Effective Adsorption of Methyl Orange on Organo-Silica Nanoparticles Functionalized by a Multi-Hydroxyl-Containing Gemini Surfactant: A Joint Experimental and Theoretical Study.

A novel multi-hydroxyl-containing gemini surfactant (G16) is first designed for modifying silica precursors (SiNPs), with the purpose of fabricating organic adsorbents targeted at methyl orange (MO). The purity of G16 and structural character of the resultant G16-SiNPs are unveiled through Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetry-derivative thermogravimetry, scanning electron microscopy, and surface analysis (BET). Compared with SiNPs, G16-SiNPs exhibit enhanced hydrophobicity, enlarged interlayer spacing, and increased thermal weight losses with the modifier availability reaching as high as 100%. Enhanced MO adsorption is obtained from the higher adsorption capacity of G16-SiNPs (401.88 mg/g) than SiNPs (64.72 mg/g), which is more effective than most of the existing silica-based adsorbents. Pseudo-second-order and Langmuir models conform to all adsorption processes, indicating that the adsorption mainly relies on the availability of adsorption sites and characterized by a homogeneous adsorption form. By combining the experimental study and theoretical calculation methods, it can be demonstrated that the as-synthesized adsorbent G16-SiNPs own multi-active sites that contribute to multi-adsorption mechanisms. The partition process, electrostatic interactions, and OH-π interactions are all responsible for the adsorption performance of G16-SiNPs. This study throws light on the exploration of the superb MO adsorbent in aspects of not only the novel structured modifier and precursor but also theoretical analysis for gaining insights into the adsorption mechanism.

Introduction

Water pollution has become a big challenge to the global environment and the human race.[1,2] Among the various pollutions to the water environment, dyes used in textiles, chemicals, and other fields pose a threat to the ecosystem that cannot be ignored.[3] Dyes are generally persistent to biodegradation, pyrolysis, and photolysis, posing non-negligible harmful potentialities such as carcinogenicity, mutagenicity, and dysfunction to the human kidney, brain, and liver.[4] Therefore, purification of dye-contaminated wastewater is of great challenge in the contemporary purification technology.[5]

Methyl orange (MO), a typical anionic mono-azo dye, is commonly used in textile and printing industries. Due to the toxic effects of MO degradation intermediates, excessive release of MO results in severe harmful effects. If not properly treated, MO intermediates would remain for a long period of time, leading to potential hazards to the living systems. Numerous purification methods, such as chemical and biological oxidation, photocatalysis, and adsorption, have been explored to deal with MO wastewaters.[6] From the practical perspective, adsorption is recognized as highly selective, facile, and economic in terms of design, operation, and regeneration.[7]

Optimization and enhancement of the adsorbent ability are crucial parts in the adsorption technology. The potentiality of biomass, clays, silicate materials, polymeric and carbonaceous materials, etc., has been extensively exploited for the removal of organic pollutants and dyes.[7,8] Particularly, rigid artificial silica with a controllable structure that can be synthesized simply and quickly (especially 3D silica nanoparticles) emerges as a superb candidate for preparing excellent organic adsorbents.[9,10] The abundant hydroxyl groups and hydrophilic nature within silica nanoparticles, however, limit its affinity and adsorption capacity toward most hydrophobic organic pollutants, which, on the other hand, could be reinforced by specific functionalization routes and/or organic reagents.[11] In the monomeric modifier system, organic modification of the silica precursor (SiNP) has been proven more effective than covalent grafting for the enrichment of smaller dye molecules.[12]

During the whole organic modification and adsorption processes, the modifier structure plays a vital role in aspects of (i) modulating the precursor microstructure, (ii) increasing the precursor hydrophobicity, and (iii) providing additional intermolecular interactions between the adsorbent and targeted adsorbate.[13,14] Gemini surfactants are generally characterized by the linkage of a covalently bonded spacer with two hydrophobic alkyl chains and head groups,[15] whose superiority over the monomeric one has been revealed on bentonite, montmorillonite, and vermiculite in terms of larger interlayer spacing, stronger hydrophobicity, and higher adsorption capacity.[16] The partition process and electrostatic interactions are mainly responsible for the retention of organic pollutants. Notably, adsorption reinforcement could be further achieved through hydrogen bonds, XH−π interactions (CH−π, OH−π, NH−π, etc.), π–π interactions between the aromatic rings, and interactions between the aromatic ring and heteroaromatic ring.[17−19] The introduction of the hydroxyl group into the modifier structure provides not only hydrogen bonds but also non-negligible OH−π interactions, which have been proven effective for the retention of sulfamethoxazole (SMX) and phenols. Moreover, the number of hydroxyl groups in the modifier is positively correlated with the capacity, application effect, and loading capacity of the adsorbent.[20]

