New ion-imprinted polymer-functionalized mesoporous SBA-15 for selective separation and preconcentration of Cr(III) ions: modeling and optimization

Abstract

A new Cr(III) ion-imprinted polymer (Cr(III)-IIP) was prepared by functionalizing the surface of an SBA-15 support with dithizone (DZ) indicator followed by an imprinting polymerization method. The Cr(III)-IIP was characterized using FTIR, EDS and SEM techniques. Using dispersive solid phase extraction coupled with flame atomic absorption spectrometry (DSPE-FAAS), the ultrasound-assisted adsorption properties of the Cr(III)-IIP were investigated and compared with those of a non-imprinted polymer (NIP), using the batch method. Central composite design under response surface methodology was used for the evaluation of the effect of variables, individually, as well as their possible interaction effects, on the adsorption process. Variables such as sonication time, Cr(III)-IIP mass, concentration of Cr(III) and pH were involved in this study. Under optimum experimental conditions, this DSPE-FAAS method exhibited a linear range of 2–950 μg L⁻¹ for the Cr(III) ion concentration with a detection limit of 0.06 μg L⁻¹. The relative standard deviation for the analyte was found to be lower than 2.44%. The selectivity of the method was tested by analyzing the interference of ions such as Pb²⁺, Co²⁺, Ag⁺, Zn²⁺, Ni²⁺ and Fe²⁺, and the method showed good selectivity. The method was successfully applied to the separation and preconcentration of Cr(III) ions from tap, river and spring water. The IIP adsorption capacity was found to be 131.4 mg g⁻¹. The enrichment and preconcentration factors were found to be 97 and 147.7, respectively.

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1. Introduction

Cr(III) is one of the toxic metals and is widely used in various industrial processes such as textile manufacture, leather tanning, electroplating, surface finishing, pigment production and manufacturing batteries.^1,2 This means that plenty of Cr(III) ions can easily enter the environment via the wastewater of these industries, and thus they continue to enter food chain organisms, which may lead to carcinogenic, allergenic and mutagenic effects.^3,4 Therefore, it is necessary to design a safe and clean procedure for the efficient separation and determination of Cr(III) ions before their arrival in organisms.

There are several separation procedures for trace Cr(III) ion determination such as liquid–liquid extraction^5,6 and solid phase extraction (SPE).^7,8 Among these techniques, SPE has been widely used to remove and determine Cr(III) ions from different samples.^9,10 This method is assumed to be superior to other methods in terms of the initial cost, the flexibility and simplicity of design, the small volumes of organic solvent used, and the ease of operation and sensitivity.^1,11 A key part of the SPE performance is the selection of an appropriate adsorbent. Criteria such as the adsorption being fast and quantitative, and the adsorbent having a high surface area and adsorption capacity as well as good regenerability and selectivity are of importance to consider.^12,13 Many adsorbents such as active carbon, graphene, carbon nanotubes, zeolites, modified ion exchange resin and ion imprinted polymers have been widely used in the SPE method for the separation of metal ions.^12,14,15

Among these adsorbents, ion-imprinted polymers (IIPs) are more efficient than the others in the removal and determination of specific metal ions due to their good selectivity.^16–20 IIPs prepared using conventional methods suffer from disadvantages such as a low available surface area and adsorption capacity, a slow mass transfer rate and incomplete template removal.^17,21 In the past decade, some IIPs have been proposed with improvements in some of the above-mentioned characteristics.^22,23 However, it is necessary to develop novel adsorbents with binding sites located at the surface of the support matrix with many advantages such as the high and easy accessibility of sites for the target species and fast binding kinetics.^24,25 To achieve these purposes, the design of novel supports is of high importance to improve the figures of merit of the process. Recently, the use of support materials including silicon and carbon-based materials and magnetic ones has been reported.^22,25 While silicon-based materials have been of more interest, conventional silicon-based IIPs have disadvantages such as low surface area and chemical restrictions which decrease the adsorption capacity and mass transfer rate.^26,27 SBA-15, a mesoporous silicon-based material, satisfies all the above-mentioned requirements and thus exhibits excellent adsorption–desorption properties which motivated us to use it as a highly suitable and efficient support.^28,29

