Adsorption and Removal of Asphaltene Using Synthesized Maghemite

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Adsorption and Removal of Asphaltene using Synthesized Maghemite and Hematite Nanoparticles Nazila Naghdi Shayan, and Behruz Mirzayi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef502494d • Publication Date (Web): 23 Feb 2015 Downloaded from http://pubs.acs.org on February 24, 2015

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Adsorption and Removal of Asphaltene using Synthesized Maghemite and Hematite Nanoparticles

Nazila Naghdi Shayan, Behruz Mirzayi* Chemical Engineering Department, University of Mohaghegh Ardabili, P.O. Box 179, Ardabil, Iran * Email: [email protected], Tel: +98-45-33512910; Fax +98-45-33512904

ABSTRACT In this study the synthetized maghemite (γ-Fe2O3) and hematite (α-Fe2O3) nanoparticles were used for the adsorption and removal of asphaltene from prepared solution. The co-precipitation of ferric and ferrous ions method was applied to synthesis the maghemite nanoparticles (MNPs) and followed by the hematite nanoparticles (HNPs) synthesis via calcination of the MNPs. Both of the synthesized nanoparticles were characterized by X-ray diffraction (XRD), vibrating sample magnetometry (VSM), transmission electron microscopy (TEM) and Fourier transform infrared spectrum (FT-IR). The prepared nanoparticles were used for the removal of asphaltene in adsorption process. The processes were carried out systematically by batch experiments to investigate the influence of different factors, such as temperature, contact time, initial concentration of asphaltene, and type of the nanoparticles. FT-IR spectrum of nanoparticles containing the adsorbed asphaltene (i.e. MNP/Asph and HNP/Asph) confirmed that asphaltene adsorb well on the nanoparticles. The experimental data of asphaltene adsorption isotherms were adequately fitted by the solid−liquid equilibrium (SLE) thermodynamic model indicating asphaltene self-association and multilayer adsorption onto the nanoparticles. The results also showed that maximum adsorption 1 ACS Paragon Plus Environment

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capacity of MNPs and HNPs is 108.1 and 45.8 mg/g, respectively. The kinetic data of adsorption of asphaltene on the synthesized nano-adsorbents were best described double-exponential model (DEM). The overall initial mass transfer rate calculation was indicated that the adsorption of asphaltene onto HNPs is faster than MNPs, and therefore the process in the presence of HNPs quickly reaches the equilibrium compared to MNPs. Furthermore, the calculated Gibbs free energies, entropies and enthalpies demonstrated that the adsorption of asphaltene onto MNPs and HNPs is endothermic and exothermic, respectively; and the process is spontaneous. This work showed that synthesized MNPs and HNPs can be considered as asphaltene nano-adsorbents, although MNPs is more effective. KEYWORDS: Asphaltene; Removal; Adsorption; Maghemite; Hematite; Nanoparticle

1. INTRODUCTION Crude oil is a continuum phase that includes thousands of different hydrocarbons and related substances, all with different physical and chemical properties. As such, determination of different features of hydrocarbon molecules is a difficult task. The SARA test separates crude oil in four main chemical fractions: saturates (S), aromatics (A), resins (R) and the asphaltenes (A). Asphaltenes are defined as a fraction of crude oil precipitating in light alkanes like pentane, hexane or heptane but this precipitate is soluble in aromatic solvents such as toluene and benzene.1 The asphaltene fraction contains the largest amount of heteroatoms (N, O, and S) and organometallic constituents (Ni, V, Fe) in crude oil. These atoms cause the polar structure of asphaltene, which can associate in crude oil and form aggregate.1 The strong tendency of asphaltene molecules to selfassociate leads to their precipitation from crude oil, and subsequently, to the formation of solid particles and deposits.1,

2

Asphaltene may deposit in oil reservoir rocks, oil wells, and adjacent

surface facilities. These molecules also enhance stability of water-in-oil emulsion and precipitate as a sticky layer in transportation pipelines and petroleum transport vessels.2 Precipitation of asphaltene as a solid phase and adsorption on solid surfaces is due to change in the process 2 ACS Paragon Plus Environment

