Amino-functionalized hierarchical porous SiO2-AlOOH composite nanosheets with enhanced adsorption performance
ABSTRACT: Hierarchical porous SiO2AlOOH composite nanosheets (HPSA) with a three- dimensional (3D) structure were prepared from two-dimensional (2D) layered mineral kaolinite (A12Si2O5(OH)4) via a template-free structural reorganization method. The obtained material
was subjected to homogeneous and effective amino-functionalization by grafting it with (3- aminopropyl) triethoxysilane. Owing to the enhanced 3D hierarchical meso-macroporous structure containing highly dispersed protonated amino groups (–NH3+), the as-prepared amino- functionalized HPSA (NH2HPSA) showed unique adsorption performance towards the congo red anionic dye. It provides feasibilities to fabricate other functional hierarchical porous materials from clay minerals, which can offer potential applications in adsorption, separation, catalysis and other environmental remediation fields.
1.Introduction
The presence of toxic organic dyes (such as congo red, rhodamine B, and methylene blue) and heavy metal ions (such as Cr3+, Cd2+, and Pb2+) in wastewater has become a severe environmental and public health problem. As an effective approach, adsorption has been extensively employed for water treatment; hence, a variety of advanced materials have been studied as potential adsorbents, including mesoporous silicas,[1, 2] porous carbons,[3] and organic-inorganic hybrid materials.[1, 2, 4] The latter contain bi-functional amino groups (–NH2) that possess the ability to easily remove both the cationic and anionic pollutants from aqueous solutions.[1, 5-8] And the grafting method is commonly used for surface amino-functionalization by covalently linking organosilane species with surface hydroxyl groups.[6, 9] However, the existed grafting method has some disadvantages including (1) significantly reduced surface area,pore volume, and pore size due to the attachment of a layer of organic functional moieties onto the material surface, leading to a less desirable product with deteriorated diffusion characteristics;[6, 10] and (2) relatively high reaction time and low efficiency because of the poor dispersibility of hydrophilic mesoporous silica powder in hydrophobic toluene, which limits the number of accessible surface hydroxyl groups and thus the concentration of the attached APTES species.[10] Hence, an appropriate matrix material and novel grafting method must be developed to alleviate the described problems.
Recently, hierarchical porous materials have attracted significant attention as prospective functionalized materials.[11-13] In general,
these compounds are highly porous and exhibit large surface area and accessible space, variable chemical composition, and the presence of interconnected hierarchical pores with different lengths. They are suitable for mass loading and diffusion, transport of electrons, ions, and organic molecules, and light harvesting; hence, hierarchical porous materials can be potentially used in energy storage and conversion, adsorption, separation, catalysis, photocatalysis, gas sensing, and biomedical applications.[13] Therefore, such materials appear to be particularly suitable for potential use as matrix materials that are capable of mitigating the decrease in surface area and porosity after amino- functionalization. Moreover, hierarchical porous materials containing multi-level pores exhibit enhanced adsorption performance as compared to that of single-sized porous materials due to their improved accessibility.[13] Over the past decades, various approaches (mainly soft/hard templating methods[14, 15]) to the preparation of hierarchical porous materials have been developed. In particular, an innovative and flexible self-formation method based on the chemistry of metal alkoxides and other precursors is now widely used for the fabrication and design of hierarchical porous structures.[13, 16, 17]
In this work, novel hierarchical porous SiO2AlOOH composite nanosheets (HPSA) with a three-dimensional (3D) structure were prepared from 2D layered mineral kaolinite (A12Si2O5(OH)4) by using a template-free structural reorganization method. The self-formation of 3D hierarchical porous structure depends on the chemistry of Al hydroxides and morphology of 2D nanosheets, which is characteristic of the kaolinite structure on the basis of the features of layered clay minerals.[18-29] Afterwards, homogeneous and effective amino-functionalization was implemented with (3-aminopropyl) triethoxysilane (APTES) via a novel grafting method. In contrast to the commonly used grafting procedure involving prolonged heating under reflux, anhydrous methanol was used to pre-disperse HPSA powder in toluene, which allowed faster grafting APTES species at a relatively low temperature. In order to characterize the adsorption performance of the amino-functionalized HPSA (NH2HPSA), the congo red anionic dye was selected as a model toxic pollutant in wastewater.
