Electronic Supplementary Material

Aluminum(III)-doped ZnO@Fe3O4 nanocomposite as a magnetic sorbent for preconcentration of cadmium(II)

Hossein Abdolmohammad-Zadeh*, ElahehRahimpour, Ali Hosseinzadeh, MonirehZamani-Kalajahi

Department of Chemistry, Faculty of Sciences, AzarbaijanShahidMadani University, 35 Km Tabriz-Marageh Road, P.O. Box 53714-161, Tabriz, Iran

* Corresponding author. Tel.: +98 4134327500, Fax: +98 4134327541.

E-mail addresses: ,

Characterization of the nano-sorbent

The crystal structures of the obtained products were characterized by X-ray diffraction (XRD) analysis. Figure 1S shows XRD patterns of Fe3O4, Al-doped ZnO and Al-doped ZnO@Fe3O4, respectively. Figure 1S(I) and 1S(II) indicate the XRD patterns and the characteristic reflections of the Fe3O4 and Al-doped ZnO nanoparticles, respectively, which are in accordance to those of the reported XRD patterns [17]. The XRD pattern of Fig. 1S(III) shows Al-doped ZnO and Fe3O4 diffraction peaks, which indicates that the existence of both of them in the nanocomposite sample.

Fig. 1S:XRD patterns of (I) Fe3O4, (II) Al-doped ZnO and (III) Al-doped ZnO@Fe3O4 nanoparticles.

To characterize the surface nature of the nanosorbent, the infrared absorption spectroscopy was used. Figure 2S shows the FT-IR spectra of Fe3O4, Al-doped ZnO and Al-doped ZnO@Fe3O4 in the region from 400 to 4000 cm−1. It can be seen from Fig. 2S(I), the characteristic absorption of Fe–O bond (stretching vibration) and Fe–O–H bond stretching vibration are around 596 cm−1 and 3320 cm−1, respectively. The peaks around 1618 cm−1 and 1400 cm−1 are assigned to the symmetric and asymmetric stretching of C=O in the COO–Fe bond, which indicates that the citrate has been successfully grafted onto the surface of Fe3O4 nanoparticles. The obvious peak found at 556 cm−1 in Fig. 2S(II)is assigned to stretching vibration of Zn–O bond. Comparing the FT-IR spectra of the Fe3O4, Al-doped ZnO and Al-doped ZnO@Fe3O4 nanoparticles (Fig. 2S(III)), indicates that the Al-doped ZnO nanoparticles were successfully bound to the citrated Fe3O4 nanoparticles.

Fig. 2S:The FT-IR spectra of (I) Fe3O4, (II) Al-doped ZnO and (III) Al-doped ZnO@Fe3O4 nanoparticles.

Field emission scanning electron microscopy (FESEM) was employed to explore the morphology of the synthesized material. FESEM image of Al-doped ZnO@Fe3O4 nanocomposite, shown in Fig. 3S, demonstrates an aggregate that consists of Al-doped ZnO crystallites were collected as small pseudo-spherical particles with approximate sizes in the range of 20–50 nm are stacking with each other, and whose surface was distinctly enwrapped with Fe3O4 nanoparticles. Moreover, TEM analysis of the synthesized Al-doped ZnO@Fe3O4 NC showed fine dispersion of black particles (Al-doped ZnO) on gray Fe3O4surface (Fig. 4S). The average size of the synthesized Al-doped ZnO@Fe3O4 NC was found to be less than 50 nm. TEM studies were in good agreement with XRD and FESEM analysis.

Fig. 3S:FESEM image of Al-doped ZnO@Fe3O4 nanoparticles.

Fig. 4S:TEM image of Al-doped ZnO@Fe3O4 nanoparticles.

Optimization of method

Effect of pH

The influence of the pH value on the recovery of Cd(II) ion was studied by adjusting the pH values of sample solution in the range of 4–10 with minimal volume of 0.01 mol L−1 HNO3 and/or NaOH. In this study, solutions of pH < 4 were not tested because of the probability of the Fe3O4 nanoparticles dissolving in strongly acidic media. As shown in Fig.5S, the recovery of the analyte increases with increasing the pH from 4 to 6 and decreases at pH values higher than 8. An increase in the concentration of the OH− anions and precipitation of Cd(II) ions as hydroxide form at pH>8 might be ascribed to the observed decrease in the recovery. Therefore, pH 7 was selected as the optimum pH for further experiments. The pH was adjusted by using 0.1 mol L−1 phosphate buffer solution.