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variables like composition, temperature or pressure which can disturb the thermodynamic equilibrium among the crude oil.2 There are some functional groups in asphaltene structure, like carboxlic, thiophenic, pyridinic, pyrrolic type, that they are responsible for the asphaltene adsorption.3 In several works the adsorption process of asphaltene on different non-metallic surfaces such as clays, rocks and silica4-6 and on metallic surfaces like iron, alumina, nickel powders7-11 was investigated. Ashtari et al. investigated separation of asphaltene from crude oils using asymmetric, ceramic monolith membranes with the pore sizes of 0.2 µm and 50 nm.12 The results showed that asphaltene particles are adsorbed on the membrane surface and through gradual aggregation a gel layer is formed on the surface of the membrane which effectively separated asphaltene particles from crude oil.12 Balabin et al. investigated adsorption of asphaltene from benzene solution on iron surface by combination of near-infrared (NIR) spectroscopy, Raman microscopy and atomic force microscopy (AFM).7 Nowadays, nanotechnology is among the fastest growing areas of science and technology that is cutting across many traditional boundaries. Reducing the size of adsorbents to the nano size range causes the enhancement of surface area and the capacity of nanoparticles in the adsorption processes. Nassar et al. used several metal nanoparticles for adsorption and cracking of asphaltene molecules during several studies.8,

13-17

They also investigated the effect of some aspects of nanoparticles such as their size,18 acidity, basicity and surface area on asphaltene adsorption process.9 Franco et al.10 and Abu Tarboush et al.19 reported adsorption, kinetic and thermodynamic equilibrium of asphaltene onto nickel oxide nanoparticles. The existing of NiO in the SHS (supported hygroscopic salt) samples resulted increasing in the number of binding sites with high asphaltene affinity.10 Among the metal oxides, iron oxides are easily found in nature and also synthesized readily in the laboratory. In this research we used two iron oxide nanoparticles (i.e. maghemite and hematite) for asphaltene adsorption process. Due to four unpaired electrons in 3d orbitals of iron atoms, they have strong magnetite state moment and in the result of subjecting external magnetic field they attain a net magnetic moment. If the size of particles decreased to smaller than about 20 nm, they would 3 ACS Paragon Plus Environment

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display superparamagnetic behavior.20 In the past decade, because of many technological applications of iron oxide nanoparticles, a lot of attention has been paid for the synthesis and utilization of these nanoparticles. According to available literature, they were applied in removal of heavy metal ions,21-23 dyes, pigments and viruses24, 25 from aqueous solutions, drug delivery26 and they have had also significant medical applications.27-29 Up to now several methods have been reported to synthesize maghemite and hematite nanoparticles: sol-gel,20 microemulsion,30 thermal decomposition, chemical reduction and sonochemical.31-33 In this work the co-precipitation method as a simple and cheap method was applied to synthesis maghemite nanoparticles. Hematite nanoparticles were synthesized by calcination of MNPs in muffle furnace. The synthetized nanoparticles were used for the adsorption and removal of asphaltene from toluene solution. The adsorption isotherm, kinetics and thermodynamics of the process were also studied. To the best of our knowledge this is the first report on asphaltene removal by γ-Fe2O3 and α-Fe2O3 nanoparticles, although the other type of nanoparticles has already been reported by several authors. 2. MATERIALS AND METHODS 2.1.

Materials

The materials used in this work were: FeCl3 (ferric chloride), FeCl2.4H2O (ferrous chloride tetra hydrate), HCl (hydrochloric acid, 37%), ammonium hydroxide (NH4OH, 25% of ammonia), methylene blue, n-heptane and toluene. All chemicals were of analytical grade and prepared from Merck Company. 2.2.

Preparation of asphaltene- toluene solutions

Asphaltene was extracted from crude oil sample according to IP-143 standard. Briefly, a specified amount of the crude oil sample was mixed with n-heptane at a 1:40 (g/L) ratio. Then the mixture was refluxed for three times in an extractor and then the mixture was kept in a dark place overnight. The asphaltene, waxy substances and inorganic material were collected by using Whatman (grade 42) filter paper. The waxy substances were removed by washing the filtrate with hot heptane in an 4 ACS Paragon Plus Environment

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extractor. After removal of waxy substances, the asphaltene were separated from inorganic materials by dissolution in hot toluene. The extraction solvent was evaporated under temperature of 110 °C. The asphaltene- toluene solution samples for the batch adsorption experiments were prepared by dissolving appropriate amount of the extracted asphaltene in toluene. All samples were prepared from a stock solution containing 1500 mg/L asphaltene diluted with different amounts of toluene. 2.3.

Preparation of nanoparticles

Maghemite nanoparticles (MNPs) were synthesized using the co-precipitation method.34 Briefly, the solutions of FeCl2.4H2O and FeCl3 with concentrations of 1 and 2 M were prepared, respectively. Then HCl solution with a concentration of 2 M was added to the FeCl2.4H2O solution. Subsequently, the resulting solution was placed on a magnetic stirrer and then NH3.H2O solution was added drop wise to the above mentioned solution. Consequently, the resultant black precipitate (MNPs) was separated by applying a magnetic field and rinsed with deionized water five times and with ethanol three times, respectively. Finally, the MNPs were dispersed in 40 mL ethanol and stored under refrigeration until their use. The hematite nanoparticles (HNPs) were also synthesized by calcination of MNPs in muffle furnace at 500 °C for 1 h. 2.4.

