Adsorption of benzothiophene sulfone over clay mineral adsorbents in the frame of oxidative desulfurization

Fuel 205 (2017) 153–160

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Full Length Article

Adsorption of benzothiophene sulfone over clay mineral adsorbents in the frame of oxidative desulfurization Angelo Earvin Sy Choi a,b, Susan Roces a, Nathaniel Dugos a, Meng-Wei Wan c,⇑ a

Chemical Engineering Department, De La Salle University, 2401 Taft Ave, Manila 0922, Philippines Center for Clean Technology and Resource Recycling, University of Ulsan, 93 Dehakro, Ulsan 680-749, South Korea c Department of Environmental Resources Management, Chia-Nan University of Pharmacy and Science, Tainan 71710, Taiwan b

h i g h l i g h t s
Clay mineral adsorbents were characterized using BET and FTIR.
Activated clay surpasses bentonite and kaolinite based on adsorption capacity.
Benzothiophene sulfone adsorption followed Langmuir and Freundlich models.
Adsorption process demonstrated a chemical adsorption and an endothermic process.
Activated clay showed a promising performance over activated carbon and alumina.

a r t i c l e

i n f o

Article history: Received 27 December 2016 Received in revised form 22 May 2017 Accepted 24 May 2017

Keywords: Activated clay Batch adsorption Bentonite Benzothiophene sulfone Oxidative desulfurization Kaolinite

a b s t r a c t The adsorption of benzothiophene sulfone (BTO) from model fuel oil was investigated using three different clay mineral adsorbents. The adsorption characteristics of clay mineral adsorbents such as activated clay, bentonite and kaolinite were evaluated using Fourier transform infrared spectroscopy and Brunauer, Emmett and Teller surface area analyzer. A batch process was conducted to determine the adsorption performances at varying contact time, reaction temperature and initial concentration. Increasing adsorption capacities followed the order of kaolinite < bentonite < activated clay. The equilibrium isotherms using Langmuir and Freundlich models yielded a good fit (R2 > 0.98) indicating a monolayer and heterogeneous adsorption. A second order reaction kinetic model showed high suitability (R2 > 0.97) based on the experimental data. Results showed that adsorption follows a two-step process: (1) fast adsorption rate for the first two hours and (2) markedly slow adsorption rate until equilibrium. The clay minerals have different functional groups present in its surface which determines the essential adsorption characteristics. The thermodynamic parameters for BTO adsorption onto clay mineral adsorbents indicated an endothermic reaction. Activated clay and kaolinite were spontaneous and non-spontaneous, respectively, while bentonite was found to be only non-spontaneous at 25 °C. In comparison with conventional adsorbents, activated clay was found to be superior in the application of sulfone adsorption in fuel oil. Ó 2017 Published by Elsevier Ltd.

1. Introduction Sulfur compounds in the form of sulfides, thiols, alkylated benzothiophene (BT) and among others are naturally present in raw petroleum products. The presence of sulfur are able to promote catalyst poisoning in engines, corroding parts for internal combustion and lowering the efficiency for combustion [1]. The formation of sulfur oxides (SOx) from the ignition of fuel oil leads to various problems such as acid rain, smog, pulmonary problems [2–4]. ⇑ Corresponding author. E-mail address: [email protected] (M.-W. Wan). http://dx.doi.org/10.1016/j.fuel.2017.05.070 0016-2361/Ó 2017 Published by Elsevier Ltd.

Many countries are implementing stringent environmental regulation to limit the total sulfur content in fuel products. The European Union and United States Environmental Protection Agency has set maximum sulfur standards for diesel oil at 10 and 15 ppmw, respectively [5,6]. Hydrodesulfurization (HDS) is currently the conventional method for unwanted sulfur removal in fuel oil. In order to achieve low sulfur concentrations in fuel oil, HDS operates at high hydrogen pressure (3.0–5.0 MPa), high temperature (300–450 °C) and requires expensive catalysts (NiMo/Al2O3 or CoMo/Al2O3) [4]. This hydrotreatment refining process when used at large-scale incurs high operating cost [7]. The mechanism in HDS is explained by

