Photochemical Degradation of Cyanides and Thiocyanates from an Industrial Wastewater.

Juan Jose Viña Mediavilla,Conchi O Ania,Maria C Fernandez De Cordoba,Begoña Fernandez Perez,Julia Ayala Espina

doi:10.3390/molecules24071373

Abstract

We have explored the simultaneous degradation of cyanides and thiocyanate present in wastewaters from a cokemaking factory using photoassisted methods under varied illumination conditions (from simulated solar light to UV light). Overall, the photochemical degradation of cyanides was more efficient than that of thiocyanates, regardless of the illumination conditions, the effect being more pronounced in the absence of a photocatalyst. This is due to their different degradation mechanism that in the case of thiocyanates is dominated by fast recombination reactions and/or charge transfer reactions to electron scavengers. In all cases, cyanate, ammonia, nitrates, and nitrites were formed at different amounts depending on the illumination conditions. The conversion yield under simulated solar light was almost complete for cyanides and quite high for thiocyanates after 6 h of illumination. Regarding toxicity, photochemical oxidation at 254 nm and under simulated solar light decreased significantly the toxicity of the pristine wastewater, showing a correlation with the intensity of the irradiation source. This indicate that simulated light can be effectively used to reduce the toxicity of industrial effluents, opening an interesting perspective for optimizing cyanide detoxification systems based on natural light.

Highlights

Cyanides and thiocyanates are formed in several industrial processes such as gold mining activities and the production of coke in steel factories
We have investigated the simultaneous photooxidation of cyanides and thiocyanates from industrial wastewater with moderate concentrations of both species using various irradiation sources
The photochemical degradation of cyanides was more efficient and faster than that of thiocyanates, following a similar trend than that observed in synthetic solutions of the individual components

Summary

Introduction

Cyanides and thiocyanates are formed in several industrial processes such as gold mining activities and the production of coke in steel factories. Both pose major environmental and health concerns as they can be generated in high concentrations in industrial effluents [1,2]. The concentrations of cyanides and thiocyanates in mill effluents in gold mining activities can vary considerably between 40 to 600 mg/L [3]. Thiocyanate is one of the major constituents of wastewater with concentrations in the range of 20–1500 mg/L. Thiocyanate can be formed biologically from the detoxification of cyanides [3]. Thiocyanate is less toxic than cyanide, it is more stable and more difficult to degrade

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