Amino-Functionalized Silica@Resorcinol–Formaldehyde Nanocomposites for the Removal of Cr(VI) from Aqueous Solutions

Amino-functionalized silica@resorcinol–formaldehyde nanocomposites (NH2-SiO2@RF) were synthesized for the removal of Cr(VI) from aqueous solutions using the sol–gel technique with two simple preparation steps, including the one-pot synthesis of SiO2@RF using the Stöber method and (3-aminopropyl)triethoxysilane (APTES) modification. The morphology, particle size, functional group, and thermal stability of the obtained nanocomposites were systematically characterized, with the results indicating a uniform sphericity with a particle size of 200 nm and high thermal stability. The adsorption results demonstrated that the preferred pH value was 2, and the data were well fitted with the Langmuir and Temkin isotherm models and quasi-second-order kinetic equation, indicating a high adsorption capacity. The maximum Cr(VI) adsorption capacity from the nonlinear form of the Langmuir model was 272.6 mg·g−1. The intra-particle diffusion model accurately described the adsorption of Cr(VI) onto NH2-SiO2@RF. The changes in Gibb’s free energy, enthalpy, and entropy revealed that Cr(VI) adsorption onto NH2-SiO2@RF was a spontaneous and endothermic process. Furthermore, high selectivity was demonstrated in the material for the removal of Cr(VI) from commonly coexisting ions. The obtained nanocomposites had good regeneration properties and maintained a removal rate above 85% in the fifth adsorption–desorption experiments. Moreover, under the optimized adsorption conditions, the obtained nanocomposites were preliminarily applied to tannery wastewater, demonstrating an excellent removal effect, which indicates their potential application value.


Introduction
Hexavalent chromium (Cr(VI)) is widely used in industrial processes including tanning, electroplating, wood preserving, and pigmentation [1].Cr(VI) is found not as free ions, but in a complex form in water, and all Cr(VI) species are highly soluble oxides (i.e., hydrochromate (HCrO 4 − ), chromate (CrO 4 2− ) and dichromate (Cr 2 O 7 2− ) anions) [2].Cr(VI) is highly toxic to plants, animals, and the human body and the Global Burden of Disease (GBD) study categorized Cr(VI) as an occupational carcinogen [3,4].China has stipulated that the emission concentration of Cr(VI) in industrial wastewater discharges must not exceed 0.1 mg•L −1 [5].Therefore, it is important to control the emission of Cr(VI).
Currently, various technologies, including chemical treatment [6], membrane separation [7], biodegradation [8], electrochemical technology [9], and adsorption [10] are used for processing Cr(VI) in sewage.Among these technologies, the adsorption method is simple, highly efficient, and has a good cost performance and low risk of secondary pollution, making it a very promising method [11].For various adsorption methods, the adsorption effects are strongly affected by the type of adsorbent.Currently, the commonly used adsorbents Polymers 2023, 15, 4094 2 of 16 mainly include activated carbon, zeolite, nanomaterials, biosorbents, etc. [12][13][14].However, the weak sorption selectivity and low surface areas/site ratio obviously limits their practical utility.Organic-inorganic nanostructured composites with a high surface area are used in a variety of fields, especially for water treatment, as the supply of the adsorption sites and the skeleton are supported by the organic part and the inorganic part, respectively [11,15].Amino-functionalized silica@resorcinol-formaldehyde nanocomposites (NH 2 -SiO 2 @RF) are a kind of organic-inorganic nanostructured composite, synthesized using the sol-gel technique with two simple preparation steps, including using the Stöber method for the one-pot synthesis of SiO 2 @RF, as well as the (3-aminopropyl) triethoxysilane (APTES) modification of SiO 2 @RF.
Silica spheres with a controllable narrow particle size distribution can be synthesized using alcohol-water solvents and an ammonia catalyst through hydrolysis and the condensation of silicon alkanes, which is known as the Stöber method [16].Resorcinolformaldehyde (RF) resin can also be synthesized using an ethanol-water-ammonia system [17].From this, a SiO 2 @RF complex can be synthesized in the Stöber system through innovative methods such as prolonging the reaction time [18,19].Nitrogen-containing functional groups such as aliphatic amines and aromatic amines can effectively enhance the adsorption effect of Cr(VI) [20,21].The surface amine group tends to be positively charged, and such a tendency provides a good amine-positive Cr(VI) interaction.Therefore, amine-functionalized silica has attracted a great deal of attention [20][21][22][23].For example, Jang et al. demonstrated the adsorption of Cr(VI) by amino-functionalized amorphous silica nanoparticles and mesoporous silica nanoparticles as well as the removal of Cr(VI) from water environments.The results suggested a structural and chemical rationale for enhanced Cr(VI) adsorption and removal.However, the adsorption capacity was not high, and there was a lack of practical applications for the adsorbent [21].Granular N-(3-trimethoxysilylpropyl)diethylenetriamine-grafted mesoporous silica SBA-15 was synthesized, and the maximum Cr(VI) adsorption capacity was determined to be 330.9mg•g −1 using the Langmuir model.To a certain extent, this overcame the limitations of powder adsorbents for large-scale applications (hydraulic and separation problems) [22].However, the exact adsorption behavior of Cr(VI) on functionalized silica remains unclear, and many aspects have yet to be systematically studied.
Taking these factors into account, amino-functionalized silica@resorcinol-formaldehyde nanocomposites (NH 2 -SiO 2 @RF) were synthesized in two simple preparation steps, involving the silane reagent APTES-functionalized SiO 2 @RF.The nanocomposites were thoroughly characterized using scanning electron microscopy (SEM), a laser particle sizer, Fourier transform infrared (FT-IR) spectroscopy, and thermogravimetric analysis (TGA).In addition to the effects of the adsorbent dosage, solution pH, contact time, initial concentration, and temperature, its regeneration properties and coexistent ions were all systematically investigated.Both the nonlinearized and linearized forms of the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) isotherm models were used to fit the test data and adsorption kinetic models (the pseudo-first-order model and pseudo-second-order model) were used to analyze the data.Finally, the selectivity and reusability of the materials were evaluated, and the prepared nanocomposites were used for the adsorption of actual tannery wastewater, showing a potential practical application value.

