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. 2022 Oct 7;7(41):36697–36711. doi: 10.1021/acsomega.2c04985

Insight into the Effect of Sulfonation Techniques on the Adsorption Properties of −SO3H Surface-Functionalized Coal as Adsorbent for Malachite Green Dye: Steric and Energetic Investigation

Ali A AlHammadi †,‡, Rania Nasser §, Mohamed S Shaban ∥, Sarah I Othman ⊥, Jong Seong Khim #, Jamaan S Ajarem , Ahmed A Allam ○, Mostafa R Abukhadra ∇,10,*
PMCID: PMC9583338  PMID: 36278107

Abstract

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Natural coal (N.C) was sulfonated with sulfuric acid by normal stirring (MS.C) and sonication waves (SS.C) to obtain −SO3H functionalized coal as enhanced adsorbents of malachite green dye (MG). The sulfonated products exhibit enhanced surface area (MS.C (27.2 m2/g) and SS.C (45.8 m2/g)) as compared to N.C. SS.C achieved higher acid density (14.2 mmol/g) and sulfur content (13.2 wt. %) as compared to MS.C. The impact of the sulfonation processes on the adsorption of MG was assessed based on the monolayer isotherm model of one energy. The MG Qsat of N.C (121.3 mg/g), MS.C (226.3 mg/g), and SS.C (296.4 mg/g) validate the significant effect of the sulfonation processes by the sonication waves. This is in agreement with the active site densities that reflect the saturation of SS.C by more active sites (180.74 mg/g) than MS.C (120.38 mg/g) and N.C (70.84 mg/g). The MS.C and SS.C can adsorb three MG molecules as compared to two molecules per site of N.C. The Gaussian energy (<8 kJ/mol) and adsorption energy (<40 kJ/mol)) reflects the physisorption of MG involving van der Waals forces, hydrogen bonding, and dipole bonding forces. The thermodynamic functions demonstrate the uptake of MG by exothermic, spontaneous, feasible reactions.

Introduction

The water contamination phenomenon represents the most critical concern that threatens strongly the safety of the contemporary world and the healthy life of the population.1,2 The discharged quantities of sewage water and industrial wastewater increased strongly at an uncontrolled rate as a result of the extensive increase in agricultural, mining, industrial, and overpopulation activities.3 Numerous types of dissolved organic (pharmaceutical residuals, dyes, pesticides, and petrochemicals) and inorganic species (heavy metals, phosphate, ammonium, sulfate, and nitrate) were identified as water pollutants of remarkable health and environmental side impacts.2−5

Synthetic dyes were introduced into the industrial communities as a vital aromatic for different applications. However, most of the commonly used dyes exhibit the chemical structure of significant biodegradable resistance and toxicity.6,7 Moreover, their discharge (7 × 105 tons per year) into fresh water is associated with some health problems such as cancer, dermatitis, allergies, skin irritation, and kidney damage.8,9 Environmentally, the existence of such synthetic organic structures in the water systems causes significant disruption to the bioactivity and photosynthesis activity of the aquatic organisms by blocking the penetration efficiency of the sunlight.9,10 Among the commonly studied dyes malachite green (C23H26N2OCI42) (MG) is a widely detected synthetic dye in water resources. However, MG dye is of wide applications in food, aquaculture, and medical disinfectant industries, it was classified as a highly toxic compound.10,11 Carcinogenesis, mutagenesis, chromosomal fractures, respiratory toxicity, and teratogenicity were identified as health side effects and diseases related to the MG contaminated water.12,13 Considering the previous environmental and health issues, the decolorization of such synthetic dyes is a critical challenge for the researchers and responsible authorities.

Therefore, the decolorization of the dye molecules by low-cost and innovative techniques was investigated recently in several studies recently.14,15 The adsorption processes as a decolorization technique of the dissolved synthetic dyes were endorsed strongly for their high efficiency, simplicity, recyclability, and production cost considerations as compared to the other investigated methods including filtration, ion exchange processes, chemical precipitation, and coagulation.16,17 Availability, fabrication cost, recyclability, biodegradability, safety, and adsorption capacity are the main parameter that controls the suitability of the selected adsorbent and its efficiency.18 Recently, natural-based carbonaceous materials and raw coal were assessed widely as potential adsorbents of some synthetic dyes as well as other types of dissolved chemicals.19−22 The raw coal was applied effectively in the removal of some synthetic dyes such as methylene blue,20 methyl orange,23 basic blue 3 (BB3) dye,24 and basic yellow 2 (BY2) dye.24

As a geological term, coal refers to a sedimentary rock of high worldwide natural reserves and is composed of different types of organic components (cellulose, resin, lignite, and macerals) and inorganic ashes.22,25,26 As a material, it was identified as a series of natural polycyclic aromatic hydrocarbons, and the aromatic rings in its structure are linked with each other by types of oxygen-bearing functional groups (carbonyl, carboxyl, phenolic, and hydroxyl groups).27,28 The existence of such active oxygen-bearing chemical groups, especially the carboxylic and phenolic groups, makes it a promising adsorbent for different species of water-dissolved chemical compounds.29 The previous studies demonstrate that the physicochemical properties of coals as adsorbents depend essentially on their surface properties such as the dominant chemical groups, surface area, and porosity properties.22,30 Therefore, several studies were introduced to follow the impact of the different surface modification processes on the chemical and textural properties of coal. Such modification processes involved demineralization, chemical activation, metal loading, and thermal activation of coal fractions.19,22

The chemical activation of coal and carbonaceous materials is the most applied technique to support their surfaces by additional or new active chemical groups especially the oxygenated chemical groups such as the hydroxyl groups (−OH) and carboxyl groups (−COOH).20,31 The sulfonation of coal was reported as a very effective method that can support its structure by different active oxygenated chemical groups in addition to the significant enhancement in its surface electronegativity.19,20,31 The sulfonation efficiency and the physicochemical properties of the modified coal as adsorbent depend essentially on the type of the used chemical activators, the activation conditions, and the used activation techniques.28,32 The sulfonation of coal as an acidic heterogeneous catalyst by sulfuric acid (H2SO4) as cost-effective and highly oxidizing acid resulted in a significant enhancement in its acid density and textural properties during its application in the transesterification reactions.30 Moreover, conducting the sulfonation reactions under the sonication source resulted in different surficial properties than the sulfonated product by normal stirring processes.19,32,33