Batch adsorption experiments

The maximum peak of absorption for asphaltene- toluene solution was determined at wavelength of 297 nm using UV-vis spectrophotometer (Model PG 80+ Instrument Ltd). For batch adsorption experiments, the appropriate amount of nanoparticles was added to asphaltene- toluene solutions. The vials containing solution samples were shaken at 250 rpm in an incubator at 25 °C for 24 hours. Then the nanoparticles containing adsorbed asphaltene were separated via centrifugal force at 2500 rpm for 10 min. The concentrations of the asphaltene remaining in the supernatant were evaluated using UV-vis spectrophotometer. The amount of asphaltene adsorbed onto nanoparticles, qt (mg/g), was calculated using the mass balance according to Eq. 1:

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qt =

(C0 − Ct )V

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(1)

m

where V is the sample volume (L), C0 is the initial concentration of asphaltene in the solution (mg/L), Ct is the concentration of asphaltene in the solution at time t (mg/L) and m is the mass of the nanoparticles (g). To confirm the trends of obtained results, all the experiments were performed three times and the average of the data was applied. 3. RESULTS AND DISCUSSION 3.1.

Characterization of the adsorbents

3.1.1. X-ray diffraction (XRD) The crystalline structure of the nanoparticles were characterized by X-Ray diffraction (Philips X Pert equipment) with Kα radiation of Cu (λ=1.54 ˚A). X-ray diffraction patterns for MNPs and HNPs have been shown in Figure 1(a) and (b), respectively. The results indicated that synthesized nanoparticles have crystalline structure and they consist of pure phase. The mean crystallite size of nanoparticles was estimated to be about 10 nm for MNPs and 29 nm for HNPs using DebyeScherrer equation:

D=

kλ β cosθ

(2)

where, k=0.9 is the Scherrer constant, λ=0.154 nm is X-ray wavelength, θ is the diffraction angle in degrees and β is the peak width at half maximum height of the peak. 3.1.2. The size and morphology of nanoparticles The synthesis experiments were repeated for several times. The results showed that by adjusting the pH and the ionic strength of the precipitation medium, it is possible to control the size of the particles. The size and morphology of nanoparticles were studied using transmission electron microscopy (TEM, Zeiss - EM10C) at the operating voltage of 80 kV. The TEM image of MNPs (γFe2O3) is presented in Figure 2(a). Particles with almost spherical shape are observed. Figure 2(b) shows the TEM images of HNPs (α-Fe2O3). Irregularly shaped particles can be seen in this figure. 6 ACS Paragon Plus Environment

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The mean size of particles was also calculated using TEM observations. By averaging at least 30 particle diameters using ImageJ software the average MNPs and HNPs diameters were estimated 8 nm and 32 nm, respectively. The values were nearly coincident with the results obtained from the XRD analysis (∼10 nm for MNPs and ~29 nm for HNPs). 3.1.3. Estimation of surface area of nanoparticles In this study the method of methylene blue adsorption in liquid phase35-38 was applied for measuring the specific surface area of MNPs and HNPs. Determination of specific surface area using this method for various natural solids such as activated carbon, charcoal, graphite, and silica has shown that the Langmuir isotherm is an adequate description of the adsorption of the methylene blue onto adsorbent.35-38 The following equation was used to calculate the specific surface area of adsorbent:

S MB =

aMB N Aqm × 10−23 M

(3)

where, SMB is the specific surface area in m2/g, aMB is the occupied surface area of one molecule of methylene blue=197.2 oA²,39 NA is Avogadro’s number=6.023×1023 molecules/mole, M is the molecular weight of methyleneblue=319.86 g/mole and qm is the number of molecules of methylene blue adsorbed at the monolayer of nanoparticles in mg/g. qm was calculated using Langmuir equation. The approximated Langmuir model can be expressed by the following equation:

qe = q m

K L Ce 1 + K L Ce

(4)

where qe is the amount of adsorbate (methylene blue) adsorption onto the nanoparticles at equilibrium (mg/g), qm is the maximum adsorbed amount of methylene blue onto the nanoparticles to form a monolayer of the adsorbate (mg/g), Ce is the equilibrium concentration of adsorbate in solution phase (mg/L), KL is the Langmuir equilibrium adsorption constant related to the affinity of binding sites (L/mg). By combination of Eqs. 3 and 4, the specific surface area for MNPs and HNPs was ca. 47.36 and 45.15 m2/g, respectively. 3.1.4. Magnetic properties


