Environ. Sci. Technol. 2003, 37, 5034-5039
Oil Removal from Water by Sorption on Hydrophobic Cotton Fibers. 2. Study of Sorption Properties in Dynamic Mode
In the same manner, we demonstrated that cotton treated by acylation of the cellulose with fatty acids (5) showed good sorbing properties for oil (6). To complete this work, we have studied the efficiency of this treated cotton during the recovery of oil from an oilin-water emulsion and the phenomena related to this separation.
GERALD DESCHAMPS,§ HERVE CARUEL,§ M A R I E - E L I S A B E T H B O R R E D O N , * ,§ CLAIRE ALBASI,† JEAN-PIERRE RIBA,† CHRISTOPHE BONNIN,# AND CHRISTIAN VIGNOLES‡ Laboratoire de Chimie Agro-Industrielle, UMR INRA/INP, ENSIACET, 118 Route de Narbonne, 31077 Toulouse Cedex 4, France, Laboratoire de Ge´nie Chimique, UMR CNRS/INP, 5 Rue Paulin Talabot, BP 1301, 31106 Toulouse Cedex 1, France, Anjou Recherche, Chemin de la digue, 78603 Maisons Laffitte, France, and Ge´ne´rale des Eaux, ZAC La Plaine, 22 Avenue Marcel Dassault, BP 5873, 31506 Toulouse Cedex 5, France
Experimental Section
The recovery of oil from an oil-in-water emulsion, during a flow through a bed of cotton rendered hydrophobic by acylation of cellulose was defined by sorption and coalescence phenomena. During percolation, the column “hold-up” (difference between injected and rejected oil) became constant at the equilibrium volume, i.e., as soon as the instant oil concentration in the effluent (C) was equal to the oil concentration in the initial emulsion (C0). This equilibrium permitted the measurement of the cotton sorption capacity (SC), which increased with C0 up to the cotton saturation. The oil-water separation improved at a lower temperature, lower flow, a deeper medium, and larger oil drops. The system was modeled as a piston flowthrough in order to generalize the results.
Introduction The water contamination by fats is a major problem. The contaminants often exist as small dispersed droplets, whose recovery is difficult. In addition to the resulting ecological disasters, this pollution also reduces the performance of purification plants, leading to a more severe impact. Therefore, for many years, several pollution control techniques have been developed. Flotation (1) is widely used for wastewater treatment. Hydrocyclones are mentioned (1), but they are not useful because they do not generate enough force to separate emulsions. A simple settling is also possible (2), but this procedure is slow and not really efficient. Ultrafiltration (2), not suitable to high concentrations, is also mentioned. Another technique consists of the use of vegetable sorbents as filtration beds. Some studies have been undertaken on the recovery of oil from an emulsion after flow through a bed of kenaf (3) or peat (4). * Corresponding author phone: +33 5 62 88 57 26; fax: +33 5 62 88 57 30; e-mail:
[email protected]. † UMR CNRS/INP. ‡ Ge ´ ne´rale des Eaux. § UMR INRA/INP, ENSIACET. # Anjou Recherche. 5034
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Type of Sorbent. The treated cotton (length between 12 and 35 mm, diameter between 20 and 25 µm) was obtained by acylation of the cellulose with octanoic acid, under microwaves radiation (5). The degree of substitution (DS) value was 0.15, and the degree of polymerization (DP) value was 655. The cotton oil sorption capacity (SC), in the static mode, was 20 g/g (6). Type of Pollutant. The tested pollutant was a vegetable oil (ISIO 4 from LESIEUR). It was a mix of saturated (10%) and unsaturated (42% of monounsaturated and 48% of polyunsaturated) fatty acids. Its density was 0.91, and its dynamic viscosity was 0.065 Pa‚s (T ) 20 °C). The emulsion was obtained by successive introduction of water, an anionic surfactant (sodium dodecyl sulfate (SDS) in a 10% aqueous solution, the amount of SDS solution represents 20% of the oil quantity), and oil. It was then magnetically stirred at 1100 rpm during 30 min until the oil drop diameter did not vary any more. The drop size measurement was carried out with a Turbiscan-On-Line (apparatus with a light source and two detectors, one in transmission, the other in backscattering), used to characterize concentrated dispersions on line by measuring the evolution of the transmitted flux through the solution and thus the instantaneous diameter of oil drops. Procedure. The glass column, with a length of 400 mm and an internal diameter of 30 mm was jacketed to regulate the temperature. The bed was constituted of 4 g of treated cotton of 90 mm height and 30 mm diameter. Void was estimated to be 75%. The emulsion was injected in the column, with a peristaltic pump, at fixed temperature and flow, with stirring. The effluent was collected by fractions of varying volumes. Oil Quantification. The oil sorbed by the cotton was calculated by the difference between the amount of oil introduced and the oil quantity value in the effluent. The latter and the instant oil concentration (C) were measured by weighing after extraction with hexane and sodium chloride (7), drying on sodium sulfate, filtration, and solvent evaporation.