Unfortunately, the adsorption properties of sulfonated raw coal were not covered by the satisfied studies yet considering the sulfonation agent, sulfonation conditions, sulfonation techniques, and types of dissolved chemical contaminants. The sulfonation of raw coal without precarbonization processes will result in enhanced low-cost adsorbent of numerous active functional groups and high adsorption capacities to be used effectively in the decontamination of organic pollutants. Also, advanced mixing techniques such as ultrasonic methods can induce the entrapping efficiency of the effective oxygenated chemical groups as well as the surface area of the modified coal as potential adsorbents of significant capacities. Therefore, the introduced study involved a detailed investigation of the adsorption properties of two sulfonated coal products prepared by two activation techniques (normal stirring and sonication) for malachite green basic dye. The adsorption studies were accomplished considering all the experimental variables, classic kinetic studies, traditional equilibrium modeling, and advanced equilibrium model based on the statistical physics theory. The change in the adsorption properties will be assessed based on steric parameters (occupied active sites, number of adsorbed dyes molecules by site, and saturation adsorption capacities of the sulfonated coal) in addition to the energetic parameters (adsorption energies, internal energy, enthalpy, and entropy).

2. Results and Discussion

2.1. Characterization of the Studied Adsorbents

The XRD patterns of raw coal as well as the sulfonated samples demonstrate considerable changes in the structures after the sulfonation reactions (Figure 1). The obtained XRD pattern of coal precursor reflects the amorphous structure that is related to the aromatic sheets of the structural carbon of the coal. This was identified based on the identified broad peaks from 8 to 30° (002) and from 40 to 50° (101) (Figure 1A).34,35 For the sulfonated samples, the resulting pattern shows a remarkable deviation of the identified main peaks (10–30° for MS.C (Figure 1B) and 15–36° for SS.C (Figure 1C)) to be in higher positions as compared to the raw sample. This suggests strong structural changes after the sulfonation reactions and a significant increase in the amorphization properties of the coal. Such structural impacts are credited to the destructive effect of the sulfuric acid on the C–O–C bonds in addition to its dehydration impact on the carbon-bearing functional groups.34,36

Figure 1.

Figure 1

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XRD patterns of raw coal (A), sulfonated coal by magnetic stirring (MS.C) (B), and sulfonated coal by sonication waves (SS.C) (C).

The changes in the chemical properties and the formed groups during the sulfonation reactions were followed based on the FT-IR spectra (Figure 2). The identified structural groups of the studied coals are phenolic O–H (3403 cm–1), symmetric aliphatic C–H (2858 cm–1), asymmetric aliphatic C–H (2930 cm–1), C=C (1620 cm–1), carboxylic OH and/or C–H of a methylene group (1432 cm–1), C–H of the methyl group (1302 cm–1), C–O (1000–1300 cm–1), and distorted C–H of the aromatic planes (500–900 cm–1) (Figure 2A).22,25 Regarding the spectrum of MS.C, the increase in the intensity and broadness of the OH identification bands (3398 cm–1) and the identified C = O bond of the carboxylic group (1712 cm–1) reflect the formation of COOH groups as oxidized products during the sulfonation reactions (Figure 2B).35,37 The existence of sulfur-bearing chemical groups was detected by the SO3 stretching of −SO3H (1133 cm–1), symmetric O=S=O (1025 cm–1), asymmetric O=S=O (1001 cm–1), and C–S groups (578 cm–1) (Figure 2B).28,37 The oxidation and deformation effect of sulfonation reactions on the methyl groups and cellulose components appeared in the remarkable reduction of the identification bands of C=C (1610 cm–1) and C–H (1304 cm–1).34,36 The same observations were detected for the SS.C sample but with a significant increase in the intensity of the sulfur-bearing functional groups, and the COOH group suggested a higher impact on the sulfonation reactions in the existence of the sonication waves (Figure 2C). This was supported by the estimated analysis of the samples as the coal exhibited a significant increment in the sulfur and oxygen content after the sulfonation reactions, especially in the existence of the sonication waves.

Figure 2.

Figure 2

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FT-IR spectra of raw coal (A), modified MS.C product (B), and modified SS.C product (C).

The effect of the sulfonation reaction on the surficial features was studied based on the SEM images (Figure 3). The coal precursor appeared as agglomerated particles composed of several compacted layers that are of massive form and random orientation (Figure 3A). The layered structure is related to the compacted wood tissues and macerals as the main components of the studied sub-bituminous sample. The obtained particles of MS.C exhibit different surfaces of rugged topography with numerous micro nudes and micropores that might be related to the dissolved inorganic components during reactions with H2SO4 (Figure 3, parts B and C). Regarding the SEM images of SS.C, the images demonstrated a significant disintegration effect for the sonication waves on the coal grain into smaller particles. Also, there is a remarkable detection of tabular or needle-like particles related to the separation of some of the present macerals (liptinite) from the coal layers (Figure 3D). The disintegrated particles still exhibit rugged topography and enhanced porosity as compared to raw coal and MS.C. Such morphological features affected strongly the determined surface area of the products as presented in Table 1. The SS.C sample exhibits a surface area of 45.8 m2/g which is higher than the determined value for MS.C (27.2 m2/g).

Figure 3.

Figure 3

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SEM images of raw coal (A), modified MS.C product (B and C), and modified SS.C product (D).

Table 1. Chemical Composition, Acid Density, and Surface Area of N.Coal, MS.C, and SS.C samples.

  N.Coal MS.C SS.C
C (wt. %) 68.4 50.2 33.1
S (wt. %) 2.32 7.6 13.2
N (wt %) 2.53 7.8 10.1
H (wt %) 6.3 10.8 14.2
O (wt %) 10.6 24.4 29.3
acid density (mmol/g) 0.064 8.87 14.2
surface area (m2/g) 5.4 27.2 45.8
av pore diameter 50.2 44.2 23.6
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2.2. Adsorption of MG Dye

2.2.1. Effect of pH

Adjusting the pH value of the prepared aqueous solutions affects strongly the properties of the adsorption reactions either by directing the ionization speciation of the dissolved compounds or the surface charges of the coal products. Investigation of the effect of pH as a variable during the conducted adsorption tests of MG molecules by N.C, MS.C, and SS.C was followed from pH 2 up to pH 9. The other experimental variables were selected to be at 240 min (contact time), 0.25 g/L (coal dosage), 100 mg/L (dye concentration), 293 K (temperature), and 200 mL (volume). The determined MG adsorption efficiencies by N.C, MS.C, and SS.C exhibit noticeable enhancement with the regular rise in the solution pH up to the highest tested value (pH 9). The MG adsorbed quantities by N.C, MS.C, and SS.C at pH 9 are 45.8, 60.2, and 72.5 mg/g, respectively (Figure 4). This was ascribed to the dominant charges on the surfaces of N.C, MS.C, and SS.C particles as well as the ionization properties of MG molecules that exhibit positive charges within the assessed pH range.38

Figure 4.