Preparation of Amino
Functionalized Silica@resorcinol-Formaldehyde Nanocomposites (NH 2 -SiO 2 @RF) SiO 2 @RF nanocomposites were prepared using the one-step Stöber method, as schematically shown in Figure 1a.In brief, a 40 mL mixture of EtOH/H 2 O (3:1, volume) and 1.25 mL NH 3 •H 2 O were added into a three-necked flask with vigorous stirring for 30 min at 30 • C.Then, 1.4 mL TEOS, 0.25 g resorcinol, and 350 µL formaldehyde aqueous solution were added, in order, at 30 min intervals; stirring was carried out continuously for 24 h.Then, the mixture was centrifuged at 7500 rpm for 10 min.To remove the unreacted solvent, the mixture was washed several times using absolute ethanol and ultrapure water.Finally, the mixture was dried at 50 • C for 12 h.About 0.575 g of SiO 2 @RF was obtained after this step.

SiO2@RF)
SiO2@RF nanocomposites were prepared using the one-step Stöber method, as schematically shown in Figure 1a.In brief, a 40 mL mixture of EtOH/H2O (3:1, volume) and 1.25 mL NH3•H 2O were added into a three-necked flask with vigorous stirring for 30 min at 30 °C.Then, 1.4 mL TEOS, 0.25 g resorcinol, and 350 μL formaldehyde aqueous solution were added, in order, at 30 min intervals; stirring was carried out continuously for 24 h.Then, the mixture was centrifuged at 7500 rpm for 10 min.To remove the unreacted solvent, the mixture was washed several times using absolute ethanol and ultrapure water.Finally, the mixture was dried at 50 ºC for 12 h.About 0.575 g of SiO2@RF was obtained after this step.
As shown in Figure 1b, the NH2-SiO2@RF nanocomposites were manufactured via the sol-gel method using APTES for the modification of SiO2@RF.In brief, 200 mg SiO2@RF was dispersed into a 90 mL mixture of EtOH/H2O (8:1, volume) via sonication, and 2 mL NH3•H 2O was added into the solution while stirring at 30 °C.Then, 2 mL APTES was added dropwise.After 12 h of vigorous stirring, the mixture was centrifuged at 7500 rpm for 10 min.Then, the mixture was washed several times using absolute ethanol and ultrapure water.The final products were dried at 50 ºC for 12 h.Finally, about 0.217 g NH2-SiO2@RF was obtained.

Characterization of NH2-SiO2@RF
The morphology of the NH2-SiO2@RF nanocomposites was characterized via scanning electron microscopy (SEM, Hitachi S-4800, Hitachi, Japan).The particle sizes were tested using a laser particle size meter (Nano ZS90, Malvern, UK).The specific surface area and porosity were measured using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods (Bethdard, Beijing, China).The infrared spectra of the As shown in Figure 1b, the NH 2 -SiO 2 @RF nanocomposites were manufactured via the sol-gel method using APTES for the modification of SiO 2 @RF.In brief, 200 mg SiO 2 @RF was dispersed into a 90 mL mixture of EtOH/H 2 O (8:1, volume) via sonication, and 2 mL NH 3 •H 2 O was added into the solution while stirring at 30 • C.Then, 2 mL APTES was added dropwise.After 12 h of vigorous stirring, the mixture was centrifuged at 7500 rpm for 10 min.Then, the mixture was washed several times using absolute ethanol and ultrapure water.The final products were dried at 50 • C for 12 h.Finally, about 0.217 g NH 2 -SiO 2 @RF was obtained.

Characterization of NH 2 -SiO 2 @RF
The morphology of the NH 2 -SiO 2 @RF nanocomposites was characterized via scanning electron microscopy (SEM, Hitachi S-4800, Hitachi, Japan).The particle sizes were tested using a laser particle size meter (Nano ZS90, Malvern, UK).The specific surface area and porosity were measured using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods (Bethdard, Beijing, China).The infrared spectra of the nanocomposites were examined using a Fourier transform infrared (FT-IR) spectrometer (Thermo Nicolet Corporation, Madison, WI, USA), with wavenumbers ranging from 400 to 4000 cm −1 .Thermogravimetric analysis in a nitrogen atmosphere (Mettler 5MP/PF7548/MET/400W thermal analyzer, Mettler Toledo, Columbus, OH, USA) was used to assess the thermal stability of the nanocomposites from 40 to 700 • C with the condition of a nitrogen flow rate of 50 mL•min −1 and heating rate of 20 • C•min −1 .

