Transfer Phenomena Laboratory, Faculty of Mechanical Engineering and Process Engineering, University of Sciences and Technology,ouari Boumediène, Bab Ezzouar, USTHB, BP 32 EL Alia, 16111 Algiers, AlgeriaInstitut Science de l’Ingénieur, Centre Universitaire Yahia Fares, Ain D’Heb, 26001 Médéa, Algeria
r t i c l e i n f o
rticle history:eceived 3 May 2008eceived in revised form 7 August 2008ccepted 6 September 2008vailable online 13 September 2008
a b s t r a c t
Soil contamination with petroleum hydrocarbons has caused critical environmental and health defectsand increasing attention has been paid for developing innovative technology for cleaning up this con-tamination. In this work, the washing process of a soil column by ionic surfactant sodium dodecyl sulfate(SDS) was investigated. Water flow rate and the contamination duration (age) have been studied. The per-formance of water in the removal of diesel fuel was found to be non-negligible, while water contributed
eywords:oil remediationurfactantiesel fuelashing process
by 24.7% in the global elimination of n-alkanes. The effect of SDS is significant beyond a concentrationof 8 mM. After 4 h of treatment with surfactant solution, the diesel soil content remains constant, whichshows the existence of a necessary contact time needed to the surfactant to be efficient. The soil washingprocess at a rate of 3.2 mL/min has removed 97% of the diesel fuel. This surfactant soil remediation processwas shown to be governed by the first-order kinetics. These results are of practical interest in developing
iatio
osssfi
ttdetchc
effective surfactant remed
. Introduction
Soil contamination by hydrophobic components is one of theain types of pollution [1]. Up to now, various remediation
echniques have been developed [2]; among them, the washingrocesses with surfactants and biosurfactants are the most used [3].he basic principle of these methods consists of the mobilizationnd/or the solubilization of hydrocarbons by lowering the inter-acial tension at the soil/organic phase and water/organic phasenterfaces [4]. In general, surfactant soil remediation is done byetaching organic molecules adsorbed on soil and trapped in theores, followed by their encapsulation within micelles formed at aoncentration greater than the critical micelle concentration (CMC)5].
Recently, many types of surfactants have been studied [6,7].
hang et al. found that, 73.6 up to 100% of polycyclic aromaticydrocarbons (PAHs) were eliminated in the presence of sodiumodecyl sulfate (SDS), while 30–80% when using only water [8]. Theolubility of hydrocarbons being dependent on the concentration
f surfactant, Liu et al. [9] showed that the increase in the apparentolubility of some PAHs in the presence of anionic and non-ionicurfactants increases significantly beyond the CMC. According toeveral studies, the solubility of hydrocarbons depends on manyactors: the type and quantity of surfactant, and the age of contam-nation [10–13].
In the bioremediation processes, the surfactants are widely usedo increase hydrocarbons solubility, and subsequently improveheir bioavailability and biodegradability [14–16]. Surfactant reme-iation of polluted soil can be done in situ or ex situ. Dwarakanatht al. [17] analyzed the effect of numerous factors (precipita-ion and adsorption of surfactant on the ground, solubilization ofontaminant and soil hydraulic conductivity) on the removal ofydrocarbons by anionic surfactants during a treatment of a soilolumn. In order to improve the removal of hydrocarbons from soil,ifferent surfactant/additive systems have been proposed [18–20].
Lee et al. studied the influence of soil texture and quality onhe removal of organic contaminants, obtaining a removal ratio of3 and 95% in batch and column experiments, respectively. Theynergistic effect between anionic and non-ionic surfactants wasnvestigated in Refs. [21,22]. Rodriguez et al. investigated the rela-ion of SDS and Triton X100 efficiency with organic matter content
nd mineral fractions [23].
