Adsorption of Sulfur Compounds from Diesel with Ion-Impregnated

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Adsorption of Sulfur Compounds from Diesel with Ion-Impregnated Activated Carbons Teng-Chien Chen,† Michelle L. Agripa,‡ Ming-Chun Lu,*,§ and Maria Lourdes P. Dalida‡ †

Metal Industries Research & Development Centre, Kaohsiung 811, Taiwan Department of Chemical Engineering, University of the Philippines, Quezon City, 1101, Philippines § Department of Environmental Resources Management, Chia Nan University of Pharmacy and Science, Tainan 717, Taiwan ‡

ABSTRACT: The adsorption of benzothiophene sulfone (BTO) and dibenzothiophene sulfone (DBTO) from model diesel fuel were evaluated using granular activated carbon (GAC) and modified activated carbon. Modified activated carbon was impregnated with metal ions, Cu2+, Fe3+, and Ni2+, and the effect on its adsorption characteristics was determined. Adsorption performance was investigated using batch process at room temperature and agitation speed of 120 rpm. Results indicated a significant increase in the adsorption capacity compared to that of the unmodified GAC. Increasing adsorption capacity followed the order Cu2+/AC < Ni2+/AC < Fe3+/AC for BTO, whereas for DBTO, increasing order is Ni2+/AC < Fe3+/AC < Cu2+/AC. Evaluation of equilibrium isotherms using Langmuir and Freundlich models yielded a good fit with values of R2 > 0.90. Adsorption process seemed to proceed in a two-step process. First, a very fast rate for the first few minutes, then a markedly reduced rate until equilibrium is reached. All the adsorbents used have several functional groups present on its surface, and it is the nature of these surface groups that determine the character of these adsorbents. the sulfur compounds from the hydrocarbons.4 Several oxidants have been used in oxidative desulfurization, namely, hydrogen peroxide, organic peroxides, nitric acid, ozone, molecular oxygen, and potassium superoxide. Among these oxidants, hydrogen peroxide has been extensively used because of its highly reactive nature and because it produces only water as byproduct.5 Separation by adsorption is based on three distinct mechanisms: steric, equilibrium, and kinetic. The steric mechanism is based on the principle that porous solid has pores that allow small molecules to enter while excluding large molecules from entering. In the equilibrium mechanism, solids have different abilities to accommodate different species that the stronger adsorbing species is preferentially removed by the solid. The kinetic mechanism is equilibria-based on different rates of diffusion of different species into the pore; therefore, the time of exposure can be controlled so that the faster diffusing species can be removed first by the solid.6 Adsorptive desulfurization utilizes an active adsorbent, which is a porous, nonreactive substrate that has high surface area for the adsorption of sulfur compounds. The sulfur molecules are adsorbed by attaching to the adsorbent, separating itself from the fuel. Adsorptive desulfurization faces the challenge of developing adsorbents that have high adsorption capacity, and high selectivity for the refractory aromatic sulfur compounds that are not removed during the hydrodesulfurization process.7 On the basis of previous studies, many adsorbents have been invented and applied to different purposes of contaminations removal. Commercial adsorbents activated carbon (AC) and aluminum oxide (ALU) have been known as effective

1. INTRODUCTION The production of hydrocarbon fuels with very low sulfur content is one of the challenges faced by the petroleum industry. Recently, the U.S. Environmental Protection Agency has set the limits of sulfur content in diesel fuel to only 15 mg/ kg and 30 mg/kg for gasoline fuels.1 This challenge is due to not only stringent environmental regulations being imposed on transportation fuels but also the great importance it plays for fuel cell applications.2 Hydrodesulfurization is the major refining process that removes the unwanted sulfur from hydrocarbon fuels. As such, it plays a crucial part in the petroleum industry. Hydrodesulfurization has been efficient in removing thiols, sulfides, and disulfides, but it has been less effective for benzothiophene, dibenzothiophene, and their alkylated derivatives.3 A disadvantage for this process is that it operates at high temperatures (around 300 °C) and high pressures (20−100 H2 atm) and utilizes Co−Mo/Al2O3 or Ni−Mo/Al2O3 catalyst. Hightemperature and -pressure operating conditions always entail high operating costs. The use of noble catalysts further increases the high expenses of the refinery. Studies are therefore being conducted on other treatment technologies that would result in lower cost but with the same or better removal efficiencies. Other desulfurization techniques, such as oxidative desulfurization (ODS) and adsorptive desulfurization, are being investigated to be able to produce ultraclean fuel. In ODS, oxidized sulfur compounds such as benzothiophene sulfone (BTO) and dibenzothiophene sulfone (DBTO) are produced from benzothiophene (BT) and dibenzothiophene (DBT), and these can be processed easily downstream. Oxidized sulfur compounds are slightly more polar than hydrocarbons of similar structure. Using a combined process of selective oxidation and solid adsorption allows the selective removal of © XXXX American Chemical Society