Results and Discussion Oil-Water Separation. An oil-in-water emulsion at C0 ) 5 g of oil per liter of water was introduced at 20 °C into the system at 95 mL/min. The evolution of the oil quantity in the effluent was then registered (Figure 1 and Table 1). The system breakthrough was considered to occur when a part of the introduced oil passed into the effluent (8). According to Aurelle (9), the system is ruled by two distinct regimes: (1) a transitional regime divided in two phases [(a) a pure sorption phase, in which the introduced oil is sorbed (phase 1) and (b) a mixed phase, in which a part of the oil begins to be released and sorption decreases. During this regime, the column “hold-up” increases (phase 2).] and (2) a permanent regime, reached at equilibrium volume in which the released oil quantity equals the introduced one. The “hold-up” becomes then constant. Moreover, the cotton 10.1021/es020249b CCC: $25.00
2003 American Chemical Society Published on Web 10/03/2003
FIGURE 1. Column “hold-up” evolution during oil-in-water emulsion percolation. Key: [ injected oil; 9 released oil. (D ) 95 mL/min; T ) 20 °C).
FIGURE 2. Breakthrough curve of the oil-in-water emulsion percolationsystem. (D ) 95 mL/min; T ) 20 °C). behaves as a coalescer, and, in the effluent, the oil drop size greatly increases. Thus, settling of the effluent becomes really easy. In these conditions, the breakthrough volume was 3.2 L. For an equilibrium volume of 14 L, the “hold-up” and so the sorption capacity was 7.5 g per g of cotton, for a C0 ) 5 g/L emulsion (Figure 1 and Table 1). These values were confirmed by the breakthrough curve of the system (Figure 2). Influence of Temperature. Temperature influenced oil recovery from an oil-in-water emulsion (Figure 3). Indeed, whatever the initial oil concentration C0, the higher the temperature, the higher the oil quantity in the effluent was, despite an increase of the breakthrough volume and thus a breakthrough delay (Table 2).
According to Aurelle (9), when temperature increases, the viscosities of the dispersed and the continuous phases decrease. It is the same for the superficial tensions of each component of the emulsion and for the interfacial tension. The variations of these properties lead to two opposite effects on coalescence. The decrease in interfacial and superficial tensions is unfavorable for coalescence. On the contrary, during the emulsion percolation through the bed, the viscosity reduction of the continuous phase makes it easier for the collision of oil drops, and their contact with the solid, which is favorable for coalescence. In our case, the temperature has a great influence on viscosity (for T ) 20, 30, and 40 °C, η ) 0.065, 0.045, and 0.023 Pa.s, respectively), which may explain why the oil fixation on the cotton was easier for higher temperatures (coalescence was favored), delaying the breakVOL. 37, NO. 21, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Influence of temperature on oil-water separation; D ) 95 mL/min. (percolation of 4 L of emulsion). Key: [ T ) 20 °C; 9 T ) 30 °C; 2 T ) 40 °C.
FIGURE 4. Influence of circulation flow on oil-water separation; T ) 20 °C. (percolation of 4 L of emulsion). Key: [ D ) 50 mL/min; 9 D ) 75 mL/min; 2 D ) 95 mL/min.
FIGURE 5. Influence of cotton bed height on oil-water separation (D ) 95 mL/min; T ) 20 °C). Key: 9 h ) 90 mm (BV ) 63.6 cm3); b h ) 120 mm (BV ) 84.8 cm3). through. When the latter occurred, the presence of bigger oil heaps (favored by coalescence) led to a higher release of oil into the effluent. Influence of Circulation Flow. The emulsion flow had a greater influence on the released oil quantity when the oil concentrations C0 were low (Figure 4). However, the lower the flow was, the lower was the oil quantity in the effluent. The contact time between cotton and oil was then longer, favoring its fixation onto the hydrophobic material. 5036
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Influence of Cotton Bed Height. An increase of the bed height (with the same cotton amount), thus of its volume (Bed Volume: BV) increased the contact time between oil and cotton, which favored sorbent swelling and oil retention in interfiber cavities. The oil-water separation was thus enhanced (Figure 5). These results are in accordance with the observations of Varghese and Cleveland (3) on a kenaf bed. Influence of Oil Drops Size. The quantity of released oil in the effluent increased when oil drops were smaller (Figure
FIGURE 6. Influence of oil drops size on oil-water separation. (D ) 95 mL/min; T ) 20 °C). Key: [ d ) 120 µm; 9 d ) 85 µm.