Figure 4

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Influence of solutions pH on the adsorption of MG dye by N.C, MS.C, and SS.C (240 min (contact time), 0.25 g/L (coal dosage), 100 mg/L (dye concentration), 293 K (temperature), and 200 mL (volume)).

The main chemical groups of the raw coal and sulfonated coal products are affected by strong deprotonation reactions under the basic environment which create numerous hydroxyl ions of negative charges. Therefore, the basic conditions during the adsorption of the dyes keep the surfaces of the coal products with numerous negative charges, which are a favorable state for the significant electrostatic attractions of MG molecules that are of positive charges.39,40 This was supported by the determined pH(ZPC) values of N.C (6.2), MS.C (5.7), and SS.C (5.2) that demonstrate the high saturation of their surface s with negative charges beyond these values. Therefore, pH 9 was selected to conduct the further MG adsorption tests either by N.C, MS.C, and SS.C.

2.2.2. Kinetic Studies

2.2.2.1. Time Interval

The MG adsorption behaviors in the presence of N.C, MS.C, and SS.C as adsorbents were followed considering different contact intervals from 30 min up to 960 min. The other experimental variables were selected to be at pH 9 (pH of the solutions), 0.25 g/L (coal dosage), 100 mg/L (dyes concentration), 293 K (temperature), and 200 mL (volume). The recognized MG uptake curves by N.C, MS.C, and SS.C reveal a noticeable segmental shape which normally signifies remarkable changes in the actual MG uptake rates with expanding the contact intervals (Figure 5A). The first observed segments or sections in the curves exhibit remarkable steep slopes signifying the fast adsorption rates of MG and strong escalation in its adsorbed quantities by the N.C, MS.C, and SS.C samples with increasing the assessed contact intervals (Figure 5A). The previously described segments were identified up to 540 min for N.C, MS.C, and SS.C. The second MG uptake segments were observed within the tested range from the previously reported contact intervals up to 960 min. During these states, the actual adsorption of MG by N.C, MS.C, and SS.C occurred at remarkable slow rates with a faint or neglected increase in their adsorbed quantities (Figure 5A). Such adsorption rates and behaviors reflect the equilibration states of N.C, MS.C, and SS.C as the used adsorbents of MG dye. The detected MG uptake equilibrium capacities by N.C, MS.C, and SS.C are 71.8, 93.6, and 113.3 mg/g, respectively (Figure 5A). The previously reported MG adsorption behaviors were ascribed to the regular and continuous adsorption of the dyes by the distributed free adsorption receptors on the N.C, MS.C, and SS.C particles with time. Subsequently, the quantities or the numbers of these receptors decline frequently with expanding tested contact intervals causing a diminution in the MG uptake rates by N.C, MS.C, and SS.C particles. After specific contact intervals, all the present receptor sites on the surfaces of N.C, MS.C, and SS.C particles were occupied or consumed by MG as adsorbed molecules and in turn, the modified coal products attend their actual or experimental equilibrium states and they cannot adsorb more dyes molecules.41

Figure 5.

Figure 5

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Effect of the time intervals on the on the adsorption of MG by N.C, MS.C, and SS.C (A), the intraparticle diffusion curves of the MG adsorption processes (B), fitting of the MG adsorption results with pseudo-first-order kinetics (C), and fitting of the MG adsorption results with pseudo-second order kinetics (D) (pH 9 (pH of the solutions), 0.25 g/L (coal dosage), 100 mg/L (dyes concentration), 293 K (temperature), and 200 mL (volume)).

2.2.2.2. Intraparticle Diffusion Behavior

The intraparticle diffusion curves of MG adsorption processes by N.C, MS.C, and SS.C adsorbents include three obvious sections or segments that exhibit different slopes without detectable crossings through the original points (Figure 5B). This validates the operation of more than one MG uptake mechanism during its adsorption by N.C, MS.C, and SS.C rather than the diffusion of its molecules to be in contact with the surfaces of the coal products.3 The perfect segmentation of the curves into three sections suggested essential changes in the MG uptake mechanisms by expanding the evaluated contact intervals (Figure 5B). The starting section during the initial contact intervals reflects the impact of the surficial or external MG adsorption processes by N.C, MS.C, and SS.C considering their surficial active groups. After certain intervals, the detection of the second sections of the curves reflects a strong fading of the impact of the external MG uptake mechanisms and the operation of another mechanism which is known as layered adsorption mechanisms42 (Figure 5B). The third section which appeared by the end of the adsorption tests and represents the equilibrium parts of the curves suggested the formation of thick layers of the adsorbed MG on the surfaces of N.C, MS.C, and SS.C (Figure 5B). Additionally, the realization of this section reveals the occurrence of interionic attraction or molecular association or both of them as the controlling MG adsorption mechanisms.18

2.2.2.3. Kinetic Modeling

The kinetic modeling of the MG adsorption systems by N.C, MS.C, and SS.C as the applied adsorbents was performed based on the nonlinear fitting results with the reported hypothetical kinetic of pseudo-first-order (Figure 5C) as well as pseudo-second-order models (Figure 5D). The R2 and χ2 values were used to detect the fitting degrees of MG adsorption behaviors with the assessed kinetic models (Figure 5, parts C and D; Table 2). According to the fitting parameters, the MG uptake processes by N.C, MS.C, and SS.C follow the kinetic behaviors of the pseudo-first-order hypothesis rather than the pseudo-second-order hypothesis (Table 2; Figure 5, parts C and D). This signifies the dominant influence of the common physisorption mechanisms during the performed adsorption reactions of MG by N.C, MS.C, and SS.C.43,44 The significant fitting degrees between the MG uptake behaviors and the assumptions of both presented models validate the considerable role of the weak chemisorption processes (chemical complexes and/or ion exchange) as assistant mechanisms during the physisorption uptake of MG by N.C, MS.C, and SS.C.

