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204 M. Aoudia et al. / Desalination and Water Treatment 29 (2011) 203–209management during 2000–2008 for disposal into the deep Omani native soil (sand) in removing aromatic crude oilreservoirs alone . This has led PDO to investigate ways components from Nimr oil field (Oman) produced water.of treating oil field produced water for use as drinking Our focus on aromatic crude oil components is justified bywater, irrigation etc. One approach was based on the their known extreme toxicity even at very low concentra-use of reed bed-cultivated at Nimr . Most of the con- tions and their adverse effect on human health [20,21]. Totaminating oil was removed by the action of plant roots. the best of our knowledge, most studies reported on theHowever, concern remains that contaminants could enter removal of organic solutes by surfactant-modified soilsin the human food chain. Accordingly, a second process- were carried out by pumping the contaminated water intoing phase involving “solar dew” distillation technology the column packed with an already surfactant-modifiedwas used to yield demineralized water to eliminate the soil. In our approach, surfactant will be added to thefood-chain risk. But the economic viability of the entire contaminated water and the resulting aqueous surfactantprocess has not been worked out . A joint investigation solution will be forced to percolate through the columnby PDO and Sultan Qaboos University  was carried out packed with native soil. Thus, in this approach, soilusing combined air-flotation and adsorption (anthracite modification by the surfactant molecules occurs duringand activated carbon) to remove oil. The process reduced the water treatment process.the oil content from 100–300 ppm to less than 0.5 ppm.Such process, however, seems to be inappropriate in a 2. Experimentallarge scale application. More recently , we showedthat micellar enhanced ultrafiltration (MEUF) could be 2.1. Materialseffective in reducing oil content to less than 0.5 ppm, but Native soil (sandstone) from Dhank (Sultanate ofthe cost of the membrane could be a serious limitation. Oman) was used in this study. The sand was sieved to ob- Extensive research is currently underway on the tain particle sizes in the range 100–200 mm. The sand wasuse of surfactant-modified clays and soils to treat water then washed with de-ionized water and heated at 120°Ccontaminated with organic pollutants [6–15], a promis- overnight. Dodecyltrimethylammonium (DDTMA) bro-ing approach justified by the fact that natural soils are mide (> 99% pure) was purchased from Acros Organics,ineffective sorbents for hydrophobic solutes due to USA. This cationic surfactant was used without furthertheir hydrophilic nature. This is due to the fact that the purification. Its critical micelle concentration (CMC) inadsorption of hydrophobic solutes onto polar soil miner- distilled water is 1.6×10–2 M .als is suppressed by the strong competitive adsorption Produced water from Nimr oil field (South Oman) wasof water. Accordingly, surfactants are used to modify provided by Petroleum Development Oman (PDO). Thethe soil nature from hydrophilic to hydrophobic upon produced water, a turbid suspension of crude oil droplets,adsorption of the surfactant at the substrate surface. contains about 8.2 g/L total dissolved solids (TDS).Surfactant adsorption onto solid minerals has been ex- RF-5301PC spectrofluorimeter (Shimadzu, Japan) wastensively studied and was shown to depend on many used for measuring fluorescence spectra.parameters: i) the nature of the substrate, ii) the molecularstructure of the surfactant being adsorbed, and iii) the 2.2. Procedureenvironment of the aqueous phase [16,17]. In particular,the mechanisms by which ionic surfactants may adsorb 2.2.1. Column leaching testfrom aqueous solution onto an oppositely charged solid Column experiment was carried out at room tempera-substrate is a complex process, during which adsorp- ture (Fig. 1). The porous plate inside the column beneathtion of the surfactant may occur successively by ion the soil prevented loss of soil during leaching. 