REMARKABLE REMOVAL OF SULFUR-CONTAINING COMPOUNDS FROM

MODEL FUELS WITH MODIFIED CLAYS

Jeong Ho Ha

Ross School

Date of Birth: 04/27/1995

Conducted in Summer 2011 under the mentorship of:

Abel Navarro, Ph.D.

Adjunct Instructor

Department of Chemistry

New York University

ABSTRACT

The presence of sulfur-containing compounds in the fuels have been an important

concern in the last decade as an environmental risk because of the increase of greenhouse

gases in the atmosphere and accentuation of acid rain. This study evaluates chemically

modified clays as adsorbents for the removal of dibenzothiophene (BT) and 4,6-

dibenzothiophene (DBT). The adsorption by the modified clay was investigated in a

batch system with synthetic fuels (gasoline and diesel) as a function of type of chemical

modification, adsorbent dosage, initial concentration of the pollutants, desorption, and

isotherm modeling. Maximum adsorption was observed with clays modified with

benzyltrimethylammonium salts (BM) due to its similarity with the pollutants (aromatic

compounds). The maximum adsorption capacity (qmax) was achieved by BT with a qmax

of 11.3 mg/g in gasoline and 31.3mg/g in diesel. The formation of Van der Waals

interaction as well as aromatic forces are the main mechanism proposed based on the

results. A solution of 0.1N HCl recovered up to 40% of the pollutant for their use in

repetitive cycles. This present work highlights the potential use of modified clay in the

elimination of sulfur-containing compounds from model fuels as a low-cost and

environmentally friendly purification technique.

EXECUTIVE SUMMARY

Greenhouse effect, global warming and acid rain have been environmental problems with

a great impact in the human being, not only affecting the agriculture, but also the ocean

and dramatic changes in temperature. One cause of these problems is the emission and

burning of sulfur-containing substances to the atmosphere. Government is currently

regulating transportation fuels to control these emissions, mainly the amount of a family

of compounds named dibenzothiophenes. These molecules are very insoluble in water

and this difficulties their extraction by common techniques. This project utilized the low-

cost clays to remove these dibenzothiophenes from model fuels like gasoline and diesel.

A chemical modification was carried out to improve the low polarity of clays and

enhance the adsorption of the pollutants. The incorporation of a bulky modification

created the optimum adsorbent for dibenzothiophenes. Equilibrium studies on adsorbent

dose, concentration of the pollutants, desorption and isotherm model were also carried

out to understand and maximize the adsorption. Positive results were determined by

means of mathematical modeling, when it was discovered that 11.3mg and 31.3 mg of the

pollutant were removed with only 1 gram of the modified clay from gasoline and diesel,

respectively. Finally, up to 40% of the initial amount of the pollutant was recovered from

the adsorbent by using a common acid, allowing this adsorbent to be re-used in repetitive

cycles. This present work highlights that modified clays are “clean” and low-cost

adsorbents can be potentially applied in the removal of sulfur-containing substances from

transportation fuels. This research opens new strategies in the development of more

applicable adsorbents without jeopardizing other resources or environment.

1. INTRODUCTION

The abatement of the sulfur concentration in fuels is one of the most important priorities

for oil refineries and subject of active research worldwide in the last 10 years because the

production and use of more environmentally friendly transportation fuels have been

attracting increasing attention in many countries. The majority of sulfur-containing

compounds present in these fuels include thiophenes, benzothiophenes and

dibenzothiophenes that are highly concentrated in the heaviest crude fractions and are

partially converted during the refining process. The removal of these compounds is

becoming more relevant due to the creation of more restrictive environmental rules. The

maximum allowable sulfur content in highway diesel fuel was reduced to less than 15

ppmw in 2006 in the US from the current maximum of 500 ppmw in 20051. The potential

risk of these sulfur-containing compounds during any combustion process resides on their

transformation in sulfur oxides such as SO2 and SO3 which are, amongst others,

important contributors to acid rain2. Likewise, the presence of sulfur promotes the

poisoning of catalysts that are used in the treatment of gases from automobiles.

