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. 2019 Oct 18;4(18):17880–17889. doi: 10.1021/acsomega.9b02768

Understanding Mechanism of Adsorption in the Decolorization of Aqueous Methyl Violet (6B) Solution by Okra Polysaccharides: Experiment and Theory

Mengdan Wang †, Qun Gu §, Yanlong Luo †,‡,*, Danil Bukhvalov †, Xiaofeng Ma †,‡, Lijun Zhu †,‡, Gefei Li ∥, Zhenyang Luo †,‡
PMCID: PMC6822221  PMID: 31681897

Abstract

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Optimal conditions for ultrasonic-assisted extraction of polysaccharide from Chinese okra were found using response surface methodology. The okra polysaccharide (OPS) was used for the adsorption of methyl violet 6B (MV). Conditions for maximal adsorption efficiency of MV were established. The mechanism of MV adsorption was investigated by the characterization and physicochemical analysis of OPS before and after the adsorption of MV. Both infrared (IR) analysis and molecular dynamics (MD) simulation suggest that MV adsorption by OPS was an electrostatic interaction between MV and oxygen-containing groups of OPS. Further, the results of first-principles calculation were in agreement with IR spectroscopy measurements and MD simulation, which were all consistent with the suggested adsorption mechanism. Optimization of okra extraction conditions, maximized efficiency of MV adsorption by OPS, and the understanding of the adsorption mechanism are the highlights of this work, providing a reference for promising applications of OPS in the treatment of wastewater in textile, paper, and other industries.

1. Introduction

Pollution of dyes has been a great concern for industries employing large quantity of dyes for their products, such as paper, textile, and plastics. Dye-contaminated water can be highly visible even at a low dye concentration,1,2 causing public concern of health risks.3 Therefore, much attention has been paid to dye removal.4 Several methods using chemical agents for dye decolorization have been reported,2 but they have not been widely applied in textile and paper industries due to high cost and difficulty of disposal after treatment.5 Recently, decolorization methods of dye waste water using materials from modified or unmodified biomass sources have been reported.6,7 Biosorbents based on chitin, lignin, chitosan, peat, yeast, fungi, or other biomasses are employed as chelating and complexing agents to remove dyes from effluents.4,8

We use okra polysaccharides (OPS) to treat water containing the dye methyl violet 6B (MV, Table 1). As a biosorbent, OPS has its intrinsic advantages of abundance and easiness of processing. Polysaccharide molecules contain many polar groups (such as hydroxyl and carboxyl groups), providing potential sites for the adsorption of dyes. Compared to traditional methods, this kind of biosorption is more selective, effective, and economic.

Table 1. Chemical Formulas and Some Properties of MV 6B.

1.

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Recently, progress has been made in the investigation of adsorption behavior and mechanism by molecular dynamics (MD) simulation.9,10 For the polysaccharide–dye system in our study, we use a novel combination of MD simulation, first-principles calculation, and infrared (IR) analyses to establish the mechanism of OPS–MV interaction.

Okra (Abelmoschus esculentus L. Moench) is a perennial plant in the family of Malvaceae, which has a long history of cultivation and is currently grown in different temperate zones. Its edible seed-pods contain high content of polysaccharides and trace elements. Its seeds, flowers, and other parts are rich in flavonoids, amino acids, and vitamins, providing high nutritional values. Among these components, polysaccharide is of great significance, showing immunomodulatory,11,12 antineoplastic,13,14 and antihyperlipidemic effects.15

