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Applied Clay Science

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Research paper

Thermoexfoliated and hydrophobized vermiculites for oleic acid removal

Celia Marcos⁎, Roberto Menéndez, Irene RodríguezDpto. Geología e Instituto de Organometálica “Enrique Moles”, Universidad de Oviedo, C/. Jesús Arias de Velasco s/n, 33005 Oviedo, Spain

A R T I C L E I N F O

Keywords:VermiculiteOleic acid adsorptionThermoexfoliationHydrophobization

A B S T R A C T

The aim of the present study was to investigate the removal efficiency of oleic acid from aqueous solutions bythermally expanded and hydrophobized vermiculites. The expansion of the samples was carried out in a furnaceat 1000 °C for 2 min. The hydrophobization using polymethyl-hydro-siloxane as hydrophobing derivatizingreagent was performed by immersion and hydrophobizing atmosphere methods. The investigation was startedwith two vermiculites from Brazil, the Goiás and Piauí states, respectively. After some previous tests the Goiásvermiculite was selected to investigate five effects: 1) Adsorption at different pH. 2) Adsorption at differentconcentrations of hydrophobizing reactive and different weights of vermiculite. 3) Adsorbent mass. 4) Influenceof the number of stages of adsorption. 5) Influence of salinity. Oleic acid uptake was quantitatively evaluatedusing the Langmuir, Freundlich and Dubinin–Kaganer–Radushkevich (DKR) models. In addition, the adsorptionequilibrium was described well by the DKR isotherm model, indicative of a cooperative process. The maximumadsorption capacity obtained with DKR model was of 1.29 mol/g of oleic acid. The hydrophobization Goiásvermiculite and the adsorption of oleic acid were verified using infrared spectroscopy. The findings of this studyshowed that this thermally expanded and hydrophobized vermiculite: 1) was very suitable for the recovery ofoleic acid in water; 2) the adsorption increased with the increase of the salinity of the aqueous medium; 3) it waspossible to reuse vermiculite unsaturated to recover more oleic acid.

1. Introduction

There are many industrial activities (mining, steel mills, refineries,etc.) and services that have problems with water contaminated with oil.Adsorption and ion exchange processes can be used for the removal oforganic substances from water. A series of clay minerals can exhibitadsorption properties as montmorillonite, vermiculite, hectorite andmicas (e.g. Schlegel et al., 1999; Álvarez-Ayuso and García-Sánchez,2003; Juang et al., 2004). Some authors (e.g. Marcos et al., 2009 andMarcos and Rodríguez, 2010) appointed that vermiculites with highmica-like content, i.e. layered structures similar to mica, expanded byabrupt heating, might be an optimal product for the adsorption ofsubstances of environmental impact, like aromatic compounds, toxicmetals or pesticides, due to their high expansive capacity and highspecific surface.

The importance of vermiculite is based on its layered structure(Shirozu and Bailey, 1966; Calle et al., 1988; Argüelles et al., 2010). Itis well known that when vermiculite is strongly heated at high tem-perature (≈1000 °C) during a short period of time, the water situatedbetween layers is quickly converted into steam, exerting a disruptiveeffect upon the structure (Walker, 1961; Wada, 1973a,b; Baumeisterand Hahn, 1976; Obut and Girgin, 2002; Marcos et al., 2009; Marcos

and Rodríguez, 2010). As a consequence, a highly porous thermal-ex-panded material is formed. The thermal exfoliation occurs in a directionapproximately perpendicular to the layers. This property of exfoliationis explained as a consequence of mosaic distribution of the differentmineral phases within the commercial vermiculite and the lateralboundaries between vermiculite, mica and chlorite layers are postu-lated to prevent or impede the escape of gas from a particle, resulting inexfoliation when the pressure exceeds the interlayer bonding forces thathold the layers together (Hillier et al., 2013). Expanded vermiculiteabsorbs a considerable amount of water, but does not wet easily. On theother hand, considering its adsorption capacity, vermiculite can behydrophobized and used to adsorb organic substances such as oleicacid. Authors as Silva da et al. (2003) or Silveira et al. (2006) in-vestigated the adsorption capacity of oleic acid with thermoexfoliatedvermiculites and with hydrophobized vermiculites, focusing its re-search on the adsorption capacity of oleic acid using vermiculites withdifferent granulometries.

