Synthesis, Characterization and Application of Organoclay for Adsorptive Desulfurization: Isothermal, kinetic and Thermodynamics Studies

The present study encompasses the application of cost effective, organo-modied bentonite material for ecient desulfurization of model oil and real fuel. For the adsorptive desulfurization of oil, dibenzothiophene (DBT) was used as model compound. Various experimental parameters (time, temperature, adsorbent-amount and DBT concentration) were thoroughly investigated. The synthesized material was characterized via X-ray diffraction (XRD), X-ray Fluorescence (XRF), Scanning electron microscopy (SEM), Energy dispersive x-ray (EDX), Thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR). The modication exhibits the increase in interlayer spacing of clay as conrmed from XRD and modied material shows interesting morphology as compared to unmodied bentonite. The results showed that >90% of DBT removal was achieved under optimized conditions for B-BTC, B-BTB and B-DSS and >80% for B-BEHA, for model fuel oil. Additionally, the ndings from desulfurization of real fuel oil declare that 96.76% and 95.83% removal eciency was achieved for kerosene and diesel oil respectively, at optimized conditions and fuel properties follow ASTM specications. The obtained ndings well tted with thermodynamic, isothermal (Langmuir) with adsorption capacity (70.8 (B-BTC), 66 (B-BTB), 61.2 (B-DSS) and 55.2 (B-BEHA) in mg/g) and pseudo-second-order kinetics. In thermodynamic studies, negative sign ( ΔG ∘ ) species the spontaneity whereas, ΔH ∘ endothermic and positive sign (ΔS ∘ ) show randomness after DBT adsorption. interlayer the adsorption capacity of for targets. So, benzethonium (BTC) and (2-ethylhexyl) (BEHA) modied clay was observed to have improved hydrophobicity to interact strongly with the organic matter providing ecient adsorption and removal of DBT from model fuel oil and real fuel oil. In addition, the unmodied and modied clay was characterized via FT-IR, XRF, XRD, SEM, EDX and TGA to investigate the composition of material. Adsorption studies reveal the desulfurization of mg/L level of DBT in model fuel and real fuel oil. To further investigate the adsorption capacity of the clay, adsorption kinetics (pseudo rst order, pseudo second order and intraparticle diffusion model) and isotherms (Langmuir, Freundlich and Temkin model) were also studied and ecacy was monitored at different optimized conditions.