Adsorption Studies
First, 1000 mg•L −1 of Cr(VI) stock solution was prepared by adding an appropriate amount of K 2 Cr 2 O 7 to deionized water.Then, the desired concentrations of the Cr(VI) solution were prepared by dissolving the stock solutions.Batch adsorption studies were carried out by mixing varying dosages of NH 2 -SiO 2 @RF with 5 mL Cr(VI) solutions of varying concentrations in 10 mL round-bottomed centrifugal tubes.Then, the tubes were sealed and shaken in a vapor-bathing vibrator at 150 rpm.The various adsorption parameters, such as adsorbent dosages (0.3~4.0 g•L −1 ), pH value (1~9), concentrations (5~400 mg•L −1 ), and contact time (10~350 min), were varied independently to optimize the adsorption capacity.The pH value was regulated using HCl (1 mol•L −1 ) and NaOH (1 mol•L −1 ).After the above-mentioned adsorption procedures were carried out, the tubes containing the above-described mixture were centrifuged at 7500 rpm for 10 min.Then, the supernatant liquors were acidized using phosphoric acid and complexed with DPC, which was detected using a UV-vis spectrophotometer (Thermo Scientific NanoDrop 2000/2000C, Waltham, MA, USA) at the wavelength of 540 nm.All the adsorption experiments were performed three times, and the average value was taken for the final analysis.
The removal efficiency R (%) of Cr(VI) and the equilibrium adsorption capacity q e (mg•g −1 ) were calculated using Equations ( 1) and ( 2 where C 0 and C e (mg•L −1 ) denote the initial concentration and adsorption equilibrium concentration of Cr(VI), respectively, V (mL) is the volume of the Cr(VI) solution, and m (mg) is the adsorbent mass used in the experiments.

Desorption Studies
To evaluate the reuse performance of NH 2 -SiO 2 @RF nanocomposites, adsorptiondesorption experiments were carried out.BFirstly, 50 mg NH 2 -SiO 2 @RF nanocomposites was dispersed into 50 mL Cr(VI) with concentration of 30 mg•L −1 at pH 2.0, and the mixture was shaken for 4 h at 150 rpm and 25 • C.Then, the adsorbents were collected via centrifugation and rinsed with ultrapure water until reaching a neutral state.After being desorbed with 100 mL NaOH at a concentration of 0.5 mol•L −1 , the adsorbents were collected and rinsed with ultrapure water until reaching a neutral state and dried in a vacuum oven at 50 • C for 12 h.Then, the regenerated NH 2 -SiO 2 @RF nanocomposites were reused in the next adsorption-desorption cycle.

