In order to affirm the presence of functional groups at the adsorbent surface, FTIR spectral analysis of both modified and unmodified material (BT) was carried out (range 4000400cm−1) as shown in Figure 1. The 3480 cm−1 band that appears only in unmodified material due to interlayer water molecules i.e., corresponds to –OH stretching vibrations. The band in the range of 14701350cm−1 and 29502800 cm−1 appears only for modified material is characteristic and corresponds to CH bending and stretching vibrations, thus absent in unmodified material. The sp3 CH stretch for BBEHA is (2876 cm−1), sp2 CH stretch for BBEHA (2964 cm−1), CH3 bending vibration for BBEHA is (1379 cm−1), CH2 bending vibration for BBEHA (1481 cm−1). These stretching and bending vibrations are confirms the functional groups of the adsorbent surface that has been modified and are similar to the FTIR described previously [21, 26, 27].
Powder Xray Diffraction studies was carried out to investigate the peak shifting towards lower 2θ value in modified clay that increase the interlayered dspacing of clay material [21, 26, 28]. The Figure 2 shows the pXRD pattern of pristine clay (Fig. 2a) along with the modified clays, most of the peaks in modified pXRD pattern is same as the pristine bentonite clay as confirmed by the Joint Committee on Powder Diffraction Standards card (JCPDS No. 000030015) except one peak that is observed at 2θ = 9.23° with basal dspacing 0.96 nm in pristine clay. If there is a shifting of this peak observed towards lower 2θ values, it shows the increase in interlayer spacing due to intercalation of organic molecule [21, 26, 28]. The modified bentonite shows the peak shifting of BDSS (2θ = 6.44°, 1.37 nm), BBTC (2θ = 6.97°, 1.26 nm), BBTB (2θ = 6.79°, 1.30 nm), BBEHA (2θ = 7.61°, 1.15 nm) (Fig. 2be), which are similar to previous reported studies [21, 26, 28].
Chemical composition of pristine clay (unmodified) as determined via XRF study was found to be: CaO = 5.01%, Na2O = 2.1%, Al2O3 = 23.9%, MgO = 2.9%, SiO2 = 55.1% and K2O = 3.48%. This data indicates that; Na, Mg, K and Ca are prime exchangeable cations [19].
The micrographs of unmodified and modified bentonite material were observed via Scanning electron microscopy (SEM Nano NOVA) to confirm the presence of organic moieties into galleries of clay particles and changes in morphology after modification. It seems that the pristine clay exhibits the grasps foliated with massive curved like plates and tightly held as shown in Figure which become foamy, fluffy and more porous after modification as given in Figure 3 [29, 30]. Moreover, in modified clay there are bigger porous aggregates that provide more residence to adsorbate and have extra available bonding sites for adsorption of DBT. Additionally, regarding as quantification of elemental analysis in modified clay material via EDX, more carbon content was observed in organoclay than carbon content in unmodified bentonite material. It indicates that the synthesized material is effectively modified that results in increase in adsorption capacity of DBT.
The stability of modified and unmodified material was determined by analyzing weight loss over a range of temperature (i.e., 40840°C) under inert atmosphere via TGA as given in Figure 4. Transitions of unmodified material during thermal degradation were at low temperature the surface adsorbed water that volatilize (below 140°C), at high temperature (450600°C) due to –OH group dehydroxylation of water occurred. The four regions of thermal degradation in modified materials occurred as following; the physically adsorbed gaseous substances and water evolved (below 150°C), the organic specie (BTC, BTB, DSS and BEHA) decomposed (between 200450°C), structural water loss caused dehydroxylation (450600°C) and the carbonaceous organic products evolved (between 600700°C) [19, 21]. The increased adsorption capacity of modified materials has successfully been demonstrated by the comparison of modified and unmodified material.
Effect of contact Time and Temperature on DBT adsorption
The effect of contact time on the removal of DBT onto BBTC, BDSS, BBTB and BBEHA was observed at various time intervals by keeping the adsorbent dose 0.5 g, volume = 30 mL and DBT concentration 1000 mg/L as constant. The adsorption of DBT onto organoclay increases with increase of contact time. At the beginning the adsorption process is fast and gradually slows down in order to attain stability. The maximum adsorption occurs at 60 min. Hence 60 min is marked for higher efficiency of adsorption process as shown in Figure 5(a) [12]. Furthermore, to find out the optimum temperature the desulfurization of DBT was carried out by varying temperature in the range of 2560 \(℃\) and other parameters were remained constant. The results depict that the adsorption efficiency has direct relation with temperature (Fig. 5b). At higher temperature, Dibenzothiophene (DBT) is more mobile due to reducing the viscosity as well as higher temperature lead the widens of adsorbent pores to some extent and results in decrease the activation energy [14].