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Magnetic properties of MNPs and HNPs were studied using vibrating sample magnetometry (VSM, MDK6, Daghigh-Kavir, Iran) at room temperature. The VSM curves of γ-Fe2O3 and α-Fe2O3 nanoparticles are shown in Figure 3(a) and (b), respectively. As it can be seen in Figure 3(a) there is no hysteresis, remanence and coercivity for MNPs, indicating that they have superparamagnetic behavior. In this figure, the maximum saturation magnetization of MNPs is also found about 59.62 emu/g. On the other hand in Figure 3(b) the magnetization curve of HNPs shows that the saturation is not reached the maximum value and the coercive field is ca. 1900 Oe. The VSM results indicated that contrary to the MNPs the HNPs have ferromagnetic behavior. 3.1.5. FT-IR spectrum In order to identify the presence of certain functional groups, the chemical structures of MNP, HNP, MNP/Asph, HNP/Asph and pure asphaltene were investigated using Fourier transform infrared (FTIR) spectroscopy (Perkin-Elmer RX-I) in the frequency range of 4000–400 cm-1. The KBr pellet technique was used for sample preparation. Figure 4(a) and (b) show FT-IR spectrum of MNP and HNP, respectively. As seen in Figure 4(a), for MNPs, the absorption peak around 473 and 601 cm-1 could be related to stretching vibrations of the Fe-O.40 Two other peaks in Figure 4(a) and (b) appeared at 1632 and 3388 cm-1, which attributed to absorption of water on both nanoparticles and the presence of hydroxyl functional group.41 Furthermore, according to Figure 4(b) the bands around 558 and 473 cm−1 are the characteristic absorption bands of the Fe–O of α-Fe2O3, which confirms the hematite phase.42 Figure 5(a), (b) and (c) show FT-IR spectrum of asphaltene and adsorbed asphaltene onto MNPs and HNPs, respectively. In Figure 5(a) the weak peak at 1033 cm-1 belongs to ethers or esters linkage present in the asphaltene molecules. The peak around 1376 cm-1 can be attributed to methyl bending vibrations. The sharp peak at 1455 cm-1 is due to CH2 or CH3 bending modes and the peaks at 1595 cm-1 and 1692 cm-1 can be assigned to the aromatic and carboxyl stretching vibrations, respectively. As shown in Figure 5(b) and (c), the peak at 1260 cm-1 corresponds to ethers or esters linkage. Three other peaks at 1378, 1456 and 1733 cm-1 attribute to methyl bending vibrations, CH2 or CH3 bending modes and carboxyl stretching vibrations, 8 ACS Paragon Plus Environment

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respectively. These peaks on Figure 5(b) and (c) confirmed asphaltene molecules adsorption onto both MNPs and HNPs. To test the materials solubility, the pure MNP and HNP were placed in the toluene solution three days and after sampling, it was analyzed by FT-IR. The result showed no changes in FT-IR spectrum of toluene solution indicating insolubility of synthetized nanoparticles. The result is in agreement with the reason below: Due to existence of hydroxyl groups on iron oxides surfaces, they have hydrophilic structure43 and therefore are insoluble in toluene. 3.2.

Optimization of amount of nanoparticles

To determine the optimum amount of nanoparticles for use in adsorption process, adsorption efficiency (E) and capacity (q) should be evaluated. These parameters were calculated by Eqs. 5 and 6, respectively:

E=

q=

(C0 − Ce ) ×100 C0

(C0 − Ce ) m

V

(5)

(6)

where, C0 and Ce are the initial and final concentrations of asphaltene solutions (mg/L), respectively, m is mass of nanoparticles (g) and V is volume of asphaltene solution (L). The various amounts of nanoparticles were added to the asphaltene solutions and the system was allowed to reach equilibrium. The range of nanoparticle dose was between 5 and 15 gnanoparticle/Lsolution. Figure S1(a) and (b) (see supplementary data) show adsorption efficiency versus mass of both MNPs and HNPs at different initial concentration of asphaltene solutions. According to the results, in the operating range of nanoparticle dose, increasing in mass of nanoparticles led to higher adsorption efficiency. The adsorption capacity in terms of amount of added nanoparticles is illustrated in Figure S2(a) and (b) (see supplementary data). The results showed that in the nanoparticle dose of about 10 g/L, maximum adsorption capacity is obtained for both MNPs and HNPs. In other words, the optimum amount of nanoparticles was 10 g per 1 liter of solution. This result is consistent with the results of previous works,8, 10, 14, 16 because increasing the amount of nanoparticles led to their


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accumulation and thereby the active surface sites for asphaltene adsorption were limited, resulting in a reduction of adsorption capacity. 3.3.

