New Smart Magnetic Ionic Liquid Nanocomposites Based on Chemically Bonded Imidazole Silica for Water Treatment.

New magnetic silica imidazolium ionic liquid nanocomposites were synthesized by a sol-gel technique. The (3-aminopropyl)triethoxysilane (APTS) was condensed with glyoxal and p-hydroxybenzaldehyde in acetic acid to produce an amino-modified silica ionic liquid (Si-IIL). The APTS was condensed with TEOS in ethanol and water to prepare amino-modified SiO2 nanoparticles. The produced amino-modified SiO2 silica was condensed with glyoxal and p-hydroxybenzaldehyde in acetic acid to produce chemically bonded silica SiO2-IIL. The SiO2-IIL and Si-IIL were used as capping agents during and after the formation of magnetite nanoparticles in ammonia to produce magnetic SiO2-IIL-Fe3O4 and Fe3O4-Si-IIL adsorbents, respectively. Their chemical structure, morphology, crystalline lattice structure, surface charges, particle sizes, and magnetic characteristics elucidated the formation of core-shell and highly dispersed magnetic nanocomposites. The saturation magnetization values of Fe3O4-Si-IIL and SiO2-IIL-Fe3O4 were 35.3 and 30.8, respectively. The uniform dispersed disconnected spherical morphologies appeared for Fe3O4-Si-IIL hybrid and the core-shell spherical morphology obtained with SiO2-IIL-Fe3O4 hybrid NPs. The Fe3O4-Si-IIL and SiO2-IIL-Fe3O4 show an excellent high chemical adsorption capacities as 460.3 and 300.9 mg·g-1, respectively (not reported in the literature) when used as an adsorbent to remove CB-R250 water pollutant under optimum conditions. Their applicability and reusability as fast and highly effective adsorbents for Coomassie blue (CB-R250) organic water pollutants were investigated.

Introduction

Organic reactive dyes were widely used in large quantities by several industries such as leather tanning, textile, paper, and food technologies.[1−3] These reactive dyes contain reactive functional groups (such as azo, oxazine, thiazine, quinone, and amino) transformed to carcinogenic aromatic amines that are very dangerous for the environment.[4] There are many different conventional physicochemicals and biological methods that were used to remove organic dye water pollutants, such as precipitation, carbon adsorbents, evaporation, membrane processes, electrochemicals, coagulation, and flocculation.[5] Some conventional methods were energetically, environmentally, and economically infeasible, and they can also lead to produce secondary water pollutants from the large amounts of toxic chemicals such as hypochlorite in the precipitation and coagulants in coagulation and flocculation techniques.[6] Recently, new technologies based on ultrasonication,[7] solar energy,[8] antimicrobial polymeric membranes,[9] and nanomaterials[10] were proposed as more efficient and time-saving methods for water purification techniques. The nanomaterials based on carbon nanotubes,[11] graphene oxide,[12] metal and metal oxides,[13,14] nanocomposites,[15] and intelligent nanoadsorbents[16] have been widely used for the water treatment. They were used as adsorbents, membranes, photocatalysis, and antimicrobial materials for desalination, decontamination, and purification of water from water inorganic and/or organic pollutants. The nanomaterials achieved the excellent photocatalytic activity, adsorption efficiency, and substance specificity, but some nanomaterials are not yet ready for the market due to their technical challenges such as scale up and system set up, cost-effectiveness, and environmental concerns.[17,18] Moreover, the release of nanomaterials to the surrounding environments and their accumulation for a long period of time are challenges that prevent their marketing to apply in water purification. In this respect, the present work aims to design nontoxic nanomaterials based on silica and magnetite, which they chemically bonded with ionic liquids (ILs) to apply as adsorbents for water purification from the reactive organic dye. The chemical bonds occurred among silica, magnetite, and ILs solve the problem relative to scale up of nanomaterials because they can be used directly without separation of nanomaterials rather than the organic capping agents.

The ionic liquids (IL) have received great scrutiny for application in water purification, desalination, and heavy metal separation due to their unique thermal and electrical properties and low toxicity.[19−21] The especially functionalized mesoporous silica-based nanomaterials have been used as adsorbents for the toxic inorganic metal, heavy metal, and metal oxide removal in water purification and desalination.[22−25] Silica adsorbents were not easily used to recover the organic dye water pollutants. The hydrophilicity of silica surfaces limits the adsorption process of reactive dyes due to hydrogen bonding formed with water; besides, they cannot easily separate from the aqueous medium.[26] Based on these findings, the objective of the present work is based on modification of the silica by chemical bonding with an imidazole-based IL (IIL) and magnetite to improve their performance as an adsorbent for organic dyes. The magnetite was used as nanomaterials to blend with the modified silica IIL to form nanocomposites.[27] The environmentally friendly chemically silica bonded IIL-functionalized composites is synthesized by a sol–gel technique.[28] The present work aims to use amino-modified silica, produced from the reaction of (3-aminopropyl)triethoxysilane (APTS) with tetraethyl orthosilicate (TEOS), as a precursor to condense with p-hydroxybenzaldehyde (HBA) and formaldehyde to produce silica imidazole nanocomposites (Si-IIL). The magnetic characteristics of Si-IIL was modified by reacting with magnetite nanoparticles either during the formation of Fe3O4 or Si-IIL. These materials were not reported in the literature to apply as adsorbents for water pollutants. The synthesized magnetic Si-IIL could solve the problem of nanocomposite separation from the aqueous medium. The IIL was not only used to increase the dispersion of Si-IIL in water but could also adsorb the Coomassie blue (CB-R250) as organic pollutants from water by an electrostatic attraction force. The selection of the optimum conditions such as the time of adsorption, initial CB-R250 concentration, and pH of the solution to apply magnetic Si-IIL as adsorbents to remove CB-R250 is another goal of the present work. The objective of the work extended to investigate the catalytic activity of magnetic Si-IIL to degrade the basic organic dye.