Effect of Adsorbent dose on adsorption
The effect of adsorbent dosage (organoclay) on the desulfurization of DBT was investigated by varying the amount of dose (0.251.25 g) and the DBT concentration of 1000 mg/L, time = 60 min and volume = 30 mL at 45 \(℃\) as shown in Figure 5 (c). The results declared that adsorption efficiency (%) is directly proportional to adsorbent dose and inversely proportional to adsorption capacity. This decrease in adsorption capacity of DBT with increased adsorbent dose is because of larger number of adsorption sites. Hence at lesser adsorbent dosage the adsorption capacity is maximum as observed [14].
Effect of DBT Concentration
Adsorption capacity of organoclays is altered by varying the concentration of DBT. Five different concentrations (5002000 mg/L) were used for the investigation of DBT concentration effects on adsorption of DBT keeping remaining parameters as constant (adsorbent dose = 0.5 g, volume = 30 mL, time = 60 min and at 45 \(℃\)). In general, adsorption capacity increases with increasing concentration of DBT until the availability of the adsorbent. Yet the efficiency is affected, as it limits the available sites of adsorbent for the DBT molecules (at high levels) but can still work with reduced efficiency. DBT removal and adsorption capacity of organoclay illustrate opposite fashion, which can be elucidated as binding sites of organoclay are fixed. When less concentration of DBT is available the faster will be the adsorption and the percent removal will be high, as higher numbers of binding sites are present on the organoclay [31]. More the presence of binding sites on organoclay, lesser the concentration of the DBT molecules, the most efficient will be the adsorption process as shown in Figure 5(d).
Kinetic Study
For understanding the mechanism of adsorption process, kinetic study is of prime importance. During adsorption of DBT on to adsorbent (BBTC, BDSS, BBTB and BBEHA), undergoes various processes from bulk solution onto organoclay surface (adsorbent). For adsorption mechanism, pseudofirst order and pseudosecond order kinetic models were applied. Pseudofirst order is valid for adsorption of adsorbate from aqueous solution (physisorption). The integral form of Pseudofirst order kinetic model is represented as eq. (3) [32]:
$$\text{log}\left(q\text{ₑ}qt\right)=\text{log}\left(qₑ\right)\frac{K₁}{2.303}t$$
3
Where qt is amount of DBT adsorbed at time, qₑ is at equilibrium the amount of DBT adsorbed, K₁ pseudofirst order constant. Rate constant, intercept and slope were calculated from linear plot (log (qₑqt) vs. time).
The pseudosecond order kinetic model is based on “the rate involves forces for sharing or exchanging of electrons between adsorbate and adsorbent (chemisorption)”. Pseudosecond order kinetic model is represented as eq. (4) [32]:
$$\frac{d{q}_{t}}{dt}={k}_{2}({q}_{e}{q}_{t}{)}^{2}$$
4
Rearranging eq. 4 by integrating within boundary conditions at qt=0 to t=0 and qt=qt to t=t, (5):
$$\frac{t}{{q}_{t}}=\frac{1}{{k}_{2}{{q}_{e}}^{2}}+\frac{t}{{q}_{e}}$$
5
Where \({k}_{2}\) pseudosecond order constant, Slope (\(\frac{t}{{q}_{t}}\)) and intercept (\(\frac{1}{{k}_{2}{qe}^{2}}\)) were used for calculating \({k}_{2}\) and qe.
The optimized situations for the studies contain 0.5 g of organoclay, 1000 mg/L of DBT concentration with range of time scale as given in Table 1. The regression coefficient (\({R}^{2}\)) of pseudosecond order is better than pseudofirst order for the organoclay. So, the data indicates the chemisorption mechanism for the adsorption of specific DBT as shown in Figure 6. In addition, the calculated adsorption capacity (qm) for pseudo second order kinetics is greater than experimental (qm) for the adsorption of DBT via organoclay.
To study the mass transfer rate comparison for the adsorption of DBT, the intraparticle diffusion model (Fickiandiffusion) model was also applied and it represents as eq. (6) [32]:
qt = \({K}_{p}{t}^{1/2}+C\) (6)
Where C is intercept (determined from qt vs t1/2 plot) and kp is rate constant (mgg−1 min−1/2) as values are given in Table 1. This Fickiandiffusion model is studied to evaluate the rate controlling step (t = 10  100 min) and the plot is not linear as well as R2 (0.881, 0.899, 0.936 and 0.931 for BBTC, BBTB, BDSS and BBEHA respectively) which is quite lower than pseudo second order kinetics. The obtained findings declared that this model is not fitted well as compared to pseudo second order kinetic model, but it also indicate that the DBT adsorption onto organoclay may also be followed by intraparticle diffusion model.
Table 1
Kinetics results of Pseudo first order, Pseudo second order and Intraparticle diffusion model for the adsorption of DBT onto BBTC, BDSS, BBTB and BBEHA
Adsorbent