154

A.E.S. Choi et al. / Fuel 205 (2017) 153–160

the destruction of the carbon to sulfur bond utilizing hydrogen with a catalyst to form a sulfur free hydrocarbon and hydrogen sulfide [8]. However, a distinct disadvantage in conventional HDS is its inefficiency towards heterocyclic sulfur compounds such as alkylated BT [4,9,10]. Other desulfurization treatment technologies are therefore explored to find alternatives to HDS that could improve removal efficiency and reduce cost. Recent studies such as the oxidative desulfurization (ODS) technology combines a selective oxidation and extraction process at low operating condition to achieve low sulfur fuel oil [7,11,12]. This process is frequently coupled with an ultrasound probe [7,13–21] and a high shear mixer [12,22–24] to enhance oxidation efficiencies that produces clean fuels. The ODS process is able to selectively oxidize heterocyclic sulfur compounds to its sulfone forms through electrophilic addition of oxygen towards sulfur without breaking C–C bonds [25]. Oxidized sulfur compounds such as benzothiophene sulfone (BTO) are produced from BT; this has a higher polarity in comparison with the hydrocarbons present in fuel oils that can be easily removed by solvent extraction or solid adsorption [14,26]. Solvent extraction is able to separate compounds based on the relative solubility of two immiscible liquids. Highly polar sulfones can therefore be separated from the nonpolar constituents present in oxidized fuel oil using a polar solvents such as acetonitrile and dimethylsulfoxide [7,27]. Several disadvantages associated in the solvents utilized includes its explosiveness and toxicity which are a major problem in solvent extraction [7]. Its recovery and recyclability also proves to be challenging due to similar boiling point properties with the extracted compounds [27]. Furthermore, the resulting fuel properties in the extracted product falls outside the required specifications [27]. An alternative method in the removal of sulfones is through solid adsorption. This extractive process does not require the use of additional chemicals [28]. The adsorption process is favored over solvent extraction due to being more environmentally friendly and having a higher removal capacity [7,11,12]. According to Lu et al. [12], a complete sulfone removal was observed in an adsorption process but retained 216 ppmw in a solvent extraction process with a 1430 ppmw initial sulfur concentration in diesel oil. Chen et al. [7] showed higher sulfone removal efficiency using adsorption over solvent extraction by a difference of 37.9% for diesel oil. In the study of Etemadi & Yen [11], the adsorbent utilized in the adsorption process is thirtythree (33) times less consumed in terms of material usage than that of the solvent used in the solvent extraction of sulfones. Past studies have used different adsorbents such as modified chitosan [29–31], granular activated carbon [30,32] and modified activated carbon [30,32] and alumina [11,30,33] for sulfone adsorption. Nevertheless, the use of these adsorbents is costly and adsorbent modifications require complicated preparative procedures and additional costs. Clay mineral adsorbents have relatively large specific surface areas and are made of hydrous aluminosilicates sheets [34,35]. This type of adsorbents are abundant, low-cost and mechanically and chemically stable [34,35]. Clay mineral adsorbents are extensively used to aid heavy metals removal such as copper [35–37], nickel [34,36], lead [36], arsenic [38] and indium [39]. Currently, there is no information available in literatures that study the mechanism and use of different clay mineral adsorbents to remove sulfones such as BTO in an adsorption process. In this study, the affinity of BTO as a model fuel oil was investigated towards clay mineral adsorbents. Clay mineral adsorbents of activated clay, bentonite and kaolinite were examined in batch studies. The effect of reaction temperature and initial concentration were studied to determine the adsorption capacities of clay mineral adsorbents in BTO at various parameters. A comparative study based on the physical properties, kinetic models, isotherm

models and thermodynamic parameters were examined to evaluate the BTO adsorption mechanisms onto clay mineral adsorbents. A comparative assessment of the adsorption capacity in BTO adsorption was also compared to various adsorbents cited in literatures.

2. Experimental 2.1. Materials Activated clay was procured by Xinxin Chemical Co., Ltd. (Tainan, Taiwan). Bentonite and kaolinite were acquired from SigmaAldrich (USA). Benzothiophene sulfone (C8H6O2S, 98% purity) was supplied from Alfa Aesar (Taiwan). Hydrochloric acid (HCl, 37 vol.% fuming), toluene (C7H8, 0.99 mass fractions) and potassium bromide (KBr) were obtained from Merck Chemical Company (USA). Sodium hydroxide (NaOH, 98% purity) was purchased from Shimakyu’s Pure Chemicals (Osaka, Japan). All chemicals used were without further purification and analytical grade.

2.2. Instrumental analysis The surface area and average pore size of clay mineral adsorbents were measured by a surface area analysis of Brunauer, Emmett, and Teller (BET) multipoint technique at 77 K using N2 adsorption in a Beckman Coulter SA 3100 Surface Area and Pore Size Analyzer. The different clay mineral adsorbents before and after adsorption were analyzed using a Fourier transform infrared spectrophotometer (FTIR, JASCO FT/IR 410) operated at wavenumbers of 400 to 4000 cm1. The samples were homogenized in KBr (1:20). The concentration of BTO were quantified using an Agilent 7890A gas chromatograph (GC, California, USA) equipped with fused-silica capillary column HP-5 ms having 0.25 mm film thickness (J & W Scientific, USA) and an Agilent 355 sulfur chemiluminescence detector (SCD). The GC oven temperature was set at an initial temperature of 150 °C for 1 min, heated at a rate of 20 °C/ min to 220 °C, and retained for 1 min.

2.3. Batch adsorption studies Kinetic studies were conducted by agitating 1.0 g of the respective clay mineral adsorbents and 10 mL BTO solution (500 mg/L) using 120 rpm at 25–55 °C. Toluene was used as the solvent to make the BTO solution. Samples were taken at pre-determined time intervals (5 min to 48 h). Equilibrium isotherm studies were carried out by placing 1.0 g of the respective adsorbent in a 10 to 1000 mg/L BTO solution under an agitation speed of 120 rpm at 25 °C for 48 h. Thermodynamic studies were performed using an initial concentration of 500 mg/L BTO, agitated at 120 rpm for 48 h. The thermal effects of BTO adsorption onto clay mineral adsorbents were studied under various temperatures settings (25 to 55 °C). The experimental data gathered for the BTO adsorption onto clay mineral adsorbents were quantified using a GC-SCD and measured using the adsorption capacity in Eq. (1):

qe ¼

Co  Ce V m

ð1Þ

where V is the volume of solution (L), Co and Ce are the initial and equilibrium concentration (mg/L), respectively, and m is the mass of clay mineral adsorbent (g).