Results and Discussion

The present work aims to synthesize the magnetic chemically bonded silica imidazolium IL as represented in Schemes and 2. In this respect, APTS is selected as a silane precursor and basic catalyst to hydrolyze the TEOS with the formation of the silica functionalized with −OH and −NH2 groups (Scheme ). The amino group of silica nanoparticles can be condensed with p-hydroxybenzaldehyde (PHB) and glyoxal (GA) under acetic acid conditions to produce Si-IIL (Scheme ). It was previously reported that the alkyl amines condensed with GA and a hydrophobic aldehyde in acetic medium to produce hydrophobic dialkyl imidazole as an IL.[29] The amino propyl group at the terminal end can also condense to form the terminal imidazolium IL. The magnetite can be reacted with the Si-IIL during the formation of magnetite nanoparticles, as presented in the Experimental Section, to form Si-IIL-Fe3O4. Scheme illustrates the formation of the disiloxyimidazolium ionic liquid (SIMIL) by reacting APTS with PHB and GA in acetic conditions. The end triethoxysilane groups of the SIMIL hydrolyzed with TEOS and magnetite in the presence of ammonia to form the Fe3O4-Si-IIL hybrid. It is expected that the presence of hydroxyl groups on the magnetite[29] and silica links their surfaces to form silica–magnetite IIL during the hydrolysis of the SIMIL under basic conditions.

Scheme 1

Synthesis of SiO2-1-IIL-Fe3O4 Hybrid NPs

Scheme 2

Synthesis of Fe3O4-Si-IIL Hybrid NPs

Characterization

The 1H NMR spectrum of the SIMIL, represented in Figure S1, seen in the Supporting Information, is used to elucidate its chemical structure. The appearance of a singlet peak at 6.8 ppm, referred to two hydrogen olefin protons, confirms the formation of imidazolium cations. Moreover, the disappearance of aldehyde protons of HBA at 9.3 ppm with the presence of new peaks at 7.6, 7.3, and 2.3 ppm (referred to four protons of p-substituted phenyl rings and −OH phenolic) elucidate the condensation of HBA with APTS to form the SIMIL. The peaks at 4.19 (m, 2H, CH2–N+), 3.64 (m, 2H, CH2–N), 3.00 (m, 12H, [CH2–O]3), 1.89 (m, 8H, 2(CH2)2), 1.62 (s, 3H, CH3COO–), and 0.55 ppm (m, 18H, 2(CH3)3) also confirm the condensation of APTS without hydrolysis under acidic conditions with HBA to form the SIMIL (Figure S1).

The Fourier transform infrared (FTIR) spectra of silica, SiO2-1-IIL-Fe3O4, and Fe3O4-Si-IIL hybrid NPs are summarized in Figure S2a–f to elucidate their chemical and surface structures. The formation of Si NPs by variation of the mol ratios of APTS/TEOS from 0.1 to 2 (Figure S2a–c) was confirmed from the appearance of bands observed at 472–475 cm–1 (Si–O rocking vibration), 755 cm–1 (Si–O bending vibration), 870–910 cm–1 (Si–O–H···H2O bending vibration), 1076 cm–1 (Si–O–Si stretching vibration), 1635 cm–1 (OH bending vibration of the adsorbed water), and 3450 cm–1 (OH stretching vibration, hydrogen bonded).[30] Moreover, the presence of the amino propyl group at the surfaces of Si NPs was proved from the appearance of a new band at 1550–1580 cm–1 (Figure S2b,c), referred to NH bending. Its intensity was increased more at an APTS/TEOS mol ratio of 1 (Figure S2b). The formation of SiO2-1-IIL-Fe3O4 (Scheme ) was elucidated from the FTIR spectrum (Figure S2e), confirming the appearance of three peaks at 584, 1580, and 3400 cm–1 corresponding to Fe–O vibration, C=NH stretching of imidazole, and O–H stretching vibration, respectively. The same bands observed with a lower intensity in the spectrum of Fe3O4-Si-IIL (Figure S2f) confirm the presence of magnetite with a lower content than Si-IIL-Fe3O4 after the removal of the uncoated magnetite as reported in the Experimental Section. It was also observed that the intensity of O–H stretching vibration (at 3400 cm–1; Figure S2e,f) was reduced to confirm the bonding of Si–OH of Si-IL with the Fe–OH groups of magnetite. Accordingly, the FTIR spectra of SiO2-IIL-Fe3O4 and Fe3O4-Si-IIL elucidate that the magnetite NPs was linked and coated with silica. Moreover, the magnetite content was increased when Si-IIL used as a capping agent during the formation of magnetite.

The crystallinity and lattice structures of the prepared Fe3O4, SiO2, Si-IIL- Fe3O4, and Fe3O4-Si-IIL hybrid NPs were elucidated from their X-ray diffraction (XRD) diffractograms represented in Figure S3a–d. Both XRD patterns of Fe3O4-Si-IIL (Figure S3a) and SiO2-1-IIL-Fe3O4 (Figure S3c) hybrid NPs match with diffraction peaks of magnetite (Figure S3b) and SiO2 (Figure S3d). The magnetite planes appeared as (220), (311), (400), (422), (511), (440), and (522).[31] The wider peak at 2θ = 15° elucidates the formation of SiO2 and indicates that the Si-IIL shell was linked with Fe3O4 as a core to prepare SiO2-1-IIL-Fe3O4 and Fe3O4-Si-IIL hybrid NPs. The formation of magnetite without other contaminated iron oxides confirms that the Si-IIL stabilizes the magnetite against further oxidation to other iron oxides such as maghemite and hematite. The peak width of plane (311) is used to determine the average crystallite sizes of Fe3O4, SiO2-1-IIL-Fe3O4, and Fe3O4-Si-IIL hybrid NPs by applying Scherrer’s equation and calculated to be 8.9, 18.9, and 25.6 nm, respectively. These data elucidate that the magnetite NPs is single-crystalline, and the crystallinity of magnetite persists well in the presence of Si-IIL as a coating shell during the formation of magnetite as a core. Moreover, the linking of the Si-IIL shell with the magnetite core does not affect the crystal structure of magnetite NPs.[32]