Introduction
Demand for the utilization of more eco-friendly fuels and their production is increasing due to the implementation of legislation requiring strict regulation of green-house gas emissions. Many countries are currently enforcing a strict control of sulfur content in liquid fuels to ultralow levels, making the production of deep desulfurization processes an important research objective [1]. Among the main industrial processes for the removal of sulfur from liquid fuels, the most important is referred as adsorptive desulfurization (ADS) and operates with microporous and mesoporous materials [2].
As the stringent sulfur content has emerged, desulfurization of fuel has received massive attention of the world.
Thiols, sul des and disul des can be e ciently removed using conventional adsorbents [3]. Various important features of adsorptive desulfurization (ADS) which have grasped attention as an alternative technology are high e ciency, moderate operation conditions and its economical rates. Similar technique is photocatalytic oxidative desulfurization (ODS) in which sulfur compounds are converted to SO 4 −2 , sulfoxides and sulfones under photooxidation [4]. These polarized compounds can be separated from non-polar oils into extractants i.e., water, acetonitrile or ionic liquids (ILs). Numerous studies have been conducted for desulfurization of fuel oils by combining UV irradiation and liquid-liquid extraction. The center of their research (adsorptive and oxidative) lies on product identi cation and the effect of sulfur removal [5].
These days many of the re neries utilize hydrodesulfurization method (HDS) for the removal of sulfur compounds from petroleum products. The process contains drastic conditions as it operates at high temperatures, elevated concentrations of hydrogen gas and conventional catalyst to convert the organo sulfur compounds into hydrocarbons and H 2 S. Despite its severe conditions the process is unable to achieve current sulfur speci cations [5,6]. Due to this reason the researchers are working to develop alternate desulfurization methods. Many methods are reported for sulfur removal such as oxidative desulfurization (ODS), bio-desulfurization (BDS) and adsorptive desulfurization (ADS) [4]. Desulfurization assisted by adsorption a potential method to remove sulfur compounds ( ) from liquid fuels. During the last few decades, adsorptive removal has gained considerable research interests upon modi cation that result in increased the adsorption capacity. Based on experimental results, about 99% of the sulfur compounds can be removed from model diesel fuels to reach promising desulfurization via adsorption process [7,8]. Adsorption is considered to be one of most popular technique for sulfur removal owing to its high e cacy, cost e ciency, simple operation, and tolerant of processing conditions [9]. Commonly used adsorbents are grapheme nanoplates [10], mesoporous silica [11], magnetic carbon [12], activated alumina [13], activated charcoal [14], Tin (Sn) impregnated activated charcoal [15], activated carbon manganese oxide nanocomposite [16], mesoporous carbon [17], activated carbon [18], etc. but they exhibit lower adsorption capacity as compared to clay material.
Clay (low-cost and eco-friendly) is a naturally available adsorbent that has been used for the removal of dyes, heavy metals, organic pollutants, mycotoxins, sulfur content etc. form decades [19][20][21]. Its effectiveness is because of its layered structure that bears strong a nity regarding cations and anions and has exchangeable ions that play vital role in adsorption. Thus, it works as host material for the adsorbents with an increased surface area as well as it has signi cant applications in synthesis of biodiesel [22,23]. However, the adsorption capacity of raw clay is not as good as compared to synthetically modi ed clay. Once the clay surface is modi ed with surfactant based organic molecule, its adsorption capacity could be enhanced by leaps and bounds. For the past two decades removal of organic contaminants using modi ed solid material has gained much attention [24,25]. The clay was modi ed with organic molecules to form micelle-like structures on its surface that had the capability to solubilize organic pollutant such as DBT.
We herein report the removal of DBT desulfurization by a low-cost adsorbent i.e., organoclay. But hydrophilicity of interlayer surface restricts the adsorption capacity of clay for organic targets. So, benzyl tri-n-butyl ammonium bromide (BTB), Dioctyl sodium sulfosuccinate (DSS), benzethonium chloride (BTC) and Bis (2-ethylhexyl) amine (BEHA) based modi ed clay was observed to have improved hydrophobicity to interact strongly with the organic matter providing e cient adsorption and removal of DBT from model fuel oil and real fuel oil. In addition, the unmodi ed and modi ed clay was characterized via FT-IR, XRF, XRD, SEM, EDX and TGA to investigate the composition of material. Adsorption studies reveal the desulfurization of mg/L level of DBT in model fuel and real fuel oil. To further investigate the adsorption capacity of the clay, adsorption kinetics (pseudo rst order, pseudo second order and intraparticle diffusion model) and isotherms (Langmuir, Freundlich and Temkin model) were also studied and e cacy was monitored at different optimized conditions.

Material Synthesis
Modi ed bentonite (B-BTB, B-DSS, B-BTC and B-BEHA) were synthesized via modi cation regent by dispersing the 1 g of bentonite (BT) in 50 mL of water through sonication (for the dispersion of clay particles into water) for 30 min at room temperature, reported previously [21]. The pH (4) was maintained by using 0.5 M HCl to make clear suspension of above solution. 0.3 g of modi cation reagent (BTB, DSS, BTC or BEHA) was dissolved in 50 mL of water and added into above suspension and re uxed stirring was carried out for 6 hours at 120 °C. The synthesized material was subjected to ltration, dried at 120 °C in oven and grounds via pestle mortar.
Adsorption studies method Here, C e and C i = equilibrium and initial DBT concentrations (mg/L), qe = adsorption capacity of synthesized organoclay (mg/g), V = solution volume, m = mass of organoclay by weight (g).
Moreover, PETRA-X-ray Fluorescence (XRF) sulfur analyzer (ASTM D-4294) used to analyze the residual amount of sulfur compounds.

