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. 2020 Mar 21;5(12):6566–6575. doi: 10.1021/acsomega.9b04304

Green Fabrication of Tannic Acid-Inspired Magnetic Composite Nanoparticles toward Cationic Dye Capture and Selective Degradation

Yihui Qian †, Shengqiu Chen †,‡, Chao He †, Chen Ye §, Weifeng Zhao †, Shudong Sun †, Yi Xie †,*, Changsheng Zhao †,∥,*
PMCID: PMC7114688  PMID: 32258892

Abstract

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An environmental strategy for developing sustainable materials presents an attractive prospect for wastewater remediation. Herein, a facile, green, and economical strategy is proposed to fabricate magnetic composite nanoparticles (NPs) toward cationic dye adsorption and selective degradation. To prepare the composite TiO2-PEI-TA@Fe3O4 NPs, tannic acid (TA) and polyethyleneimine (PEI) were first used to decorate Fe3O4 NPs at aqueous solution, and then TiO2 NPs were anchored onto the surfaces of Fe3O4 NPs based on the catecholamine chemistry. The chemical composition and microstructure of the obtained NPs were systematically characterized. The NPs not only exhibited adsorption ability for the cationic dye of methylene blue (MB) but also responded to ultraviolet light to selectively degrade the adsorbed MB, and the removal (adsorption and/or degradation) ratio for MB could reach 95%. In addition, cyclic experiments showed that the removal ratio of the composite NPs for MB could still be maintained more than 85% even after five cycles. Given by the above-mentioned advantages, such a green and facile strategy for combining the adsorption and degradation methods to construct magnetic nanocomposites exhibits potential applications in cationic dye selective removal and sustainable wastewater remediation.

Introduction

Water is an extremely valuable natural resource for maintaining a stable natural environment, meeting the basic needs of life and promoting the development of the human society.1 However, water pollution reduces the use value and even does harm to human health.2 Dyes account for a large proportion of organic pollutants in water environment. In practice, both the produced and applied processes of dyes will cause the dye enter the water environment and then pollute the water body. The environmental pollution by dyes would induce carcinogenicity (such as dyes of benzidine series),3 change the water color which hinders the penetration of sunlight in water and weakens the photosynthesis of aquatic organisms,4 consume oxygen while dyes are decomposed by microorganisms and result in anoxic water body, and affect the growth of aquatic animals and plants.5 Meanwhile, most dyes can cause irritation to human skin and mucosa, have influences on the respiratory system, and even cause cancer.6−8 Thus, developing efficient and convenient methods to deal with dyes in wastewater has gained worldwide attention.

There have been many studies relating to sewage treatment. Owing to the easy operation and simple design, the adsorption method to deal with wastewater has been widely used.9−11 Song et al. fabricated amino-coated particles which exhibited diverse adsorption capacities to Congo red, Cu2+, and bilirubin.12 Xu et al. took advantages of the surface migration strategy of functional groups to fabricate an ultrafast adsorption nanofibrous membrane for removing organic dyes.13 When these materials are reused, acid, alkali, and other organic solvents are often added for the desorption of dyes from the adsorbents, and the acidic or alkaline wastewater still needs to be treated.14,15 Thus, if adsorbents can be reused without adding other reagents, it will have greater potential for application in sustainable wastewater treatment.

Another method for sewage treatment is to decompose toxic substances into harmless compounds directly.16 Semiconductor photocatalysts are widely used to decompose dyes,17−19 and many semiconductor materials, such as titanium dioxide (TiO2), zinc sulfide (ZnS), and zinc oxide (ZnO), have been reported to degrade complex chemicals, especially dyes.20,21 Possessing ideal properties such as nontoxicity and low cost and displaying a photocatalytic activity under the irradiation of ultraviolet light (UV light),22−24 TiO2 nanoparticles (TiO2 NPs) also have the ability to decompose dyes into soluble nontoxic ions and small molecules.25 Thanks to the large specific surface area, TiO2 NPs have been studied in the field of water treatment.26,27 However, the separation of TiO2 NPs from the water is too cumbersome, limiting the application of TiO2 NPs in commercial or industrial water treatment.28 Many researches have been reported on the immobilization of TiO2 NPs onto the surfaces of silicon wafers, glasses, films, and other macroscopic materials;29−31 however, the immobilization will reduce the useable area of TiO2 NPs. Furthermore, the recycling processes, such as filtration and centrifugation, require complex operations and postprocessing.32,33 The other way is synthesizing magnetic TiO2 NPs which contain magnetic cores, and the obtained composite NPs can be simply separated from suspensions by magnetic fields.34

