Polarity adjustment of a nanosilica-functionalized polyamine modified by ionic liquid for removal of Cu²⁺ from aqueous solutions

Abstract

The current work deals with modification of the surface of a silica–polyamine with amine-based ionic liquid (IL) and its application for removal of Cu²⁺ from an aqueous media. The absorbent is simply prepared from commercially available reagents. The material is characterized by FT-IR spectroscopy, thermogravimetric analysis (TGA), N₂ adsorption–desorption, and transmission electronic microscopy (TEM). Effects of several parameters on absorption efficiency, as well as activity of the absorbent to other metals, are also investigated. Moreover, the reusability of the absorbent is studied at the optimized condition.

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Introduction

Owing to shortage of clean water sources, purification of contaminated water and wastewater is a hot topic of environmental science.¹ One type of hazardous impurity is the presence of heavy metal ions in water. Human activities such as plating, mining, and battery manufacturing are the major sources of releasing toxic heavy metal ions in the environment.² Heavy metals can easily enter in the food chain and their accumulation in living organisms leads to various illnesses which can cause serious risk to health of humans, plants, and marine animals.³ However, by separating or reducing amounts of heavy metals to an acceptable level, the remaining purified wastewater can be “reused” for non-potable applications such as agriculture and industrial uses. Or the purified water can be released to the environment without negative impact concerns for soil, streams, and underground water.

Copper is a valuable metal widely used in the manufacture of brass and in petroleum refining. Wastewater and mud from these industries contain high amounts of Cu²⁺ ion. Copper has an essentially negative impact to animal metabolism, and excess amounts of Cu²⁺ brings about serious toxicological effects such as vomiting, cramps, convulsions, or even death. Moreover, an excess adsorption of copper causes Wilson's⁴ or Alzheimer's diseases.⁵

Several methods and techniques have been introduced for the removal of heavy metal ions; for example, pyridine–pyrazole ligands attached on polymethylhydrosiloxane,⁶ or polyvinyl alcohol/polysaccharides hydrogel graft materials.⁷ Other methods include using magnetic Co–Fe bimetallic nanoparticle containing modifiable microgels.⁸ Recently, Chen et al. reported the influence of structural characteristics of various carbon nanotubes on the adsorption–desorption hysteresis of metal cations such as Cu²⁺.⁹ In an interesting example, Zhang and co-workers introduced multi-group modified mesoporous organosilica as a highly efficient material for absorption of both organic (hydrophobic specie) and inorganic metal ions (aqueous solutions).¹⁰

From materials which were developed for metal ions absorption, functionalized silica gel with organic groups is a well-known method with widespread applications for separation of various types of heavy metals.¹¹ The major benefits of silica materials are attributed to high surface area, simple functionalization, high thermal stability, and low preparation cost. However, incorporation of organic groups on the surface of silica increases hydrophobicity of the surface. Since the experiments were performed in aqueous media, the efficiency of absorption decreases through prevention of diffusion of the aqueous solution containing metal ions on the hydrophobic surface of the absorbent. In other words, the inherent absorption might be inhibited from hydrophobic prevention by the surface and not from the capacity of the absorbent. On the other hand, the loading of organic groups on the surface in most cases is less than 2 mmol g⁻¹ of the support. Thus, the total absorption capacity of silica-functionalized materials is not sufficient for higher amounts of metal ion solutions.

Ionic liquids (ILs) have been used as versatile chemicals in various fields of chemistry from catalysts and gas separation to biological applications.¹² Among the most important advantages of ILs are their tunable structures and polarity. Their structures can be manipulated to gain ILs with a wide range of functional groups. Moreover, their polarity can be tuned by incorporating hydrophobic hydrocarbon chains or hydrophilic functionalities.

In this study, we report the application of silica-functionalized polyamine species which was confined by a task-specific IL 1-(2-aminoethyl)-3-methylimidazolim tetrafluoroborate (IL 2) as a new material for Cu²⁺ removal from an aqueous solution. The applied IL has two roles in this protocol. (1) Increase the polarity of the surface of the absorbent material for suitable aqueous experiments, and (2) increase the absorption capacity via the side chain amine functionality.