2.Materials and methods
Natural kaolinite (A12Si2O5(OH)4) was obtained from China Kaolin Clay Co., Ltd (Suzhou, China). Its major composition (wt.%) is SiO2(53.27%), Al2O3(42.46%), Fe2O3(0.48%), TiO2(0.33%), K2O (0.54%), CaO (0.09%), MgO (0.16%), P2O5(0.26%), and SO3(2.19%).
Kaolinite is classified as a 1:1 type clay mineral composed of stacked layers of SiO4 tetrahedral sheets and AlO2(OH)4 octahedral sheets. The layers are less tightly bound together by hydrogen- bonds between hydroxyl sites in the alumina octahedral sheet and the oxygen of silica tetrahedral sheet, which can be separated.[18] APTES (98%) was purchased from Aladdin Reagent Co., Ltd(Shanghai, China). Kaolinite was first converted to anhydrous metakaolinite via calcination at 700 °C for 2 h. After that, 2 g of metakaolinite was treated with 100 mL of 2 M HCl solution at 85 °C for 8 h. After cooling to room temperature, the supernatant liquid was extracted temporarily, and the precipitate was dispersed in 100 mL of the mixed solution of deionized water and ethanol (volume ratio 1:1) followed by sonication for 2 h (400 W, 24 KHz). After that, the large particles were removed by centrifugation at 800 r/min for 1 min, and the fully sonicated precipitate was obtained by further centrifugation at 8000 r/min for 3 min. After mixing with the extracted supernatant liquid containing Al ions, the leached Al ions were reprecipitated by the gradual addition of aqueous ammonia under vigorous stirring until reaching a pH of 8.5. Afterwards, the suspension was transferred to a Teflon-lined stainless steel autoclave and heated to 180 °C for 12 h. The resultant precipitate was filtrated, repeatedly washed with deionized water, dried at 110 °C for 12 h, and ground into fine powder. The obtained product contained hierarchical porous HPSA composite nanosheets.
The amino-functionalization procedure based on the grafting method was implemented as follows. 500 mg of HPSA was sonicated in 25 mL of anhydrous methanol for 30 min, and then 50 mL of anhydrous toluene was added to the suspension under vigorous stirring. After evaporating methanol under nitrogen gas flow (30 mL/min) at 75 °C for 1.5 h, 5 mL of APTES was dropwise added to the suspension under the same conditions (the reaction time was 1 h). The obtained precipitate was extracted by centrifugation (5000 r/min, 3 min), washed several times successively with anhydrous toluene and methanol, dried for 12 h at 70 °C under vacuum, and milled to fine powder. The resulting product was NH2HPSA.X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker-AXS D8 Advance diffractometer with Cu Kα radiation (0.15406 nm) operated at a voltage of 40 kV and current of 40 mA in the 2θ scanning range of 365°. Sample compositions were determined using a PANalytical Axios mAX wavelength dispersive X-ray fluorescence spectrometer. scanning electron microscopy (SEM) observations combined with energy-dispersive X-ray spectroscopy (EDS) analysis were conducted on a Tescan Mira3 LMU instrument. Scanning transmission electron microscopy (STEM) observations combined with EDS analysis were performed using a Titan G2 60-300 microscope operated at an accelerating voltage of 300 kV.