Fe3O4 NPs have attracted much attention because of their outstanding magnetic property, low toxicity, and high chemical stability.35 Based on Fe3O4 NPs and TiO2 NPs, magnetic composites have also been studied. Liu et al. prepared Fe3O4/TiO2/C nanocomposites, which could catalyze the decomposition of H2O2 and the subsequently generated radicals could almost completely oxidize methylene blue (MB).36 In this study, TiO2 NPs were precipitated onto Fe3O4 cores via the hydrolysis of titanium salt. For loading TiO2 NPs onto the insulated carbon surface of the magnetic core (Fe3O4/C), Aghamali et al. employed a vapor phase hydrolysis method.37 Liu et al. prepared multifunctional composite microspheres with spinel Fe3O4 cores and anatase TiO2 shells by combining the solvothermal reaction and calcination process.38 In these studies, Fe3O4 NPs and TiO2 NPs were obtained through heat treatment or hydrolysis processes, and high temperature (450 °C) was needed.

Based on the catecholamine chemistry, polyphenol and its derivatives have been shown to adhere onto a variety of material surfaces through a simple one-step process, such as 2D or 3D porous materials’ superhydrophobic or superoleophobic surfaces, and this method has been identified as green surface chemistry.39−41 More importantly, the catecholamine layer shows intrinsic chemical reactivity because of the existence of catechol quinone moieties and catechol radical species.42 Recently, tannic acid (TA) has been investigated as a functional natural material for surface/interface modification owing to the high content of catechol/pyrogallol groups,43,44 and the catechol/pyrogallol groups made TA negatively charged. By cooperating with FeIII, TA could adhere easily on multi interfaces within 20 s for post-functionalization.45 Liu et al. modified the oxidized graphene surface with TA as the reducing agent and capping agent to change the physicochemical properties of the graphene surface and improve the adsorption efficiency for rhodamine via the intensive π–π stacking interaction and electrostatic attraction.46 Kim et al. used iron(III) and TA coordination chemistry to modify the surface of polydopamine (PDA); poly(ethylene glycol) was fixed on the surface of PDA via the hydrogen bond interaction with TA, and then the decorated PDA was fixed on the surfaces of stainless steel and nylon to inhibit the adhesion of diatoms for marine antifouling applications.47

Herein, based on the catecholamine chemistry, we anticipated employing a facile, green, economical, and energy-saving method to prepare magnetic composite NPs. Cationic dyes would be adsorbed on the surface of the NPs by TA and then degraded by TiO2 NPs. The magnetic composite NPs could be recycled by a magnetic field. Thus, the composite NPs have self-cleaning, reusable abilities. In this study, Fe3O4 NPs were employed as the cores of composite NPs. TA and polyethyleneimine (PEI) were employed to decorate the Fe3O4 NPs at room temperature, and TiO2 NPs were anchored onto the surface of the Fe3O4 NP core to prepare composite TiO2-PEI-TA@Fe3O4 NPs. The compositions of the obtained composite NPs were characterized using X-ray photoelectron spectroscopy (XPS), energy-dispersive spectroscopy (EDS), and Fourier transform infrared spectroscopy (FTIR). The microstructure was investigated via transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Besides, a vibrating sample magnetometer was used to test its magnetic properties. Furthermore, the adsorption and degradation abilities for dyes were systematically studied, and the recycle property was also investigated.

Experimental Section

Preparation of TA@Fe3O4 NPs

Fe3O4 NPs were immersed in aqueous TA solution for 5 min with shaking. Then, the NPs were transferred into aqueous FeCl3 solution and shaken for another 5 min. After that, the obtained NPs were washed several times with deionized (DI) water and named TA@Fe3O4 NPs.