In general, methods for deep desulfurization of liquid hydrocarbon fuels can be classified

into catalytic hydrodesulfurization with improved new catalyst, chemical oxidation,

adsorption, extractive desulfurization, and biodesulfurization by using special bacteria3,4.

Among them, selective adsorption seems to be one of the most promising techniques for

the removal of sulfur from fuels due to their low price and realistic operational conditions

at room temperature and atmospheric pressure.

Nowadays, the most common adsorption techniques are based on the use of activated

carbon. Activated carbon is well known for its adsorption properties, including phenolic

compounds, dyes, metals and organic molecules in general. Therefore, its application to

the elimination of sulfured compounds has been subject of research in the past5-7.

Unfortunately, the next question raises: isn’t activated carbon prepared by burning wood

and other organic materials? Is this process not also by-producing carbon dioxide which

is a greenhouse and acid rain gas? Perhaps, one of the most important tasks that need to

be studied is the recycling of the activated carbon or to improve the experimental

conditions to recover the pollutant and re-use the same adsorbent in sequential cycles.

Conversely, natural clays are naturally occurring resources whose extraction does not

require sophisticated equipment and does not produce environmentally toxic substances.

Clays are molecularly composed of up to two silica tetrahedral sheets with a central

octahedral sheet. The main structure of clays is negatively charged that is balanced by the

presence of exchangeable cations (Na, K, Ca, etc.). The presence of these ions imparts a

high surface polarity that is not chemically friendly to apolar environments like gasoline

and diesel. Moreover, the target pollutants to be adsorbed are very hydrophobic. For this

reason, a chemical modification is needed to improve the hydrophobic affinity of natural

clay to aim these apolar molecules. This is done by taking advantage of the exchangeable

ions that reside on the porous surface of clays. Apolar quaternary ammonium ions have

been used in the past to improve the adsorption of phenolic compounds on bentonite (a

type of clay)8,9. The positive results showed the reversal of polarity of the surface and

enhancement of affinity towards apolar substrates.

The objective of the research summarized in this project is to investigate the most

important equilibrium parameters on the adsorption of 2,4-dimethyldibenzothiophene

(DBT) and dibenzothiophene (BT) as model sulfur-containing compounds present in

transportation fuels. Batch adsorption experiments under normal conditions were carried

out in synthetic gasoline and diesel fuels to compare the strength of modified clays in

different types of fuels. We expect that these positive results can be transmitted to the

scientific community to further investigate the viability of these “clean” adsorbents on the

removal of pollutants from apolar solutions such as gasoline and diesel.

2. MATERIALS AND METHODS

2.1 Conditioning of the Adsorbents

Bentonites are clays from the montmorillonite group and smectite family of mineral. It

was purchased as raw mineral (Sigma-aldrich, USA). The clay was chemically modified

with three different ammonium salts: benzyltrimethylammonium chloride (BM),

tetramethylammonium chloride (TM), and with hexadecyltrimethylammonium chlroride

(HM). These quaternary ammonium ions have demonstrated high affinity towards

organic compounds8,9, to enhance their adsorption capacity. The exchange procedure was

as follows: 20g of clay was suspended in 2L of distilled water and combined with two

equivalents of its cation exchange capacity (CEC) of the ammonium salts (99% Sigma

Aldrich, USA). The final suspension was stirred at 250 rpm during 24h at room

temperature. The modified clays were decanted, vacuum filtered and vigorously rinsed

with distilled water to eliminate the excess of ammonium salts , dried at 60°C in an oven

for 48h and stored until their use. The clays were identified according to the ammonium

salt used in their modification, for example: clay treated with

hexadecyltrimethylammonium chloride was simply referred as HM clay. The CEC of the

clay (0.06mmol/g) was determined using the methylene blue standard method (ASTM C-

837-81).

