Electrical and chemical demulsification techniques for microemulsion liquid membranes

journal of

M.EEEE Journal of Membrane Science 9 1 ( 1994) 23 l-248

ELSEVIER

Electrical and chemical demulsification techniques for microemulsion liquid membranes Karen Larson, Bhavani Raghuraman,

John Wiencek’

Department of Chemicaland Biochemical Engineering, Rutgers University,Piscataway,NJ 08855, USA

(Received July 16, 1993; accepted in revised form December 20, 1993)

Abstract Recent work has focused on removing mercury from contaminated water via microemulsion liquid membranes. Electrostatic coalescence and butanol addition are evaluated as potential demulsification techniques for recovery of the components of mercury-rich microemulsions. Unlike coarse emulsions, electrical demulsiflcation with heat is not effective for microemulsions due to the smaller size of the internal phase droplets in microemulsions. This explanation is confirmed by the photomicrographs of the two emulsion systems. Microemulsions can be demulsitied using butanol as an additive. The demulsification kinetics are proportional to the butanol concentration and the temperature and inversely proportional to the surfactant concentration. Reduced mercury extraction efficiency with microemulsions formulated from recovered oil phase is attributed to some surfactant degradation during the demulsification processing as well as residual butanol in the recycled organic phase. Keywords: Demulsitication; Microemulsions; Facilitated transport; Liquid membranes; Water treatment

1. Introduction

Emulsion liquid membranes (ELMS) have been successfully utilized to treat aqueous streams contaminated with heavy metal ions such

as copper, zinc, cadmium, nickel, mercury, lead and chromium [ l-6 1. ELMS, first reported by Li [ 7 1, are made by forming an emulsion between two immiscible phases. Usually stabilized by surfactants, the water-in-oil emulsion contains the metal extracting agent in the oil phase and the stripping reagent in the aqueous receiving phase. This emulsion is then dispersed by mechanical agitation into a feed phase containing the metal to be extracted. Fig. 1 is a sche*Corresponding author.

matic representation of an ELM extraction of mercury (II ) . Combining the extraction and stripping processes removes equilibrium limitations and reduces the metal concentrations in the feed to very low levels. Demulsification by application of high voltage electric fields has proved to be the most efficient means of recovering the internal aqueous phase from the ELM [8]. Heavy metals concentrated in the receiving phase can be recovered by electroplating or crystallization and the oil phase can be recycled. In the systems described above, the emulsions typically used are coarse emulsions. The internal phase droplet sizes are in the range of l-10 p. Energy must be expended to form the emulsion and, although it may have some kinetic stability, it is thermodynamically unstable. Leakage of the

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K. Larson et al. /Journal ofMembrane Science 91(1994) 231-248

232 RFJXJMNG PHASE glow

pri to strip)

.MlCRODROPS

IHigh DdRODROPS

pHto Extract)

’ MEMBJWh’E PHASE lOlLI

Fig. 1. Schematic representation of mercury ion extraction with an emulsion liquid membrane. Mercury(II) is transported to the emulsion/feed phase interface and reacts with the complexing agent (CH ) to form a soluble mercury complex (HgQ). This complex diffuses to the interior of the emulsion droplet until it encounters a microdroplet of the internal phase where the metal ion is exchanged for a hydrogen ion. The net effect is a unidirectional mass transport of the cation from the original feed to the receiving phase with countertransport of hydrogen ions. The dispersion is then allowed to settle and the lower aqueous stream is withdrawn for discharge. The upper emulsion phase is then demulsified to split the membrane and the enriched stripping phases.

internal phase to the feed phase using coarse ELMS can reduce the efficiency of the extraction. A microemulsion, on the other hand, is a thermodynamically stable dispersion of oil and water stabilized by a surfactant. It forms spontaneously when the various components are brought in contact. A microemulsion has a microstructure of small oil and water domains ( N 100 A) separated by a monolayer of surfactant [91. A microemulsion employed as a liquid membrane has a separation mechanism similar to that described for coarse emulsions; however, there are several advantages. The low interfacial tensions which are characteristic of microemulsions lead to smaller macrodrops, and thus, faster mass transfer rates due to increased surface area per unit volume. Over time, the microdrops dis-

persed in a coarse emulsion will coalesce and phase separate. Such phase separation will result in undesirable leakage of the receiving phase into the feed phase. Microemulsions do not show such phase separation due to their thermodynamic stability and offer a more stable liquid membrane. Microemulsion liquid membranes have been used to successfully extract copper and mercury from aqueous streams [ 10,111. Electrostatic coalescence has been successfully implemented for the demulsification of spent coarse emulsion formulations; however, this technique has yet to be applied to microemulsion systems. This paper focuses on the last stage of the microemulsion extraction process, namely, demulsification and recovery of the metal [ mercury(I1) in this case]. The internal stripping phase concentrated in mercury must be recovered from the emulsion, while the organic components (solvent, surfactant and ion exchanger) must be reclaimed for reuse. Demulsitication techniques considered are heat treatment, electrostatic coalescence and addition of chemicals (butanol) , Material balances of mercury extraction experiments are presented along with preliminary data on organic phase recycle. 1.1. Heat treatment Heat treatment of the emulsion is an effective method of demulsification because it reduces the viscosity and density of the oil. The oil density decreases faster than the density of the water (oils have a larger coefficient of expansion than water). Elevated temperature also increases the solubility of the surfactants in both the oil and water phases, thus weakening the interfacial film [ 12 1. For microemulsions in particular, the amount of solubilized water decreases with increasing temperature. Consequently, heat treatment of a spent microemulsion is considered the primary technique for demulsification. The drawback of heat treatment alone is slow demulsification (i.e., phase disengagement) kinetics. For this reason, heat treatment in conjunction with some other technique which improves coalescence kinetics is required.

