Applied Clay Science xxx (2014) xxx–xxx

CLAY-03083; No of Pages 9

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Applied Clay Science

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Research paper

Oil–water separation process with organoclays: A comparative analysis

Mariaugusta F. Mota a, Meiry Gláucia F. Rodrigues a, Fabricio Machado b

a Unidade Acadêmica de Engenharia Química, Universidade Federal de Campina Grande, Av. Aprígio Veloso, 882 – Bodocongó, CEP 58429-970, Campina Grande, Brazilb Instituto de Química, Universidade de Brasília, Campus Universitário Darcy Ribeiro, CP 04478, CEP 70910-900, Brasília, DF, Brazil

E-mail addresses: meiry@deq.ufcg.edu.br (M.G.F. Rodr(F. Machado).

http://dx.doi.org/10.1016/j.clay.2014.06.0390169-1317/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: Mota, M.F., et al., Odx.doi.org/10.1016/j.clay.2014.06.039

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 August 2012Received in revised form 15 June 2014Accepted 17 June 2014Available online xxxx

Keywords:OrganoclaysQuaternary ammonium saltsOil–water separation processAdsorption and removal capacityMultiple linear regression modelFactorial experimental design

This work presents an experimental study focusing on the preparation and characterization of modified greenclay with quaternary ammonium salts alkyl dimethyl benzyl ammonium chloride (ADMBAC) and distearyldimethyl ammonium chloride (DSDMAC) intended to be used as adsorbent in the process of removing oilemulsion in an oil–water system using finite bath. X-ray diffraction (XRD), infrared spectroscopy (IR) andexpansion test (adsorption capacity and Foster swelling) measurements were performed in order to evaluatethe performance of the ion exchange reactions and the degree of affinitywith oil products. There was an increasein the XRDbasal spacing of themodified clays (1.96 nmand 2.25 nm for DSDMAC andADMBAC salts, respectively)in comparison to the observed value (1.56 nm) for the unmodified clay. The IR results revealed that salts weresuccessfully incorporated to clay structure. Based on the expansion tests the organoclays presented the bestefficiency of separation, independent on the kind of solvent used in comparison with the unmodified clayperformance. The modified clays exhibit a very high capacity of adsorption. The predictions of multiple linearregression models determined based on the factorial design of experiments are excellent.

igues), fmachado@unb.br

il–water separation process with organoclays: A

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

The importance of oil and natural gas in modern civilization is wellknown, however, most activities, such as oil and gas production pro-cesses normally generate large volumes of liquid waste. As oil–wateremulsion effluents found in the oil industry during its production, refin-ing, and throughout the process (generally named production water orwater produced)may containmany organic and inorganic components,the discharge of this producedwater can lead to pollution of surface andgroundwater, and soil (Fakhru'l-Razi et al., 2009).

In face of the increasing global demand for oil and its derivatives, theproduction of oil–water emulsions can be regarded as one of the mainproblems found in this field. In this context, the treatment of undesir-able process effluent plays a fundamental role before its final disposal,due to legal restrictions for emission to the environment and technicalrequirements for injection into oil wells.

Bentonites are generally used as adsorbents, because they presenthigh adsorption properties and elevated potential for ion exchange.Additionally, these clays are found in abundance and extremely cheap(e.g. 20 times cheaper than activated carbon) (Zhou et al., 2011). Theintercalation of cationic surfactant molecules in the interlayer spacethrough ion exchange is responsible for the improvement of the proper-ties of the clay, as a consequence of an increase in the basal spacing andexposure of new adsorption sites of the clay (Park et al., 2011).

Due to their versatility both the unmodified and the modified claysare employed for several technological application purposes, as forinstance, precursors of polymeric nanocomposites, adsorbents inpollution prevention and environmental remediation associated to thetreatment of spills, wastewater and hazardous waste landfills; removalof organic contaminants from contaminated; coating reservoirs forwaste disposal; and treatment of effluent in tanks of oil or gasolineand landfill liners (Guangyao and Boyd, 1998; Zhang et al., 1993).

In recent years, organoclays (OC) have attracted great interest due totheir academic and industrial importance (Bergaya and Lagaly, 2001;Sinha Ray and Okamoto, 2003). It is well-known that OC are an impor-tant type ofmodified clay materials, obtained by intercalation of cation-ic surfactants such as quaternary ammonium compounds (QAC) intothe interlayer space of clays through ion exchange reactions (Chaiko,2002; Natalie and Susan, 2001). As a result of the hydration of inorganiccations on exchange sites, themineral clay surface becomes hydrophilic,thereby rendering unmodified clays as ineffective adsorbents foradsorbing organic compounds (Kim et al., 2003). The treatment ofclays with cationic surfactants makes them hydrophobic, increasingtheir adsorption capacity for organic pollutants characterized by lowwater solubility or polarity (Boyd et al., 1988a,b; Xu et al., 1997).

In an interesting work, Moazed and Viraraghavan (2005a) havedeveloped several studies focusing on the capacity of oil adsorption bycommercial OC. According to the experimental results, different oil-in-water emulsions, including standard mineral oil, Kutwell45 (KUT45),Valcool (VAL), refinery effluent and wastewater from production wellswere successfully treated by the modified clay, leading to a completeremoval of the oils from the emulsions.

comparative analysis, Appl. Clay Sci. (2014), http://

2 M.F. Mota et al. / Applied Clay Science xxx (2014) xxx–xxx

High separation efficiency has also been observedwhen oil-in-wateremulsions were treated with organobentonite (or bentone)/anthracitemixture (consisting of 30 mass% OC and 70 mass% anthracite). Accord-ing to Moazed and Viraraghavan (2005b), oil removal efficiencies lyingin the range of 72% to 98% can easily be obtained, showing that that themixture OC/anthracite can be considered as an excellent medium fortreating oily wastewaters.