Experimental

Materials and equipment

All chemicals were supplied from Acros, Sigma-Aldrich and Merck and used without further purification. ¹H NMR and ¹³C NMR spectra were recorded with a Bruker DRX 500 MHz. FT-IR spectra were recorded with an ABB Bomem MB100 Fourier transform infrared analyzer. Thermogravimetric analysis (TGA) was acquired under a nitrogen atmosphere with a TGA Q 50 thermogravimetric analyzer. N₂ adsorption–desorption was performed by a Belsorp Mini II (Bel Japan). Measurements of the absorptions were determined utilizing atomic absorption spectroscopy (AAS) Varian SpectrAA 220. Britton–Robinson buffer was used as pH buffer which is suitable in the range pH 2 to pH 12.¹³ The procedure comprises the addition of the following compounds: 0.04 M H₃BO₃, 0.04 M H₃PO₄, and 0.04 M CH₃CO₂H titrated to the desired pH with 0.2 M NaOH.

Preparation of the silica-grafted polyamine

Silica gel (Davisil grade 635, average pore diameter 60 Å, pore volume 0.75 cm³ g⁻¹, surface area 480 m² g⁻¹) was activated by refluxing in 6 M hydrochloric acid (HCl). After 24 h, the mixture was filtered and washed thoroughly with deionized water to adjust the pH of the solution to 6–7 and dried in vacuum. Then, 1 g of silica gel was treated with 3 mmol of 2-[2-(3-trimethoxysilylpropylamino)ethylamino]ethylamine in dry toluene. The mixture was heated to reflux for 24 h after which the solid materials were filtered and washed in a continuous extraction apparatus (Soxhlet) to afford surface-bonded polyamine 1 (Scheme 1).

Scheme 1 Schematic preparation of absorbent 3.

Preparation of the 1-(2-aminoethyl)-3-methylimidazolim tetrafluoroborate (IL 2)

The IL was prepared according to literature procedures¹⁴ with some modifications. To a round bottom flask equipped with a condenser, 1-methylimidazol and 2-bromoethylamine hydrobromide were added and the mixture was refluxed in ethanol for two days. Then, the solvent was evaporated by applying vacuum and the residue was dissolved in minimum amounts of water. The pH of the solution was increased to nearly 8 by adding powdered KOH to release free amine functionality of the IL. Residual KBr was precipitated by dissolving the IL in a minimum volume of ethanol. Anion exchange of the resulting IL was performed with NaBF₄ in ethanol at room temperature. IL 2: ¹H NMR 500 MHz D₂O: 3.19 (2H), 3.38 (2H), 3.73 (3H), 4.42 (2H), 7.31 (1H), 7.40 (1H), 8.69 (1H); ¹⁹F NMR 500 MHz D₂O: −148.9. FT-IR (liquid film): 3486, 3145, 3081, 2859, 2066, 1635, 1564, 1464, 1378, 1166 and 1021.

Preparation of the silica-grafted polyamine 3

An ethanol solution comprising 0.2 g of IL 2 was added to a mixture of 1 g of material 1 and 30 mL ethanol over 1 h. The reaction mixture was stirred for an additional 2 h and then the solvent was removed under reduced pressure to afford material 3 as a yellow powder (Scheme 1).

Absorption experiment

A certain amount of absorbent 3 was added to a solution of Cu(NO₃)₂ at various reaction conditions and stirred until the reaction was complete by utilizing a 1 cm magnet stirring at 800 rpm. Then, the mixtures were filtered using filter paper (Whatman 0.45 μm) and the collected solutions were analyzed using atomic absorption spectroscopy.

Results and discussion

The schematic representation of 3 is shown in Scheme 1. With the first step, the polyamine precursor is grafted on the surface of the silica gel. Then, IL 2 was supported on the surface of the material 1 to form the final absorbent.

FT-IR spectra of the material 1 and absorbent 3 are shown in Fig. 1. The bands at 2852 and 2926 cm⁻¹ are attributed to C–H stretching frequencies of the surface polyamine functionalities.¹⁵ Additional bands, which were observed at 2961, 3080, and 3148 in cm⁻¹ in 3, belong to N–H and aromatic C–H stretching frequencies of the supported IL 2. The bands observed near 1464 cm⁻¹ in both 1 and 3 are attributed to N–H bending frequencies. Moreover, the band at 1567 cm⁻¹ belongs to C=N aromatic frequency of the imidazolium ring of the supported IL 2.

Fig. 1 FT-IR spectra of 1 and 3.

Thermogravimetric analyses (TGA) of 1 and 3 are shown in Fig. 2. The thermograms included a small weight loss (≈3%) at 100 °C for 1. This is attributed to the releasing of water from 1 (details of these analyses were added to the ESI†).

Fig. 2 TGA of 1 and 3.