Ultrasound waves can make acoustic cavitations (the growth and collapse of tiny bubbles) which cause high pressure variation through a liquid. This phenomenon enhances the diffusion coefficient, the mixing and dispersion of the adsorbent, as well as the mass transfer in solution. Therefore, ultrasound-assisted dispersive solid phase extraction coupled with flame atomic absorption spectrometry (DSPE-FAAS) is applied to greatly improve the separation and preconcentration processes.^30,31

In this work, a novel Cr(III) ion-imprinted polymer (Cr(III)-IIP) was prepared by functionalizing the surface of an SBA-15 support with dithizone (DZ) indicator followed by the imprinting polymerization method. The Cr(III)-IIP was characterized using FTIR, EDS and SEM techniques. Ultrasound-assisted DSPE-FAAS using SBA-15-based Cr(III)-IIP was successfully applied to the efficient adsorption of Cr(III) ions by running systematic experiments designed using central composite design (CCD) under response surface methodology (RSM).^32,33 The method was successfully applied and optimized for the separation and preconcentration of Cr(III) ions from tap, river and spring water. A low detection limit, wide linear calibration range and good selectivity were obtained. The IIP adsorption capacity as well as the enrichment and preconcentration factors were determined.

2. Experimental

2.1. Reagents and instruments

Chemical reagents including Pluronic P123 (M_w = 5800), chloropropyltriethoxysilane (CPTES), methacrylic acid (MAA), ethylene glycol dimethacrylate (EGDMA) and 2,2′-azobisisobutyronitrile (AIBN) were purchased from the Sigma Aldrich company (St. Louis, MO, USA). Chromium(III) nitrate nonahydrate, tetraethylorthosilicate (TEOS) and dithizone were purchased from Merck (Darmstadt, Germany). All other required chemicals of analytical grade (with the highest purity available) were provided by Merck and used as received. The pH was adjusted using 0.1 M HNO₃ or 0.1 M NaOH solution. Double distilled deionized water was used throughout.

The pH was measured using a pH/Ion meter model 686 (Metrohm, Switzerland, Swiss). The concentration of Cr(III) ions was determined using a 680 flame atomic absorption spectrometer (FAAS) (Shimadzu, Japan) equipped with a hollow cathode lamp and a deuterium background corrector using an air–acetylene flame. The frequency and power of the ultrasonic bath (Tecno-GAZ SPA Ultrasonic system, Italy) were set to be 60 Hz and 130 W, respectively. The morphology of the samples was studied using scanning electron microscopy (SEM: KYKY-EM3200) under an acceleration voltage of 26 kV. FTIR absorption spectra of the samples were taken using a FTIR 6300 in the region 400–4000 cm⁻¹.