Table 2. Mathematical Parameters of the Studied Kinetic Models.
kinetic models
  model parameters  
N.C pseudo-first-order K1(1/min) 0.0052
Qe(Cal)(mg/g) 76.2
R2 0.95
X2 1.7
pseudo-second-order k2(mg/g min) 4.29 × 10–5
Qe(Cal)(mg/g) 100.2
R2 0.93
X2 2.4
MS.C pseudo-first-order K1(1/min) 0.0065
Qe(Cal)(mg/g) 96.3
R2 0.97
X2 1.01
pseudo-second-order k2(mg/g min) 4.94 × 10–5
Qe(Cal)(mg/g) 120.54
R2 0.95
X2 1.68
SS.C pseudo-first-order K1(1/min) 0.0071
Qe(Cal)(mg/g) 113.48
R2 0.98
X2 0.47
pseudo-second-order k2(mg/g min) 4.84 × 10–5
Qe(Cal)(mg/g) 139.6
R2 0.97
X2 0.99
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2.2.3. Equilibrium Studies

2.2.3.1. Effect of MG Concentration

Conducting the MG uptake tests considering different starting concentrations of its dissolved molecules (100 up to 800 mg/L) can be denoted its maximum adsorbed quantities by N.C, MS.C, and SS.C and the equilibrium properties of the reactions. The other experimental variables were selected to be at pH 9 (pH of the solutions), 0.25 g/L (coal dosage), 780 min (contact time), and 200 mL (volume) considering three values for the adjusted temperature (293, 303, and 313 K). The adsorbed quantities of MG molecules by N.C, MS.C, and SS.C depend strongly on the starting dye concentrations (Figure 6, parts A–C). The high MG concentrations induce the efficiencies of the N.C, MS.C, and SS.C particles as the evaluated solid adsorbents. The previous literature illustrated this experimental behavior as a result of the predicted acceleration in the MG driving forces and/or diffusion speed with dissolving the dye molecules at high concentrations in the reaction system.3 The proportionate relation between the starting MG concentrations and its adsorbed quantities by N.C, MS.C, and SS.C were observed clearly up to specific concentrations of 500, 600, and 600 mg/L using N.C (Figure 6A), MS.C (Figure 6B), and SS.C (Figure 6C), respectively. These MG concentrations are the equilibration concentrations at which the N.C, MS.C, and SS.C products were saturated by the adsorbed molecules of the dye and realized their actual or experimental maximum capacities. The detected best MG uptake capacities by N.C are 112.3 mg/g (293 K), 98.2 mg/g (303 K), and 78.8 mg/g (313 K) (Figure 6A). For MS.C, the measured MG capacities are 203.1 mg/g (293 K), 174.6 mg/g (303 K), and 131.4 mg/g (313 K) (Figure 6B) while the detected values by SS.C are 254.5 mg/g (293 K), 204 mg/g (303 K), and 156.7 mg/g (313 K) (Figure 6C). The detectable decrease in the N.C, MS.C, and SS.C adsorption efficiencies for MG dye with the tested values of temperature reveals the exothermic properties of their MG adsorption systems.

Figure 6.

Figure 6

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Effect of the MG concentration on its uptake capacities by N.C (A), MS.C (B), and SS.C adsorbents (C), the fitting of the MG adsorption results with the Langmuir model (D–F), fitting of the MG adsorption results with the Freundlich model (G–I), and fitting of the MG adsorption results with the D-R model (J–L) (pH 9 (pH of the solutions), at 0.25 g/L (coal dosage), 780 min (contact time), and 200 mL (volume)).

2.2.3.2. Classic Isotherm Models

The classic isotherm studies of MG adsorption by N.C, MS.C, and SS.C were accomplished based on the nonlinear fitting of the results with the theoretical hypotheses of Langmuir (L-G) (Figure 6, parts D–F), Freundlich (F-R) (Figure 6, parts G–I), and Dubinin–Radushkevich (D-R) (Figure 6, parts J–L) as traditional equilibrium models. The fitting parameters of R2, as well as χ2, revealed the adsorption of MG by the N.C, MS.C, and SS.C particles according to the traditional equilibrium hypothesis of Langmuir isotherm as compared to the hypothesis of the Freundlich Isotherm (Table 3). Considering this equilibrium behavior, the adsorbed MG molecules characterizes by homogeneous and monolayer properties on the surfaces of the N.C, MS.C, and SS.C particles.16 Based on the estimated parameters of the Langmuir model, the theoretical maximum capacities of N.C, MS.C, and SS.C as adsorbents for MG at the best temperature value (293 K) are 153.3, 391.3, and 454.3 mg/g, respectively (Table 3). The Gaussian energies of the N.C, MS.C, and SS.C adsorption systems for MG dye were recognized as parameters for the addressed D-R isotherm model (Table 3). The obtained energies for N.C (3.02 to 3.6 kJ/mol), MS.C (2.55 to 2.73 kJ/mol), and SS.C (2.7 to 3.01 kJ/mol) authorize dominant effect for the physisorption mechanisms (<8 kJ/mol) during their adsorption reactions for MG molecules.45