Adsorption isotherms

Adsorption isotherms of asphaltene onto MNPs and HNPs were modeled using the Langmuir (Eq. 4), Freundlich (Eq. 7), modified Langmuir (Eq. 8) models44 and solid−liquid equilibrium (SLE) model based on the Chemical Theory45 (Eq. 9) at different temperatures (i.e. 25, 40 and 50 oC).

qe = K F Ce1/ n qe = q m

C=

KCeX 1 + KCeX

ψ  ψH  exp 1 + K cψ  Nm 

(7) (8)

(9)

where, qe is the amount of asphaltene adsorbed onto the nanoparticles at equilibrium (mg/g), Ce is the equilibrium concentration of asphaltene in solution phase (mg/L), KF is the Freundlich equilibrium adsorption constant related to the adsorption capacity ((mg/g)(L/mg)1/n) and 1/n is the adsorption intensity factor (dimensionless). qm is the maximum adsorbed amount onto nanoparticles to form a monolayer of the adsorbate (mg/g), K is the modified Langmuir model constant and X is an exponent indicating the level of concentration dependence. In equation (9), C is the asphaltene concentration in the bulk phase (mg/g), H is the measured Henry’s law constant, which is an indicator of the adsorption affinity of asphaltene onto solid surface (mg/g), Nm is the maximum adsorption capacity of asphaltene (g/g), Kc is constant and represents the rapid association of asphaltene molecules once the primary sites are occupied (g/g). The parameters Kc and ψ are defined by:

Kc =

K T RT S MB

(10)


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ψ=

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− 1 + 1 + 4 K cξ

(11)

2K c

KT is the reaction constant for dimer formation, ξ is a constant (defined as ξ = N m N /( N m − N ) ) and N is the amount adsorbed (g/g). The root-mean-square error (RMSE) of isotherm models prediction is defined as follows:

∑i =1 (Ycal ,i − Yexp,i )

2

n

RMSE =

n

(12)

where Yexp and Ycal are respectively experimental and modeled values of variables (qe or C) in Eqs. 4 and 7-9. Tables 1 and 2 show the calculated parameters of isotherm models for asphaltene adsorption onto MNPs and HNPs, respectively. As indicated by the values of R2, the experimental data were adequately fitted by SLE model. According to Figures 6 to 8 at low concentrations of asphaltene all the models could predict well the experimental data. While, at high concentration the prediction of modified Langmuir model is acceptable, the SLE is superior to the other models. This indicates the dominate potential of SLE model in describing adsorption of complex asphaltene molecules onto nanoparticles. Comparison of calculated RMSE for all the models confirmed the mentioned good performance of SLE model. Excellent agreement between the SLE model and experimental results also demonstrated high tendency of asphaltene molecules to dimerization and aggregation. Accordingly, it can be inferred that the asphaltene adsorption onto nanoparticles was multi-layered. In other words, after forming the first layer, due to high affinity of asphaltene molecules for aggregating, the later molecules are attached on the first layer and the multilayer adsorption is provided. Furthermore, based on the SLE model and comparing the amount of asphaltene adsorbed on the nanoparticles (parameter Nm in Tables 1 and 2), it was found that, maximum adsorption capacity of MNPs is more than that for HNPs (108.1 against 45.8 mg/g). The difference was attributed to the smaller size and larger surface area of MNPs compared with HNPs. Another important factor that 11 ACS Paragon Plus Environment

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may contribute to the adsorption capacity could be the acidity of the surface of MNPs.9 According to the calculation of pH at the point of zero charge (pHpzc) by using the acidity constants (pKa1 and pKa2), it was shown by Cornell and Schwertmann that maghemite and hematite has the acidic and basic surface, respectively.43 If we set pKa1=3.8, pKa2=9.4 for maghemite and pKa1=8.86, pKa2=10.1 for hematite, the parameter pHpzc=(pKa1+ pKa2) can be calculated as 6.6 (acid range pH) for maghemite and 9.48 (base range pH) for hematite. Similar observations were reported by other researchers for the adsorption of asphaltenes onto three types of aluminas in the following order: acidic > basic and neutral.9 In addition, comparison of X values of modified Langmuir model for asphaltene adsorbed onto the nanoparticles showed that X value for MNP/Asph is larger than that of HNP/Asph, indicating high concentration dependence of MNP/Asph. Thus, for a given adsorption capacity, asphaltene adsorption on the surface of MNPs is greater than HNPs. This finding is consistent with the result obtained by comparing the amount of parameters Nm, Kc and H in SLE model for MNPs and HNPs. The trends of the values of Kc and H in Tables 1 and 2 suggest that the adsorption affinity in the case of MNP/Asph is stronger than that of HNP/Asph. The large value of parameter H for MNPs indicates its high adsorption affinity, and the low value of Kc suggests that the extent of asphaltene self-association on the MNPs is greater than that on HNPs. As a result, MNPs showed high asphaltene absorption capacity. Furthermore, the difference in adsorption capacity between MNPs and HNPs could be attributed to the different degree of interactions between the nanoparticle surface and asphaltene molecules. The existence of functional groups in asphaltene structure, like carboxlic, thiophenic, pyridinic, pyrrolic and sulfite type is responsible for favorable intraparticle interaction between asphaltene and the surface.9 It can be inferred that the orientation of such various functional groups when they are exposed to chemically different surfaces (i.e. MNPs and HNPs) is one of the major factors that could contribute to surface adsorption capacity.