Samples were dispersed in ethanol by ultrasonication for 20 min and then deposited onto holey carbon-coated copper grids. N2 adsorption measurements were performed at a temperature of 77 K using a Micromeritics ASAP 2020 gas sorption instrument. Before measurements, the samples were degassed at 120 °C for more than 6 h. The specific surface areas were calculated using the multi- point BrunauerEmmettTeller (BET) method, and the total pore volumes (Vtot) were estimated from the adsorbed volumes at a relative pressure (P/P0) of 0.995. The pore size distributions were determined from the adsorption branches of the obtained isotherms using the Barett–Joyner– Halenda (BJH) method. The micropore volumes (Vmicro) were obtained via t-plot analysis. Thermogravimetry (TG) and Fourier transform infrared (FTIR) spectroscopy for the on-line analysis of the volatile compounds formed during TG runs was conducted on a PerkinElmer STA 8000 simultaneous thermal analyzer connected to a PerkinElmer Spectrum™ 100 infrared spectrometer via a TL 8000 transfer line containing a 10-cm gas cell. The sample was heated at a rate of 10 °C/min from 40 to 800 °C under pure nitrogen flow (40 mL/min). Transmission FTIR spectra of the samples were recorded on a Nicolet Nexus 670 infrared spectrometer at a spectral resolution of 4 cm1. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo Scientific Escalab 250Xi spectrometer equipped with an Al Kα monochromated X-ray source. The obtained binding energies were calibrated with respect to the C 1s peak (284.6 eV). Tapping-mode atomic force microscope (AFM) images were obtained under ambient conditions using an Agilent 5500 AFM/SPM system. For AFM imaging, silicon probes with a 10 nm radius of curvature were used in the tapping mode.
Batch congo red dye adsorption tests were performed using a traditional bottle-point method. The detailed experimental procedure can be described as follows.50 mg of NH2HPSA powder was added to 250-mL glass bottles containing 100 mL of 100 mg/L congo red dye solutions. 1% NaOH and 1% HNO3 solutions were used to adjust the pH of the mixtures (3.012.0). The obtained solutions were stirred at 25 °C for more than 12 h to achieve sorption equilibrium. Finally, the resulting mixtures were extracted by centrifugation, and the corresponding congo red contents were determined.The sorption isotherms of the HPSA and NH2HPSA samples were obtained by adding 50 mg of adsorbent into 100 mL solutions with congo red concentrations of 20, 50, 100, 150, 200,250, 300, 350, and 400 mg/L, respectively. The obtained mixtures were stirred at 25 °C for 12 h to achieve sorption equilibrium. The equilibrium solutions were then filtered, and their congo red contents were determined. By conducting these experiments, the actual sorption capacities and equilibrium congo red concentrations were obtained.
Sorption kinetic tests were performed at 25 °C inside 1000-mL bottles by sampling 5 mL of the studied solution at various time intervals. The initial solution volume was 500 mL, and it contained 100 mg/L of congo red and 250 mg of adsorbent.To evaluate the regeneration properties of NH2HPSA, 100 mg of sorbent powder was added to 200 mL of the 100 mg/L congo red solution. After stirring for 12 h, the amounts of adsorbed congo red were determined. Subsequently, a high-velocity centrifuge was used for achieving solid-liquid separation, and the resulting congo red-loaded NH2HPSA system was regenerated with 100 mL of 0.01 mol/L NaOH solution. After stirring for 1 h, the amount of the desorbed congo red was determined, and the corresponding regeneration efficiency was calculated.Congo red concentrations were measured at λmax = 497.0 nm using a Unico UV-2600 UV- vis spectrophotometer. Its calibration curve was obtained by measuring the absorbances at various predetermined congo red concentrations. The adsorption performance was evaluated using the following expressions(mg/g) is the equilibrium adsorption capacity, qt (mg/g) is the adsorption capacity at time t (min), V (L) is the volume of the solution, and m (g) is the mass of the adsorbent.