Received: January 30, 2016 Revised: March 31, 2016

A

DOI: 10.1021/acs.energyfuels.6b00230 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels adsorbents for many applications.8 Chitosan-coated bentonite (CHB) has been tested in removing heavy metals from aqueous solutions.9,10 Metal-ion-impregnated activated carbons have been used in removing BT and DBT.11 Activated carbon is a microporous adsorbent that can be produced from different carboneous materials, including wood, coconut shells, sugar, coal, and lignin. Its exceptional adsorption properties result from its micropores, high surface area, and wide range of surface functional groups. The structure of activated carbon comprises carbon atoms ordered in parallel stacks of hexagonal layers extensively cross-linked and tetrahedrally bonded.12 Heteroatoms are bound to the surface of the adsorbent, and it assumes the character of the functional groups usually found in aromatic compounds. These surface groups play an important role in the surface chemistry of the activated carbon, and they play a key role in adsorption in aqueous solutions and the catalytic properties of the carbon. The function groups on the surface of activated carbon were acidic and basic in nature. Figure 1a shows the acidic surface groups on activated carbon. Figure 1b shows the basic surface function groups presenting onto activated carbon.

Activated carbon is an amorphous form of carbon that has highly developed internal pore structure and a large surface area. It has industrial applications in decolorization, purification, sugar refining, and other food industries. It also has uses in pollution control and wastewater treatment.15 Aluminum oxide has been utilized in removing organic compounds from aqueous solutions. Other researchers have used it for the adsorptive removal of phosphate and metal ions. In this study, novel adsorbents were synthesized, and ion impregnation of the granular activated carbon was done to improve its adsorption efficiency. Metal ions used were copper(II) ion, iron(III) ion, and nickel(II) ion. The names used for the adsorbents were Cu2+/AC for the copper(II) ionimpregnated activated carbon, Fe3+/AC for the iron(III)-ion impregnated activated carbon, and Ni2+/AC for the nickel(II) ion-impregnated activated carbon. The effect of impregnating metal ions onto the activated carbon on its adsorption performance was explored. Benzothiophene sulfone and dibenzothiophene sulfone were used to simulate the oxidation products from benzothiophene and dibenzothiophene in the oxidative desulfurization process. Adsorption batch studies were conducted to remove benzothiophene sulfone and dibenzothiophene sulfone from model diesel fuel for all mentioned adsorbents. Langmuir and Freundlich isotherm models were employed to interpret the data. Kinetic studies were also conducted to determine how fast the rate of adsorption was, and finally a possible adsorption mechanism was proposed.

2. MATERIALS AND METHODS 2.1. Chemicals and Reagents. Benzothiophene sulfone (98%) and dibenzothiophe sulfone (98%) were obtained from Sigma Alfa Aesar. Dibenzothiophene (98%) and benzothiophene (99.9%) were supplied by Sigma Alfa Aesar. Toluene (98%) was purchased from Merck. Hydrogen peroxide was obtained from Pancreac (50%). Phosphotungstic acid (98%) and tetraoctalynammonium bromide (98%) were purchased from Sigma-Aldrich. For the metal nitrate solutions, analyticalgrade cupric nitrate 2.5 hydrate (Cu(NO3)2·2.5(H2O), 98%) was purchased from J. T. Baker, ferric nitrate nonahydrate (Fe(NO3)3·9(H2O), 98%) and nickel(II) nitrate hexahydrate (Ni(NO3)2·6(H2O), 98%) were analytical reagents from Merck. 2.2. Synthesis of the Ion-Impregnated Activated Carbons. Preparation of the ion-impregnated activated carbons followed a previously published work by Xiao and colleagues.11 About 10 g of granular activated carbon (40−60 mesh) was weighed and placed in each of three 250 mL Erlenmeyer flasks. A 100 mL aliquot of the previously prepared solutions of the metal nitrates, 0.1 M Cu(NO3)2, 0.1 M Fe(NO3)3, 0.1 M Ni(NO3)2 was added, and the flasks were covered with parafilm to prevent evaporation of the solution. The flasks containing the carbon and metal solutions were then placed in a temperature controlled shaker and bath and agitated at a speed of 120 rpm and constant temperature of 25 °C. The set up was left for 12 h with continuous shaking to facilitate the reaction between the activated carbon and the metal nitrates. After 12 h, the samples were then filtered using No. 40 Whatman filter paper. The treated activated carbons were then placed in a glass Petri dish and dried in an oven at 393 K for 5 h to remove the moisture clinging to the carbon. Afterward, the dried ion-impregnated activated carbon samples were then transferred to the SATA Moisture Buster to cool and let its

Figure 1. (a) Simple schematic of acidic surface function groups on activated carbon. (b) Schematic of possible basic groups in activated carbon.