155

A.E.S. Choi et al. / Fuel 205 (2017) 153–160

3. Results and discussion 3.1. Specific surface area analysis In Table 1, the physical properties of activated clay, bentonite and kaolinite are listed. Results after BTO adsorption were under the temperature setting of 55 °C. The t-plot and Barrett-JoynerHalenda (BJH) methods were used to determine the micropore area and pore diameter, respectively. For the BJH method, the adsorption branch was utilized. Based on the International Union of Pure and Applied Chemistry, the size of the pore diameter (d) is divided into three categories: macropores (d > 50 nm), mesopores (2 < d < 50 nm) and micropores (d < 2 nm) [34]. Results indicated that activated clay and kaolinite are under the mesoporous category, while bentonite is a macroporous material prior to BTO adsorption. The physical properties of total pore volume and surface area for the clay mineral adsorbent are arranged in the order of activated clay > bentonite > kaolinite. The physical properties for each clay mineral adsorbent decreased after the adsorption process due to the BTO molecules covering the surface and blocking the pores of the adsorbent. This is attributed to the adhesion of BTO adsorbates in the surface of each respective adsorbent. 3.2. FTIR analysis Adsorption capacity depends also on the chemical reactivity of the functional groups at the surface of the adsorbent [40]. Fig. 1 shows the FTIR spectra of clay mineral adsorbents before and after BTO adsorption at a spectral resolution of 4 cm–1 and temperature setting of 55 °C. The bands at 3433 to 3628 cm1 and 2863 to 2953 cm1 is due to –OH stretching and –C–H stretching, respectively [41,42]. The carboxyl group at a peak of 1734 cm1 is determined by the –C@O stretching [43]. The peak at 1638 to 1643 cm1 indicates the amide functional group through –N–H bending. The bands at 1511 to 1524 cm1 denotes to a –N–O asymmetric stretching. The –C–N stretching and –C–O stretching corresponds to peaks at 1103 to 1280 cm1 and 1027 to 1049 cm1, respectively. The peaks from 919 to 923 cm1 and from 788 to 797 cm1 is attributed by a –O–H bending and –R–NH2 stretching vibrations, respectively. The –C–Cl stretching vibration is in between 695 to 714 cm1 while –C–Br stretching refers to the bands at 525 to 617 cm1. The essential functional groups involved in the adsorption process in clay mineral adsorbents are determined by the shift of vibrations (>±4 cm1) after BTO adsorption. Activated clay displayed a notable shift in the peak of 3436 cm1 to a lower wavelength of 3430 cm1. This suggests that the hydroxyl (–OH) functional groups were involved the adsorption of BTO. The adsorption of BTO onto bentonite illustrated a –N–H bending which infers as the functional group responsible for the adsorption to occur due to a markedly shift of peak from 1643 cm1 to lower wavelength of 1638 cm1. The –N–H groups were also involved in the BTO adsorption utilizing kaolinite. This is due to the peak at 1638 cm1 noticeably shifted to a higher wavelength of 1647 cm1 implying the occurrence of –N–H bending.

Fig. 1. FT-IR analysis before and after BTO adsorption using (a) activated clay, (b) bentonite and (c) kaolinite.

3.3. Kinetic studies A kinetic analysis for the adsorption of BTO onto clay mineral adsorbents is essential in process design to determine the rate of adsorption. The results in the experimental data illustrated in Fig. 2 can aid in indicating the type of adsorption mechanism that occurs based on its rate-limiting step. The pseudo-first order and pseudo-second order were the kinetic models utilized to define the kinetic rate constant and adsorption mechanism. The pseudo-first order equation is in the form given in Eq. (2) [44]:

lnðqe  qt Þ ¼ ln qe  k1 t

ð2Þ

Table 1 Physical properties before and after BTO adsorption utilizing activated clay, bentonite and kaolinite. Physical properties

Total pore volume (cm3/g) Average pore diameter (nm) Micropore area (m2/g) BET surface area (m2/g) Langmuir surface area (m2/g)

Activated Clay

Bentonite

Kaolinite

Before

After

Before

After

Before

After

0.40 16 20 292 277

0.36 14 2 262 248

0.12 134 <0.01 24 23

0.10 55 <0.01 16 14

0.04 45 <0.01 10 7

0.03 35 <0.01 8 6

156

A.E.S. Choi et al. / Fuel 205 (2017) 153–160

1.0

activated clay > bentonite > kaolinite. The R2 values observed a poor correlation due to lower values for pseudo-first order (R2 < 0.90) kinetic model, and the theoretical and experimental qe were considerably different which proves that this model cannot appropriately described the rate-limiting step of BTO adsorption. It is seen that the experimental data gave an evident fit towards the pseudo-second order kinetic model due to a high R2 value (R2 > 0.97) and close theoretical and experimental qe values. This implies that BTO adsorption is best described by chemisorption as the rate determining step of the adsorption process that utilizes activated clay, bentonite and kaolinite from 25 to 55 °C. This also suggest that the formation of covalent bonds enables the sharing of electrons between the binding sites of the adsorbents and the adsorbate [36,41,48]. The kinetic rate constants, k2, determines how fast the adsorption occurrence of the molecules onto the surface of the adsorbent. Although the pseudo-second order showed the best fit, this has still no physical meaning. A fast adsorption rate was observed for all the adsorbents for the first two hours after which the adsorption rate noticeably slowed down until reaching equilibrium. The observed two-step adsorption process indicates the presence of physisorption and chemisorption due to high correlation for both Langmuir and Freundlich models discussed in Section 3.5. In the first two hours, the high adsorption rate was due to the physical adsorption that only involves covalent forces. Afterward, the observable slow rate of adsorption until reaching equilibrium was due to the chemical adsorption that involves the formation of chemical bonds. Activated clay had the highest rate of BTO adsorbed among the three adsorbents in all the temperature settings.