Preparation of PEI-TA@Fe3O4 NPs

TA@Fe3O4 NPs were immersed into a PEI phosphate-buffered solution (pH = 8.5). After shaking overnight, the obtained NPs were washed several times with DI water and named PEI-TA@Fe3O4 NPs.

Preparation of TiO2-PEI-TA@Fe3O4 NPs

PEI-TA@Fe3O4 NPs were added to a suspension of TiO2 NPs. After shaking for 12 h, the NPs were washed several times with DI water to remove the unstable TiO2. Finally, the magnetite composite TiO2-PEI-TA@Fe3O4 NPs were obtained.

The obtained NPs were characterized and used for the dye removal experiment. Materials, preparation, and characterization of methods, dye adsorption experiments, and degradation experiments are all supplied in the Supporting Information.

Results and Discussion

Preparation and Characterizations of TiO2-PEI-TA@Fe3O4 NPs

To prepare the UV and magnetic dual-responsive TiO2-PEI-TA@Fe3O4 NPs, TA molecules with a high content of catechol/pyrogallol groups were first coated onto the surface of the pure Fe3O4 NPs through the strong interaction between the FeIII–TA complex and the material surface,48,49 and then the amino group-enriched PEI molecules were further introduced to form a stable functional layer via the Schiff-based reaction and Michael addition between TA and PEI,50−52 as shown in Figure 1a, and the obtained samples were named PEI-TA@Fe3O4 NPs. Finally, the target TiO2 NPs were anchored onto the surfaces of PEI-modified TA@Fe3O4 NPs,53 and the obtained composite NPs were named TiO2-PEI-TA@Fe3O4 NPs.

Figure 1.

Figure 1

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(a) Illustration of the preparation route of TiO2-PEI-TA@Fe3O4 NPs. (b–d) TEM images of Fe3O4, PEI-TA@Fe3O4, and TiO2-PEI-TA@Fe3O4 NPs. (e) High-resolution scanning TEM elemental mapping images for TiO2-PEI-TA@Fe3O4 NPs. (f) Powder XRD patterns, (g) FTIR, and (h) XPS wide spectra of TiO2, Fe3O4, PEI-TA@Fe3O4, and TiO2-PEI-TA@Fe3O4 NPs, respectively.

The morphologies for the as-prepared magnetic NPs were observed by TEM and SEM, as shown in Figures 1b and S1. The pure Fe3O4 NPs presented the average diameter of around 100 nm and a clear single-layer structure. After being modified with TA and PEI, a remarkable shell layer (around 5 nm in thickness) on the surface of Fe3O4 NPs is observed, as shown in Figure 1c. For TiO2-PEI-TA@Fe3O4 NPs, as shown in Figure 1d, high-resolution TEM images further disclose the NP-anchored structure such that many grain-shaped particles with an average diameter of around 25 nm were aggregated onto the surface, which were consistent with the SEM results. Moreover, elemental mapping analysis as presented in Figure 1e provided direct evidence to confirm the introduction of TiO2 NPs, where Ti and N elements were distributed onto the surface, whereas the Fe element was only probed in the core. Furthermore, the lattice spacing of 0.356 nm corresponded to the (101) plane of the anatase TiO2,54 whereas the lattice spacing of 0.245 nm corresponded to the (004) plane of anatase TiO2.55 The planes were also detected by XRD, as shown in Figure 1f. The obvious diffraction peaks for Fe3O4, PEI-TA@Fe3O4, and TiO2-PEI-TA@Fe3O4 NPs at 2θ = 30.1, 35.5, 43.2, and 57.0° were ascribed to the (220), (311), (400), and (511) planes of Fe3O4, respectively. The diffraction peaks for TiO2-PEI-TA@Fe3O4 NPs at 2θ = 25.3, 38.0, 47.9, 54.2, and 68.9° were ascribed to the (101), (004), (200), (211), and (116) planes of TiO2 NPs, respectively.56 The results revealed that the original phases of Fe3O4 and TiO2 within composite NPs were not changed after coating.