Rodrigues et al. (2010) have studied the swelling behavior ofBrazilian OC in different solvents from the petroleum industry,through modification of gray clay by using a quaternary ammoniumsalt (distearyl dimethyl ammonium chloride). According to XRDmeasurements, it was possible to determine the significant increasein basal spacing in comparison to unmodified clay, which indicatesproper intercalation of quaternary ammonium salt between thelayers of the clay.

Queiroz et al. (2010) have performed the modification of both theBrasgel clay and gray clay through cation exchange reaction by usingthe quaternary ammonium salt (cetyltrimethyl ammonium bromide).According to the authors, the compatibility of the modified Brasgeland gray clays with solvents such as gasoline, diesel, toluene andlubricating oil was improved. The quaternary ammonium salt wasintercalated into the clay structure, leading to the similar organophilicfeaturewhen themodified Brasgel and gray clays are compared. Resultsfrom expansion test application also demonstrated the high affinity ofmodified clays by organic compounds.

Recently, Mota et al. (2011) have evaluated the performance of claymodified with the quaternary ammonium salt (alkyl dimethyl benzylammonium chloride) as an organophilic adsorbent in the oil–waterseparation process. It was shown that a removal efficiency equal to95% and a removal capacity of 38mg/g can be easily achieved, indicatingthe high potential of this kind of modified clay for the oil–waterseparation purposes.

In this work, Brazilian clay (green calcium bentonite–aluminum clayminerals, consisting mostly of montmorillonite) from the State ofParaíba wasmodified using quaternary ammonium salts and its perfor-mance for oil–water separation was evaluated. Two different types ofammonium salts, namely: alkyl dimethyl benzyl ammonium chloride(ADMBAC), distearyl dimethyl ammonium chloride (DSDMAC) and acombination of ADMBAC/DSDMAC was employed. It is very importantto emphasize that this paper also intends to review and compare theresults obtained by Mota et al. (2011) with the new one obtainedhere. In this sense, a critical evaluation of the main features of the OCobtained from ion exchange reactions with individual quaternary am-monium salts and the combination of ADMBAC/DSDMAC is presentedin this work. In order to evaluate the efficiency of the modifiedBrazilian clay, swelling test and adsorption capacity were performedwith oil–water emulsions of gasoline, diesel and kerosene and amultiple linear regression statistical technique based on the two-levelfactorial design was employed to predict the process behavior.

2. Experimental

2.1. Materials

Green calcium bentonite, kindly donated by Dolomil (Brazil), wasused during the preparation of the OC. The chemical composition ofthis clay shows a high content of silica, alumina and iron, and smallquantities of magnesium, potassium, titanium, calcium and sulfur,whose fractions when expressed as element oxides were determinedas 55.3% of SiO2, 22.8% of Al2O3, 13.4% of Fe2O3, 2.5% of MgO, 2.4% ofK2O, 1.5% of TiO2, 0.9% of CaO, 0.6% of SO3 and 0.5% of other compounds.The clay sample was sieved according to the Tyler standard sieve 200Mesh (sieve opening equal to 0.074 mm). The ammonium quaternarysalts: alkyl dimethyl benzyl ammonium chloride (ADMBAC) (a cationicsurfactant named Dodigen 1611, supplied as an aqueous dispersionwith 50% of activity) and distearyl dimethyl ammonium chloride

Please cite this article as: Mota, M.F., et al., Oil–water separation process wdx.doi.org/10.1016/j.clay.2014.06.039

(a cationic surfactant, named Praepagen WB–DSDMAC, dispersedin aqueous isopropanol with an active content of 80%) werepurchased from Clariant. Sodium carbonate with purity of 99% andchloroform with 99.5% purity were supplied by Synth (Brazil).

2.2. Preparation of the samples

The method used involves cation exchange reactions carriedout based on a conventional method, as reported elsewhere(Valenzuela-Diaz, 2001). Initially, the green calcium bentonite(4 mass%) are dispersed in distilled water and treated with a concen-trated solution of sodium carbonate at 95 °C under rigorous magneticagitation for 20 min. The sample is then allowed to settle for 24 h,after which 20.4 g of ammonium quaternary salts are added to thedispersion and stirred for 20 min. After another 24-h settling period,the solid sediments are filtered out, dried at 60 °C ± 5 °C for 48 h andsieved in a Tyler 200 Mesh (sieve opening equal to 0.074 mm). Thesample mixture composed of the two salts was prepared by adding adispersion of 10.2 g of salt DSDMAC and 10.2 g of salt ADMBAC. Themethod used in this work corresponds to a modification of the oneproposed by Valenzuela-Diaz (2001), named “direct method”, whichwas developed by Pereira et al. (2005). It presents as main advantagethe reduction of the preparation time to 1 day, since the steps compris-ing the 24-h settling periods are not performed.

2.3. Characterization of the samples

2.3.1. X-ray powder diffraction (XRD)Samples of sieved clays were placed in an aluminum sample door

for the XRD measurements. The XRD patterns of the materials wereobtained on a Shimadzu XRD 6000 equipment (Shimadzu ScientificInstruments, USA), using copper Kα radiation at 40 kV/30 mA, with agoniometer velocity of 2°/min and a step of 0.02° in the range of 2θscanning from 2° to 45°. The only d-values of interest in the X-raypatterns were the basal spacing along the c axis.

