Effect of scavengers on the photocatalytic digestion of organic matter in water samples assisted by TiO2 in suspension for the voltammetric determination of heavy metals

Journal of the Brazilian Chemical Society

Cavicchioli, Andrea¹  Universidade de São Paulo, São Paulo, Brazil¹  Gutz, Ivano G. R.¹ 

关键词: photodigestion;    TiO2;    scavengers;    voltammetry;   

DOI  :  10.1590/S0103-50532002000400006

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

In recent years, intense investigation has been undertaken on the TiO2-assisted photochemical mineralisation of organic substances by UV radiation, either with the semiconductor catalyst in suspension or immobilized on a solid support.1-26 The method was originally contemplated as a possibility to employ solar radiation as an energy source in water decontamination processes in connection with the treatment of industrial effluents and with water potabilisation.5,11,18,21 At a certain stage, however, studies and trials have markedly shifted towards the use of artificial UV source, mainly mercury vapour lamps, because of the scarce efficacy obtained with the solar radiation, the practical inconvenient of having to rely on the unpredictable availability of a natural light source and, not least, the poor overlapping of the solar spectrum with the TiO2 absorption range.7 Nonetheless, to date, such photocatalytic degradation has been successfully tested on a wide variety of organic substrates,7,16 both at laboratory and industrial ¾ or at least semi-industrial ¾ scale.15,21 This decomposition method has also set foot in the area of analytical chemistry. Initially, in the post-treatment of toxic reagents and products used in or generated by analytical procedures,22 looking at developing clean analytical routines, but still as an effluent treatment process, though at a small-scale. Then, in an attempt to upgrade the conventional UV photolysis in TOC determination devices.24 More notably, the authors of this article have recently proposed and evaluated the use of TiO2 suspensions in the photocatalytic pre-treatment of water samples aiming at the determination of heavy metals by voltammetric detection on mercury drop electrodes (MDE).26 It is well known that the presence of dissolved organic matter, present in natural waters, e.g. as humic substances, impairs the determination of total metal due to formation of stable and inert adducts between organic species and metal ions.27-28 For this reason, to access the total metal concentration, sample digestion is generally required and the UV photodegradation seems to represent a very suitable option when the organic load is not to high.29 This technique is, as a rule, carried out under temperature and pressure conditions closer to ambient, hence it is more easily manageable than the irradiation in microwave ovens. Therefore, the implementation of in-line (e.g. FIA) and in-situ procedures with UV digestion30 is favoured and there is also strictly no need for the addition of concentrated acids and reagents, potential sources of sample contamination. The experiments hitherto conducted with model samples containing Cd(II) (down to concentrations of 30 ppb) and dissodium ethylenediaminotetracetate (at concentrations of 10 to 500 fold in excess the Cd(II) content) have proven the efficacy of the photocatalytic destruction of the ligand and the complete recovery of the voltammetric metal signal, without prior filtration of the TiO2 suspension. Beside displaying the already mentioned benefits of the conventional UV digestion, the photocatalytic variant yields to the enhancement of the decomposition reaction speed and to widening the effective radiation range towards less short wavelengths, what enables the use of PTFE tubing in the irradiation chamber (instead of the customary quartz ones, more expensive and unpractical) and a likely wider range of light sources.26 Nevertheless, in order to extend the results obtained with a somewhat well-behaved model solution to real samples, it is of primary importance to assess the influence of other species present in the sample in the photocatalytic mechanism. This is centred on the formation of an electron-hole pair on the TiO2 (anatase) surface upon the incidence of a photon with energy equal to or higher than the gap between the valence and the conduction band within the semiconductor crystal (3.23 eV):1-8An electron is promoted to the conduction band (CB) while a positive hole is formed in the valence band (VB). In order for this process to have a net chemical effect, the recombination of the electron-hole pair has to be precluded so that the energy stored in chemical form is not dissipated as heat.7,8 The natural fate for h+VB and e-CB is the extraction of the electron to an external electric circuit,21,25 as it happens solar cells based on this principle, or the removal of either of them by means of redox reactions involving species present at the semiconductor surface, i.e. in the solid-liquid interface in the

【 授权许可】

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