3.Results and discussion
The schematic illustration of the preparation of HPSA composite nanosheets and NH2HPSA is displayed in Fig. 1. First, SiO2 nanosheets were obtained from natural kaolinite (A12Si2O5(OH)4) through thermal activation and Al-leaching treatment[30] followed by ultrasonic dispersion. Subsequently, the leached Al3+ ions were converted into AlOOH nanosheets and simultaneously assembled with SiO2 nanosheets using a hydrothermal crystallization method. The assembly of the curved AlOOH nanosheets among SiO2 nanosheets would effectively prevent the 2D plane aggregation tendency of SiO2 nanosheets, leading to the assembly of SiO2 nanosheets in 3D space, thus building a 3D hierarchical porous nanostructure, which could be confirmed by the following morphology (Fig. 2) and N2 adsorption analysis (Fig. 3a). In conclusion, the 3D hierarchical porous SiO2AlOOH composite nanosheets were derived from the 2D layered crystal structure of natural kaolinite via a template-free structural reorganization method.[28, 30] The resulting hierarchical porous SiO2AlOOH composite nanosheets (HPSA) containing abundant hydroxyl groups were used as an ideal substrate for homogeneous and effective amino- functionalization. Anhydrous toluene was used as a solvent for the monolayer grafting of APTES onto the HPSA surface.[31] In order to improve the dispersibility of hydrophilic HPSA in hydrophobic toluene, anhydrous methanol was introduced to pre-disperse HPSA powder prior to its mixing with toluene. After the methanol evaporation step conducted at 75 °C (the boiling points of methanol and toluene were 64.7 and 110.6 °C, respectively), HPSA species were highly dispersed in toluene.
Subsequently, APTES was added under N2 atmosphere at 75 °C to react with the surface hydroxyl groups of HPSA. Some grafted amino groups further reacted with the surface hydroxyl groups to form protonated amino groups (–NH3+).[9, 32, 33] The ethanol by- product (with a boiling point of 78.3 °C) was further removed from the reaction system at 75 °C to promote the formation of NH2HPSA.
As indicated by the XRD patterns (Fig. S1), the as-received kaolinite corresponded to kaolinite-1A (JCPDS 140164) containing trace impurities of alunite (KAl3(SO4)2(OH)6) and quartz.[30] After modification, the reflections of kaolinite and alunite disappeared, while the reflections of quartz became very weak due to its partial removal during the preparation process, and the obtained HPSA exhibited a broad diffraction peak centered at 2θ ≈ 23°, which could be attributed to amorphous SiO2, and the weak reflections attributed to the boehmite (γ-AlOOH) crystal phase (JCPDS 211307), which might be partially covered with amorphous SiO2. The obtained results indicate that natural kaolinite (Al2Si2O5(OH)4) was converted to SiO2AlOOH composite after structural reorganization. Note that the XRD pattern of NH2HPSA was similar to that of HPSA, confirming that amino-functionalization did not destroy the crystalline structure of HPSA.
The SEM images of the sample powders and STEM images of the samples pre-dispersed in the liquid phase are shown in Fig. 2(al), respectively. Fig. 2a displays a typical pseudo- hexagonal layered structure of kaolinite with a size of 200~500 nm. After structural reorganization (Fig. 2b), the layered structure disappeared, and the resulting HPSA sample was characterized by a loose 3D porous structure likely formed by the assembly of SiO2 nanosheets (which were similar to kaolinite individual sheets) and smaller thin AlOOH nanosheets. Further, the NH2HPSA sample exhibited a more clear and dense 3D porous structure (Fig. 2c). Unlike the SEM observations, the TEM images of HPSA (Fig. 2d&e) show the well dispersed SiO2 nanosheets (with sizes of 200500 nm) and large numbers of the curved AlOOH nanosheets with a uniform size of around 50 nm, indicating that the agglomeration of HPSA solid powders is reversible in liquid solution (this observation is very important for achieving the homogeneous and effective amino-functionalization). The corresponding SAED pattern (see the inset of Fig. 2e) and HRTEM image (Fig. 2f) further confirm the amorphous framework of SiO2 nanosheets and boehmite crystalline structure of AlOOH nanosheets. The lattice fringes of AlOOH nanosheets are not clear and complete due to the interference of amorphous SiO2 nanosheets, which is consistent with the XRD spectra of HPSA (Fig. S1). The TEM image of NH2HPSA is similar to its SEM observation (Fig. 2g) and HAADFSTEM image presented in Fig. 2h, which exhibits a dense 3D porous structure. The EDS spectrum reveals the presence of C, N, O, Al, and Si elements on the sample surface (the inset of Fig. 2h). The corresponding quantitative EDS maps obtained for the C, N, Al, and Si elements display the homogeneous distributions of C and N on the NH2HPSA surface (Fig. 2(i-l)), indicating the uniform grafting of APTES molecules and relatively high dispersion of surface amino groups.
