The theory and computations described by Zepp and Cline (1977) were experimentally tested in predicting the direct photolysis rates of dilute hexacyanoferrate (II) and (III) solutions in the aquatic environment. Essential information for these calculations includes the quantum yield for the photoreaction, molar extinction coefficients of the complex ions for wavelengths > 295 nm, solar irradiance data used to calculate specific sunlight absorption rates, and the assumption that the photolysis reaction obeys a first-order kinetic rate expression. Direct photolysis rates of the irreversible photochemical reactions are calculated as a function of the time of year, latitude, time of day, meteorological conditions, and depth in natural water bodies. Light of wavelengths < 480 nm is active in the photolysis reactions, and pH, temperature, and concentration all affect the reaction to varying degrees. Assuming first-order kinetics, in which the rate constant was approximately concentration independent within the range of 25-100 micrograms/l total cyanide, the minimum quantum yields of HCN formation were 0.14 and 0.0023 for the iron (II) and (III) complexes, respectively. These values correspond to minimum, nearsurface, midday half-lives at midsummer of about 18 and 64 min at St. Paul, Minn. The photolysis rate at various fixed depths in a natural water column, when compared with that at the surface, decreases exponentially with depth. It is suggested that the photolysis reactions are enhanced by suspended material in turbid waters because of the forward scattering of light when compared with that theoretically calculated from beam attenuation coefficients. Hexacyanoferrate (II) and (III) solutions of equal initial total cyanide concentration respond photochemically quite differently from one another in solutions prepared with deionized water, but respond in a similar manner for solutions prepared with natural waters. The potentially rapid photodecomposition of iron-cyanides with formation of HCN suggests that this phenomenon may be of toxicological importance under certain environmental conditions.

"January 1980." Includes bibliographical references (pages 45-47). Grant no.