2.3.2. Infrared spectroscopy (IR)The OC obtained were sieved on Tyler 200 Mesh (74 μm) and then

submitted to a physical treatment in accordance with the KBr method,which consists ofmixing 0.007 g of the sample and 0.1 g of KBr, grindingand pressing the solid mixture to 5 ton during 30 s in order to form apastille that allows the passage of light. The characterization wasrealized using an Nicolet Avatar infrared spectrophotometer AVATARTM 360 FT-IR E.S.P. (Thermo Electron Corporation, USA), in thewavelengths in the range from 4000 to 400 cm−1, with incrementsof 500 cm−1 and a resolution of 4 cm−1.

2.3.3. Differential thermal analysis and thermogravimetric analysis(DTA/TG)

Thermal analyses were performed on a DTG Shimadzu-60H, operat-ing the heating rates of 10 °C/min from room temperature to 1000 °Cunder a nitrogen atmosphere at a flow rate of 50 ml min−1.

2.3.4. Foster swelling testThis technique is employed to carry out studies focusing on the

compatibility of specific modified and unmodified clays with gasoline,diesel oil and kerosene (Foster, 1951, 1953). This method is based onthe “Standard Test Method for Swell Index of Clay Mineral Componentof Geosynthetic Clay Liners” (ASTM D 5890-95). The test consisted ofadding 1 g of the powder sample to 50 ml of solvent in a test tube.After 24 h of rest, the reading of the clay column volume (swelling ofthe clay without agitation) was registered. Afterwards, the content ofthe test tube was stirred with a glass stick during 5 min and, after afurther 24-h settling period, a new reading of the clay column volume(swelling of the claywith agitation)was done. The followingparametershave been fixed for the interpretation of the test: non-swelling

ith organoclays: A comparative analysis, Appl. Clay Sci. (2014), http://

5 10 15 20 25 30 35 40 45 50

5 10 15 20 25 30 35 40 45 50

2 (°)

DSDMAC / ADMBAC

ADMBAC

Inte

nsity

(a.u

.)

DSDMAC

Natural clay

Fig. 1. XRD of natural clay and organoclays: DSDMAC, ADMBAC and DSDMAC/ADMBAC.The experimental data regarding to the natural clay and the organoclay modified withADMBAC correspond to the one published by Mota et al. (2011).

3M.F. Mota et al. / Applied Clay Science xxx (2014) xxx–xxx

(minor or equal to 2 ml/g of the clay), low swelling (between 3 and5 ml/g of the clay), intermediate swelling (between 6 and 8 ml/g ofthe clay) and high swelling (more than 8 ml/g of the clay) (Pereiraet al., 2005).

2.3.5. Adsorption capacityThe gasoline, diesel, kerosene and lubricating oil adsorption

capacity was measured following a method based on the “StandardMethods of Testing Sorbent Performance of Adsorbents (ASTMF716-82 and ASTM F 726-99).” The adsorbent sample is weighed,with a minimum weight of 1 g, and the value is recorded. The testcell is filled with an initial layer of test liquid. The adsorbent is placedin a basket (a 200mesh stainless steel basket), which is then loweredinto the test cell. The adsorbent should be allowed to float freelywithin the test cell. After 15 min ± 0.33 min the basket with theadsorbent is removed manually in a vertical direction and allowedto drain for 15 s ± 3 s. A pre-weighed (tared) weighing support isplaced under the adsorbent sample/mesh basket set to catch anyadditional drips and the entire system is weighed. All the testsare conducted in duplicate with a minimum of two runs used forcalculations. Based on the data obtained, the adsorbed oil is calculatedas the ratio of adsorbed oil to dry adsorbent mass.

The adsorption capacity is obtained based on Eq. (2)

Ad ¼ P1−P2

P2

� �� 100 ð1Þ

where, P1 is mass of the material after adsorption, expressed in g; P2 isthe mass of dry adsorbent material, expressed in g; and Ad is theefficiency of adsorption to adsorbent tested. The adsorption capacity isreported in grams of solvent adsorbed per gram of clay.

2.4. Factorial experimental design

A two-level factorial experimental design with three replicates inthe central point (repetitions at the central point allows for estimationof the experimental errors and nonlinear effects of curvature) was ap-plied in order to evaluate qualitatively and quantitatively the influenceof individual factor on the variables being analyzed. The purpose of thisstudywas to evaluate the trend towardmaximizing the efficiency of theprocess, i.e., to perform a joint estimation of the best values for each var-iable (Montgomery and Runger, 2003). Two independent variableswere selected: the initial concentration of oil–water solutions (C0),evaluated in the range of 100 to 500 mg/L and the stirring speed(A) kept within the range from 100 to 200 rpm. According to theproposed factorial experimental design, the factor levels werecoded as follows: i) 100 mg/L and 100 rpm associated to the highlevel, denoted “+1”; ii) 500 mg/L and 200 rpm, corresponding tothe low level, denoted “−1”; and iii) the central points, 300 mg/Land 150 rpm, denoted “0.”

2.5. Batch experiments

The experiments were performed simultaneously. Initially, 0.5 g claywasweighed anddispersed in 50ml solution containing oil–water emul-sion with concentrations of 100, 300 and 500 mg/L. The set (emulsionoil–water/clay) was kept under mechanical ranging from 100 to200 rpm for 6 h to ensure the system equilibrium (Rodrigues et al.,2010). A similar procedure was performed to the process performedwithout agitation.