To further characterize the thickness and roughness of the nanosheets structure, AFM topographic images of kaolinite and NH2HPSA samples on smooth Si wafer substrate were displayed in Fig. S2, Before observation, the samples were first dispersed in ethanol by ultrasonication for 20 min and then deposited onto the smooth Si wafer substrate. The kaolinite nanosheets were observed highly dispersed (Fig. S2a), and the NH2HPSA samples maintained the 3D structure without clear nanosheets (Fig. S2b), which was coincident with the SEM and TEM observation (Fig. 2). What’s more, the height profile analysis of kaolinite nanosheets (inset in Fig. S2a) showed their thickness range from 2.2 to 10.3 nm, and the corresponding average height roughness was only 5.1 nm. In contrary, the average height roughness of NH2HPSA sample could be up to 72.7 nm due to the existence of 3D porous structure.” Fig. 3a shows the N2 adsorption-desorption isotherms and pore size distribution curves obtained for the HPSA and NH2HPSA samples. The relevant textural properties (summarized in Table 1) reveal that the surface areas and pore volumes of both the HPSA (311 m2/g and 1.09 cm3/g) and NH2-HPSA (250 m2/g and 1.26 cm3/g) samples were significantly larger than those of the as-received kaolinite and SiO2AlOOH composite nanosheets prepared in our previous work.[30] The N2 adsorption-desorption isotherms obtained for the HPSA and NH2HPSA samples correspond to the type IIB isotherms[34] characterized by a significant hysteresis pattern, indicating that the studied materials contain both mesopores and macropores. For HPSA, the H1-type hysteresis and capillary condensation (which occur at high relative pressures) indicate relatively large mesopore sizes reaching the macropore size range (about 200 nm). Comparatively, NH2HPSA exhibits the H3-type hysteresis, which represents a broader macropore size range and is consistent with the pore size distribution curves depicted in the inset of Fig. 3a. Meanwhile, the surface area of HPSA decreased while its pore volume increased after amino-functionalization (Table 1), which likely occurred due to the bridging effect of the grafted APTES molecules on the structure framework of HPSA. In addition, both the N2 adsorption- desorption isotherms and corresponding pore size distribution curves show the presence of micropores with sizes below 2 nm in the HPSA and NH2HPSA samples.
However, their micropore volumes estimated by the t-plot method were equal to zero (Table 1), indicating that the number of micropores was negligible as compared to those of mesopores and macropores. Therefore, it can be concluded that both the HPSA and NH2HPSA samples possess the hierarchical meso-macroporous structure.In order to study the thermal properties of NH2HPSA and confirm the successful grafting of APTES, TGDSC and FTIR techniques were used. First, the TGDSC curves of HPSA (dotted lines, Fig. S3a) exhibit a continuous weight loss of 19.5% during heating from 38 to 795°C. The weight loss of 3.2% between 38 and 165 °C accompanied by the DSC endothermic peak centered at 64 °C is due to the desorption of physically adsorbed water species, and that of 16.3% observed between 165 and 795 °C can be attributed to the dehydroxylation of the SiO2AlOOH composite. The endothermic DSC peaks detected at 452, 540, and 604 °C likely correspond to the transition from the γ-AlOOH to γ-Al2O3 crystalline phase. As compared to HPSA, the TGDSC curves of NH2HPSA (solid lines, Fig. S3a) show a slightly higher weight loss (21.0%) in the temperature range of 38795 °C, indicating a relatively small amount of grafted APTES. The weight loss of 4.3% that occurred after heating from 38 to 165 °C with a stronger endothermic peak centered at 70 °C is due to the removal of physically adsorbed water and grafted APTES, while the higher weight loss of 16.7% observed between 165 and 795 °C is attributed to the dehydroxylation of the SiO2AlOOH composite and decomposition of the grafted APTES species. The DSC endothermic peak centered at 280 °C indicates the removal of nitrogenous fragments from APTES, while the endothermic peak at 437 °C likely originates from the decomposition of the oxyethyl groups (–OC2H5) of APTES molecules.[35]