Polysaccharides can be extracted from plants using hot water. The disadvantages include substantial consumption of water, energy, and time.16 Alternative methods such as ultrasonic-, microwave-, and infrared-assisted extractions have been reported.17−19 We use ultrasonic-assisted extraction of polysaccharide from okra because of its remarkable advantages including moderate solvent consumption, short extraction time, and high extraction yield.20 To investigate the overall impacts of independent variables on extraction efficiency, classical method of studying one variable at a time can be ineffective. Therefore, we use an optimization strategy that examines the combined effects of all independent variables on the extraction efficiency, along with interactions between these variables. Response surface methodology (RSM)21−23 is a collection of statistical and mathematical tools we choose to find conditions for optimal extraction efficiency. Collectively, we utilized the ultrasonic-assisted extraction technology, along with RSM, to optimize polysaccharide extraction from okra. To carry out RSM, the Box–Behnken design with three independent variables (extraction time, liquid–solid ratio, and extraction temperature) was employed to find the best set of variables for optimal extraction yield. Further physicochemical analysis of OPS was performed to study the decolorization efficiency of OPS under various conditions. The varying factors include temperature, contact time, polysaccharide concentration, and initial dye concentration in the solution (pH value was kept at 11 for all trials). The mechanism of adsorption in the decolorization process was discussed extensively using the information obtained in IR analyses, MD simulation, and first-principle calculations.

2. Materials

All materials/apparatus used for extraction and characterization of polysaccharide are presented in the Supporting Information. Devices and instruments used include a thermostatic heating magnetic stirrer (DF-101Z), a low-temperature stirring reaction bath (DHJF-2005), a TG16-WS bench centrifuge, and a UV–vis spectrophotometer (UV-1200).

3. Experimental Section

3.1. Extraction and Physicochemical Analysis of Polysaccharide

The polysaccharide was extracted from freeze-dried okra by ultrasonic-assisted technology under optimal conditions established in this work (extraction time of 31 min, liquid–solid ratio of 51:1, and temperature of 63 °C), creating a yield of 22.35% (w/w, unless indicated otherwise). Based on physicochemical analysis, the molar ratio of rhamnose, galactose, and glucose in the OPS was 1:0.56:0.13. The content of uronic acid in OPS was 10.71%, and the average molecular weight of OPS was 68 010 Da. Details of optimization and physiochemical analysis are presented in the Supporting Information.

3.2. Adsorption Experiments

OPS belongs to anionic natural macromolecules, as indicated by its negative values of ζ-potential (see the Supporting Information). Studies have shown that the presence of cations (such as Al3+, Fe3+) improves the flocculation of anionic polymers.24 These cations can provide a bridging between negative charges on the surface of the anionic polymer and promote flocculation. In our work, Fe3+ ions facilitate the bridging effect between the negative hydroxyl and/or carboxyl groups in OPS, promoting the flocculation and precipitation (hence decolorization) of OPS–MV complexes.

Adsorption experiments was carried out by adding OPS to MV solution treated with Fe2(SO4)3 and adjusted to targeted pH value; each component in the mixture was added based on planned amount or concentration. Four independent variables, contact time (stirring time + rest time), initial concentration of MV, concentration of OPS, and temperature, were individually investigated, with other factors remaining unchanged.

A certain amount of sodium hydroxide solution was added to a 10 mL of MV solution of certain concentration, so that its pH was adjusted to 11. Then, a certain amount of Fe2(SO4)3 was added to the solution so that the final concentration of ferric sulfate was maintained at 50 mg/L. Certain amount of dried, powdery OPS was added to the above solution. The mixture was stirred for 20 min and then rested for a recorded time at a controlled temperature. Finally, the solution was centrifuged for 20 min at 600 rpm, the supernatant was retrieved for absorbance measurements at λmax of 580 nm.

The concentration of residual MV was determined based on a MV calibration curve. The dye removal efficiency (%) and the relative amount of MV adsorbed at equilibrium (qe, mg/g) was calculated as follows

3.2. 1
3.2. 2

where V is volume of the solution; m is the mass of dried OPS; and C0 and Cf are initial and final (equilibrium) concentrations of MV, respectively. The standard curve of MV is presented in Figure 1. Its linear regression equation, y = 0.00722 + 0.01869x (R2 = 0.99883), was the basis of further calculation.

Figure 1.

Figure 1

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Standard curve of MV.