(a)

0.8

C/Co

0.6 Activated Clay Bentonite Kaolinite

0.4 0.2 0.0 0

1000

2000

3000

Time (min) 1.0

(b)

0.8

C/Co

0.6

Activated Clay Bentonite Kaolinite

0.4 0.2 0.0 0

1000

2000

3000

Time (min) 1.0

(c)

0.8

3.4. Equilibrium isotherm studies

C/Co

0.6 Activated Clay Bentonite Kaolinite

0.4 0.2 0.0 0

1000

2000

3000

Time (min) Fig. 2. BTO adsorption using clay mineral adsorbents at different temperatures of (a) 25 °C, (b) 40 °C and (c) 55 °C.

where qe and qt are the adsorption capacities (mg/g) at equilibrium and at a predetermined time interval, respectively, k1 is the pseudofirst order rate constant (min1) and t is the reaction time (min). The pseudo-first order (Lagergren) equation was the earliest adsorption rate model and the most typical model that describes a reversible equilibrium on the adsorbate and adsorbent [45]. The Lagergren kinetic model has an assumption that the rate of occupation of adsorption sites is proportional to the number of unoccupied sites [46]. The pseudo-second order rate equation is expressed in Eq. (3) [39]:

t 1 t ¼  qt k2 q2e qe

ð3Þ

where k2 (g/mg-min) denotes the pseudo-second order adsorption rate constant. Table 2 lists a summary of the corresponding kinetic parameters and correlation coefficients (R2) of the pseudo-first order and pseudo-second order kinetic models. Results indicate that the adsorption capacity of BTO towards clay mineral adsorbents were higher at higher temperatures. Based on the experimental adsorption capacity for the respective adsorbent, it followed a trend of

The adsorption experimental data for BTO adsorption of clay mineral adsorbents were evaluated by the Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherms at 25 °C. The equilibrium isotherm models essentially describe the relationship between adsorbent and adsorbate upon reaching equilibrium at different initial concentrations illustrated in Fig. 3. The adsorption equilibrium models give insights on the surface properties, adsorption mechanism and adsorbent affinity based on the equation parameters and its underlying thermodynamic assumptions. The Langmuir model assumes a saturated monolayer of solute molecules on the adsorbent surface for its maximum adsorption, no transmigration of adsorbed molecules on the surface of the adsorbent and no lateral interaction between adsorbed molecules [49]. This model is expressed in Eq. (4):



 1 1 1 1 ¼ þ qe qmax bqmax Ce

ð4Þ

where qe is the adsorption capacity at equilibrium (mg/g), qmax is the maximum adsorption capacity at a monolayer coverage (mg/ g), b is the constant related to the free energy of adsorption (mL/ mg) and Ce is the equilibrium concentration of BTO (mg/L). The Freundlich model assumes a multilayer adsorption on energetically heterogeneous surface which is applicable for a non-ideal adsorption process [50]. The Freundlich equation is given in Eq. (5):

log qe ¼ log kf þ

1 log C e n

ð5Þ

where kf is the relative adsorption capacity (mg/g) and n refers to the favorability of the adsorption process (L1). Values of n < 1 signifies physical adsorption, while n > 1 represent the adsorption process as chemical in nature [49]. The Temkin model accounts for the effects of indirect adsorbate/adsorbate interactions that suggests the heat of adsorption

157

A.E.S. Choi et al. / Fuel 205 (2017) 153–160 Table 2 Kinetic parameters at different temperatures for BTO adsorption onto activated clay, bentonite and kaolinite. Temp. (°C)

Adsorbent

qe, expt’l. (mg/g)

Pseudo first-order model 1

k1 (min 25

Activated Clay Bentonite Kaolinite Activated Clay Bentonite Kaolinite Activated Clay Bentonite Kaolinite

40

qe (mg/g)

55

8 7 6 5 4 3 2 1 0

4.0052 2.2883 1.7255 4.4630 3.7076 2.8931 4.7670 4.3681 3.8097

)

0.0015 – – 0.0011 – – 0.0012 – –

200

400

600

800

Fig. 3. Effect of initial concentration on BTO adsorption using activated clay, bentonite and kaolinite.

of all adsorbed molecules in the layer decreases linearly with the coverage [51]. The Temkin equation is given in Eq. (6):

ð6Þ

where b is related to the heat of adsorption (J/mol) and a denotes the Temkin isotherm binding constant (L/g). The Dubinin–Radushkevich model equation is used to determine whether the process of adsorption is physical or chemical in nature which is shown in Eq. (7) [36]:

ln qe ¼ ln X m  Be2

ð7Þ

where Xm is the theoretical saturation capacity (mg/g), B is the Dubinin–Radushkevich constant (kJ2/mol2) and e is the Polanyi potential shown in Eq. (8):




1 e ¼ RT ln 1 þ Ce



ð8Þ

where R is the universal gas constant (8.314 J/mol-K) and T is the absolute temperature (K). The B constant is related to the mean free energy of adsorption which is given in Eq. (9):

1 E ¼ pffiffiffiffiffiffi 2B

R

0.3648 1.1784 0.6024 0.8436 2.1641 1.9566 0.3663 1.3133 0.9967

0.8927 0.8106 0.8971 0.8989 0.8985 0.8956 0.5199 0.6551 0.8483

k2 (g/mg min)

qe, theo (mg/g)

R2

0.0273 0.0037 0.0176 0.0081 0.0023 0.0054 0.0312 0.0062 0.0116

4.0051 2.1733 1.7436 4.4437 3.7306 2.9389 4.7545 4.3621 3.8295

0.9999 0.9792 0.9999 0.9991 0.9959 0.9989 0.9999 0.9994 0.9999

3.5. Thermodynamic studies

1000

BTO Concentration (mg/L)

qe ¼ b ln a þ b ln C e

qe, theo (mg/g)

chemical adsorption. Results indicate that activated clay has the lowest n and highest kf values over bentonite and kaolinite. Langmuir model cannot be completely disregarded due to also attaining high R2 values (R2 > 0.98). Thus, heterogeneous adsorption is not solely the mechanism involved in BTO adsorption. This means that clay mineral adsorbents also have an adsorption process of a homogenous monolayer adsorption and the adsorbed particles do not interact with each other.