Table 3. Mathematical Parameters of the Addressed Kinetic and Classic Isotherm Models.
isotherm models
  model parameters 293.13 K 303.13 K 313.13 K
N.C Langmuir model Qmax(mg/g) 153.37 141.25 124.4
b (L/mg) 0.0047 0.0038 0.0028
R2 0.96 0.96 0.96
X2 0.48 0.48 0.53
Freundlich model 1/n 0.42 0.46 0.54
kF(mg/g) 7.52 4.93 2.44
R2 0.89 0.90 0.90
X2 1.48 1.35 1.3
D-R model β (mol2/KJ2) 0.0376 0.0436 0.0546
Qm(mg/g) 111.2 96.6 77.53
R2 0.95 0.94 0.95
X2 0.58 0.83 0.61
E (kJ/mol) 3.64 3.38 3.02
MS.C Langmuir model Qmax(mg/g) 391.03 348.36 268.67
b (L/mg) 0.0017 0.0016 0.0015
R2 0.96 0.96 0.93
X2 1.87 1.82 2.48
Freundlich model 1/n 0.66 0.68 0.69
kF(mg/g) 2.85 2.22 1.54
R2 0.92 0.92 0.89
X2 3.77 3.47 4.07
D-R model β (mol2/KJ2) 0.0671 0.0724 0.0766
Qm(mg/g) 196.35 170.28 132.25
R2 0.95 0.95 0.97
X2 2.58 2.31 0.98
E (kJ/mol) 2.73 2.63 2.55
SS.C Langmuir model Qmax(mg/g) 454.34 345.9 291.47
b(L/mg) 0.0020 0.0023 0.0019
R2 0.97 0.96 0.94
X2 1.46 1.34 2.16
Freundlich model 1/n 0.62 0.59 0.65
kF(mg/g) 4.64 4.56 2.46
R2 0.93 0.92 0.90
X2 3.48 3.24 3.9
D-R model β (mol2/KJ2) 0.0578 0.0549 0.06653
Qm(mg/g) 240.6 196.5 155.11
R2 0.91 0.94 0.97
X2 4.6 2.54 1.07
E (kJ/mol) 2.94 3.01 2.74
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2.2.3.3. Advanced Isotherm Models

Recently, statistical physics theory was applied to develop advanced isotherm models of more significance about the nature and mechanisms of the adsorption reactions. The adsorption properties of N.C, MS.C, and SS.C particles during the MG uptake reactions were evaluated and discussed considering the theoretical hypothesis of the Monolayer model with one energy site in addition to the obtained mathematical parameters either the steric parameters (number of adsorbed MG molecules per site (n), the receptor sites density (Nm), and saturation adsorption capacity of the coal products (Qsat)) or the energetic parameters (internal energy (Eint), adsorption energy (ΔE), free enthalpy (G), and entropy (Sa)) (Figure 7A–C; Table 4).

Figure 7.

Figure 7

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Fitting of the MG adsorption results with Monolayer model of one energy site (A–C), the change in the number of adsorbed ions per site with temperature (D), the change in the active sites density with temperature (E), and the change in the saturation adsorption capacities with temperature (F) (pH 9 (pH of the solutions), 0.25 g/L (coal dosage), 780 min (contact time), and 200 mL (volume)).

Table 4. Estimated Mathematical Parameters for the Fitting Process with a Monolayer Model of One Energy.
steric and energetic parameters
  temp (K) n Nm(mg/g) Qsat(mg/g) C1/2(mg/L) ΔE(kJ/mol)
N.C 293 1.71 70.84 121.3 137.57 –3.01
303 1.76 65.35 115.6 157.72 –3.45
313 1.82 47.36 86.43 176.74 –3.86
MS.C 293 1.88 120.38 226.3 227.33 –4.23
303 1.92 101.52 195.52 212.7 –4.21
313 2.3 61.07 141.9 195.61 –4.13
SS.C 293 1.64 180.74 296.4 212.68 –4.07
303 1.75 130.79 228.88 191.04 –3.94
313 2.14 77.93 166.77 186.76 –4.01
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2.2.3.3.1. Steric Parameters
2.2.3.3.1.1. Number of Adsorbed MG Molecules per Site (n)

The numbers of adsorbed MG molecules per each active receptor on the surfaces of N.C, MS.C, and SS.C particles can validate the orientation of the dyes on their surfaces as well as the affected mechanisms. The number of adsorbed MG molecules per each site on the surfaces of N.C (n = 1.71–1.82), MS.C (n = 1.88–2.3), and SS.C (n = 1.64–2.14) are higher than 1 (Figure 7D; Table 4). This signifies the uptake of MG on the surfaces of the coal products in nonparallel and/or vertical orientation by multimolecular mechanisms.46,47 Moreover, the previous values validate the suitability of the active sites on the surfaces of MS.C and SS.C to adsorb up to three molecules of MG as compared to two molecules by the active sites of N.C. The remarkable increase in the number of MG molecules that were adsorbed by the active sites of N.C, MS.C, and SS.C demonstrates a considerable increase in the aggregation behaviors of the dye molecules (Figure 7D).47−49

2.2.3.3.1.2. Density of the Receptor Sites (Nm)

The density of the receptor sites (Nm) on the surfaces of N.C, MS.C, and SS.C particles reflects strongly the quantities of the present free active sites which can be occupied by the adsorbed MG molecules (Figure 7E; Table 4). The receptor density values (Nm) during the MG adsorption by N.C are 70.84 mg/g (293 K), 65.35 mg/g (303 K), and 47.36 mg/g (313 K) and the estimated values of MS.C are 120.38 mg/g (293 K), 101.5 mg/g (303 K), and 61.01 mg/g (313 K). For SS.C, the determined values are 180.74 mg/g (293 K), 130.79 mg/g (303 K), and 77.93 mg/g (313 K). The estimated Nm parameter of SS.C and MS.C show higher values than N.C, and the values of SS.C are higher than MS.C. This demonstrates the effect of the sulfonation modification reactions in inducing the formation or activation of more active adsorption sites on the surface of the coal, which induces its adsorption capacity. Additionally, the results validate the higher effect of the sonication waves in inducing the oxidation and sulfonation efficiency of the coal components which is in agreement with the FT-IR and chemical analysis findings in addition to its effect on enhancing the surface area and the porosity properties. Therefore, the sulfonation activation of coal by the sonication waves induces the adsorption properties of coal by enhancing the quantities and exposures of the active adsorption sites. Moreover, the observable decrease in the Nm values of N.C, MS.C, and SS.C with the adsorption temperature is in significant agreement with the estimated values of the n parameter (Figure 7E; Table 4). The increase in the aggregation efficiencies of the MG molecules during their adsorption by N.C, MS.C, and SS.C is associated with declination in the quantities of the occupied active sites.47

2.2.3.3.1.3. The Adsorption Capacities at the Saturation State of Qsat

The adsorption capacities of N.C, MS.C, and SS.C as adsorbents for MG dye at the saturation states of their incorporated particles (Qsat) are the most appropriate values that represent their maximum adsorption capacities as compared to the recognized findings of the classic isotherm models (Figure 7F; Table 4). The values of Qsat depend essentially on the density of the active sites (Nm) and/or the number of adsorbed MG molecules per each site on the surfaces of N.C, MS.C, and SS.C. The attained Qsat values of N.C are 121.3 mg/g (293 K), 115.6 mg/g (303 K), and 86.43 mg/g (313 K) while the calculated values of MS.C are 226.3 mg/g (293 K), 195.5 mg/g (303 K), and 141.9 mg/g (313 K) (Figure 7F; Table 4). The Qsat of SS.C during the uptake of MG are 296.4 mg/g (293 K), 228.8 mg/g (303 K), and 166.7 mg/g (313 K) (Figure 7F; Table 4). This agreement between the detected Qsat values and the estimated Nm values as a function of the adsorption temperature as compared to the n parameter validates the controlling role of the active sites densities on the adsorption efficiencies of the MG dye by N.C, MS.C, and SS.C. Also this explains the reported higher actual uptake capacities of the sulfonated products as compared to the raw sample especially the sulfonated sample in the presence of the sonication waves (SS.C).

