Trace determination and chemical speciation of selenium in environmental water samples using catalytic kinetic spectrophotometric method

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Abstract

A catalytic kinetic method is described for the determination of Se(IV), Se(VI) and total inorganic selenium in water based on the catalytic effect of Se(IV) on the reduction of bromate by hydrazine dihydrochloride in acidic media. The generated bromine decolorized methyl orange (MO) and the reaction was monitored spectrophotometrically at 507 nm as a function of time. The initial rate and fixed time methods were adopted for the determination and speciation of inorganic selenium. Under two optimum conditions, the calibration graphs are linear in the range 0–126.3 and 0–789.6 μg L−1 of Se(IV) for the initial rate method and 0–315.8 and 0–789.6 μg L−1 of Se(IV) for the fixed time method. The detection limits were 1.3 and 14.7 μg L−1 for the initial rate and fixed time methods, respectively. The proposed methods were validated statistically and through recovery studies in environmental water samples. The relative standard deviation in the determination of 31.6–94.8 μg L−1 of Se(IV) and Se(VI) was less than 6%. Analyses of standard reference materials for selenium using initial rate and fixed time methods showed that the proposed methods have good accuracy. Se(IV), Se(VI) and total inorganic selenium in environmental water samples have been successfully determined by this method after selective reduction of Se(VI) to Se(IV).

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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