Activated Clay Bentonite Kaolinite

0

Pseudo second-order model 2

ð9Þ

where E is the mean adsorption energy (kJ/mol). The values of E < 40 kJ/mol suggest physical adsorption, while E > 40 kJ/mol imply chemical adsorption [52]. Table 3 lists a summary of the adsorption parameters and R2 of Langmuir, Freundlich, Temkin and Dubinin–Radushkevich equilibrium isotherm models. According to the computed R2 values, BTO adsorption onto clay mineral adsorbents did not suitably fit with Temkin (R2 < 0.77) and Dubinin–Radushkevich (R2 < 0.62) models. The Freundlich model best describes the adsorption process based on having the highest R2 value (R2 > 0.99). This implies a heterogeneous adsorption. Clay mineral adsorbents showed values of n > 1 which appropriately supports the results in the kinetic study of following a pseudo-second order model reiterating the occurrence of

The nature of BTO adsorption of activated clay, bentonite and kaolinite can be determined through the thermodynamic parameters of Gibbs free energy (DG) (kJ/mol), enthalpy (DH) (kJ/mol) and entropy (DS) (kJ/mol-K) at different temperature settings. The equilibrium constant Kc (mg-L/g-mol) is shown in Eq. (10):

Kc ¼

C ads Ce

ð10Þ

where Cads is the amount of BTO adsorbed by the clay mineral adsorbent at equilibrium (mg/L) and Ce is the amount of BTO remaining in the solution at equilibrium (mg/L). The van’t Hoff equation is used widely used to estimate the thermodynamic parameters [36,39]. This equation is used to determine the spontaneity of adsorption expressed in Eq. (11):

DG ¼ RT ln K c

ð11Þ

The relationship of DG, DS and DH is given in Eq. (12).

ln K c ¼

DS DH  R RT

ð12Þ

Fig. 4 illustrates the van’t Hoff linear plot of ln Kc against 1/T is used to compute the values of DH and DS based on its slope and intercept. Results indicate high suitability of the van’t Hoff equation that can determine the respective thermodynamic properties of activated clay, bentonite and kaolinite due to its high correlation of R2 > 0.99. Table 4 lists the observed thermodynamics properties using various clay minerals at 25 to 55 °C. Utilizing activated clay as the adsorbent for BTO adsorption indicated negative values of DG which means the adsorption process is spontaneous and feasible in all temperature settings, while using kaolinite showed positive values of DG which suggests that BTO adsorption is not spontaneous. For bentonite, BTO adsorption is only non-spontaneous at 25 °C but spontaneous at 40 and 55 °C. The decreasing values of DG with increasing temperature settings correspond to the favorability of adsorption at higher temperatures. The lower the negative values in DG imply higher spontaneity and favorability of BTO adsorption onto the adsorbent. Activated clay has the lowest DG values at all temperature settings. This supports the results of achieving higher adsorption capacity of activated clay over bentonite and kaolinite in the kinetic study. For all clay mineral adsor-

158

A.E.S. Choi et al. / Fuel 205 (2017) 153–160

Table 3 Adsorption isotherm model parameters for BTO adsorption onto activated clay, bentonite and kaolinite. Isotherms

Adsorbents

Langmuir

b (mL/mg) qmax (mg/g) R2 kf (mg/g) n (L1) R2 a (L/g) b (J/mol) R2 Xm (mg/g) B (kJ2/mol2) E (kJ/mol) R2

Freundlich

Temkin

ln Kc

Dubinin-Radushkevich

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 0.003

Activated Clay

Bentonite

Kaolinite

0.074 0.172 0.9885 0.134 1.36 0.9987 0.445 1.15 0.7340 2.27 5.1  107 991.9 0.6014

0.024 0.040 0.9895 0.048 1.42 0.9993 0.158 0.68 0.7694 1.44 3.0  106 407.7 0.6139

0.003 0.002 0.9862 0.001 1.05 0.9931 0.039 0.19 0.7190 0.26 4.2  105 108.9 0.5365

Table 5 Comparison of research findings for BTO adsorption using various types of adsorbents.

Actived Clay Bentonite Kaolinite

Type of adsorbent

Adsorption capacity (mg/g)

Equilibrium concentration (mg/L)

Reference

Granulated activated carbon Copper impregnated activated carbon Iron (III) impregnated activated carbon Nickel impregnated activated carbon Aluminum oxide

3.52

324.0

3.34

333.0

3.59

320.5

3.52

324.0

2.79

360.5

Activated clay Bentonite Kaolinite

4.01 2.29 1.73

99.5 271.2 327.0

Chen et al. (2016) Chen et al. (2016) Chen et al. (2016) Chen et al. (2016) Lu et al. (2013) This study This study This study

R² = 0.9919 R² = 0.9956

R² = 0.9906 0.0031

0.0032

0.0033

0.0034

1/T (1/K) Fig. 4. The van’t Hoff plot of BTO adsorption using activated clay, bentonite and kaolinite.

bents, an endothermic adsorption process was observed due to having positive values of DH. Hence, this further supports the findings of higher BTO adsorption capacity at higher temperatures. Positive values of DS imply an increased in the degrees of freedom of the adsorbed BTO onto the clay mineral adsorbents.

backs include the complicated and additional preparative procedure in its synthesis that incurs additional costs. Clay minerals are well-known for being abundant and cheap, which makes it an attractive adsorbent material in the oxidative desulfurization of fuel oil.

