Smart Materials for Environmental Remediation Based on Two-Component Gels: Room-Temperature-Phase-Selective Gelation for the Removal of Organic Pollutants Including Nitrobenzene/O-Dichlorobenzene, and Dye Molecules from the Wastewater
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Novel two-component gel systems based on aliphatic acid–hydroxy/base interaction were developed as smart materials for environmental remediation. The G1-A16 gelator could be used directly as a powder form to selectively gel aromatic solvents (nitrobenzene and o-dichlorobenzene) from their mixtures with wastewater (containing 0.5 M sodium nitrate and 0.5 M sodium sulfate) via a simple shaking strategy at room temperature without employing co-solvents and a heating–cooling process. Meanwhile, the two-component gel system can efficiently remove the toxic dyes from the aqueous solution. The dominant factors that drive gelation in the case of the gelator and nitrobenzene or water have been studied using FT-IR, 1H NMR, and XRD. Overall, our research provides an efficient two-component approach for facilely tuning the properties of one-component gel for the realization of high-performance functionalities of gels. At the same time, our study demonstrates potential industrial application prospect in removing pollutants efficiently (such as aromatic solvents and toxic dye removal).
KeywordsRoom-phase-selective-gelator (RPSG) Self-healing Dye removal
- 1H NMR
H nuclear magnetic resonance
Field-emission scanning electron microscope
Fourier transform infrared spectroscopy
Powder X-ray diffraction
The multicomponent supramolecular gel systems, as intriguing soft materials formed via H-bonding, donor–acceptor, metal ion coordination, and acid–base interactions, provide a flexible method to realize the functionalities of gels [1, 2, 3]. They have attracted more and more attention owing to their potential applications in the fields such as pollutant collection [4, 5, 6], oil spill treatment [5, 7], and advanced materials.
Dyes are useful in the textile industry (e.g., paints, printing, drugs, and cosmetics). Most dyes are non-biodegradable and even at low concentrations, dyes still threaten the environment and the ecosystem [30, 31, 32, 33, 34]. Eliminating toxic dyes from the polluted water is therefore an important goal.
We recently reported the amine-acid two-component gelators Bn-Am (Scheme 1), which exhibited excellent gelation abilities towards certain organic solvents. Herein, based on these previous works [27, 28, 35], a few novel D-gluconic acetal-based derivatives Gn with a free amino or hydroxy group at the terminal position of a long alkyl chain and amino group in the middle of a long carbon chain were designed and synthesized. The amino or hydroxy group provides sites for H-bond interaction with aliphatic acids and for the formation of two-component gelators (Scheme 1).
The gelation properties and the gel performance could be flexibly tuned by changing the chain length of the alkyl group in aliphatic acid. Surprisingly, compared with Bn-Am, the two-component gel system Gn-Am could gel not only some organic solvents but also water at room temperature. Moreover, as shown in Additional file 1: Table S5, Bn-Am could gel organic solvents which have smaller densities than water (e.g., toluene, o-xylene, 1, 3, 5-trimethylbenzene and ethylbenzene) and Gn-Am showed strong gelation properties towards organic solvents which are heavier than water (e.g., nitrobenzene, o-dichlorobenzene, dichloromethane). Additionally, Gn-Am showed strong abilities to selectively gel nitrobenzene/o-dichlorobenzene from the mixture with wastewater (containing 0.5 M sodium nitrate and 0.5 M sodium sulfate) without employing co-solvents and a heating-cooling process. Moreover, the gel system Gn-Am at room temperature displayed high-efficiency self-healing properties. To the best of our knowledge, it was the very few examples of two-component gelators reported to be excellent room-temperature PSGs that could gel organic solvents and water. It was also the first example of a two-component gel system reported as excellent room-temperature PSGs used to remove nitrobenzene/o-dichlorobenzene from the wastewater directly in a powder form. Furthermore, other excellent functions of two-component gels including dye removal are also demonstrated. These findings provide a simple method for the design of multifunctional supramolecular gelators via an effective two-component gel strategy.