The endothermic peaks detected at 361, 452, and 570 °C are related to the crystal transition from the γ-AlOOH to γ-Al2O3 phase. For the NH2HPSA sample, the corresponding FTIR spectra of the gaseous products obtained at different temperatures are shown in Fig. S3b. The peaks at 3720 cm1 and in the range of 13001800 cm1 are due to the presence of water vapor resulting from the dehydroxylation of the SiO2AlOOH composite and decomposition of the grafted APTES. According to the results of TG analysis, water vapor is expected to be the most gaseous product. However, the peaks assigned to water vapor are not as strong as the peaks obtained for other gaseous products at various temperatures, indicating that the dehydroxylation process of the SiO2AlOOH composite is very slow and confirming the thermal stability of the HPSA matrix. Furthermore, the major peaks observed in the wavenumber ranges of 27903160, 22502430, and 8501020 cm1 can be attributed to various alkanes, CO2, and NH3, respectively.[36, 37] These species mainly originate from the decomposition of grafted APTES species, and the peak centered at 1738 cm1 is due to the stretching vibration of the C=O groups of various aldehydes.[36, 38] It can be observed that the peak assigned to aldehydes first appears at 280 °C, while the peaks attributed to alkanes, CO2, and NH3 disappear at 570 °C, indicating that the decomposition of grafted APTES species mainly occurs in the temperature range of 280570 °C. The strongest peaks assigned to CO2 were detected at 437 °C, suggesting that the DSC endothermic peak observed at 437 °C was due to the decomposition of the oxyethyl groups (– OC2H5) of APTES molecules. At the same time, the strongest peaks corresponding to water vapor, alkanes, and NH3 were observed in the range of 437452 °C, indicating that the main decomposition temperature of the NH2HPSA sample was 437452 °C, thus further confirming its high thermal stability.
The incorporation of amino groups into the HPSA framework can be qualitatively described by the FTIR spectra depicted in Fig. 3b. According to the FTIR spectrum of NH2HPSA, a weak peak due to the bending of N–H bonds is detected at 679 cm1, and the symmetrical bending of protonated amino groups (–NH3+) is observed at 1514 cm1, indicating the existence of amino groups in the NH2HPSA sample.[10] The C–N stretching vibration maybe overlapped with the IR absorption bands of Si–O–Si at 1075 cm1 and Si–CH2–R in the range of 12001250 cm1 (the obtained results are consistent with the broader and stronger peaks detected in the range of 11001250 cm−1 for the NH2HPSA sample). One weak peak centered at 2930 cm1 likely represents the stretching of CH2 groups in NH2HPSA species.[39, 40] The shoulder at 3128 cm−1 and sharp peak at 1075 cm−1can also be attributed to the stretching and bending vibrations of Al−OH bonds, respectively,[41] while the peaks detected at 731, 638, and 476 cm−1 represent the vibration modes of AlO6,[42, 43] demonstrating the presence of the boehmite phase in all samples. In addition, the peaks at 731 and 638 cm−1 can also be attributed to the Si−O−Al translation,[35] and a weak peak at 3734 cm-1 due to silanol groups was observed for all samples.[30] The sharp peak detected at 1405 cm−1 for HPSA, which represented the vibration overtone of the surface hydroxyl groups,[44] was significantly reduced for the NH2HPSA sample, indicating a strong reaction of the HPSA surface hydroxyl groups with APTES.