2.5.1. Oil–water separation testThe concentration of oil present in the aqueous phase was

determined by analysis of absorbance using spectrophotometer(UV–visible/UV 1800 Shimadzu). A calibration curve of absorbanceversus concentration using different concentrations of oil ranging

Please cite this article as: Mota, M.F., et al., Oil–water separation process wdx.doi.org/10.1016/j.clay.2014.06.039

from 0 to 100 ppm was built with a coefficient of determination (R2)equal to 0.9974. Chloroformwas used as solvent for extraction, becauseof excellent response (significant peak) at a wavelength of 262 nm forthe samples analyzed. The absorbance at this wavelength is commonlyused to estimate the concentration of oil in water samples (Clesceriet al., 1999; Henderson et al., 1999) and also in water produced. Thiswavelength is measured bands of aromatic CH in the medium.This implementation procedure was designed to standardize thedetermination of oil and grease.

2.5.2. Determination of the amount of oilThe percentage of removal (%Rem) as well as the removal capacity

will be obtained through Eqs. (3) and (4), respectively:

%Rem ¼ C0−Ceq

C0

� �� 100 ð2Þ

where %Rem is the percentage of oil removed from the oil–wateremulsion, C0 is initial concentration, expressed in ppm and Ceq is finalconcentration of the oil–water emulsion, expressed in mg/L.

A relatively simple calculation makes it possible to obtain the oiladsorbed expressed in mg of aqueous solution per gram of adsorbentsolid, given by the following relation:

qeq ¼Vm

C0−Ceq

� �ð3Þ

where qeq is removal capacity at equilibrium, expressed mg of oil/gof adsorbent, C0 is initial concentration of the oil–water emulsion,expressed in mg/L, Ceq is final concentration of the oil–water emulsion,expressedmg/L,V is the volumeof the adsorbate, expressed inml andmis the mass of adsorbent, expressed in g.

3. Results and discussion

Fig. 1 presents the X-ray patterns of the natural (unmodified) clayand modified clays (treated with different alkyl quaternary ammoniumsalts, DSDMAC, ADMBAC and DSDMAC/ADMBAC). Qualitatively green

ith organoclays: A comparative analysis, Appl. Clay Sci. (2014), http://

80

85

90

95

100

t Los

s (%

)

Natural ClayDSDMACADMBACDSDMAC / ADMBAC

4 M.F. Mota et al. / Applied Clay Science xxx (2014) xxx–xxx

clay presents high levels of smectite, however, the presence of kaoliniteand quartz is also observed. It is very important to emphasize thatthese results are consistent with those found in the open literature(Rodrigues, 2003).

It can be observed that after treatment of the claywith the quaterna-ry ammonium salts, DSDMAC and ADMBAC, changes occur in the basalspacing when compared with that of the clay. Such changes can beattributed to the difference in spacing. The d001 value of the raw clayis 1.56 nm, and for the organoclays the d001 values are 1.96 nmand 2.25 Å when the clay is modified with DSDMAC and ADMBAC,respectively. The interlayer space is equal to 0.40 nm for DSDMAC,0.69 nm for ADMBAC and 0.40 nm for DSDMAC/ADMBAC.

The XRDpatterns indicate that themodification of the claywith qua-ternary ammonium salts (DSDMAC, ADMBAC and DSDMAC/ADMBAC)leads to the expansion of the montmorillonite (Akçay, 2004; Chittumet al., 1997; Queiroz et al., 2010; Rodrigues et al., 2010; Ruiz-Hitzkyand Meerbeek, 2006; Shen, 2001; Wang et al., 2004; Xi et al., 2004).In addition, the reflection is shifted to lower values of angles, whichconfirms the intercalation of cations between the silicate layers.

Fig. 2 illustrates typical infrared spectra of clay and modifiedgreen clay with different alkyl quaternary ammonium salts (DSDMAC,ADMBAC and DSDMAC/ADMBAC). The unmodified clays exhibit bandsin the region of 3640–3400 cm−1 and 1651 cm−1, these bands areassigned to stretching vibrations of structural hydroxyl group and theOH group on the adsorbed water. Bands are observed for both theunmodified and the organophilic clays in the region between 1100 and1045 cm−1, characteristic of Si–O–Si and about 915 and 523 cm−1

corresponding to the octahedral layers of aluminosilicate Al–O–Si.These results are in agreement with the literature (Pereira et al., 2005;Russel and Fraser, 1994). It is also possible to detect vibrations of theSi–O group (1045 cm−1) and octahedral layers (between 523 and525 cm−1) for both samples. The organophilic clays also exhibit the char-acteristic peaks of clay minerals, although of low intensity, which clearlyindicates that the clay was properly modified (Bala et al., 2000; ElMessabeb-Ouali et al., 2009).

4000 3500 3000 2500 2000 1500 1000

4000 3500 3000 2500 2000 1500 1000

Wavenumber (cm-1)

DSDMAC / ADMBAC

ADMBAC

Abs

orba

nce

(a.u

.)

DSDMAC

Natural clay

Fig. 2. Infrared spectra of natural and modified clays: DSDMAC, ADMBAC andDSDMAC/ADMBAC. The experimental data regarding to the natural clay and theorganoclaymodifiedwith ADMBAC correspond to the one published byMota et al. (2011).

Please cite this article as: Mota, M.F., et al., Oil–water separation process wdx.doi.org/10.1016/j.clay.2014.06.039

The OC also ehxibited at 2923 cm−1 a band corresponding toasymmetric stretching vibrations of the CH2 group. A characteristicpeak at 2850 cm−1 corresponding to a group CH2 symmetric stretchand 1470 cm−1. The asymmetric angular deformation of the CH3

groups occurs at about 1459 cm−1. The presence of CH2 and CH3

groups in the infrared spectrum is an evidence of the intercalationof the quaternary ammonium cations within the interlamellar spacesof the sample (Kozak and Domka, 2004; Pereira et al., 2005; Rodrigueset al., 2010).