2.2. Synthetic fuels and Solutions

Synthetic gasoline and diesel were prepared based on their major components, according

to the literature5-7. Hexadecane was chosen as synthetic gasoline and a 1:1 mixture of

decane and hexadecane was used for diesel. All the solvents were of a purity of 99% or

higher (Acros Organic, USA). DBT- and BT-containing solutions were freshly made

prior every single experiment, to avoid changes in concentrations due to spontaneous

evaporation of the fuels. DBT and BT were purchased from Aldrich, USA (Reagent

grade) and used without further purification.

2.3 Adsorption experiments

Batch experiments were carried out in duplicate at room temperature combining variable

masses of the clays with 20 mL of a solution the fuels with orbital agitation at 200 rpm

for 24 h. The adsorption time was determined by preliminary experiments. Thereafter, the

suspensions were centrifuged and the remaining concentration of DBT and BT were

determined through UV spectrophotometry (Agilent 8453) at a wavelength of 330 nm.

This quantitative analysis was performed right after the end of the experiment to avoid

changed in the concentration of the pollutants due to evaporation. No spectrometric

interference was noticed from other substances, improving the resolution of the spectra.

Although the spectra showed no interference from the presence of clays and fuels,

nevertheless for the desorption experiment, some adjustments were needed, where some

of the cosolvents showed light absorption at the worked wavelength such as acetone.

2.3.1 Best modified clay

Three different modified clays were prepared as mentioned in section 2.1. However, the

best clay was chosen based on an experiment were all the clays, including the non-

modified clay (NC) were exposed to DBT and BT. The purpose of this experiment was

to discover any chemical or physical difference between the TM, BM, HM and NC clays.

2.3.2 Adsorption isotherms and effect of adsorbent dosage

Solutions of DBT and BT were prepared at different concentrations. Then, a variable

mass of the best clay was added. Adsorption isotherms are needed to obtain a

mathematical modelling of the process.

2.4 Desorption experiments

Batch experiment with the optimized parameters of best clay, adsorbent dose and

concentration of pollutant was carried out with both fuels. The adsorbents were

centrifuged, carefully rinsed with the same fuel to eliminate any residual DBT or BT

from the adsorbents and air-dried overnight. Then, 25 mg of the dry pollutant-loaded

adsorbent were put in contact with co-solvents during 2h to identify the best eluting

solution. The co-solvents included solutions of Methanol, Acetone, Toluene, NaOH

(0.1N), and HCl (0.1N). Finally, the samples were centrifuged and the supernatant was

taken for quantification of residual amounts of DBT and BT by UV spectrophotometry.

2.5 Data analysis

The amount of the DBT and BT adsorbed on the clays were expressed as Adsorption

Capacity (q, mg/g) and calculated as shown in equation (1):

(Ci – Ceq) * V

q = (1) m

where m is the mass of the adsorbent expressed in g, V is the volume of the solution in L

and Ci and Ceq are the initial and at the equilibrium concentrations of lead (II) expressed

in mg/L, respectively.

A different, commonly used, quantitative indication of the adsorption is expressed as

percentage. In this research, (%DES) was used to quantify the amount of pollutant that

was recovered during the desorption experiment, and was determined according to

equation (2):

%(DES) = (Cd / Cl) * 100 (2)

where Cd represents the concentration of DBT or BT present in the eluting solution and

Cl is the concentration of the pollutant that was loaded in the adsorbent after the

adsorption experiment.

3. RESULTS

3.1 Best modified clay

Three different types of quaternary ammonium salts were compared against natural clay

to determine the improvement in the adsorption due to the reversal of surface polarity.

The structures of the three different ammonium ions are shown in Figure 1. As observed,

we can categorize these three ions according to the dimensions of the chain as: short

chain (TM), bulky chain (BM) and long chain (HM). We expected substantial changes in

adsorption with chain size due to the specific and somehow problematic hydrophobic

interaction.