3.3. Molecular Dynamic Simulations

In the simulation, the geometric optimization procedure with energy convergence tolerance of 10–5 kcal/mol was performed to obtain low potential energy characteristics for each cell by using smart minimized method. After this step, the amorphous cell was annealed at the pressure of one bar; five annealing cycles from the initial temperature of 300 K to the mid-cycle temperature of 600 K were repeated. Finally, the cell was subjected to 1 ns of NVT (constant number of particles, volume, and temperature) ensemble and 1 ns of NPT (constant number of particles, pressure, and temperature) ensemble. In each simulation, periodic boundary condition was applied. The pressure and temperature were controlled by Andersen barostat25 and Berendsen thermostat,26 respectively. The Verlet velocity time integration method27 with a time step of 1 fs was used to integrate the Newtonian equation of motion. The van der Waals interactions were calculated by the Lennard–Jones function with a cutoff radius of 1.25 nm, and the electrostatic interactions were approximated by the Ewald method.28 All the simulations were performed with Material Studio software applying a universal force field.

3.4. Density Functional Theory Based Modeling

Modeling was performed using density functional theory (DFT), implemented by means of the pseudopotential code SIESTA,29 as was done in our previous work on adsorption of molecules to metal–organic frameworks.30 All calculations were performed using generalized gradient approximation (Perdew–Burke–Ernzerhof),31 including spin polarization with consideration of van der Waals correction.32 Full optimization of the atomic positions was also carried out. During this optimization, the ion cores were described by norm-conserving nonrelativistic pseudopotentials33 with cutoff radii of 1.14, 1.48, 1.47, and 1.25 au for C, N, O, and H, respectively. The wave functions were expanded with a double-ζ plus polarization basis of localized orbitals for nonhydrogen atoms and with a double-ζ basis for hydrogen atoms. Optimizations of the force and total energy were performed with the accuracies of 0.04 eV/Å and 0.001 eV, respectively.

4. Results and Discussion

4.1. Factors Affecting the Removal Efficiency of Dye

4.1.1. Contact Time

As shown in Figure 2a, at the initial stage of adsorption, MV molecules were easily adsorbed to the active sites of the polysaccharides, thus the adsorption rate was relatively fast. With the extension of time, adsorption rate decreased gradually as less number of active sites became available, until the adsorption equilibrium was reached. At the contact time of 8 h, the adsorption capacity of OPS was reached, and the maximal dye removal efficiency was achieved. In all the following sections, measurements were performed after maximal contact time (more than 24 h).

Figure 2.

Figure 2

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Impacts of four factors on dye removal efficiency (removal percent) (a, b, d, e). (a) Contact time: concentrations of OPS and initial MV are both 50 mg/L, room temperature (RT). (b) OPS concentration: initial MV concentration is 50 mg/L, contact time >24 h, under RT. (d) Concentration of initial MV: concentration of OPS is 50 mg/L, contact time >24 h, under RT. (e) Temperature: concentrations of OPS and initial MV are both 50 mg/L, contact time >24 h. Other facts (c, f). (c) Effect of OPS concentration on ζ-potential value. (f) Decoloration results: a pure MV solution (right) compared to a sample after decolorization (left).

4.1.2. OPS Concentration

The effects of OPS concentration on the adsorption rate and ζ-potential of MV solution are shown in Figure 2b,c. In general, as the concentration of polysaccharides increases, the absolute value of ζ-potential increases (becomes more negative) significantly due to more negative charges of the OPS (an anionic polymer). However, this monotonous trend (Figure 2c) does not echo in the trend of dye removal efficiency (Figure 2b) as OPS concentration increases.

When OPS concentration increased to a certain level, ζ-potential became more negative, to about −8 mV; during this period, dye removal efficiency increased. It was reported that in this range of ζ values, coagulation–flocculation is thermodynamically favored.34,35 When the concentration of OPS in the MV solution increased further, that is, to a level >50 mg/L, the absolute value of the ζ-potential became too large, which was in favor of the formation of a stable colloidal solution. In other words, coagulation–flocculation became more difficult, hence dye removal efficiency decreased.