The results of the differential thermal and thermogravimetricanalyses of the material samples are shown in Fig. 3. The mass lossesof the materials were determined to be equal to 15.4 mass% for theunmodified clay, 38.4 mass% for OC treated with salt DSDMAC,30.4 mass% for ADMBAC OC and 38.1 mass% for DSDMAC/ADMBACOC. The thermogravimetric curve of the unmodified clay exhibits twocharacteristics of mass-losing within the temperature range from30 °C to 250 °C and from 400 °C to 600 °C, as a result of the release ofadsorbed water. According to Fig. 3A, the OC present three significantmass-losses: the first loss reflects the release of adsorbed water in therange of 30 °C and 140 °C and the second and third losses, regardingthe decomposition of the quaternary ammonium salts ranged from150 °C to 850 °C.

The behavior of DTA curves are shown in Fig. 3B. In the particular caseof the OC, a similar behavior is observed, concerning the decompositionof the quaternary ammonium salts represented by two overlappingexothermic peaks, the first one refers to the dehydroxylation of the OC,

0 100 200 300 400 500 600 700 800 900 100060

65

70

75

Wei

gh

Temperature (°C)

(A)

0 100 200 300 400 500 600 700 800 900 1000

-20

0

20

40

60

80

100 (B)

ecnereffiD

erutarepmeT

(°C/

mg)

Temperature (°C)

Natural clay DSDMAC ADMBAC DSDMAC / ADMBAC

Fig. 3. Thermal measurements of the green natural clay. (A) Thermogravimetric analysis(TGA) and (B) differential thermal analysis (DTA) of the natural and surfactant-treatedclays: DSDMAC, ADMBAC and DSDMAC/ADMBAC. The experimental data regarding tothe natural clay and the organoclay modified with ADMBAC correspond to the onepublished by Mota et al. (2011).

ith organoclays: A comparative analysis, Appl. Clay Sci. (2014), http://

Table 1Results of the Foster swelling tests performed in different organic solvents, without andwith agitation, for green clay before and after modification with different quaternaryammonium salts (DSDMAC, ADMBAC and DSDMAC/ADMBAC). The experimental dataregarding to the organoclay modified with ADMBAC correspond to the one published byMota et al. (2011).

Solvents Organoclay(DSDMAC)

Organoclay(ADMBAC)

Organoclay(DSDMAC/ADMBAC)

Withoutagitation

Withagitation

Withoutagitation

Withagitation

Withoutagitation

Withagitation

Gasoline 8 10 9 13 10 12Diesel 9 12 8 10,5 12 9Kerosene 9 10 3 4 11 12

5M.F. Mota et al. / Applied Clay Science xxx (2014) xxx–xxx

and second one, is concerning the combustion of carbonaceous residuefromburning before (Natali Sora et al., 2005). Characteristic endothermicpeaks are also present in the both unmodified andmodified clays. In thecase of unmodified clay, a strong endothermic peak occurring between34 °C and 140 °C, accompanied by the loss of free water is observed.These changes are due to water adsorbed by clay minerals. When theOC are considered, two endothermic peaks are noticed within therange from 250 °C to 400 ºC (depending on the type of quaternaryammonium salt), probably due to the water coordinated to cationsdehydrating to form the anhydrous cation.

Adsorption tests were carried out in order to assess the potentialfor adsorption of unmodified and organophilic clays in organicsolvents, such as, gasoline, kerosene and diesel according to ASTMF716-82 and ASTM F726-99. The results of the adsorption capacityof the different green clay and modified clays (DSDMAC, ADMBACand DSDMAC/ADMBAC) are shown in Fig. 4. Based on theseresults it seems reasonable to assume that organophilic clays exhibitbetter adsorption capacity in all solvents when compared withunmodified clay.

In the particular case of the clay treatedwith DSDMAC, the adsorptioncapacity follows the decreasing order: diesel N kerosene N gasoline. Thebest result is obtainedwith the diesel organic solvent, which probably in-dicates that there is a better interaction of the OC with the diesel solvent.This interaction is directly related to the chemical composition of com-pounds (gasoline: C5–C10; kerosene: C11–C12; diesel: C13–C17). The resultsmay also be related to the viscosity of the solvent, given that diesel oil isthemost viscous: diesel, 8 CP (600 rpm) and 3.5 CP (300 rpm); kerosene,CP (600 rpm) and a CP (300 rpm), gasoline, 2 CP (600 rpm) and a CP(300 rpm). In the case of ADMBAC treated clay, the order is the following:gasoline≈ diesel N kerosene.

Comparatively, the observed results of adsorption capacity arequite different for the ADMBAC OC regarding to clay treated withDSDMAC OC, which may reflect a strong interaction of the OC withthe solvents used.

Table 1 presents the results of the Foster swelling tests of thegreen clay aftermodificationwith different alkyl quaternary ammoniumsalts (DSDMAC, ADMBAC and DSDMAC/ADMBAC). In these assays,all samples are allowed to interact with different organic solvents(gasoline, diesel and kerosene).