Activated carbon can be modified with the intention on increasing its performance as an adsorbent for specific type of substance. Selvavathi and his colleagues13 modified activated carbon with nitric acid followed by argon treatment to alter the surface functional groups and surface area. Their results showed better adsorption capacity of refractory sulfur compounds for the modified activated carbons compared with that of asreceived carbon. Wang and fellow researchers14 modified activated carbon by loading it with zinc oxide. Prepared adsorbent was then tested in adsorptive desulfurization of bioethanol that contained dimethylsulfide (DMS) as sulfur impurity. Results indicated that the amount of DMS adsorbed was doubled compared to that of the unmodified activated carbon. B


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where Co and Ce are the initial and equilibrium concentrations, respectively. 2.5.2. Isotherm Models. The Langmuir and Freundlich isotherm models were used to describe the adsorption equilibrium. From these models, adsorbents for adsorption performance were evaluated. All adsorbents for adsorption capacity were calculated, and equation constants that also gave indications of adsorption performance were also determined. The linear form of the Langmuir equation used was

moisture stabilize. The samples were then placed in separate clean plastic bottles prior to use. 2.3. Experimental Methods. 2.3.1. Preparation of the Model Diesel Fuel Benzothiophe Sulfone. A small amount of benzothiophene, 2.093 g, was weighed and dissolved in 1 L of toluene in a volumetric flask to make a 500 mg/L BT solution. An aliquot of 20 mL was measured, to which 0.15 g of phosphotungstic acid and 0.1 g of tetraoctylammonium bromide was added. The mixture was then placed under ultrasound radiation in a sonicator for 15 min. The mixture was centrifuged to separate the sulfone from the oxidant. 2.3.2. Adsorption Process. Approximately 1.0 g of adsorbent was placed in stoppered 250 mL Erlenmeyer flasks. A 10 mL aliquot of the model diesel fuel (benzothiophene sulfone or dibenzothiophene sulfone) was added to each flask. The mixtures were then placed in a temperature-controlled shaker and were agitated at 120 rpm at a constant temperature of 25 °C. The flasks were then stoppered to prevent evaporation of the toluene solvent. The solutions and adsorbents were shaken for predetermined contact time ranging from 5 min to 4 h (5, 10, 15, 30, 45, 60, 90, 120, 280, and 240 min). After the desired contact time, the solution was then filtered using 0.2 μm GHP membrane. The remaining sulfur content was analyzed using the gas chromatograph−mass spectrometer. 2.3.3. Equilibrium Process. About 1 g of adsorbent was placed in Erlenmeyer flasks, and 20 mL of the model fuel was added. The flasks were then stoppered to prevent evaporation of the solvent. A contact time of 48 h between solution and adsorbent ensured that equilibrium was reached. Different initial concentrations were used in the range of 100−1000 ppm of the model fuel. After the required contact time, solution was then filtered with 0.2 μm GHP membrane. Analysis of the remaining sulfur was performed using the GC-MS. 2.4. Analytical Method. The concentrations of benzothiophene sulfone and dibenzothiophene sulfone before and after adsorption were analyzed using the Agilent 7890A gas chromatograph equipped with a fused-silica capillary HP-5 ms column (30 m) having a thickness of 0.25 mm film (J & W Scientific, Folsom, CA USA). The GC was connected to a sulfur chemiluminescence detector for higher selectivity and sensitivity toward ultralow sulfur concentration. The GC temperature was initially set to 100 °C for 3 min and ramped to 300 °C at increasing rate of 20 °C/min. An Agilent 5975C mass spectrometer was used. A calibration curve was established to give the relationship between the concentration of the sulfur compounds and the corresponding area given by the equipment for specific concentrations. Values that were below the detection limits were considered as zero concentration to simplify calculations. 2.5. Mathematical Methods. 2.5.1. Adsorption Equilibrium Studies. The amount of material adsorbed at equilibrium, q, was computed using the equation q=

(Co − Ce)V M

1 b 1 = + qe qmax qmax Ce

where qe is the amount adsorbed at equilibrium (mg/g), Ce is the amount of the adsorbate in solutionat equilibrium (mg/L), qmax is the maximum monolayer capacity of the adsorbent (mg/ g), and b is the affinity of the adsorbates to the binding site (L/ mg). A plot of 1/qe versus 1/Ce was used to determine the Langmuir constants. The Freundlich equation used the linear form: log qe = log k f +

1 log Ce n

(4)

where qe (mg/g) is the amount of sulfur compound adsorbed at equilibrium, Ce (mg/L) is the remaining concentration of the solution at equilibrium, kf is an indicator of the adsorption capacity, and n is related to the magnitude of the adsorption driving force and to the distribution of the energy sites on the adsorbent Plotting log qe versus log Ce would enable the calculation of the isotherm constants. 2.5.3. Kinetic Studies. Kinetic studies were performed to determine the rate of adsorption and have an idea on the mechanism of adsorption. Calculated rates would indicate whether the adsorption was fast or slow. The higher the calculated rate constant, the higher the expected adsorption. The Lagergren’s model that was used is as follows: pseudo-first-order ln(qe − q) = ln qe − k1t

(5)

pseudo-second-order 1 1 t = + t 2 q qe k 2qe

(6)

A plot of ln (qe − q) versus t was used to determine the rate constant and theoretical qe for the pseudo-first-order. For the pseudo-second-order, t/q versus t was plotted to get the needed constants.