200 g ofexchange, ion pairing, and hydrophobic bounding. At sand was incrementally packed to a height of 15.5 cm inlow concentration, surfactant monomers are adsorbed glass columns (3.5 cm diameter and 35 cm height), Next,by ion exchange and eventually form a monolayer with distilled water was pumped into the packed column atthe hydrophilic surfactant head adsorbed on the surface a constant flow rate of 1.65 ml/min overnight to saturateand the hydrophobic surfactant tail oriented towards the the column. The feed solution (Nimr produced wateraqueous phase, thereby modifying the soil surface from with and without added surfactant) was pumped intohydrophilic to hydrophobic. As the amount of surfactant the column at the same rate. Permeates were collectedincreases, hydrophobic interaction among hydrocarbon in individual 100 mL for analysis.tails causes the formation of bilayer (a hydrocarbonphase) that function as a powerful solubilizing medium 2.2.2. Chemical analysis[18,19] for organic compounds. At this stage, adsorptionis usually complete, and in many cases this occurs in the Aromatic compounds are known for their strongneighborhood of the critical micelle concentration . fluorescence emission. Therefore, fluorescence was used In the present study, our main objective is to demon- to estimate the relative concentration of aromatics in thestrate laboratory feasibility of using surfactant-modified non-treated oil field produced water and in the treated
M. Aoudia et al. / Desalination and Water Treatment 29 (2011) 203–209 205 C p = b ⋅ Ap and C0 = b ⋅ A0 (2) where b is a constant (instrumental parameter). Substitu- tion of Eq. (2) into Eq. (1) leads to % R = 1 − Ap / A0 × 100 ( ) (3) The validity of Eq. (3) assumes that the total fluo- rescence area varies linearly with the concentration of the fluorescent species. Thus, fluorescence spectra were measured for untreated Nimr produced water diluted with distilled water to obtain different volume fractions (Fig. 2). Areas of the total emission intensity (At) were extracted from spectra shown in Fig. 2 and plotted versus untreated Nimr produced water diluted with distilled water at different mixture compositions in Fig. 3. Clearly, a linear relationship is observed, with an intercept at the origin Ar = 645.58, reflecting a residual emission even in the absence of aromatics at infinite dilution. In fact, residual emission intensity from distilled water was also observed (Ar = 598.281). Hence, the total emission inten- sity from the different samples appears to be the sum of two contributions, namely a contribution from the true fluorescence emission (Af) from the excited singlet statesFig. 1. Schematic representation of column leaching (S1) of the aromatic molecules to their correspondingexperiment. ground states (S0) and a contribution from scattering emis- sion (Asc) intensity. Accordingly, the true fluorescence area could be estimated from the following relationwater (permeate) from the area of their respective fluores- A f = At − Asc (4)cence spectra. All samples were excited at lexc = 235 nm. Plot of the true fluorescence Af vs. Nimr produced 2.2.3. Retention of aromatics water concentration is shown in Fig. 3 (inset). An excel- In all cases, the parameter which quantifies the ef- lent linear fit (R2 = 0.99687) is observed with an interceptficiency of oil removal is defined by the percentage of at the origin equal to zero, suggesting correctness of as-rejection R factor (% R) as expressed by sumptions made in Eq. (2). Therefore, in this study, the ( )% R = 1 − C p / C0 × 100 (1) 180where Cp and C0 refer to the analytical concentration of 10pollutants in the collected permeate and the non-treated 160 9produced water, respectively. However, the determina- 140 Fluorsecence Intensity (au) 8tion of Cp and C0 in many complicated systems such as 7 120crude oil contaminated water can be a very time-con- 6suming and challenging task. Accordingly, in this work 100 5we propose a simple and efficient method to estimate the 80rejection of crude oil aromatic components by fluores- 4cence emission measurements from the untreated water 60 3and the treated water (permeate). In this approach, the 40 2ratio (Cp/C0) is replaced by the ratio AP/A0, where AP and 1 20A0 are the total fluorescence area of the permeate and the 0feed solution (untreated Nimr water), respectively. This 0 250 300 350 400 450is because, under similar experimental conditions, total Wavelength/nmconcentration of aromatic components in the permeate(Cp) and the corresponding concentration in the untreated Fig. 2. Fluorescence spectra for diluted Nimr produced water:Nimr water (C0) can assumed to be proportional to the distilled water (0), 10% (1), 20% (2), 30% (3), 40% (4), 50% (5),areas of their respective fluorescence spectrum 60% (6), 70% (7), 80% (8), 90% (9) and 100% Nimr water (10).