3.6. Comparative assessment of research findings 4. Conclusions Clay mineral adsorbents in the present study were compared to conventional adsorbents used in oxidative desulfurization in terms of BTO removal. From Table 5, activated clay has the highest adsorption capacity (4.01 mg/g) and lowest BTO equilibrium concentration (99.5 mg/L) in comparison to granulated activated carbon (3.52 mg/g, 324.0 mg/L), metal impregnated activated carbon (3.34–3.59 mg/g, 320.5–333.0 mg/L) and aluminum oxide (2.79 mg/g, 360.5 mg/L). A disadvantage of using the commercially available adsorbents of activated carbon and aluminum oxide is its expensive cost. For the metal impregnated activated carbon, draw-

In this study, different clay mineral adsorbents were utilized for the batch adsorption of BTO. Activated clay has the highest adsorption capacity and surface area in comparison to bentonite and kaolinite. Various kinetic and equilibrium isotherm models were used to assess the experimental data. The pseudo-second order model best describes the BTO adsorption mechanism in activated clay, bentonite and kaolinite implying the occurrence of chemical adsorption. The Freundlich and Langmuir models appropriately fit the equilibrium experimental data that indicates a heteroge-

Table 4 Thermodynamic parameters at different temperatures for BTO adsorption onto activated clay, bentonite and kaolinite. Adsorbent

Temperature (°C)

DG (kJ/mol)

DS (kJ/mol-K)

DH (kJ/mol)

Activated Clay

25 40 55 25 40 55 25 40 55

3.451 5.510 8.231 0.421 2.742 5.272 3.389 1.722 0.567

0.1586

43.9259

0.1904

57.0739

0.0946

31.5002

Bentonite

Kaolinite

A.E.S. Choi et al. / Fuel 205 (2017) 153–160

neous and monolayer adsorption. Thermodynamic parameters indicated an endothermic adsorption of BTO onto clay mineral adsorbents. Activated clay is spontaneous while kaolinite is not spontaneous in BTO adsorption at all temperature settings. BTO adsorption using bentonite is only not spontaneous at 25 °C but spontaneous at 40 and 55 °C. Activated clay has proven to be a promising low-cost adsorbent and superior over conventional adsorbent in terms of adsorption capacities which are ideal for the oxidative desulfurization process. Acknowledgments The authors would like to acknowledge the Taiwan Military of Science and Technology (MOST 105-2221-E-041-002-MY3 and MOST 104-2221-E-041-002) and the Engineering Research and Development for Technology – Department of Science and Technology (ERDT-DOST), Philippines for the financial support in this research.

[19]

[20]

[21]

[22]

[23]

[24]

[25]

References [1] Jiang B, Yang H, Zhang L, Zhang R, Sun Y, Huang Y. Efficient oxidative desulfurization of diesel fuel using amide-based ionic liquids. Chem Eng J 2016;283:89–96. http://dx.doi.org/10.1016/j.cej.2015.07.070. [2] Sharma VK, Luther GW, Millero FJ. Mechanisms of oxidation of organosulfur compounds by ferrate(VI). Chemosphere 2011;82:1083–9. http://dx.doi.org/ 10.1016/j.chemosphere.2010.12.053. [3] Hampel R, Peters A, Beelen R, Brunekreef B, Cyrys J, de Faire U, et al. Long-term effects of elemental composition of particulate matter on inflammatory blood markers in European cohorts. Environ Int 2015;82:76–84. http://dx.doi.org/ 10.1016/j.envint.2015.05.008. [4] Srivastava VC. An evaluation of desulfurization technologies for sulfur removal from liquid fuels. RSC Adv 2012;2:759. http://dx.doi.org/10.1039/c1ra00309g. [5] EUR-Lex. Sulphur content of certain liquid fuels 2007. http://eur-lex.europa. eu/legal-content/EN/TXT/?uri=URISERV:l21050 (accessed September 16, 2015). [6] EPA. Emission Standards Reference Guide Highway, Nonroad, Locomotive, and Marine Diesel Fuel Sulfur Standards 2012. http://www.epa.gov/ otaq/standards/fuels/diesel-sulfur.htm (accessed August 31, 2015). [7] Chen TC, Shen YH, Lee WJ, Lin CC, Wan MW. The study of ultrasound-assisted oxidative desulfurization process applied to the utilization of pyrolysis oil from waste tires. J Clean Prod 2010;18:1850–8. http://dx.doi.org/10.1016/j. jclepro.2010.07.019. [8] Song C, Ma X. Ultra-deep desulfurization of liquid hydrocarbon fuels: chemistry and process. Int J Green Energy 2004;1:167–91. http://dx.doi.org/ 10.1081/GE-120038751. [9] Xiao J, Li Z, Liu B, Xia Q, Yu M. Adsorption of benzothiophene and dibenzothiophene on ion-impregnated activated carbons and ion-exchanged Y zeolites. Energy Fuels 2008;22:3858–63. http://dx.doi.org/10.1021/ ef800437e. [10] Hasan Z, Jeon J, Jhung SH. Oxidative desulfurization of benzothiophene and thiophene with WOx/ZrO2 catalysts: Effect of calcination temperature of catalysts. J Hazard Mater 2012;205–206:216–21. http://dx.doi.org/10.1016/j. jhazmat.2011.12.059. [11] Etemadi O, Yen TF. Selective adsorption in ultrasound-assisted oxidative desulfurization process for fuel cell reformer applications. Energy Fuels 2007;21:2250–7. http://dx.doi.org/10.1021/ef0700174. [12] Lu M-C, Biel LCC, Wan M-W, de Leon R, Arco S. The oxidative desulfurization of fuels with a transition metal catalyst: a comparative assessment of different mixing techniques. Int J Green Energy 2014;11:833–48. http://dx.doi.org/ 10.1080/15435075.2013.830260. [13] Chen T-C, Shen Y-H, Lee W-J, Lin C-C, Wan M-W. Optimization of thiophene removal by an ultrasound-assisted oxidative desulfurization process. Environ Eng Sci 2012;29:623–9. http://dx.doi.org/10.1089/ees.2011.0123. [14] Mello PDA, Duarte FA, Nunes MAG, Alencar MS, Moreira EM, Korn M, et al. Ultrasound-assisted oxidative process for sulfur removal from petroleum product feedstock. Ultrason Sonochem 2009;16:732–6. http://dx.doi.org/ 10.1016/j.ultsonch.2009.03.002. [15] Dai Y, Qi Y, Zhao D, Zhang H. An oxidative desulfurization method using ultrasound/Fenton’s reagent for obtaining low and/or ultra-low sulfur diesel fuel. Fuel Process Technol 2008;89:927–32. http://dx.doi.org/10.1016/ j.fuproc.2008.03.009. [16] Bolla MK, Choudhury HA, Moholkar VS. Mechanistic features of ultrasoundassisted oxidative desulfurization of liquid fuels. Ind Eng Chem Res 2012;51:9705–12. http://dx.doi.org/10.1021/ie300807a. [17] Wu Z, Ondruschka B. Ultrasound-assisted oxidative desulfurization of liquid fuels and its industrial application. Ultrason Sonochem 2010;17:1027–32. http://dx.doi.org/10.1016/j.ultsonch.2009.11.005. [18] Choi AES, Roces S, Dugos N, Futalan CM, Lin S-S, Wan M-W. Optimization of ultrasound-assisted oxidative desulfurization of model sulfur compounds