3. RESULTS AND DISCUSSION 3.1. Adsorption of Benzothiophene Sulfone and Dibenzothiophene Sulfone with Ion-Impregnated Activated Carbon. A modification to the granular activated carbon (GAC) was performed with the intention of increasing its capacity in adsorbing benzothiophene sulfone and dibenzothiophene sulfone. Xiao and colleagues11 impregnated activated carbon with metal ions to increase dibenzothiophene (DBT) adsorption. Their results gave an increase in adsorbing DBT after impregnation as compared with that of the unmodified carbon (GAC). Following the same impregnation method, the modified carbons were tested in removing BTO and DBTO from the model diesel. Batch adsorption processes were conducted at

(1)

where Co, Ce, V, and M represent the initial concentration (mg/ L), concentration at equilibrium (mg/L), volume of the solution used (L), and mass of adsorbent used (g), respectively. The percentage removal of adsorbate is computed as follows: ⎛ C − Ce ⎞ % removal = ⎜ o ⎟ × 100 ⎝ Co ⎠

(3)

(2) C


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Energy & Fuels agitation speed of 120 rpm at 25 °C. Contact time was set at 48 h. Table 1 lists the content of metal elements on the activated carbons prepared separately by impregnation with solutions of

versus 1/Ce show how the data fit to the Langmuir model. The results show a relatively good fit to the model, which means that the adsorption process occurs on a monolayer surface and that adsorbed particles do not have any interaction with each other. Comparing the performance of unmodified GAC and the ion-impregnated carbons from Table 4, the modification gave a large effect to the capacity of the GAC. Original BTO uptake of unmodified GAC was only 15.7 mg/g, but upon ionimpregnation, rose to 39.8 mg/g for Cu2+/AC, 63.7 mg/g for Fe3+/AC, and 50.1 mg/g for Ni2+/AC. BTO uptake increased more than 100%. Increasing qe values are of the order Cu2+/AC < Ni2+/AC < Fe3+/AC. For the values of the binding affinity, b decreased after the ion-impregnation of the GAC. Figure 2b gives the Langmuir fit for DBTO adsorption using GAC and the ion-impregnated carbons, Cu2+/AC, Fe3+/AC, and Ni2+/AC. From the graph, it can be seen that after ion impregnation, DBTO uptake of the activated carbon increased. Values of qe in Table 5 were higher for the modified GAC. Increasing qe follows the order Ni2+/AC < Fe2+/AC < Cu2+/ AC. As with BTO, the values for the binding affinity, n, also decreased. 3.3. Freundlich Isotherm Model. Figure 3a gives the Freundlich fit for BTO adsorption using GAC and the ionimpregnated carbons, Cu2+/AC, Fe3+/AC, and Ni2+/AC. Figure 3b shows the plot of log qe versus log Ce, a fit for the Freundlich isotherm model, for DBTO adsorption. Adsorbents used were GAC, Cu2+/AC, Fe3+/AC, and Ni2+/AC. The Langmuir model better fits the experimental data, but the Freundlich constants were calculated nonetheless and are shown in Table 3. Figure 3b shows the experimental data fit of DBTO adsorption onto unmodified and modified activated carbon to the Freundlich model. The adsorption capacities or BTO/DBTO uptake reported have been milligrams adsorbate per gram of adsorbent. It can also be expressed in terms of unit surface area. Table 4 shows the comparison of the capacities of GAC and the ionimpregnated activated carbons. Surface areas of Cu2+/AC, Fe3+/AC, and Ni2+/AC are taken from literature.11 It is interesting to note that in terms of unit surface area the

Table 1. Content of the Metal Element Loaded on the ACs sorbent

metal ion impregnated

content of metal element loaded (mmol/g)

surface area (m2)

GAC Cu2+/AC Ni2+/AC Fe3+/AC

Cu2+ Ni2+ Fe3+

0.49 0.48 0.50

1100 1110 1077 1145

nitrate salts. It can be seen that the metal elements, Cu2+, Ni2+, and Fe3+, had been loaded onto the activated carbons, and each of the activated carbons has a close content value for the corresponding element components. Table 2 shows the raw data gathered from the equilibrium studies of BTO and DBTO adsorption using the ionimpregnated carbon. Included also are the data for GAC for comparison. The isotherm constants for the adsorption of the oxidized sulfur compounds using GAC and the ion-impregnated activated carbon are shown in Table 3. The values for the separation factor, RL, are also shown in the same table. The range for RL gave values less than 1, which indicated favorable adsorption of BTO and DBTO on the ion-impregnated activated carbon. This was confirmed with the values of the Freundlich constant n being larger than 1, except for Ni2+/AC. Looking at the values of R2, higher values for the Langmuir isotherm than that for the Freundlich isotherm indicated that the adsorption process was better described using the Langmuir model, meaning adsorption occurred through a homogeneous monolayer on the surface of the adsorbents. 3.2. Langmuir Isotherm Model. Figure 2a shows data fit to the Langmuir model of BTO adsorption. Adsorbents used were unmodified GAC and the ion impregnated-activated carbons, Cu2+/AC, Fe3+/AC, and Ni2+/AC. These plots of 1/qe

