Chromatе surface-imprinted silica gel sorbent for speciation of Cr in surface waters

: This study is focused on the synthesis of chromate anion imprinted sorbent supported on silica gel for non-chromatographic Cr speciation in surface waters. The preparation procedure is based on grafting of 3-methyl-1-trimethoxysilylpropylimidazolium, preliminarily coordinated to CrO 42− as a template ion, onto the surface of silica gel. Sorption and desorption characteristics of surface imprinted sorbent toward Cr(III) and Cr(VI) were examined by batch solid phase extraction. An excellent separation of Cr(VI), selectively retained on the sorbent, from Cr(III) remained in the solution was achieved at pH 2-3 for 20 minutes. A freshly prepared mixture of ascorbic acid and nitric acid was selected as the most efficient eluent for quantitative desorption of the retained Cr(VI). An analytical procedure for Cr speciation in surface waters was developed and validated through analysis of certified reference materials. Detection limits achieved and relative standard deviations for typical concentration levels of Cr(VI) in surface waters matched the requirements of analytical procedures used in monitoring programs.


Introduction
Nowadays, it is commonly accepted that a lot of elements can naturally exist in the environment under various chemical forms with considerably different properties and behaviour in the environment, which resulted insubstantial distinctions in their toxicity, mobility and bioavailability. Undoubtedly this means that, determination of total element concentrations is generally not sufficient for comprehensive clinical and environmental considerations. Therefore, speciation analysis has reasonably become an important topic of the present-day analytical research.
During the recent years, one of the most investigated problems is the speciation analysis of chromium, mainly because of totally contrasting physiological effects of its predominantly existing chemical forms, i.e. Cr(III) and Cr(VI). The former is identified as an essential nutrient for humans, required for glucose and fats metabolism 1 , while the latter and its compounds are definitely recognized as carcinogenic and mutagenic substances. 2 From practical point of view, application of expensive and complicated hyphenated methods such as chromatographic separation followed by ICP-MS measurement might be replaced by simple off line quantitative separation of Cr species followed again by instrumental measurement. Solid phase extraction (SPE) is a widely used separation technique that offers several significant benefits such as low solvent consumption, high enrichment factors, fastness, simple operation in batch and column mode, good reproducibility as well as relatively low cost compared to other methods. 3 Furthermore, the correct selection of suitable sorbent is very important because the reliable species separation is a crucial stage of each successful SPE procedure. A large variety of materials were proposed as effective sorbents in non-chromatographic speciation analysis of chromium, e.g.

Synthesis and characterization of chromate surface-imprinted silica gel sorbent (Cr(VI)-SIS)
The synthesis of chromate anion surface-imprinted silica gel sorbent (Cr(VI)-SIS) via a multistep procedure is described in Section 3.3. and shown in Figure 1. After successful leaching of Cr(VI) from the surface of the synthesized sorbent, some specific binding sites with functional groups in a predetermined orientation and cavities with special size of CrO 2were formed. Non-imprinted sorbent (NIS) is synthesized in the same way as described above, in the absence of template.
To evaluate the degree of 1-methylimidazole incorporation, the elemental analysis of the synthesized  Cr(OH)2 + and Cr(OH) 2+ . 30 Accordingly, the range of pH 2−3 was selected as an optimal for quantitative separation of Cr(VI) from Cr(III).
Kinetic of sorption was investigated under optimal conditions (pH 2-3), the sorbent 50 mg Cr(VI)-SIS particles was mixed with 10 mL aqueous solution, containing 2 µg Cr(VI) and then vigorously shaken for 40 minutes. Aliquots (0.2 mL) of the supernatant solution were recurrently removed and Cr was measured by ETAAS. Results obtained showed that retention of Cr(VI) is a relatively fast process and 20 minutes are completely enough to achieve quantitative sorption.

Capacity and adsorption isotherms
The experimental adsorption capacity (Q) of the Cr(VI)-SIS was determined after saturation of the sorbents with chromate anions under optimum conditions at room temperature ( Figure 3). For this purpose, increasing amounts (2 µg -30 µg) of Cr(VI) anions were added to 50 mg sorbent and the equilibrium chromium concentration after adsorption was measured by FAAS. The sorption capacity Q was calculated using the following equation: where Q is the mass of chromate anions adsorbed per unit mass of the sorbent, µmol g −1 ; V is volume of the solution, L; m is the mass of the sorbent, g; C0 and Ce are the initial and equilibrium concentrations after adsorption of the chromium anions in aqueous solution, respectively, µmol L −1 .
The results presented in Figure  Langmuir isotherm model was used for curve fitting to derived adsorption data. According to the Langmuir isotherm theory the sorption process occurs in a surface monolayer of homogenous sites which number is fixed. 31 The expression of the linearized Langmuir isotherm (Eq. (2)) is: Where Ce is the equilibrium concentration of chromate anions in the solution, µmol L −1 ; Qe is the adsorption capacity of the adsorbed chromium ions onto the sorbents at equilibrium, µmol g −1 ; Qmax is the maximum adsorption capacity, µmol g −1 ; b is the Langmuir constant that relates to the affinity of binding sites, L µmol −1 . Calculated coefficients of the Langmuir model for the isotherms presented in Figure 3(B) were Qmax = 6.54 µmol g −1 and b =1.18 L µmol −1 and the obtained regression coefficient was R 2 = 0.992. The high R 2 value achieved for the adsorption of chromium anions onto Cr(VI)-SIS show that the Langmuir equation gives a good mathematical fit to the adsorption isotherm.
The experimental value of sorption capacity, determined according to the procedure described in paragraph 3.5 was 6.42 µmol g −1 sorbent, very close to the calculated value by Langmuir model (Eq.