To further clarify the effect produced by the amino functional groups, XPS analysis was performed. The C 1s spectra of the HPSA and NH2HPSA samples (Fig. 3c) exhibit a strong peak at around 284.8 eV, which can be assigned to both the reference C−C and C−H bonds. For HPSA, the appearance of the small peaks at 286.2 and 288.6 eV, which are attributed to the C−OH/C−O−C and C−O−Si species, respectively,[10] was not expected. These C−O species most likely resulted from the weak ethanol carbonization that occurred during the high-power ultrasonic treatment of SiO2 nanosheets in the ethanol-water solution (Fig. 1). After amino- functionalization, the same three peaks with significantly larger intensities were also detected for the NH2HPSA sample. Among these features, the peak at about 286.1 eV can be assigned to not only the C−OH/C−O−C bonds, but also to the C−N bonds of APTES,[9, 10] indicating the successful grafting of APTES species onto the HPSA surface. The additional peak originated from the C−Si bonds of −Si−(CH2)3−NH2 in APTES species was also observed at 284.4 eV. The N 1s spectrum of the NH2HPSA sample is shown in Fig. 3d. The two peaks at 400.0 and 401.9 eV are attributed to the free amino (–NH2) and protonated amino groups (–NH3+), respectively.[9, 10] The ratio of the corresponding peak areas (–NH3+:–NH2) is about 11:1, indicating that almost all the amino functional groups on the NH2HPSA surface exist in the protonated form (–NH3+). Hence, the majority of the grafted amino groups (–NH2) further reacted with the surface hydroxyl groups of HPSA (see Fig. 1) due to the large number of accessible surface hydroxyl groups in the highly dispersed reaction system utilized in this study. The corresponding surface elemental compositions of the HPSA and NH2HPSA samples are listed in Table 2. It shows that the contents of C and N elements increase after amino- functionalization, indicating the successful grafting of APTES molecules. Here the N content of NH2HPSA equal to 1.45 at.% was determined via XPS analysis, the magnitude of 1.09 at.% was obtained via SEMEDS analysis, and the value of 1.28 at.% was determined via STEMEDS analysis.
It is well known that the amino groups (–NH2) exhibit bi-functional properties, which allow the removal of both the cationic and anionic pollutants from aqueous media depending on the solution pH.[5, 6] For example, at high pH values (>7.2), the amino groups can remove cationic pollutants, while at low pH values (˂7.2), they form protonated amino groups (–NH3+) in the presence of free H+, which can absorb anionic pollutants due to electrostatic attraction. Therefore, the adsorption activity of the commonly used amino-functionalized materials toward the congo red anionic dye usually decreased with increasing pH. The influence of the initial solution pH (312; the pH of the natural congo red solution is 7.6) on the adsorption activity of NH2HPSA toward congo red is shown in Fig. 4. The obtained results indicate that the measured uptake efficiency remained at a level of about 98% in the pH range of 37.6 and then decreased with increasing pH. The uptake efficiency was still equal to 87% at pH=9, but decreased significantly to 21% at pH=10 and finally reached the minimum (6%) at pH=12. According to the results of FTIR and XPS analyses (Fig. 3(b-d)), the NH2HPSA sample contains large amounts of protonated amino groups (–NH3+); hence, unlike other amino-functionalized materials,[5, 45, 46] its anionic dye adsorption behavior does not depend on the solution acidity. Thus, NH2HPSA could exhibit high adsorption activity toward anionic dyes in the broad pH range of 39, and the exhausted materials can be regenerated by their treatment in alkaline solutions (with pH ≥ 12).