Notably, theoretical simulation methods are powerful tools for understanding the mechanism in various adsorption systems.[21] Insights have been gained from frontier molecular orbitals, adsorption isosteric heat, and bond lengths.[22] Possible active sites and intermolecular interactions could be predicted by the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) through accepting or donating electrons based on density functional theory (DFT),[23] which have been successfully applied to analyze interactions between organo-SiNPs and tritoluene dyes (malachite green, crystal violet, and bromophenol blue).[12] To date, theoretical investigations about the modification of organo-SiNPs are limited at hexadecyl trimethyl ammonium bromide and hexadecyl trimethoxysilane, which are seldomly extended to the modification and simulation about organic modification of SiNPs by functional gemini surfactants.[12]

To explore the potentiality of the SiNP-based adsorbent, to enrich the existing modifier species, and to pursue the novel adsorbent with prominent properties, a multi-hydroxyl-containing gemini surfactant (G16) is first adopted to functionalize SiNPs for preparing organic adsorbents toward MO. The structural property, thermostability, modifier arrangement, and availability of the resultant G16-SiNPs are revealed through a series of probing techniques. The influencing factors of the adsorption process were explored. On this basis, the kinetic and thermodynamic properties of the adsorption process were obtained, and the adsorption mechanism was clarified. The relationships between G16-SiNPs and MO are elucidated not only by geometry optimization but also by frontier molecular orbitals by DFT. Finally, considering the recyclability of the adsorbent, research studies on recovery and regeneration were carried out. The results in this study not only present a detailed and systematic view of a superb MO adsorbent but also provide guidelines for designing novel silica-based adsorbents and throwing light on multi-hydroxyl groups for adsorption enhancement.

Experimental Section

Materials

Na2SiO3·9H2O is provided by Sigma-Aldrich. The raw materials for synthesizing G16, N,N-dimethylhexadecylammonium (DHA), and 1,4-dibromo-2,3-butanediol (DBBD) are obtained from Sigma-Aldrich. HNO3, ether, ethanol, and n-propanol are supplied by Beijing Chemical Works. The pH-adjusted reagents HCl and NaOH are supplied by Beijing Chemical Works. MO is obtained from J&K Scientific Ltd. All chemicals are of analytical grade and used without further purification.

Experimental Procedures

G16 is synthesized as follows: certain amounts of DHA (17.6 mmol, excess 20%) dissolved in 35 mL of n-propanol are placed in a 100 mL round-bottomed flask. After adding 7.33 mmol of DBBD dropwise, the mixture is stirred at 95 °C for 64 h. The raw products are obtained by vacuum evaporation and recrystallization from ether/ethanol mixtures. The purity of G16 is examined by the melting point (mp), elemental analysis (EA), and Fourier transform infrared spectroscopy (FT-IR).

The synthesis procedure of SiNPs is to dissolve 6.5 g of Na2SiO3·9H2O in deionized water (the molar ratio of H2O to Na2SiO3 is 60:1). Subsequently, 3 mol/L of HNO3 is added dropwise into the mixture under vigorous stirring until the pH reaches 7 under 30 °C. Then, the mixture is aged at 60 °C for 20 h. The final product is obtained by washing with deionized water, centrifuging five times, and drying at 100 °C overnight. The cationic exchange capacity (CEC) of SiNPs is 42 mmol/100 g.[2]

G16-SiNPs are prepared as follows: after separately dissolving certain amounts of G16 (0.42, 0.84, 1.26, 1.68, and 2.10 mmol, corresponding to 1, 2, 3, 4, and 5 CEC of SiNPs, respectively) and 1 g of SiNPs in deionized water (40 and 10 mL, respectively), the solution is stirred for 1 h under 70 °C. Organic modification is proceeded by mixing G16 and SiNP solutions and reacting the mixture under 70 °C for 3 h. The final products are obtained by centrifuging, washing, and drying overnight.