The aim of this investigation was to evaluate the adsorption capa-city of hydrophobic compounds, of low solubility in water, by ther-moexfoliated and hydrophobized commercial vermiculites using poly-methyl-hydro-siloxane as hydrophobizing reagent. To evaluate thisadsorption capacity the topics investigated were: 1) pH, 2)

http://dx.doi.org/10.1016/j.clay.2017.09.026Received 31 December 2016; Received in revised form 19 September 2017; Accepted 20 September 2017

⁎ Corresponding author.E-mail address: cmarcos@uniovi.es (C. Marcos).

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0169-1317/ © 2017 Elsevier B.V. All rights reserved.

MARK

concentration of hydrophobizing reactive and weight of vermiculite, 3)adsorbent mass, 4) influence of the number of stages of adsorption, 5)influence of salinity, 6) accuracy of the adsorption process. The oleicacid (representative of the alkanes group) was taken as model moleculeof adsorbent substances with high hydrophobic character.

2. Experimental

2.1. Materials

Vermiculites from the Piauí state (P) and Goiás state (G) of Brasilwere used as the starting materials for this study. They were present inform of small packets of a golden brown color the Goiás sample andbrownish green the Piauí sample, with maximum dimensions of ca.1–2 mm in diameter and 0.5–1 mm thickness.

Chemicals: Polymethyl-hydro-siloxane (viscosity to 20°14–40 mPas, d = 1.004 g/m, refractive index 1.398). Oleic acid 90%.Acetone rectapur prolab. Solutions: HCl 0.1 M; NaOH 0.1 M; 0.05, 0.1;NaCl 0.5, 0.75 and 1 M. All chemicals used were of analytical reagentgrade and not made them any purification prior to use.

2.2. Methodology

The methodology followed in this work consisted of the followingsteps: 1) Cleaning of some of the vermiculites. 2) Modification bythermoexfoliation and hydrophobization of cleaned and uncleanedvermiculites. 3) Oleic acid adsorption by cleaned and uncleaned ther-moexfoliated and hydrophobized vermiculites. 4) Characterization ofthermoexfoliated and hydrophobized vermiculites and oleic acid ad-sorption.

2.2.1. Cleaning processThe elimination of other minerals for obtaining cleaned samples was

made by two methods. The hand-picking method which consisted ofseparating easily identifiable vermiculite from impurities like ironoxides, quartzt, etc. picked by hand. The flotation method consists ofdispersing the samples in a beaker of 400 mL with distilled water. Afterreaching the equilibrium between the sample and the aqueous phase(15–30 min) the floating ore was filtered through a filter paper andallowed to dry at 60 °C in a furnace for 24 h.

2.2.2. Modification of vermiculites2.2.2.1. Thermoexfoliation. The samples placed into porcelain crucibleswere heated at 1000 °C in an furnace for 2 min to expand them, oncestabilized the temperature (for 2 h approximately). The possibleimpurities present in the thermoexfoliated samples from Piauí andGoiás (henceforth PE and GE, respectively) were eliminated by thefloating method.

2.2.2.2. Hydrophobization. The thermoexfoliated samples weresubsequently submitted to a hydrophobization process. Two methodswere carried out using polymethyl-hydro-siloxane as hydrophobingderivatizing reagent.

1. Immersion method, based on BR Patent 39004025 (Martins, 1990):1) 1–1.5 g of each thermoexfoliated sample, PE and GE sampleswere deposited into both glass vessel. 2) The hydrophobicizing re-agent dissolved in acetone was added slowly, ensuring a thickness of1 cm reagent above the level of solid sample. 3) Both vessels wereplaced in a desiccator with sodium carbonate, keeping them in op-timal humidity (15–20% HR) and temperature (15–20 °C) for 72 h.4) The reagent excess was carefully removed and samples washedseveral times with acetone, removing in each sample liquid excess.The hydrophobized and washed samples were kept in the furnace(60–80 °C) for 24 h.