Pseudofirst order

Pseudosecond order

Intraparticle diffusion model


q (exp)
(mg/g)

q(calc)
(mg/g)

K1(min1)

R2

q(calc)
(mg/g)

K2(g/mg min1)

R2

Kp

C

R2

BBTC

58.8

67.7

0.0575

0.953

78

5.99\(\times\)104

0.983

5.02

15.46

0.881

BBTB

58.2

74.38

0.0576

0.918

81

4.75\(\times\)104

0.972

5.42

11.38

0.899

BDSS

54.6

60.25

0.0511

0.973

75

5.32\(\times\)104

0.984

5.41

9.081

0.936

BBEHA

50.4

59.06

0.0525

0.960

71

5.16\(\times\)104

0.989

5.06

7.587

0.931

Adsorption Isotherm Models
Equilibrium adsorption was analyzed by different isotherm models. The amount of adsorbed DBT and its concentration in solution were shown by Langmuir and Freundlich isotherm. Langmuir model shows the adsorption of DBT molecules on to adsorbent surface, happens on homogenous monolayer surface deprived of any interaction with neighboring adsorbed molecules. Langmuir isotherm equation is given as eq. (7) [19]:
$$\frac{Cₑ}{qₑ}=\frac{Cₑ}{{q}_{m}}+\frac{1}{{K}_{L}{q}_{m}}$$
7
Where \({K}_{L}\) is Langmuir adsorption equilibrium constant associated with free energy and \({q}_{m}\) is maximum adsorption capacity of organoclay. The adsorption isotherm is plotted by \(\frac{Cₑ}{qₑ} vs Cₑ\), and data should give straight line, which is appropriate for this model where, slope is \(\frac{1}{{q}_{m}}\) and intercept \(\frac{1}{{K}_{L}{q}_{m}}\). Regarding as Langmuir Isotherm model; there is no interaction among the adsorbed molecules. The distinctiveness of Langmuir isotherm model was expressed via RL parameter (dimensionless constant) that is expressed in equation (8):
$${R}_{L}=\frac{1}{1+KL.Co}$$
8
RL parameter point out the nature of DBT adsorption onto organoclay either; irreversible (RL = 0), unfavorable (RL> 1), favorable (0 < RL< 1) and linear (RL = 1).
Freundlich adsorption isotherm is for multilayer formation that occurs due to heterogeneous adsorption. Freundlich adsorption isotherm equation is given as eq. (9) [31]:
$$\text{ln}{q}_{e}=\text{ln}{K}_{F}+\frac{1}{{n}_{F}}\text{ln}{C}_{e}$$
9
Where \({q}_{e}\) is the amount of adsorbate, adsorbed on the surface of adsorbent at equilibrium, \({C}_{e}\) is equilibrium adsorbate concentration, \({K}_{F}\) is Freundlich constant and \(\frac{1}{{n}_{F}}\) is heterogeneity factor. For favorable adsorption \({n}_{F}\) is large than 1. The values for slope \(\frac{1}{{n}_{F}}\) and intercept \(\text{ln}{K}_{F}\) were obtained from linear plot of\(\text{ ln}{q}_{e}vs\text{ln}{C}_{e}\).
Table 2 and Figure 7 include the experimental results of isothermal models, though the regression coefficient (\({R}^{2}\)) of Langmuir model is best for DBT removal, which indicates the monolayer adsorption of sulfur molecules on the surface of organoclay. Moreover, the calculated value of qm = 70.8 (BBTC), 66 (BBTB), 61.2 (BDSS) and 55.2 (BBEHA) in (mg/g) by using Langmuir model is very close to the experimental observed qm = 71.43 (BBTC), 66.66 (BBTB), 62.50 (BDSS) and 57.14 (BBEHA) mg/g which clearly indicated the monolayer adsorption phenomenon. The RL value is lesser than 1 which indicating the favorability of DBT adsorption onto organoclay. Moreover, the order of adsorptive desulfurization is BBTC > BBTB > BDSS > BBEHA.
To examine the adsorbateadsorbent interaction for the adsorption of DBT onto modified bentonite, we further applied a Temkin model. The general representing of Temkin model is given in eq (10) [14]:
$${q}_{e}=Bln{C}_{e}+BlnA$$
10
Where A (Temkin constant (L/g) that is related to adsorbateadsorbate interaction), B (Heat of adsorption in J/mol) and qe (equilibrium adsorbed amount (mg/g)). The heat of adsorption and regression coefficient (R2) is given in Table 2 which indicates that the Temkin model is not well fitted as compared to Langmuir isotherm model.
Table 2
Isotherm studies for the adsorption of DBT onto modified bentonite
Adsorbent

Langmuir

Freundlich

Temkin Model


qm (mg/g)

Exp.qm (mg/g)

KL(L/mg)

R2

RL

nF

Kf (mg/g)