However, after physisorption of IL 2 on the surface of the material, the related weight loss of water in 3 was increased to nearly 6% around 100 °C. This means that 3 has a higher content of water and consequently, the surface hydrophilicity of 3 is increased and the final absorbent has better affinity in aqueous media.

N₂ adsorption–desorption analysis was performed at 77 K for the absorbent 3 (Fig. 3). The result of BET (Brunauer–Emmett–Teller) showed that the surface area of 3 was decreased from the initial 480 m² g⁻¹ to 141 m² g⁻¹. Moreover, the pore volume of 3 was reduced from the initial 0.75 cm³ g⁻¹ to 0.31 cm³ g⁻¹.

Fig. 3 N₂ adsorption–desorption of 3.

BJH (Barrett–Joyner–Halenda) average pore diameter analysis of 3 was calculated to be 7.06 nm. Decreasing the surface area and total pore volumes in the N₂ adsorption–desorption analysis of 3 provided reasonable evidence for successful confinement of IL 2 on the surface of the silica–polyamine.

The efficiency of 3 for separating Cu²⁺ from an aqueous solution was investigated considering several reaction conditions. The adsorption percent of the Cu²⁺ ion was calculated using following equation:¹⁶

In this equation, C₀ and C_e are initial and equilibrium concentrations (mg L⁻¹), respectively.

In the first step, the effect of IL 2 loading on the absorbent efficiency was investigated. To study this parameter, four absorbents with different loadings of IL 2 were prepared (Fig. 4). These loadings are attributed to the mass of the IL 2 used during the absorbent preparation. The results indicated that the best result was obtained when 3 was prepared with confinement of 0.2 g of IL 2 on the surface of 1 g of silica–polyamine. Increasing loading of the IL improved the absorption efficiency of Cu²⁺ removal. However, at the higher loading values, the accumulation of IL 2 on the surface reduced absorption efficiency. Our investigation on the absorption efficiency of 1 showed that its removal efficiency was 36%. Moreover, the absorption capacity of a material which was prepared from the confinement of IL 2 on the surface of the “un-functionalized” silica gel was also tested. It was found that absorption capacity was obtained up to 40%. However, the combined efficiency was lower than that in 3. A plausible explanation may be attributed to balancing the surface polarity in 3 and lower leaching of IL 2 from the surface of absorbent 3 compared to supported IL 2 on the surface of un-functionalized silica gel.

Fig. 4 Effect of the loading of IL 2 on removal efficiency of Cu²⁺. Experimental conditions: 3 (30 mg), pH = 5, 10 mL solution of 100 ppm Cu(NO₃)₂ at room temperature and 24 h.

Having these interesting data in hand, we set up experiments to study the effect of reaction pH on the absorption efficiency (Fig. 5). At pH = 2, both the surface-bonded polyamine moiety and the amine specie of IL 2 are protonated and form ammonium species. Thus, no retention of the Cu²⁺ ions was observed in this situation. At this pH it was found that pure activated silica gel (silica gel without polyamine functionalities or IL 2) did not have an absorption property; this may be attributed to protonation of the surface silanol (OH) groups. When the pH of the solution was raised to 2.5, a significant jump in absorption was observed of about 70%. The removal percentages were 82%, 87%, and 96% for pH 3, 4, and 5, respectively. For pH > 5, formation of insoluble copper^(II) hydroxide may interfere the experiments. Hence, pH = 5 was selected for the experiments. However, in this protocol the absorbent 3 had desirable activity even at pH = 3. The activity of 3 in acidic media is very interesting since most metal wastes have acidic pH.

Fig. 5 Effect of the solution pH on removal efficiency of Cu²⁺. Experimental conditions: 3 (30 mg), 10 mL solution of 100 ppm Cu(NO₃)₂ at room temperature and 24 h.

Contact time was also assessed for Cu²⁺ removal. As shown in Fig. 6, after 24 h absorption reached maximum value. These results demonstrate that the material could smoothly absorb the Cu²⁺ ions. It is worthy to note that rapid absorption in most cases is not suitable since the desorption rate is reduced due to strong interaction of the metal to the surface.

Fig. 6 Effect of reaction time on removal efficiency of Cu²⁺. Experimental conditions: 3 (30 mg), pH = 5, 10 mL solution of 100 ppm Cu(NO₃)₂ at room temperature.

Next, the effect of initial concentration of Cu²⁺ on the absorption efficiency was also monitored. These results are shown in Fig. 7. The removal percentage was increased to the highest value in the 100 ppm solution. Then, it was gradually reduced to 74% for the solution with an initial concentration of 350 ppm.