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For comparing the ability of synthetized nanoparticles with some previously used commercial ones the maximum amount of adsorbed asphaltenes per unit surface area of nanoparticles (Qm, mg/m2) is reported in Table S1 (see supporting information). 3.4.

Adsorption kinetics

A double-exponential model (DEM) proposed by Wilczak and Keinath46 was used to investigate asphaltene adsorption kinetic onto the nanoparticles. Adsorption kinetic was evaluated at different initial concentrations of asphaltene (100, 500 and 1000 mg/L) and 25 °C for contact time of 12 hours. The so-called double-exponential model which, also can be considered as a diffusion model is presented as follows:47

qt = qe −

D1 D exp (− K1t ) − 2 exp (− K 2 t ) ma ma

(12)

where qe and qt are the amount of adsorbed asphaltene onto nanoparticles (in mg/g) at equilibrium state and time t, respectively; t represents the time in hours, D1 and D2 (in mg/L) are the adsorption coefficients, K1 and K2 (in h-1) are the mass transfer coefficients and ma is the amount of adsorbent in the solution (g/L). This model describes well the mechanism of adsorption and mas transfer diffusion through the two-step kinetics of the adsorption: the rapid and the slow step. In this equation the parameters with subscript 1 and 2 represents the rapid and slow step, respectively. During the rapid step the most of the adsorbate diffuses toward adsorbent and adsorption happens within a few minutes, whereas on the second step, adsorption is more slowly. Based on the adsorption mechanism of DEM the rapid step involves both external and internal diffusion however the slow step is controlled only by the intraparticle diffusion. By using the experimental kinetic adsorption data (qt vs. t) and applying the mathematical curve fitting method, the parameters of Eq. 12 (D1, D2, K1 and K2) were determined at the optimum dose of nanoparticles (ma=10 g/L). Table 3, shows the obtained kinetic parameters. According to the values of R2, the experimental data were fitted well by double-exponential model. The asphaltene adsorption kinetic data (experimental and modeling) in Figure S3(a) and (b) (see supplementary 13 ACS Paragon Plus Environment

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data) was suggested that a two-step mechanism occurs for both MNPs and HNPs. In the first portion a rapid adsorption occurs in less than 1 h then the process goes toward equilibrium slowly. However, with increasing initial concentration of asphaltenes this time period was shorter. According to the results about 90% removal for aspahltene was happened within 1 h. By using the calculated parameters and derivation form of Eq. 12, the overall initial mass transfer rate (r0) can be determined:47

r=

dqt D1 D = K1 exp (− K1t ) + 2 K 2 exp (− K 2 t ) dt ma ma

(13)

For initial conditions, t=0:

r = r0 =

D1 D K1 + 2 K 2 ma ma

(14)

The overall initial mass transfer rate for MNPs and HNPs are reported in Table 3. According to the obtained results the adsorption of asphaltene onto HNPs is faster than MNPs. In other words, the asphaltene adsorption process onto HNPs quickly reaches equilibrium compared to MNPs. The shape of curves in Figure S3(a) and (b) (see supplementary data) confirms this result. This can be due to relatively high adsorption capacity of MNPs, as explained in Adsorption isotherms section, which needs longer time to reach the surface saturation and equilibration. 3.5.