Characterization

Fourier transform infrared (FT-IR) spectroscopy (MO, G16, SiNPs, G16-SiNPs, and adsorbed G16-SiNPs), X-ray diffraction (XRD; XRD patterns of SiNPs and G16-SiNPs), thermogravimetric analysis [thermogravimetry-derivative TG (TG-DTG) curves of G16, SiNPs, and G16-SiNPs], EA (EA of G16, SiNPs, and G16-SiNPs), and scanning electron microscopy (SEM; SEM images of SiNPs and G16-SiNPs) are conducted to characterize the structural properties. The values of zeta potential of G16-SiNPs are recorded by immersing 0.03 g of G16-SiNPs in water solutions at pH of 3, 5, 7, and 9. Specific operating parameters are presented in the Supporting Information.

Adsorption Experiment

Batch adsorption experiments are adopted for evaluation of the adsorbent performance and clarification of the adsorption mechanism. Analysis about the adsorption mechanisms (such as kinetics, isotherms, and thermodynamics) is conducted by mixing G16-SiNPs and MO solution under varying time intervals, MO concentrations, and temperatures, respectively. The effect of pH on adsorption is conducted at the values of 3, 5, 7, and 9. The adsorption experiments are performed using the batch equilibration technique. 0.03 g of G16-SiNPs is dispersed into 30 mL of MO solution in a 50 mL conical flask (with a sealed cap). Also, the mixtures are shaken at a speed of 200 rpm in a constant temperature water bath for 3 h. After centrifugation, the supernatant is analyzed with a UV-2550 spectrophotometer at 464 nm at a pH of 7. The concentration of MO is calculated by the standard curve equation. The adsorption capacity of G16-SiNPs toward MO is obtained by the following equation:where qe (mg/g) is the adsorption capacity of G16-SiNPs and C0 and Ce (mg/L) are the initial and equilibrium concentrations of MO solution, respectively. m (g) is the mass of the adsorbent and V (L) is the volume of MO solution.

The regeneration experiment is conducted by immersing 1 g of the spent adsorbent in 50 mL of ethanol and stirring for 3 h under room temperature. Conditions of the re-adsorption are the same as those of adsorption experiments.

Theoretical Calculations

All the quantum chemical calculations are performed by using Material Studio 8.0. The calculation of the frontier orbitals and optimized structures of molecules is based on the DFT. Calculations are conducted using the Dmol3 module, in which the process using the function GGA&BLYP (generalized gradient approximation and Becke–Lee–Yang–Parr) is used. To obtain the optimized adsorbent–adsorbate structure, the Adsorption Locator module is used, and the force field is set to universal.

Results and Discussion

Characterization of G16

The FT-IR spectra of G16 are shown in Figure a. Peaks at 2919 and 2850 cm–1 are associated with the stretching vibrations of the alkyl chain within G16, whose bending vibration is characterized by the peak at 1468 cm–1. Peaks around 3250 (3183) and 3517 cm–1 are resulted from the OH groups (two OH groups with different conformations) and water molecules within the G16 structure.

Figure 1

FT-IR spectra (a), XRD patterns (b), and TG-DTG curves (c) of G16, SiNPs, and G16-SiNPs.

EA results of G16: calculated value: C 63.80%, H 10.89%, N 4.30%. Test value: C 64.21%, H 10.06%, N 4.82%. The mp of G16 is measured to be 196.3–197.0 °C. Combining the results from FT-IR, EA, and mp, it can be concluded that G16 is the target product with a high purity.

Characterization of G16-SiNPs

FT-IR Analysis

The FT-IR spectra of SiNPs and G16-SiNPs are depicted in Figure a. In the spectrum of SiNPs, the anti-symmetric and symmetric stretching vibrations of Si–O bonds are observed at 1097 and 835 cm–1, respectively, representing the existence of the Si–O framework.[24,25] The peak at 1359 cm–1 is related to the N=O stretching vibrations derived from HNO3 (i.e., the solvent for synthesizing SiNPs),[26] indicating the existence of interactions between N=O (i.e., NO3–) and the surface of SiNPs, which is consistent with the considerable N content in SiNPs (Table ). Moreover, due to the negative charge of NO3–, its existence on the SiNP surface may be responsible for the CEC of SiNPs, offering considerable active sites for the combination of the cationic modifier G16.[12]

Table 1

Elemental Analysis Results (wt %) of SiNPs, G16, and G16-SiNPs.

sample	C	H	N
SiNPs	0.12	2.32	2.73
G16	64.21	10.06	4.82
G16-SiNPs	40.22	6.94	5.66

The spectrum of G16-SiNPs exhibits combined characteristic peaks of G16 and SiNPs, supporting the successful modification process. As shown in the FT-IR spectra of G16-SiNPs, peaks at 2919, 2850, and 1468 cm–1 indicate the incorporation of the alkyl chain into the structure of G16-SiNPs. Compared with the raw SiNPs, the shift of the OH-associated peak (from 3481 to 3391 cm–1) and the structural Si–OH peak (from 1359 to 1468 cm–1) in G16-SiNPs indicate the hydrophobicity variation due to the intercalation of G16.[27] Moreover, variation in the location and strength of the N=O peak in G16-SiNPs compared with SiNPs (from 1359 to 1371 cm–1) indicates the participation of NO3– for bonding with G16 in the modification process.