According to the results shown in Table 1, the OC treated withDSDMAC present high swelling capacity (with and without agitation)for all organic solvents (gasoline, diesel and kerosene). OC modifiedwith ADMBAC also present high swelling (with and without agitation)when tested in gasoline and diesel organic solvents. In spite ofthe great affinity of the OC modified with ADMBAC with organiccompounds such as gasoline and diesel, the observed results forkerosene demonstrated that this OC present low affinity, and as aresult swellings were not significant, independent on the agitation

0.92

5.53 5.62

8.39

1.95

9.70

5.37

8.21

1.98

8.06

2.52

6.95

Natural DEDMA ADMBA DEDMA + ADMBA

Adsorption Capacity (gof adsorbed material / g of material solvent)

Gasoline Diesel Kerosene

Fig. 4.Adsorption capacity in organic solvents (gasoline, kerosene and diesel). Natural andsurfactant-treated clays: DSDMAC, ADMBAC and DSDMAC+ ADMBAC. The experimentaldata regarding to the natural clay and the organoclay modifiedwith ADMBAC correspondto the one published by Mota et al. (2011).

Please cite this article as: Mota, M.F., et al., Oil–water separation process wdx.doi.org/10.1016/j.clay.2014.06.039

speed of the medium (Pereira et al., 2005; Rodrigues et al., 2010;Valenzuela-Diaz, 2001).

The potential of OC in removing oil was evaluated in a finitebath system, based on the 22 factorial design presented in Table 2,in particular, C0 refers to the theoretical initial concentration of oil inthe emulsion and A corresponds to the stirring speed. The multiplelinear regression model accounting for the importance of processvariables can be represented generically as:

yj ¼ b0 þXki¼1

bixji þ ε j j ¼ 1;2;…;n ð4Þ

where yj represents the dependent variable (response) in the experi-mental condition j, bi (i= 0, 1,…, k) is the linear regression coefficient,xji is the independent variable and εi is the error associated with eachexperimental condition j.

The least square method was employed to estimate the regressioncoefficient bi. In this case, the least square function to be minimizedcan be expressed as follows:

ℑ ¼Xni¼1

ε2 ¼Xni¼1

yj−b0 þXki¼1

bixji

!2

: ð5Þ

Table 2 illustrates the experimental results obtained for claymodified with DSDMAC. Statistical analysis demonstrated a significanteffect of C0 on the removal capacity. It can be observed that removalcapacity around 93% is easily obtained, independent of the adoptedstirring speed. The regression models (see Eqs. (6) and (7)) indicatethat the system is strongly dependent on C0, contributing positively tothe removal of oil. When the effect of curvature (quadratic contributionat the midpoint of the experimental design, δcurv) is considered, themodel fit is significantly improved, leading to coefficients (R2) equal to99.62% for %Rem and 99.98% for qeq, respectively.

%Rem ¼ 83:22þ 9:95 � C0 þ 9:50 � δcurv ð6Þ

qeq ¼ 27:44þ 20:26 � C0 þ 6:68 � δcurv ð7Þ

Table 2Results of the 22 factorial experimental design for green clay treated with distearyldimethyl ammonium chloride (DSDMAC).

Entry C0 (mg/L) A (rpm) Oil–water(mg/L)

%Rem qeq(mg/g)

1 100 0 27.93 71.50 7.012 500 0 38.47 92.49 47.353 500 200 31.34 93.88 48.074 100 200 24.46 75.04 7.355 300 100 31.13 91.54 33.696 300 100 24.74 93.28 34.337 300 100 24.46 93.35 34.35

ith organoclays: A comparative analysis, Appl. Clay Sci. (2014), http://

-1.05

2.40

12.13

19.42

p=.05

Standardized Effect Estimate (Absolute Value)

C0 by A

A

Curvature

C0

-1.05

2.40

(A)

.51

1.41

23.29

107.96

p=.05

Standardized Effect Estimate (Absolute Value)

C0 by A

A

Curvature

C0

.51

1.41

(B)

Fig. 5. Pareto chart for standardized effects for the initial concentration and the stirringspeed for DSDMAC Clay. (A) Percentage of oil removed from the oil–water emulsion,%Rem. (B) Removal capacity at equilibrium, qeq. The symbol “C0” refers to the theoreticalinitial concentration of oil in the emulsion, “Curvature” corresponds to effect of curvatureδcurv and “A” corresponds to the stirring speed.

65 70 75 80 85 90 95 100

Observed Values - %Rem

65

70

75

80

85

90

95

100

Pre

dict

ed V

alue

s - %

Rem

(A)

0 5 10 15 20 25 30 35 40 45 50 55

Observed Values - qeq

0

5

10

15

20

25

30

35

40

45

50

55

Pre

dict

ed V

alue

s -

q eq

(B)

Fig. 6. Prediction quality of the regression model for experimental data for DSDMAC clay.(A) Percentage of oil removed from the oil–water emulsion, %Rem. (B) Removal capacityat equilibrium, qeq. The dashed lines indicate the regression bands created with 95 %confidence level.

6 M.F. Mota et al. / Applied Clay Science xxx (2014) xxx–xxx

where

δcurv ¼ 1 if the point is a center point0 otherwise :

�ð8Þ

Fig. 5 illustrates a standardized Pareto chart for effects (initialconcentration and stirring speed) on responses (removal capacityat equilibrium and percentage of removal). The initial concentrationC0 plays a dominant effect on the percentage removal in the evaluatedexperimental conditions range, and in both cases, there is no significanteffect of stirring speed on the response variables.