TABLE 1. Column “Hold-Up” Evolution as a Function of Oil-in-Water Emulsion Volumea percolated emulsion volume (L)
injected oil quantity (g)
released oil quantity (g)
hold-up (g)
2 4 6 8 10 12 15 20 30 50
10 20 30 40 50 60 75 100 150 250
0 1 8 17 26 34 47.5 69 121 219
10 19 22 23 24 26 27.5 31 29 31
a
D ) 95 mL/min, T ) 20 °C.
FIGURE 7. Sorption isotherm for oil-in-water emulsion percolation system. (D ) 95 mL/min; T ) 20 °C).
TABLE 2. Influence of Temperature on Breakthrough Volume (VB)a oil concentration C0 (g/L)
VB at 20 °C (L)
VB at 30 °C (L)
VB at 40 °C (L)
5 6.25 7.5 8.75 10 11.25 12.5
3.2 2.7 2.3 2 1.7 1.5 1
3.5 3.05 2.5 2.1
3.8 3.15 2.8 2.3 1.9
1.8 1.5
1.3
D ) 95 mL/min. Standard deviation ) 0.02 for each C0 value (3 experiments). a
6). So the oil-water separation was enhanced with larger drops. The contact with cotton fibers was thus favored, which led to a better interception and a higher sorption. These observations are in agreement with those of Varghese and Cleveland (3) and models proposed by Yao, Habibian, and O’Melia (10, 11) and used by Aurelle (9) and Tapaneeyangkul (12) for a theoretical study of drop interception by a bed. According to these authors, the separation efficiency (ηT) varies with the oil drop diameter d, according to the equation
ηT ) kdn
(1)
with n ) 2 for drops with a diameter higher than 5 µm and k ) constant. The substrate bed efficiency (1 - CS/C0) is expressed by the following relation
1 - CS/C0 ) 1 - exp (-3/2(1-)RηTH/dP)
(2)
FIGURE 8. Theoretical and experimental breakthrough curves, C0 ) 10 g/L. (D ) 95 mL/min; T ) 20 °C). Key: 9 experimental; theoretical. where C0 is the initial oil concentration in emulsion, CS is the residual oil concentration at bed outlet, H is the bed height, is the bed void fraction, R is the contact efficiency factor: ratio between oil shocks number onto the collector, leading to adhesion and total shocks number, and dP is the solid coalescer diameter. Equation 2 is valid for a granular bed. An extension of this equation to our case would only modify the values of constants but variables would be the same. Thus, when oil drops size increases, the separation efficiency ηT is better and CS/C0 decreases. The separation is therefore favored by larger oil drops. Sorption Isotherm. To draw sorption isotherm of the system, the oil concentration C0 in emulsion was varied between 0.5 and 10 g/L, and then the corresponding sorption capacities, SC (measured at equilibrium, i.e., at constant VOL. 37, NO. 21, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 9. Theoretical and experimental breakthrough curves, C0 ) 7.5 g/L. (D ) 95 mL/min; T ) 20 °C). Key: × experimental; theoretical.
value would increase up to the cotton saturation, under these operating conditions, i.e., 14.5 g/g (measured by throughflow of pure oil). This sorption isotherm plays a crucial part in sorption modeling and in the analysis and development of sorption systems. Percolation System Modeling. The phenomena governing the oil percolation through cotton bed with time can be described by a mathematical relation between the quantity of solute sorbed in solid phase (q) and the amount of solute in liquid phase (c) at equilibrium. We considered that the column behaves like a series of small reactors in succession. In each reactor, the emulsion at a concentration C0 is introduced with a flow D. The amount of oil retained by the cotton is equal to q, and the emulsion leaves the reactor at a concentration CS. Thus, for each reactor, inlet is equal to outlet plus accumulation, represented by sorbed oil and oil delayed in the reactor because of axial dispersion. The percolation of a sodium carbonate aqueous solution (sodium carbonate is a salt with no affinity for hydrophobic cotton and was used as a marker) showed that this axial dispersion was negligible. Therefore, for each reactor, the mass balance equation at equilibrium was as follows:
m
FIGURE 10. Theoretical and experimental breakthrough curves, C0 ) 5 g/L. (D ) 95 mL/min; T ) 20 °C). Key: 2 experimental; theoretical.