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34] [35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

159

using commercial ferrate (VI). J Taiwan Inst Chem Eng 2014;45:2935–42. http://dx.doi.org/10.1016/j.jtice.2014.08.003. Wan M-W, Biel LCC, Lu M-C, de Leon R, Arco S. Ultrasound-assisted oxidative desulfurization (UAOD) using phosphotungstic acid: effect of process parameters on sulfur removal. Desalin Water Treat 2012;47:96–104. http:// dx.doi.org/10.1080/19443994.2012.696802. Mei H, Mei BW, Yen TF. A new method for obtaining ultra-low sulfur diesel fuel via ultrasound assisted oxidative desulfurization. Fuel 2003;82:405–14. http://dx.doi.org/10.1016/S0016-2361(02)00318-6. Choi AES, Roces S, Dugos N, Wan MW. Operating cost study through a Paretooptimal fuzzy analysis using commercial ferrate (VI) in an ultrasound-assisted oxidative desulfurization of model sulfur compounds. Clean Technol Environ Policy 2016;18:1433–41. http://dx.doi.org/10.1007/s10098-015-1079-6. Choi AES, Roces S, Dugos N, Futalan CM, Wan M-W. Optimization analysis of mixing-assisted oxidative desulfurization of model sulfur compounds using commercial ferrate(VI). Desalin Water Treat 2015;3994:1–8. http://dx.doi.org/ 10.1080/19443994.2015.1088475. Choi AES, Roces S, Dugos N, Wan M-W. Mixing-assisted oxidative desulfurization of model sulfur compounds using polyoxometalate/H2O2 catalytic system. Sustain Environ Res 2016;26:184–90. http://dx.doi.org/ 10.1016/j.serj.2015.11.005. Fox BR, Brinich BL, Male JL, Hubbard RL, Siddiqui MN, Saleh TA, et al. Enhanced oxidative desulfurization in a film-shear reactor. Fuel 2015;156:142–7. http:// dx.doi.org/10.1016/j.fuel.2015.04.028. Song C, Ma X. New design approaches to ultra-clean diesel fuels by deep desulfurization and deep dearomatization. Appl Catal B Environ 2003;41:207–38. http://dx.doi.org/10.1016/S0926-3373(02)00212-6. Liu S, Wang B, Cui B, Sun L. Deep desulfurization of diesel oil oxidized by Fe (VI) systems. Fuel 2008;87:422–8. http://dx.doi.org/10.1016/ j.fuel.2007.05.029. Campos-Martin JM, Capel-Sanchez MC, Perez-Presas P, Fierro JLG. Oxidative processes of desulfurization of liquid fuels. J Chem Technol Biotechnol 2010;85:879–90. http://dx.doi.org/10.1002/jctb.2371. Etemadi O, Yen TF. Aspects of selective adsorption among oxidized sulfur compounds in fossil fuels. Energy Fuels 2007;21:1622–7. http://dx.doi.org/ 10.1021/ef070016b. Lu M-C, Biel LCC, Wan M-W, de Leon R, Arco S, Futalan CM. Adsorption of dibenzothiophene sulfone from fuel using chitosan-coated bentonite (CCB) as biosorbent. Desalin Water Treat 2015:1–11. http://dx.doi.org/10.1080/ 19443994.2014.996773. Lu M-C, Agripa ML, Wan M-W, Dalida MLP. Removal of oxidized sulfur compounds using different types of activated carbon, aluminum oxide, and chitosan-coated bentonite. Desalin Water Treat 2013;52:873–9. http://dx.doi. org/10.1080/19443994.2013.826330. Ogunlaja AS, Coombes MJ, Torto N, Tshentu ZR. The adsorptive extraction of oxidized sulfur-containing compounds from fuels by using molecularly imprinted chitosan materials. React Funct Polym 2014;81:61–76. http://dx. doi.org/10.1016/j.reactfunctpolym.2014.04.006. Chen T, Agripa ML, Lu M, Dalida MLP. Adsorption of sulfur compounds from fiesel with ion-impregnated activated carbons. Energy & Fuels (ACS) 2016:3870. http://dx.doi.org/10.1021/acs.energyfuels.6b00230. Etemadi O, Yen TF. Surface characterization of adsorbents in ultrasoundassisted oxidative desulfurization process of fossil fuels. J Colloid Interface Sci 2007;313:18–25. http://dx.doi.org/10.1016/j.jcis.2007.04.033. Futalan CM, Kan C, Dalida MLP. Nickel removal from aqueous solution in fixed bed using chitosan-coated bentonite. Sustain Environ Res 2011;21:361–7. Futalan CM, Kan CC, Dalida ML, Pascua C, Wan MW. Fixed-bed column studies on the removal of copper using chitosan immobilized on bentonite. Carbohydr Polym 2011;83:697–704. http://dx.doi.org/10.1016/j.carbpol.2010.08.043. Futalan CM, Kan CC, Dalida ML, Hsien KJ, Pascua C, Wan MW. Comparative and competitive adsorption of copper, lead, and nickel using chitosan immobilized on bentonite. Carbohydr Polym 2011;83:528–36. http://dx.doi.org/10.1016/ j.carbpol.2010.08.013. Chen I, Kan C, Futalan CM, MJ C, Lin S, Tsai WC, et al. Batch and fixed bed studies: Removal of copper (II) using chitosan-coated kaolinite beads from aqueous solution Sustain. Environ Res 2015;25:73–81. Arida CVJ, de Luna MDG, Futalan CM, Wan M-W. Optimization of As(V) removal using chitosan-coated bentonite from groundwater using BoxBehnken Design: Effects of adsorbent mass, flow rate and initial concentration. Desalin Water Treat 2016;57:18739–47. Calagui MJC, Senoro DB, Kan CC, Salvacion JWL, Futalan CM, Wan MW. Adsorption of indium(III) ions from aqueous solution using chitosan-coated bentonite beads. J Hazard Mater 2014;277:120–6. http://dx.doi.org/10.1016/j. jhazmat.2014.04.043. Senthil Kumar P, Ramalingam S, Senthamarai C, Niranjanaa M, Vijayalakshmi P, Sivanesan S. Adsorption of dye from aqueous solution by cashew nut shell: Studies on equilibrium isotherm, kinetics and thermodynamics of interactions. Desalination 2010;261:52–60. http://dx.doi.org/10.1016/j.desal.2010.05.032. de Luna MDG, Flores ED, Genuino DAD, Futalan CM, Wan MW. Adsorption of Eriochrome Black T (EBT) dye using activated carbon prepared from waste rice hulls-Optimization, isotherm and kinetic studies. J Taiwan Inst Chem Eng 2013;44:646–53. http://dx.doi.org/10.1016/j.jtice.2013.01.010. Xiao H, Peng H, Deng S, Yang X, Zhang Y, Li Y. Preparation of activated carbon from edible fungi residue by microwave assisted K2CO3 activation-Application in reactive black 5 adsorption from aqueous solution. Bioresour Technol 2012;111:127–33. http://dx.doi.org/10.1016/j.biortech.2012.02.054.