Fig. 5S: Effect of pH on adsorption of Cd(II) ions by Al-doped ZnO@Fe3O4nano-sorbent. Utilized conditions: sample volume: 25.0 mL, Cd(II) ions concentration: 40 ng mL−1, amount of the nanosorbent: 200 mg, extraction time :15 min, elution condition: 2.0 mL of 1 mol L−1 acetic acid, desorption time: 15 min.

Effect of the Al-doped ZnO@Fe3O4 nanocomposite amount

The influence of the sorbent amount on extraction efficiency was tested in the range of 50–300 mg. The results indicated that a quantitative recovery of analyte was obtained by using at least 150 mg of the nanosorbent (Fig. 6S). Therefore, 200 mg of the nano-sorbent were used for further experiments.

Optimization of elution conditions

In this work, desorption of the extracted analytes from the nano-sorbent was examined using various reagents such as EDTA,citrate, acetic acid, nitric acid, hydrochloric acid and sulfuric acid. As shown in Fig. 7Sa, the best recovery was achieved when acetic acid was used as an eluent. The concentration of acetic acid was also optimized. For this purpose, various concentrations of acetic acid (0.1–4.0 mol L−1) were tested for the elution. Based on the obtained results, 2.0 mol L−1 acetic

Fig. 6S: Effect of the nano-sorbent amount on adsorption of Cd(II) ions by Al-doped ZnO@Fe3O4nano-sorbent. Utilized conditions: sample volume: 25.0 mL, pH: 7, Cd(II) ions concentration: 40 ng mL−1, extraction time :15 min, desorption time: 15 min, elution condition: 2.0 mL of 1 mol L−1 acetic acid.

Fig. 7S:Effect of (a) eluent type and (b) eluent concentration on desorption of the analyte from Al-doped ZnO@Fe3O4nano-sorbent. Utilized conditions: sample volume: 25.0 mL, pH: 7, Cd(II) ions concentration: 40 ng mL−1, amount of the nanosorbent: 200 mg, extraction time :15 min, desorption time: 15 min.

acid was sufficient for complete elution of the adsorbed analytes (Fig. 7Sb). By keeping the eluent concentration of 2.0 mol L−1 acetic acid, the effect of elution volume on the recovery was investigated within the range of 1.0–4.0 mL. Based on the obtained results, the recovery of Cd(II) ions increased by increasing the volume of eluent up to 2.0 mL. Therefore, optimum volume of the eluent was chosen as 2.0 mL.

Effect of adsorption/desorption time

Due to the superparamagnetic property of the Al-doped ZnO@Fe3O4, the sorbent can be separated rapidly from the sample solution using an external magnetic field instead of filtration or centrifugation. Therefore, the effect of adsorption/desorption time on the recovery of analyte was investigated as analysis time. Both the adsorption and desorption time was varied in the range of 5–25 min. According to the obtained results (results are not shown), 15 min was sufficient for each step.

Sample volume and preconcentration factor

The possibility of enriching low concentrations of Cd(II) ions from large volumes of samples was examined by studying the effect of sample volume on the recovery of the analyte. For this aim, 25.0, 50.0, 100.0, 150.0 and 200.0 mL of sample solutions containing 1.6 μg of Cd(II) ions were studied. Recovery of Cd(II) ions was found to be quantitative when sample volume was chosen between 25.0 and 100.0 mL. Above 100.0 mL, the recovery decreased for the analyte (Fig. 8S). So, by analyzing 2.0 mL of the final solution after the preconcentration of 100.0 mL of sample solution, an enrichment factor was found as 50.

Fig. 8S: Effect of sample volume on adsorption of Cd(II) ions by Al-doped ZnO@Fe3O4nano-sorbent. Utilized conditions: pH: 7, Cd(II) ions concentration: 40 ng mL−1, amount of the nanosorbent: 200.0 mg, extraction time: 15 min, desorption time: 15 min, elution conditions: 2.0 mL of 2 mol L−1 acetic acid.