Fig. 7 Effect of initial concentration on removal efficiency of Cu²⁺. Experimental conditions: 3 (30 mg), pH = 5, 10 mL solution of Cu(NO₃)₂ at room temperature and 24 h.

To explore effects of the absorbent on copper ion removal, several experiments were set up and the results are depicted in Fig. 8. By increasing the amounts of 3, the removal percentages were increased to 96% when 30 mg of 3 was used. Surprisingly, the removal efficiency showed a small decrease when the absorbent amount was increased. A plausible explanation for this phenomenon may be attributed to electrostatic interference of the functionalized nano particles of the silica gel. Moreover, increasing amounts of 3 led to aggregation of the absorbent and thus decreased the contact surface area. Similar observations were shown in a previous research reference for the removal of the heavy metals.¹⁷

Fig. 8 Effect of absorbent amounts on removal efficiency of Cu²⁺. Experimental conditions: 3 (mg), pH = 5, 10 mL solution of 100 ppm Cu(NO₃)₂ at room temperature and 24 h.

The effect of reaction temperature is a very important issue in metal ion separations. Synthesis of an absorbent with retention of activity in a high thermal range is of great interest. The results of Cu²⁺ removal at various temperatures using 3 as an absorbent are shown in Fig. 9. As expected, by increasing the experimental temperature, the ability of 3 to remove Cu²⁺ ions from an aqueous solution was decreased. However, it still had good efficiency at 70 °C with 68% removal.

Fig. 9 Effect of the experiment temperature on the removal efficiency of Cu²⁺. Experimental conditions: 3 (30 mg), pH = 5, 10 mL solution of 100 ppm Cu(NO₃)₂ and 24 h.

To find whether absorbent 3 is usable for other metal separations, at the next stage some experiments were conducted with solutions containing the following metals: 100 ppm solutions of sulfate salts were prepared by adding CuSO₄·5H₂O, FeSO₄·7H₂O, CdSO₄·H₂O, and MnSO₄·4H₂O to double distilled water. Results of the removal efficiency of 3 gave excellent efficiency for Cu²⁺ (96.5%), Fe²⁺ (96.7%) and Mn²⁺ (96.8%). However, in the case of Cd²⁺, the removal efficiency was a lesser 87.65%. A plausible explanation for lower absorption efficiency of Cd²⁺ may be interpreted via differences in the atomic radii; values for these metal ions are: Cu²⁺ (73 pm), Fe²⁺ (70 pm), Mn²⁺ (70 pm), and Cd²⁺ (95 pm). Regardless of Cd²⁺, the others have smaller and relatively identical ionic radii, thus they have better interactions with the surface functionalities. These interactions mainly occur through the complexation of both surface amine and the amine group of IL 2. The second way for absorption is from electrostatic attractions of ions with the anion of IL 2.

Reusability of the absorbent for separation is an important part of designing new materials for metal separations.¹⁶ To investigate the possibility of absorbent reusability, after an absorption experiment the mixture was filtrated and subjected to a 0.3 molar solution of HCl with 10 min of sonication. Then, it was filtrated and treated with a pH = 7 buffer. Finally, it was filtered, dried, and then used for the next reaction. Results of the recycling experiment using 3 after the first run showed that it could be reused for an additional four experiments with high efficiency. The results of the first to the fifth run were 94%, 92%, 89%, 87%, and 86%, respectively. After each experiment, the absorbent was isolated and dried. The weight of the absorbent after recycling experiments was determined to be 30, 30, 30, 27, and 25 mg. Analysis of 3 after the fifth run did not show significant changes in the absorbent structure. TEM images of 3 after initial preparation and after the fifth run of the reaction are shown in Fig. 10.

Fig. 10 TEM image after initial preparation (a) and after fifth run of the reusability experiment (b).

The mean pore diameter distributions of the recovered materials were 7.2 nm. Compared to the initial un-functionalized silica gel (6 nm) and absorbent 3 (7.06 nm), a small increase in pore diameter of the recovered absorbent was found.

Conclusions

In summary, the surface of the silica–polyamine was modified by amine-based IL 2. The resulting absorbent was used for efficient Cu²⁺ separation from aqueous media. Effects of several parameters on the absorption efficiency were investigated. It was found that absorbent 3 is reusable for at least five absorption experiments without significant loss of activity. Moreover, absorbent 3 gave excellent activity for the removal of other cations such as Fe²⁺, Mn²⁺, and Cd²⁺. Increasing removal capacity and activity are attributed to the adjustment of polarity in absorbent 3 after the confinement of IL 2.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra20179a

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