N

N N

Cl

Cl

Cl

Figure 1: Chemical structure of the quaternary ammonium ions used to modify the clay. Top left: TM, Top right: BM and Bottom: HM

The chemical modification involved the replacement of some positively charged ions

such as Na and Ca with these ammonium cations. The positive charge located on nitrogen

tightly bind to the clay surface, while the hydrophobic side chain interacts with apolar

substances such as DBT and BT.

The adsorption results on gasoline and diesel are shown in Figure 2. From the results, we

chose BM as the most efficient ammonium salts for the adsorption of DBT and BT in

both fuels. It seems like the bulky chain preferentially binds to bulky chains. On the other

hand, diesel also shows a better environment for the adsorption of both benzothiophenes.

With exception of BT on BM in gasoline, most of the adsorption capacities are in the

range 2-4 mg/g, whereas diesel presents adsorptions in the range 6-7 mg/g. Moreover,

HM shows the smallest affinity towards DBT and BT in both fuels. As expected, the non

treated clay (NC) presented an almost negligible adsorption. To summarize, we could

describe the side chain dependence as follows: BM˃TM˃HM˃NC. Therefore, BM clay

was chosen for the next experiments.

NC

TM

BM

HM

0 2 4 6 8 10

Cla

y

q (mg pollutant / g adsorbent)

BT DBT

NC

TM

BM

HM

0 1 2 3 4 5 6 7 8C

lay

q (mg pollutant / g adsorbent)

BT DBT

Figure 2: Determination of the best adsorbent for DBT and BT in gasoline (left) and diesel (right).

3.2 Effect of Adsorbent dosage

The minimum amount of adsorbent is crucial for scaled-up processes. Minimizing costs

we can attract inversions and compete with other, perhaps, more efficient adsorbents.

Adsorbent dosage effect tried to optimize the mass of BM clay used to adsorb DBT and

BT. Results are shown in Figure 3. Negligible adsorption changes are observed at

adsorbent masses higher than 75mg. However the higher adsorption was obtained with

25mg, being this value taken as the optimized mass dosage for the future experiments.

20 40 60 80 100 120 140 160 180 200

1

2

3

4

5

6

7

8

9

10

DBT BT

q (m

g po

lluta

nt /

g ad

sorb

ent)

Adsorbent Dose (mg)

20 40 60 80 100 120 140 160 180 2000

1

2

3

4

5

6

7

8

DBT BT

q (m

g po

lluta

nt /

g ad

sorb

ent)

Adsorbent Dose (mg)

Figure 3: Adsorbent Dosage effect for DBT and BT in gasoline (left) and diesel (right).

3.3 Effect of the Concentration of Pollutant – Adsorption Isotherms

The range of concentration at which is adsorption is optimum is important to determine

the saturation of the fuels. There is a maximum amount of pollutant the adsorbent can

handle and where no more DBT or BT will be adsorbed. This effect was solved and the

isotherms are shown in Figure 4. From the results, we can conclude that most of the

adsorbent reach a plateau where no more adsorption is observed, which means that all the

active sites were occupied by the pollutant.

0 50 100 150 200

3

4

5

6

7

8

9

10

11

DBT BT

q (m

g po

lluta

nt /

g ad

sorb

ent)

Ce (mg/L)

-20 0 20 40 60 80 100 120 140 160 180 2000

5

10

15

20

25

30

35

40 DBT BT

q (m

g po

lluta

nt /

g ad

sorb

ent)

Ce (mg/L)

Figure 4: Adsorption Isotherms for DBT and BT in gasoline (left) and diesel (right) at room temperature.

Concentration effect experiment also allows us to determine important parameters that

explain and predict energetic and equilibrium behavior of the interactions between

adsorbent and adsorbate. Different theories have been developed to explain the physical

and chemical forces in heterogeneous and homogenous mixtures that hold an adsorbate to

an adsorbent.