The numbers indicated in the histograms in Fig. 5 correspond to thevalue of the computed t-test statistic, whose statistical significances arecompared to thep-value equal to 0.05 (criterion for statistical significance,which indicates the minimum magnitude for a significant effect).According to the p-values presented in Fig. 5A, the regressors thatcontribute most to the model are the curvature (δcurv) with p-valueequal to 0.0067 and the initial concentration (C0) with p-value of0.0026. The stirring speed (A) and the combined effect of C0 by A werenot statistically important, presenting p-values equal to 0.1381 and0.4045, respectively.

A similar behavior is observed when the removal capacity atequilibrium (qeq) is considered (see Fig. 5B). In this case, the observedp-values for δcurv and C0 were determined to be equal to 0.0018 and

Please cite this article as: Mota, M.F., et al., Oil–water separation process wdx.doi.org/10.1016/j.clay.2014.06.039

0.00009, respectively, and the less important effects A and thetwo-way interaction C0 by A presented p-values equal to 0.2935and 0.6631, respectively. Fig. 6 illustrates the quality of the predictionof the regression model, agreeing very well with the experimentaldata, exhibiting an excellent correlation coefficient (R2) equal to99.62% for %Rem and 99.98% for qeq.

Figs. 7 and 8 present the standardized Pareto chart for effects (initialconcentration and stirring speed) on responses (removal capacity atequilibrium and percentage of removal) and the prediction of theregression model, respectively, based on the experimental resultsobtained by Mota et al. (2011) concerning the OC obtained from ionexchange reaction with alkyl dimethyl benzyl ammonium chloride(ADMBAC). The experimental conditions are similar to the oneemployed in the present work. Additionally, a table with the experi-mental results obtained byMota et al. (2011) is not presented here inorder to avoid repetition, and the reader is referred to this originalreference for additional information.

According to Fig. 7A, both the initial concentration (C0) and stirringspeed (A) significantly affect the percentage removal in the evaluatedexperimental conditions range. The removal capacity at equilibrium(qeq) is also dependent on the C0 and A, however the combined effectseems to be less important, as shown in Fig. 7B. The quality of theprediction of the regression model is depicted in Fig. 8. When thepercentage of oil removal (%Rem) is evaluated, the factors C0, A,the combination C0 by A (negative response) and the curvature

ith organoclays: A comparative analysis, Appl. Clay Sci. (2014), http://

7.68

-8.82

69.24

125.87

p=.05

Standardized Effect Estimate (Absolute Value)

A

C0 by A

Curvature

C0

(A)

1.45

-2.96

107.51

529.09

p=.05

Standardized Effect Estimate (Absolute Value)

A

C0 by A

Curvature

C0

1.45

-2.96

(B)

Fig. 7. Pareto chart for standardized effects for the initial concentration and the stirringspeed for ADMBAC clay. (A) Percentage of oil removed from the oil–water emulsion,%Rem. (B) Removal capacity at equilibrium, qeq. The symbol “C0” refers to the theoreticalinitial concentration of oil in the emulsion, “Curvature” corresponds to effect of curvatureδcurv and “A” corresponds to the stirring speed.

60 65 70 75 80 85 90 95 10060

65

70

75

80

85

90

95

100

(A)

0 5 10 15 20 25 30 35 40 45 50 550

5

10

15

20

25

30

35

40

45

50

55

(B)

Observed Values - %Rem

Pre

dict

ed V

alue

s - %

Rem

Observed Values - qeq

Pre

dict

ed V

alue

s -

q eq

Fig. 8. Prediction quality of the regression model for experimental data for ADMBAC clay.(A) Percentage of oil removed from the oil–water emulsion, %Rem. (B) Removal capacityat equilibrium, qeq. The dashed lines indicate the regression bands created with 95 %confidence level.

Table 3Results of the 22 factorial experimental design for green clay treated with distearyldimethyl ammonium chloride and alkyl dimethyl benzyl ammonium chloride(DSDMAC/ADMBAC) mixture.

Entry C0 (mg/L) A (rpm) Oil–water(mg/L)

%Rem qeq(mg/g)

1 100 0 26.38 73.08 7.162 500 0 23.77 95.36 48.823 500 200 25.50 95.02 48.654 100 200 25.56 73.92 7.245 300 100 25.64 93.03 34.236 300 100 27.81 92.44 34.027 300 100 25.09 93.18 34.29

7M.F. Mota et al. / Applied Clay Science xxx (2014) xxx–xxx

effect δcurv exhibit statistical significance with p-values equal to0.00006, 0.01652, 0.0126, and 0.0002, respectively. In the particularcase of the capacity of removal at equilibrium (qeq), only the variablesC0 (p = 0.000004) and δcurv (p = 0.00009) presented p-values belowthe significance level 0.05, whereas the factor A presented p = 0.2844and the two-way interaction C0 by A presented p = 0.0977. Accordingto Fig. 8, the regression models (see Eqs. (9) and (10)) predict verywell the experimental data and present a high correlation coefficient(R2) equal to 99.99% for both the %Rem and the qeq.

%Rem ¼ 81:10þ 13:80 � C0 þ 0:84 � A−0:97 � C0 � Aþ 11:60 � δcurv ð9Þ

qeq ¼ 27:59þ 21:00 � C0 þ 0:06 � A−0:12 � C0 � Aþ 6:52 � δcurv ð10Þ

Table 3 presents the experimental results obtained for claymodifiedwith the combination of DSDMAC and ADMBAC. Themaximum value ofthe removal capacity for the DSDMAC/ADMBAC was obtained as equalto 95%. Statistical analysis demonstrated a significant effect of C0 onthe removal capacity. On the other hand, stirring speed seems tonot affect the behavior of separation process. As in the previouscase (DSDMAC clay), the regression models (see Eqs. (11) and(12)) indicate that the system is strongly dependent on C0, decisively

Please cite this article as: Mota, M.F., et al., Oil–water separation process wdx.doi.org/10.1016/j.clay.2014.06.039

contributing to the removal of oil from the oil–water emulsions.When the effect of curvature (quadratic contribution at themidpointof the experimental design) is considered, the model fit is significantlyimproved, leading to coefficients (R2) equal to 99.95% for %Rem and99.998% for qeq, respectively.