2.2.3.3.2. Energetic Properties
2.2.3.3.2.1. Adsorption Energy

The adsorption energies (ΔE) of N.C, MS.C, and SS.C as adsorption systems during the MG uptake reactions are of remarkable significance to the affecting mechanisms. The ΔE values for the MG adsorption by the coal products were determined directly from eq 1 using the values of the obtained steric parameters where C1/2 is the concentration of MG at the half-saturation, S is the solubility of MG dye, R is the gas constant, and T is the adsorption temperature.47

2.2.3.3.2.1. 1

The adsorption systems that are characterized by adsorption energies less than 40 kJ/mol display a dominant effect for the physisorption mechanisms while the adsorption systems that exhibit energies beyond 80 kJ/mol display strong chemisorption mechanisms. The physisorption mechanisms involved different subclasses of different energy ranges such as van der Waals forces (4–10 kJ/mol), dipole bonding forces (2–29 kJ/mol), hydrophobic bonding (5 kJ/mol), coordination exchange (40 kJ/mol), and hydrogen bonding (<30 kJ/mol).47,50 The determined MG adsorption energies using N.C (−3.01 to −3.86 kJ/mol), MS.C (−4.13 to −4.23 kJ/mol), and SS.C (−3.94 kJ/mol to −4.07 kJ/mol) are within the identified range of the physisorption mechanisms (Table 4). These signify the significant effects of the van der Waals forces in addition to the dipole bonding forces and hydrogen bonding as the operating mechanisms during the MG adsorption processes by N.C, MS.C, and SS.C.51 The negative signs of the estimated ΔE values during the MG adsorption by N.C, MS.C, and SS.C suggest the exothermic behaviors of these reactions.

2.2.3.3.2.2. Thermodynamic Functions
2.2.3.3.2.2.1. Entropy

The MG adsorption entropy (Sa) by N.C, MS.C, and SS.C considering the temperature of the tests as well as the concentrations of MG can signify the surficial properties of the coal particles (order and disorder). The Sa values of the MG adsorption by N.C, MS.C, and SS.C were calculated according to eq 2 using the values of the previously estimated steric parameters.51

2.2.3.3.2.2.1. 2

The estimated Sa values of the MG adsorption by N.C, MS.C, and SS.C exhibit reversible trends with the equilibrium concentrations of the dye without any impact of the temperature on these behaviors (Figure 8A–C). Consequently, there is an observable enhancement in the disorder properties of the N.C, MS.C, and SS.C surfaces during the MG uptake reaction at low concentrations of its molecules in the systems. Moreover, these behaviors demonstrate strong docking of the MG molecules on the distributed adsorption sites on the surfaces of N.C, MS.C, and SS.C by conducting the experiments at low dye concentrations.47,48 The equilibrium MG concentrations that are corresponding to the maximum Sa values of the N.C adsorption system are 182.05 mg/L (293 K), 185.52 mg/L (303 K), and 188.9 mg/L (313 K) (Figure 8A). For the MS.C system, the identified concentrations are 176.6 mg/L (293 K), 180.3 mg/L (303 K), and 184.15 mg/L (313 K) (Figure 8B). Regarding the MG uptake system by SS.C, the identified concentrations are 171.6 mg/L (293 K), 174.8 mg/g (303 K), and 180.15 mg/g (313 K) (Figure 8C). These MG equilibration concentrations exhibit values close to estimated values of the dye concentrations during the half-saturation states of N.C, MS.C, and SS.C adsorbents. Consequently, the previously reported MG concentrations reflect mostly the complete occupation of the distributed active sites on the surfaces of N.C, MS.C, and SS.C with the MG molecules, and in turn, no further dye can be docked. The observed sharp dropping in the values of Sa of the N.C, MS.C, and SS.C adsorption systems for MG dye beyond the previously reported equilibrium concentrations is associated with a decrease in the adsorption sites’ availability and freedom degrees in addition to the diffusions of the dye molecules.46

Figure 8.

Figure 8

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Change in the entropy properties with temperature and the tested concentrations (A–C), the change in the internal energy properties with temperature and the tested concentrations (D–F), and the change in the free enthalpy properties with temperature and the tested concentrations (G–I) (pH 9 (pH of the solutions), 0.25 g/L (coal dosage), 780 min (contact time), and 200 mL (volume)).

2.2.3.3.2.2.2. Internal Energy and Free Enthalpy

The internal energy (Eint) as a thermodynamic function of the MG adsorption by N.C, MS.C, and SS.C were estimated utilizing eq 3 which depends mainly on the translation partition (Zv) and the values of other steric parameters.51

2.2.3.3.2.2.2. 3

The Eint values of the N.C, MS.C, and SS.C adsorption systems for MG exhibit negative signs either at the different values of the dye concentrations or the tested temperature values which signify the spontaneous properties of these processes (Figure 8D–F). Moreover, the decrease in these values for the tests which were conducted under high-temperature conditions demonstrates the exothermic properties of the MG adsorption by N.C, MS.C, and SS.C (Figure 8D–F). Moreover, the negative signs of the free enthalpy (G) (eq 4) considering the corresponding MG concentrations and the temperature of the tests confirm the spontaneous and exothermic properties of the MG adsorption by N.C, MS.C, and SS.C (Figure 8G–I).