D-Gluconic acid, 3, 4-dichlorobenzaldehyde, and β-hydroxyethylenediamine were purchased from Shanghai Jingchen Scientific Co., Ltd. The chemical reagents were commercially available and directly utilized without further purification. 2, 4-(3, 4-Dichloro) benzylidene methyl-d-gluconate was synthesized by the methods reported previously. . Characterizations of a new compound Gn are provided in Additional file 1. Synthetic routes of Gn are shown in Additional file 1: Scheme S1. The detailed synthetic procedures and characterization data of Gn are given in Additional file 1.
Preparation of the Two-Component Hydrogels
Gn (5 mmol) and aliphatic acid (5 mmol) were simultaneously added to 10 mL methanol. The resultant mixture was subsequently heated to reflux for 10 min leading to a clear solution and finally a white or faint yellow solid via vacuum evaporation. A certain amount of the two-component gelator was weighed in a test tube. The corresponding solvents was subsequently added, which was shook for 1 min and then the test tube stood for 8 h. Finally, the test tube was inversed to observe whether the solution inside could still flow . Gelation was considered to have occurred when a homogeneous substance was obtained which exhibited no gravitational flow, and it was denoted by “G”. Solution and solid-like gel may coexist within a system as “partial gels (PG)”. Systems, in which only solution was obtained, were referred to as solution (S). In an insoluble system (I), gelators could not be dissolved. The critical gelation concentrations (CGCs) mean the minimum amount of gelators required to immobilize 1 mL of solvent.
Field-emission scanning electron microscope (FESEM)
The morphologies of the xerogels were obtained by a Hitachi S-4800 SEM instrument operating at 3–5 kV. Samples were prepared by dropping the diluted solution of gels on the thin aluminum sheets and then dried under vacuum for 24 h. We coated the samples with a thin layer of Au before the experiment.
IR spectra were collected by a FTS3000 spectrometer with KBr pellets. The xerogels were prepared by drying chlorobenzene gels on glass slides under vacuum for 24 h.
Powder X-Ray Diffraction (PXRD)
PXRD diagrams of xerogels which were prepared from hydrogels were obtained by using a Bruker D8-S4 (CuKα radiation, λ = 1.546 Å). The d spacing values were calculated by Bragg’s law (nλ = 2d sinθ).
Rheology experiments were carried out with a strain-controlled rheometer (Anton Paar Physica MCR 301) equipped with steel-coated parallel-plate geometry (15 mm diameter). The gap distance was fixed at 0.5 mm. A solvent trapping device was placed above the plate and measurement was set at 20 °C in order to avoid solvent evaporation. The frequency sweep at a constant strain of 0.1% was obtained from 0.1 to 100 rad s− 1. Strain sweep was performed in the 0.01–1000% range at a constant frequency (1 Hz). The time sweep was conducted to observe the recovery property of the gel. First, a constant strain of 0.1% was applied on the sample. Then a constant strain of 100% was applied to destroy the sample. And then a constant strain (0.1%) was applied again. The storage modulus G’ and the loss modulus G” of the sample were monitored as functions of time in this experiment.
Dye Removal Experiments
Five milligrams of the xerogels (G1-Am) prepared from the hydrogels by freeze drying was immersed in a single dye solution (5 mL, 0.1 mM) for 24 h containing the anionic dyes (e.g., acid fuchsin (AF), eosin Y (EY), methyl orange (MO)) or cationic dyes (e.g., malachite green (MG), methylene blue (MB), Rhodamine (RB)). The resultant mixture was subsequently centrifuged, and the concentration of dyes in the supernatant was monitored by UV–vis spectroscopy. The removal rate was calculated as (C0 − Ce)/C0, where C0 (mg L− 1) was the initial concentration of dye in the solution and Ce (mg L− 1) was the equilibrium concentration. The maximum amount of dyes adsorbed at equilibrium qe (mg g− 1) was calculated as qe = (C0 − Ce) × V/W, where V (L) was the solution volume and W (g) was the mass of xerogels. The changes in the dye concentration were also monitored by UV–vis spectroscopy. The adsorption isotherm was determined by immersing 5 mg of the xerogel into a MO or CV solution with varying concentrations for a week, and subsequently calculating the equilibrium adsorption capacity and concentration.
