Reduction of ferrate(VI) and oxidation of cyanate in a Fe(VI)–TiO 2–UV–NCO − system

Abstract

The aqueous photocatalytic degradation of cyanate (NCO −), which is a long-lived neurotoxin formed during the remediation of cyanide in industrial waste streams, was studied in the ferrate(VI)–UV–TiO 2–NCO − system. Kinetics measurements of the photocatalytic reduction of ferrate(VI) were carried out as a function of [NCO −], [ferrate(VI)], [O 2], light intensity ( I o), and amount of TiO 2 in suspensions at pH 9.0. The photocatalytic reduction rate of ferrate(VI) in the studied system can be expressed as - d [ Fe ( VI ) ] / d t = kI o 0.5 [NCO −] [TiO 2]. The rate of photocatalytic oxidation of cyanate with ferrate(VI) was greater than the rate in the analogous system without ferrate(VI). The possibility of involvement of reactive ferrate(V) species for this enhancement was determined by studying the reactivity of ferrate(V) with NCO − in a homogeneous solution using a premix pulse radiolysis technique. The rate constant for the reaction of ferrate(V) and NCO − in alkaline medium was estimated to be (9.60 ± 0.07) × 10 2 M −1 s −1, which is much slower than the ferrate(VI) self-decomposition reaction ( k ≈ 10 7 M −1 s −1). An analysis of the kinetic data in the Fe(VI)–UV–TiO 2–NCO − system suggests that ferrate(V) is not directly participating in the oxidation of cyanate. Possible reactions in the system are presented to explain results of ferrate(VI) reduction and oxidation of cyanate.