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Hydrogen Cyanide and Thiocyanate Transformations in Anoxic Aquatic Systems

Final Report Summary - THIOCYANOX (Hydrogen Cyanide and Thiocyanate Transformations in Anoxic Aquatic Systems)

The ThioCyAnOx project was designed to study the pathways of cyanide and thiocyanate formation and their biogeochemical transformations in various types of oxygen-depleted aquatic systems: salt marshes, mono- and meromictic lakes, coastal and deep sea settings.
Three main objectives of this project were:
• To study the rates of reactions of different forms of cyanide (cyanide adsorbed on various solid phases, cyanide complexes, and hydrogen cyanide) with various sulfur compounds (solid sulfur, colloidal sulfur, dissolved sulfur, polysulfides, polythionates, thiosulfate and organic polysulfanes).
• To determine how cyanide and its transformation products are distributed in anoxic systems on the global scale in various types of oxygen-depleted aquatic systems.
• To identify the biogeochemical parameters that control the formation, release and transformations of cyanide and thiocyanate in the three natural aquatic systems mentioned above.
All planned experiments on measuring the rates of reactions of reduced sulfur species with hydrogen cyanide and metallo-cyanide complexes were successfully performed. Results of these experiments demonstrated that: 1) free hydrogen cyanide reacts with sulfur species faster than metallo-cyanide complexes; 2) in a presence of metals, sulfur species react preferably with metallo-cyanide complexes rather than with free cyanide released by dissociation of the complexes; 3) reactivity of sulfur species toward hydrogen cyanide and metallo-cyanide complexes decrease in the order polysulfide > tetrathionate > colloidal S0 > dimethyltrisulfide > thiosulfate.
During the project execution, samples from various locations characterized by different biogeochemical settings (e.g. polluted and no-polluted sea-water, iron- and sulfide-rich lake waters and anoxic pore-waters of salt-marsh and lake) were sampled and screened for presence of hydrogen cyanide and thiocyanate. Detailed research on biogeochemistry of hydrogen cyanide and thiocyanate was performed in three natural aquatic systems.
In the sediments of the Gulf of Aqaba, Red Sea, hydrogen cyanide and thiocyanate were not detected.
In the Jade Bay, the highest concentrations of hydrogen cyanide were found in the pore waters (up to 800 nM) and in the whole sediment (up to 15 µmol kg-1) in of the muddy cohesive sediments, which are vegetated by Salicornia stricta. Thiocyante (up to 800 nM) and reactive sulfur species (e.g. polysulfides in micromolar concentrations) were detected in this sediment as well. In the roots of Salicornia stricta, up to 5 µmol kg-1 of free hydrogen cyanide up to 32 µmol kg-1 of total cyanide was detected. On the other hand, in the sediments populated with Triglochin maritima, which has much higher free and total cyanide content in roots (up to 69 µmol kg-1 and up to 204 µmol kg-1, respectively), hydrogen cyanide, was not detected in sedimentary pore waters as well as in the solid phase. Based on these observations, it may be concluded that the main control on hydrogen cyanide concentrations in the sediments is its consumption rate rather than its release from the roots. In more oxidized sediments (permeable, bioturbated or roots-rich), hydrogen cyanide is consumed fast. On opposite, in anoxic, especially sulfide-poor sediments, hydrogen cyanide, which is released from the roots, is preserved for longer time, and its concentrations rise.
In Lake Kinneret concentrations of hydrogen cyanide in pore-waters of non-vegetated sediments, both on shore and at the deepest point the lake (38 m water depth) were <1 µM. In the shore sediments populated by Cyperus articulates, concentrations of hydrogen cyanide in pore-waters and sediments was as high as 4 µM and 8 µmol kg-1, respectively. Concentrations of thiocyanate were low in all samples (<200 nM) due to low concentrations of hydrogen sulfide and its reactive oxidation intermediates.
This observation, in combination with previously published data, leads to conclusions that (1) hydrogen cyanide in aquatic systems is present mostly in plant-populated coastal sediments and (2) transformation of hydrogen cyanide to thiocyanate is possible only in the presence of significant concentrations of hydrogen sulfide.
In the course of the research, the problem of paleoceanographic consequences of the reaction of hydrogen cyanide with zero-valent sulfur species was raised. Model for evaluation of composition of Archean ocean, which takes into account reaction between hydrogen cyanide and elemental sulfur, which are produced in atmosphere and deposited to the ocean, was developed. Results of calculations show that the concentration of thiocyanate in the Archean was possibly higher than 10 mM. In this case, thiocyanate was the most abundant dissolved sulfur species.