The morphology of Fe3O4, SiO2, SiO2-1-IIL-Fe3O4, and Fe3O4-Si-IIL hybrid NPs was investigated by transmission electron microscopy (TEM), as summarized in Figure a–d, respectively. The cubic Fe3O4 NPs (Figure a) with an average particle size of 10 nm were obtained. The connected spherical SiO2 NPs agglomerate (Figure b) morphology is observed when the mol ratio of APTS/TEOS equals or more than 1. This observation can be referred to the elongation of APTS at the silica surfaces to form loose bridges or the formation of a hydrated interparticle layer.[29] This tightly bounded water layer disappeared when the IIL formed on the surface of Si-IIL that was elucidated from the appearance of dispersed disconnected spherical morphologies of Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 hybrid NPs (Figure c,d). This observation indicates that the Si-IIL have successfully coated the Fe3O4 NPs during their formation. The uniform spherical morphology obtained with SiO2-1-IIL-Fe3O4 hybrid NPs (Figure d) more than Fe3O4-Si-IIL (Figure c). The Fe3O4 NP core thickness appeared as slightly black dots of Si-IIL-Fe3O4 (Figure d) determined as 42.2 ± 8 nm and the bright shell thickness is 5.3 ± 2.8 nm to elucidate that the core–shell SiO2-1-IIL-Fe3O4 formed without Si-IIL core-free silica nanospheres. The distribution of magnetite in the spherical morphology of Fe3O4-Si-IIL (Figure c) elucidates that the linking of the hydroxyl groups on the magnetite surfaces with the hydroxyl groups of Si-IIL. The microstructure and surface morphologies of Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 hybrid NPs are clarified in scanning electron microscopy (SEM) images represented in Figure a,b, respectively. The rough surface was clearly observed in SiO2-1-IIL-Fe3O4 hybrid NPs (Figure b) than Fe3O4-Si-IIL (Figure a). The less rough surfaces of Fe3O4-Si-IIL elucidate that the linking of magnetite with Si-IIL without the formation of the core–shell morphology reduces the roughness of the spherical particles of Fe3O4-Si-IIL.

Figure 1

TEM micrographs of (a) Fe3O4 NPs, (b) SiO2, (c) Fe3O4-Si-IIL, and (d) SiO2-1-IIL- Fe3O4.

Figure 2

SEM micrographs of (a) Fe3O4-Si-IIL and (b) SiO2-1-IIL-Fe3O4 hybrid NPs.

The nitrogen adsorption–desorption isotherms of Si-IL, Fe3O4-Si-IL, and SiO2-1-IIL-Fe3O4 were evaluated at 77 K after removal of the water humidity after heating under vacuum at 423 K and are represented in Figure a,b to determine their pore structure and surface area. The isotherms of Si-IL, Fe3O4-Si-IL, and SiO2-1-IIL-Fe3O4 (Figure a,b) obeyed type I. The Brunauer–Emmett–Teller (BET) surface area (SBET; m2·g–1), pore sizes (D; nm), and pore volume (Vtotal; cm3·g–1) of Si-IL are 64, 10.56, and 0.0869, respectively. The D, SBET, and Vtotal values of Fe3O4-Si-IL are 92 m2·g–1, 14.63 nm, and 0.1729 cm3·g–1, respectively. The SiO2-1-IIL-Fe3O4 adsorption–desorption (Figure b) shows a much longer straight line portion of the curve where its starting point is known as the adsorption capacity of the monolayer due to Brunauer and Emmett.[33] The D, SBET, and Vtotal values of SiO2-1-IIL-Fe3O4 are 8.36 nm, 220.39 m2·g–1, and 14.097 cm3·g–1, respectively. The larger surface area of SiO2-1-IIL-Fe3O4 and larger Vtotal confirm the rough surface observed in SiO2-1-IIL-Fe3O4 hybrid NPs (SEM; Figure b).

Figure 3

BET data of the prepared (a) Si-IL, Fe3O4-Si-IL and (b) SiO2-1-IIL-Fe3O4.

The room temperature magnetization curves of Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 are determined from the vibrating sample magnetometer (VSM) hysteresis loop and are represented in Figure a,b, respectively. Their magnetic separation ability was tested in ethanol by placing a magnet near the solution as represented on top of the hysteresis loop (Figure a,b). The black particles were attracted toward the magnet within 5 s to confirm that these nanospheres possess magnetic properties. The saturation magnetization (Ms; emu/g), coercivity (Hc; G), and remnant magnetization (Mr; emu/g) are used to evaluate the magnetic properties of the hybrid NPs. The Ms, Hc, and Mr of Fe3O4-Si-IIL are 35.3 emu/g, 20.71 G, and 0.38 emu/g, respectively. The Ms, Hc, and Mr of SiO2-1-IIL-Fe3O4 are 30.8 emu/g, 14.50 G, and 0.56 emu/g, respectively. These data elucidate that the Fe3O4-Si-IIL NPs have strong magnetic properties than SiO2-1-IIL-Fe3O4 nanocomposites, having core–shell morphology, and they possess a strong magnetic field than other magnetite capped with silica nanoparticles.[34,35] The low Hc value of SiO2-1-IIL-Fe3O4 confirms their superparamagnetic properties due to the lower particle sizes of dispersed magnetite and lower thickness of Si-IIL shell.[34,35]

Figure 4

VSM hysteresis loops of (a) Fe3O4-Si-IIL and (b) SiO2-1-IIL-Fe3O4 at room temperature.

