Introduction

In the environment chromium occurs mainly in two different oxidation states, chromium(III) and chromium(VI). Chromium(III) is considered one of the essential elements for living organisms, whereas chromium(VI) is toxic to human, and was found to be carcinogenic. In recent years, chromium enters the environment from the discharge of steel, electroplating, tanning industries etc., leading to serious problems and hazardous risks for human health. The analysis of total chromium cannot be utilized to evaluate the environmental impact. For speciation analysis in environmental studies, the determination of redox species (Cr(VI)/Cr(III)) is very important. Therefore, the development of methods for the determination of the two species of Cr is required [1–4].

The direct determination of noxious trace elements in real samples may not be possible because of low concentrations and/or matrix interferences, so a separation/preconcentration technique were necessary [5]. Various separation/analysis methods have been employed to speciation analysis of Cr(III) and Cr(VI), such as liquid–liquid extraction-GFAAS [6], coprecipitation-FAAS [7, 8], cloud point extraction-FAAS [9], cloud point extraction-HPLC [10], cloud point extraction-ETAAS [11], ion exchange-FAAS [12], and solid phase extraction-FAAS [3, 13, 14], solid phase extraction-ICP-AES [15], etc. In our previous publication, the speciation of chromium by cloud point extraction-ETAAS [11] and by in situ separation and sequential determination with electrothermal vaporization inductively coupled plasma atomic emission spectrometry [16] were researched.

Solid phase extraction (SPE) technique is widely applied to preconcentration and speciation of trace metals because of its advantages of high enrichment factor, high recovery, rapid phase separation, low consumption of organic solvents and the ability of combination with different detection techniques in the form of on-line or off-line mode [17]. Nowadays, many adsorption materials, such as activated carbon [18], alumina [19, 20], chelating resins [21, 22] and nanometer material [15] were employed on the speciation studies of chromium. β-CDCP synthesized by the reaction of β-CD and cross-linked agent[23] was a spherical or grainy solid subject, which retained the inclusion property of β-CD. β-CDCP as a solid phase extraction material had been applied to separation/preconcentration the trace metal element [23, 24]. But β-CDCP-micro-column used as SPE material to speciation of Cr(III) and Cr(VI) seem to be lacking.

The present work aimed at investigating the separation Cr(III)/Cr(VI), the complex of Cr(III) with 4-(2-pyridylazo)-resorcinol (PAR) could been adsorbed on β-CDCP micro-column, whereas the Cr(VI) remained in aqueous phase. Thus, an in situ separation of Cr(VI) and Cr(III) could be realized. Cr(VI) was calculated by subtracting of Cr(III) from the total chromium. Total chromium was determined after reducing Cr(VI) to Cr(III). The developed method was applied to the speciation analysis of Cr(VI) to Cr(III) in tap water with satisfactory results.

Experimental

Apparatus

A ZEEnit 700 graphite furnace atomic absorption spectrometer (analytikjena, Germany) was used for the determination of Cr(III) and Cr(VI). The operating conditions for GFAAS are listed in Table 1. UV-2550 spectrophotometer (Shimadzu, Japan) and digital constant temperature water-bath (Guohua Limited Company, China) were used. The pH values were measured with a pHS-25 pH-meter (Shanghai, China). A BT-001 peristaltic pump (Shanghai Zhisun Instrument Factory, China) was used in separation and preconcentration process. A self-made PTFE micro-column (20 mm × 2.0 mm i.d.) was packed with β-CDCP in the manifold for separation/preconcentration.

Table 1 Operating condition for GFAAS
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Reagents

All chemicals used were of analytical grade. Cr(III) stock standard solution (1.0 mg L−1) was prepared by dissolution of 0.1 g metallic chromium (Tokyo, Japan 5N) in concentrated hydrochloric acid and dilution with water to 100 mL. Cr(VI) stock standard (1.0 mg L−1) solution was prepared by dissolution of 0.2830 g K2Cr2O7 (Shanghai Reagent Factory, Shanghai, China) in water and dilution to 100 mL. 4-(2-Pyridylazo) resorcinol (PAR) solution of 1.0 × 10−3 mol L−1 was prepared from regent grade PAR (A.R., Shanghai Reagent Factory, Shanghai, China) by dissolving 21.5 mg in 100 mL of alcohol. The required pH adjustments were made by the CH3COOH/CH3COONH4 buffer solutions. The reduction of Cr(VI) to Cr(III) was carried out using hydroxylamine hydrochloride (Nantong, China) as the reducing agent.