2. Hydrophobizing atmosphere method: 1) In a nebulizer device

containing 5 mL of hydrophobizing reagent was passed a stream ofair nebulizing the reagent by Venturi effect, forming a fine aerosolthat filled the flasks containing 0.37 g of samples (PE and GE, re-spectively). 2) After 30 min the samples were washed several timeswith acetone and then kept in a furnace for 24 h.

2.2.3. AdsorptionFor a first evaluation of the behaviour of both Goiás and Piauí

vermiculites on the adsorption of oleic acid two experiments wereconducted: 1) Oleic acid adsorption at different pH and concentrationsof hydrophobizing reactive with uncleaned and cleaned Goiás vermi-culite and uncleaned Piauí vermiculite previous to thermoexfoliationand hydrofobizing modification. The elimination of other minerals forobtaining cleaned samples was made by hand-picking. 2) Oleic acidadsorption with cleaned vermiculites from Goiás and Piauí. Thecleaning process was made by the both hand-picking and flotationmethods. After thermoexfoliation at 1000 °C for 2 min the hydro-phobizing process was made with polydimethylsiloxane 5% (because itwas the best result of adsorption obtained in the first experiment) byimmersion method. The adsorption was carried out at pH 4.

According to the first results of adsorption (Table 1) the authorsdecided to continue the investigation with Goiás vermiculite, makingthe following experiments: 1) Adsorption at different pH, at 19.5 °C and69% of relative humidity. 2) Adsorption at different concentrations ofhydrophobizing reactive and different weights of vermiculite at 19.5 °Cand 69% of relative humidity, to investigate the retention capacitymaximum of vermiculite. 3) Adsorbent mass, to check the minimumamount of vermiculite necessary for full recovery of a certain amount ofoleic acid; experiments were performed at pH 4, 17.7 °C and 47% re-lative humidity. 4) Influence of the number of stages of adsorption at21.1 °C and 45% of relative humidity, for the purpose of investigate ifvermiculite unsaturated of adsorbate can be used to recover more oleicacid without previous washing of the same. For this, 0.3 g of vermi-culite from Goiás and 0.15 g of oleic acid were used in the first ad-sorption step; in the second stage the same sample as in the first stageand other 0.15 g of oleic acid were used. 5) Influence of salinity, inorder to investigate the influence that the salt content in a marine en-vironment can have on the oleic acid adsorption. The experiments werecarried out at 16.6 °C and 57.7% of relative humidity. 6) Accuracy ofthe adsorption process. For this, two independent experiments wereperformed at 18.2 and 21 °C and 61 and 55% of relative humidity,respectively, and the repeatability and reproducibility of the adsorptionprocess was evaluated.

The experiments were conducted as follows: In a beaker 50 mL ofdistilled water and 0.3 g of oleic acid (0.15 g in each step in the ex-periment 4; 0.48 g in the second experiment 6) were added, trying toform a drop of acid in the beaker center and preventing adhesion to thewalls. Later, the content was poured in a flask with 50 mL of distilledwater, ensuring that the acid drop pass full. Finally, distilled water wasadded to obtain 300 mL of solution. Adjustment of pH was performed

Table 1Oleic acid absorbed by thermoexfoliated and hydrophobized vermiculites at different pHand concentrations of hydrophobizing reactive.

Sample Hydrophobizing reactive (%) pH oleic acid adsorbed (%)

Goiás uncleaned 5 2 67.65Goiás cleaned 94.13Piauí 74.16Goiás uncleaned 4 64.90Piauí 68.66Goiás uncleaned 20 49.08Goiás cleaned 48.83Piauí 37.19Goiás uncleaned 70 2 20.30Goiás cleaned 21.00Piauí 19.90

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by adding the amount of HCl or NaOH required to reach the desired pHas shown in Table 2. The flask was manually stirred for 3 min so that anemulsion of oleic acid in water was formed. Then 0.3 g of exfoliated andhydrophobized vermiculite were added and stirred continued another5 min. Elapsed that time, the sample was filtered through filter paperand dried in an furnace (60–80°) for 24 h. The sample was weighed atroom temperature and the oleic acid retained by the vermiculite wasobtained by difference in weight percentage. All experiments werecarried out up to five times, three in the experiment 4, the concentra-tions were given as average values, and the standard deviations did notexceed 5% of the respective means.