206 M. Aoudia et al. / Desalination and Water Treatment 29 (2011) 203–209 10000 140 Fluorescence Emission (au) 8000 120 Af 6000 8000Total Emision Area (au) 4000 100 2000 0 80 6000 0.0 0.2 0.4 0.6 0.8 1.0 Dilution factor 60 4000 40 20 2000 0 250 300 350 400 450 0 Wavelength/nm 0.0 0.2 0.4 0.6 0.8 1.0 Dilution Factor Fig. 4. Fluorescence spectra for: untreated Nimr water (), successive individual 100 mL permeate samples (o) and dis- Fig. 3. Calibration curve showing the variation of the total tilled water (+) treated with unmodified native sand (200 g, emission (At) area vs. dilution factor. Inset: variation of the 100–200 mm). true fluorescence emission (Af) with dilution factor. 100 percentage of rejection (% R) was calculated from the true 90 fluorescence of the sample (corrected from scattering) by using Eq. (3). 80 70 %R 60 3. Results and discussion 50 3.1. Retention of aromatic components by native sand 40 In this experiment, we investigated the rejection of 30 aromatic components from Nimr oilfield produced water 20 with unmodified native sand (200 g, 100–200 mm). Perme- ates were collected in ten individual 100 mL samples. 10 All fluorescence spectra are presented in Fig. 4. True 0 fluorescence areas (Af = At – A0) were extracted from 0 200 400 600 800 1000 these spectra and rejections of aromatic pollutants were VP/mL calculated according to Eq. (3) and are shown in Fig. 5. Interestingly, the first 100 mL permeate sample showed Fig. 5. Variation of the percent rejection of aromatic compo- a significant rejection of 83.95%. However, the rejection nents with cumulated volume of permeate collected in the capacity of the natural sand was reduced to 8.52% as the treatment of Nimr oil field produced water with: unmodified amount of cumulative volume of collected permeate is native sand (o), DDTMA (10–3 M)-modified sand (), DDTMA increased from 100 to 1000 mL. Two mechanisms were (5×10–4 M)-modified sand (×), and DDTMA (10–4 M)-modified sand (+), 200 g sand (100–200 mm). proposed to account for the sorption of hydrocarbon solutes in soil–water systems. First, the sorption might be attributed to the adsorption of aromatics onto the soil surface . The second mechanism could be associated and to the saturation of soil surface with the adsorbed with a partition process involving hydrophobic interac- aromatic molecules. tions between soil organic matter (OM) and the aromatic pollutant, in which the OM acts as a natural solubilizing 3.2. Retention of aromatic components by surfactant- medium for the pollutant. In fact, X-ray fluorescence modified sand analysis (Table 1) shows that the sand may contain as much as 6.11% natural organic matter. The decrease Surfactant adsorption onto solid surfaces is strongly in pollutant retention with the increase in cumulative influenced by the surfactant concentration. In particu- volume of permeate is probably due to the decreasing lar, a plateau adsorption generally occurs at or near the capacity of the soil OM in solubilizing aromatic molecules critical micelle concentration (CMC) and is characterized
M. Aoudia et al. / Desalination and Water Treatment 29 (2011) 203–209 207Table 1X-ray fluorescence (XRF) analysis for the native sand collected 46from Dhank, Oman 44 Surface Tension/mNm-1Compound % wt.SiO2 64.11 42Al2O3 5.63Fe2O3 1.81 40CaO 13.81MgO 6.43 38SO3 0.08K2O 1.00 36Na2O 0.98Mn2O3 0.01 2.0x10-4 4.0x10-4 6.0x10-4 8.0x10-4 1.0x10-3TiO2 0.01 [DDTMA]/mol L -1P2O5 0.01Cl 0.00 Fig. 6. Variation of surface tension with DDTMA concentrationTotal* 93.89 in Nimr oil field produced water at 23°C.*The remaining 6.11% may be organic matter (OM) 140by no increase in adsorption with increasing surfactant Fluorescence Emission (au) 120concentration. The critical micelle concentration (CMC)of DDTMA in distilled water is reported to be 1.6×10–2 M 100at 25°C . However, the presence of electrolyte in Nimr 80water may affect drastically the CMC . Accordingly,we estimated the CMC of DDTMA in Nimr water from the 60variation of surface tension with surfactant concentration 40(Fig. 6). As shown in this figure, the micelle formationin Nimr water occurs at 5.3×10–4 M, considerably less 20than the CMC value of the surfactant in distilled water 0(1.6×10–2 M). This depression in the CMC is attributed 250 300 350 400 450to the decrease in the electrical repulsion between the Wavelength/nmsurfactant cationic head groups in the micelles in the Fig. 7. Fluorescence spectra for: untreated Nimr water (), suc-presence of electrolyte . cessive individual 100 mL permeate samples (o) and distilled In this experiment, dodecyltrimethylammonium water (+) treated with DDTMA (10–3 M)-modified sand (200 g,bromide (DDTMA) was added to the untreated Nimr 100–200 mm).produced water at a concentration of 10–3 M, concentra-tion above its CMC. Permeates were collected in succes-sive individual 100 mL samples to get a total volume or provide a powerful partition medium for organic pollut-1000 mL. All fluorescence spectra are presented in Fig. 7. ants. Hence, this significant surfactant-induced rejectionRejections of aromatic components of the crude oil in enhancement may be attributed to the combined effectNimr water were calculated according to Eq. (3) and are of the two mechanisms mentioned above, namely ionplotted in Fig. 5. Interestingly, almost complete rejections exchange and hydrophobic–hydrophobic interactions.