2.2. Synthesis of Cr(III) IIP based on a mesoporous SBA-15 support

SBA-15 was synthesized using the hydrothermal method.³⁴ Briefly, a solution including 4.0 g of Pluronic P123 (M_w = 5800), 90 mL of 2.0 M hydrochloric acid and 21 mL of deionized water was prepared and stirred for 5.0 hours at room temperature. Then, 6.8 mL of TEOS was slowly added to the solution while stirring at room temperature followed by the raising of the temperature to 40 °C, where it was kept for 24 hours. Subsequently, the resulting white mixture was transferred to an autoclave and was put in the oven at 100 °C for 24 hours. Next, the product was filtered, washed with 250 mL of distilled water and dried at room temperature. Finally, the resulting white precipitate was calcinated at 600 °C for 6.0 h to remove the templates. For functionalizing the SBA-15 support, it was first chlorinated as follows: 5 g of SBA-15 was stirred in acetonitrile for 1 h followed by the addition of 20.76 mmol of CPTES and the mixture was refluxed in an oil bath at 80 °C for 24 h under a neutral atmosphere. The resulting compound was filtered, washed and dried at 110 °C for 4 h in an oven under vacuum. Finally, 2 g of the chlorinated SBA-15 (Cl-SBA-15) was stirred in acetonitrile for 1 h and 0.25 g of dithizone was subsequently added and this was refluxed for 12 h in an oil bath at 80 °C. Then, the solid, obtained as dithizone-SBA-15 (DZ-SBA-15), was filtered, rinsed with ethanol and dried under vacuum at 50 °C. The Cr(III)-IIP was prepared by the surface imprinting of the DZ-SBA-15 as follows: a mixture of 50 mg of DZ-SBA-15 and 50 mL of acetonitrile/methanol (60/40 v/v) was prepared and dispersed in a round bottom flask. Then, 0.2 mmol of Cr(NO₃)₃·9H₂O, 1.0 mmol of MAA, 3 mmol of EGDMA and 40 mg of AIBN were added to the flask. To deoxygenate the solution, as for the polymerization mixture, it was purged with N₂ for 10 min. Then the flask was well-sealed and stirred in an oil bath at 60 °C for 24 h. After the surface polymerization was finished, the solution was washed using methanol repeatedly and finally was washed with distilled water to remove the unreacted materials. Then, to leach the ions trapped in the polymer, 3 M HCl was used until a Cr(III) ion-free filtrated solution was achieved. Finally, it was repeatedly washed with deionized water to achieve a filtrate with neutral pH. The final powdered product, Cr(III)-IIP, was dried at 40 °C under vacuum for 12 h (the synthesis procedure is schematically shown in Scheme 1). For comparison, a non-imprinted polymer (NIP) was also prepared as a blank in parallel but without the addition of Cr(NO₃)₃·9H₂O.

Scheme 1 All steps of the preparation of Cr(III)-IIP based on an SBA-15 support.

2.3. Dispersive solid-phase extraction experiments

The ultrasound-assisted adsorption experiments were performed in batch mode to simultaneously increase the diffusion coefficient of the Cr(III) ions and the mass transfer as follows: Cr(III) ion solutions with various concentrations at pH 6 (optimum value) were added into 50 mL Erlenmeyer flasks containing specific amounts of Cr(III)-IIP and were dispersed thoroughly over 6 min in an ultrasonic bath at room temperature. Finally, the analyte-containing Cr(III)-IIP extracted from the sample solution was immediately centrifuged (3000 rpm, 5 min) and the liquid phase was discarded. In order to remove interfering compounds and loosely retained Cr(III) ions, the IIP particles were washed several times with double distilled water. Afterwards, for the desorption of Cr(III) ions as an analyte, the analyte-containing Cr(III)-IIP was eluted using 2 mL of HCl for 2 minutes (optimal elution time). Finally, the solution was immediately centrifuged (3000 rpm, 5 min) to separate the Cr(III) ions from the Cr(III)-IIP, followed by the collection of the liquid phase. The total concentration of Cr(III) ions in this phase was measured using a flame atomic absorption spectrometer. To evaluate the performance of the method, the preconcentration factor percentage, enrichment factor (EF) and extraction recovery percentage (ER%) were calculated.

2.4. Experimental design

Central composite design under response surface methodology is one of the most applicable approaches of simultaneous optimization, which is superior to optimizing one variable at a time in terms of the lower number of required experiments and cost-effectiveness, as well as its ability to estimate the individual effect of variables and their interactions on the performance of the ultrasound-assisted SPE of Cr(III) ions through the response. STATISTICA software (version 10.0) was used to design a five-level central composite design (Table 1) including four independent variables (initial concentration of Cr(III) ions (X₁), pH (X₂), amount of Cr(III)-IIP (X₃) and sonication time (X₄)) in a randomized fashion to minimize the effects of the uncontrolled factors. The central composite design matrix as well as the observed and predicted response values are given in Table 1.