4.1.3. Initial Dye Concentration

A given amount of adsorbent can only adsorb a fixed amount of adsorbate, hence the initial concentration of adsorbate is important.36 Initial dye concentration affects the rate of adsorption and relative number of sites on OPS available for adsorbing MV molecules.37

According to Figure 2d, at first, the decolorization rate increased steeply with the increase of MV concentration. Before the MV concentration becomes too large, there is a relatively large number of unoccupied adsorption sites on the OPS molecules, allowing easy adsorption of MV. As the MV concentration increases to an upper limit, much less adsorption sites are available, leading to an adsorption–desorption equilibrium. As the MV concentration further increases beyond the upper limit, there is no further adsorption, leading to a decrease of removal efficiency due to the increase of the denominator, as shown in eq 1.

4.1.4. Temperature

Temperature is another significant parameter because it affects the adsorption–desorption equilibrium between the adsorbent and the adsorbate.38 From 10 to 20 °C, kinetic factor dominates: as the temperature increases, there is a slight increase of decolorization efficiency, as shown in Figure 2e. Beyond 20 °C, thermodynamic factor dominates: adsorption is an exothermic process (see Section 4.2); therefore, increase in temperature leads to decrease of the adsorption capacity.

Decolorization of the MV solution is shown in Figure 2f. The color of the dye solution turned from purple to near limpidity, indicating a near complete removal of MV. The precipitation at the bottom of the left-side sample vial was the result of coagulation–flocculation after MV molecules were adsorbed by OPS.

It is worth mentioning that in studying the efficiency of MV adsorption by OPS, we kept the pH value constant at 11 for all trials. This is because the change of pH value has a dramatic impact on the protonation–deprotonation equilibrium of the MV molecules and hence the nature of the dye. Reducing the pH will lead to the protonation of MV and alter its color toward yellow. Increasing the pH prevents the protonation and helps maintain MV in the form shown in Table 1, to the extent of pH 11. Beyond pH 11, MV will become deprotonated, leading to a shift of its absorption peak. Under this condition, evaluation of MV decolorization by OPS using absorbance at 580 nm becomes impossible.

4.2. Thermodynamic Analysis of Adsorption

Adsorption isotherm models39 are fundamental in describing the interactions between adsorbents and adsorbates. We use these models to investigate the mechanism of adsorption in our study. The equilibrium experimental data were analyzed using the Langmuir and Freundlich isotherm models as follows:

(a) Langmuir isotherm model

4.2. 3

where ρe is the concentration of MV at an equilibrium (mg/L), qe is the amount of MV adsorbed by OPS at an equilibrium (mg/g), qm is the theoretical maximum adsorption capacity (mg/g) corresponding to monolayer coverage, and KL is the Langmuir isotherm constant (L/mg).

To decide whether the adsorption process is favorable or not, a dimensionless constant, separation factor RL, is defined by

4.2. 4

where ρ0 is the initial MV concentration (mg/L).

The Langmuir model is based on the assumption of monolayer adsorption on a structurally homogeneous adsorbent, where all the adsorption sites are identical and energetically equivalent.40

(b) Freundlich isotherm model

4.2. 5

where KF and 1/n are constants of the Freundlich isotherm model.

The Langmuir and Freundlich isotherm plots are shown in Figure 3, and relevant parameters are shown in Table 2.

Figure 3.

Figure 3

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(a) Langmuir isotherm model and (b) Freundlich isotherm model.

Table 2. Isotherm Parameters of Adsorption.

  temperature (K)
isotherm model 293 298 301
Langmuir
qm (mg/g) 1132.46 1096.75 1071.18
KL (L/mg) 1.2318 1.0805 0.9968
RL 0.0160 0.1817 0.0197
R2 0.9931 0.9971 0.9982
Freundlich
KF (mg/g(mg/L)1/n) 24.15 23.51 23.40
1/n 0.1570 0.1550 0.1569
R2 0.9952 0.9948 0.9947
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As shown in Table 2, the correlation coefficients, R2, of the Langmuir isotherms were all higher than 0.99 at three different temperatures, indicating that adsorption processes were best described by the Langmuir isotherm model. Further, RL represents the adsorption capability of adsorbent at certain temperature. If RL > 1, adsorption is unfavorable; if RL = 1, adsorption process is linear, that is, desorption occurs simultaneously with adsorption; if 0 ≤ RL < 1, adsorption is favorable. The calculated values of RL were all in the range of 0–1, thereby confirming that adsorption processes are favorable under all these temperatures.