Table 2. BTO and DBTO Adsorption Equilibrium Data Comparison for GAC and the Ion-Impregnated ACs Cu2+/AC

GAC

BTO

DBTO

Ce (mg/L)

qe (mg/g)

4.30 9.06 19.0 37.1 59.5 65.9 55.6 73.6

1.90 3.80 5.61 7.24 8.78 10.7 12.9 18.5

5.16 7.33 16.2 21.0 26.7 29.2 37.8 46.0 48.2

3.90 5.84 7.66 9.56 11.5 13.4 15.2 17.1 19.0

Fe3+/AC

Ce (mg/L)

qe (mg/g)

10.9 19. 0 30.5 59.9 59.5 37.3 32.4 30.5 56.9

2.00 3.78 5.61 7.37 8.80 10.8 13.2 15.3 17.4 18.8

8.98 11.9 10.0 20.9 26.4 19.2 20.7 31.4

5.81 7.76 9.80 11.5 13.5 15.6 17.6 19.4 D

Ni2+/AC

Ce (mg/L)

qe (mg/g)

Ce (mg/L)

qe (mg/g)

10.3 17.1 29. 5 74.5 27.5 27.5 28.9 28.0 42.6 5.29 6.23 8.80 9.71 16.9 18.1 19.7 22.5 27.3

2.00 3.79 5.65 7.39 8.48 11.4 13.4 15.4 17.4 19.1 3.89 5.86 7.81 9.81 11.7 13.6 15.6 17.5 19.4

4.80 11.6 22.1 30.6 73.1 23.3 29.3 30.5 25.1 49.5 5.29 6.23 8.80 9.71 16.9 18.1 19.7 22.5 27.3

1.90 3.75 5.54 7.37 8.51 11.5 13.4 15.4 17.4 19.0 3.90 5.88 7.81 9.81 11.7 13.6 15.6 17.5 19.4


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Energy & Fuels Table 3. Calculated Isotherm Constants for BTO and DBTO Adsorption Using the Ion-Impregnated AC Langmuir model sulfur compound

BTO

DBTO

Freundlich model

separation factor

adsorbent

b (L/mg)

qe,max (mg/g)

R2

kf

n

R2

RL

GAC Cu2+/AC Fe3+/AC Ni2+/AC GAC Cu2+/AC Fe3+/AC Ni2+/AC

0.03 0.01 0.01 0.01 0.04 0.01 0.02 0.03

15.7 39.8 63.7 50.1 24.2 69.9 51.3 35.2

0.98 0.75 0.66 0.90 0.97 0.74 0.97 0.90

0.80 0.96 1.51 4.05 1.36 1.25 1.08 0.50

1.55 1.49 1.76 1.28 1.51 1.27 1.13 0.97

0.91 0.41 0.22 0.54 0.98 0.72 0.95 0.80

0.03 0.09−0.50 0.13−0.60 0.10−0.55 0.03−0.21 0.08−0.46 0.04−0.32 0.03−0.27

Table 4. Comparison of BTO and DBTO Adsorption Capacities for GAC and Ion-Impregnated ACs in Terms of Mass and Surface Area sulfur compound

BTO

DBTO

adsorbent

adsorption capacity (mg/g)

surface area (m2)

adsorption capacity (mg/m2)


15.7 39.8 63.7 50.1 24.2 69.9 51.3 35.2

1100 1110 1077 1145 1100 1110 1077 1145

0.01 0.04 0.06 0.04 0.02 0.06 0.05 0.03

adsorption capacities of the ion-impregnated activated carbon are all higher than the adsorption capacity of the unmodified GAC. This supports the result that impregnating activated carbon with metal ions increases its BTO and DBTO uptake or removal. 3.4. Kinetic Studies. The Lagergren model is one of the most typical models used to describe kinetics. It fits data to pseudo-first-order rate or pseudo-second-order rate models.16 The results can give an indication of what type of mechanism occurs in the process. Kinetic studies were performed where samples of BTO and DBTO were allowed to come in contact with the adsorbent at specific times. Equilibrium was almost reached after 1 h, but equilibrium time was taken to be at maximum contact time of 4 h. Table 5 shows the comparison of all kinetic model constants calculated from the Lagergren model equations and experimental and calculated values of qe. Comparing the values for the correlation factor (R2), it is seen that the data gave a good fit to the pseudo-second-order rate model. R2 values were 0.99. For the pseudo-first-order fit, values for R2 only gave 0.17−0.85. This showed that BTO and DBTO adsorption on the ionimpregnated carbons were also better described by the pseudosecond-order kinetic model. This was confirmed by the calculated values of the equilibrium concentration, qe. The columns for qe,exptl and qe,cal for the pseudo-second-order model gave figures that did not differ much, as compared to the qe,cal values of the pseudo-first-order rate model where qe,exptl is the experimental value and qe,cal is the calculation value. Decreasing qe was of the order Fe3+/AC > Ni2+/AC > Cu2+/AC for benzothiophene sulfone (BTO). This order was confirmed by the experimental values of qe. For dibenzothiophene sulfone, decreasing qe,cal values were in the order Ni2+/AC > Fe3+/AC > Cu2+/AC. Experimental values of qe validated this order. The rate constants, k2, give an idea on how fast the adsorption of the molecules on the surface of the adsorbent is