Table 1 Matrix for the central composite design

	Factors	Levels
		−α	Low (−1)	Central (−1)	High (−1)	+α
(X1)	Cr(III) ion concentration (μg L^−1)	20	30	40	50	60
(X2)	pH	2	4.0	6	8	10
(X3)	Adsorbent dosage (mg)	5	7.5	10	12.5	15
(X4)	Sonication time (min)	2	4.0	6	8	10

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Runs	x1	x2	x3	x4	Observed ER%	Predicted ER%
1	50	4	7.50	8	76.00	75.64
2	40	2	10.0	6	96.00	95.81
3	30	8	7.50	8	96.00	95.64
4	40	6	10.0	6	97.20	97.74
5	40	6	10.0	6	98.40	97.74
6	30	8	12.5	8	86.00	86.73
7	30	4	7.50	4	87.50	87.14
8	40	6	5.00	6	73.00	73.90
9	40	6	10.0	10	97.10	96.91
10	50	8	12.5	4	93.00	93.73
11	40	6	15.0	6	87.60	86.33
12	40	6	10.0	6	96.70	97.74
13	40	6	10.0	6	98.30	97.74
14	60	6	10.0	6	68.00	67.81
15	40	6	10.0	2	72.00	71.81
16	20	6	10.0	6	97.00	96.81
17	40	6	10.0	6	98.40	97.74
18	50	8	7.50	4	70.00	69.64
19	40	6	10.0	6	98.20	97.74
20	30	4	12.5	4	80.00	80.73
21	50	4	12.5	8	91.00	91.73
22	40	6	10.0	6	97.00	97.74
23	40	10	10.0	6	91.80	91.61

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In general, the following full quadratic equation may be applied to model a response versus the independent variables.³⁵ It may account for the possible interactions between the variables.

where Y is the predicted response (extraction recovery percentage); β₀ is the intercept; β_i values are the coefficients of the linear terms; β_ii values are the coefficients of the quadratic terms and β_ij values are the coefficients of the interaction terms. The analysis of variance (ANOVA) was performed to investigate the level of significance of each term (Table 2). 3D response surfaces were plotted to visualize the variation of the response versus the terms involved.³⁶

Table 2 ANOVA and model statistics summary and quality of the quadratic model for the separation and preconcentration of Cr(III) ions

Source of variation	Sum of squares	Degree of freedom	Mean square	F-value	p-value	Coefficient estimate	Standard error
Model	2395.400	14	171.1000	158.9700	0.000000	6.613390	9.657190
x1	420.5000	1	420.5000	760.5943	0.000000	−0.893930	0.222398
x1^1	410.2580	1	410.2576	742.0680	0.000000	−0.038580	0.001416
x2	8.820000	1	8.820000	15.95350	0.007166	0.416520	1.156730
x2^2	27.98900	1	27.98930	50.62660	0.000388	−0.251900	0.035403
x3	154.3810	1	154.3806	279.2414	0.000003	4.996790	0.766041
x3^3	535.5830	1	535.5831	968.7550	0.000000	−0.705210	0.022658
x4	315.0050	1	315.0050	569.7765	0.000000	27.74152	1.156730
x4^4	308.4890	1	308.4893	557.9909	0.000000	−0.836270	0.035403
x1x2	62.80600	1	62.80560	113.6019	0.000040	0.198130	0.018589
x1x3	385.0310	1	385.0313	696.4390	0.000000	0.277500	0.010515
x1x4	22.32600	1	22.32560	40.38230	0.000712	−0.118120	0.018589
x2x3	3.781000	1	3.781200	6.839500	0.039840	0.137500	0.052576
x2x4	92.64100	1	92.64060	167.5670	0.000013	−1.203120	0.092943
x3x4	13.78100	1	13.78120	24.92730	0.002470	−0.262500	0.052576
Lack of fit	5.293000	2	2.646600	4.790000	0.057180
Pure error	3.317000	6	0.552900
Cor total	2404.012	22

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Statistics quality of model
Standard deviation	1.0400	R-squared	0.9964
Mean	88.970	Adjusted R-squared	0.9902
Coefficient variation%	1.1700	Predicted R-squared	0.8948
PRESS	252.84	Adequate precision	35.725