All these isotherms can be mathematically described and their constants can be

determined. However, due to the complexity of their mathematical treatment, a linearized

version of these isotherm models are prefered and perfectly adjust to the equilibrium

results that are observed in the experiment12-13.

These theories and their equation are:

Langmuir Model 1/q = 1/qmax + (1/ qmax* b) * 1/Ceq) (3)

where qmax is the maximum adsorption capacity under these working conditions, b is a

constant related to the affinity between adsorbent and the pollutant.

Freundlich Model ln(q) = ln(k) + 1/n * ln Ceq (4)

where k y n are the Freundlich constants related to the adsorption capacity and the

adsorption intensity, respectively.

Dubinin-Radushkevich (D-R) Model

ln(q) = ln(qmax) - KR2T2 * ln2(1+(1/Ceq) (5)

where K is the activity coefficient related to the Mean Adsorption Energy (mol2/J2). R is

the Universal gas constant in J/K*mole, and T the temperature in the Kelvin scale. The

Mean Adsorption Energy, E (kJ/mol) can be calculated with the equation: E = (2*K)-1/2.

And finally:

Temkin Model: q = RT/bt ln at + RT/bt * lnCe (6)

in which R is the gas constant, T is absolute temperature in Kelvin, bt is a constant related

to the heat of adsorption and at is the Temkin isotherm constant.

The mathematical models produced equilibrium constants that are summarized in Table

1. Coefficient of correlation is also shown as an indication of how appropriate is the

fitting of that theory to the experimental data.

Table 1: Parameters and Constants for the Adsorption Isotherms of DBT and BT in gasoline and diesel at room temperature.

ISOTHERM

MODEL AND

PARAMETERS

GASOLINE DIESEL

DBT BT DBT BT

Langmuir Model

qmax (mg/g)

b (L/mg)

R2

4.74

0.103

0.979

11.311

0.245

0.999

9.404

1.407

0.998

31.32

0.015

0.964

Freundlich Model

k (L/g)

n

2.222

7.465

3.242

4.041

4.928

7.562

0.631

1.286

R2 0.966 0.979 0.954 0.982

D-R Model

qmax (mg/g)

k

E (J/mole)

R2

4.582

2.18 x10-5

151.41

0.999

10.867

1.07 x 10-6

684.02

0.999

9.317

1.56 x 10-7

1789.68

0.998

34.834

5.08 x 10-4

31.362

0.955

Temkin Model

at

bt

R2

44.900

4857.62

0.955

4.237

1433.57

0.985

220.145

2715.39

0.936

0.072

195.736

0.891

3.4 Desorption Experiments

The success of any adsorbent resides on the easiness and capability of releasing the

adsorbed pollutant to be potentially used in repetitive cycles. The transfer of the pollutant

from the exposure to humans and environment to a safe place where the levels of

concentration can be controlled gives the adsorption process a great advantage compared

to other techniques. This difficult task was accomplished by treating pollutant-loaded BM

clays with several solvents. These solvents were chosen based on their chemical

properties such as acid/base behavior, and polarity. The low solubility of DBT and BT

predicted little or no desorption with aqueous solutions, however, as observed in Figure

5, a 0.1M solution of HCl provides the desorbing conditions that maximize the release of

DBT and BT. Conversely, the low polarity solvents like ethanol and acetone, expected to

be powerful desorbers, did not present enough strength to liberate the pollutants.

HCl

NaOH

MeOH

Acetone

Toluene

0 5 10 15 20 25

Des

orbe

nt

Desorption Percentage (%DES)

BT DBT

HCl

NaOH

MeOH

Acetone

Toluene

0 10 20 30 40

Des

orbe

nt

Desorption Percentage (%DES)

BT DBT

Figure 5: Desorption properties of cosolvents for DBT and BT in adsorbent pollutant-loaded in gasoline (left) and diesel (right).

4. DISCUSSION OF RESULTS

4.1 Best modified clay

Although TM, BM and HM are highly hydrophobic compared to the natural clay (NC),

DBT and BT show preference with bulky side chain, like the one in BM in both fuels.

