The THIOCYANOX (Hydrogen cyanide and thiocyanate transformations in anoxic aquatic systems) project was established to investigate pathways of cyanide and thiocyanate formation and their biogeochemical transformation in different types of oxygen-depleted aquatic systems. Rates of reactions of different types of cyanide with various sulphur compounds were studied to determine how cyanide and its transformation products are distributed in different types of oxygen-depleted systems around the world. Researchers also identified the biogeochemical parameters that control the formation, release and transformation of cyanide and thiocyanate. Samples from various locations and biogeochemical settings were sampled and screened for the presence of relatively stable products of hydrogen cyanide transformation. They included and thiocyanate and strong metallo-cyanide complexes. Detailed studies were also conducted into the biogeochemical mechanisms of cyanide release and transformations in three selected systems: the Gulf of Aqaba in the Red Sea, Jade Bay on the North Sea coast of Germany, and Lake Kinneret (Sea of Galilee). The studies were accompanied by laboratory experiments to determine the rates of reactions between metallo-complexed and adsorbed cyanide in natural aquatic systems. THIOCYANOX results indicated that the main control on hydrogen cyanide concentrations in sediments was its consumption rate, rather than release from plant roots. In more oxidised sediments hydrogen cyanide is rapidly consumed. However, in anoxic, particularly sulphide-poor sediments, hydrogen cyanide, which is released from the roots, is preserved for a longer time and its concentration rises. These findings, together with previously published data show that hydrogen cyanide in aquatic systems is mainly present in plant-populated coastal sediments. Furthermore, transformation of hydrogen cyanide to thiocyanate is only possible in the presence of sufficient quantities of hydrogen sulphide.
Hydrogen cyanide, THIOCYANOX, thiocyanate, metallo-cyanide complexes, sulphides