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3. Results and discussion

3.1. Characterization of samples

The mesoporous SBA-15, SBA-15-Cl, SBA-15-ditizon, and the unleached and leached Cr(III)-IIP based on an SBA-15 support were characterized using SEM, EDS and FTIR. The FTIR spectra of these materials (Fig. 1) exhibit a broad band at 3000–3700 cm⁻¹ attributed to the O–H bond stretching of surface silanol groups. The peaks located at 1089, 808 and 465 cm⁻¹ belong to the stretching and bending vibrations of the O–Si–O and Si–O of the silanol groups from SBA-15. After the functionalization of the SBA-15 with CPTES, a peak emerges at 2935 cm⁻¹ corresponding to the vibration of C–H in the propyl group. The stretching vibration of the C–Cl bond is observed at 709 cm⁻¹. The appearance of peaks at 1522 cm⁻¹ attributed to C=S stretching in DZ and the disappearance of the C–Cl stretching band after the reaction of SBA-15-Cl with N–H in DZ imply the successful functionalization of SBA-15-Cl with DZ and the production of SBA-15-DZ. After ion imprinting polymerization, the absorption bands around 1738 and 2970 cm⁻¹ correspond to the bending vibrations of C=O and C–H bonds, respectively, in MAA. The stretching vibration of C=S is shifted to 1641 cm⁻¹. The peaks appearing at 2930 cm⁻¹ prove the successful formation of an IIP. After leaching the Cr(III) ions, the peaks corresponding to the C=S group in DZ, the OH group in MAA and the N–H group in DZ are expected to be slightly shifted, which is not resolved using FTIR.³⁷ Therefore, EDS analysis was applied to verify this.

Fig. 1 FTIR spectra of all samples.

The N₂ adsorption–desorption isotherms of SBA-15 (Fig. 2) reveal that type IV isotherms according to IUPAC, adsorption–desorption isotherms with H1-type hysteresis loops, are obtained, which is in good agreement with the defined behavior of SBA-15. The BJH pore size distribution was shown to be narrow around 8 nm (inset of Fig. 2).³⁴

Fig. 2 N₂ adsorption–desorption isotherms and BJH pore size distribution of SBA-15.

EDS analysis was performed to identify the elemental composition of the unleached and leached Cr(III)-IIP (Fig. 3a and b). As shown in Fig. 3a, the elements of C, O, Si, N, Cr, Cl and S are observed in the unleached Cr(III)-IIP while no trace of Cr is found in the leached Cr(III)-IIP (Fig. 3b).

Fig. 3 EDS taken from unleached Cr(III)-IIP (a) and leached Cr(III)-IIP (b).

FE-SEM and SEM were used to study the morphology of the samples. The SEM image of the NIP (Fig. 4a) shows little cylindrical structures with a relatively sharp size distribution. According to this, the polymerization process of SBA-15 in the absence of Cr(III) ions as target ions is an incomplete polymerization process. From Fig. 4b and c, it is seen that the leached and unleached Cr(III)-IIP become rough and stick together. It is also seen that their ordered morphologies are strongly destroyed after the surface ion imprinting polymerization.²⁸

Fig. 4 SEM images of NIP (a), unleached Cr(III)-IIP (b), and leached Cr(III)-IIP (c).

3.2. Effect of sample volume

The sample volume is one of the important parameters influencing the enrichment factor, preconcentration factor and extraction recovery percentage (ER%) in the analysis of real samples. The ER% was studied by the addition of a given volume of Cr(III) ion solution (25–225 mL) to Cr(III)-IIP (see Fig. 5a for the results), while the total amount of Cr(III)-IIP used as adsorbent was kept constant (10.0 mg) under optimum experimental conditions. The results demonstrated that an increase in the sample volume up to 225 mL causes no significant change in the ER%, while at higher sample volumes, the ER% decreases significantly. The successful application of this adsorbent at higher volumes was observed. However, the sample volume was set to 100 mL to make the experimental process fast and easy to handle.