Figure 2. Langmuir model fit for (a) BTO and (b) DBTO adsorption using GAC, Cu2+/AC, Fe3+/AC, and Ni2+/AC.

occurring. Among the three adsorbents, Cu2+/AC had the highest rate for adsorbing BTO, with a value of 0.0899, followed by Ni2+/AC with 0.0538, then Fe3+/AC with k2 of 0.0475. In adsorbing DBTO, Cu2+/AC had the fastest rate with 12.00, next is Fe3+/AC with k2 of 4.99, and last is Ni2+/AC with k2 of 0.47. Figure 4a,b gives the pseudo-first-order fit of BTO and DBTO adsorption on the modified carbons, Cu2+/AC, Fe3+/ AC, and Ni2+/AC, and also that of unmodified GAC. As the graphs indicated, the data on BTO and DBTO adsorption on Cu2+/AC, Fe3+/AC, and Ni2+/AC did not give a good correlation with the pseudo-first-order model. Table 5 shows E


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Table 5. Comparison of Pseudo-First-Order and Pseudo-Second-Order Kinetic Constants for BTO and DBTO Adsorption for the Ion-Impregnated ACsa pseudo-first-order constants sulfur compound

BTO

DBTO a

pseudo-second-order constants

adsorbent

qe,exptl (mg/g)

k1

qe,cal (mg/g)

R2

k2

qe,cal (mg/g)

R2


3.52 3.34 3.59 3.52 3.50 4.90 4.92 4.94

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

0.65 0.49 0.73 0.73 0.68 0.03 0.02 0.10

0.66 0.47 0.66 0.23 0.42 0.17 0.19 0.85

0.06 0.09 0.05 0.05 0.05 12.00 4.99 0.47

3.49 3.34 3.59 3.39 3.48 4.88 4.92 4.94

0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99

Exptl: experimental. Cal: calculation.

Figure 3. Freundlich model fit for (a) BTO and (b) DBTO adsorption using GAC, Cu2+/AC, Fe3+/AC, and Ni2+/AC.

Figure 4. Pseudo-first-order kinetics of (a) BTO and (b) DBTO on GAC, Cu2+/AC, Fe3+/AC, and Ni2+/AC.

that the pseudo-second-order rate gave high R2 values for the adsorption of the sulfur compounds onto the ion-impregnated carbons. This the implies that this model describes the kinetics better than the other model. Figure 5a,b shows the individual t/q versus t plots for the pseudo-second-order fits of BTO, DBTO adsorption onto GAC, Cu2+/AC, Fe3+/AC, and Ni2+/AC. With the plots almost overlapping, it can be said that the GAC and the modified activated carbon did not very much in adsorption performance. The graph shows how the ion-impregnated activated carbon improved with regards to DBTO uptake. But in comparing the

performances of the three modified carbons, overlapping plots mean their DBTO uptakes do not vary significantly. Figure 6 gives a plot of the fraction of BTO or DBTO remaining in the solution after the desired contact time. Comparing the two graphs, the ion-impregnated activated carbon showed better adsorption of DBTO than BTO. 3.5. Comparison Adsorption Efficiency between Activated Carbon and Modified Activated Carbon. Table 6 summarizes the efficiency of GAC and the ionimpregnated GAC in removing BTO and DBTO from F


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Figure 5. Pseudo-second-order kinetics of (a) BTO and (b) DBTO on GAC, Cu2+/AC, Fe3+/AC, and Ni2+/AC.

Figure 6. Fraction of (a) BTO and (b) DBTO remaining versus contact time for unmodified GAC and ion-impregnated activated carbon.

synthetic fuel. As was indicated by the previous graphs, the modified activated carbon adsorbed DBTO better, with up to 99% removal. For BTO removal, the impregnation method did not improve the adsorption capacity at all. As a summary, the graphs are plotted to compare the performance of all adsorbents in terms of its efficiency in removing the oxidized sulfur compounds BTO and DBTO. It is of interest to note that the adsorption process seems to proceed in a two-step process, as shown in Figure 6a,b. At first, there is a sharp drop in the remaining concentration in the first 10 min of the adsorption, and then the process continues in a markedly slowed rate until equilibrium is reached. This applied to all adsorbents and for both BTO and DBTO removal and may be due to the mechanism involved. Having large values of correlation factor (R2) for both Langmuir and Freundlich, the models may indicate the presence of both chemisorption and physisorption as mechanisms of adsorption. Chemisorption proceeds in a slow rate because it involves overcoming certain energies related to chemical bond formation. In contrast, physisorption occurs at a relatively fast pace because only covalent forces are involved. In the first few minutes, physical adsorption may have been involved in addition to chemical adsorption, which led to a relatively fast adsorption process. Afterward, there might have been only chemisorption involved until equilibrium because the rate suddenly decreased and adsorption continued at a slower pace.