The hydrodynamic diameter (nm) and polydispersity index (PDI) of SiO2-1-IIL, Si-IIL, Fe3O4-Si-IIL, and SiO2-1-IIL-Fe3O4 are determined in aqueous solutions at different pHs (4, 7, and 9) from dynamic light scattering (DLS) measurements and are represented in Figure a–d. It is very important to study the effect of pHs either acidic or alkaline on the stability of SiO2-1-IIL, Si-IIL, Fe3O4-Si-IIL, and SiO2-1-IIL-Fe3O4 in their aqueous solutions. The measurements were made in dilute aqueous solutions of the prepared nanohybrid materials having 0.01 M ionic strength to evaluate their aggregation in dust-free aqueous solution dispersions. It was noticed that the sizes of SiO2-1-IIL and SiO2-1-IIL-Fe3O4 determined by TEM (Figure ) and SEM (Figure ) are smaller than that determined by DLS measurements (Figure ). This observation does not occur for Si-IIL and Fe3O4-Si-IIL, which shows the agreement between their microscope particle diameters and DLS data. The chemical structures of SiO2-1-IIL and SiO2-1-IIL-Fe3O4 (Scheme ) proposed that the IIL located as end groups, while the Si-IIL and Fe3O4-Si-IIL proposed that the IIL will be located at the center of their chemical structures (Scheme ). This means that the presence of IIL at end groups affects their behaviors in their aqueous solutions and increases their shrinkage under an electron beam.[36]The PDI values were used to elucidate the monodispersity or polydispersity of the nanomaterials in the solution. It is confirmed that the lower PDI value of less than 0.7 elucidates the monodispersity of the uniform particle sizes, which increases with the lowering of the PDI value nearest 0.001,while the greater PDI value of more than 0.7 confirm the polydispersity of nonuniform particle sizes that increases with increasing the PDI value of more than 1. It was noticed that the SiO2-1-IIL and SiO2-1-IIL-Fe3O4 have a lower PDI (<0.2) than Si-IIL and Fe3O4-Si-IIL (PDI > 0.2–0.7) in acidic pH aqueous solutions to confirm the protonation of remaining amine groups of SiO2-1-IIL and SiO2-1-IIL-Fe3O4 of IIL and APTS (Scheme ). It is expected that the distribution of magnetite into Fe3O4-Si-IIL reduces their particle sizes. The presence of magnetite as a core into SiO2-1-IIL-Fe3O4 increases their particles sizes due to the presence of reactive IIL groups as the shell due to ionic interactions and hydrogen bonding of IIL with water.

Figure 5

DLS data of (a) SiO2-1-IIL, (b) Si-IIL, (c) Fe3O4-Si-IIL, and (d) SiO2-1-IIL-Fe3O4 dispersed in 0.001 M KCl aqueous solution at room temperature and different pHs (4, 7, and 9) from left to right.

The prepared SiO2-1, SiO2-1-IIL, Si-IIL, Fe3O4-Si-IIL, and SiO2-1-IIL-Fe3O4 hybrids have different reactive groups that affect their surface charges such as negative charges of Si–O, phenoxy, acetate, and Fe–O groups. The positive charges IIL are also responsible for the occurrence of the positive charges for the prepared materials. In this respect, the surface charges of the particles are evaluated at different pHs to determine the isoelectric point of the prepared materials (pH has zero surface charges) as measured from their zeta potential values (mV), as represented in Figure . The isoelectric points of Si-IIL, Fe3O4-Si-IIL, SiO2-1, SiO2-1-IIL, and SiO2-1-IIL-Fe3O4 hybrids are obtained at pHs 4.8, 6.6, 7.83, 7.2, and 7.78, respectively (Figure ). This means that the linking of the hydroxyl group of magnetite during the hydrolysis of silica in the Si-IIL and Fe3O4-Si-IIL hybrids facilitates the orientation of imidazolium cations into the core and their hydroxyl groups as a shell in slightly acidic and neutral aqueous solutions, while the presence of amine groups of APTS and those of IIL into SiO2-1, SiO2-1-IIL, and SiO2-1-IIL-Fe3O4 hybrids orients the imidazolium cations into the core and their hydroxyl groups as a shell at a slightly basic aqueous solution. It was previously reported that[28] the protonation of the amino groups for the silica modified with APTS also led to a more basic environment in the small region around the particles to form electrostatic attraction and deposition of apatite on their surfaces. The higher negative charges of SiO2-1, SiO2-1-IIL, and SiO2-1-IIL-Fe3O4 hybrids in the basic medium can be referred to the presence of the hydrated silica layer that could serve to increase the negativity of silica surfaces.[37]

Figure 6

Zeta potentials of the prepared composites in aqueous solution at room temperature and different pHs.

Sequestration and Optimization of CB-R250 Dye Adsorption

CB-R250 is a toxic acidic dye used in the textile and wool industry and to stain protein.[38] It is a nondegradable dye and accumulated in an aquatic system to be hazard pollutants as it is toxic in nature. It was reported that the consumption of this dye in the water system caused severe eye problems and irritation to mucus membranes and upper respiratory of living organisms besides its effect on inhalation, skin, and ingestion.[39] In the present work, the Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 are used as CB-R250 adsorbents due to their strong magnetic properties that facilitate their isolation from the water by applying an external magnetic field. Moreover, the lower particle sizes of SiO2-1, SiO2-1-IIL, or Si-IIL and their higher dispersion in water affect their isolation from the aqueous medium even with filtration. The literature survey for using different adsorbents to remove CB-R250 was reported in Table to compare their maximum removal capacity (qmax) with the prepared Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4. It was found that their qmax values ranged from 22.89 to 67.6 mg·g–1 (Table ).[40−45] The present work aims to fulfill the optimum conditions to obtain the highest maximum adsorption capacities, qmax, to remove CB-R250. The adsorbents were dispersed into the CB-R250 aqueous solutions, and their equilibrium qmax (mg·g–1) and their adsorption efficiency E (%) can be calculated as:

Table 4

Thermodynamic Parameters of SiO2-1-IIL-Fe3O4 and Fe3O4-Si-IIL Adsorbents for the Removal of CB-R250 from Aqueous Solutions at Different Temperatures