Fig. 5 Effect of initial sample volume (a), type of eluent (b), volume of eluent (c) and concentration of eluent (d) on the ER% of Cr(III) ions.

3.3. Effect of type, volume and concentration of eluent

The selection of the type, volume and concentration of eluent is very important for the separation and preconcentration processes. In this work, after the extraction of Cr(III) ions from 50 mL of aqueous solution, different eluents including HCl, HNO₃ and CH₃COOH were used for removing the Cr(III) ions from the Cr(III)-IIP (Fig. 5b). As seen, compared to other eluents, HCl is more effective due to its higher polarity disrupting the electrostatic interactions between the Cr(III) ions and the adsorbent in addition to weakening the coordination bindings. Therefore, HCl was used as an eluent thereafter. Subsequently, the Cr(III) ions were removed using different HCl volumes and concentrations, the optimum values of which were found to be 1.2 mL (Fig. 5c) and 2.0 mol L⁻¹ (Fig. 5d), respectively.

3.4. Central composite design

According to Table 1, the experimental and model predicted values are very consistent, with a high correlation coefficient, which indicates well the applicability of the model. ANOVA (Table 2) was performed to assess the model quality and obtain the most important variables as well as to investigate the extent to which the variables interact with each other. The model F-value 158.97 and very low p-value (<0.0001) show the significance of the model. Moreover, the p-values of all terms were less than 0.05 indicating their significance. The “Lack of Fit” F-value of 4.79 also confirms the suitability of the full quadratic model for predicting the real behavior of the preconcentration process with values of 0.9964, 0.8948 and 0.9902 for the determination coefficient R², predicted R² and adjusted R², respectively. Additional confirmation of the applicability of the model for predicting the performance of Cr(III) ion extraction is found in the low and acceptable standard deviation values, low PRESS value, low coefficient of variation, low standard error, high “Adeq precision” value and high mean value of the ER% (Table 2). Therefore, the following semi-empirical predictive expression applies for the extraction recovery (ER%) in terms of the significant parameters:

ER% = 6.61339 − 0.89393x₁ − 0.03858x₂ + 0.41652x₃ − 0.25190x₄ + 4.99679x₁² − 0.70521x₂² + 27.74152x₃² − 0.83627x₄² + 0.19813x₁x₂ + 0.27750x₁x₃ − 0.11812x₁x₄ + 0.13750x₂x₃ − 1.20312x₂x₄ − 0.26250x₃x₄ (2)

3.5. Optimization of CCD by desirability function for the preconcentration of Cr(III) ions

By using STATISTICAL 10.0 software, the desirability function was applied to optimize variables including the adsorbent mass, sonication time, initial Cr(III) ion concentration and pH. The minimum, middle and maximum values of desirability were configured as 0.0, 0.5 and 1.0, respectively. A value closer to 1.0 means that the corresponding operating condition is optimum. Optimum values of 40 μg L⁻¹, 6, 10 mg and 6 min were obtained for the initial concentration of Cr(III) ions, pH, adsorbent mass and sonication time, respectively with a desirability of 0.9783 (Fig. 6).

Fig. 6 Profile for predicted values and desirability function for the ER% of Cr(III) ions.

3.6. Response surfaces

Three dimensional response surfaces give good information about the interactions between variables. 3D surface plots (Fig. 7) indicate the simultaneous effects of two of the factors on the percentage of extraction recovery at zero level of the other variables.

Fig. 7 Response surfaces.

The effect of the initial Cr(III) ion concentration on the ER% is shown in Fig. 7c, which clearly indicates a high ER% at lower Cr(III) ion concentrations due to a low ratio of Cr(III) ions to the available surface area, while the ER% decreases at higher Cr(III) ion concentrations probably due to the saturation of the adsorbent.