XRD Analysis

SiNPs were present as an agglomerated morphology with silica particles orderly connecting to each other.[2] After incorporation with G16, intermolecular interactions between adjacent modifiers on different precursor surfaces would further facilitate the aggregation into a staked whole, thus leading to the expansion of interlayer space and emergence of interlayer spacing in the XRD patterns of G16-SiNPs (Figure b).

Modifier arrangement within SiNPs could be deduced from the combined information of the G16 molecular size and interlayer spacing of G16-SiNPs.[28] The lengths of the alkyl chain and the spacer and the height of G16 are approximately 1.98, 0.63, and 0.31 nm, respectively. As seen from Figure b, the interlayer spacing of G16-SiNPs increases from 4.11 nm (0.42 mmol) to 4.74, 3.08, and 1.52 nm (0.84–2.10 mmol). It can be deduced that the arrangement of G16 is evolved from the paraffin-bilayer arrangement to the coexistence of the pseudotrimolecular layer, paraffin bilayer, and lateral bilayer arrangements. The more the modifier dosage, the greater the diversity of modifier molecules arranged within the interlayer space of SiNPs. The XRD patterns of G16-SiNPs remain constant at the modifier dosage of 0.84 mmol, suggesting the saturated quantity of G16 on SiNPs.

TG-DTG Analysis and EA Results

TG-DTG analysis is adopted to verify the hydrophobic variation during the modification process and to estimate the thermostability of the resultant G16-SiNPs. The TG-DTG curves of SiNPs and G16-SiNPs are shown in Figure c, and the TG-DTG curves of G16 are depicted in Figure S1.

As shown in Figure S1, a sharp weight loss of G16 is observed at around 305.8 °C, indicating that the decomposition of G16 starts at as high as 300 °C. Compared with the weight losses in Figure c, organic functionalization decreases the thermostability of G16, which may be due to the involvement of interactions between the organic modifier and SiNPs. The intercalation manner of G16 into SiNPs and the thermostability of G16-SiNPs are evaluated by TG-DTG curves (Figure c). Two distinct weight loss stages are observed in the raw SiNPs (52.9 and 718.0 °C), which mean the desorption of physically adsorbed water molecules and dihydroxylation of Si–OH units, respectively.[29] With the increase of temperature, the hydroxyl groups start to get removed after the loss of physically adsorbed water molecules.[30] At the temperature of around 700–1000 °C, almost all the silica surface would condense into the siloxane condition. In the dehydroxylation stage, the removal of one water molecule means that two hydroxyl groups are reduced.[31] The existence of enormous structural Si–OH is validated by considerable weight losses around 718 °C (nearly 40.2%).

Compared with SiNPs, disappeared peaks at 52.9 and 718.0 °C verify (i) the increased hydrophobicity of G16-SiNPs due to organic modification and (ii) the important role of Si– OH in the organic modification process. Interactions between the hydroxyl groups in G16 and the structural Si–OH in SiNPs would affect the stability of Si–OH, leading to the disappeared dehydroxylation process in G16-SiNPs. The loss of G16 in SiNPs is not a simple desorption procedure but also involves a complicated decomposition process, where the surfactants would decompose into small organic molecules with the increase of temperature.[32] In this study, interactions between G16 and the structural Si–OH may lead to the advanced loss of hydrogen atoms, which impedes the dehydroxylation process with the increase of temperature.

Moreover, additional weight losses between 200 and 500 °C in G16-SiNPs are resulted from the organic modifier. Organic modification is mainly achieved through electrostatic interactions (between the N+ head group in G16 and NO3– in SiNPs), physical adsorption (between the OH in G16 and Si–OH in SiNPs), and van der Waals forces (between the alkyl chains in G16).[33] The strengths of binding from the corresponding temperatures of three weight loss stages are as follows: electrostatic interactions (388.4 °C) > physical adsorption (278.4 °C) > van der Waals forces (245.8 °C).