The empirical Freundlich equation is applicable to describe the adsorption and interactions on heterogeneous surfaces. This equation is not restricted to the formation of a monolayer.41 It can be seen from Table 2 that the R2 of Freundlich isotherm models were all higher than 0.99, which indicates that the experimental data agree well with the Freundlich model. Freundlich constant KF represents the adsorption capacity of the adsorbent, and a higher value indicates that the adsorbent has a higher affinity to the adsorbate. 1/n is the empirical parameter related to the strength of adsorption, which varies with the heterogeneity of the material.39 If 0.1 < 1/n < 1, the adsorption is favorable. As shown in Table 2, all the values of 1/n support the favorability of the adsorption.

Thermodynamic parameters can be decided using the adsorption constant KL (L/mol) obtained from the Langmuir isotherm. The values of ΔG0, ΔH0, and ΔS0 for the adsorption process were calculated using the following equations

4.2. 6
4.2. 7

where R is the universal gas constant and T is the temperature in Kelvin. The values of ΔH0 and ΔS0 can be obtained from the slope and intercept of the plot of ΔG0 against T. The results of ΔG0, ΔH0, and ΔS0 are shown in Table 3. A positive ΔS0 can be related to an increased number of vibrational frequencies in the OPS + MV system (see Sections 4.3 and 4.5). The negative values of ΔG0 suggested that the adsorption of MV was thermodynamically favored. Energy required for the decoupling of MV from OPS is −ΔG0. On the other hand, an increased temperature will lead to an increased decoupling rate, according to the Arrhenius equation, k = Ae–Ea/RT.

Table 3. Thermodynamic Parameters for MV Adsorption.

temperature (K) 293 298 301
ΔG0 (kJ/mol) –31.89 –32.11 –32.23
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Based on the intercept and slope of ΔG0–T plot, ΔH0 = −19.39 kJ/mol, and ΔS0= 42.65 J/(mol/K).

4.3. Fourier Transform Infrared (FT-IR) Spectroscopy of Materials and Product

FT-IR spectra of OPS, MV, and precipitates are shown in Figure 4. Detailed peak frequencies are reported in Table 4. The broad peak of OPS at about 3400 cm–1 was assigned to O–H stretching vibrations of intramolecular and intermolecular hydrogen bondings of saccharides. This peak disappeared in the IR spectrum of the product and was replaced by two sharper peaks at 3444 and 3344 cm–1. This phenomenon indicated that the MV molecule and the OPS were bound together by the bridging of the hydroxyl group. The split of the peak was caused by the formation of two types of hydroxyl groups on sugar rings of OPS: bonded and nonbonded (with MV). Due to the C=O stretching vibration of carboxylic groups, the OPS produced a strong peak at 1740 cm–1. However, in the IR spectrum of the product, this peak was weakened, indicating that the carboxylic groups were also involved in the adsorption, which may be combined with certain cationic MV molecules. The peaks related with C–N–R stretching at 1364 cm–1 in MV and C–H asymmetric vibrations in methyl groups of OPS at 2938 cm–1 are smeared. The strong peaks at 1462, 1629, and 1677 cm–1 were all due to the skeletal vibrations of the benzene rings, which were derived from the triphenylmethane structure in MV. The absorption peak of 1171 cm–1 related to the C–N stretching vibration in pure MV is shifted to 1158 cm–1 in the products. The absorption peak at 1033 cm–1 was caused by C–O–C stretching vibration of glycosidic linkage on the sugar ring. The IR spectrum of the products retained certain characteristic peaks of both MV and OPS, indicating that the two were combined by physical adsorption and no chemical reaction occurred. Further theoretical modeling (see Section 4.5) confirmed this interpretation of the IR spectra.