The amount of IIP significantly affects the Cr(III) ion ER% (Fig. 7b, d and f). The ER% of Cr(III) ions is positively related to the Cr(III)-IIP mass, so that an increase in the amount of Cr(III)-IIP enhances the ER%. As seen, the ER% significantly decreases at a lower Cr(III)-IIP mass because of a high ratio of Cr(III) ions to vacant Cr(III)-IIP sites. The rapid increase in the ER% at higher Cr(III)-IIP values is attributed to a higher surface area and the availability of more adsorption sites.

Fig. 7a, d and e show the effect of the initial solution pH on the ER%. As seen, the ER% of Cr(III) ions decreases at lower pH due to the probable protonation of DZ and the appearance of positive charge on the adsorbent surface. At pH 6, the deprotonation of DZ causes an enhancement in complex formation and thus an improvement in mass transfer to the adsorbent surface. At a pH higher than 6, a significant decrease in the ER% is observed due to the probable competition of hydroxide ions with DZ for complexation with the Cr(III) ions, which leads to the precipitation of the Cr(III) ions.

The variation in the ER% of Cr(III) ions versus the sonication time is presented in Fig. 7c, e and f. It is observed that the maximum ER% of Cr(III) ions is achieved in a short sonication time because of the ultrasound-assisted mass transfer. The Cr(III) ion ER% efficiency may also be due to the enhancement in the diffusion coefficient, and the mixing and dispersion of adsorbent in the solution.

3.7. Analytical figures of merit

Cr(III) ion calibration curves were made at the optimal conditions for the IIP and NIP. They were constructed by plotting the intensity of the signal acquired using FAAS as a function of the Cr(III) ion concentration and the following linear calibration equations were obtained in the range of 2–950 μg L⁻¹ and 700–900 μg L⁻¹ for the IIP and NIP, respectively.

y = 4.4667[Cr(III)] + 0.0075 (3)

y = 0.4109[Cr(III)] + 0.0075 (4)

The correlation coefficient (R²), limit of detection (LOD), enrichment factor and preconcentration factor for the IIP were found to be 0.9998, 0.06 μg L⁻¹, 97 and 147.7, respectively, while they were found to be 0.9977, 670 μg L⁻¹, 22 and 30 for the NIP. The adsorption capacities of the Cr(III)-IIP and NIP were found to be 131.4 and 25 mg g⁻¹, respectively.

3.8. Repeatability and regeneration of Cr(III)-IIP

In this work, the highly important factors of the response repeatability and the regeneration of the adsorbent were studied. The repeatability was evaluated using five standard solutions of Cr(III) with a concentration of 40 μg L⁻¹ at pH 6 followed by the addition of 10 mg of Cr(III)-IIP to each solution and the implementation of the adsorption process under 6 min of ultrasonication. Finally, the relative standard deviation of the Cr(III)-IIP adsorbent response was obtained to be 2.44%. To evaluate the regeneration of the adsorbent, it was washed with 50 mL of 2.0 M HCl. Then, it was neutralized by washing with double distilled water. Subsequently, it was successfully reused in the adsorption–desorption process 10 times (Fig. 8) with very little change in the adsorption efficiency which indicates that it is highly recoverable and reusable.

Fig. 8 Effect of the number of recoveries of adsorbent on the ER% of Cr(III) ions.

3.9. Investigation of interference

One of the most important characteristics of an adsorbent is its relative response towards the primary analyte in the presence of other analytes in the sample. To determine the selectivity of Cr(III)-IIP, the adsorbent was tested using Cr(III) ions with a concentration of 40 μg L⁻¹ at pH 6 in the presence of other ions at different concentrations. The results shown in Table 3 indicate no significant interference in the extraction and determination of Cr(III) ions in the presence of the ions studied.

Table 3 Tolerance limits of interfering species in the determination of Cr(III) ions

Interference	Tolerance ratio
CO3^2−, SO4^2−, SCN^−	1100
Na^+, K^+, Ca^2+, Mg^2+	1000
Zn^2+, Cu^2+, Pb^2+, Co^2+, Ag^+, Ni^2+	750
Fe^2+, Fe^3+	500

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3.10. Application to spiked samples, real samples and recovery tests

Cr(III)-IIP was used as an adsorbent in the dispersive solid-phase extraction of Cr(III) ions in tap, river and spring water through the spiking method with triplicate measurements (Table 4). As seen, the amount of the Cr(III) ions added to the water samples could be successfully separated and concentrated by this adsorption process. Using these samples, the recovery tests were accurately performed (between 96.3 and 104.9%).