(~100%) were observed in the presence of the surfactant.This drastic increase in surfactant-modified rejection of 3.3. Effect of surfactant concentration on the rejectionaromatics can be attributed to the known fact that soilswith cation-exchange capacity (CEC) might be modified The above experiment was repeated at the surfactantby ion exchange with large surfactant molecules to in- CMC (5×10–4 M) and below the CMC (10–4 M). Permeatescrease their organic matter contents, thereby providing were collected in individual 100 mL samples. Fluorescenceadditional organophilic moiety favorable for the sorption spectra from these treated samples are given in Fig. 8of organic contaminants. Furthermore, surfactant tails (5×10–4 M) and Fig. 9 (10–4 M). Plots of R% vs. cumulativecan interact with each other (hydrophobic–hydrophobic volume of permeate collected (VP) are displayed in Fig. 5.bounding) and form a hydrophobic bilayer that can also Interestingly, rejections observed with surfactant system
208 M. Aoudia et al. / Desalination and Water Treatment 29 (2011) 203–209 160 CMC and in the absence of surfactant (Fig. 5). This invari- 140 ance may be attributed to the occurrence of an equilibrium between adsorption and desorption of aromatic solutes Fluorescence Emission (au) 120 onto the solid substrate. Previous studies showed that 100 hexadecyltrimethylammonium bromide (HDTMA) held 80 at ion-exchange sites was resistant to desorption, whereas the sorption of surfactant held via hydrophobic bonding 60 was more readily desorbed [24,25]. Current work on the 40 dynamic adsorption–desorption of DDTMA onto the 20 native sand is in progress in our laboratory. Finally, it is worth to note that highest rejections were 0 250 300 350 400 450 obtained with surfactant systems above the critical micelle Wavelength/nm concentration. This appears to indicate that hydrophobic– hydrophobic interaction among surfactant hydrocarbon Fig. 8. Fluorescence spectra for: untreated Nimr water (), suc- plays a major role in the rejection efficiency. This interac- cessive individual 100 mL permeate samples (o) and distilled tion causes the formation of bilayer (a hydrocarbon pool) water (+) treated with DDTMA (5×10–4 M)-modified sand that function as a powerful solubilizing medium for the (200 g, 100–200 mm). aromatic solutes [18,19]. At this stage, surfactant adsorp- tion reaches a plateau, and in many cases this occurs in the neighborhood of the critical micelle concentration . Accordingly, surfactant systems having very low critical 350 micelle concentrations can be suitable for low-cost water treatment processes.Fluorescence Emission (au) 300 250 200 4. Conclusion 150 Rejection of aromatic hydrocarbon components from produced oil field (Nimr) water by native and surfac- 100 tant (DDTMA)-modified native sand was investigated. 50 Excellent rejections (~100%) were achieved at surfactant concentrations at/and above the critical micelle concentra- 0 tion (5×10–4 M and 1×10–3 M). In the absence of surfactant 250 300 350 400 450 Wavelength/nm (unmodified native sand) and in the presence of surfac- tant below its CMC (1×10–4 M), the rejection first decreases Fig. 9. Fluorescence spectra for: untreated Nimr water (), suc- with the cumulative volume of permeate collected (VP) cessive individual 100 mL permeate samples (o) and distilled and then reaches a plateau (10% with native sand and water (+) treated with DDTMA (10–4 M)-modified sand (200 g, 50% surfactant-modified sand), at a particular value of 100–200 mm). VP. Surfactant concentration is therefore a vital parameter for a successful surfactant-enhanced water treatment process. In particular, surfactant systems above the criti- at the CMC were similar to those observed with the cal micelle concentration (CMC) can be very effective in surfactant system above the CMC (10–3 M). On the other solute rejection. Clearly, surfactant-modified soils could hand, at 10–4 M surfactant concentration (below the CMC), be successfully used to enhance removal of aromatic a relatively high rejection (~90%) was observed only with components from contaminated oil field produced water. the first individual 100 mL permeate sample collected, followed by a decrease by a decrease as the cumulative permeate volume is increased to reach a limiting value Acknowledgment (~50%) at VP around 600 mL. These results clearly indicate We express word of appreciation to Mike Thomson, that maximum rejections can be achieved only above the Head of MAF laboratory at PDO for providing Nimr surfactant critical micelle concentration. It is, therefore, produced water and crude oil samples. essential to use surfactants characterized by low CMCs in order to reduce the amount of surfactant required in a large scale water treatment process. It is also interesting to References note the invariance of the rejection at a critical cumulative  Petroleum Development Oman (PDO), Water, water everywhere. volume of permeate (VP) for surfactant system below the Al mandhal, 1 (2009) 2–6.