2.2.3.3.2.2.2. 4

2.2.4. Recyclability

The recyclability of N.C, MS.C, and SS.C as adsorbents for MG dye was studied as essential factors during the assessment of the products for commercial and realistic scale applications. The spent particles of N.C, MS.C, and SS.C were washed strongly, utilizing distilled water for 10 min, and this was repeated for five cycles. After that, the washed N.C, MS.C, and SS.C were dried at 60 °C for 8 h to be reused again in other MG adsorption cycles. The conducted recyclability tests of the coal adsorbents for MG were performed considering the adsorption variables at pH 9 (pH of the solutions), 0.25 g/L (coal dosage), 780 min (contact time), 200 mL (volume), 100 mg/L (MG concentration), and 293 K (temperature). The determined MG adsorption efficiencies using N.C as well as MS.C and SS.C adsorbents reflect significant stability and remarkable recyclability values of the products of the modified coal considering the three investigated recyclability runs (Figure 9). The recyclability of N.C in the five MG uptake cycles resulted in adsorption efficiencies of 71.9 mg/g (cycle 1), 71 mg/g (cycle 2), 66.3 mg/g% (cycle 3), 61.2 mg/g (cycle 4), and 54.7 mg/g (cycle 5) (Figure 9). For the recyclability of MS.C, the adsorbed quantities are 93.8 mg/g (cycle 1), 93 mg/g (cycle 2), 90.2 mg/g% (cycle 3), 85.6 mg/g (cycle 4), and 78.4 mg/g (cycle 5) (Figure 9). For SS.C, its determined capacities of MG are 113.3 mg/g (cycle 1), 113 mg/g (cycle 2), 111.4 mg/g% (cycle 3), 107.2 mg/g (cycle 4), and 101.4 mg/g (cycle 5). There are small declinations in determined MG adsorption efficiencies of N.C, MS.C, and SS.C within the regular repeating of the recyclability cycles. This might be assigned to the predicted leaching of the coal components during the washing processes and/or the continuous formation of chemical complexes between the MG molecules and the chemical structures of the coal components.

Figure 9.

Figure 9

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Recyclability properties of N.C, MS.C, and SS.C during the adsorption of MG dye pH 9 (pH of the solutions), 0.25 g/L (coal dosage), 780 min (contact time), 200 mL (volume), 100 mg/L (MG concentration), and 293 K (temperature)).

2.2.5. The Formation and Operation Mechanism of Sulfonated Coal

The sulfonation of coal as a heterogeneous organic structure of different organic components including cellulose, lignin, and resin is controlled by the availability of effective electrophilic species. Such species are the main components in the systems that can functionalize and affect chemically the aromatic components of coal.52 The interaction between the chemical activator (H2SO4) and the organic structure of coal resulted in significant grabbing of the hydrogen ions by the −OH groups of the incorporated acid molecules. This in turn causes remarkable destruction of the present oxygen–hydrogen bond and the formation of H2O molecules (Figure 10).26 During this reaction, another interaction occurs between the protonated oxygen and lone pair electrons resulting in a new π bond with the incorporated sulfur ions creating protonated trioxide.26 The sulfur ions and the formed sulfur trioxide exhibit significant electronegativity therefore it acts as effective electrophiles during the performed sulfonation reactions.25 Consequently, the sulfur ions attack the structural benzene rings which are of destructive effects on the double bonds of the aromatic rings which in turn induce the entrapping of the S-bearing chemical groups (−SO3H) (Figure 10). At the same time, the interaction between the H2SO4 acid resulted in HSO4– radicals which act as base centers for effective removal of the hydrogen protons.52 Moreover, the progressive oxidation of the coal structure with the sulfuric acid causes a significant transformation of the present −OH groups into active carboxyl groups (COOH). Therefore, the sulfonated coal structure becomes enriched in COOH–, OH–, and −SO3H groups which are very active sites during the adsorption reactions especially the cationic ions or molecules (Figure 10).

Figure 10.

Figure 10

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Schematic diagram for the adsorption mechanism of MG dye by the sulfonated coal samples.

The sonication waves during the sulfonation process induce strongly the efficiency of the reaction and the entrapment of the acidic groups. This was assigned to the impact of the sonication waves in inducing mixing properties and the homogeneity between the different reactants and in turn the interaction between the acid molecules and the coal fractions.53,54 This is credited to the cavitation phenomenon which is associated with the sonication waves that involve the formation of fine acid droplets or bubbles along the interface between the acid and the coals particles.53,54 The sudden collapse of these droplets creates additional pressure centers, microjets, shockwaves, and microstreams, which induce the mixing and interaction efficiency.55 Moreover, the presence of the sonication waves induces the disintegration of the present coal components into finer particles of enhanced surface area.56

Considering the present functional chemical groups and the previous findings of the adsorption mechanism based on the adsorption energy and Gaussian energy, the uptake of MG dye by the sulfonated coal products might be involved three mechanisms (Figure 10). The first mechanism involved the electrostatic attraction between the positively charged MG dye molecules and the negatively charged groups of coal. The second mechanism involved the formation of hydrogen bonds between the free hydrogen of the sulfonated coal surfaces and the nitrogen atoms of the MG dye chemical structure. The third mechanism involved π–π interactions between the π-electron system of the sulfonated coal and the aromatic rings of MG molecules involving a combination of dispersion and dipole–induced dipole interactions (Figure 10).

2.2.6. Comparison Study

The estimated capacities of sulfonated coal adsorbents were compared with other studied adsorbents in the literature (Table 5). The present results demonstrate the higher capacities of the sulfonated coal products (MS.C and SS.C) as compared to the presented materials including graphene oxide/Fe3O4 composite, modified CNTs, Fe-Fe2O3@PDA, ZnO-NRs-AC, and halloysite nanotubes (Table 5). This validates the value of the prepared sulfonation oxidation of coal especially using the sonication waves in developing enhanced adsorbents of high adsorption capacities, facile synthesis procedures, significant recyclability, low cost, and available precursors.

