INTRODUCTION

In last decade, the speciation study was extensively used for the investigation of essential and toxic levels of element ions in the living organisms since the biological role of an element can vary greatly depending on its chemical forms. Because the toxicity effects of Cr, As and Sb depended on their oxidation states, they are the most investigated elements in the speciation studies [1]. It has been known that Cr(III) is not toxic while Cr(VI) is toxic causing biological properties change for human being and living organisms. The Cr(III) is involved in the important physiological functions for human body taking part in carbohydrate, lipid and protein metabolisms [2] and it is considered as an indispensable trace element in the human diet [3]. On the other hand, Cr(VI) causes a stimulant of skin dermatitis and it is also carcinogenic and mutagenic for living organisms [4]. Therefore, determination of chromium species are more significant than total chromium in water, food, environmental samples [3].

In general, some instrumental methods, such as atomic absorption spectrometry (AAS), electrothermal atomic absorption spectrometry (ETAAS), inductively coupled plasma-atomic emission spectrometry (ICP-AES), inductively coupled plasma mass spectrometry (ICP-MS), are the mostly used for the determination of metals in the different matrixes [5]. Therefore, some preconcentration and separation methods such as dispersive liquid–liquid microextraction (DLLME) [6], could point extraction (CPE) [7] and solid phase extraction (SPE) [8], should be performed before detection step for the speciation of inorganic chromium species. SPE is the most preferred separation and preconcentration method due to its some advantages, such as higher recovery, precision, accuracy and enrichment factor, shorter analysis time, and lower consumption of organic solvents [9]. For the speciation of inorganic chromium species with SPE method, activated carbons, alumina, and some chelating resins are commonly used as adsorbent materials [10].

The permitted limits in drinking water by the International Agency for Research on Cancer (USEPA) and the World Health Organization (WHO) for total chromium and Cr(VI) are reported as 100 and 50 μg L–1, respectively [11]. In this study, the speciation of inorganic chromium species is performed in a column containing Amberlite CG-120 resin used as an adsorbent and AAS is used for the determination of these inorganic chromium species in water samples. Some experimental parameters such as, pH, eluent type, eluent concentration, sample flow rate, eluent flow rate, adsorbent amount and matrix effect, were optimized to obtain better experimental conditions for the preconcentration, separation and detection of inorganic chromium species in water samples. Total chromium is determined after reducing Cr(VI) to Cr(III) in aqueous medium. The Cr(VI) is calculated by the differences between the total Cr and Cr(III) concentrations in both standards and water samples. The proposed procedure was also applied to a certified reference material, TMDA-70.2 Ontario Lake Water and the results was in good agreement with certified values for chromium.

EXPERIMENTAL

Apparatus and Reagents

ATI Unicam 939 model atomic absorption spectrometry is used for the total content of Cr(III) and Cr(VI). The operating conditions are as follows: lamp current; 12 mA, band pass 0.5 nm, wavelength; 357.6 nm and acetylene/air flow rates: 1.5 L min–1. pH measurements are carried out by using digital pH meter (Thermo, Orion Star). 18.2 MOhm cm deionized water is used to prepare all solutions (PURIS, Expe-up). Cr(NO3)3⋅9H2O supplied from Merck and K2Cr2O7 supplied from Chem-Lab are used to prepare 1000 mg L–1 stock solution of Cr(III) and Cr(VI) standard solutions, respectively. The used acid solutions are HNO3 (65%, Merck), HCl (37%, Sigma–Aldrich) and HClO4 (60%, Riedel-de Haen).

General Preconcentration Procedure for Inorganic Chromium Species in a Column

The preparation of the column system used in this study is carried out with reference to our previous study [12]. The column is firstly preconditioned at pH 1 using HCl. 25 mL of a model solution containing 25 μg of Cr(III) is also adjusted to pH 1 using same HCl solution. Then, the model solution is passed through the column at a flow rate of 5 mL min–1. The adsorbed Cr(III) ions on the column are eluted with 5 mL of 4 mol L–1 HCl at a flow rate of 5 mL min–1. The eluent solution is analyzed by FAAS. The same experiment is also carried out using Cr(VI) standard synthetic solution but it is not adsorbed into the column at the same experimental conditions. The column filled with Amberlite CG-120 resin is used repeatedly after washing with 10 mL of 4 mol L–1 HCl and deionized water.

To determine the total chromium, 25 mL of 25 μg Cr(III) and 25 μg Cr(VI) are prepared and then Cr(VI) in the solution are reduced to Cr(III) by adding 0.5 mL of 2 mol L–1 HCl and 0.5 mL of 5% (w/v) hydroxylamine hydrochloride in turn and the reduction procedure is finished in 10 min [13, 14]. After reduction procedure, this solution was adjusted at pH 1 and passed through to resin at a flow rate of 5 mL min–1. The adsorbed Cr(III) ions on the resin are eluted with 5 mL of 4 mol L–1 HCl at a flow rate of 5 mL min–1. The eluent solution is analyzed by FAAS. The concentration of Cr(VI) is found by the differences between the total chromium and Cr(III) concentrations.

RESULTS AND DISCUSSION

In this study, some experimental parameters, such as pH, eluent type, eluent concentration, sample flow rate, eluent flow rate, adsorbent amount and matrix effect, are optimized to obtain a high recovery.

pH Effect

pH is a significant parameter in the solid phase extraction method. The pH effect on the recovery of Cr(III) and Cr(VI) is investigated using model solutions. For this aim, 25 mL of 50 μg Cr(III) and 50 μg Cr(VI) are separately send to the column containing 0.5 g of resin at different pH values. Cr(III) and/or Cr(VI) adsorbed on the resin is recovered with 10 mL of 4 mol L–1 HCl followed by AAS detection. The recovery yield of Cr(III) is quite high (>95%) for both pH 1 and 2 values shown in Fig. 1. In other respect, that of Cr(VI) is quite low (<10%) for all pH values and different HCl concentrations. Based on the results, pH 1 is selected as the optimum pH value so that the highest recovery yield for Cr(III) is obtained [15].