Thermodynamic studies

Study of asphaltene adsorption onto MNPs and HNPs from thermodynamics point of view helps us to better understand the temperature effect on this process. By using Van't Hoff equations (Eqs. 15 and 16) thermodynamic studies of adsorption process can be evaluated:

∆ Gοads = − RT ln(K )

(15)

∆ H οads ∆ Sοads ln(K ) = − + RT R

(16)

where R is the universal ideal gas constant (=8.314 J /mol.K), T is the temperature (K), and K is the adsorption equilibrium constant (dimensionless). K can be expressed as KLCs, where KL is the


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equilibrium Langmuir constant (L/mmol) and Cs is the solvent molar concentration (mM), which can be calculated from the density and molecular mass of toluene. Molecular mass of asphaltene in the range of 750-5000 g/mol48 was used to determine the thermodynamic parameters and also the equilibrium Langmuir constant. The calculated parameters ° ° are given in Table 4. The sign of calculated Gibbs free energy (∆ and entropy (∆ are

negative and positive, respectively, that indicating of feasibility and spontaneity of thermodynamic ° process for both the nanoparticles. The positive and negative values of the enthalpy (∆ ,

respectively, for MNPs and HNPs indicated that asphaltene adsorption onto MNPs is endothermic while onto HNPs is exothermic in nature. The surface acidity or basicity of the nanoparticles may be responsible for this opposite thermodynamic behavior. The acidic surface of maghemite nanoparticles increases their tendency to attract the hydroxyl groups of asphaltene molecules. While hematite nanoparticles tend to adsorb proton (H+) due to their basic surface. Since proton adsorption onto iron oxides exhibited heat production,43 asphaltene adsorption on hematite is exothermic while it is endothermic on the maghemite. Thermodynamic studies also showed that as temperature and asphaltene molecular mass increase, the standard Gibbs free energy decreases. These values are in well-agreement with the results reported in the literature on the adsorption of asphaltene onto metal surfaces.8, 49 4. CONCLUSION In this work, maghemite and hematite nanoparticles were prepared by a facile and cheap method and were employed, for the first time, for asphaltene removal from toluene solution. The optimum value for dose of nanoparticles (mass of the nanoparticles/volume of solution) was obtained about 10 g/L. Temperature, initial concentration, time, particle’s surface area and surface acidity dependency of asphaltene adsorption onto MNPs and HNPs were studied. The adsorption isotherms were determined by Freundlich, Langmuir, modified Langmuir and solid-liquid equilibrium (SLE) models, at different temperatures. For both MNPs and HNPs, the adsorption data were fitted well by the models SLE model. The results showed high adsorption affinity of MNPs and large amount of 15 ACS Paragon Plus Environment

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asphaltene self-association onto MNPs. Kinetic experiments showed that, asphaltene adsorption onto MNPs and HNPs followed double-exponential model. According to this model the kinetics of the adsorption include two-step: the rapid and the slow step. The obtained results revealed that in the first portion a rapid adsorption was occurred in less than 1 h and about 90% removal for aspahltene is happened in this period of time. In addition, the overall initial mass transfer rate calculation showed that the adsorption of asphaltene onto HNPs is faster than MNPs. While for HNPs

the equilibrium could be achieved faster than MNPs, higher adsorption took place onto the MNPs (108.1 against 45.8 mg/g), owing to the small size, high surface area, acidity of the MNPs surface and the orientation of various functional groups present in the structure of asphaltene molecules. The results confirmed the ability and high adsorption capacity of super paramagnetic MNPs in comparison with HNPs and some before used nanoparticles. Furthermore, calculation of thermodynamics parameters (Gibbs free energy, entropy and enthalpy) of asphaltene adsorption onto MNPs and HNPs showed: (1) feasibility and spontaneity of adsorption process, (2) exothermic and endothermic nature of process for HNPs and MNPs, respectively. The opposite thermodynamic behavior of two nanoparticles was attributed to the surface acidity and basicity of MNPs and HNPs, respectively.

ACKNOWLEDGMENT We would like to give special thanks to Dr. Ali Nematollahzadeh for help in writing and finishing this paper. SUPPORTING INFORMATION AVAILABLE This information is available free of charge via the Internet at http://pubs.acs.org/.


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Table 1. Parameters of Isotherms for Asphaltene Adsorption onto γ-Fe2O3 Nanoparticles

models Freundlich

temperature (oC) 25 40 50

Langmuir 25 40 50 modified Langmuir 25 40 50 SLE model 25 40 50

1/n 0.398 0.399 0.388 qm(mg/g) 57.61 61.91 61.31 qm(mg/g)

parameters of isotherms KF((mg/g)(L/mg)1/n) 4.823 5.316 5.726 KL(L/mg) 0.016 0.0175 0.0195 K ((L/mg)1/X)