Results And Discussion
Material Characterization

FT-IR
In order to a rm the presence of functional groups at the adsorbent surface, FT-IR spectral analysis of both modi ed and unmodi ed material (BT) was carried out (range 4000-400cm −1 ) as shown in Figure

XRD and XRF
Powder X-ray Diffraction studies was carried out to investigate the peak shifting towards lower 2θ value in modi ed clay that increase the interlayered d-spacing of clay material [21,26,28]. The Figure  Chemical composition of pristine clay (unmodi ed) as determined via XRF study was found to be: CaO = 5.01%,

SEM
The micrographs of unmodi ed and modi ed bentonite material were observed via Scanning electron microscopy (SEM Nano NOVA) to con rm the presence of organic moieties into galleries of clay particles and changes in morphology after modi cation. 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, uffy and more porous after modi cation as given in Figure 3 [29,30]. Moreover, in modi ed 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 quanti cation of elemental analysis in modi ed clay material via EDX, more carbon content was observed in organoclay than carbon content in unmodi ed bentonite material. It indicates that the synthesized material is effectively modi ed that results in increase in adsorption capacity of DBT.

TGA
The stability of modi ed and unmodi ed material was determined by analyzing weight loss over a range of temperature (i.e., 40-840°C) under inert atmosphere via TGA as given in Figure 4. Transitions of unmodi ed material during thermal degradation were at low temperature the surface adsorbed water that volatilize (below 140°C), at high temperature (450-600°C) due to -OH group de-hydroxylation of water occurred. The four regions of thermal degradation in modi ed 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 200-450°C), structural water loss caused de-hydroxylation (450-600°C) and the carbonaceous organic products evolved (between 600-700°C) [19,21]. at 60 min. Hence 60 min is marked for higher e ciency of adsorption process as shown in Figure 5(a) [12].
Furthermore, to nd out the optimum temperature the desulfurization of DBT was carried out by varying temperature in the range of 25-60 ℃ and other parameters were remained constant. The results depict that the adsorption e ciency 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 (0.25-1.25 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 e ciency (%) 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 Yet the e ciency is affected, as it limits the available sites of adsorbent for the DBT molecules (at high levels) but can still work with reduced e ciency. DBT removal and adsorption capacity of organoclay illustrate opposite fashion, which can be elucidated as binding sites of organoclay are xed. 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 e cient 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 (B-BTC, B-DSS, B-BTB and B-BEHA), undergoes various processes from bulk solution onto organoclay surface (adsorbent). For adsorption mechanism, pseudo-rst order and pseudo-second order kinetic models were applied. Pseudo-rst order is valid for adsorption of adsorbate from aqueous solution (physisorption). The integral form of Pseudo-rst order kinetic model is represented as eq. (3) [32]: Where q t is amount of DBT adsorbed at time, q is at equilibrium the amount of DBT adsorbed, K pseudo-rst order constant. Rate constant, intercept and slope were calculated from linear plot (log (q -q t ) vs. time).
The pseudo-second order kinetic model is based on "the rate involves forces for sharing or exchanging of electrons between adsorbate and adsorbent (chemisorption)". Pseudo-second order kinetic model is represented as eq. (4) [32]: Rearranging eq. 4 by integrating within boundary conditions at qt=0 to t=0 and qt=qt to t=t, (5): Where k 2 pseudo-second order constant, Slope ( t q t ) and intercept ( 1 k 2 qe 2 ) were used for calculating k 2 and q e .
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 coe cient (R 2 ) of pseudo-second order is better than pseudo-rst order for the organoclay. So, the data indicates the chemisorption mechanism for the adsorption of speci c DBT as shown in Figure 6. In addition, the calculated adsorption capacity (q m ) for pseudo second order kinetics is greater than experimental (q m ) 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]: Where C is intercept (determined from q t vs t 1/2 plot) and k p is rate constant (mgg −1 min −1/2 ) as values are given in Table 1. This Fickian-diffusion model is studied to evaluate the rate controlling step (t = 10 -100 min) and the plot is not linear as well as R 2 (0.881, 0.899, 0.936 and 0.931 for B-BTC, B-BTB, B-DSS and B-BEHA respectively) which is quite lower than pseudo second order kinetics. The obtained ndings declared that this model is not tted 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.
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 Freundlich adsorption isotherm is for multilayer formation that occurs due to heterogeneous adsorption. Freundlich adsorption isotherm equation is given as eq. (9) [31]: 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 To examine the adsorbate-adsorbent interaction for the adsorption of DBT onto modi ed bentonite, we further applied a Temkin model. The general representing of Temkin model is given in eq (10) [14]: q e = BlnC e + BlnA