The observed ranking in both fuels and for both pollutants can be described as:

BM˃TM˃HM˃NC. NC was expected to display the lowest affinity due to its high

polarity. This polarity is the product of electrostatic interactions due to the presence of

numerous ions on the clay surface. A plausible explanation to the differences observed

with the 3 modified clays is not only attributed to the size of the side chain, but also to the

type of interaction they can offer to DBT and BT. The structures of these two pollutants

are shows in Figure 6.

S

Figure 6: Chemical structures of DBT (left) and BT (right), adsorption targets in this work.

The structure of both pollutants are very similar with the exception of the extra two

methyl observed in DBT, which would increase the hydrophobicity somewhat. The

relatively low adsorption capacity of HM could be attributed to the poor affinity towards

DBT and BT. Even though they are both hydrophobic, Van der Waals interactions are,

amongst all the intermolecular forces, the weakest and the least contributing in molecular

interactions. Moreover, the crowding that is going on in the passages and pores of the

clay bends and coils the long hydrophobic chain of HM and decreases the effective

molecular surface that this side chain has. It might look more like a twine instead of a

straight chain of carbons. Therefore the adsorbent-adsorbate interaction is decreased. TM

shows a medium adsorption in gasoline and diesel, the small side chains create enough

hydrophobicity to retain some molecules of DBT and BT, but not enough to form several

layers of the pollutants. The short distance between the polar walls of the clays and the

pollutant might be affecting the interactions and repelling divergent polarities.

Conversely, BM is a medium sized side chain and, in addition to this property, it presents

an aromatic ring, which looks like the rings present in DBT and BT (conjugated double

bonds in a ring). In chemistry, it is known the rule: “like dissolves like” therefore,

aromatic interactions attract each other and enhance the adsorption14.

S

4.2 Effect of Adsorbent Dosage

The results shown in Figure 3 demonstrate that there is a saturation of adsorbent in the

system. Therefore, the addition of more adsorbent does not change the overall adsorption.

This could be understood as the saturation of the adsorbent surface under these

experimental conditions. Rubin et. al.10 studied the adsorption of phenols , observing that

500mg of marine seaweed leveled off the adsorption percentage. They consider that the

plateau observed in the adsorption at high biomass concentration is a consequence of

partial aggregation of the adsorbent in solution that reduces the number of available

adsorption sites. The fact that the adsorbents are inmersed in 20mL of solution, the higher

number of particles in the same solution volume promotes the aggregation of adsorbent

preventing the pollutants to occupy the active centers. Moreover, clays coated with

hydrophobic substances have a tendecy to aggregate.

4.3 Adsorption Isotherms

From the equilibrium results, BT is strongly adsorbed on HM clay displaying the highest

adsorption capacity (qmax = 11.3 and 31.3 mg/g in gasoline and diesel, respectively).

DBT was also decently adsorbed in both fuels with qmax values of 4.7 and 9.4 mg/g in

gasoline and diesel, respectively. Both pollutants in both fuels adjusted to all the

adsorption theories of Langmuir, DR, Temkin and Freundlich, suggesting a mixed

adsorptive mechanism. From the Langmuir isotherm, the constant b predicts a high

affinity, reporting the adsorption of DBT in diesel, as the better and faster reached

adsorption. According to the Freundlich theorie, n values larger than 1 reflect a positive

adsorbate-adsorbent contacts. As observed in Table 1, all the adsorbents presented n

larger than 1. Other adsorption studies of DBT and BT with activated carbon5-7 show

lower adsorption capacities than the ones reported in this work, with q values around 3

mg/g at low pollutant concentration5. On the other hand, other research group found

Freundlich “n” values between 0.78 and 1.15, which represent hardly favourable

adsorption according to the Freundlich model6. BM clay present n values between 1.3

and 7.6, showing its affinity towards the pollutants.