%Rem ¼ 84:35þ 10:85 � C0 þ 8:54 � δcurv ð11Þ

qeq ¼ 27:97þ 20:77 � C0 þ 6:21 � δcurv ð12Þ

ith organoclays: A comparative analysis, Appl. Clay Sci. (2014), http://

.64

-1.51

28.58

55.45

p=.05

Standardized Effect Estimate (Absolute Value)

A

C0 by A

Curvature

C0

.64

-1.51

(A)

-.32

-.88

57.37

292.97

p=.05Standardized Effect Estimate (Absolute Value)

A

C0 by A

Curvature

C0

-.32

-.88

(B)

Fig. 9. Pareto chart for standardized effects for the initial concentration and thestirring speed for DSDMAC/ADMBAC clay. (A) Percentage of oil removed fromthe oil–water emulsion, %Rem. (B) Removal capacity at equilibrium, qeq. Thesymbol “C0” refers to the theoretical initial concentration of oil in the emulsion,“Curvature” corresponds to effect of curvature δcurv and “A” corresponds to thestirring speed.

70 75 80 85 90 95 10070

75

80

85

90

95

100

(A)

0 5 10 15 20 25 30 35 40 45 50 550

5

10

15

20

25

30

35

40

45

50

55

(B)

Observed Values - %Rem

Pre

dict

ed V

alue

s - %

Rem

Observed Values - qeq

Pre

dict

ed V

alue

s -

q eq

Fig. 10. Prediction quality of the regression model for experimental data forDSDMAC/ADMBAC clay. (A) Percentage of oil removed from the oil–water emulsion,%Rem. (B) Removal capacity at equilibrium, qeq. The dashed lines indicate the regressionbands created with 95 % confidence level.

8 M.F. Mota et al. / Applied Clay Science xxx (2014) xxx–xxx

Fig. 9 presents a standardized Pareto chart for effects (initialconcentration and stirring speed) on responses (removal capacityat equilibrium and percentage of removal) when DSDMAC/ADMBACclay are used for separation of oil present in the oil–water emulsions.The initial concentration C0 plays a significant effect on both the per-centage removal and the removal capacity at equilibrium in the eval-uated experimental conditions range, and in both cases there is nosignificant effect of stirring speed on the response variables, whosep-values were determined to be higher than the significance level0.05 (p = 0.5882 for %Rem and p = 0.7810 for qeq). A similar behav-ior was observed for the two-way interaction C0 by A (p= 0.2705 for%Rem and p = 0.4710 for qeq). As depicted in Fig. 9, C0 (p = 0.0003for %Rem and p = 0.00001 for qeq) and δcurv (p = 0.0012 for %Remand p = 0.0003 for qeq) are the main factors (positive responses)with statistical significance based on p-value for the DSDMAC/ADMBAC clay. The quality of the prediction of the regression model(see Eqs. (12) and (13)) is shown in Fig. 10. The obtained model isable to satisfactorily describe the experimental data. Based on theexperimental results one can observe that both the DSDMAC andthe DSDMAC/ADMBAC clays lead to a proper removal of oil presentin the oil–water emulsions, confirming the interaction betweenclay and oil.

Please cite this article as: Mota, M.F., et al., Oil–water separation process wdx.doi.org/10.1016/j.clay.2014.06.039

4. Conclusions

The Brazilian clays modified with two different quaternaryammonium salts were properly obtained. The X-ray diffractogramsexhibited that the basal spacing (d001) of the green clay increasesafter the treatment with the two quaternary ammonium salts(DSDMAC and ADMBAC), which confirms the intercalation of theammonium cations inside the clay. The largest basal spacing isobserved in the clays treated with ADMBAC salt. Infrared spectraexhibited new bands of CH2 and CH3 vibrations, which correspondto the presence of ammonium salts inside the clay structure.

The efficiency of Na+ ion exchange by the quaternary ammoniumsalt was evaluated by thermogravimetric measurements, showing twostages of degradation, corresponding to residual water, and decomposi-tion of the quaternary ammonium salt. The improved efficiencies of OCindependent of the solvent used (gasoline, kerosene and diesel), wereproven by adsorption capacity and Foster swelling tests.

The OC from the mixture of salts (DSDMAC/ADMBAC) exhibitedperformance slightly higher than the one obtained through modifi-cation with individual salt (DSDMAC), which justifies its use in oil–water removal system. The predictions of themultiple linear regressionmodels developed based on the 22 factorial designwere excellent in theexperimental range evaluated, independent on the kind ofmodifiedOC.Finally, the adsorption capacity results confirmed the great promise of

ith organoclays: A comparative analysis, Appl. Clay Sci. (2014), http://

9M.F. Mota et al. / Applied Clay Science xxx (2014) xxx–xxx

the use of OC as adsorbents for organic solvents in adsorption for theeffluent treatment.

Acknowledgments

The authors gratefully acknowledge the Conselho Nacional dePesquisa e Desenvolvimento (CNPq), Coordenação de Aperfeiçoamentode Pessoal de Nível Superior (CAPES), Dolomil and Petrobras.

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ith organoclays: A comparative analysis, Appl. Clay Sci. (2014), http://