INFLUENCE OF pH ON THE PHOTOCATALYTIC TREATMENT OF CYANIDE

The toxicity of cyanide in water, air and soil is an excepted fact. However to find serious exposure limits is a difficult task. Data for the evaluation is often old and difficult to use in contemporary requirements, on the other hand, studies about the toxicity of cyanide are getting more complicated due to the growing knowledge of its hazardousness. One of the basic arguments for the definition of exposure limits in air is the detoxification rate of cyanide in the human body. Two values are still in discussion in recent literature. The rate of 17 μg/kg body weight/min is based on studies from Loevenhart (1918) [1], who tested the impact of NaCN seriously ill patients, ignoring the fact that free cyanide is not stable in solutions at high pH. The rate of 1 mg/kg body weight/min is found in a more recent publication from Schulz (1982) [2], although no explanation was found for citations of fatalities after consumption of similar cyanide concentrations in the same paper. Hence, an amount of cyanide being metabolised in the body must be below this value. The volatilisation rate of HCN in different pH values is a crucial point in the estimation of the hazardousness of cyanide in the environment and further treatment opportunities. The photoelectrocatalytic treatment, using TiO2 on conducting glass (FTO) was investigated by the authors in previous studies. Treatment of solutions with a pH value of 10.5 resulted in a low degradation rate, compared to experiments with other substances [3]. In plotting the degradation rate against the distance of the redox potential to the energy of the valence band, a correlation is found (Fig. 1)