The non-linearity of the isotherms confirms the competitive adsorption of the pollutants

for the adsorption sites of the clay11. This is understandable due to the mixed nature of

clays, with polar skeleton that is coated with a apolar molecule. Under this case, DBT and

BT will run away from the polar walls of the clay looking for a non-polar substrate to

bind.

The D-R and Temkin isotherms are commonly used to determine the energetic factor in

the adsorption process. According to the results, DBT in diesel has the most exothermic

energy (1.8 kJ/mole) and therefore the most favourable interaction with the adsorbent

from the thermodynamic point of view. The value of these energy might seem relatively

low (ranging between 0.2 to 1.8 kJ/mole), but it is necessary to remember that we are

dealing with hydrophobic interactions and this type of forces do not release as much

energy as the polar or ionic interaction, mostly due to dehydration/hydration phenomena.

Finally, qmax obtained with the D-R model completely agrees with the values observed

with the Langmuir theory, confirming the combined adsorption mechanism with the

adsorbents13.

4.4 Desorption Experiments

Desorption expectations were highly placed in toluene as an aromatic solvent and

therefore with a higher affinity towards aromatic molecules (DBT and BT). However,

HCl showed a higher desorption properties with both pollutants and fuels. A possible

explanation to this observed “contradiction” could be related to the positive nature of H+

ions from HCl. Since the modification of the clays was done by ionic exchange of

positively charged ions (like Na and Ca) from the surface of the clay with quaternary

ammonium ions, the reverse situation could be happening. In other words, since H+ ions

are more hydrophilic than ammonium ions, they could be re-exchanging the BM ions and

therefore freeing DBT and BT to the solution by desorption not of the pollutants

themselves, but of the ammonium salt that is holding them. An alternative explanation is

due to the usually forgotten cation-π interaction that exists between an aromatic

compound (like DBT and BT) and positively charged ions like H+. This affinity could

override the weak hydrophobic interaction on the clay surface and release the pollutants

to the solution. This desorption, even though does not 100% of the initial amount of

pollutant in the adsorbent, but recovers up to 40%. Toluene also shows some desorption

properties, it could be understood as an aromatic interaction or “like dissolves like” with

the pollutants. The medium low capacity of acetone and methanol could be explained due

to their amphipathicity which is not friendly with the non polar character of DBT and

BT13.

5. CONCLUSIONS

This present work demonstrates that chemically exchanged bentonite clay can potentially

be utilized as adsorbent of dimethyldibenzothiophene (DBT) and

dimethyldibenzothiophene (BT) from synthetic fuels at room temperature. Batch

experiments exhibit a strong ammonium salt as chemical exchanger, reporting a

maximum adsorption capacity around 10mg/g of BT in gasoline and 8mg/g for DBT in

diesel. These results align with the intermolecular interactions the

benzyltrimethylammonium ion (BM) presents. The effect of the adsorbent dosage was

studied as well, showing a decreasing adsorption at increasing masses of adsorbent. This

is explained by the formation of adsorbent aggregates that diminish the number of active

sites and effective surface. Moreover, the mathematical models of Freundlich, Dubinin-

Radushkevich, Temkin and Langmuir were applied to the experimental data. The result

was a mixed mechanism description by all the theories for pollutants. The calculated

parameters from the isotherms demonstrate an interesting affinity between adsorbate and

adsorbent and a high adsorption capacity when compared to those reported in literature

explaining their applicability for sulfur-containing real fuels. Finally, the use of 0.1N HCl

as an eluting solvent, removed about 40% of BT and 25% of DBT from diesel and

gasoline, respectively. This advantage will allow the re-use of adsorbents in consecutive

cycles. Overall, this project constitutes a major contribution for the biotechnology for the

development of cost-efficient adsorbents for the removal of pollutants in general. In this

particular case, the versatility of modified clays as adsorbent sulfur-containing

compounds opens the doors for their extensive use in new techniques of purification at

low cost.