Figure 4.

Figure 4

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FT-IR infrared spectra of (A) MV, (B) OPS, and (C) the product.

Table 4. IR Peak Assignment and Calculated Energy Changes of Selected Vibrational Modes for Pure System (MV or OPS) and the Product (MV + OPS)a,b.

  wavenumber vibrational mode ΔE (kJ/mol) remarks
MV 1171 C–N stretching 32.3  
1364 C–N–R stretching    
1480 (weak)/1584/1680 (weak) C–C skeleton vibration of benzene ring    
3233 (weak) N–H stretching    
OPS 1043 C–O–C stretching of glycosidic linkage    
1253 C–O stretching vibration of sugar ring    
1428 C–C skeleton vibration    
1638 C=O asymmetric stretching vibration of carboxylic group    
1742 C=O stretching vibration of carbonyl ester 70.7 Figure 8a
2938 C–H asymmetric stretching    
3432 O–H stretching 30.9 (type I-a) Figure 8a
26.3 (type I-b)
30.6 (type II-a)
28.9 (type II-b)
product 1033 C–O–C stretching of glycosidic linkage    
1158 C–N stretching (compare to peak 1171 of MV) 31.2 Figure 8b
28.1 Figure 8c
30.0 Figure 8d
1250–1300 (weak) C–O stretching of sugar ring    
1462/1604/1677 C–C skeleton vibration of benzene ring    
1740 C=O stretching of carbonyl ester (compare to peak 1742 of OPS) 51.5 Figure 8b
55.6 Figure 8c
3344 N–H stretching    
3444 O–H stretching (compare to peak 3432 of OPS) 29.7 (type I-a) Figure 8b
30.2 (type I-b)
26.6 (type II-a)
44.5 (type II-b)
25.0 (type I-a) Figure 8c
26.8 (type I-b)
24.7 (type II-a) Figure 8d
45.1 (type II-b)
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a

The types of hydroxyl groups on sugar rings are presented in Figure 5. The columns of remarks indicate the configuration of sugar ring as indicated in Figure 8.

b

Note: (1) Due to the simplification of OPS structure, only limited number of vibrational frequencies can be calculated with DFT modeling. (2) The existence of chloride ions in the MV solution would cause slight shifting of some peaks,42 but it would not disturb the analysis. (3) Types I and II refer to two types of sugar rings shown in Figures 5 and 8, respectively. The letters (a and b) refer to two types of hydroxyls participating in the adsorption of MV.

For the theoretical evaluation of the effect of noncovalent bonds formed between MV and OPS, we calculate the change of the total energy of the system as a result of the stretching of 0.04 Å by selected bonds. The value of ΔE is the difference of the total energy of the system before and after increasing the length of selected bonds. These calculations were performed for pure MV, pure model OPS system (Figure 8a), and various types of attachments of MV to sugar rings of OPS (see Figure 8b–d). Calculation results presented in Table 4 are discussed in Section 4.5.

Figure 8.

Figure 8

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Types of sugar rings and their interactions with MV. Optimized atomic structure of two types of sugar rings with different chemical compositions (a). MV interacted with both types (b) or one type of these sugar rings (c, d). Note that the sugar ring is in the front of the page and MV is the back of the page in (d).