X

77.58 75.13 72.91 H(mg/g) 1.64 1.25 1.17

0.0289 0.0278 0.0303 Kc(g/g) 0.005006 0.005354 0.00528

0.703 0.774 0.782 Nm(mg/g) 108.1 106.7 105.5

R2

RMSE

0.9803 0.9752 0.9725

2.823 3.402 3.605

0.9835 0.9859 0.9848

2.589 2.566 2.675

0.9894 0.9895 0.9881

2.268 2.425 2.300

0.9935 0.9964 0.9949

0.017 0.012 0.012

Table 2. Parameters of Isotherms for Asphaltene Adsorption onto α-Fe2O3 Nanoparticles

models Freundlich

temperature (oC) 25 40 50

Langmuir 25 40 50 modified Langmuir 25 40 50 SLE model 25 40 50

1/n 0.282 0.259 0.253 qm(mg/g) 29.97 25.31 24.04 qm(mg/g)

parameters of isotherms KF((mg/g)(L/mg)1/n) 4.867 4.81 4.745 KL(L/mg) 0.0215 0.0256 0.0244 K ((L/mg)1/X)

X

49.31 49.07 34.05 H(mg/g) 1.87 2.49 1.68

0.0638 0.0746 0.0763 Kc(g/g) 0.002073 0.001879 0.001889

0.488 0.411 0.531 Nm(mg/g) 45.8 42.0 36.7


R2

RMSE

0.9883 0.9886 0.9828

1.187 0.993 1.163

0.9663 0.9651 0.9747

2.012 1.741 1.413

0.9938 0.9909 0.9899

0.945 0.945 0.975

0.9940 0.9933 0.9855

0.026 0.029 0.057

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Table 3. Kinetic Model Parameters for the Adsorption of Asphaltene onto γ-Fe2O3 and α-Fe2O3 Nanoparticles

nanoparticles γ-Fe2O3 α-Fe2O3

C0 (mg/L) 100 500 1000 100 500 1000

qe,exp (mg/g) 9.1444 36.8074 56.0780 8.8755 26.7668 30.6456

qe (mg/g) 9.0238 36.4036 56.0989 8.8426 26.4430 30.2453

K1 (h-1) 0.5758 0.3581 0.3456 0.5883 0.4652 0.4344

Double-exponential model K2 D1 D2 -1 (h ) (mg/L) (mg/L) 0.0026 83.8110 7.6300 0.0048 239.5960 127.5110 0.0141 280.2580 280.4460 0.0036 84.2510 4.4980 0.0046 225.8400 41.7240 0.0165 275.6340 70.6920

R2 0.9987 0.9981 0.9996 0.9994 0.9974 0.9973

r0 mg/g.h 4.83 8.64 10.08 4.96 10.52 12.09

Table 4. Calculated Parameters of van't Hoff Equation for The Adsorption of Asphaltene onto γ-Fe2O3 and αFe2O3 Nanoparticles at Different Temperatures

nanoparticles γ-Fe2O3

asphaltene molecular mass (g/mol) 800 2000 4500

α-Fe2O3

800 2000 4500

temperature (K)

K

-∆ ° (kJ/mol)

-∆° (kJ/mol)

∆° (J/mol.K)

R2

298 313 323 298 313 323 298 313 323 298 313 323 298 313 323 298 313 323

121539.4 132341.2 147373.1 303848.5 330853 368432.3 683659.1 744419.4 828973.8 184839.6 193148.7 194281.7 462099 482871.7 485704.4 1039723 1086461 1092835

29 30.68 31.95 31.27 33.07 34.41 33.28 35.18 36.59 30.04 31.67 32.7 32.31 34.05 35.16 33.74 35.38 36.48

-5.99

117.39

0.96

-5.99

125.04

0.96

-5.99

131.77

0.96

1.66

106.42

0.93

1.66

114.07

0.93

1.66

120.802

0.93


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Figure 1. XRD patterns of γ-Fe2O3 (a) and α-Fe2O3 (b) nanoparticles 70x21mm (600 x 600 DPI)

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Figure 2. TEM images of the γ-Fe2O3 (a) and α-Fe2O3 (b) nanoparticles 234x89mm (300 x 300 DPI)


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Figure 3. VSM magnetization curve of γ-Fe2O3 (a) and α-Fe2O3 (b) nanoparticles 81x30mm (600 x 600 DPI)


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Figure 4. FT-IR spectrum of γ-Fe2O3 (a) and α-Fe2O3 (b) 116x88mm (600 x 600 DPI)


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Figure 5. FT-IR spectrum of asphaltene (a) adsorbed asphaltene onto γ-Fe2O3 (b) and α-Fe2O3 (c) 124x100mm (600 x 600 DPI)


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Figure 6. Adsorption isotherm of asphaltene onto MNPs (a) and HNPs (b) at 25 oC 66x18mm (600 x 600 DPI)


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Graphical Abstract 104x77mm (600 x 600 DPI)


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Adsorption and Removal of Asphaltene Using Synthesized Maghemite

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