We investigated how to reuse water from cyanide wastewater by combining UV–ozone oxidation with a titanium dioxide (TiO2) catalyst and de-ionization by applying an ion exchange resin method. A TiO2 catalytic film was formed on the surface of quartz tubes by a novel sol-gel method. During UV–ozone oxidation with the TiO2 catalyst, cyanide ion (CN-) was oxidized to cyanate ion (OCN-) in a relatively short time without the application of any other chemicals. The processed water was smoothly de-ionized by an ion exchange resin method. The cyanate ion was decomposed into NH4+ and CO2 by hydrolysis under acidic conditions in a cation exchange resin column. Ammonium ion (NH4+) adsorbed to the cation exchange resin. The electric conductivity of the effluent from the ion exchange resin column was less than 1×10-1 mS m-1. This de-ionized water could be reused as rinsing water in the plating process. These experimental results suggested that toxic cyanide wastewater could be reused as de-ionized water.

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Wastewater contaminated with iron-complex cyanides was processed by UV photodecomposition accompanied by an iron elimination process using an iron-adsorbent. The wastewater processed by UV photodecomposition was oxidized by ozone combined with UV irradiation. The treated water was deionized by an ion-exchange resin method. This combined processes further increased the production of the pure water volume compared to a single process using an ion-exchange resin. The technique developed in this study can be summarized as follows. First, the iron-complex cyanides in plating wastewater were converted into aquapentacyanoferrate(III) [Fe(CN)5(H2O)]3− ion. Following the application of UV irradiation, it was decomposed into iron and cyanide (CN−) ion. The iron was removed from the water in the form of Fe(OH)3 by processing the wastewater with ozone and an iron-adsorbent. The CN− ion was oxidized to the cyanate (OCN−) form by UV ozone oxidation in a relatively short time. The processed water was de-ionized by passing through cation and anion-exchange resins. The deionized water could be reused as rinsing water in a plating process. The results reported here suggest that wastewater contaminated with chemically stable iron-complex cyanides can be effectively recycled.