2.2.4. Adsorption isothermsIsotherms allow to derive the nature of the processes acting at the

surface of a material and as such are extensively used to evaluate theeffectiveness of adsorption both in different conditions and with dif-ferent adsorbents and adsorbates (Talaat et al., 2011; Yavuz et al.,2003; Hinz, 2001). These isotherms are mathematical models that de-scribe the distribution of the adsorbate species among liquid and ad-sorbent, based on a set of assumptions that are mainly related to theheterogeneity/homogeneity of adsorbents, the type of coverage andpossibility of interaction between the adsorbate species. Langmuir(Langmuir, 1916), Freundlich (Freundlich, 1906) and Dubinin–Kaga-ner–Radushkevich (DKR) (Dubinin and Radushkevich, 1986) isothermmodels were fitted to the adsorption data and their constants wereevaluated. Satisfactory conformity between experimental data and themodel-predicted values was expressed by the correlation coefficient(R2).

The linear expression of Langmuir isotherm:

= ⎡⎣⎢

⎤⎦⎥

+q q K C q1 1 1 1

e m L e m (1)

The linear expression of Freundlich isotherm:

= +q Kn

Clog log 1 logm F e (2)

The adsorption capacity qe (μg/Kg) of the oleic acid by PE and GEvermiculites, was calculated by using the formula:

= −q C Cm

V( )e

i e(3)

In Eqs. (1)–(3): qe is the corresponding adsorption capacity (mg/g)of adsorbent; Ce is the solute concentration in the equilibrium (mg/L);KL (L/mg) and qm (mg/g), maximum adsorption capacity, are constantsrelated to the energy of adsorption and energy or net enthalpy of ad-sorption, respectively; KF (mg/g) and n are the constants, which mea-sure the adsorption capacity and intensity and an indication of howfavorable the adsorption processes is, respectively; Ci is the initialconcentration of solute in the solution (mg/L); m is the mass of ad-sorbent in Kg and V is the dissolution volume in mL.

The Dubinin–Kaganer–Radushkevich (DKR) isotherm has the form:

= −C X βεln lnads m2 (4)

where Cads is the number of metal ions adsorbed per unit weight ofadsorbent (mol/g), Xm (mol/g) is the maximum adsorption capacity, β(mol2/J2) is the activity coefficient related to mean adsorption energy,and ε is the Polanyi potential, which is equal to:

= +( )ε RT Cln 1 1e (5)

where R is the gas constant (8.314 kJ/mol K) and T is the temperature(K). The saturation limit Xm may represent the total specific micro-porevolume of the sorbent. The slope of the plot of ln Cads versus ε2 gives β(mol2/J2) and the intercept yields the adsorption capacity, Xm (mol/g).

The adsorption energy was calculated using the following re-lationship:

= −E β1

2 (6)

2.3. Characterization

The raw and thermo exfoliated vermiculites were previously de-scribed and characterized by X-ray diffraction and electron microprobeanalysis (Marcos et al., 2009). The total iron content was higher in theG (9.58%) sample than in the P sample (6.69%).

The Scanning Electron Microscopy (SEM) using a JEOL-6100equipment was used to study thermoexfoliated vermiculites afterheating abruptly at 1000 °C during 1 min.

The infrared spectra (IR) were used to verify the hydrophobizationof Goias vermiculite and the adsorption of oleic acid. The IR were re-corded on a Perkin Elmer FT-IR PARAGON 1000 spectrophotometer bydispersing the solid sample in KBr at a ratio of approximately 1 partsample to 100 parts KBr, in the range of 4000–400 cm−1. MettlerAnalytical Balance Model AE163 was used to obtain the oleic acid re-tained in the adsorption experiments.