Column preparation

Fifty milligrams of β-CDCP was introduced into a PTFE micro-column (20 mm × 2.0 mm i.d.) plugged with a small portion of glass-wool at both ends. The column was washed thoroughly with distilled water and then preconditioned to the desired pH with CH3COOH/CH3COONH4 buffer before passing chromium ions containing solutions.

General preconcentration procedure for Cr(III)

The amount of Cr(III) (1.5 mL, 100.0 μg L−1), 0.25 mL PAR (1.0 × 10−3 mol L−1, ethanol) was mixed, controlling the appropriate acidity. The column was preconditioned by passing the buffer solution with pH 6.0 through the micro-column and then the standard solutions were passed through the column at a flow rate of 1.0 mL min−1. The Cr(III)-PAR complex retained on β-CDCP was eluted with 1.0 mL of 1.0 mol L−1 HCl solution. The concentration of the chromium was determined by GFAAS.

Reduction of Cr(VI) to Cr(III) and determination of total chromium

Reduction of Cr(VI) ions to Cr(III) was performed by using the procedure given in the literature [2, 25]. 1.0 mL of 2.0 mol L−1 HCl and then 0.5 mL of 5% (w/v) hydroxylamine hydrochloride were added to 3.0 mL of sample solution containing 150 ng Cr(III) and 150 ng Cr(VI), respectively. The solution was left at room temperature for 45 min. After the reduction of Cr(VI) to Cr(III), the described preconcentration procedure was applied and total chromium was determined by GFAAS. The Cr(VI) concentration was calculated by substracting the Cr(III) concentration from total chromium concentration.

Results and discussion

Spectral characteristics

The UV–vis spectra of Cr(III)-PAR and Cr(VI)-PAR at pH 6.0 were shown in Fig. 1. As could be seen that the λmax of Cr(III)-PAR complex was at 542 nm. On the contrary, there was no absorbance for Cr(VI)/PAR due to no complexation between Cr(VI) and PAR.

Fig. 1
figure 1

UV–vis spectrometry of Cr and PAR (curve 1:Cr(III) + PAR; curve 2: Cr(VI) + PAR; cCr(III): 20 mg L−1, cCr(VI): 20 mg L−1)

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Optimization of adsorption conditions

Effect of pH

The effect of pH (4.0–7.5) on the adsorption of Cr(III) and Cr(VI) on the β-CDCP column was studied (Fig. 2). It was showed that the quantitative adsorption (≥90%) for Cr(III) was found in the range of the pH 5.5–6.0, whereas the adsorption efficiency for Cr(VI) at this pH range was rather low (<10%). For all subsequent experiments, pH 6.0 was selected as optimal.

Fig. 2
figure 2

Effect of pH on the adsorption efficiency (30.0 μg L−1 of Cr(III) and Cr(VI))

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Effect of flow rate of sample solution

The flow rate of sample solution affected the adsorption of complex and the duration of analysis. The flow rate (1.0–2.0 mL min−1, Fig. 3) of sample solution was examined under the optimum condition. It was found that the adsorption of Cr(III)-PAR was quantitative when the flow rate <1.25 mL min−1. So, 1.0 mL min−1 was chosen as the sample flow rate in subsequent experiments.

Fig. 3
figure 3

Effect of flow rate of sample solutions on the adsorption efficiency (30.0 μg L−1 of Cr(III) and Cr(VI))

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Effect of the size of β-CDCP

The effect of the size of β-CDCP (100–300 mesh) on the adsorption efficiency was investigated (Fig. 4). With the decrease of the size of β-CDCP, the adsorption efficiency of Cr(III)-PAR complex on β-CDCP increased. When the mesh size of β-CDCP was larger than 200 mesh, Cr(III)-PAR could be quantitatively adsorbed. So, the size of 300 mesh was chosen.