Table 6. Comparison of BTO Removal for Adsorbents GAC and the Ion-Impregnated ACs removal (%) +2

sulfur compound

GAC

Cu /AC

Fe+3/AC

Ni+2/AC

BTO DBTO

70.5 70.3

66.9 97.8

71.9 98.3

65.4 99.0

3.6. Adsorption Mechanism. All the adsorbents used have several functional groups present on its surface, and it is the nature of these surface groups that determine the character of these adsorbents. It is these surface functional groups that interact with the adsorbates. The adsorption process is also affected by the nature of the adsorbates themselves. The sulfur compounds underwent oxidation and were converted to sulfones to become more polar compounds, which is taken advantage of during adsorptive desulfurization. It is this polarity of BTO and DBTO that makes them easier to be adsorbed compared to just benzothiophene (BT) and dibenzothiophene (DBT). An adsorption mechanism of BTO and DBTO onto all the adsorbents used is therefore proposed, and is shown as follows. To illustrate how the sulfur compounds, BTO and DBTO, adsorb on the surface of the adsorbents, the concept of the G


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Energy & Fuels Gibb’s free energy of adsorption will be used. The free energy ΔG abs is a summation of different adsorption energy components:17 ΔGads = ΔGe + ΔGi + ΔGc + ΔGid + ΔG hb + ΔG hp (7)

ΔGe represents contributions from electrostatic interaction, where electrostatic interactions are interactions between charged particles; ΔGi relates to ion exchange between the adsorbent and adsorbate. ΔGc refers to coordination by surface metal cations, where coordination bonds are bonds formed by sharing of electrons between metal ions and an electronegative atom; ΔGid is for ion−dipole interactions, where ion−dipole interactions involve interactions between a charged ion and a polar molecule. ΔGhb represents hydrogen-bonding contributions. A hydrogen bond is both an attractive force between a lone pair of electron of an electronegative atom and a hydrogen atom that is bonded to nitrogen, oxygen, or fluorine. Last, ΔGhp relates to hydrophobic interactions, which deal with interactions with water. Because BTO and DBTO are un-ionizable compounds, the first two terms of the equations are negligible. BTO and DBTO possess an aromatic ring; being in a benzene solution renders it improbable to ionize the two compounds. This also implies that the ion−dipole interactions between the adsorbents’ surface and the compounds are also negligible. Surface metal cations are present only on the ion-impregnated activated carbons; therefore, only in them would this mechanism be relevant. The model fuels used were BTO in benzene and DBTO in benzene solvent; thus, hydrophobic interactions are also taken to be minimal. Granular activated carbon has several functional groups present on its surface. For functional groups present in activated carbon, only ΔGhb would have a relevant contribution. A schematic diagram of how BTO and DBTO adsorb on GAC is shown in Figure 7a. The presence of the electronegative oxygen atoms on the sulfones makes them polar molecules, which makes them the target site of interactions with the functional groups on the adsorbent’s surface. The oxygen’s lone pair of electrons is shared with the hydrogen of the hydroxyl group, forming a hydrogen bond. The modification of the activated carbon embedded metal cations, Cu2+, Fe3+, and Ni2+, on the surface of the solid carbon.11 The presence of these cations would add to the interactions between the carbon and the sulfones. In addition to a hydrogen bond, other forces of attraction would contribute to the adsorption mechanism. The oxygen has lone pairs of electrons, and the sulfone shares it with the metal ion, forming a coordination bond. ΔGc would then have a contribution. ΔGid would also have a contribution because there are now interactions between a charged ion (cation) and a polar molecule (sulfone). Figure 7a−d demonstrates the proposed mechanism of the sulfones adsorbing onto the modified carbons. Metal ion impregnation onto the surface of the activated carbon greatly increased the capacity of the solid to adsorb BTO and DBTO, as was shown in the results of the equilibrium studies. Increase in adsorbent capacity can be attributed to these additional interactions provided by the impregnated ions.