The obtained sorption isotherms (Fig. 5a) reveal that NH2HPSA exhibits larger sorption capacities (around 506 mg/g at pH=7.6, 25 °C) as compared to those of HPSA (around 104 mg/g) and other congo red adsorbents (Table 3), indicating significant potential of NH2HPSA for anionic dyes adsorption. Furthermore, Fig. 5b shows that the uptake efficiency of NH2HPSA exceeds 92% at initial congo red concentrations ranging from 20 to 250 mg/L and then slowly decreased to the minimum value (63%) at 400 mg/L, suggesting the high applicability of NH2HPSA in a broad concentration range of congo red in wastewater.Kinetic experiments were conducted by measuring the contents of congo red in solution at various time intervals. NH2HPSA reached 89% of the maximum adsorption capacity within 1 min and then achieved sorption equilibrium within the next 30 min (Fig. 5c). In comparison, HPSA achieved sorption equilibrium within 1 min due to the relatively small sorption capacity. The observed fast kinetics can be mainly attributed to the 3D hierarchical porous nanostructures of the HPSA and NH2HPSA samples, which favor the diffusion of congo red molecules. The absorption spectra recorded for the congo red solution treated with the NH2HPSA sample for specified time intervals and corresponding digital photographs (Fig. 5d) visually illustrate the efficient adsorption of congo red on the NH2HPSA surface. From the results presented above, it was determined that the exhausted NH2HPSA could be regenerated in alkaline solutions (at pH ≥12). Hence, 0.01 mol/L NaOH solution (with pH=12) was utilized as a regenerant, which resulted in the desorption efficiency of congo red equal to 95.4%.
To further investigate the adsorption characteristics of NH2HPSA towards congo red dye, the adsorption isotherm (Langmuir and Freundlich equations) and kinetic (pseudo-first order and pseudo-second order) models were proposed according to the experimental data (in Fig. 6). The corresponding equations were given: where qm (mg/g) and KL (L/mg) are Langmuir isotherm coefficients; KF (mg/g) and n are Freundlich constants; K1 (min−1) and K2 (g/(mg·min)) are the pseudo-first order and pseudo- second order rate constants, respectively. The fittings of experimental data to Langmuir and Freundlich isotherm models were shown in Fig. 6a. The values of R2 (the coefficient of determination) of Langmuir and Freundlich models were 0.92772 and 0.76885, respectively, indicating that the Langmuir isotherm model well represented the equilibrium adsorption of congo red onto NH2HPSA nanocomposite, with the monolayer coverage of congo red dye on NH2HPSA surface. The qm value of congo red on NH2HPSA was 518.17 mg/g, close to the experimental data (506 mg/g).Adsorption kinetics was carried out to evaluate the rate and mechanism of the dye molecules transfer from the liquid solution onto the NH2HPSA surface. The data and fittings of pseudo-first order and pseudo-second order kinetic models were shown in Fig. 6b. The values of R2 of pseudo-first order and pseudo-second order models were 0.02528 and 0.99996, respectively, obviously showing that the pseudo-second order kinetic model well described the adsorption of congo red onto NH2HPSA, and indicating the chemisorption nature of congo red dye on NH2HPSA surface. Thus, the adsorption rate constant of NH2HPSA nanocomposite towards congo red dye could be calculated from the pseudo-second order kinetic model with the K2 value of 4.9×10-3 g/(mg·min).Based on the characterization, adsorption performance, and adsorption isotherm and kinetics model analysis, the enhanced adsorption property of NH2HPSA nanocomposite could be attributed to the highly dispersed protonated amino groups (–NH3+), which showed the strong electrostatic attraction with the congo red anionic dye (Fig. 4). Additionally, the 3D hierarchical meso-macroporous structure of nanocomposite was also helpful to the diffusion and transport of congo red molecules (Fig. 5c).
4.Conclusions
In summary, novel hierarchical porous HPSA nanosheets with a 3D structure were obtained from kaolinite using the template-free structural reorganization method. In particular, this 3D hierarchical meso-macroporous structure was formed through the innovative self-formation approach based on the chemistry of Al hydroxides and 2D nanosheet morphology of natural kaolinite. After that, a novel grafting method was developed for the homogeneous and effective amino-functionalization of HPSA, resulting in the formation of the enhanced 3D hierarchical porous structure and highly dispersed protonated amino groups (–NH3+). As a result, the as- prepared NH2HPSA exhibited high uptake efficiency to congo red anionic dye under broad conditions, fast sorption kinetics and efficient sorbent regeneration. Thus, the described route can be utilized for the preparation of other functional hierarchical porous materials from clay minerals, which can find numerous potential applications in environmental remediation Congo Red fields.