Preparation and Characterization of NH 2 -SiO 2 @RF
In the process of synthesizing silica using the Stöber method, silica particles can be hydrolyzed by TEOS one hour after the reaction begins.The preparation of RF microspheres via the Stöber method under the same reaction conditions (e.g., reaction solvent, catalyst, temperature, etc.) takes longer (about 24 h).In this work, the NH 2 -SiO 2 @RF nanocomposites were prepared by using ethanol/water as a reaction solvent and concentrated ammonia as a catalyst at 30 • C, as shown in Figure 1.At the beginning of the reaction, the silica particles were manufactured through the hydrolysis and condensation of TEOS, around which there were a large number of NH 4 + ions.At the same time, different RF materials were formed during the catalysis of OH − ions.Thus, the shell was formed outside the silica particles due to the electrostatic attraction of NH 4 + and OH − layer by layer.In order to obtain nanocomposites with uniform particle size and spherical morphology, various chemical characteristics (such as the ethanol/water volumetric ratio, the amount of ammonia water, the TEOS dosage, and the resorcinol/formaldehyde molar concentration ratio) were studied.The optimum preparation conditions were an ethanol/water volumetric ratio of 3:1 (total volume 40 mL), an ammonia water volume of 1.25 mL, a TEOS volume of 1.4 mL, and a resorcinol/formaldehyde concentration ratio of 3:1 (0.9 mmol:1.8mmol).When using the sol-gel method, this mainly occurs in the reaction between the hydroxyl of RF and the free amino group of APTES in the aqueous solution [24,25].
The morphological structures and particle size of SiO 2 @RF and NH 2 -SiO 2 @RF were characterized and measured using SEM and dynamic light scattering, and the results are shown in Figure 2. It can be clearly seen that SiO 2 @RF (Figure 2a,b) and NH 2 -SiO 2 @RF (Figure 2d,e) were spherical in terms of morphology, with a relatively rough surface; they had an approximate size of 150-200 nm.The average particle size of the SiO 2 @RF was about 203 nm (Figure 2c), and that of NH 2 -SiO 2 @RF was about 211 nm (Figure 2f).
Polymers 2023, 14, x FOR PEER REVIEW 5 of 17 nanocomposites were prepared by using ethanol/water as a reaction solvent and concentrated ammonia as a catalyst at 30°C, as shown in Figure 1.At the beginning of the reaction, the silica particles were manufactured through the hydrolysis and condensation of TEOS, around which there were a large number of NH4 + ions.At the same time, different RF materials were formed during the catalysis of OH − ions.Thus, the shell was formed outside the silica particles due to the electrostatic attraction of NH4 + and OH -layer by layer.In order to obtain nanocomposites with uniform particle size and spherical morphology, various chemical characteristics (such as the ethanol/water volumetric ratio, the amount of ammonia water, the TEOS dosage, and the resorcinol/formaldehyde molar concentration ratio) were studied.The optimum preparation conditions were an ethanol/water volumetric ratio of 3:1 (total volume 40 mL), an ammonia water volume of 1.25 mL, a TEOS volume of 1.4 mL, and a resorcinol/formaldehyde concentration ratio of 3:1 (0.9 mmol:1.8mmol).When using the sol--gel method, this mainly occurs in the reaction between the hydroxyl of RF and the free amino group of APTES in the aqueous solution [24,25].
The morphological structures and particle size of SiO2@RF and NH2-SiO2@RF were characterized and measured using SEM and dynamic light scattering, and the results are shown in Figure 2. It can be clearly seen that SiO2@RF (Figure 2a, b) and NH2-SiO2@RF (Figure 2d, e) were spherical in terms of morphology, with a relatively rough surface; they had an approximate size of 150-200 nm.The average particle size of the SiO2@RF was about 203 nm (Figure 2c), and that of NH2-SiO2@RF was about 211 nm (Figure 2f).The surface area and pore size distribution of the nanocomposites were analyzed and tested using BET [26].The results are given in Figure 2g,h and Table 1.The specific surface area and pore volume of the nanocomposites was reduced from 32.46 m 2 •g −1 and 0.217 mL•g −1 to 29.42 m 2 •g −1 and 0.109 mL•g −1 , respectively, after SiO 2 @RF amino modification; this is probably because of the introduction of the surface amino group, which occupies part of the pore volume of the SiO 2 @RF material.Figure 2g displays the N 2 adsorption-desorption isotherm of the material before and after aminoization, exhibiting typical IV isotherms with a sharp capillary condensation step at a high relative pressure (P/P 0 > 0.9), suggesting the existence of a mesoporous structure [19].The concentration of pore size distribution and the small average pore size were confirmed by the corresponding BJH pore size distributions (Figure 2h).The BJH pore size of the nanocomposites increased from 2.48 nm to 2.61 nm after SiO 2 @RF amino modification.FT-IR spectral analysis was carried out on the materials and the results are outlined in Figure 3a.The peaks around 3421 cm −1 , 1633 cm −1 , and 1400 cm −1 were the stretching vibrations of O-H.The presence of an Si-OH group is confirmed by the peak of O-H bending vibrations at 953 cm −1 .The peak at 1095 cm −1 was assigned to Si-O-Si anti-symmetric stretching vibrations.The peaks at 798 cm −1 and 467 cm −1 were assigned to Si-O symmetric stretching, and Si-O-Si bending vibrations, respectively [20,27].For SiO 2 @RF and NH 2 -SiO 2 @RF, the O-H stretching vibration of Si-OH almost disappeared, and the peak at 1624 cm −1 represents a C=C stretching vibration of resorcinol on the aromatic ring [12,19].The peaks at 690 cm −1 and 1459 cm −1 were assigned to N-H bending vibrations and N-H asymmetric bending vibrations, respectively [28,29].The FT-IR spectra of NH 2 -SiO 2 @RF showed IR bands at 1469 cm −1 , which were assigned to the C-H bending vibrations of the alkyl chain of APTES [11,21].The N-H, C-H of the alkyl chain, and absenting O-H indicated that the RF shell formed on the surface of SiO 2 and APTES was successfully modified on the surface of SiO 2 @RF.The surface area and pore size distribution of the nanocomposites were analyzed and tested using BET [26].The results are given in Figure 2g,h and Table 1.The specific surface area and pore volume of the nanocomposites was reduced from 32.46 m 2 •g −1 and 0.217 mL•g −1 to 29.42 m 2 •g −1 and 0.109 mL•g −1 , respectively, after SiO2@RF amino modification; this is probably because of the introduction of the surface amino group, which occupies part of the pore volume of the SiO2@RF material.Figure 2g displays the N2 adsorptiondesorption isotherm of the material before and after aminoization, exhibiting typical IV isotherms with a sharp capillary condensation step at a high relative pressure (P/P0 > 0.9), suggesting the existence of a mesoporous structure [19].The concentration of pore size distribution and the small average pore size were confirmed by the corresponding BJH pore size distributions (Figure 2h).The BJH pore size of the nanocomposites increased from 2.48 nm to 2.61 nm after SiO2@RF amino modification.FT-IR spectral analysis was carried out on the materials and the results are outlined in Figure 3a.The peaks around 3421 cm −1 , 1633 cm − 1, and 1400 cm −1 were the stretching vibrations of O-H.The presence of an Si-OH group is confirmed by the peak of O-H bending vibrations at 953 cm −1 .The peak at 1095 cm −1 was assigned to Si-O-Si anti-symmetric stretching vibrations.The peaks at 798 cm −1 and 467 cm −1 were assigned to Si-O symmetric stretching, and Si-O-Si bending vibrations, respectively [20,27].For SiO2@RF and NH2-SiO2@RF, the O-H stretching vibration of Si-OH almost disappeared, and the peak at 1624 cm −1 represents a C=C stretching vibration of resorcinol on the aromatic ring [12,19].The peaks at 690 cm −1 and 1459 cm −1 were assigned to N-H bending vibrations and N-H asymmetric bending vibrations, respectively [28,29].The FT-IR spectra of NH2-SiO2@RF showed IR bands at 1469 cm −1 , which were assigned to the C-H bending vibrations of the alkyl chain of APTES [11,21].The N-H, C-H of the alkyl chain, and absenting O-H indicated that the RF shell formed on the surface of SiO2 and APTES was successfully modified on the surface of SiO2@RF.The thermal stability of SiO 2 @RF and NH 2 -SiO 2 @RF was investigated using thermogravimetric analysis (TGA).The curve of SiO 2 @RF showed that the degradation process occurred between 40 • C and 700 • C and the weight loss was 27.5% (Figure 3b), due to the decomposition of the RF shell on the silica surface [30].After being modified by APTES, the thermal stability of NH 2 -SiO 2 @RF presumably increased, and the mass loss decreased to 25.2%.It is speculated that this is because the surface amino groups are decomposed before the carbonization of the RF shell on the silica surface.
3.2.Effect of NH 2 -SiO 2 @RF Dosage Among the factors influencing adsorption efficiency, the adsorbent dosage is one of the most important.A high dose leads to a high adsorbent cost, which means that the adsorbent cannot be used widely.The adsorption experiment was carried out using the method described above, and the results are shown in Figure 4a.The removal efficiency increased gradually with the increase in the adsorbent amount from 0.2 g•L −1 to 1.0 g•L −1 .As the amount of adsorbent continued to increase from 1.0 g•L −1 to 4.0 g•L −1 , there was almost no further increase in removal efficiency.This is possibly because when the amount of adsorbent used was small, there were fewer active adsorbent sites and they could not completely adsorb Cr(VI) in the solution.With the increase in the amount of adsorbent used, the number of active sites available gradually increased, and the removal efficiency reached the maximum when the active sites of NH 2 -SiO 2 @RF can completely adsorb Cr(VI) in the solution, reaching adsorption equilibrium [6,20,26].When the amount of the adsorbent continued to increase, the removal efficiency was no longer changed.Therefore, in subsequent experiments, 1.0 g•L −1 was selected as the appropriate adsorbent dosage.
Polymers 2023, 15, x FOR PEER REVIEW 7 of 17 The thermal stability of SiO2@RF and NH2-SiO2@RF was investigated using thermogravimetric analysis (TGA).The curve of SiO2@RF showed that the degradation process occurred between 40 °C and 700 °C and the weight loss was 27.5% (Figure 3b), due to the decomposition of the RF shell on the silica surface [30].After being modified by APTES, the thermal stability of NH2-SiO2@RF presumably increased, and the mass loss decreased to 25.2%.It is speculated that this is because the surface amino groups are decomposed before the carbonization of the RF shell on the silica surface.