As shown in Table , the actual modifier loading could be calculated from the higher carbon content in G16-SiNPs (40.22%) than SiNPs (0.12%). Given the actual carbon percentage of G16, the amount of the modifier loaded is calculated to be 52.63% on G16-SiNPs, which is nearly consistent with the first two weight losses in TG-DTG (51.1%), indicating that G16 molecules are bonded mainly through physical adsorption and van der Waals forces. Notably, the modifier availability (ratio of modifier molecules loaded to added) of G16 on SiNPs reaches nearly 100%, demonstrating the great potentiality and promising aspect of SiNPs for submitting organic modification processes.

SEM Images and BET Analysis

The SEM image and BET surface area of SiNPs and G16-SiNPs are measured and shown in Figure . SiNPs exhibit an unordered morphology with silica nanoparticles stacking irregularly on the incompact surface. After organic modification, the surface of G16-SiNPs is denser. Interactions (van der Waals forces and hydrogen bonds) between G16 molecules on different surfaces could aggregate the particles of G16-SiNPs into a stacked whole, thus leading to the aggregation of neighboring G16-SiNPs into a large and irregular shape.[20]

Figure 2

SEM images of SiNPs (a) and G16-SiNPs (b).

SiNPs and G16-SiNPs exhibit the surface areas of 62 and 134 m2/g, respectively. In the relative high P/P0 region, the adsorption/desorption branches of SiNPs and G16-SiNPs have a slight adsorption hysteresis, which manifest that the curves are in accordance with a type IV isotherm with type H1 (Figure S2). The BET-N2 surface area and the total pore volume (Table S1) of G16-SiNPs exhibit a slight decrease over the pristine one, which may be due to the coverage of the precursor surface by the long alkyl chains of G16, which hinder the approach of nitrogen to G16-SiNPs.

Adsorption Tests

Effect of G16 Dosage on Adsorption

Variation of MO adsorption with the modifier dosage is depicted in Figure (the initial MO concentration is 500 mg/L). As shown in Figure , MO adsorption amounts are greatly improved after the functionalization of G16 on SiNPs (approximately 300–420 mg/g compared with the adsorption capacity of the raw SiNPs of 64.72 mg/g), indicating the great improvement of G16 for MO adsorption.

Figure 3

Effect of modifier dosage on the adsorption of MO onto G16-SiNPs.

In terms of G16-SiNPs, MO adsorption keeps increasing, followed by gradual decrements with G16 feeding. These trends could be explained as follows: on one hand, the inserted G16 enhances the hydrophobicity of the alkyl chain and at the same time generates hydrogen bonds and OH−π interactions. This series of interactions produces a large number of activation points on the adsorbent surface that can be used for adsorption, which promotes the upward trend of adsorption.[34] On the other hand, excess G16 molecules would result in severe blocking of the adsorbent gallery and porosity, which impedes the adsorption process for a further degree.[35]

Given the adsorption capacity and mole ratio of MO to G16, 0.84 mmol is set as the saturated modifier dosage in the subsequent adsorption experiments, which is consistent with XRD results.

Effect of Time and Adsorption Kinetics

A slow adsorption equilibrium is observed with prolonged time (Figure a), and the required equilibrium time is approximately 120 min. To further clarify the kinetic characters, pseudo-first-order, pseudo-second-order, and intra-particle diffusion models are adopted to fit the adsorption data (Table ).

Figure 4

Effects of time (a), MO concentration (d), pseudo-second order (b), intra-particle diffusion (c), and Langmuir isotherm (e) on the adsorption of MO onto G16-SiNPs.

Table 2

Kinetic Parameters for the Adsorption of MO onto G16-SiNPsa

		pseudo-first order				pseudo-second order					intra-particle diffusion
adsorbents	qe,exp	qe,cal	k1 (10–2)	R2	SSE (10–4)	qe,cal	k2 (10–2)	k2qe	R2	SSE (10–4)	kid	C (102)	R2	SSE
G16-SiNPs	401.88	44.79	2.5	0.8815	3.01	401.61	0.249	1.51	0.9999	0.19	2.93	3.68	0.9941	0.21

qe,exp (mg g–1), qe,cal (mg g–1), k1 (min–1), k2 (g mg–1 min–1), kid (mg g–1 min1/2), C (mg g–1).