160

A.E.S. Choi et al. / Fuel 205 (2017) 153–160

[43] Deng H, Yang L, Tao G, Dai J. Preparation and characterization of activated carbon from cotton stalk by microwave assisted chemical activationApplication in methylene blue adsorption from aqueous solution. J Hazard Mater 2009;166:1514–21. http://dx.doi.org/10.1016/j.jhazmat.2008.12.080. [44] Hameed BH, Tan IAW, Ahmad AL. Adsorption isotherm, kinetic modeling and mechanism of 2,4,6-trichlorophenol on coconut husk-based activated carbon. Chem Eng J 2008;144:235–44. http://dx.doi.org/10.1016/j.cej.2008.01.028. [45] Vijaya Y, Popuri SR, Boddu VM. Krishnaiah a. Modified chitosan and calcium alginate biopolymer sorbents for removal of nickel (II) through adsorption. Carbohydr Polym 2008;72:261–71. http://dx.doi.org/10.1016/ j.carbpol.2007.08.010. [46] Chou WL, Wang CT, Huang KY, Chang YC, Shu CM. Investigation of indium ions removal from aqueous solutions using spent coffee grounds. Int J Phys Sci 2012;7:2445–54.

[48] Taffarel SR, Rubio J. Removal of Mn2+ from aqueous solution by manganese oxide coated zeolite. Miner Eng 2010;23:1131–8. http://dx.doi.org/10.1016/j. mineng.2010.07.007. [49] Chou W-L, Wang C-T, Chang W-C, Chang S-Y. Adsorption treatment of oxide chemical mechanical polishing wastewater from a semiconductor manufacturing plant by electrocoagulation. J Hazard Mater 2010;180:217–24. http://dx.doi.org/10.1016/j.jhazmat.2010.04.017. [50] Kamari A, Wan Ngah WS. Isotherm, kinetic and thermodynamic studies of lead and copper uptake by H2SO4 modified chitosan. Colloids Surf B Biointerfaces 2009;73:257–66. http://dx.doi.org/10.1016/j.colsurfb.2009.05.024. [51] Allen SJ, Mckay G, Porter JF. Adsorption isotherm models for basic dye adsorption by peat in single and binary component systems. J Colloid Interface Sci 2004;280:322–33. http://dx.doi.org/10.1016/j.jcis.2004.08.078. [52] Al-anber MA. Thermodynamics Approach in the Adsorption of Heavy Metals. Thermodyn – Interact Stud – Solids, Liq Gases; 2011.