Photochemical Changes in Cyanide Speciation in Drainage from a Precious Metal Ore Heap

Abstract

In drainage from an inactive ore heap at a former gold mine, the speciation of cyanide and the concentrations of several metals were found to follow diurnal cycles. Concentrations of the hexacyanoferrate complex, iron, manganese, and ammonium were higher at night than during the day, whereas weak-acid-dissociable cyanide, silver, gold, copper, nitrite, and pH displayed the reverse behavior. The changes in cyanide speciation, iron, and trace metals can be explained by photodissociation of iron and cobalt cyanocomplexes as the solutions emerged from the heap into sunlight-exposed channels. At midday, environmentally significant concentrations of free cyanide were produced in a matter of minutes, causing trace copper, silver, and gold to be mobilized as cyanocomplexes from solids. Whether rapid photodissociation is a general phenomenon common to other sites will be important to determine in reaching a general understanding of the environmental risks posed by routine or accidental water discharges from precious metal mining facilities.

Mine Wastes

“…In the presence of UV radiation, even strong cyanide complexes such as the stable Fe 2+ and Fe 3+ cyanide complexes ferrocyanide (Fe(CN) 6 4-) and ferricyanide (Fe(CN) 6 3-) can be broken down to yield free cyanide (Johnson et al 2002). In the presence of UV radiation, even strong cyanide complexes such as the stable Fe 2+ and Fe 3+ cyanide complexes ferrocyanide (Fe(CN) 6 4-) and ferricyanide (Fe(CN) 6 3-) can be broken down to yield free cyanide (Johnson et al 2002).…”

Effects of pH on the degradation of aqueous ferricyanide by photolysis and photocatalysis under solar radiation

“…These aqueous complexes dissociate extremely slowly in darkness [3,4]; whereas, upon exposure to strong UV light they photodissociate releasing free cyanide [5][6][7] to the environment. The primary steps of the photodissociation of ferrocyanide and ferricyanide involve the formation of iron aquapentacyano complexes together to the release of free cyanide ions [8][9][10]. Also, in alkaline aqueous solutions, the photodissociation of these iron cyanide complexes results in the formation of iron hydroxopentacyano species.…”

The formation of an antitubercular complex [Fe(CN)5(INH)]3− through mercury(II)‐catalyzed ligand substitution reaction: A kinetic and mechanistic study

“…The kinetic, thermodynamic, and volume of activation data from numerous mechanistic studies are consistent with a dissociative mechanism for ligand substitution reactions in these complexes [7,8]. Because of their high stabilities in dark [9], the hexacyanoferrate(II/III) is considered nontoxic [10], but undergo photolysis in acidic medium [11] when exposed to ultraviolet (UV) radiation, producing [Fe(CN) 5 The INH, an antitubercular drug, was first reported to be effective in the treatment of tuberculosis [12,13] but strains of Mycobacterium tuberculosis, a causative agent of disease resistant to INH, were also reported shortly after its introduction [14]. The mutations in drug target genes were held responsible for INH resistance [15][16][17].…”