3. Results and discussion

3.1. Oleic acid adsorption

Percentages of the oleic acid adsorption by thermoexfoliated andhydrophobized vermiculites from Goiás and Piauí, both cleaned anduncleaned, at different pH and concentration of hydrophobizing re-active are in Table 1. From its analysis could extract that: 1) The ad-sorption of oleic acid (from 20 to 94%) was regardless of the vermi-culite origin. 2) The minimum adsorption for all vermiculites, around20%, and the pH did not seem to be a critical factor in the adsorptionprocess. 3) Best results for oleic acid adsorption were obtained withpH 2 and a hydrophobicizing reagent at 5% in acetone, as at higherconcentrations the sample was saturated and the adsorption was about20%. In the latter conditions, the presence of impurities caused greateradsorption of the oleic acid because the exfoliation capacity is greaterand so its adsorption capacity also (e.g. Marcos et al., 2009 and Marcosand Rodríguez, 2010).

Results of the oleic acid adsorption by the investigated samples inrelation to the cleaning procedure of impurities (Table 3) showed thatthe adsorption percentage was higher: 1) for the Goiás sample than forthe Piauí one; 2) and for the Goiás vermiculite cleaned by flotationprocedure than by hand-picking. Therefore, it follows that the influenceof the cleaning procedure of impurities in vermiculite was critical foreffective retention of oleic acid. On the other hand, in the samplescleaned by flotation the non-exfoliated fraction of vermiculite was alsoseparated because its structural characteristics were considered not

Table 2Adjustment of pH with HCl or NaOH.

pH 2 4 7 9 11 12

Reagent HCl NaOHAmount (μl) 15,000 200 4000 500 250 150Molarity 10−1 10−1 10−1 5 × 10−2 10−1 5 × 10−2

Table 3Influence of the cleaning procedure of impurities on the oleic acid adsorption.

Sample Cleaning procedure Oleic acid adsorbed (%)

GoiásPiauí

Hand-picking 69.168.9

GoiásPiauí

Flotation 83.976.0

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suitable for adsorption. This would again evidence (Hillier et al., 2013)that vermiculite without exfoliate would correspond to vermiculite instrict sense or vermiculite with very few interestratified phases andtherefore with lower exfoliation and adsorption capacity.

The oleic acid adsorption at different pH (Fig. 1) with thermo-exfoliated and hydrophobicized vermiculite from Goiás, uncleaned andcleaned by flotation, showed that at pH greater than 7 oleic acid ad-sorption substantially decreased due to emulsification problems andsoap formation, resulting in the preferential solubilization of oleic acidin aqueous medium. No significant variation in the oleic acid adsorp-tion between pH 2–7 was observed, which would be advantageous sincethe pH of natural waters varies between 6 and 8.

The oleic acid adsorption at different concentrations of hydro-phobizing reactive plotted in Fig. 2 showed that vermiculite saturatedwhen ratio in weight vermiculite/oleic acid was equal to 1, i.e. 0.3 g.However, in the experiments with higher amounts of oleic acid and thesame weight of vermiculite (0.3 g) the saturation of the mineral wasreached when the weight ratio of oleic acid/vermiculite was equal to1.5. The ratio in weight vermiculite/oleic acid, defined as adsorptionfactor (AF) by Silva da et al. (2003), was obtained with lower mass ofoleic and vermiculite than the obtained by these authors with vermi-culite thermoexfoliated or hydrofobized, stating that the adsorption is

higher when vermiculite is both thermoexfoliated and hydrophobized.The results of the experiments varying adsorbent dosage (Fig. 3)

showed that the maximum adsorption was of 86%, when the amount ofvermiculite was similar to that of oleic acid (about 0.3 g). Adsorptiondid not reach 100% probably due to the strong adsorption of oleic acid,about 10%, by glass vessel where the experiment was carried out.

In relation to the influence of the number of stages of oleic acidadsorption by Goiás vermiculite in the first stage was 69.1% (3.1) andin a second step was 66.5% (3.5), being the total 67.8% (3.3), in-dicating that it was possible to reuse vermiculite unsaturated of ad-sorbate to recover more oleic acid without previous washing of thesame, considering that about 10% was lost, probably due to the strongadsorption of oleic acid by glass vessel where the experiment wascarried out, as it was previously mentioned.