The equations of pseudo-first-order (2), pseudo-second order (3), and intra-particle diffusion (4) models are expressed as follows:[20]where k1 (min–1) and k2 (g (mg min)−1) are the rate constants. qe (mg g–1) and qt (mg g–1) stand for the adsorption capacities at equilibrium and at time t (min), respectively. All these unknown parameters can be determined from plots of ln(qe – qt) against t and t/qt against t. kid (mg g–1 min–1/2) is the rate constant of the intra-particle diffusion, and the values of C and kid can be determined from the intercept and slope of the linear plot of qt against t1/2, respectively.

The pseudo-second-order model (Figure b and Table ) provides the best correlation for the adsorption process with a higher correlation coefficient (the value of R2 is 0.9999), a lower SSE value (0.19), and greater consistency of qe,cal to qe,exp (401.61 to 401.88 mg/g), suggesting that the adsorption processes may be mainly affected by chemical adsorption.

Moreover, three sections during the adsorption process could be observed from the intra-particle diffusion model (Figure c).[36] The high concentration of MO solution diffuses to the adsorbent surface rapidly, gradually occupying the active binding sites on the adsorbent surface and weakening the driving force by concentration gradient between MO solution and the adsorbent surface. The gradual diffusion of MO into G16-SiNPs could account for the slowing down of the second stage in the intra-particle diffusion model. The adsorption capacity attains the final equilibrium in third stage.[37] Intra-particle diffusion is not the only reason that affects the rate. The adsorption rate is controlled by multiple processes, which is supported by the fact that all the lines do not pass through the origin (Figure c).

Concentration of MO and Isotherms

To unveil the adsorptive effect of G16-SiNPs, one of the UV–vis spectra of neat MO solution before and after adsorption is measured and presented in Figure S3. Considering the measuring range of UV–vis, an initial MO concentration of 100 mg/L is chosen in the measurement of UV–vis spectra. As shown in Figure S3, the wide and strong characteristic peak of the spiked MO sample is greatly weakened after treatment by G16-SiNPs, indicating the effective removal performance of G16-SiNPs. Meanwhile, the color difference between MO solution before and after adsorption reveals the obvious adsorptive effect of G16-SiNPs from a visual perspective. This result further confirms that G16-SiNPs are a kind of efficient adsorbent for MO.

The relationship between the MO concentration and the amount of adsorption is given in Figure d. It can be clearly seen that as the MO concentration increases, the amount of adsorption increases too. The great concentration difference between MO solution and the surface of the adsorbent enhances the driving force of adsorption and other interactions (such as OH−π interactions between G16-SiNPs and MO or π–π stacking between MO molecules). The highest adsorption amounts of MO could reach as high as 665 mg/g under the tested conditions (298 K with the initial MO concentration of 700 mg/L).

The Langmuir, Freundlich, and Redlich–Peterson isotherms are expressed as follows:[20]where qe (mg/g) is the adsorption capacity onto per unit mass of the adsorbent at equilibrium, Ce is the solute equilibrium concentration (mg/L), and qm (mg/g) is the maximum theoretical adsorbed amount. kL (L/mg), kf (mg/g), and n represent the constants of Langmuir and Freundlich, respectively. A (L/g) and B ((L/mg)) are the Redlich–Peterson model constants. g fluctuates between 0 and 1.

Compared with the linear formed Langmuir isotherms, the nonlinear Langmuir model (Figure e) has been demonstrated to be the most accurate for representing the experimental data.[38] The nonlinear equilibrium isotherms are summarized in Table . The correlation coefficients R2 calculated by the Langmuir model at all tested temperatures are all greater than 0.99, which means a better fitting than the Freundlich model, suggesting that MO molecules are captured by specific sites within G16-SiNPs and arranged in a monolayer form. Due to the high modifier availability, G16-SiNPs could offer abundant active sites for MO adsorption. From the isotherm results, it can be deduced that MO is mainly immobilized and interacts with G16 through hydrophobic and OH−π interactions, which is characterized by monolayer arrangement. Higher affinity between G16 and MO than that between adsorbed MO and dissolved MO molecules weakens the secondary interactions (i.e., π–π stacking),[39] leading to the preferable monolayer arrangement, as fitted by the Langmuir isotherm.