To further verify the chemical structures and detailed composition of these magnetic Fe3O4 NPs, FTIR, XPS, EDS, and zeta potential tests were performed. As shown in Figure 1g, in contrast to the original Fe3O4 NPs, three peaks were observed on the spectra of PEI-TA@Fe3O4 NPs and TiO2-PEI-TA@Fe3O4 NPs. These three peaks at 475, 584, as well as 2920 and 2979 cm–1 were ascribed to Ti–O bond vibration from TiO2 NPs,37 Fe–O stretching vibration from Fe3O4 NPs, and the −CH2– stretching vibration from PEI, respectively. Moreover, the Fe, N, and Ti signals were probed from the XPS survey scan, as shown in Figure 1h. As expected, the Fe 2p peak was observed on Fe3O4, PEI-TA@Fe3O4, and TiO2-PEI-TA@Fe3O4, while the N 1s peak was observed only for PEI-TA@Fe3O4 and TiO2-PEI-TA@Fe3O4 NPs, ascribed to the introduced PEI. After anchoring TiO2 NPs, the Ti 2p and Ti 2s peaks were probed only on TiO2-PEI-TA@Fe3O4 NPs. The estimated content of the Ti element in TiO2-PEI-TA@Fe3O4 NPs was 16.8 at. %, as listed in Table S1. Furthermore, in the Ti 2p spectra, two characteristic peaks of TiO2 located at 458.3 and 464.0 eV36 were probed for TiO2-PEI-TA@Fe3O4 NPs in Figure 2a. The corresponding Fe 2p spectra of TiO2-PEI-TA@Fe3O4 NPs are presented in Figure S2; Fe atoms were composed of Fe3+ and Fe2+, which were provided by Fe3O4 and FeCl3. Meanwhile, the high-resolution XPS O 1s spectra in Figure 2b can be fitted to three peaks (Fe–O, C–O, −OH)57,58 for TA@Fe3O4 NPs and PEI-TA@Fe3O4 NPs, while only two peaks could be probed on the pure Fe3O4 NPs: Fe–O group (529.4 eV) and surface hydroxyl (−OH, 530.5 eV) groups. Furthermore, in contrast to Fe3O4 and PEI-TA@Fe3O4 NPs, the Ti signal was probed for TiO2-PEI-TA@Fe3O4 NPs by the EDS pattern, as shown in Figures 2c and S3. The corresponding element contents determined by the EDS pattern are provided in Figure 2d, which was consistent with the result estimated by XPS analysis. Overall, these results indicated the successful preparation of TiO2-PEI-TA@Fe3O4 NPs.

Figure 2.

Figure 2

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(a) High-resolution XPS Ti 2p spectra of TiO2-PEI-TA@Fe3O4 NPs. (b) High-resolution XPS O 1s spectra of Fe3O4, TA@Fe3O4, and PEI-TA@Fe3O4 NPs. (c) EDS pattern of TiO2-PEI-TA@Fe3O4 NPs. (d) Elemental contents estimated by EDS and (e) hysteresis loops of Fe3O4, PEI-TA@Fe3O4, and TiO2-PEI-TA@Fe3O4 NPs. (f) Zeta potential for Fe3O4, TA@Fe3O4, PEI-TA@Fe3O4, and TiO2-PEI-TA@Fe3O4 NPs.

Furthermore, to investigate the recyclability by magnetism of TiO2-PEI-TA@Fe3O4 NPs, the hysteresis loop tests were performed. As shown in Figure 2e, both Fe3O4 and TiO2-PEI-TA@Fe3O4 NPs presented little hysteresis, coercivity, and remanence, which suggested that Fe3O4 and TiO2-PEI-TA@Fe3O4 NPs were superparamagnetic. The saturation magnetization of Fe3O4 NPs was 68.24 emu/g; however, the saturation magnetization of TiO2-PEI-TA@Fe3O4 NPs was 46.54 emu/g, and the decrease was caused by the nonmagnetic TiO2 NPs. Nevertheless, the magnetism of TiO2-PEI-TA@Fe3O4 NPs was still high enough for separation.59 As shown in the inserted picture, TiO2-PEI-TA@Fe3O4 NPs could be conveniently separated from solution through a magnet, which made it possible to reuse the magnetic composite NPs after being used for dye removal in wastewater.

