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Kinetics of decomposition of thiocyanate in natural aquatic systems Irina Kurashova, Itay Halevy, and Alexey Kamyshny Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04723 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on January 2, 2018
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Environmental Science & Technology
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Kinetics of decomposition of thiocyanate in natural
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aquatic systems
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Irina Kurashova1, Itay Halevy2, and Alexey Kamyshny1,*
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Beer Sheva, Israel.
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Israel.
Department of Geological and Environmental Sciences, Ben-Gurion University of the Negev,
Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot,
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* Corresponding author:
[email protected] 11
KEYWORDS
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Thiocyanate, decomposition kinetics, microbial degradation, sulfidic, ferruginous, natural self-
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cleansing, Archean ocean
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ABSTRACT
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Rates of thiocyanate degradation were measured in waters and sediments of marine and
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limnic systems under various redox conditions, oxic, anoxic (non-sulfidic, non-ferruginous,
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non-manganous), ferruginous, sulfidic, and manganous, for up to 200-day period at micromolar
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concentrations of thiocyanate. The decomposition rates in natural aquatic systems were found
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to be controlled by microbial processes under both oxic and anoxic conditions. The Michaelis-
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Menten model was applied for description of the decomposition kinetics. The decomposition
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rate in the sediments was found to be faster than in the water samples. Under oxic conditions,
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thiocyanate degradation was faster than under anaerobic conditions. In the presence of
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hydrogen sulfide, the decomposition rate increased compared to anoxic non-sulfidic
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conditions, whereas in the presence of iron(II) or manganese(II), the rate decreased. Depending
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on environmental conditions, half-lives of thiocyanate in sediments and water columns were in
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the ranges of hours to few dozens of days, and from days to years, respectively. Application of
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kinetic parameters presented in this research allows estimation of rates of thiocyanate cycling
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and its concentrations in the Archean ocean.
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INTRODUCTION
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Thiocyanate (NCS−) is formed in various natural and industrial processes. It was found in
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waste waters from coal and oil processing, steel manufacturing, and the petrochemical
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industry1-5. Thiocyanate concentrations of up to 17 mmol·L−1 were detected in coal plant
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wastewaters4,6 and in gold extraction wastewaters7,8. In electroplating, dyeing, photo-finishing,
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thiourea and pesticide production the concentration of NCS− in wastewater effluents is in the
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range of 0.09-2 mmol·L−1 (9). Thiocyanate is toxic to aquatic species with LC50 values of 0.01 to
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0.5 mmol·L–1 reported for Daphnia magna10.
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Natural sources of thiocyanate include plants, biological and abiotic decomposition of
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organic matter, and in vivo detoxification of cyanide11-14. Several species of bacteria, algae,
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fungi, plants and animals are physiologically capable of detoxifying cyanide, and in most cases
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one of the end products of detoxification is thiocyanate10,15. Another important mechanism of
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thiocyanate formation in natural aquatic systems is the reaction between hydrogen cyanide and
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sulfide oxidation intermediates, which contain sulfur-sulfur bond, such as colloidal sulfur16,
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polysulfides17, thiosulfate and tetrathionate18,19. Reaction with thiosulfate is ~1000 times
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slower than reaction with polysulfides17,20,21, and is catalyzed by Cu(II) and sulfur transferases
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from the rhodanese family15,22. Concentrations of hydrogen cyanide in polluted aquatic systems
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may reach levels toxic to aquatic life23. In non-polluted aqueous systems hydrogen cyanide
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concentrations are usually very low due to fast degradation by plants, fungi and bacteria in
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aquatic systems, as well as to chemical transformations, volatilization and adsorption24-26.
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The presence of thiocyanate was recorded in various natural non-polluted aquatic
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systems, for example in stratified lakes Rogoznica in Croatia (up to 288 nmol·L−1) and Green
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Lake (NY, USA) (up to 274 nmol·L−1)19. In these lakes thiocyanate is produced in the anoxic,
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sulfide-rich sediments and diffuses to the chemocline where it is consumed by oxidative
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processes, which are likely biologically enhanced. In the oxic North Sea water thiocyanate was
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detected at concentrations up to 13 nmol·L−1(16). In salt marsh sediment of the Delaware Great
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Marsh, thiocyanate concentrations are up to 2.28 µmol·L−1 in pore-water and up to 15.6
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µmol·kg−1 in wet sediment26. In these sediments, concentrations of thiocyanate precursors,
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cyanide-reactive zero-valent sulfur and free hydrogen cyanide are as high as 78 µmol·L−1 and
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1.9 µmol·L−1, respectively. Co-existence of hydrogen cyanide, polysulfides and thiocyanate
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allows attribution of thiocyanate formation to the abiotic reactions between hydrogen cyanide
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and reduced sulfur species. Thiocyanate was also found in concentrations 23-40 µmol·L−1 in
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the Red Sea Atlantis II Deep brine27. Reactions between abiotically formed hydrogen cyanide 3 ACS Paragon Plus Environment
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and reduced sulfur species were proposed as the source of thiocyanate in the brine. The same
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processes were proposed to result in thiocyanate formation in the hydrothermal springs and
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pools in the Yellowstone National Park, where it was detected at concentrations 133-791
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µmol·L−1 (28).