Effect of NH2-SiO2@RF dosage
Among the factors influencing adsorption efficiency, the adsorbent dosage is one of the most important.A high dose leads to a high adsorbent cost, which means that the adsorbent cannot be used widely.The adsorption experiment was carried out using the method described above, and the results are shown in Figure 4a.The removal efficiency increased gradually with the increase in the adsorbent amount from 0.2 g•L −1 to 1.0 g•L −1 .As the amount of adsorbent continued to increase from 1.0 g•L −1 to 4.0 g•L −1 , there was almost no further increase in removal efficiency.This is possibly because when the amount of adsorbent used was small, there were fewer active adsorbent sites and they could not completely adsorb Cr(VI) in the solution.With the increase in the amount of adsorbent used, the number of active sites available gradually increased, and the removal efficiency reached the maximum when the active sites of NH2-SiO2@RF can completely adsorb Cr(VI) in the solution, reaching adsorption equilibrium [6,20,26].When the amount of the adsorbent continued to increase, the removal efficiency was no longer changed.Therefore, in subsequent experiments, 1.0 g•L −1 was selected as the appropriate adsorbent dosage.

Effect of Solution pH
The solution pH is a key factor affecting the adsorption efficiency of Cr(VI), and the results of adsorption experiments are shown in Figure 4b.As observed, when the solution pH ranged from 1.0 to 2.0, the removal efficiency of Cr(VI) by NH 2 -SiO 2 @RF increased slowly, with the highest removal efficiency (about 98%) found at pH 2.0.Then, the removal efficiency decreased rapidly as the pH increased from 2.0 to 4.0 and continued decreasing to about 9% from pH values of 4.0 to 9.0.The results may be due to the altered morphology of Cr(VI) in solutions at different pH values.Cr(VI) was usually in the form of dichromate (Cr 2 O 7 2− ) and hydrogen chromate (HCrO 4 − ) at a pH of 2.0-6.0.When the pH was above 6.0, it was chromate (CrO 4 2− ), while below 1.0, it was H 2 CrO 4 [2,12,28].This phenomenon could likely to be explained by the amino groups on the surface of the material, which were protonated into positively charged −NH 3 + at a low pH, meaning that Cr(VI) can be removed via the electrostatic adsorption of HCrO 4 − and Cr 2 O 7 2− in the solution.The degree of protonation of NH 2 -SiO 2 @RF decreased when the pH of the solution increased, and the number of OH − ions increased in the admixture.There is competition between OH − ions and the Cr(VI) of the anionic morphology for the recognition site of the adsorbent, which results in a decrease in the removal efficiency of Cr(VI).Therefore, the pH was selected as 2.0 in the solution for Cr(VI) removal in subsequent experiments, which is consistent with previous studies [21].