R2

B

R2

BBTC

71.43

70.8

0.104

0.99

0.009

6.71

28.8

0.753

7.19

0.8166

BBTB

66.66

66.00

0.129

0.99

0.007

6.72

26.9

0.724

6.89

0.7724

BDSS

62.50

61.20

0.072

0.99

0.013

5.23

18.9

0.782

8.36

0.8175

BBEHA

57.14

55.2

0.040

0.99

0.024

4.72

14.4

0.788

8.54

0.8146

Thermodynamic studies of DBT adsorption
To understand the effect of different temperatures for the removal of DBT from fuel oil via organoclay, various thermodynamic paramters i.e., standard entropy, standard enthalpy and standard Gibbs Free energy has been thoroughly studied. The above procedure was conducted with 10 mL of (100 mg/L) initail (DBT) solution at various temperatures (298.5, 303.5, 318.5, and 333.5 K) along with 0.5 g (modified clay) for an hour. Using following equation (11) Gibbs Free energy was calculated [14]:
$$\varDelta G^\circ = RT\text{ln}{K}_{c}$$
11
Moreover, Standard entropy was calculated using Vant’s Hoff equation (12) by plotting InKc vs 1/T and standard enthalpy was calculated using equation (13):
$$ln{K}_{c}=\frac{\varDelta H^\circ }{RT}+\frac{\varDelta S^\circ }{R}$$
12
$$\varDelta G^\circ = \varDelta H^\circ T\varDelta S^\circ$$
13
Where Kc = Organoclay retention ability to hold the (DBT) which is calculated through equation (14):
$${K}_{c}=\frac{{q}_{e}}{{C}_{e}}$$
14
Here Ce is the adsorbed Organoclay equilibrium concentration while qe is the (DBT) equilibrium concentration.
The van der wall forces exist between organoclay (BBTC, BDSS, BBTB and BBEHA) and DBT molecules were reduced by triggers the weak interaction at low temperature and at high temperature optimum adsorption was observed ,thus it gives neagtive values of \(\varDelta G^\circ\). However, positive value of enthalpy was justified the endothermic nature of adsorption. Besides this during the adsorption of (DBT), the positive values of \(\varDelta S^\circ\) indicates the irregularity in randomness onto synthesized oragnoclay as given in Table 3.
Table 3
Thermodynamic parameters for the adsorption of Dibenzothiphene (DBT) on modified clay
Adsorptive desulfurization (ADS) of real fuel oil
The adsorptive desulfurization of commercially available fuel samples (Kerosene and Diesel) was also investigated via modified bentonite material (BBTC) under optimized conditions (time = 120 min, adsorbent = 1 g, volume = 50 mL and temperature = 45 \(℃\)) and before desulfurization the total sulfur content in kerosene and diesel oil was 2848 mg/L and 4468 mg/L respectively. To quantify the amount of sulfur components PETRA XRay Fluorescence Spectrophotometer (XRF) (ppm, ASTM D4294) was used. Moreover, other fuel properties such as specific gravity, water content and distillation were also conducted via Hydrometer (g/mL @ 15.6 \(℃\), ASTM D1298), Water content tester (China PTD40068929A) (vol. %, ASTM D4006) and Distillation tester (PMD 110, PAC) (ASTM D86). The findings declared that 96.76 %and 95.83 %removal efficiency was achieved for kerosene and diesel oil respectively and the other fuel characteristics before and after ADS are given in Table 4. Moreover, the unmodified bentonite was also tested for the desulfurization of fuel oil but due to lower interlayer spacing of clay the results was not efficient as compared to modified bentonite material. Upon modification the increase in dspacing and development of interesting morphology (porous and fluffy) results in the increase in adsorption capacity.
Table 4
ADS of kerosene and diesel oil onto BBTC
Tests

Method No.

Kerosene

Diesel Oil

Before ADS

After ADS

Before ADS

After ADS

Specific Gravity (g/mL @ 15.6 \(℃\) )

ASTM D1298

0.834

0.830

0.882

0.875

Total Sulfur by Petra XRay, wt. % (ppm)

ASTM D4294

2848

92

4468

186

Water Content by distillation (vol. %)

ASTM D4006

Nil.

Nil.

Nil.

Nil.

Distillation
(\(℃)\)

50%

ASTM D86

240

240

280

279

90%

301

299

346

342

Compariosn with other reported methods
Due to limited available data for the DBT desulfurization via organoclay based modified materials, we can not make comparison for the adsorption efficiency effectively. Moreover, we have compared adsorpton capacity (mg/g) for the DBT removal with other modified adsorbents as given in Table 5. It can be seen that the proposed method shows better adsorption capacity than the reported methods.
Table 5
Comparison of adsorption capacity (mg/g) of various adsorbents for desulfurization
Adsorbent

Adsorption Capacity
(mg/g)

Initial Sulfur Conc. (mg/L)

Referemce

Granulated activatd carbon

3.52

324

[33]

Copper impregnated activated carbon

3.34

333

[33]

Iron impregnated activated carbon

3.59

320

[33]

Nickel impregnated activated carbon

3.52

324

[33]

Aluminium Oxide

2.79

360.5

[34]

Bentonite

2.29

271.2

[5]

Activated clay

4.01

99.5

[5]

Kaolinite

1.73

327

[5]

Carbon nanotubes

21.50

250

[35]

ACTD

8.60

150

[36]

Carbon silica nanocomposite via Cumodified

13.95

960

[37]

Carbon aerogels

15.10

250

[38]

Magnetic mesoporous carbon

62

1000

[39]

BBTC

70.8

1000

Present Study

BBTB

66.00

1000

Present Study

BDSS

61.20

1000

Present Study

BBEHA

55.2

1000

Present Study