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Although concentrations of thiocyanate in modern aquatic systems are low16,19,26,28, this
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may be not the case for the iron-rich water column of the Archean ocean. Existence of large
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atmospheric fluxes of both hydrogen cyanide and zero-valent sulfur to Archean ocean has been
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suggested29-34. These works provide constraints on cyanide and zero-valent sulfur fluxes but
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lack of data regarding rates of thiocyanate degradation under environmental conditions in the
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presence of dissolved iron(II) prevents estimation of thiocyanate concentrations and its fate in
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the Archean ocean.
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Multiple investigations focused on the isolation and identification of microorganisms that
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are responsible for thiocyanate biodegradation under controlled conditions35-52. Thiocyanate-
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degrading bacteria have been isolated from activated sludge41, steel plant wastewater53, gold
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mine tailing sites43,50,51, and extreme alkaline54 and hypersaline55,56 environments. The end
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products of NCS− degradation are ammonium, carbon dioxide, and sulfate, subsequent to the
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formation of cyanate57-62 or carbonyl sulfide63-67 as intermediates. A variety of autotrophic
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bacteria are capable of using NCS− as their sole energy source via sulfur oxidation.
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Chemolithotrophic bacteria utilize hydrogen sulfide formed on the initial step of NCS−
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degradation and products of its incomplete oxidation (polysulfide, elemental sulfur or
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thiosulfate) as an energy source for growth, oxidizing sulfide to sulfate41,63,68,69. Utilization of
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nitrogen released from thiocyanate as ammonia has also been reported58. A number of
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heterotrophic bacteria utilize NCS− as a source of nitrogen and sulfur46,50,52,70,71 as well as
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energy source38,44. Heterotrophic bacteria were shown to utilize thiocyanate at concentrations
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as high as 100 mmol·L–1 (70). 4 ACS Paragon Plus Environment
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In biotechnological systems for thiocyanate removal, which are operating under high
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concentrations of thiocyanate (up to 120 mmol·L−1 and oxic conditions10,36,43,72-79), rates of
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thiocyanate decomposition are high. Under oxic conditions the rates may be as high as 83
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mmol·L−1·d−1 at 1 mmol·L−1 concentration of thiocyanate (Table 1). Thiobacillus species
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isolated from steel plant wastewater, are able to degrade 5.2 mmol·L−1 of NCS− over 8 days
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under anaerobic denitrifying conditions53. Rates of thiocyanate degradation depend not only on
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oxygen concentrations3,53,80,81, but also on concentrations of nutrients. It was shown that
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concentrations of NH3 >70 µmol·L–1 may inhibit76 and phosphate concentrations >500 µmol·L–
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may stimulate thiocyanate biodegradation8.
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Currently, metabolic pathways for thiocyanate degradation are known and the rates of its
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degradation in bioreactors are published, but no data on kinetics of decomposition of
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thiocyanate in natural aquatic systems is available. The main aim of the present research is to
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fill this gap and to provide constraints on rates of decomposition of thiocyanate in limnic and
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marine water columns and sediments at various redox conditions, which are relevant for
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modern and ancient natural aquatic systems. In order to achieve this goal, we have determined
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kinetic parameters of thiocyanate degradation in natural aquatic and sedimentary systems at
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various salinities and concentrations of thiocyanate, oxygen, hydrogen sulfide, and iron(II).
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MATERIALS AND METHODS
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Materials. Solutions for measurements of the kinetics parameters of thiocyanate degradation
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were prepared in Milli-Q water, Mediterranean Sea water, Red Sea water, Lake Kinneret (Lake
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Tiberias) water. Slurries were prepared from Red Sea and Lake Kinneret sediments. All
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reagents were purchased from Sigma-Aldrich and were at least 98% purity. Potassium
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thiocyanate (KSCN) (99%), sodium sulfide nonahydrate (Na2S·9H2O) (98%), and iron(II)
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sulfate heptahydrate (FeSO4·7H2O) (99%) were used without further purification. Nitrogen gas 5 ACS Paragon Plus Environment
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(99.999%) was purchased from Maxima LTD, Israel. Test tubes, screw caps with hole and
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septum, and aluminum caps were purchased from Romical LTD, Israel and Labco LTD, UK;
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butyl rubber stoppers were purchased from Geo-Microbial Technologies Inc, USA.
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Samples preparation and treatment. Water and sediment samples were collected from
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various locations in Israel (Tables 2 and 3). Sediment cores and water samples were collected
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from the Red Sea (29º30'N, 34º55'E), the Mediterranean Sea (32º49'N, 34º59'E) and Lake
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Kinneret (32º50'N, 35º35'E) in May 2015 - December 2016. Sediment samplings were
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performed with a multicorer, Lake Kinneret and Red Sea waters were sampled with Niskin
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bottles, Mediterranean Sea water was sampled manually near the shore. The water and cores
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were stored at 25±1oC in the dark and processed in