Widely used in the world, the diesel fuel represents a permanentource of soil and water pollution [24,25]. Resulting from the crudeistillation, diesel fuel is a mixture of more than 2000 compounds,hich cannot be all separated by chromatography [26,27]. Diesel
1180 R. Khalladi et al. / Journal of Hazardous Materials 164 (2009) 1179–1184
Nomenclature
Cf final soil concentration of diesel fuel after the sur-factant washing (mg/g)
Ci initial soil concentration of diesel fuel (mg/g)Ct soil concentration of diesel fuel at instant t (mg/g)Cw final soil concentration of diesel fuel after water
washing (mg/g)Cliquid liquid phase concentration of diesel fuel (mg/g)Csoil soil concentration of diesel fuel (mg/g)kp liquid and solid partition coefficient of diesel fuel
R coefficient of linear regressiont time of washing (h)
uel is composed of 60% of saturated hydrocarbons (n-alkanes andaphtenes) and 40% of aromatics [28]. In this work, we studiedhe removal of diesel fuel from soil in a glass column by a contin-ous washing process. Focused objectives were: (1) to study theemoval of the diesel fuel from a soil column using water and sur-actant solution; (2) to determine the kinetic model of the dieseluel desorption from soil.
. Experimental
.1. Materials
An anionic surfactant SDS purchased from FLUKA was usedithout further treatment (purity ≥99% and 288.38 g/mol molareight). Its critical micelle concentration (CMC = 8 mM) was deter-ined through the surface tension measures. The surface tensionas measured by an automatic thin plate Schlumberger tension-eter Pro lab. The hexane used in the extraction step was obtained
s analytical grade solvent. Distilled water was used for the tests.
.2. Soil contamination
The soil sample for the experiments was collected according tostandard procedure (AFNOR X31100) from a non-contaminated
rea located in Bordj-El-Kiffan near Algiers. The soil was air dried,omogenized and sieved to remove large particles (>0.8 mm), theninsed by acid solution followed by distilled water to avoid micro-ial culture. It was then sterilized by autoclaving at 105 ◦C for 24 h.fter washing and sterilization, the soil was artificially pollutedy a diesel-oil with a rate 7 wt.% (diesel density 0.83 g/cm3). Theontaminated soil was kept out of light in a closed vessel at room
emperature for 10 and 15 days. Soil analysis by X-ray fluorescencePanalytical MagixPro) revealed the presence of different inorganicompounds. The soil properties are given in Table 1.
cheme 1. Representation of the experimental set-up. (1) Water/surfactant solutionank; (2) polyvinyl tubing; (3) potentiometer; (4) pump; (5 and 6) valves; (7) flow
eter; (8) soil column; (9) samples tubes.
.3. Experimental set-up and procedure
The scheme of the laboratory experimental set-up is shown incheme 1. A cylindrical glass column of 150 mm length, 40 mmuter diameter and 25 mm inner diameter was used. To preventoil elution, two stainless grids (50 �m diameter) were fixed on thenput and the output points of the column. Contaminated soil waslaced in the column maintaining a constant porosity of 0.42. Aentrifugal pump (MARQUIS MKP60) was used to pump water andurfactant solution into the soil column. The pump was monitoredy a potentiometer (RH7), and the flow rate was measured withflow meter apparatus (Orang Burg, NY, USA). The effluent sam-les were collected in a glass test tube and kept cold (at 4 ◦C) fornalysis.
The treatment of contaminated soil was carried out in two steps:ashing with distilled water then by surfactant solution. During theater washing process, the removal of diesel fuel from soil and the
ontribution of water in the overall process was determined fromhe following equations, respectively:
limination rate (%) = Ci − Cw
Ci× 100 (1)
here Ci (mg/g) is the initial soil concentration of n-alkanes, andw (mg/g) is the final soil concentration of n-alkanes after waterashing.
ontribution rate (%) = Ci − Cw
Ci − Cf× 100 (2)
here Cf (mg/g) is the final diesel fuel concentration in soil (at thend of the surfactant treatment).