The result obtained on the influence that the salt content in a marineenvironment can have on the oleic acid adsorption (Fig. 4) showed thatas the salinity of the medium increased so did the oleic acid adsorptionby vermiculite. This increase could be explained by the fact that as theaqueous medium became more ionic an “salting-out” effect originated,i.e., the oleic acid solubility in water decreased and the hydrophobicinteractions between oleic acid and less ionic or polar regions of ver-miculite were favored.

Fig. 1. Adsorption at different pH, at 19.5 °C and 69% of relative humidity.

Fig. 2. Adsorption at different concentrations of hydrophobizing reactive and differentweights of vermiculite at 19.5 °C and 69% of relative humidity.

Fig. 3. Oleic acid adsorption depending on GEH mass (g) to pH 4, 17.7 °C and 47% re-lative humidity.

Fig. 4. Influence of salinity, at 16.6 °C and 57.7% of relative humidity.

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3.2. Adsorption isotherms

The DKR model fitted the experimental data best by linear analysis(R2 = 0.995) (Fig. 5), while the Langmuir and Freundlich fitted verypoorly (R2 Langmuir was of 0.34 and R2 Freundlich was of 0.55). Thissuggested that the adsorption process would be cooperative, whereadsorbed adsorbate would have an effect on the adsorption of “new”adsorbate molecules. Therefore, cooperativity is common for adsorp-tions. The Xm value calculated from DKR model was 1.29 mol/g, the βparameters was 5.16 × 10−1 mol2/J2. The value 1.42 kJ/mol of theapparent adsorption energy E, below 8 kJ/mol, would explain physicaladsorption (Krishna et al., 2000; Lin and Juang, 2002; Wang et al.,2004).

3.3. Characterization

3.3.1. SEMThe increase of specific surface of vermiculite caused with the ex-

pansion and exfoliation after abrupt heating at 1000 °C produced anoptimum product for the adsorption of oleic acid. In addition, a largenumber of pores of different sizes were formed (see Figs. 6–7). The poresize ranged from 3.7 × 104 to 28.9 × 104 Å in width and 6.7 × 104 to138.7 × 104 Å in length for PE and from 3.6 × 104 to 30.2 × 104 Å inwidth and 10.6 × 104 to 156.2 × 104 Å in length for GE. These poresmay be retention sites of adsorbed ions and the possibility that someoleic acid was trapped in them was not discarded.

3.3.2. Infrared spectraThe infrared spectra of the samples measured are in Fig. 8. In the

spectrum of the untreated sample (Fig. 8-1) adsorption characteristicsof trioctahedral phyllosilicates can be observed (Russell and Fraser,1994). The main infrared absorption bands were located at 3470 and1642 cm−1, ascribed to the characteristic OeH stretching and bendingvibrations of the hydration water molecule, respectively; 1001 cm−1

band attributed to the SieOeSi and SieOeAl stretching vibrations(Farmer, 1974); 669 cm−1 band attributed to deformation vibration ofthe SieO; cm−1 band ascribed to the bending vibration of SieOeM(where M can be Si, Mg, Al or Fe,) (Farmer, 1974; Madejová, 2003). Anabsorption at 2400 cm−1 is due to the CO2 of the air and that at1400 cm−1 of low-intensity was related to the presence of a sodiuminterlayer cation (Madejová, 2003).

The infrared spectrum of the thermoexfoliated vermiculite (Fig. 8-2)was similar to the untreated one, with the difference that showed morenoise and the hydration water molecule bands were less intense due towater loss with thermoexfoliation and indicating that the vermiculitehas not suffered a total dehydroxylation. The infrared spectrum of theexfoliated vermiculite and hydrophilized by the hydrophobizing at-mosphere method using 5% polymethyl-hydro-siloxane (Fig. 8-3)showed a band of weak intensity at 2160 cm−1 corresponding to SieHof hydrophobizing reactive, confirming its presence in the said vermi-culite. In the spectrum of the thermoexfoliated.

vermiculite and hydrophilized by immersion method using 5%polymethyl-hydro-siloxane (Fig. 8-4) the band at 2160 cm−1 corre-sponding to SieH of hydrophobizing reactive was more intense than

Fig. 5. Linear fit of experimental data obtained using DKR adsorption isotherm for theadsorption of oleic acid onto GEH.