10
Where A (Temkin constant (L/g) that is related to adsorbate-adsorbate interaction), B (Heat of adsorption in J/mol) and q e (equilibrium adsorbed amount (mg/g)). The heat of adsorption and regression coe cient (R 2 ) is given in Table 2 which indicates that the Temkin model is not well tted as compared to Langmuir isotherm model. 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 (modi ed clay) for an hour. Using following equation (11) Gibbs Free energy was calculated [14]: Moreover, Standard entropy was calculated using Vant's Hoff equation (12) by plotting InK c vs 1/T and standard enthalpy was calculated using equation (13): Where K c = Organoclay retention ability to hold the (DBT) which is calculated through equation (14): Here C e is the adsorbed Organoclay equilibrium concentration while q e is the (DBT) equilibrium concentration.
The van der wall forces exist between organoclay (B-BTC, B-DSS, B-BTB and B-BEHA) 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 ΔG ∘ . However, positive value of enthalpy was justi ed the endothermic nature of adsorption. Besides this during the adsorption of (DBT), the positive values of ΔS ∘ indicates the irregularity in randomness onto synthesized oragnoclay as given in Table 3. Table 3 Thermodynamic parameters for the adsorption of Dibenzothiphene (DBT) on modi ed clay Adsorptive desulfurization (ADS) of real fuel oil The adsorptive desulfurization of commercially available fuel samples (Kerosene and Diesel) was also investigated via modi ed bentonite material (B-BTC) 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 X-Ray Fluorescence Spectrophotometer (XRF) (ppm, ASTM D-4294) was used. Moreover, other fuel properties such as speci c gravity, water content and distillation were also conducted via Hydrometer (g/mL @ 15.6 ℃, ASTM D-1298), Water content tester (China PT-D4006-8929A) (vol. %, ASTM D-4006) and Distillation tester (PMD 110, PAC) (ASTM D-86). The ndings declared that 96.76 %and 95.83 %removal e ciency was achieved for kerosene and diesel oil respectively and the other fuel characteristics before and after ADS are given in Table 4. Moreover, the unmodi ed bentonite was also tested for the desulfurization of fuel oil but due to lower interlayer spacing of clay the results was not e cient as compared to modi ed bentonite material. Upon modi cation the increase in dspacing and development of interesting morphology (porous and uffy) results in the increase in adsorption capacity. Compariosn with other reported methods Due to limited available data for the DBT desulfurization via organo-clay based modi ed materials, we can not make comparison for the adsorption e ciency effectively. Moreover, we have compared adsorpton capacity (mg/g) for the DBT removal with other modi ed adsorbents as given in Table 5. It can be seen that the proposed method shows better adsorption capacity than the reported methods.

Conclusions
The cost-effective material has been developed via benzyl tri-n-butyl ammonium bromide (BTB), Dioctyl sodium shows direct relation of adsorption e ciency with time, temperature and adsorbent amount and is indirectly related to DBT concentration. Adsorption kinetics study follows pseudo-second-order kinetic model (regression coe cient R 2 = 0.98) which shows chemisorption behavior of DBT adsorption. However, present data ts very well with isothermal (Langmuir Model where 0<R L <1 shows favorable adsorption and R 2 = 0.99) and thermodynamic studies (endothermic and spontaneity in system). Thus, the whole analytical study con rms the prominence of developed organoclay material for better adsorptive desulfurization.

Con ict of Interest
All the authors declares no con ict of interest.   Pseudo second order kinetics for the adsorption of DBT onto organoclay