Fig. 1.
figure 1

The effect of pH on the recovery of Cr(III) and Cr(VI).

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Eluent Effect

Since the concentration and volume of the eluent solution are of great importance to obtain quantitative recovery for Cr(III) determination, 10 mL of different eluent solutions, such as 2 mol L–1 HNO3, 2 mol L–1 HClO4, 2 mol L–1 HCl and pure ethanol are sent into the column and the highest recovery yield is achieved using 2 mol L–1 HCl. In order to reach highest enrichment factor for Cr(III) detection, the optimum HCl concentration is also investigated using 5 mL of eluent solution. To this purpose, various HCl concentrations ranged from 2 to 5 mol L–1 of HCl are investigated. Quantitative recovery is obtained by using 4 mol L–1 HCl. Hence 5 mL of 4 mol L–1 HCl is selected as eluent for further experiments.

Flow Rate of Sample and Eluent Solution

Since optimization of flow rate is a substantial factor which influences the recovery yield, the flow rates of the sample and eluent solutions are separately investigated to obtain the highest recovery yield for Cr(III) determination in this study. Additionally, both sample and eluent flow rates are also of great importance to determine the lowest analysis time and highest recovery yield. For this, the flow rates are varied from 0.25 to 5 mL min–1 for both sample and eluent solutions and the optimum flow rates are found to be 5 mL min–1 for both sample and eluent solutions based on the highest recovery yield shown in Fig. 2. At these conditions, Cr(III) is fastly and efficiently adsorbed on the column and easily recovered by 4 mol L–1 HCl used as eluent solution.

Fig. 2.
figure 2

The effect of sample and eluent flow rate on the recovery of Cr(III).

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Amberlite CG-120 Amount

Various amounts of Amberlite CG-120 resin ranged from 0 to 0.5 g are used to obtain quantitative recovery for Cr(III) determination. Since the recovery yield of Cr(III) is reduced when the adsorbent amounts are lower than 0.5 g, the amount of adsorbent is selected as 0.5 g (Fig. 3).

Fig. 3.
figure 3

The effect of adsorbent amount on the recovery of Cr(III).

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Sample Volume

In general, sample volume should be selected as high as possible so that the enrichment factor is increased for chromium determination in any samples including the low chromium. To determine enrichment factor of Cr(III), sample solutions between 25 and 4000 mL including 25 μg Cr(III) are passed through the resin at the optimum conditions. The Cr(III) in the solution is quantitative recovered (≥95%) for sample volumes ranged from 25 to 3000 mL but the recovery value is declined to 88% when sample volume is 4000 mL. Therefore, 3000 mL is chosen as the maximum sample volume; and since the eluent volume is chosen as 5 mL of 4 mol L–1 HCl, 600-fold enrichment factor is obtained in this study.

Interference Studies

Study of interference should be performed since environmental samples containing complex matrix may affect the analyte signal positively and/or negatively. For this reason, 25 mL of synthetic solution are prepared including 25 μg of Cr(III) and different cations prepared from salts with chloride and nitrate ions are added individually to the Cr(III) synthetic solution and then the developed preconcentration method is applied. The maximum tolerances of the investigated ions are given in Table 1. There is no obvious influence on the recovery of Cr(III) for studied matrix ions.

Table 1.   Interference effects of some ions on the recovery of Cr(III)
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Method Validation

To check method validation, the certified reference material (TMDA-70.2 Lake Ontario water) in which chromium is given as total chromium is used for the determination of inorganic chromium species. At first, Cr(III) amount is found to be 413 ± 9 μg L–1 in the certified reference material without reduction. After reduction procedure is applied, total Cr amount is found to be 415 ± 10 μg L–1. As the amount of Cr(III) is not significantly different from that of total Cr, Cr(VI) amount is not detected in certified reference material. There is a good agreement between found and certified values (total chromium, 400 ± 24 μg L–1) at 95% confidence level with about 4% of relative error.

Analysis of Real Samples

The proposed method is used for the determination of inorganic chromium species under optimum experimental conditions in the different spring waters supplied from Isparta province (Eyüpler Village) and Burdur province (Ağlasun County), the commercial drinking water samples purchased from local market in Burdur province, and the waste water supplied from Isparta province (Süleyman Demirel Organized Industrial Region) at Turkey. The method validation is also used for the determination of chromium in water samples and the accuracy of the method is checked by the determination of the recovery of spiked chromium in the water samples (Table 2).

Table 2.   Determination of Cr(III), Cr(VI) and total Cr in the water samples, 100 mL
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The analytical performance of the proposed method is compared with that of other methods in the literature (Table 3).

Table 3.   Comparison of this method with other methods for chromium determination
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CONCLUSIONS

This developed method for inorganic chromium speciation with AAS detection is an accurate, precise, rapid, economic and simple and method in different water samples. This method has several advantages. For example, there is no necessity of using chelating agents and buffer for the separation and preconcentration of inorganic chromium species at the strongly acidic medium, pH 1. Using this method, the limit of detection and enrichment factor are found to be 0.3 μg L–1 and 600-fold, respectively. The concentration of inorganic chromium species is successfully calculated for spring, drinking and waste water samples and also certified reference material, TMDA-70.2 Ontario Lake Water, at a 95% confidence level.