Results and Discussion
The Gelation Abilities of the Two-Component Gel System at Room Temperature
Rheology of Self-Sealing Two-Component Gel System
Room-Temperature-Phase-Selective Gelation in the Removal of Nitrobenzene
Removal of Dyes
Powder X-Ray Diffraction (PXRD)
The XRD patterns of G1-A16 nitrobenzene xerogel showed d spacing values of 1.25, 0.64, 0.43, and 0.32 nm (Fig. 5e) in a ratio of 1:1/2:1/3:1/4, indicating the lamellar arrangements [46, 47]. Similarly, the XRD patterns of G1-A16/water xerogel and G1-A16 powder suggested the lamellar arrangements. Additionally, the d values of 0.38 nm was the characteristic of π–π stacking force of the benzene rings [48, 49, 50]. Moreover, the d value of 0.43 nm was ascribed to the packing of the alkyl chains [51, 52]. It reveals that the driving forces for the self-assembly involve the π–π stacking force of the benzene rings and van der Waals force of the alkyl chains in the solution system.
In order to gain further understanding of the possible driving force in the self-assembly of G1-A16 for gel formation, temperature-dependent 1H NMR spectroscopic studies were also performed in D2O-d6. As shown in Fig. 5f, 1H NMR spectra of the G1-A16 (2.0% w/v) in D2O at different temperatures were compared. One of the H-shifts on the benzene ring of G1-A16 hydrogel appeared at 8.288 ppm in pure D2O at 80 °C. The aromatic proton signals showed an up-field shift (blue line in Fig. 5f) when the temperature decreased, which provided the support for the existence of π–π interactions between the phenyl groups of G1-A16 in the gel state. Concurrently, the 1H NMR spectral signals of the alkyl chain protons were observed to shift up-field when the temperature reduced (orange line in Fig. 5f). Accordingly, these results reveal that the main driving force for the self-assembly of G1-A16 in water is the combined interactions of π–π and van der Waals.
These results suggested that the excellent properties of the two-component G1-Am gels originated from their highly ordered structures which formed based on the synergistic effect of intermolecular hydrogen bonding, van der Waals, and π–π stacking.
In summary, we have designed a novel multifunctional two-component gel system, which exhibited highly efficient self-healing and room temperature-phase-selective properties, and potential applications in the fields of waste water treatment. The viscoelasticity and self-healing properties of gels can be successfully tuned by changing the length of the alkyl chain of the aliphatic acid component. Surprisingly, the G1-Am gel system could gel four organic solvents and water at room temperature. Moreover, the powders of the G1-A16 could directly gel nitrobenzene/o-dichlorobenzene from their biphasic mixtures with wastewater at room temperature via simple mechanical shaking. In addition, the xerogels obtained from G1-A16 gel can be used to effectively remove toxic dyes (anionic dyes: AF, EY; cationic dyes: MG, MB, RB) from their aqueous solutions. Further studies on the relationship of gel properties and the component structure and exploring applications of these materials are still in progress.
All authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (grant nos. 21676185) in the design of the study, collection, and analysis. The fundings from Tianjin science and technology innovation platform program (no. 14TXGCCX00017) and Tianjin municipal education commission scientific research project (no. 2018KJ265) are also acknowledged.
This research was supported financially by the National Natural Science Foundation of China (grant nos. 21676185), the Tianjin science and technology innovation platform program (no. 14TXGCCX00017) and Tianjin municipal education commission scientific research project (no. 2018KJ265).
Availability of data and materials
All data generated or analyzed during this study are included in this published article and in Additional file 1.
JZ conceived and carried out the experiments, analyzed the data, and wrote the manuscript. JL, CT, SC, and BaohaoZ carried out the experiments and analyzed the data. BaoZ and JS designed the study, analyzed the data, and wrote the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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