			ΔG0 (kJ mol–1)
adsorbents	ΔH0 (kJ mol–1)	ΔS0 (J mol–1)	298 K	308 K	313 K	323 K
Fe3O4-Si-IIL	242.03	0.856	–13.16	–21.7	–26.01	–34.76
SiO2-1-IIL-Fe3O4	169.20	0.599	–9.43	–15.43	–18.43	–24.42

Table 1

Adsorption Data of Different Adsorbents Used To Remove CB-R250 Dye at a Temperature of 298 K

system	qmax (mg·g–1)	equilibrium time (min)	reference
IL-modified ZnO nanoparticles	53.7	90	(40)
BMTF-functionalized Zn	59.9	90	(41)
porous nanocrystalline cobalt ferrite (PNCoFe) composit-120	46.06	50	(42)
iron oxide nanoparticles with polyarginine	67.6		(43)
acid-treated clays	22.89		(44)
starch/poly(alginicacid-cl-acrylamide) nanohydrogel	31.24	120	(45)
Fe3O4-Si-IIL	460.3	275	present work
SiO2-1-IIL-Fe3O4	306.9	275	present work

The Co, Ce, V, and m are the initial CB-R250 concentration in aqueous solutions, equilibrium CB-R250 concentration (mg·L–1), the volume of aqueous solution (L), and the adsorbent mass (g), respectively. In this respect, the effects of adsorbents weights, CB-R250 concentration, pH of the aqueous solution, and temperature on the %E and qmax are investigated to determine the optimum conditions to remove the CB-R250 pollutant.

The optimum weight of adsorbents to remove 100 mg·L–1 CB-R250 was determined from the relation of qmax (mg·g–1) and weight of adsorbent (mg) as represented in Figure a,b. It was noticed that the optimum weights of Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 to remove CB-R250 are 8 and 5 mg, respectively. The effects of the initial CB-R250 concentration on its removal from the aqueous solution in the presence of the optimum weights of Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 are represented in Figure c,d, respectively. The %E values confirmed that the Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbed 100% at the initial concentration of CB-R250 100 and 50 mg·L–1, respectively. It is clear that the %E of Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 decreased progressively with increasing the initial CB-R250 concentration. The higher reactive sites of Fe3O4-Si-IIL increase the concentration of the adsorbed CB-R250 on its surfaces than SiO2-1-IIL-Fe3O4.[46]

Figure 7

Relations of adsorption parameters of (a, b) Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbents weights, (c, d) CB-R250 concentrations, and (e, f) pHs of aqueous solution at room temperature.

The optimum weights of Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbents and optimum initial CB-R250 concentrations were used to investigate the effect of pH on the removal efficiencies (%E) of CB-R250 from the aqueous solution as described in Figure e,f. The data confirmed that the optimum pH of the aqueous solution is 4 to remove CB-R250 for both Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbents. The effect of pH on the surface charges of the Fe3O4-Si-IIL adsorbent before and after adsorption of CB-R250 is represented in Figure S4a–c. The positive surface charges of Fe3O4-Si-IIL before CB-R250 adsorption at pH 4 (19 ± 6 mV) changed to slightly negative values after CB-R250 adsorption (Figure S4a). pHs 7 and 9 (Figure S4b,c) do not show any changes for the surface charges of Fe3O4-Si-IIL before and after CB-R250 adsorption. The presence of phenol in the chemical structure of Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbents (Schemes and 2) produces negative charges in the basic condition due to the formation of the phenoxy groups that repulse with the negative charges on anionic CB-R250 to reduce its adsorption.[45] The data represented in Figure a,b elucidate that the diverse interactions (such as ion exchange, chemical bonding, hydrophobic, van der Waals interactions, hydrogen bonding, and physical adsorption) occurred between CB-R250 and the prepared adsorbents. These interactions were increased at a pH range from 2 to 5 and decreased in the basic pH due to increasing the CB-R250 dye–dye interaction.[47] Consequently, pH 4 is selected as the optimum pH for the removal of CB-R250 using Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbents to ensure that the magnetite adsorbents are well dispersed without magnetite leaching or distortion in acidic conditions below pH 4.[48] These data confirm the stability of the magnetite to both acidic and alkaline solutions and confirm that the chemical bonding of the magnetite nanoparticles either with silica or IIL protects them from leaching and oxidation.

The effect of contact time on the removal of CB-R250 using Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbents at pH 4 is plotted in Figure S5a,b. It was noticed that the CB-R250 was completely removed from the aqueous solution using both Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbents for 275 min with the highest adsorption capacities that are not reported in the literature (Table ).[40−45] The Fe3O4-Si-IIL shows a higher CB-R250 removal efficiency (45%) than SiO2-1-IIL-Fe3O4 (33.3%) after the first 5 min. This can be referred to the presence of the more active site on the Fe3O4-Si-IIL surfaces than SiO2-1-IIL-Fe3O4 due to the good dispersion of magnetite on the Fe3O4-Si-IIL as elucidated from TEM micrographs (Figure ).

The optimum temperature to remove CB-R250 using Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbents is determined from the data represented in Figure at the optimum contact agitation time, adsorbent weight (mg), CB-R250 dye concentration, and pH 4. It is observed that both Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbents achieved higher qmax (mg·g–1) at a temperature of 313 K. It was noticed that the increasing temperature of the aqueous solution increases the qmax, which referred to activation of Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 surfaces. Moreover, the diffusion and interaction of CB-R250 into adsorbents increases with increasing the temperature of the aqueous solution. The increasing of temperature above 318 K decreases qmax of both Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4, which was attributed to breaking the hydrogen bonding between CB-R250 adsorbate and Fe3O4-Si-IIL or SiO2-1-IIL-Fe3O4 adsorbents.

Figure 8

Effect of solution temperature on the removal of CB-R250 using (a) Fe3O4-Si-IIL and (b) SiO2-1-IIL-Fe3O4 adsorbents.