When using surfactant, the elimination of n-alkanes from soily surfactant was determined as follows:
limination rate (%) = Cw − Cf
Ci× 100 (3)
.4. Extraction and analysis
Samples to be analyzed were of two types: solid and liquid. Theolid samples (2 g) were crushed and mixed with anhydrous sodiumulfate (4:5 w/w), then placed with hexane in a Soxhlet extrac-or for 6 h. 5 mL of effluent were mixed with 2 mL of hexane in a
eparating funnel; stirred for 2 min then left at rest for separation.upernatant phase was analyzed for diesel fuel. In the case of sam-les containing surfactant, the addition of organic solvent produceshe formation of a trouble solution (supposed to be an emulsion). Aentrifugation at 6000 rpm was realized during 20 min to separate
rdous Materials 164 (2009) 1179–1184 1181
tap(
sii6tigimia
3
3
lSswto
descoefsabdfe
F
Table 2Diesel fuel elimination rate by water (15 days contamination)
Time of washing (h) Flow rate (mL/min) Elimination rate (%)
33
3
fu
raoflrrflwbtttTd
rrtsiF
3
R. Khalladi et al. / Journal of Haza
he organic phase. The extracts were concentrated then filtered onsilica gel column to eliminate all polar compounds. All the sam-les (1 �L) were mixed with a specific amount of internal standardHeptacosan C27) and analyzed by gas chromatography.
Since their high proportion, n-alkanes were chosen as repre-entative components of diesel fuel, and their total concentrationn solid and aqueous samples was taken as that of diesel fuel. Thedentification of n-alkanes was performed by gas chromatograph890 (series II Hewlett-Packard) coupled with a mass spectrome-er model 5973 Network. The diesel fuel (n-alkanes) concentrationn solid and liquid samples was determined by a gas chromato-raph (GC; HP6890 Hewlett-Packard) equipped with a flameonization detector (FID), an integrator HP 3395, and a capillary
ethylpolysiloxane column HP1 (30 m × 0.25 mm × 0.25 �m). Thenjector and detector were kept at 280 ◦C and the column temper-ture was programmed from 35 to 300 ◦C with a rate 4 ◦C/min.
. Results and discussion
.1. Surface tension of the surfactant–soil system
Surfactant efficiency in the remediation of contaminated soils isimited by their adsorption on the solid matrix. Within this context,hen [29] had studied the dependence of surfactant sorption onoil composition. The surface tension of different solutions of SDSas measured in presence of 3 g of uncontaminated soil at room
emperature. Fig. 1 shows a plot of the surface tension as a functionf SDS concentration in presence and absence of soil.
Fig. 1 shows that the two curves are similar. The surface tensionecreases versus SDS concentration toward a constant value. How-ver, in the presence of soil, surfactant solution exhibits a lowerurface tension. The small gap observed between the two curvesan be explained by the presence of mineral molecules (Table 1)riginated from soil. According to Puisieux and Seiller [30], addinglectrolyte to a surfactant solution leads to the reduction of the sur-ace tension. Where, these electrolytes encourage the migration ofurfactant molecules to the interface solution/air and promote theirdsorption on the interface. According to the surfactant–soil system
ehavior, the presence of mineral components in soil enhances theecrease of the surface tension. Thus, the risk of losing some sur-
actant by sorption on soil and subsequently the reduction of itsffectiveness does not arise.
ig. 1. Effect of soil on the surface tension of SDS. (�) SDS and (�) SDS + soil.
sehfpbtc
crddArbdcta
mpaol
7 30.8 32.94 18.5 24.74 3.2 24.8
.2. Water washing
The soil column was first washed with distilled water using dif-erent flow rates. The removal of diesel fuel by water was quantifiedsing Eq. (1) and the results are given in Table 2.
Table 2 shows that a non-negligible amount of diesel wasemoved by water. In addition, the extracted amount of diesel fuelfter 34 h for a flow rate of 18.5 mL/min was the same as thatbtained with a flow rate of 3.2 mL/min. Thus, for a relatively highow rate, the wash time has no significant effect on the diesel fuelemoval. As a result, water was said exerting a limited effect on theemoved quantity of diesel fuel that remains constant even if theow rate and the washing time were increased. During the waterash, the effluent concentration increased significantly, and thenecame constant at the end of the process. This result allowed uso consider the effluent concentration as a constant before startinghe treatment with surfactant. Furthermore, the diesel fuel concen-ration was shown more stable at low flow rate than at higher one.he contribution of water in the overall remediation process wasetermined from Eq. (2).