Figure 7. Schematic of proposed mechanism for BTO and DBTO adsorption on (a) Cu2+/AC, (b) Ni2+/AC, and (c) Fe3+/AC.

indicated that all adsorbents used, granular activated carbon and the ion-impregnated activated carbons, have the capacity to remove benzothiophene sulfone and dibenzothiophene sulfone. This is supported by the equilibrium studies and kinetic studies performed. The ion-impregnated activated carbon adsorbed DBTO better with removal efficiency of 97.8, 98.3, and 99% for Cu2+/AC, Fe3+/AC, and Ni2+/AC, respectively. However, the adsorption capacity of the modified activated carbon was not improved with regard to the BTO removal. Adsorption isotherms tend to follow the Langmuir isotherm model, where correlation factors gave high values, R2 > 0.96. This suggests monolayer coverage of the adsorbates on the adsorbent’s surface and chemisorption as the prevailing adsorption mechanism. High values of R2 for the Freundlich model may also indicate the presence of physisorption in the system. Adsorption kinetics follows the pseudo-second-order rate model. This suggests that the main mechanism of adsorption is chemisorption, which involves the formation of strong chemical bonds between the adsorbents and the oxidized sulfur compounds. The adsorption process seemed to proceed in a two-step process: first, a very fast rate for the first few minutes and then a markedly reduced rate until equilibrium is reached. This may indicate the presence of physical adsorption during the first few minutes of the process. Modifying the

4. CONCLUSIONS On the basis of the results of the experiments conducted, the following conclusions can be stated. Adsorption experiments H


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Energy & Fuels

(16) Yuh-Shan, H. Citation review of Lagergren kinetic rate equation on adsorption reactions. Scientometrics 2004, 59 (1), 171−177. (17) Lu, M.-C.; Roam, G.-D.; Chen, J.-N.; Huang, C.-P. Adsorption characteristics of dichlorvos onto hydrous titanium dioxide surface. Water Res. 1996, 30 (7), 1670−1676.

granular activated carbon with the impregnation of the metal ions (Cu2+, Fe3+, and Ni2+) gave a significant increase in the activated carbon’s adsorption capacity as shown by the Langmuir model.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +886-6-2660489. Fax: +886-6-2663411. E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology, Taiwan, for financially supporting Contract No. NSC 102-2221-E-041-001MY3.

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REFERENCES

(1) Office of Transportation and Air Quality. Heavy-Duty Engine and Vehicle Standards and Highway Diesel Fuel Sulfur Control Requirements; Technical Report EPA420-F-00-057 prepared for the United States Environmental Protection Agency:Washington, DC, 2000. (2) Gui, J.; Liu, D.; Sun, Z.; Liu, D.; Min, D.; Song, B.; Peng, X. Deep oxidative desulfurization with task-specific ionic liquids: an experimental and computational study. J. Mol. Catal. A: Chem. 2010, 331 (1), 64−70. (3) Yang, R. T.; Hernández-Maldonado, A. J.; Yang, F. H. Desulfurization of transportation fuels with zeolites under ambient conditions. Science 2003, 301 (5629), 79−81. (4) Caero, L. C.; Hernández, E.; Pedraza, F.; Murrieta, F. Oxidative desulfurization of synthetic diesel using supported catalysts: Part I. Study of the operation conditions with a vanadium oxide based catalyst. Catal. Today 2005, 107-108, 564−569. (5) Zhang, J.; Wang, A.; Li, X.; Ma, X. Oxidative desulfurization of dibenzothiophene and diesel over [Bmim] 3 PMo 12 O 40. J. Catal. 2011, 279 (2), 269−275. (6) Do, D. D. Adsorption Analysis: Equilibria and Kinetics. Imperial College Press: London, 1998. (7) Srivastav, A.; Srivastava, V. C. Adsorptive desulfurization by activated alumina. J. Hazard. Mater. 2009, 170 (2), 1133−1140. (8) Ho, Y.; McKay, G. A comparison of chemisorption kinetic models applied to pollutant removal on various sorbents. Process Saf. Environ. Prot. 1998, 76 (4), 332−340. (9) Futalan, C. M.; Kan, C.-C.; Dalida, M. L.; Hsien, K.-J.; Pascua, C.; Wan, M.-W. Comparative and competitive adsorption of copper, lead, and nickel using chitosan immobilized on bentonite. Carbohydr. Polym. 2011, 83 (2), 528−536. (10) Futalan, C. M.; Kan, C.-C.; Dalida, M. L.; Pascua, C.; Wan, M.W. Fixed-bed column studies on the removal of copper using chitosan immobilized on bentonite. Carbohydr. Polym. 2011, 83 (2), 697−704. (11) 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 (6), 3858−3863. (12) Karanfil, T.; Kilduff, J. E. Role of granular activated carbon surface chemistry on the adsorption of organic compounds. 1. Priority pollutants. Environ. Sci. Technol. 1999, 33 (18), 3217−3224. (13) Selvavathi, V.; Chidambaram, V.; Meenakshisundaram, A.; Sairam, B.; Sivasankar, B. Adsorptive desulfurization of diesel on activated carbon and nickel supported systems. Catal. Today 2009, 141 (1), 99−102. (14) Wang, C.-T.; Chou, W.-L.; Chung, M.-H.; Kuo, Y.-M. COD removal from real dyeing wastewater by electro-Fenton technology using an activated carbon fiber cathode. Desalination 2010, 253 (1), 129−134. (15) Alhamed, Y. A. Activated Carbon from Dates’ Stone by ZnCl2 Activation. J. King Abdulaziz Univ. 2006, 17, 75. I


Adsorption of Sulfur Compounds from Diesel with Ion-Impregnated

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