Effect of Initial Concentration and Temperature
The adsorption capacities of NH 2 -SiO 2 @RF increased with the increase in the initial concentration of Cr(VI) and in the temperature, as shown in Figure 4c, which might be because increasing the concentration of Cr(VI) and the temperature was conducive to internal diffusion [1].Considering the actual usage environment, 25 • C was suitable for practical use.The maximum adsorption capacity for Cr(VI) of NH 2 -SiO 2 @RF was 151.6 mg•g −1 at an ambient temperature.

Effect of Contact Time
The adsorption of Cr(VI) (30, 50, and 100 mg•L −1 ) by NH 2 -SiO 2 @RF at different times was investigated at 25 • C to study the adsorption equilibrium time and the dynamic interaction between the adsorbent and adsorbate.As the results presented in Figure 4d show, the adsorption capacity increased rapidly with the increase in time from 0 to 30 min, and it then increased slowly until adsorption equilibrium was reached.The rapid adsorption growth stage could be attributed to the abundant active sites of the adsorbent surface, implying a close connection between NH 2 -SiO 2 @RF and Cr(VI) [13].Due to the increase in the amount of Cr(VI), the adsorption capacity increased, which implied an increase in the number of collisions between the Cr(VI) anions and the adsorbent.Meanwhile, the adsorption equilibrium time was prolonged with the increase in the Cr(VI) concentration, and it was necessary to set an appropriate contact time.

Adsorption Isotherm Analysis
The Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) adsorption isotherm models were used to analyze the test data and attempt to explain the mechanism of NH 2 -SiO 2 @RF for Cr(VI).The Langmuir adsorption isotherm model assumes that the target molecules (or ions) are monolayered, and the defined sites where they are fixed can only hold one molecule layer [31].The Langmuir isotherm model is shown in Equation (3) and its linear form in Equation (4): C e q e = C e q m + 1 q m KL (4) Polymers 2023, 15, 4094 9 of 16 where q m (mg•g −1 ) denotes the maximum adsorption capacity, and K L (L•mg −1 ) is the Langmuir constant.R L can describe the basic feature of the Langmuir isotherm model to a certain extent and can be given by Equation ( 5), The value of R L indicates the character of adsorption and the shape of the isotherm; when the adsorption is unfavorable with R L > 1 and favorable with R L < 1, the linear adsorption isotherm becomes R L = 1 [32].
Moreover, the Freundlich isotherm model is an empirical equation based on multilayer adsorption on a heterogeneous surface [33].Equation ( 6) and its linear form, given in Equation ( 7), express this empirical model: where K f (mg and n are Freundlich constants related to adsorption capacity and intensity, respectively.In addition, 1/n ranges between 0 and 1 for a suitable adsorption system.The Temkin model takes into account the effects of adsorbent-adsorbate interactions.It assumes that the heat of adsorption of all molecules in the layer decreases linearly, rather than logarithmically, with the increasing surface coverage [31].The Temkin isotherm model is shown in Equation ( 8) and its linear form is given as Equation ( 9): ) where R (8.314 J•mol −1 •K −1 ) is the ideal gas constant, T (K) is the absolute temperature, K T (L•g −1 ) is the Temkin isotherm equilibrium binding constant, and b T is the Temkin isotherm constant related to the heat of sorption (J•mol −1 ).The Dubinin-Radushkevich (D-R) model is a semi-empirical equation in which adsorption follows a pore-filling mechanism with a Gaussian energy distribution onto heterogeneous surfaces.It has been successfully used to quantitatively describe the adsorption of gases onto microporous adsorbents [34].Equation (10) and its linear form, Equation (11), express this empirical model: ln q e = ln q m − K DR RTln 1 + 1 C e 2 (11) where K DR (mol 2 •kJ −2 ) is the D-R isotherm constant, and ε (kJ•mol −1 ) is the potential Polanyi.
Both the nonlinear and linearized forms of the four models were used to obtain best fits to the adsorption equilibrium data.The obtained curves are illustrated in Figure 5a-c.Table 2 summarizes the isotherm parameters and correlation coefficients obtained from the best fits to data using the nonlinear and linearized forms of the four models, respectively.The Langmuir and Temkin models fitted the adsorption process reasonably well, followed by the Freundlich model.The maximum adsorption capacity calculated by the Langmuir model was closer to the experimental results.According to the assumption of the Langmuir adsorption isothermal model, it could be concluded that the adsorption of Cr(VI) in the solution by the NH 2 -SiO 2 @RF composite likely constituted monolayer adsorption.Meanwhile, the calculated 1/n values of the Freundlich model were smaller than 1, indicating that the adsorption reaction could easily occur.Furthermore, the values of R L were much smaller than 1 at all the three temperatures, which indicated that the adsorption of Cr(VI) on NH 2 -SiO 2 @RF was advantageous.
Both the nonlinear and linearized forms of the four models were used to obtain best fits to the adsorption equilibrium data.The obtained curves are illustrated in Figure 5a-c.Table 2 summarizes the isotherm parameters and correlation coefficients obtained from the best fits to data using the nonlinear and linearized forms of the four models, respectively.The Langmuir and Temkin models fitted the adsorption process reasonably well, followed by the Freundlich model.The maximum adsorption capacity calculated by the Langmuir model was closer to the experimental results.According to the assumption of the Langmuir adsorption isothermal model, it could be concluded that the adsorption of Cr(VI) in the solution by the NH2-SiO2@RF composite likely constituted monolayer adsorption.Meanwhile, the calculated 1/n values of the Freundlich model were smaller than 1, indicating that the adsorption reaction could easily occur.Furthermore, the values of RL were much smaller than 1 at all the three temperatures, which indicated that the adsorption of Cr(VI) on NH2-SiO2@RF was advantageous.