Fig. 4
figure 4

Effect of the size of β-CDCP on the adsorption efficiency (30.0 μg L−1 of Cr(III) and Cr(VI))

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Effect of the sample volume

The effect of the sample volumes on the adsorption efficiency of Cr(III)-PAR on β-CDCP was investigated. For this purpose, 5.0–45.0 mL volumes of sample solutions containing 150.0 ng Cr(III) were passed through the column under the optimum conditions. The results were shown that the adsorption efficiency of Cr(III) was greater than 90% in the volume of sample 5.0–25 mL, when the volume of sample was larger than 25.0 mL, the adsorption efficiency is lesser than 90%. In this work, the sample volume of 25.0 mL was adopted. The preconcentration factor was 25 (the quotient of volume before absorption and after elution).

Adsorption capacity

To determine the adsorption capacity of the β-CDCP (maximum amount of Cr(III) retained from 1.0 g of β-CDCP), 15.0 mL sample solutions(Cr(III) concentrations from 5.0 to 200.0 μg L−1) was adjusted to pH 6.0 with CH3COOH/CH3COONH4 buffer, and the separation and preconcentration procedure described above was applied. The amount of Cr(III) adsorbed at each concentration level was determined. The breakthrough curve for Cr(III) was gained by plotting the concentration (μg L−1) of Cr(III) solution versus the micrograms of Cr(III) adsorbed per gram β-CDCP. When the concentration of Cr(III) reached 120.0 μg L−1, the retention capacity arrived at its maximum value. A adsorption capacity of the polymer was calculated to be 32.5 μg g−1.

Effect of HCl concentration and volume on desorption efficiency

For the elution of adsorbed Cr(III)-PAR on β-CDCP, diluted hydrochloric acid solutions at different volumes were used as an eluent. The desorption efficiency of Cr(III)-PAR with 0.25–3.0 mol L−1 hydrochloric acid solutions and 0.5–3.0 mL of HCl was investigated. So optimum volume of HCl (1.0 mol L−1) solution chosen for this work was 1.0 mL.

Column reuse

The stability and potential regeneration of β-cyclodextrin-cross-linked polymer micro-column were investigated. The β-cyclodextrin-cross-linked polymer micro-column was reproduced 6 times as section 2.4 and its adsorption efficiency for Cr(III)-PAR is greater than 90%. It demonstrated that β-CDCP exhibited good reversibility and reproducibility, which was associated with the fact that the inclusion interaction between β-CDCP and Cr(III)-PAR was a reversible process.

Effect of interfering ions

The effect of various interfering ions found in water samples on the determination of Cr(III) was studied. With a relative error of less than ±5%, the tolerance limits for the potentially interfering substances were listed in Table 2. These results show that Cr(III) could be determined quantitatively in water samples.

Table 2 Effect of potentially interfering substances on the determination of Cr (III)(30.0 μg L−1)
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Analytical parameters

The calibration graph for Cr (III) was linear 2.0–160.0 μg L−1, the corresponding coefficient of correlation was r = 0.9998. The relative standard deviation (R.S.D) was 2.47% (n = 3,c = 30.0 μg L−1). According to the dentition of IUPAC, the detection limit (3σ) of this method was 0.056 μg L−1. The preconcentration factor was 25.

Table 3 compares the characteristic data of the present method with those reported in literatures. Generally, the enrichment factor obtained by the present method is comparable to those reported method, and the detection limit is better than them.

Table 3 Comparison of the present method to other published methods for the analysis of Cr(III)
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Accuracy and applications of the method

For the validation of the method, a standard reference material (GBW(E) 080642) water sample after 1,000-fold dilution with deionized double distilled water was analyzed and the analytical results were in good agreement with the certified values (Table 4). The speciation procedure was applied to the speciation of chromium in tap water and the analytical results along with the recovery are given in Table 5. As could be seen, recoveries in the range of 98.6–104.8% were obtained.

Table 4 Analytical results for chromium speciation in the standard reference material water sample (mean values, n = 3)
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Table 5 Analytical results for chromium speciation in tap water (mean values, n = 3)
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Conclusion

A new method is used for the speciation of chromium by SPE combined with GFAAS due to the formation of Cr(III)-PAR in this paper. Cr(III)-PAR was quantitatively retained on polymer β-CDCP at pH 6.0, while Cr(VI) remained in the solution. The method is characterized with simplicity, rapidity, selectivity, safety, low cost and high preconcentration factor, and is suitable for the determination of Cr(III) and Cr(VI) in environmental water samples.