°70 for sulfide standards in this range, the added variability probably results from the inhomogeneity of natural sediments and smearing o f the distributions during the gravity coring operation, The effects of frozen storage of subcores on ARS analytical results were determined on core LE-2. After 6

months of storage, six samples were redetermined in duplicate. The mean values after storage agreed within ± 17°70 with the average determined on fresh samples. It is concluded that storage effects on the total ASF in freshwater sediments are minimized by the sampling precautions outlined in this work. There is a dilemma here, however. Copper sulfate treatment is a common practice for controlling algal bloom, which usually leads to taste and odor problem, among others. Copper sulfate eventually sinks to the bottom. Under anoxic conditions, sulfate is reduced to sulfide, which causes yet another taste and odor problem. Lake Eureka, as a public water supply source, has experienced both sets of taste and odor problems. The high sulfide content in Lake Eureka sediments in comparison with that of Lake Paradise sediments points out the problem of this common practice.

Conclusions An improved method for the determination of acidreactive sulfide has been developed. Calibrations with zinc sulfide suspension standards and an improved H2S absorption solution demonstrate analytical precision better than ± 8070 with 95070 recovery. Application of the method to freshwater sediments yielded results comparable with past determinations of ARS in similar lacustrine environments. Storage difficulties can be minimized by observing precautions

Determination of acid-reactive sulfide a g a i n s t s u l f i d e o x i d a t i o n . T h e m e t h o d a l l o w s f o r facile

determination of acid-reactive sulfide in diverse solids which have posed significant problems for previously developed techniques, Acknowledgements-Several people contributed substantially to the field and laboratory aspects of the work. They are Edward Garske, Steven Heffelfinger, Pamela Beavers, and Joel Kleiman. The work upon which this publication is based was supported in part by the U.S. Department of the Interior through the University of Illinois Water Resources Center (A-105-Ill), as authorized under the Water Research and Development Act, 1978, P.L. 95-467. The support of the University of Illinois Graduate Research Board is also greatly appreciated.

References American Public Health Association (1980) Standard Methods for the Examination of Water and Wastewater. 15th Ed. APHA, Washington, DC. American Society for Testing and Materials (1980) Manual on water, p. 241. STP 442A, ASTM, Philadelphia, PA. Berner, R. A. (1964a) Iron sulfides formed from aqueous solution and at atmospheric pressure, J. Geol. 72, 293-306. Berner, R. A. (1964b) Distribution and diagenesis of sulfur in some sediments from the Gulf of California, Marine Geol. 1, 117-140. Black, C. S., ed. (1965)MethodsofSoilAnalysis. University of Wisconsin Press, Madison, WI. Cline, J. D. (1969) Spectrophotometric determination of hydrogen

133 sulfide in natural waters. Limnol. Oceanogr. 14, 454-458. Jackson, R. E. 0979) Development of methods for sampling, preserving and analyzing ground waters and aquifer sediments. In Annual Progress Reports and Short Research Notes 1977-1978, pp. 32-42. Hydrology Research Division Report Series No. 64, Environment Canada Inland Waters Directorate, Water Resources Branch, Ottawa, Canada. Nriagu, J. O. (1968) Sulfur metabolism and sedimentary environment: Lake Mendota, Wisconsin, Limnol. Oceanogr. 13, 430-439. Nriagu, J. O. and Hem, J. D. (1978) Chemistry of pollutant sulfur in natural waters. Sulfur in the Environment. Part II. Ecological Irapacts, J. O. Nriagu, ed., Chapter 6, Vol. 2, pp. 211-270. WileyInterscience, New York, NY. Schubel, J. R., Carter, H. H., Wilson, R. E., Wise, W. M., Heaton, M.G., and Gross, M. G. (1978) Field investigations of the nature, degree and extent of turbidity generated by open water pipeline disposal operations. Technical report D-78-30, Dredged Material Research Program, July 1978, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS. Sweeney, R. E. and Kaplan, I. R. (1973) Pyrite framboid formation: laboratory synthesis and marine sediment, Econom. Geol. 68, 618-634. U.S. Environmental Protection Agency (1979) Methods for chemical analysis of water and wastes. EPA, Cincinnati, OH. Wang, W. (1980) Fractionation of sediment oxygen demand, Water Res. 14, 603-612. Zhabina, N. N. and Volkov, I. I. (1978) A method of determination of various sulfur compounds in sea sediments and rocks. In Environmental Biogeochemistry and Geomicrobiology: Methods, Metals and Assessment, W. E. Krumbein, ed., Volume 3, Chapter 59, Ann Arbor Science, Ann Arbor, MI.