New method for complete recovery of total cyanide in water and soil and its application at the Samira gold mine (Niger)

Abstract

Cyanide is a very toxic substance, which occurs in various forms in environmental matrices and its determination requires a good extraction method. From the reference method of cyanide distillation by reflux heating under acidic conditions and entrainment of HCN gas collected as CN- ions with detection by UV-Visible spectrophotometry, an improved and complete procedure for total cyanide extraction in aqueous and solid matrices has been developed. The best conditions after optimization of complete recovery (≃ 100%) of total cyanide and elimination of interferences in natural waters and soils fortified are: heating at 181.75°C, saving of reflux time to 30 min, sulfuric acid concentration of 0.6 M, 2 to 4 times less acid, without catalyst and other special treatments of interferences relatively to existing techniques. Application to water samples (drinking, surface, underground) and effluents collected at the Samira gold mine (Niger) and its environs shows the impact of distillation extraction on total cyanide recovery. Waters, exempt of cyanide at the starting of mining activity, currently contain total cyanide contents of 1 to 5 μg L-1 and in effluents up to 37 times the discharge standard (1 mg L-1). Key words: Cyanide decomposition, distillation extraction, environmental matrices, Samira gold mine (Niger).