Ionic liquid-immobilized silica gel as a new sorbent for solid-phase extraction of heavy metal ions in water samples

Abstract

In the current study, we have developed a solid-phase extraction (SPE) method with novel C18-alkylimidazolium ionic liquid immobilized silica (SiO₂-(CH₂)₃-Im-C₁₈) for the preconcentration of trace heavy metals from aqueous samples as a prior step to their determination by inductively coupled plasma mass spectrometry (ICPMS). The material was characterized by Fourier-transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA), Energy-Dispersive X-ray Spectroscopy (EDS), and Brunauer-Emmett-Teller (BET) analysis. A mini-column packed with SiO₂-(CH₂)₃-Im-C₁₈ sorbent was used for the extraction of the metal ions complexed with 1-(2-pyridylazo)-2-naphthol (PAN) from the water sample. The effects of pH, PAN concentration, length of the alkyl chain of the ionic liquid, eluent concentration, eluent volume, and breakthrough volume have been investigated. The SiO₂-(CH₂)₃-Im-C₁₈ allows the isolation and preconcentration of the heavy metal ions with enrichment factors of 150, 60, 80, 80, and 150 for Cr³⁺, Ni²⁺, Cu²⁺, Cd²⁺, and Pb²⁺, respectively. The limits of detection (LODs) for Cr³⁺, Ni²⁺, Cu²⁺, Cd²⁺, and Pb²⁺ were 0.724, 11.329, 4.571, 0.112, and 0.819 μg L^-1, respectively with the relative standard deviation (RSD) in the range of 0.941-1.351%.

Scheme 1. Synthetic procedure of SiO₂–(CH₂)₃–Im–C₁₈.

Fig. 1. FT-IR spectra of (a) SiO₂ (1), (b) SiO₂–(CH₂)₃–Cl (2), (c) SiO₂–(CH₂)₃–Im (4), (d) SiO₂–(CH₂)₃–Im–C₁₈ (5).

Fig. 2. Thermal gravimetric analysis diagram of SiO₂–(CH₂)₃–Im–C₁₈.

Fig. 3. Energy-dispersive X-ray spectroscopy of SiO₂–(CH₂)₃–Im–C₁₈.

Fig. 4. SEM image of SiO₂–(CH₂)₃–Im–C₁₈.

Fig. 5. BET of SiO₂–(CH₂)₃–Im–C₁₈.

Fig. 6. Effect of pH on the sorption percentage. Utilized conditions: Cr³⁺, Ni²⁺, Cu²⁺, Cd²⁺, and Pb²⁺ 50 μg L; volume: 50 mL; PAN: 2 × 10⁻⁵ mol.L⁻¹; flow rate: 0.8 mL.min⁻¹.

Fig. 7. Effect of PAN concentration on SiO₂–(CH₂)₃–Im–C₁₈. Utilized conditions: Cr³⁺, Ni²⁺, Cu²⁺, Cd²⁺, and Pb²⁺ 50 μg L; volume: 50 mL; flow rate: 0.8 mL.min⁻¹.

Fig. 8. Effect of the length of alkyl chain.

Fig. 9. Effect of nitric acid concentration on the elution recovery.

Fig. 10. Effect of sample volume on sorption percent.

Fig. 11. Determination of Cr³⁺, Ni²⁺, Cu²⁺, Cd²⁺, and Pb²⁺ in water samples. Average of three determinations ± standard deviation (SD), n = 3.