The adsorption isotherms based on Langmuir and Freundlich isotherms are the most applicable models used to predict the removal efficiencies of the adsorbents, the homogeneity or heterogeneity of the adsorbent surfaces, and the assembly of the adsorbate on the adsorbent surfaces as a monolayer or multilayers.[49] In this respect, the optimum conditions such as pH, weight of adsorbent, and agitation contact time were applied to plot the Langmuir and Freundlich isotherms from the following relations:

The constants in eqs and 4 such as n (in g·L–1), Kl (in L·mg–1), and Kf [in (mg·g–1)(L·mg–1)(1/] are the empirical constant, Langmuir constant, and Freundlich constant, respectively. The adsorption capacities qe and qmax (in mg·g–1) are the determined equilibrium and theoretical maximum amounts of CB-R250 adsorbate that adsorbed on the adsorbent surfaces, respectively. The Ce (mg.L–1) is the concentration of the CB-R250 dye in the aqueous solution at equilibrium. The relations of Ce/qe versus Ce (Langmuir) and log(qe) versus log Ce (Freundlich) were plotted in Figure S6a,b. Equations and 4 should obey a linear relation with the highest linear coefficient (R2). The Langmuir and Freundlich parameters of Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbents were calculated and are summarized in Table . The data listed in Table and Figure S6 elucidate that the adsorption of the CB-R250 dye on the Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbent surfaces fits with the Langmuir isotherm that have the highest R2 values than the Freundlich isotherm. Moreover, it is determined that the qmax values for the adsorption of the CB-R250 dye on the Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbent surfaces calculated from the Langmuir model agree with the qe values determined experimentally. These data elucidate that the CB-R250 dye forms a monolayer on the homogeneous Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbent surfaces.

Table 2

Adsorption Isotherm Parameters of CB-R250 Dye Using SiO2-1-IIL-Fe3O4 and Fe3O4-Si-IIL Adsorbents at a Temperature of 298 K

	Langmuir isotherm parameters			Freundlich isotherm parameters			exp. adsorption capacity
adsorbents	qmax (mg·g–1)	Kl (L·mg–1)	R2	n (g·L–1)	Kf [(mg·g–1)(L·mg–1)(1/n)]	R2	qmax (mg·g–1)
Fe3O4-Si-IIL	454.5	0.115	0.932	5.96	185.78	0.878	460.3
SiO2-1-IIL-Fe3O4	322.58	0.168	0.998	6.08	141.50	0.912	306.9

Adsorption Kinetics and Mechanism

The adsorption rate constant of the CB-R250 dye on the Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbent surfaces and the predicted adsorption capacity (qcalc) after the adsorbents reached the equilibrium can be estimated from the pseudo-first-order and pseudo-second-order kinetics.[50] Moreover, the adsorption mechanism was exploited using the Weber–Morris model.[45] The pseudo-first-order and pseudo-second-order kinetics equations are given as:

The experimental values of pseudo-first-order and pseudo-second-order kinetics are represented in Figure S7b. Their correlation coefficient (R2), the calculated qcalc, and pseudo-first-order constant (K1; min–1) or pseudo-second-order constant (K2; g·mg–1·min–1) were determined from the linear relations of eqs and 6 and are listed in Table . The data represented in Figure S7 and Table confirm that both Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbents obey the pseudo-second-order kinetics to remove the CB-R250 dye from aqueous solutions due to the highest R2 and the agreements between their qcalc and qexp as listed in Table . It was also confirmed that the Fe3O4-Si-IIL adsorbed CB-R250 dye from aqueous solutions is faster with a higher K2 value than SiO2-1-IIL-Fe3O4 (Table ).

Table 3

Kinetic Parameters of SiO2-1-IIL-Fe3O4 and Fe3O4-Si-IIL Adsorbents for the Removal of CB-R250 from Aqueous Solutions at Temperature 298 K

		Weber–Morris parameters			pseudo-first order kinetic parameters			pseudo-second-order kinetic parameters
cryogel composites	qexp (mg·g–1)	Kdif (mg·g–1·min0.5)	C	R2	R2	qcalc (mg·g–1)	K1(min–1)	R2	qcalc (mg·g–1)	K2 (g·mg–1·min–1)
Fe3O4-Si-IIL	460.3	5.098	117.3	0.991	0.94	250.0	0.00023	0.952	385.4	0.0113
SiO2-1-IIL-Fe3O4	306.9	10.827	–7.9	0.997	0.85	212.3	0.00003	0.992	270.6	0.0073

The diffusion mechanism of CB-R250 molecules into Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbents can be estimated from the Weber–Morris equation as:

The linear relation of eq is plotted in Figure S7c and used to determine the intraparticle diffusion rate constant Kdif (mg·g–1·min0.5) from the slope and the thickness of the boundary layer (C; mg·g–1) from the intercept data of CB-R250 molecules into Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbents as summarized in Table . The greater value of C for Fe3O4-Si-IIL (Table ) reflects the boundary layer effect. It is noticed that the higher Kdif value of CB-R250 molecules into SiO2-1-IIL-Fe3O4 and lower C values than the Fe3O4-Si-IIL adsorbent concludes that the boundary layer effect increases the adsorption of CB-R250 on the surface of Fe3O4-Si-IIL than SiO2-1-IIL-Fe3O4. The intraparticle diffusion of CB-R250 molecules is the rate-limiting step for the adsorption mechanism of CB-R250 molecules into SiO2-1-IIL-Fe3O4. The data represented in Figure S7c confirm that the adsorption of CB-R250 onto Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbents exhibit three distinctive linear regions. The first phase is based on transportation of CB-R250 from the liquid phase to the adsorbent solid boundary of the adsorbent surfaces. The second phase is based on the CB-R250 film diffusion, and the third region is dominated by pore diffusion that increased by increasing the contact time.[51] The first region increased for adsorption of CB-R250 on the Fe3O4-Si-IIL surface than SiO2-1-IIL-Fe3O4. The second and third region CB-R250 rates increased on the SiO2-1-IIL-Fe3O4 surface than Fe3O4-Si-IIL. The adsorption mechanism of CB-R250 molecules into SiO2-1-IIL-Fe3O4 is clarified in Scheme . The core–shell morphology of SiO2-1-IIL-Fe3O4 (Figure ) and the presence of magnetite in the core and IIL or silica at the shell of the composites increase the intraparticle diffusion rate of CB-R250 molecules using physical and chemical interactions after the formation of the CB-R250 film. The good dispersion of magnetite at Fe3O4-Si-IIL surfaces without the formation of core–shell morphology (Figure ) increases the adsorption rate for the formation of CB-R250 on the Fe3O4-Si-IIL surfaces than SiO2-1-IIL-Fe3O4. The interactions (such as ion exchange, chemical bonding, hydrophobic, van der Waals interactions, hydrogen bonding, and physical adsorption) occurred between CB-R250, and the adsorbent surfaces are responsible for the adsorption of CB-R250 on SiO2-1-IIL-Fe3O4 surfaces (Scheme ). Consequently, it can be concluded that the adsorption mechanism of CB-R250 from the aqueous solution to Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 surfaces is controlled and preceded with a multifaceted process and intraparticle diffusion edicts of the sorption process.