Table 3

Adsorption Isotherm Constants for the Adsorption of MO onto G16-SiNPsa

	Langmuir			Freundlich			Redlich Peterson
temperature	qmax	KL	R2	Kf	n	R2	a	b	g	R2
298 K	852.2	0.093	0.9988	73.47	3.15	0.9754	120.67	0.33	0.78	0.9943
313 K	834.3	0.022	0.9964	61.74	2.06	0.9625	16.61	0.01	1.13	0.9958
328 K	764.4	0.016	0.9909	49.78	2.08	0.9570	10.95	0.01	1.14	0.9905

qmax (mg g–1), Ka (L mg–1), Kf (mg g–1), a (L g–1), b (J mol–1).

The maximum adsorption capacities (qmax) calculated by the Langmuir isotherm are 852.2, 834.3, and 764.4 mg/g under 298, 313, and 328 K, respectively, which are higher than those of most of the MO adsorbents reported recently (Table ).[40−46]

Table 4

Comparison of MO Adsorption Capacities on Several Organic Adsorbents

adsorbent	adsorption capacity (mg/g)	references
organo-vermiculite (Na)	155	(40)
chitosan-coated sodium zeolites	287	(41)
zeolitic imidazolate framework	145	(42)
activated carbon from endemic Vitis vinifera L. grape seeds	79	(43)
Fe–La oxides co-loaded MgO nanosheets	39	(44)
N-acyl thiolated chitosan	435	(45)
reduced graphene oxide-hydroxyapatite hybrid composite	49	(46)
G16-SiNPs	401	this study

Effect of Temperature and Adsorption Thermodynamics

MO adsorption exhibits a relative reverse trend with temperature (Figure d), verifying the exothermic nature during all MO adsorption processes. Thermodynamic parameters ΔH°, ΔG°, and ΔS° are calculated to test the thermodynamic characters.

Thermodynamic parameters could be calculated using the following equations:where KL is the Langmuir constant and qe and Ce have the same definitions as the above equations. R is the universal gas constant (8.3145 J/(mol K)) and T represents the absolute temperature in kelvin. The values of ΔH° and ΔS° can be extrapolated from intercept and slope of ln KL versus 1/T, respectively.

Negative values of ΔH° give evidence for the exothermic nature of MO adsorption (Table). The negative values of ΔS° reflect the decreases in randomness between the solid and solution interface. In addition, the positive ΔG° values suggest nonspontaneous adsorption processes. According to the literature that the physisorption energies are in the range of 0–20 kJ/mol, while the chemical sorption energies are between 80 and 400 kJ/mol,[47] it can be concluded that MO is physically adsorbed instead of chemical adsorption on G16-SiNPs.

Table 5

Thermodynamic Parameters for MO on G16-SiNPs

	ΔGo (kJ/mol)
adsorbents	T = 298 K	T = 313 K	T = 328 K	ΔSo (J/(mol K))	ΔHo (kJ/mol)
G16-SiNPs	6.28	9.02	11.76	–0.18	–48.12

Effect of Solution pH

The variation of MO adsorption on G16-SiNPs as a function of pH value is presented in Figure . The zeta potential of G16-SiNPs continuously decreases with increasing pH, which, on the other hand, is always kept positive under all experimental conditions.

Figure 5

Effect of pH on MO adsorption and zeta potential values of G16-SiNPs.

The important role of electrostatic interactions is verified from the effect of solution pH. As shown in Figure , the positive charge of G16-SiNPs keeps decreasing but remains positive at all the tested pH values. With the solution pH varying from 3 to 5, the molecular form of MO molecules plays an important role for the variation of adsorption capacity. Reduced amounts of positively charged MO are beneficial for their electrostatic attractions with the surface of G16-SiNPs, leading to the constantly increased adsorption trend. After the pH becomes higher than 5, the charge property of G16-SiNPs dominates the adsorption trend (432.44, 405.58, and 388.08 mg/g corresponding to the pH of 5, 7, and 9, respectively). The decreased positive charge of G16-SiNPs weakens the electrostatic interactions between the adsorbent and MO, indicating that the mainly controllable adsorption mechanism is not only electrostatic interactions but also the partition process and OH−π interactions (Figure ).

Figure 6

Schematic diagram of interactions between MO and G16.

Moreover, to further confirm the possible interactions between MO and G16-SiNPs, the FT-IR spectra of MO and adsorbed MO/G16-SiNPs are recorded and presented in Figure S4.