After 48 h of washing, the contribution of water in the overallemediation process was found to be 24.7% for the different flowates listed in Table 2. Therefore, the use of water in the remedia-ion of highly contaminated soil as a first treatment solution washown of great interest. This result proved the performance of watern the remediation of hydrocarbon-contaminated soil as found byernandez and Luque de Castro [7].
.3. Treatment by surfactant
According to Taylor et al. the removal of hydrocarbons from aandy soil by surfactants is governed by mass transfer phenom-na between solid, liquid and micellare phases. Additionally, theydrocarbon solubility is highly dependent on the flow rate of sur-actant solution [31]. In the following parts, the SDS solution wasumped at a constant rate of 3.2 mL/min. Compared to that usedy Pennell et al., this flow rate is moderately high [32]. Fig. 2 showshe plot of the effluent concentration versus time for two differentontaminations.
Given the proportional relation between organic matter soilontent and hydrocarbons sequestration, it was expected that theemoval of diesel fuel from soil will be different in the 10 and 15ays aged soil. Fig. 2 shows that, a contamination age difference of 5ays had no significant effect on the removal of n-alkanes from soil.fter 12 h of surfactant treatment, a maximum concentration waseached and maintained for more than 20 h. The similarity observedetween these two curves could be interpreted by the fact that theiesel fuel in soil needs a long time to behave differently. In thease of recent pollution incidents, we suggest that it is importanto determine the maximum tolerable time interval before startingny remediation process.
According to Fig. 2, the surfactant behavior during the treat-
ent of the diesel-polluted soil could be divided into three distinct
arts. First, the direct contact between the surfactant moleculesnd pollutant molecules initiates mobilization process. Therefore,rganic molecules were desorbed from soil and could be encapsu-ated within micelles, if formed. Consequently, n-alkanes effluent
1182 R. Khalladi et al. / Journal of Hazardous Materials 164 (2009) 1179–1184
F(
cFatttc
e
FdC
3ep
ig. 2. Effluent concentration of diesel fuel versus the washing time. (�) 10 days and�) 15 days.
oncentration increased versus time ending at a maximum value.rom that maximum, the concentration was almost constant forbout 20 h showing the existence of a different stage. Finally, inhe third part, hydrocarbons effluent content decreased graduallyo lower concentrations (0.1–0.2 mg/mL), reflecting the decrease of
he number of accessible organic molecules, explaining why hydro-arbons desorption became more and more difficult.
The diesel fuel used in this work was characterized by the pres-nce of 19 hydrocarbons (Fig. 3) ranging from C8 to C26 representing
0% of the diesel fuel. In order to investigate the effect of SDS onach n-alkane, the effluent concentration of each compound waslotted versus the washing time.
Fig. 4 shows that, these components could be divided into threeifferent classes. The first class was the minor fraction (C12–C14)epresenting from 6.5 to 11.5% of n-alkanes. The second class rep-esented n-alkanes ranging from C15 to C20 (4.1–14.9% of n-alkanes).his fraction constituted the major part detected during the SDS soilreatment, reflecting their high content in diesel fuel (Fig. 3). Theast class was that of n-alkanes ranging from C21 to C26 present withsmall amount in the diesel fuel. As a result, we deduced that theDS exerted no preferential effect on the various n-alkanes presentn diesel. However, it should be noted that, interactions between theifferent diesel components might have an effect on this results.
Following the diesel fuel concentration in soil during the sur-actant washing (result not represented), it was deduced that, afterh of SDS treatment, the soil content of hydrocarbons was almostnchanged. This result indicates the existence of a necessary con-act time for the surfactant to be active. Beyond 4 h, the hydrocarbonoil concentration decreased considerably. The elimination rate of-alkanes from soil by SDS was determined using Eq. (3).
According to Table 3, the SDS was remarkably efficient in elimi-ating n-alkanes from soil. However, it should be noted that underhe same operating conditions (age of contamination, flow rate andime of treatment), 4 mM SDS solution removed 3.5% of n-alkaneshile 8 mM solution removed 73%.