Scheme 3

CB-R250 Adsorption Mechanism Using SiO2-1-IIL-Fe3O4 and Fe3O4-Si-IIL Composites

Thermodynamic Parameters

Thermodynamic constants such as standard Gibbs energy (ΔG0; in kJ·mol–1), enthalpy (ΔH0; in kJ·mol–1), and entropy (ΔS0; in J·mol–1 K) were computed by the following equations:where CeA, R, and T are the adsorbent concentration (in mg·L–1), gas constant (8.314 J mol–1 K–1), and the aqueous solution temperature (in K), respectively. ln(CeA/Ce) is designated as the equilibrium concentration constant (ln Kc). The relation between ln Kc and 1/T (K–1) for the removal of CB-R250 using Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbents is represented in Figure . The intercept and slope of the linear relation (Figure ) are used to calculate the ΔH0 and ΔS0 values according to eq . The thermodynamic parameters for the removal of CB-R250 using Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbents are summarized in Table . The more negative values of ΔG0 (Table ) elucidate the spontaneous nature of the CB-R250 adsorption on the Fe3O4-Si-IIL than SiO2-1-IIL-Fe3O4 surfaces. The positive value of ΔH0 (Table ; more than 50 kJ·mol–1) confirms the chemical adsorption and endothermic nature of the CB-R250 adsorption process using Fe3O4-Si-IIL or SiO2-1-IIL-Fe3O4 adsorbents.[52] The positive values of ΔS0 (Table ) confirm that the randomness increased at the solid–liquid interface with the structural changes in the CB-R250 and Fe3O4-Si-IIL or SiO2-1-IIL-Fe3O4 adsorbents. The degree of freedom occurred during the adsorption of CB-R250 species increases with increasing temperature. Moreover, the water molecules displaced by CB-R250 molecules gain an extra amount of transitional entropy and permit the randomness in the adsorption system. Finally, the thermodynamic parameters elucidate the proposed mechanism (Scheme ) for the removal of the CB-R250 adsorption process using Fe3O4-Si-IIL or SiO2-1-IIL-Fe3O4 adsorbents. The CB-R250 dye was first adsorbed onto the Fe3O4-Si-IIL or SiO2-1-IIL-Fe3O4 matrix and then began to interact with the adsorbents via chemical binding by ion exchange and hydrogen bonding mechanism.

Figure 9

van’t Hoff plot for the adsorption of CB-R250 by Fe3O4-Si-IIL and SiO2-1-IIL-Fe3O4 adsorbents.

Regeneration and Reuse of the Adsorbents

The adsorption data and zeta potential data (Figure and Figure S4) elucidate that the adsorption of CB-R250 reduced in the basic medium in the presence of Fe3O4-Si-IIL or SiO2-1-IIL-Fe3O4 adsorbents. The presence of an aprotic ionic liquid and magnetite in the chemical structures of Fe3O4-Si-IIL or SiO2-1-IIL-Fe3O4 adsorbents also confirms that the alkaline ethanol is the best reagent for desorption and regeneration of these adsorbents.[19,42,53] Acetone was used as an eluent to remove CB-R250 from the prepared adsorbents and showed a longer time after 48 h. The desorption experiments of CB-R250 from Fe3O4-Si-IIL or SiO2-1-IIL-Fe3O4 adsorbents were easily completed using ethanol at pH 10 and are represented in Figure a. Their reuse data are summarized in Figure b. It can be seen that the CB-R250 dye was easily desorbed with a higher %DE in the case of Fe3O4-Si-IIL (%DE ranged from 97 to 75%) than SiO2-1-IIL-Fe3O4 (%DE ranged from 93 to 85%) after five cycles. These data can be referred to the higher IIL contents of SiO2-1-IIL-Fe3O4 than Fe3O4-Si-IIL (Schemes –3) that enhanced the chemical bonding of CB-R250. The reuse data of Fe3O4-Si-IIL or SiO2-1-IIL-Fe3O4 adsorbents to remove CB-R250 (Figure b) elucidate that the Fe3O4-Si-IIL remove the dye effectively than the SiO2-1-IIL-Fe3O4 adsorbent. This can be attributed to the presence of the dye with the SiO2-1-IIL-Fe3O4 adsorbent after desorption that reduces its %E after the reused cycle. Therefore, it can be concluded that the prepared Fe3O4-Si-IIL or SiO2-1-IIL-Fe3O4 were appealing adsorbents to remove the CB-R250 with higher effective adsorption capacities than the reported data in the literature.[40−45]

Figure 10

Cycles of CB-R250 (a) desorption in ethanol at pH 10 and (b) adsorption reuse using Fe3O4-Si-IIL or SiO2-1-IIL-Fe3O4 adsorbents.