Fig. 6. SEM image of PE sample.

Fig. 7. SEM image of GE sample.

Fig. 8. Infrared spectra of Goiás vermiculite: 1 untreated; 2 thermoexfoliated; 3 ther-moexfoliated and hydrophobized by the atmosphere method using 5% poly-methylhydrosiloxane; 4 thermoexfoliated and hydrophobized by the immersion methodusing 5% polymethylhydrosiloxane; 5 thermoexfoliated and hydrophobized by the im-mersion method using 5% polymethylhydrosiloxane and oleic acid adsorbed; 6 thermo-exfoliated and hydrophilized by the immersion method using 70% poly-methylhydrosiloxane and oleic acid adsorbed.

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with the hydrophobizing atmosphere method, which could indicate thatthe number of hydrophobized points was greater with the immersionprocess than with the hydrophobizing atmosphere or that there was asurface excess of the same. The spectrum of the thermoexfoliated ver-miculite and hydrophilized by immersion method using 5% poly-methyl-hydro-siloxane and after adsorption of oleic acid (Fig. 8-5), didnot show bands that indicate the presence of oleic acid. This could beexplained by an overlap of the bands at 1700 and 3400 cm−1 corre-sponding to carbonyl and OH groups. Also, bands corresponding to thedouble bonds of oleic acid, about 1450 cm−1, were not seen becausethey matched with bands characteristic of vermiculite. In the spectrumof the thermoexfoliated vermiculite and hydrophilized by immersionmethod using 70% polymethylhydrosiloxane and after adsorption ofoleic acid (Fig. 8-6), the bands corresponding to oleic acid, the carbonylband at 1700 cm−1 and the characteristic bands of the double bond ofoleic acid at 1450–1500 cm−1, were appreciated.

4. Conclusions

The present investigation showed that the thermoexfoliated andhydrophobized vermiculites studied were effective adsorbents for theremoval of oleic acid from aqueous solutions because its high mica-likecontent and therefore high degree of expansion, exfoliation and for-mation of larger pores. Vermiculite without exfoliate after an abruptheating at 1000 °C would correspond to vermiculite in strict sense orvermiculite with very few interestratified phases and therefore withlower exfoliation and adsorption capacity. The results of the experi-ments on the oleic acid adsorption by thermoexfoliated and hydro-phobized vermiculite from Goiás showed that: 1) pH 2–7 would beadvantageous since the pH of natural waters varies between 6 and 8. 2)For different concentrations of hydrophobizing reactive and 0.3 g ofvermiculite this saturated when ratio in weight vermiculite/oleic acidwas equal to 1, but with higher amounts of oleic acid and the sameweight of vermiculite (0.3 g) the saturation was reached when the ratiowas equal to 1.5. 3) The efficiency of oleic acid adsorption increasedwith an increase in the adsorbent dosage and the maximum was of 86%,when the amount of vermiculite was similar to that of oleic acid. 4) Itwas possible to reuse vermiculite unsaturated of adsorbate to recovermore oleic acid without previous washing of the same. 5) The salinity ofthe medium increased so did the oleic acid adsorption by vermiculite.Isotherm studies indicated that the DKR model, indicative of a co-operative process, fitted the experimental data better than Langmuirand Freundlich models in the investigated vermiculite. The maximumadsorption capacity obtained with DKR model was 1.29 mol/g of oleicacid on Goiás vermiculite thermoexfoliated and hydrophobized.

Acknowledgements

The authors wish to acknowledge the Scientific-technical services ofUniversity of Oviedo (Spain) for Infrared Spectroscopy. We are gratefulto the referees and the editor for his comments, which have sub-stantially improved the original manuscript.

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