Models T/K Linear Nonlinear
Langmuir

Adsorption Kinetic Model
Adsorption kinetic models not only describe the rate of adsorption but can also be helpful for understanding the mechanism of adsorption.The pseudo-first-order and pseudo-second-order models were used to analyze the data [15].The quasi-first-order kinetic model can be descripted by Equation ( 12): log q e − q t = logq e − k 1 t 2.303 (12) where q t (mg•g −1 ) is the adsorption capacity at time t, and k 1 /min −1 is the quasi-first-order rate constant.The quasi-second-order model can be given in the form of Equation ( 13): where k 2 ((g•mg −1 )•min −1 ) is the rate constant of quasi-second-order adsorption and the values of the constants are calculated from the straight-line plots of t/q t versus t.
The simulation results are illustrated in Figure 5f,g and Table 3.The size of the obtained linear R 2 was calculated using two kinetic equations to select the best matching model.The results showed that the R 2 values calculated from the pseudo-second-order equation were larger than those obtained from the pseudo-first-order equation.The value of the equilibrium adsorption capacity Qcal calculated from the pseudo-second order equation was similar to the experimental value Qexp.Meanwhile, the value of k 2 decreased with the increase in the Cr(VI) concentration in the solution.In short, the results suggested that the pseudo-second-order model was suitable for the description of the adsorption kinetics and that chemisorption played an important role in the adsorption process [35].

Intraparticle Diffusion Model
The intraparticle diffusion model [36], the Weber-Morris plot (q t versus t 1/2 ), is often used to measure the functional relationship in the adsorption process, which can be calculated using Equation ( 14): where k i (mg•g −1 •min −1/2 ) is the diffusion rate constant, and the intercept C relates to the thickness of the boundary layer.The plots of q t versus t 1/2 for Cr(VI) adsorption by the adsorbent, which were linearly fitted, are displayed in Figure 5h.
The fitting yields a multi-linear model composed of three parts, indicating that the adsorption process consists of three steps.The first section, the process of external diffusion, indicates that the Cr(VI) in the solution passed through the film or the boundary layer via molecular diffusion or convection diffusion into the outer surface of NH 2 -SiO 2 @RF.The second step is the internal diffusion process, and the model parameters were generally calculated using this section when Cr(VI) is transferred to the inner surface of NH 2 -SiO 2 @RF through pore diffusion.The third step is relatively simple, representing the adsorption equilibrium stage [37].As shown in Figure 5h, none of the linear straight lines passed through the origin, indicating that intraparticle diffusion was not the only a rate-limiting step, but that there were other adsorption stages.Moreover, the intraparticle diffusion rate constant k i .increased with the increase in the Cr(VI) concentration.This result likely occurred because the diffusion driving force increased with the increase in the Cr(VI) concentration, which accelerated the rate of Cr(VI) diffusion into the material surface through the hole in the solution.The intercepting C reflects the influence of the boundary layer, and the large value implied that the boundary layer effect was also large [26].

Thermodynamic Study
In order to explore the adsorption process further, a thermodynamic study was conducted at 298, 308, and 318 K. thermodynamic parameters [26].Gibbs free energy ∆G, standard entropy ∆S, and standard enthalpy changes ∆H associated with the adsorption of Cr(VI) onto NH 2 -SiO 2 @RF can be calculated from the temperature-dependent adsorption isotherm using the following equations: where K d is the solid-liquid distribution coefficient.∆S and ∆H are calculated from the intercept and slope of the linear plot of lnK d vs. 1/T (lnK d = −2.418/T+ 3.9589 correlation coefficient: 0.9923), respectively.The calculated values are tabulated in Table 4.The negative ∆G values (−9.79~−10.45kJ•mol −1 ) illustrate the feasibility and spontaneity of the adsorption process [6,36].The positive ∆H value (0.02 kJ•mol −1 ) indicates that the adsorption process was endothermic.The positive value of ∆S (32.91J•mol −1 •K −1 ) demonstrated the increase in randomness at the solid-liquid interface during Cr(VI) adsorption onto NH 2 -SiO 2 @RF [23,37].

Anti-Interference Examinations
To evaluate the potential of the adsorbent in real water samples, the effect of ions that may coexist with Cr(VI) in a solution on their adsorption efficiency was investigated.The familiar coexisting ions (K + , Na + , Mg 2+ , Ca 2+ , Fe 2+ , Cu 2+ , Pb 2+ , Co 2+ , Ni 2+ , Cr 3+ , Cl − , NO 3 − , NO 2 − , CO 3 2− , SO 4 2− and PO 4 3− ) were studied.In brief, 300 mg•L −1 of the above ions with 5 mL 30 mg•L −1 Cr(VI) solution was used for the above adsorption experiment.As the results in Figure 6a show, the influence of the concentration of cations and anions, which was studied ten times, had hardly any effect.This indicates that there was almost no competition between Cr(VI) and the aforementioned ions, which would make the recovery of the adsorbent from wastewater samples advantageous.