Conclusions

New magnetic imidazolium ionic liquid silica nanocomposites based on SiO2-1-IIL- Fe3O4 and Fe3O4-Si-IILwere prepared by a simple technique. The FTIR spectra elucidate that the magnetite NPs were linked and coated with silica and the magnetite contents increased when Si-IIL was used as the capping agent during the formation of magnetite (SiO2-1-IIL-Fe3O4). The XRD data of SiO2-1-IIL-Fe3O4 elucidate that the crystallinity of magnetite persists well in the presence of Si-IIL as the coating shell during the formation of magnetite as the core. The core–shell spherical morphology obtained with SiO2-1-IIL-Fe3O4 hybrid NPs more than Fe3O4-Si-IIL. The less rough surfaces of Fe3O4-Si-IIL elucidate that the linking of magnetite with Si-IIL without the formation of the core–shell morphology reduces the roughness of the spherical particles of Fe3O4-Si-IIL. The Fe3O4-Si-IIL NPs have strong magnetic properties than SiO2-1-IIL-Fe3O4, having core–shell morphology, and they possess a strong magnetic field than other magnetite capped with silica nanoparticles. The presence of IIL at end groups of SiO2-1-IIL-Fe3O4 affects the solution behaviors of the nanohybrid materials and increases the shrinkage of the particles under an electron beam. The isoelectric points of Si-IIL, Fe3O4-Si-IIL, SiO2-1, SiO2-1-IIL, and SiO2-1-IIL-Fe3O4 hybrids are obtained at pHs 4.8, 6.6, 7.83, 7.2, and 7.78, respectively. pH 4 was selected to apply both SiO2-1-IIL- Fe3O4 and Fe3O4-Si-IIL as effective adsorbents to remove toxic CB-R250 with high adsorption capacities that were not reported in the literature.

Experimental Section

Preparation Techniques

Preparation of Magnetic SiO2-1-IIL

The silica solution was prepared by dispersing the SiO2-1 (1.34 g; 10 mmol) in acetic acid (50 mL; 50% acetic acid). The aldehyde mixture was prepared by dissolving GA (0.38 g; 5 mmol) and PHB (0.6 g; 5 mmol) in acetic acid (50 mL; 50% acetic acid). The aldehyde mixture was added dropwise to the silica solution under vigorous stirring for 30 min, and the reaction temperature was increased to 343 K and remained constant for 4 h. The product of the reaction was collected by ultracentrifugation at 10,000 rpm for 10 min. The silica imidazolium ionic liquid (SiO2-1-IIL) product was washed several times with ethanol and dried under vacuum.

Anhydrous FeCl3 solution (4 g; dissolved in 30 mL of water) was mixed and stirred with KI solution (1.32 g; dissolved in 5 mL of water) at room temperature under a N2 atmosphere for 1 h. The precipitate (iodine byproduct) was filtered from the reaction mixture. Ammonia solution (20 mL of 25% ammonia) and SiO2-1-IIL (1 g) were added to the filtrate at 40 °C under vigorous stirring for 30 min. The Fe3O4-SiO2-1-IIL nanocomposite contaminated with Fe3O4 nanoparticles were separated from the mixture by an external magnet. The solid (1 g) was dispersed into HCl solution (1 L; 4 M) for 18 h to dissolve the uncapped SiO2-1-IIL-Fe3O4. The remained solid was separated by the external magnet, washed with ethanol mixture three times, and then dried under vacuum.

Preparation of Fe3O4-Si-IIL Nanocomposite

APTS (0.1 mol; 18.5 g) was dissolved in 50 mL of acetic acid aqueous solution (50%) at a temperature of −4 °C. GA (0.02 mol; 1.52 g) and PHB (0.02 mol; 2.4 g) were dissolved in 50 mL of acetic acid aqueous solution (50 vol %) at a temperature of −4 °C. The APTS solution was added to the aldehyde solution, and the reaction temperature rose up at 70 °C for 5 h. The reaction mixture was cooled and washed several times with diethyl ether to obtain a colorless organic phase. The solvent was separated from the reaction mixture using a rotary evaporator under vacuum. The SIMIL was formed, and its yield percentage is 95%.

Magnetite (1 g) and SIMIL (1 g) were dispersed into a 25% ammonia solution/ethanol mixture (100 mL; 50/50) by sonication for 5 min. Then, the reaction mixture was heated to 35 °C and stirred for 36 h until the complete hydrolysis of silicate to form the magnetic silica IL (Fe3O4-Si-IIL) collected using an external magnet. The solid (1 g) was dispersed into HCl solution (1 L; 4 M) for 18 h to dissolve the uncapped magnetite. The dispersed solid was separated by an external magnet and washed several time using a water/ethanol mixture (50:50). The Fe3O4-Si-IIL yield percentage of the reaction is 98.9%. The Si-IIL was prepared in the absence of magnetite under the same condition, and its reaction yield percentage is 97.6%.

CB-R250 Adsorption Measurements

Standard CB-R250 aqueous solutions (from 0.01 to 0.1 mM) were used to demonstrate the calibration curve of CB-R250 (dye concentrations and the corresponding absorbance values). A UV–visible spectrophotometer was used to determine the CB-R250 absorbance at a wavelength of 547 nm using buffer solutions having different pHs. The adsorption measurements were repeated five times, and the data averages were added in the tables and figures with an accuracy of ±0.1 to 1.3.

The desorption and regeneration of SiO2-1-IIL-Fe3O4 and Fe3O4-Si-IIL adsorbents measurements were carried out by using an external magnetic field after dispersion of the CB-R250 saturated adsorbents into ethanol at pH 11 as an effective eluent. The desorbed CB-R250 concentration in ethanol was measured to calculate the desorption efficiency (%DE). The %DE is the percentage of the CB-R250 concentration in the eluent divided by its initial concentration in the adsorbents. The measurements were repeated for five consecutive cycles.