dq ) D(C0 - CS) dt
(3)
The results relative to the percolation of the Na2CO3 aqueous solution permitted us to evaluate a theoretical plates number of 7. The model validation was represented by correlation between theory and experience for the breakthrough curves concerning the emulsions with C0 between 1 and 10 g/L (Figures 8-11). The model validation was valid, considering the use of a compactible substrate, leading to possible packing defaults. The through-flow was thus of piston type, and the relation between q and c can be represented by eq 3. The behavior of cotton, as a sorbent, during the oil-in-water emulsion percolation could be extended to other operating conditions.
Acknowledgments The authors thank Generale des Eaux and Anjou Recherche for financial support during this study and the Society Formulaction (31240 - L’Union, France) for their help during the use of Turbiscan-On-Line and measurement of oil drops size. FIGURE 11. Theoretical and experimental breakthrough curves, C0 ) 1 g/L. (D ) 95 mL/min; T ) 20 °C). Key: 2 experimental; theoretical.
TABLE 3. Values of q and c at Equilibrium (C )
a
C0)a
effluent oil concentration (c ) C0) (g/L)
cotton oil sorption (q ) SC) (g/g)
0.5 1 5 7.5 10
3.4 3.7 7.5 8.75 11.25
Sorption isotherm. D ) 95 mL/min, T ) 20 °C.
“hold-up”), were quantified (Table 3 and Figure 7). In our case, q represents the SC of cotton, and c represents the oil concentration in emulsion C0. On one hand, we note that q > c, which confirms that oil has a strong affinity for acylated cotton in an aqueous medium (13). On the other hand, the SC (q) increases with C0 (c). This 5038
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Literature Cited (1) Pushkarev, V. V.; Yuzhaninov, A. G.; Men, S. K. Treatment of oil-containing wastewater; Allerton Press: New York, 1980. (2) White, L. R. Fluid/Part. Sep. J. 1996, 9, 2, 57-61. (3) Varghese, B. K.; Cleveland, T. G. Sep. Sci. Technol. 1998, 33, 14, 2197-2220. (4) Viraraghavan, T.; Mathavan, G. N. Environ. Technol. Lett. 1989, 10, 4, 385-394. (5) Vaca-Garcia, C.; Girardeau, S.; Deschamps, G.; Nicolas, D.; Caruel, H. Borredon, ME; Gaset, A. Patent PCT WO 00/50492, 2000. (6) Deschamps, G.; Caruel, H.; Borredon, M. E.; Bonnin, C.; Vignoles, C. Environ. Sci. Technol. 2003, 37, 1013-1015. (7) Taras, M. J.; Blum, K. A. J. Water Pollut. Control Fed. 1968, 11, 2, 404-411. (8) Benefield, L. D.; Judkins, J. F. J.; Weand, B. L. Process chemistry for water and wastewater treatment; Prentice-Hall: Englewood Cliffs, NJ, 1982; 07632. (9) Aurelle, Y. Contribution a` l’e´tude du traitement des eaux pollue´es par des hydrocarbures e´mulsionne´s par coalescence sur re´sines ole´ophiles, Ph.D. Thesis INSA, Toulouse, France, 1974. (10) Yao, K. M. Influence of suspended particle size on the transport aspect of water filtration, Ph.D. Thesis University of North Carolina, Chapel Hill, U.S.A., 1968.
(11) Yao, K. M.; Habibian, M. T.; O’Melia, C. R. Environ. Sci. Technol. 1971, 5, 11, 1105-1112.
(13) Weber, W. J. J. Physicochemical processes for water quality control; Wiley-Interscience: John Wiley and Sons: 1972.
(12) Tapaneeyangkul, P. Etude et mode´lisation d'une nouvelle ge´ne´ration de coalesceurs liquide-liquide: le coalesceur a` garnissage fibreux dynamique, Ph.D. Thesis INSA, Toulouse, France, 1989.
Received for review December 30, 2002. Revised manuscript received August 1, 2003. Accepted August 29, 2003. ES020249B
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