Table 4 Extraction recoveries and RSD in different water samples and spiked levels

Sample	Cr(III) added (μg L^−1)	Cr(III) found (μg L^−1)	RSD (n = 5)	Recovery (%)
Tap water	0.0	0.023	3.3	—
	0.1	0.129	3.8	104.9
	0.4	0.411	3.2	97.20
	0.7	0.732	2.7	101.2
Water from the Gugerd spring in Dehdasht, Iran	0.0	0.051	1.9	—
	0.1	0.156	3.3	103.3
	0.4	0.445	2.7	98.70
	0.7	0.742	1.9	98.80
Water from the Beshar river in Yasouj, Iran	0.0	0.034	2.5	—
	0.1	0.129	3.4	96.30
	0.4	0.441	2.1	101.6
	0.7	0.721	3.6	98.20

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3.11. Comparison of this process with other methods

A comparison between the performance of this process with other reported SPE-based methods coupled with various detection techniques is summarized in Table 5.^38,39 It was found that the proposed method is preferable and superior to others in terms of the LOD, linear range and adsorption capacity. The enrichment factor is satisfactorily comparable to others.^38–46

Table 5 Comparison of the proposed method with published methods for the preconcentration of Cr(III) ions

Sorbent	Enrichment factor	Adsorption capacity (mg g^−1)	LOD (μg L^−1)	Linear range (μg L^−1)	Detection	Ref.
a Electrothermal atomic absorption spectroscopy.b Inductively coupled plasma.c Atomic emission spectrometry.d Mass spectrometry.e Flow injection analysis.
2,4-Dinitrophenylhydrazine-nano-γ-Al2O3	266.7	100.0	0.55	2.4–520	FAAS	38
Graphene	125.0	24.80	0.50	10–1000	FAAS	39
Saccharomyces cerevisiae immobilized on sepiolite	2.280	—	94.0	—	FAAS	40
Cr(III)–pyrrolidinedithiocarbamate ion imprinted polymer	—	0.0013	0.018	—	ETAAS^a	41
4-Aminoantipyrine immobilized bentonite	100.0	38.80	0.12	—	ICP^b-AES^c	42
1-(Di-2-pyridyl) methylene thiocarbonohydrazide–gel Amberlite	2.400	—	0.03	—	ICP-MS^d	4
Iminodiacetate resin, muromac A-1	11.50	—	0.02	0.1–20	ICP-MS	43
Epichlorohydrin cross-linked chitosan–clay composite	13.40	0.2500	16.0	25–100	FIA^e-FAAS	44
Fe3O4@ZrO2 nanoparticles	25.00	24.50	0.69	4–400	FAAS	45
Iron phosphate	8.700	8.120	0.02	0.05–2.5	ETAAS	46
Mesoporous SBA-15@Cr(III) ion imprinted polymer	97.00	131.4	0.06	2–950	FAAS	This work

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4. Conclusion

A novel mesoporous surface Cr(III) ion-imprinted polymer was successfully prepared by the surface imprinting polymerization of a dithizone indicator on a mesoporous SBA-15 support. Characterization using FTIR, EDS and SEM techniques confirmed the formation of Cr(III)-IIP. The effects of individual variables and their interactions were successfully modeled using RSM. The linear range, detection limit and RSD of the process, with high selectivity, were found to be 2.0–950 μg L⁻¹, 0.06 μg L⁻¹ and 2.44, respectively. In other words, the proposed ultrasound-assisted method is promising and highly efficient for the separation and preconcentration of Cr(III) ions from aqueous media.

Acknowledgements

The authors thank the Research Council of the Yasouj University for financial support.

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