6
(2)). The sorption capacity of NIS sorbent was found to be 4.75 µmol g −1 sorbent, around 25% lower than this obtained for imprinted particles.

Elution study
Taking into account that the separation of Cr species is based on electrostatic interactions, various solutions were tested as an appropriate eluents for quantitative desorption of Cr(VI). The initial idea was that elution of Cr(VI) could be realized by ion exchange, but the results obtained were unsatisfactory ( Figure 4). The highest degree of elution achieved by using (NH4)2CO3 as ionexchanger was a little bit above 80%. A possible explanation for the superiority of (NH4)2CO3 over the other ion exchangers used could be the stronger competitive action of the doubly charged carbonate anions at pH 9-10, but even though, quantitative elution was not acquired.
A suitable alternative to overcome this obstacle was elution based on reduction of Cr(VI) to Cr(III).
For this purpose ascorbic acid was used as a mild and environmentally friendly reducing agent. It was experimentally verified that Cr(VI) was entirely eluted (De > 98%) with freshly prepared solution of ascorbic acid (3 mmol L −1 ) in 2 mol L −1 HNO3.
Kinetics of elution process was studied after the loading of the sorbent with 10 µg Cr(VI) and subsequent elution with 10 mL 3 mmol L −1 ascorbic acid in 2 mol L −1 HNO3 for 10-40 min. Aliquot samples (0.2 mL) were taken and measured by ETAAS. The results showed that 20 min elution time ensures quantitative elution of retained Cr(VI).

Effects of competitive ions
The separation of Cr species is a result of electrostatic attraction between Cr(VI), i.e. HCrO4 -, and the positively charged methylimidazolium groups. In this regard, the extend of possible interferences of another anions, e.g. SO4 2-, HCO3 -, Cl -, PO4 3-, HPO4 2-, etc., on the extraction efficiency of Cr(VI)-SIS particles toward Cr(VI) have to be evaluated. As far as these anions exists at various concentration levels in surface waters, known amounts of Cr(VI) were directly spiked in several spring, river and mineral water samples (previously acidified to pH 2-3 by adding of HNO3) and SPE procedure was carried out under the optimized chemical conditions. Recoveries obtained for Cr(VI) for all studied samples were in the range 97 -99 %, with relative standard deviations (RSD) less than 7%, which can be accepted as an evidence for the absence of matrix interferences on the extraction efficiency of Cr(VI)-SIS sorbent toward Cr(VI) in real samples. However, the degree of sorption of Cr(VI) in the presence of Black sea water varied between 55% and 60%, which means that highly mineralized samples should be preliminary diluted in order to remove matrix interferences from high concentration of SO4 2and Clin sea water.
The batch-to-batch reproducibility of the synthesis of Cr(VI)-SIS was tested by using sorbents prepared independently from different batches. The relative standard deviation of the degree of sorption of 0.2 µg mL -l Cr(VI) with different sorbents was 4% which confirms very good reproducibility of the applied synthesis procedure. Experiments performed showed that Cr(VI)-SIS particles can be used for at least 50 sorption/desorption cycles of without significant loss of extraction efficiency.

Analytical figures of merit and applications to real samples
The accuracy and precision of the developed SPE procedure has been evaluated by the analysis of parallel samples of procedural blank (5 parallel blanks, containing 10 mL Milli-Q water and 50 mg Cr(VI)-SIS particles) and certified reference material Chromium VI-WS (Fluka) (

Conclusions
Sorbent based on surface Cr(VI) imprinted silica gel has been characterized for selective and efficient solid phase extraction of Cr(VI) and incorporated in analytical procedure developed for Cr speciation in surface waters. The synthesis procedure for sorbent preparation and enrichment procedure for Comparison of the proposed method with some other methods and strategies for Cr speciation (employing also nanomaterials as sorbents) is presented in Table 2. It is worth mention that the detection limits achieved depend on the instrumental method used and direct comparison of different procedures with different measurements methods is often misleading. The value of enrichment factor, typically, is in relation with measurement method and sorbent properties, however sample throughput has been also taken into account. It can be seen from Table 2, that the proposed analytical method for selective determination of Cr(VI) ensures detection limits, which are close to those of methods employing ETAAS as measurement method and fit well with environmentally relevant concentrations of Cr in surface waters even at background level in unpolluted sites.

Apparatus
The Flame AAS/Electrothermal AAS measurements were carried out with a Perkin-Elmer Model

Synthesis of the chromate anion surface imprinted sorbent
The synthesis scheme of chromate anion surface imprinted sorbent involves several steps ( Figure 1). This procedure was repeated until the Cr concentration (template ions) in the washing solution is below the LOD as measured by ETAAS. Finally, the prepared material was dried under vacuum at 60 °C for 8 h.

Sorbent capacity
The total sorption capacity (mg Cr(VI) g -1 sorbent) of the synthesized Cr(VI)-SIS was determined by shaking model solutions of Cr(VI) with increasing concentration with 50 mg sorbent for 20 min at optimal sorption pH=3. The amount of Cr in the effluate is determined by ETAAS.

Analytical procedure for Cr(VI) and Cr(III) determination in surface waters.
The water samples were filtered through 0.45 µm membrane filters, on site, during sampling and acidified with 1 mL 1 mol L -1 HNO3 per 100 mL sample, before transportation to the laboratory.

Determination of Cr(III):
If necessary, Cr(III) content could be simply calculated as a difference between both measurements for total Cr and Cr(VI).