Desorption and Reusability
The reusability of the material is a key factor in evaluating whether the adsorbent has potential practical application value.A lower pH was beneficial for adsorption, but conversely, alkaline conditions could be helpful for the desorption of Cr(VI).The adsorptiondesorption experiment of the above-mentioned parts, with 0.5 mol•L −1 NaOH as the desorbent, was used to investigate the reusability times of NH 2 -SiO 2 @RF.As the results in Figure 6b show, the removal efficiency of 30 mg•L −1 Cr(VI) is still maintained at more than 85% after five adsorption-desorption cycles of regeneration, which indicated that the material has good reusability and high stability for Cr(VI) adsorption in solution.
familiar coexisting ions (K + , Na + , Mg 2+ , Ca 2+ , Fe 2+ , Cu 2+ , Pb 2+ , Co 2+ , Ni 2+ , Cr 3+ , Cl − , NO3 − , NO2 − , CO3 2− , SO4 2− and PO4 3− ) were studied.In brief, 300 mg•L -1 of the above ions with 5 mL 30 mg•L −1 Cr(VI) solution was used for the above adsorption experiment.As the results in Figure 6a show, the influence of the concentration of cations and anions, which was studied ten times, had hardly any effect.This indicates that there was almost no competition between Cr(VI) and the aforementioned ions, which would make the recovery of the adsorbent from wastewater samples advantageous.

Desorption and reusability
The reusability of the material is a key factor in evaluating whether the adsorbent has potential practical application value.A lower pH was beneficial for adsorption, but conversely, alkaline conditions could be helpful for the desorption of Cr(VI).The adsorptiondesorption experiment of the above-mentioned parts, with 0.5 mol• L −1 NaOH as the desorbent, was used to investigate the reusability times of NH2-SiO2@RF.As the results in Figure 6b show, the removal efficiency of 30 mg•L −1 Cr(VI) is still maintained at more than 85% after five adsorption-desorption cycles of regeneration, which indicated that the material has good reusability and high stability for Cr(VI) adsorption in solution.

Comparison of adsorption properties
Table 5 presents a comparison of the adsorption capacities of NH2-SiO2@RF with other amino-functionalized adsorbents for Cr(VI) removal.The maximum adsorption capacity based on the Langmuir model (qm) was 272.6 mg• g −1 , which was higher than those of amine-functionalized mesoporous silicas reported in the literature (Table 5), excluding the Cr(VI) adsorption capacity (330.9 mg• g −1 ) of Kim et al. [22].

Primary Stage of Practical Application
Tannery wastewater was selected as the actual water sample to further verify the practical application of the materials.The tannery wastewater was filtered using a hydrophilic PTFE syringe filter (0.22 µm).After that, the Cr(IV) in tannery wastewater was treated under the optimum conditions selected by the batch adsorption studies.After the adsorption procedures, the tubes containing the mixture were centrifuged at 7500 rpm for 10 min.Then, the supernatant liquors were acidized using phosphoric acid and complexed with DPC; they were detected with full wavelength scanning using a UV-vis spectrophotometer.Most of the yellow color came from tannin extracts and other pigments in the wastewater.Impurities had little effect on chromium detection and were removed via filtration.The results obtained after the adsorption experiment are illustrated in Figure 7, which shows that the initial color of the sample was light brown-yellow; the color deepened after ultrasonic dispersion with the addition of the adsorbent, and, finally, the solution became clear and transparent after the adsorption reaction (Figure 7a,b).On the corresponding Uv spectrum, there were two obvious valley peaks in the initial tannery wastewater.After material adsorption, the valley peak disappeared and was basically close to the baseline.Owing to the multiple effects of the functional groups and structures of the material, complex compounds in tannery wastewater can be effectively removed, showing the material's potential practical application value.treated under the optimum conditions selected by the batch adsorption studies.After the adsorption procedures, the tubes containing the mixture were centrifuged at 7500 rpm for 10 min.Then, the supernatant liquors were acidized using phosphoric acid and complexed with DPC; they were detected with full wavelength scanning using a UV-vis spectrophotometer.Most of the yellow color came from tannin extracts and other pigments in the wastewater.Impurities had little effect on chromium detection and were removed via filtration.The results obtained after the adsorption experiment are illustrated in Figure 7, which shows that the initial color of the sample was light brown-yellow; the color deepened after ultrasonic dispersion with the addition of the adsorbent, and, finally, the solution became clear and transparent after the adsorption reaction (Figure 7a and b).On the corresponding Uv spectrum, there were two obvious valley peaks in the initial tannery wastewater.After material adsorption, the valley peak disappeared and was basically close to the baseline.Owing to the multiple effects of the functional groups and structures of the material, complex compounds in tannery wastewater can be effectively removed, showing the material's potential practical application value.

Table 1 .
BET surface areas, pore volumes and pore size of NH 2 -SiO 2 @RF.

Table 3 .
Adsorption parameters obtained from the kinetic equation and intraparticle diffusion model for the adsorption of Cr(VI) by NH 2 -SiO 2 @RF at different concentrations.

Table 5 .
Comparison of adsorption capacities with similar materials.

Table 5 .
Comparison of adsorption capacities with similar materials.