4.4. Molecular Dynamics Simulation

Under adsorption conditions of 20 °C, pH of 11, and OPS concentration of 50 mg/L, we used MD simulation to further explore the adsorption of MV by polysaccharides. In the simulation, the structure of the OPS (Figure 5) was modeled based on the literature.43 The Focite module in Materials Studio software was used to calculate the charge by QEq option. The parameter is set to QEq_charged1.0. The parameter description is that originally generated for positive metal ions and recommended for systems containing metal ions. We calculated the electronegativity oxygen (in electron Volts) in the hydroxyl, ether, and carboxyl groups of the OPS, as shown in the red fonts in Figure 5. The calculation shows that the magnitude of oxygen electronegativity is of the order −OH > −COOH > −O–. In a simulated system, the number of water molecules, Fe3+, Na+, SO42–, and OH– ions were 2000, 10, 5, 15, and 5, respectively, in accordance with the conditions of adsorption. For further exploration of the adsorption mechanism, the number of MV molecules was set to 5, 10, 15, and 20, respectively. An amorphous model of the polysaccharide-adsorbed MV aqueous solution is shown in Figure 6, and various substances have been noted in the figure.

Figure 5.

Figure 5

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Chemical structure of OPS (n is set to 2 to build the OPS model in MD simulation). Numbers in red are the calculated electronegativity of oxygen in the hydroxyl, ether, and carboxyl groups of OPS, respectively. Two types of sugar rings (type I and II) were constructed as simplified OPS models in the first-principles calculation. In each type of sugar ring, two types of hydroxyl groups, with labels (a) and (b), are involved in the adsorption of MV.

Figure 6.

Figure 6

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Amorphous cell within periodic boundary conditions with MV adsorbed by OPS in the aqueous solution in the presence of additional anions and cations.

Binding energy (Ebind) between OPS and MV is evaluated by total energy of the mixture and that of individual components, as follows

4.4. 8

where Etotal is the total energy of the system including all components used in the experiment, EOPS is the total energy of the system without MV molecules, and EMV is the total energy of the system without OPS molecules. As listed in Table 5, the results suggest that Ebind originated from the component of electrostatic interactions, Ebind(elec), is larger than Ebind and originated from the component of van der Waals interactions, Ebind(vdW). This indicates that the physical adsorption is dominated by electrostatic interactions. Thus, we add more MV in the system, this leads to increased interactions caused by hydrogen bonding and vdW forces. Further, we studied the radial distribution function of the oxygen-containing groups in polysaccharide and the nitrogen atoms in MV, as shown in Figure 7. The radial distribution function gAB(r) gives a measure of the probability of where atom B is located in an atom pair of type AB within a reference frame defined by atom A. A is located at the center of a sphere, and B is located in the spherical shell of infinitesimal thickness at a distance r from atom A. gAB(r) can be calculated by

4.4. 9

where ⟨nAB(r)⟩ is the average number of atom pairs between r and r + Δr and ρAB is the density of atom pairs of type AB. From Figure 7, gAB(r) of the hydroxyl system has the largest peak; hence, the concentration of MV in the vicinity of −OH is the largest, indicating that the adsorption to hydroxyl groups is dominant. This is consistent with the infrared analysis and the results of DFT calculations (Table 5).

Table 5. Energy Changes in the System at Different Loads of MV.

amount of MV Ebind (kcal/mol) Ebind(vdW) (kcal/mol) Ebind(elec) (kcal/mol)
5 –216.5 –89.8 –126.7
10 –245.8 –103.5 –142.3
15 –277.6 –120.6 –157.0
20 –302.8 –135.9 –166.9
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Figure 7.

Figure 7

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Radial distribution functions of atoms pairs of type AB, where A is functional groups (−OH, −O–, −COOH) on OPS and B is nitrogen atoms on MV.

4.5. First-Principles Modeling

Based on the results of IR measurements and MD simulations, we performed the first-principles simulations of the interactions between the MV molecules and the sugar rings of OPS. As the model of the OPS, we used two types of sugar rings: one with a methyl group (type I), the other with a carboxyl group (type II), see Figure 8a. We used this model to imitate alteration of these two groups in sugar rings (also see Figure 5).