Additionally, the preparation of TiO2-PEI-TA@Fe3O4 NPs was monitored by the zeta potential measurement, as shown in Figure 2f. In contrast to the raw Fe3O4 NPs with a zeta potential of around +12.3 mV, the zeta potential of TA@Fe3O4 NPs was found to be −28.8 mV. As expected, PEI-TA@Fe3O4 NPs exhibited a positive zeta potential (+25.3 mV), which was attributed to the introduction of the amino group-enriched PEI molecules. Importantly, TiO2-PEI-TA@Fe3O4 NPs had a negative zeta potential of around −12.1 mV, which has better affinity toward cationic dye molecules than anionic dye molecules. This contributed to the capture of cationic dyes and further degradation for wastewater remediation.

Single Dye Removal

As mentioned above, TiO2-PEI-TA@Fe3O4 NPs presented a negative zeta potential; thus, the NPs should have better affinity and higher removal ability toward cationic dyes than anionic dyes, as illustrated in Figure 3a. To verify this view, the different single dyes including three kinds of cationic dyes [methyl violet (MV), rhodamine B (RhB), and MB] and two kinds of anionic dyes [methyl orange (MO) and amaranth (AR)] were utilized to investigate the removal properties of TiO2-PEI-TA@Fe3O4 NPs. When the adsorption of dyes reached the equilibrium in the dark, all the test vessels were irradiated by the UV light and TiO2-PEI-TA@Fe3O4 NPs were excited to degrade the dyes. As shown in Figure 3b, TiO2-PEI-TA@Fe3O4 NPs presented 95, 66, 36, 10, and 5% removal ratios toward MB, MV, RhB, AR, and MO, respectively, which confirmed the higher removal ability of TiO2-PEI-TA@Fe3O4 NPs toward cationic dyes than anionic dyes. The result was further confirmed by the color change of the dye solution such that the color of the single MB solution changed from deep blue to colorless, while that for the single MO solution was basically unchanged, as shown in the inserted image. The corresponding UV–vis spectra variations are provided in Figure 3c,d. Obviously, TiO2-PEI-TA@Fe3O4 NPs had more significant removal ability toward the cationic dye MB than the anionic dye MO. As a control, the pure TiO2 NPs were also used to remove all the above-mentioned dyes under UV light, and the corresponding UV monitoring spectra are shown in Figure S4. The pure TiO2 NPs presented 90, 78, 73, 86, and 74% removal ratios toward MB, RhB, MV, AR, and MO, respectively. There was no clear difference in affinity between the cationic and anionic dyes by the pure TiO2 NPs under UV light. These results demonstrated that TiO2-PEI-TA@Fe3O4 NPs had higher removal ability toward the cationic dyes than the anionic dyes.

Figure 3.

Figure 3

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(a) Schematic diagram for the single cationic and anionic dye removal under UV light. (b) Removal ratios for different dyes in the dark and under UV light. UV–vis spectra of the (c) MB and (d) MO solutions before and after being UV-treated at different times, respectively. (e) Removal ratios toward MB removal in the dark and under UV light by different magnetic NPs. (The initial concentrations of the above-used dye solutions were 50 μmol/L.)

Moreover, the removal ratios toward MB by different magnetic NPs in the dark and under UV light were also investigated. In the dark, the samples only had adsorption ability but would present degradation property toward the dyes under UV light. To confirm this, as shown in Figures 3e and S5, the adsorption and degradation capacities of MB by different NPs were studied, respectively. Among these magnetic NPs, TA@Fe3O4 NPs presented the highest adsorption ability with the removal ratio as high as 94% toward MB in the dark. However, the removal ratio of TA@Fe3O4 NPs was nearly zero under UV light, indicating that TA@Fe3O4 NPs had no degradation ability. In contrast, TiO2-PEI-TA@Fe3O4 NPs presented the highest removal ratio up to 71% under UV light, which demonstrated that the dye removal by the magnetic NPs was mainly attributed to the loaded TiO2 NPs.