Table 5. Comparison between the Studied Coal-Based Adsorbents and Other Studied Adsorbents in Literature.
adsorbents Qmax(mg/g) references
Fe–Fe2O3@PDA 61.22 (57)
ZnO-NRs-AC 59.17 (58)
aerobic granules 56.8 (59)
organoclay 40.4 (60)
Halloysite nanotubes 99.6 (61)
modified carbon nano tubes 172 (62)
Limonia acidizsima 35.48 (63)
iron humate 19.2 (64)
graphene oxide/Fe3O4 179 (65)
N.C (Qsat) 121.3 this study
N.C (Qmax) 153.37 this study
MS.C (Qsat) 226.3 this study
MS.C (Qmax) 391.03 this study
SS.C (Qsat) 296.4 this study
SS.C (Qmax) 454.34 this study
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3. Conclusion

Natural coal (N.C) was treated with H2SO4 by normal stirring and by the sonication waves producing modified products of enhanced adsorption capacity for malachite green dye. The sulfonation processes enhanced the surface area (MS.C (27.2 m2/g) and SS.C (45.8 m2/g)) and functionalized the coal surface by SO3–H and other active oxygenated chemical groups. The sulfonation by sonication waves (SS.C) resulted in higher surface area and functionalization efficiencies as compared to normal stirring sulfonation (MS.C) considering the acid density value (14.2 mmol/g) and sulfur content (13.2 wt. %). The sulfonation modification induces the adsorption of MG dye by N.C (121.3 mg/g) to 226.3 and 296.4 mg/g by MS.C and SS.C, respectively. This was assigned to the increase in the quantities of the active sites which are 180.7 and 120.38 mg/g for SS.C and MS.C as compared to 70.8 mg/g for N.C. This validates the significant effect of the sonication process in inducing the adsorption activity of coal in addition to the applied sonication techniques. The uptake of MG occurred by exothermic, spontaneous, and physisorption mechanisms involving van der Waals forces, hydrogen bonds, and dipole bonding forces considering the adsorption energy (<40 kJ/mol) and thermodynamic functions.

4. Methdology

4.1. Materials

The coal sample which was used in the sulfonation processes is a sub-bituminous coal sample used in sulfonation processes obtained from the El-Maghara mine, the Sinai peninsula, Egypt. The chemical composition was determined based on the proximate and ultimate analyses (Table S1). The investigated malachite green dye (C23H26N2O,CI42) was estimated from Sigma-Aldrich; Egypt.

4.2. Sulfonation Processes

The sample was prepared based on the reported procedures by 19 (Figure S1). The raw sample was ground using a home blender to be within the size range from 20 up to 70 μm. The ground product (10 g) was mixed with sulfuric acid (95%) within two beakers, and each beaker contains 100 mL of the acid as separated systems. The homogenization process of the first system was continued for 90 min by the normal magnetic stirring process at a fixed speed of 500 rpm and an adjusted temperature of 150 °C. The homogenization of the second system (beaker) was accomplished in the presence of a sonication source (240 W) at an adjusted temperature of 150 min for 60 min. After each test, the obtained samples were washed and neutralized with distilled water for three runs, and each run consumed 10 min. This was followed by gentle drying of the samples using a digital dryer for 10 h at 65 °C. Finally, the products were collected, preserved in specific containers, and labeled as MS.C (sulfonation by magnetic stirring) and SS.C (sulfonation by sonication waves).

4.3. Analytical Techniques

The impact of the sulfonation conditions was assessed considering the changes in the structural, textural, chemical, and morphological properties. The structure of the modified samples was studied based on their XRD patterns utilizing an X-ray diffractometer (PANalytical (Empyrean)). The incorporated sulfonic groups and the impact of the acid oxidation reactions were followed based on the FT-IR spectra of the produced structures using a Fourier transform infrared spectrometer (FTIR–8400S). The morphological features of the modified coals as compared to the raw sample were studied considering the SEM images of the samples using a scanning electron microscope (Gemini, Zeiss-Ultra 55)). The impact of the sulfonation processes on the surface area was followed considering the BET surface area of the samples using the Beckman Coulter surface area analyzer (SA3100 type). The ζ-potential values were measured using Zetasizer attached to the ζ-cell (Malvern, version 7.11) at different pH, and the results were applied to determine the pH value at zero point charge (pH(ZPC)).

4.4. Adsorption Studies

The adsorption of the selected synthetic dye (malachite green (MG) (Figure S2) by normal raw coals (N.C), sulfonated coals by normal stirring (MS.C), and sulfonated coal by sonication waves (SS.C) were performed as batch tests considering the main experimental variables such as pH (2 until 9), contact time (30–960 min), and dye concentration (100–800 mg/L) at a fixed volume of 200 mL, considering the change in the temperature of the equilibrium experiments from 293 K up to 313 K. The tests were conducted in triplicate forms considering the average values for all the inserted results. The rest dye concentrations after each test were determined using a UV–vis spectrophotometer considering the value of λ max of the MG dye and the adsorption capacity (Qe) was calculated from eq 5.

4.4. 5

The nonlinear fitting degrees with the addressed classic kinetic and isotherm models (Table S2) were considered based on the correlation coefficient (R2) (eq 6) and chi-squared (χ2) value (eq 7). However, the fitting degrees with the evaluated advanced isotherm models based on the statistical physics theory (Table S2) were estimated based on the correlation coefficient (R2) and root-mean-square error, (RMSE) (eq 8) where the used symbols of m′, p, Qical, and Qiexp refer to the inserted data, the experimental variables, adsorbed quantities of MG, and actual adsorbed quantities of the MG molecules, respectively.

4.4. 6
4.4. 7
4.4. 8

Acknowledgments

The authors acknowledge Researchers Supporting Project Number RSP-2021/149, King Saud University, Riyadh, Saudi Arabia. Also, the authors acknowledge Princess Nourah bint Abdulrahman University Researchers Supporting Project Number PNURSP2022R5, Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c04985.

  • Proximate and ultimate analysis of the original coal sample (Table S1) in addition to the representative equations of the kinetic and isotherm models (Table S2) and a schematic diagram of the of the sulfonation processes (Figure S1) and the molecular structure of malachite green dye (Figure S2) (PDF)

Author Contributions

This article was written through the contributions of all authors. All authors have given approval to the final version of the manuscript

The authors declare no competing financial interest.

Notes

Recommendation. Further studies will be conducted to evaluate the efficiency of the coals based adsorbents in realistic removal of malachite green dye from real industrial wastewater based on continuous fixed bed column studies considering the competitive ions, the real pH of the water, the lifetime of the coal bed, and the adsorption capacity and regeneration properties of the bed.

Supplementary Material

ao2c04985_si_001.pdf (341.3KB, pdf)

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