.4. Kinetics of diesel removal
The plot of measured soil n-alkanes concentration versus time
sing kinetic models illustrated the diesel fuel desorption fromoil. The first- and second-order model used were expressed byhe following equations, respectively:
t = Ci exp(−k1t) (4)
able 3limination rate of diesel fuel by 8 mM SDS solution
ontamination age (day) SDS treatment yield (%) Total treatment yield (%)
15 73.1 73.710 97.2 98.0
R. Khalladi et al. / Journal of Hazardous Materials 164 (2009) 1179–1184 1183
Table 4First- and second-order coefficient
C
wtr(
mfisfkrets
ttita(
drtc
wCpa
F8
Table 5Kinetic of the soil-liquid partitioning of diesel fuel
Contamination age (day) Concentration of SDS (M) kp (h−1) R2
here Ct (mg/g), is the diesel fuel concentration in soil at instant, Ci (mg/g) is the initial concentration of soil, k1 and k2 are theate constants of the first and second order expressed in (h−1) andg g−1 h−1), respectively, t (h) is the time of washing.
The values of rate constant k for the first- and second-orderodel are given in Table 4. Based on the square regression coef-
cient R2, the first-order model fitted the experimental resultslightly more than the second-order model. Accordingly, the dieseluel removal from soil was found to be governed by first-orderinetic model. Similarly, Zhao et al. reported that hydrocarbonsemoval from soil was governed by first-order kinetics [33]. How-ver, according to the coefficient R2 values (Table 4) it was obvioushat the surfactant soil remediation could be described by theecond-order kinetic model.
The relationships of organic substances solubility with surfac-ant type and concentration had been well studied [9,10,20,32]. Inhis work, the removal of diesel fuel from soil was studied throughts partitioning between the liquid phase and soil. The ratio betweenhe diesel fuel concentration in effluent and soil was plotted as
function of washing time during the surfactant soil treatmentFig. 5).
Fig. 5 illustrates the behavior of diesel fuel versus time for threeifferent experiments. This figure indicates that there is a linearelationship between the time and the ratio of diesel fuel concen-ration in liquid and solid phases in the three cases. This relationshipould be expressed as:
Cliquid
Csoil= kpt (6)
here Cliquid (mg g−1) is the effluent concentration of diesel fuel,soil (mg g−1) is the soil concentration of diesel fuel, kp (h−1) is theartition coefficient of the hydrocarbon between the liquid phasend soil, and t is the time of washing (h). The linear regression for
ig. 5. Partitioning of diesel fuel between liquid phase and soil (Q = 3.8 mL/min): (�)mM SDS, 15 days age; (�) 8 mM SDS, 10 days age; (�) 4 mM SDS, 15 days age.
he three curves shows that, for a low SDS concentration, the parti-ion coefficient was too small compared to that obtained with theigh concentration of surfactant for both the two contaminations.owever, the age of contamination has no significant effect on theartition of the diesel. The results are given in Table 5.
From these results, it was obvious that an efficient desorption ofiesel fuel molecules from soil corresponded to the increase of theatio Cliquid/Csoil. This ratio was found depending on the solubilityf the organic substance as well as on the surfactant concentration33]. The combination of this result with those of kinetic study,eads to a well comprehension of the mass transfer that occurs inuch a system.
. Conclusion
In a surfactant soil remediation, the presence of mineralsmproved the anionic surfactant activity on the surface (SDS sur-ace activity). This result showed the effect of soil compositionn the surfactant efficiency during the remediation of such a soil.ater washing of a diesel-polluted soil could eliminate up to
4% of n-alkanes. This low percentage is of great economic inter-st, especially for important quantity of polluted soil. Therefore,water washing process can be recommended before any other
emediation process to reduce the hydrocarbon soil content andubsequently the consumed surfactant quantity. An eliminationate of 97% was achieved after a soil washing by 8 mM SDS solutionith a rate 3.2 mL/min. According to this study, for close contam-
nation ages, the effect of surfactant was approximately the same.urthermore, the diesel fuel removal was almost negligible dur-ng the first 4 h, and then it increased with time and seemed to betabilized after 15 h. Concerning the remediation process kinetics,first-order kinetic model was found to be well adapted for the
verall treatment. Partitioning of diesel molecules between liquidnd solid phases was shown linearly dependent on time.
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