The above phenomenon could be explained as follows: as seen from the zeta potential data, TiO2-PEI-TA@Fe3O4 NPs presented a negatively charged surface, which presented a high adsorption ability toward the cationic dye MB because of the charge attraction; after reaching the adsorption equilibrium, the degradation ability of TiO2-PEI-TA@Fe3O4 NPs dominated. The mechanism for the degradation of organic dyes by the TiO2 NPs has been reported in many literature studies,60−62 and the mechanism of photocatalytic degradation of MB by TiO2 is shown in the Supporting Information. TiO2 can absorb photons and be excited to produce a series of free radicals under UV light. The organic dyes (MB) are either attacked by hydroxyl radicals or degraded after being trapped and oxidized by holes directly.63 The degradation mechanism of the composite magnetic NPs is shown in Figure 4a. The produced electrons were reacted with holes onto the surface of TiO2-PEI-TA@Fe3O4 NPs, and the molecules of the cationic dye gathered around the magnetic particles, which were easier to be degraded, leading to the high removal ratio toward dyes. For comparison, the removal ability toward MB by TiO2-PEI-TA@Fe3O4 NPs with different formulations was also studied, and the results are shown in Table S2 and Figure S6. Figure S6a shows that the amount of TiO2 is a key element for the removal efficiency. With the increase of the TiO2 NP content, the removal efficiency toward MB increased. Especially when the Fe3O4/TiO2 molar ratio decreased to 1:1 (or 1:3), the removal ratio increased to over 95%. However, as shown in Figure S6b, when very high TiO2 ratio was used (as the Fe3O4/TiO2 ratio was 1:3), compared to that with the ratio of 1:1, the reusability was weakened.

Figure 4.

Figure 4

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(a) Photocatalytic mechanism toward MB degradation by TiO2-PEI-TA@Fe3O4 NPs under UV irradiation. (b) Adsorption amounts of TiO2-PEI-TA@Fe3O4 NPs with different initial concentrations toward MB; applications of (c) Langmuir and (d) Freundlich isotherm models; applications of (e) pseudo-first-order and (f) pseudo-second-order at different initial MB concentrations; and (g) calculated parameters of kinetic models for MB adsorption.

As a control, the adsorption behavior toward MB by TiO2-PEI-TA@Fe3O4 NPs was systematically studied in the dark. As shown in Figure 4b, the adsorption capacity was increased with the increase of MB concentration because of the stronger mass-transfer force at a high concentration. It was also suggested that the equilibration time increased at higher concentration, and all the adsorption reached the equilibrium within 2 h. Moreover, regardless of the concentration, the adsorption rate in the initial stage was faster. This phenomenon could be explained by the fact that much more vacant adsorbent sites and a stronger mass-transfer driving force existed at the initial stage.64 Moreover, the Langmuir and Freundlich isotherm models were employed to study the equilibrium data, and the detailed equations and parameters are provided in the Supporting Information. Figure 4c,d shows that the adsorption data fitted well with both the Langmuir adsorption isotherm and Freundlich adsorption isotherm. Therefore, the adsorption process is a monolayer adsorption process. Furthermore, the adsorption processes of TiO2-PEI-TA@Fe3O4 NPs toward MB were analyzed by pseudo-first-order and pseudo-second-order kinetic models, and the detailed equations and parameters are presented in the Supporting Information. The linear plots of different models are shown in Figure 4e,f. According to the linear equations, the constants were calculated from the slope and intercept. The r2 values of the pseudo-second-order kinetic model were all higher than 0.99 as summarized in Figure 4g, which indicated that the pseudo-second-order kinetic model was more suitable to the experimental data.

Binary Dye Removal

To investigate the removal behavior of TiO2-PEI-TA@Fe3O4 NPs toward mixed dyes (cationic dye and anionic dye), the above-mentioned degradation procedures were performed using the mixed solution of MB/MO and MB/MV. As a control, as shown in Figure 5a, the pure TiO2 NPs presented 77 and 67% removal ratios toward MB and MO, respectively. For TiO2-PEI-TA@Fe3O4 NPs, as shown in Figure 5b, after being adsorbed and exposed to UV light for 12 h, the color of the MB/MO mixed dye solution changed from green to yellow, which was almost the same as that of the single MO solution. Seen from the corresponding UV–vis spectra of the MB/MO solutions before and after UV light irradiation, the peak of the maximum UV absorption wavelength of MB gradually disappeared under UV irradiation, while the maximum UV absorption wavelength of MO did not change significantly. The removal ratios for MB and MO were 92 and 4%, respectively. It suggested that the cationic dye MB was selectively adsorbed and degraded by TiO2-PEI-TA@Fe3O4 NPs, while the anionic dye MO was barely removed. Furthermore, the adsorption and degradation behaviors toward the cationic/cationic mixed dye of MB/MV were also investigated by TiO2-PEI-TA@Fe3O4 NPs. As shown in Figure 5c, the removal ratios toward MB and MV were as high as 58 and 53%, respectively, which suggested that TiO2-PEI-TA@Fe3O4 NPs presented adsorption and degradation properties for both MB and MV. These results demonstrated that TiO2-PEI-TA@Fe3O4 NPs had selective degradation ability only toward cationic dyes for the cationic/anionic mixed dye solution and no selective degradation ability for the same charged mixed dye solution.

