Kinetics of Decomposition of Thiocyanate in Natural Aquatic Systems Irina Kurashova, † Itay Halevy, ‡ and Alexey Kamyshny, Jr.* ,† † Department of Geological and Environmental Sciences, Ben-Gurion University of the Negev, Beer Sheva, Israel 84105 ‡ Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot, Israel 76100 ABSTRACT: Rates of thiocyanate degradation were measured in waters and sediments of marine and limnic systems under various redox conditions, oxic, anoxic (nonsul fidic, nonferruginous, nonmanganous), ferruginous, sulfidic, and manganous, for up to 200-day period at micromolar concentrations of thiocyanate. The decomposition rates in natural aquatic systems were found to be controlled by microbial processes under both oxic and anoxic conditions. The Michaelis-Menten model was applied for description of the decomposition kinetics. The decomposition rate in the sediments was found to be higher than in the water samples. Under oxic conditions, thiocyanate degradation was faster than under anaerobic conditions. In the presence of hydrogen sulfide, the decomposition rate increased compared to anoxic nonsulfidic conditions, whereas in the presence of iron(II) or manganese(II), the rate decreased. Depending on environmental conditions, half-lives of thiocyanate in sediments and water columns were in the ranges of hours to few dozens of days, and from days to years, respectively. Application of kinetic parameters presented in this research allows estimation of rates of thiocyanate cycling and its concentrations in the Archean ocean. ■ INTRODUCTION Thiocyanate (NCS - ) is formed in various natural and industrial processes. It was found in waste waters from coal and oil processing, steel manufacturing, and the petrochemical industry. 1-5 Thiocyanate concentrations of up to 17 mmol· L -1 were detected in coal plant wastewaters 4,6 and in gold extraction wastewaters. 7,8 In electroplating, dyeing, photo- finishing, thiourea, and pesticide production the concentration of NCS - in wastewater effluents is in the range of 0.09-2 mmol·L -1 . 9 Thiocyanate is toxic to aquatic species with LC 50 values of 0.01 to 0.5 mmol·L -1 reported for Daphnia magna. 10 Natural sources of thiocyanate include plants, biological and abiotic decomposition of organic matter, and in vivo detoxification of cyanide. 11-14 Several species of bacteria, algae, fungi, plants, and animals are physiologically capable of detoxifying cyanide, and in most cases one of the end products of detoxi fication is thiocyanate. 10, 15 Another important mechanism of thiocyanate formation in natural aquatic systems is the reaction between hydrogen cyanide and sulfide oxidation intermediates, which contain sulfur-sulfur bonds, such as colloidal sulfur, 16 polysulfides, 17 thiosulfate, and tetrathio- nate. 18,19 Reaction with thiosulfate is ∼1000 times slower than reaction with polysulfides, 17,20,21 and is catalyzed by Cu(II) and sulfur transferases from the rhodanese family. 15,22 Concentrations of hydrogen cyanide in polluted aquatic systems may reach levels toxic to aquatic life. 23 In nonpolluted aqueous systems, hydrogen cyanide concentrations are usually very low due to fast degradation by plants, fungi and bacteria in aquatic systems, as well as to chemical transformations, volatilization and adsorption. 24-26 The presence of thiocyanate was recorded in various natural nonpolluted aquatic systems, for example in stratified lakes Rogoznica in Croatia (up to 288 nmol·L -1 ) and Green Lake (NY, USA) (up to 274 nmol·L -1 ). 19 In these lakes thiocyanate is produced in the anoxic, sulfide-rich sediments and diffuses to the chemocline, where it is consumed by oxidative processes, which are likely biologically enhanced. In the oxic North Sea water thiocyanate was detected at concentrations up to 13 nmol·L -1 . 16 In saltmarsh sediment of the Delaware Great Marsh, thiocyanate concentrations are up to 2.28 μmol·L -1 in pore-water and up to 15.6 μmol·kg -1 in wet sediment. 26 In these sediments, concentrations of thiocyanate precursors, cyanide-reactive zerovalent sulfur and free hydrogen cyanide are as high as 78 μmol·L -1 and 1.9 μmol·L -1 , respectively. Coexistence of hydrogen cyanide, polysulfides and thiocyanate allows attribution of thiocyanate formation to the abiotic reactions between hydrogen cyanide and reduced sulfur species. Thiocyanate was also found in concentrations 23-40 μmol·L -1 in the Red Sea Atlantis II Deep brine. 27 Reactions between abiotically formed hydrogen cyanide and reduced sulfur species were proposed as the source of thiocyanate in the brine. The Received: September 17, 2017 Revised: December 25, 2017 Accepted: December 28, 2017 Published: December 28, 2017 Article pubs.acs.org/est Cite This: Environ. Sci. Technol. 2018, 52, 1234-1243 © 2017 American Chemical Society 1234 DOI: 10.1021/acs.est.7b04723 Environ. Sci. Technol. 2018, 52, 1234-1243 Downloaded via WEIZMANN INST OF SCIENCE on September 20, 2018 at 06:31:37 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.