In the first step of our modeling, we considered energetics of the insertion of −N+H–CH3 part of MV between two sugar rings. The binding energy is calculated by the formula

4.5. 10

where EMV is the total energy of the MV molecule, Ehost is the total energy of two sugar rings (Figure 8a), and Eproducts is the total energy of OPS + MV (Figure 8b). The value obtained in the calculations is −45.6 kJ/mol. Because this value is about 2 times larger than the enthalpy obtained in thermodynamics analysis (Table 3), we considered interactions of MV with a single sugar ring (Figure 8c,d). In this case, Ehost in eq 10 is the total energy of the pair of sugar rings minus the total energy of the sugar ring excluded from consideration. Calculated binding energy is −14.7 kJ/mol for the interaction between MV and the sugar ring with methyl group (Figure 8c) and −24.2 kJ/mol for the interaction between MV and the sugar ring with carboxyl groups (Figure 8d). The average binding energy is nearly −19 kJ/mol, which is in quantitative agreement with the thermodynamic analysis. Note that the presence of carboxyl groups enhances the interactions. Overall, this result is in agreement with the IR spectroscopy measurements and MD simulations.

The next step of our modeling was to check the influence of interactions on the vibrational energies of selected bonds. We calculated the total energy difference between the system with optimized atomic structure and the same system where the length of one of the selected bonds was increased by 0.04 Å. Listed in Table 4 are the results of the calculations demonstrating that different types of adsorption provides different changes in C–N–R stretching energies, which is consistent with the shift of the corresponding peak in the IR spectra. The fact that the energy of the C=O bond stretching in sugar rings containing carboxyl groups has changed drastically is consistent with the disappearance of the corresponding peak in the IR spectra of the product (MV + OPS). For O–H bond energies, we have found that the stretching energies of the O–H groups (type b) participating in the interactions with MV were increasing and the stretching energies of O–H groups (type a) that does not interact with MV were decreasing. Based on the calculations, additional patterns in the stretching energies of the product correspond to increased vibration modes. This is in agreement with the positive value of entropy in Table 3.

The last step of our modeling was to check the influence of adsorption on optical properties of MV. Results of the calculations indicate that each type of considered interaction between MV and sugar ring(s) provides a shifting of the highest occupied molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LUMO) level or both (Figure 9). The nature of this shifting is the interaction of the −N+H–CH3 part of MV with the hydroxyl group(s) of sugar rings. The energy gap between HOMO and LUMO becomes diversified after MV is adsorbed on OPS. This makes possible multiple transitions with different wavelengths instead of a single dominant transition observed in MV solution (Figure 2f).

Figure 9.

Figure 9

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Total densities of states of pure MV (red) and three considered MV + OPS structures. Interaction of MV with two sugar rings (Figure 8b) are shown in blue, interaction of MV with the single sugar ring containing methyl group (Figure 8c) in green, and interaction of MV with single sugar ring containing carboxyl group (Figure 8d) in pink.

5. Conclusions

Employing RSM, we found the optimal conditions of extracting polysaccharide from okra (extraction time of 31 min, liquid–solid ratio of 51:1 (mL:g), and temperature of 63 °C). Based on physicochemical analysis, average OPS molecular weight was 68 010 Da; molar ratio of the three monosaccharides in OPS was rhamnose/galactose/glucose = 1:0.56:0.13, and the uronic acid content was 10.71%.

Using thermodynamic analysis, the adsorption was proved to be a thermodynamically favored exothermic process. The results from MD simulation and first-principles calculations reveal that the nature of this process is the physical (dominantly electrostatic) adsorption of MV on sugar rings of OPS, which is consistent with the infrared analysis.

This work can be a foundation of future study on biosorbent-dye systems. Simulation approaches used in this study can potentially be applied for further studies of adsorption mechanism of various systems and improvement of sorption and decolorization efficiency.

Acknowledgments

The authors gratefully acknowledge the National Natural Science Foundation of China (Grant No. 51903122).

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b02768.

  • Experimental procedures for the extraction of polysaccharide from okra; and characterization and physicochemical analysis of polysaccharide (PDF)

Author Contributions

⊥ M.W. and Q.G. contributed equally to this work.

The authors declare no competing financial interest.

Supplementary Material

ao9b02768_si_001.pdf (520.1KB, pdf)

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