Compared with G16-SiNPs, additional peaks emerging between 1596 and 1380 cm–1 symbolize the aromatic rings of MO within the spectrum of MO/G16-SiNPs.[48] The shift of the −OH stretching vibration from 3391 to 3434 cm–1 indicates the existence of OH−π interactions. The existence of the partition process is proved by the shift of the −CH2 bending vibration from 1468 to 1596 cm–1.[49] Moreover, interactions between MO and the silica surface are verified by the variation of the Si–O peak (from 1097 to 1085 cm–1).

Molecular Simulation and Computation

Understanding the adsorption mechanism of adsorbate molecules on the surface of the adsorbent is essential to explain the interaction between them and the optimal design of this type of adsorbent. In this section, the frontier orbital theory is used to obtain the optimal structure of MO and G16 and discuss their electronic orbital-related properties. All the optimized structures had been ensured that they were global minima through frequency calculations, and the results of frequency calculations are shown in the Supporting Information. The vibrational analysis data of the optimized structures of G16 and MO are presented in Table S2.

The optimized structures of MO, G16, and their frontier orbitals have been given in Figure . Frontier orbitals include the HOMO and the LUMO. The electron on HOMO has the highest energy and is the least bound, so it is the most active and easy to change, while LUMO has the lowest energy among all unoccupied orbitals and is the easiest to accept electrons. Therefore, these two orbitals determine the electron gain or loss and transfer ability of the molecule, which determines important chemical properties such as the spatial orientation of intermolecular reactions. In addition to the orbital structure, the orbital energies (EHOMO and ELUMO) of two molecules are also given in Table .

Figure 7

Optimized geometries, frontier molecular orbitals for the organic modifier, and MO calculated by DFT.

Table 6

Frontier Orbital Energies of the Two Molecules

molecule	EHOMO (eV)	ELUMO (eV)
G16	–4.465	–1.212
MO	–0.869	0.162

When two molecules interact with each other, the electrons in HOMOs and LUMOs would exchange. The energy difference between HOMO and LUMO determines the direction and strength of electron transfer. In Table , it is obvious that the energy difference between G16-HOMO (−4.465 eV) and MO-LUMO (0.162 eV) is much greater than the energy difference between MO-HOMO (−0.869 eV) and G16-LUMO (−1.212 eV). Therefore, electrons tend to transfer from MO to G16 during the adsorption process. It can be seen from Figure that in the MO molecule, HOMO orbitals concentrate near the sulfur and oxygen atoms. For G16, LUMO is concentrated in the central part. When G16 and MO interact during the adsorption process, the SO3 group in MO will approach N atoms which are in the middle of G16 and exchange the electrons. This conclusion is also consistent with the positions of charged groups in the two molecules.

Desorption Tests

The regeneration performance of G16-SiNPs is tested by ethanol. After three cycles, the MO adsorption decreases from 401.88 to 368.24, 354.06, and 350.98 mg/g, evidencing the considerable adsorption active sites within G16-SiNPs. Although the original adsorption sites are partially occupied due to adsorption and/or desorption, there are still new binding sites emerging on the adsorbent surface, accounting for the stability during regeneration processes.[50]

Conclusions

A novel multi-hydroxyl-containing gemini surfactant (G16) is first designed for modification of emerging SiNPs for fabricating an organic adsorbent targeted at MO. The resultant G16-SiNPs exhibit enhanced hydrophobicity, enlarged interlayer spacing, and increased thermal weight losses. The immobilization strength of G16 on SiNPs is ordered in the sequence of electrostatic interactions > physical adsorption > van der Waals forces, with the combination of which contributing to the unprecedented high modifier availability (approximately 100%).

Effective MO adsorption is obtained from the comparison between SiNPs (64.72 mg/g) and G16-SiNPs (401.88 mg/g), which is higher than those of most of the existing silica-based adsorbents. Pseudo-second-order and Langmuir models conform to all adsorption processes, indicating that the adsorption rate mainly relies on the availability of adsorption sites and is characterized by a homogeneous adsorption form. Thermodynamic parameters show the nonspontaneous and exothermic nature of the adsorption process. By combining the results of adsorption kinetics, isotherms, thermodynamics, and frontier molecular orbitals, the as-synthesized adsorbent G16-SiNPs own diverse active sites that contribute to multi-adsorption mechanisms: the partition process (derived from the alkyl chain of G16), electrostatic interactions (derived from the N+ head group of G16), and OH−π interactions (derived from the hydroxyl group of G16), contributing to the excellent adsorption performance of G16-SiNPs. It can be concluded that G16-SiNPs are a synergistic, efficient, and promising adsorbent for removing anionic dyes from contaminated water.