Elsevier

Catalysis Today

Volume 75, Issues 1–4, 3 July 2002, Pages 95-102
Catalysis Today

Removal of cyanides in wastewater by supported TiO2-based photocatalysts

https://doi.org/10.1016/S0920-5861(02)00049-4Get rights and content

Abstract

Titania supported samples on different types of silica have been prepared through a sol–gel method followed by hydrothermal processing. The photocatalytic activity of the samples was tested for free cyanides photo-oxidation. As compared to commercial TiO2 all the synthesised materials showed not only similar photocatalytic efficiencies but improved recovery properties. The degradation of iron(III) cyanocomplexes was also studied in the absence or presence of titania catalyst. In all cases, a photoinduced CN released from the complex takes place through a homogeneous process. The further oxidation of those cyanides ions to cyanate species is significantly enhanced in the presence of the catalyst in which mesostructured SBA-15 silica is used as support.

Introduction

The presence of cyanides in effluents related to coal gasification and electroplating processes, in which metal cyanides utilisation is almost essential, is a matter of environmental concern not well resolved nowadays. Among the different technical alternatives to remove cyanides from industrial wastewater, photocatalytic processes have demonstrated a high efficiency [1]. Heterogeneous photocatalysis is based on the use of a source of UV radiation to stimulate a semiconductor material on whose surface the oxidation of the pollutant is carried out. In the last years, several environmental applications have been developed in this research field [2], titanium dioxide being the semiconductor most widely used due to its high activity. To maximise TiO2 photoactivity, particles should be small enough to offer a high number of active centres by unit mass [3]. Unfortunately, a small particle size leads to high filtration costs to remove the catalyst, hindering its industrial application. For this reason, a major focus of current photocatalysis research is the achievement of titania-based materials with suitable properties to present simultaneously high photoactivity and effective separation properties. Titania nanoparticles supported on silica might be materials capable of accomplishing both requests since they do not only take the advantage of high surface areas but of improved recovery properties as compared to pure titania, which may facilitate the separation [4].

In the present work, the synthesis and characterisation of different TiO2 supported on silica photocatalysts is reported as well as their application to the photo-oxidation of both free cyanides and complexed iron cyanides in water. The interest of testing the catalysts in the degradation of the latter species lies on the fact that none of the commercially available methods used up-to-date to treat wastewaters polluted with cyanides are effective for the removal of iron cyanocomplexes species. Even though these compounds can be considered as nearly non-toxic themselves, in the presence of sunlight they are quite unstable dissociating the highly toxic free cyanide ions.

Section snippets

Synthesis

The samples were prepared via sol–gel hydrolysis–condensation of titanium tetraisopropoxide in the presence of the chosen silica support: either a commercial SiO2 (Crosfield CS 4187) or mesostructured MCM-41 and SBA-15 silica samples prepared in our laboratory. Textural properties of all the supports are summarised in Table 1. MCM-41 [5] was synthesised following a room temperature two-step sol–gel process based on a method recently reported [6]. In the first stage, a solution made up of

Catalysts characterisation

Textural properties of the photocatalysts are presented in Table 1. XRD patterns showed that anatase was the only crystalline phase present in all TiO2 supported samples. Average diameter sizes of the titania particles were calculated from the broadening of the 1 0 1 XRD anatase peak. The obtained values were consistent with those obtained by analysis of the TEM micrographs and the calculated ones from the optical band gap applying the hyperbolic band model [9]. The morphology of the catalysts

Conclusions

Titania supported over a porous silica, a host that is transparent to the employed UV radiation, is shown to be a good method for developing TiO2-based materials with reasonable activity for cyanide photocatalytic oxidation and improved separation properties. The type of the silica host affects not only the TiO2 size or the recovery properties of the catalysts but also, the activity of each sample. The use of mesostructured silica SBA-15 leads to the best activity of the supported titania,

Acknowledgements

The authors thank “Consejerı́a de Educación, Comunidad de Madrid” for the financial support of this research through the project 07M/0050/1998 and “Grupos Estratégicos de Investigación” and “CICYT” through the project PPQ2000-1287.

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