Even today, cyanide complex metals such as sodium cyanide and potassium cyanide are used in plating baths as metal salts. Moreover, cyanide compounds are used as pretreatment solutions. Among these applications are electrolytic degreasing, for which iron plates are often used as anode plates. During electrolysis, cyanide in the solution and iron eluted from the anode plate combine to form a persistent iron-cyanide complex, which cannot be treated using common cyanide degradation methods: neither the alkaline chlorine method nor the ozone oxidation method. The conventional combined ozone and ultraviolet method using a diffusion tube（hereinafter, the O₃＋UV method）also decomposes cyanide, but the reaction time is long. The dark blue method generates large amounts of sludge. Moreover, the combustion method, which requires large amounts of heat, is inefficient in other respects.

For this study, we specifically examined the use of ozone fine bubbles（OFB）, which have attracted attention in recent years for their gas solubility and reaction acceleration effect, as a method to shorten the reaction time when using the O₃＋UV method.

Furthermore, for more effective reaction, the use of a reducing agent to lower the oxidation-reduction potential（ORP）of the treated solution as a pretreatment for the combined OFB and UV method（hereinafter, OFB＋UV method）was also investigated.

Findings from this study confirmed that reduction＋OFB＋UV method can detoxify cyanide in the iron-cyanide complex about twice as efficiently as the conventional O₃＋UV method.