Figure 5.

Figure 5

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Photographs of the mixed dye solutions of MB/MO and corresponding UV–vis spectra before and after being degraded by (a) pure TiO2 NPs and (b) TiO2-PEI-TA@Fe3O4 NPs, respectively. (c) Photographs of the mixed dye solution of MB/MV and corresponding UV–vis spectra before and after being degraded by TiO2-PEI-TA@Fe3O4 NPs. (d) Schematic illustration of degradation toward mixed dyes. (e) Effects of dye solution concentration on the degradation behavior. (f) Application of the Langmuir–Hinshelwood kinetic model for the photodegradation process with different concentrations. (g) Corresponding parameters of the Langmuir–Hinshelwood kinetic model toward MB degradation.

Different from the separation by the dye molecule size as reported by Ran et al.,65 the difference in charges of the dyes is the main cause of selective degradation in our study, as illustrated in Figure 5d. The removal ratio toward MB by TiO2-PEI-TA@Fe3O4 NPs was a combined result of adsorption and degradation, and TiO2-PEI-TA@Fe3O4 NPs have the selective degradation ability toward cationic dyes.

To investigate the photocatalytic degradation behavior, TiO2-PEI-TA@Fe3O4 NPs were first immersed into the MB solution (50 μmol/L) in the dark until the adsorption equilibrium was reached. Then, the adsorbed-TiO2-PEI-TA@Fe3O4 NPs were moved into different concentrations (20, 50, 100 μmol/L) of MB solutions under UV light for degradation experiments. As shown in Figure 5e, the concentration of MB solution decreased with time under UV light. In this process, TiO2-PEI-TA@Fe3O4 NPs acted as the photocatalyst, catalyzing the degradation of MB. The degradation process of TiO2-PEI-TA@Fe3O4 NPs was analyzed by fitting with the Langmuir–Hinshelwood equation as follows

graphic file with name ao9b04304_m001.jpg 1

where CDo (μmol/L) is the concentration of MB before degradation and CDt represents the concentration of MB at different time intervals and kD represents the rate constant of degradation (h–1). The fitting curves of Langmuir–Hinshelwood of the degradation process are presented in Figure 5f; and the corresponding parameters are summarized in Figure 5g. The kD value reflected the catalytic efficiency, and with the increase of concentration, the kD value decreased. With the increase of concentration, the r2 increased, and when the concentration was 100 μmol/L, the r2 was 0.99. The results demonstrated that the MB degradation behavior by TiO2-PEI-TA@Fe3O4 NPs fitted well with the Langmuir–Hinshelwood kinetic model.

Recyclability

The stability and recyclability of composite magnetic NPs are very important parameters for wastewater treatment in practical application. The removal ability toward different concentrations of MB solutions by TiO2-PEI-TA@Fe3O4 was investigated. For each concentration, TiO2-PEI-TA@Fe3O4 NPs were used to adsorb MB in the dark for 2 h first, and then the test vessels were irradiated by UV light. As shown in Figure 6a, the removal ratios by UV degradation were significantly increased with the illumination time. Subsequently, TiO2-PEI-TA@Fe3O4 NPs were collected through a magnet and could be separated from the dye solutions, as shown in Figure 6b. The collected NPs were reused for adsorbing and degrading again. This process was repeated for five cycles, and the cycled-TiO2-PEI-TA@Fe3O4 NPs were detected by XRD. Considering the adsorption time and catalytic efficiency, 50 μmol/L of MB solution was chosen for the cycle experiments. As presented in Figures 6c and S7, the removal ratio toward MB was still up to 85% after five cycles of adsorption and degradation. The diffraction peaks for the cycled-TiO2-PEI-TA@Fe3O4 NPs matched well with the initial sample, which confirms the stability of TiO2-PEI-TA@Fe3O4 NPs. These results demonstrated the good recyclability of TiO2-PEI-TA@Fe3O4 NPs.