6. FUTURE WORK

Future goals will involve the set up of bigger scale elimination of DBT and BT for the

real application in larger volumes of fuels and perhaps in continuous systems to make it

more applicable for the industry. Temperature- and time- dependents are also needed.

The temperature will confirm the interactions between adsorbent-adsorbate by the

determination of the energy absorbed or released during the adsorption. Kinetics will

predict the mechanism by which the pollutant is adsorbed; these include superficial

diffusion or intraparticular diffusion due to the porosity of the adsorbent. These

parameters are crucial in order to scale-up this process for larger volumes of fuels and

purification columns. An adequate management of lab scale is needed prior to treat real

volumes of liquid. Up to this point, this is more than a promising adsorbent due to its

versatility. A long term plan would include the use of these adsorbents in hybrid fuels

like ethanol/gasoline mixture and even try gas fuels that also contain heteroatoms that,

when burned, produce greenhouse gases and acid rain.

REFERENCES

1. Brock W.A., Taylor M.S. (2004). Economic growth and the environment: a

review of theory and empirics. En NBER Working Paper 10854, National Bureau

of Economic Research, Cambridge MA.

2. Stern D.I. (2005). Beyond the environmental Kuznets curve: Diffusion of

emissions reducing technology. J. Environ. & Devel. 14, 101–124.

3. Dhandapani B., Clair S,T. y Oyama S.T. (1998). Simultaneous

hydrodesulfurization, hydrodeoxygenation, and hydrogenation with molybdenum

carbide. J. Catal., 168., 219–228.

4. Domínguez J.M., Hernández J.L. y Sandoval G. (2002). Surface and catalytic

properties of Al2O3–ZrO2 solid solutions prepared by sol–gel methods. Appl.

Catal. A., 197. 119–130.

5. Sederych, M., Hulicova, D., Bandosz, TJ. (2010) Effect of the incorporation of

nitrogen to a carbón matrix on the selectivity and capacity for adsorption of

dibenzothiophenes from model diesel fuel. Langmuir, 26,1, 227-233.

6. Zhou, A., Ma, X., Song, C. (2006) Liquid phase adsorption of multi-ring

thiophenic sulfur compounds on carbon materials with different surface

properties. J. Phys. Chem. B., 110, 4699-4707.

7. Deliyanni, E., Sederych, M., Bandosz, TJ. (2009). Interactions of 4,6-

dimethylbenzothiophene with the surface of activated carbons. Langmuir, 25,16,

9302-9312.

8. Navarro, AE., Cuizano, N., Lazo, J., Sun-Kou, M., Llanos, BP. (2009).

Comparative study of the removal of phenolic compounds by biological and non-

biological adsorbents. J. Hazard. Mater., 164, 1439-1446.

9. Navarro, AE., Lazo, J., Cuizano, N., Sun-Kou, M., Llanos, BP. (2009). Insights

into the removal of phenol from aqueous solutions by low cost adsorbents: clays

versus algae. Sep. Sci & Technol., 44, 2491-2509.

10. Rubin, E., Rodriguez, P., Herrero, R., Sastre de Vicente, M. (2006) Biosorption of

phenolic compounds by the Brown algae Sargassum muticum. J. Chem. Technol.

& Biotechnol., 81, 1093-1099.

11. Navarro, AE., Portales, R., Sun-Kou, R., Llanos, BP. (2008) Effect of pH on

phenol biosorption by marine seaweeds. J. Hazard. Mater., 156,405-411.

12. Liu, Y. and Wang, J. (2009) Fundamentals and Applications of Biosorption

Isotherms and Kinetics and Thermodynamics. Nova Science Publishers, Inc. New

York, US.

13. Volesky, B. (2003). Sorption and Biosorption. BV Sorbex, Inc. Quebec, Canada.

14. Chojnacka, K. (2009) Biosorption and Bioaccumulation in Practice. Nova Science

Publishers, Inc. New York, US.