Trace determination and chemical speciation of selenium in environmental water samples using catalytic kinetic spectrophotometric method
Introduction
Selenium is one of the most interesting elements from the point of view of its clinical and environmental effects because it has the narrowest tolerance window of any element [1]. Depending on the species, oxidation state and concentration, Se compounds can range from being essential to highly hazardous and toxic [2]. It is an important trace element for human health as being incorporated in proteins, shows anti carcinogenic role and prevents heavy metal toxic effects [3], [4], [5]. Naturally, Se is present in earth's crust in low amounts (0.05 μg g−1), however, its compounds are now widely spread throughout the environment from the combustion of fossil fuels, improper disposal of wastes from activities such as mining, uses in the glass and electronic industries as well as agriculture [6]. Selenium mobility in the environment, availability for biota and toxicity depends on its oxidation state [7] and so its speciation is necessary. The predominant forms of Se in water are selenite {Se(IV)} and selenate {Se(VI)}. Inorganic Se(IV) has been found to be 500 times more toxic than common organo-Se compounds [2] and is considered more dangerous to aquatic organisms than Se(VI) due to its higher solubility and bioavailablity [4]. Hence inorganic Se speciation in the environmental water is extremely desirable.
Over the past two decades various instrumental techniques including UV–visible spectrophotometry [8], atomic/molecular fluorescence spectrometry [9], [10], [11], [12], high performance liquid chromatography [11], hydride generation atomic absorption spectrometry [7], [13], [14], [15], differential pulse cathodic stripping voltammetry [16], inductively coupled plasma mass spectrometry [17], ion chromatography [18], [19] and flow injection analysis coupled with hydride generation atomic fluorescence spectrometry [20] have been developed and used for Se speciation. However, the catalytic kinetic spectrophotometric methods (CKM) offer distinct advantages for being of high sensitivity, low cost and simplicity for a large majority of analytes in water samples [21], [22], [23], [24], [25]. Few CKM for the determination of Se(IV) in water systems have been reported in the literature as presented in Table 1 [26], [27], [28], [29], [30], [31], [32], [33] but no CKM exists which allows the determination of Se(IV), Se(VI) and total inorganic Se in water samples.
The reduction of bromate by hydrazine dihydrochloride using methyl orange (MO) is quite slow. Se(IV) dramatically catalyses this reaction and has been used for the catalytic determination of Se(IV) [34]. However, the study on bromate-hydrazine-methyl orange indicator reaction has been studied at very high reactants concentrations which lack reproducibility. Also the choice of λmax and MO concentration are neither studied nor supported with literature while selection of the optimum pH is in contrast to the experimental data [34]. This prompted us to investigate the rate dependence studies of the reactant concentrations for optimization and to discover the feasible dynamic range for Se(IV) determination based on its catalytic effect on the indicator reaction and is reported in the present paper.
Section snippets
Apparatus
A PerkinElmer Lambda 16 UV–visible Spectrophotometer (model 1096) with 10 mm matched quartz cells was used for all spectral and absorbance measurements. A thermostatic water bath (Thermoline, Australia) was used to control the temperature of the reagents and reaction. A Hanna Instruments 211 microprocessor pH meter was calibrated with standard buffers (pH 4 and 7) and used for measuring pH of solutions.
Reagents
All the reagents of analytical grade were used without further purification. Distilled
Spectral studies
The spectral changes occurring for the first 7 min in the Se(IV) catalyzed reduction of bromate by hydrazine dihydrochloride followed as the decolorization of MO in acidic media with time is shown in Fig. 1. Fig. 1 clearly shows the maximum absorbance at 507 nm, which is the characteristic of the acid form of MO, and decreases with its oxidation with time [36]. Therefore the absorbance and initial rate measurements for the determination of Se(IV) were made at λmax 507 nm, which is significantly
Conclusion
The validated kinetic spectrophotometric method employed here proved to be simple, sensitive, selective, inexpensive and hence allows rapid determination of inorganic selenium at parts per billion levels in water. Its limit of detection is found to be 1.3 μg L−1 Se(IV). The proposed method is suitable for determination of trace amounts of Se(IV), Se(VI) and total inorganic selenium in environmental water in presence of other ions at natural levels. A distinct advantage of the proposed method is
Acknowledgements
The authors are grateful to the University of the South Pacific, Suva, Fiji for financial support through URC project no. 6C 067-1321. One of the authors (VC) is grateful to the University of the South Pacific, Suva, Fiji Graduate Assistance through project no. 39696-1321.
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