Figure 6.

Figure 6

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(a) Removal ratios of TiO2-PEI-TA@Fe3O4 NPs toward different concentrations of MB solutions; (b) schematic illustration of the recyclability of TiO2-PEI-TA@Fe3O4 NPs; and (c) removal ratios of five cycles toward MB (50 μmol/L).

Many researchers have been working on the methodology of adsorption, followed by photodegradation to produce materials with selective removal property to dyes. Here, some of the studies are compared. Table 1 shows the status of these studies, and Table S3 displays the cost parameters.

Table 1. Recent Studies on the Methodology of Adsorption, Followed by Photodegradation.

material preparation photocatalyst light collection refs
CuWO4, Cu3Mo2O9 Sonication CuWO4, Cu3Mo2O9 simulated daylight, 150 W centrifugation (66)
KTO molten salt flux method (850–900 °C) K2Ti6O13 simulated daylight, 300 W filtration (67)
AgX@MIL-101(Fe) microwave-solvothermal + precipitation method AgX + MIL-101(Fe) simulated daylight, 150 W centrifugation + filtration (68)
TiO2-PEI-TA@Fe3O4 commercial products + mild conditions TiO2 UV light, 48 W magnetic field  
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Some advantages of TiO2-PEI-TA@Fe3O4 NPs cannot be ignored: (1) industrial commodities are used to prepare target products in mild conditions, avoiding the complicated preparatory process; (2) commercial TiO2 NPs are cheap, and the power of the UV lamp is much lower than that of the solar simulator, which can avoid more energy consumption; (3) taking advantage of magnetic composite NPs for recycling rather than for filtration or centrifugation, TiO2-PEI-TA@Fe3O4 NPs have the potential for large-scale use.

Conclusions

In this study, TiO2-PEI-TA@Fe3O4 NPs were prepared through a polyphenol-inspired, facile, environmental-friendly, and economical method. Fe3O4 NPs acted as the cores, endowing the composite NPs with magnetic property and recyclability; TiO2 NPs acted as photocatalysts to degrade dyes; polyphenol coating was employed to combine two inorganic NPs together for the removal of cationic dyes. The removal ratio of TiO2-PEI-TA@Fe3O4 NPs toward MB, which included adsorption and degradation, could reach 95% and remained 85% even after five cycles. Owing to the adsorption ability, the NPs could selectively degrade the cationic dyes in the cationic and anionic mixed solution. Meanwhile, the degradation of MB eliminated the needs of acids, bases, or other organic substances to desorb MB, so troublesome post-processing could be avoided in adsorbent recovery, and it is anticipated that the NPs could have a great potential for cationic removal in wastewater treatment.

Acknowledgments

This work was financially sponsored by the National Natural Science Foundation of China (nos. 51503125, 51673125, 51873115, and 51903168). Y.X. acknowledges the support of China Postdoctoral Science Foundation (no. 2018M643485). We should also thank our laboratory members for their generous help and gratefully acknowledge the help of Hui Wang at Analytical and Testing Center in Sichuan University for the SEM observation.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b04304.

  • Scheme of synthetic routes for preparing the composite NPs, operation and fitting models of dye removal experiments, mechanism of degradation, SEM images of NPs, elemental contents by XPS and EDS analysis, Fe 2p XPS spectra of TiO2-PEI-TA@Fe3O4 NPs, photographs of degradation of dyes, removal ratios toward MB by TiO2-PEI-TA@Fe3O4 NPs at different molar ratios, powder XRD pattern of recollected TiO2-PEI-TA@Fe3O4 NPs, and price list of raw materials (PDF)

The authors declare no competing financial interest.

Supplementary Material

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