K. Larson et al. /Journal of Membrane Science 91(1994) 231-248

1.2. Electrostaticcoalescence One technique that has been used extensively for demulsification of water-in-oil emulsions is electrostatic coalescence. In fact, electrostatic coalescence is used industrially to demulsify spent emulsions from coarse ELM separations [ 8 1. Cottrell [ 13 ] observed that emulsions coalesce more quickly when subjected to an electric field than gravity alone. The petroleum industry uses this method to separate brine emulsified in crude oil. Electrostatic coalescence is an ideal method for improving demulsification kinetics because it is strictly a physical process which makes recycle of the organic phase feasible. A high electric field can be established across an emulsion because the oil phase is non-conducting. The field polarizes and elongates water droplets and can cause unidirectional migration of the droplets by either an electrophoresis or dielectrophoresis mechanism. When a dc field is applied, water molecules orient and move in the direction of the applied field. When an ac field is applied, motion occurs in the direction of maximum field strength (dielectrophoresis mechanism) [ 141. When Pearce [ 15 ] applied an electric field to a 10% water-in-oil emulsion, he observed under a microscope that the water molecules form chains of droplets that are oriented along the direction of the applied field. This chain formation and elongation of water droplets can also disturb the thin film between droplet surfaces and lead to coalescence [ 161. The force acting on the water droplets under an applied electric field can be described qualitatively as follows:

E2r6

fyr-

(1)

where f is the attractive force between droplets, E the electric field strength (V/cm), r the droplet radius and d the droplet separation Because the attractive force between water droplets is proportional to the square of the applied field, high voltages are commonly used in electrostatic coalescers. As the droplets coalesce and grow, they settle rapidly. In a typical experiment, the emulsion is fed continuously into a

233

vessel which contains a small volume of water/ electrolyte that is pumped out at the same rate as the coalescence rate. The high voltage electrode is placed in the organic phase while the grounded electrode is located in the aqueous phase. The electrodes are placed such that the applied field is perpendicular to the emulsion/water interface. Initial work with coalescers used non-insulated electrodes. However, when the water chain extends from one electrode to another, completion of the circuit causes sparking. This limits the maximum voltage that can be applied and results in slow coalescence kinetics as well as the formation of a stable ‘sponge’ emulsion (a high water content, 90 wt%, water-in-oil emulsion). In order to minimize the formation of this type of emulsion, the voltage must exceed a critical value. Consequently, Hsu and Li [ 173 studied a number of electrode designs that were insulated with various materials. The insulation must be non-water wetting to prevent sparking. Although the insulation consumes some of the applied ac voltage, most is transmitted through the insulation. In effect, the insulation acts as a parallel array of capacitors which helps suppress sparking. Hsu and Li [ 171 found that optimum coalescence rates occur when the dielectric constant of the insulating material is at least 4. The rate is also a function of applied voltage, position of the feed inlet, separation between the insulated electrode, and the aqueous level in the coalescer. The use of electrostatic coalescence for demulsitication is evaluated for microemulsion systems and compared to the results of similar coarse emulsion formulations. For microemulsions, electrostatic coalescence is applied in conjunction with heat. I. 3. Chemical demuZsi$cation Demulsification of emulsions by addition of chemicals is known to be effective [ 121. The disadvantage of chemical additives is the requirement of an additional separation step for the recovery and recycle of the emulsion components. Choice of a particular additive, therefore, must be governed by the ease with which it can be removed once demulsification is complete. In our

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K. Larson et al. /Journal of Membrane Science 91 (I 994) 231-248

laboratory, Vasudevan [ 181 has noted that addition of n-butanol to some microemulsion formulations causes spontaneous demulsification. Presumably, this occurs because the presence of butanol shifts the equilibrium of the system to that of separate oil and water phases. Butanol is favored over isopropanol as an additive because of its lower water solubility. Its high volatility in the microemulsion organic phase compared to those of the other components present, namely, tetradecane (solvent) and oleic acid [extracting agent for mercury( II) 1, implies that a distillation step for its recovery at the end of the demulsification may be possible. 2. Experimental Reagent grade mercuric nitrate, oleic acid, tetradecane, light mineral oil, butanol and coned sulfuric acid were obtained from Fisher Scientific. The surfactant for the coarse emulsion, ECA 5025 (polyamine), was supplied by Exxon. The surfactant for the microemulsion, Trycol DNP-8 [ polyoxyethylene ( 8 ) dinonylphenol] , was supplied by Henkel. Aqueous solutions of mercury were prepared by dissolving mercuric nitrate monohydrate in distilled water and adjusting the pH to the desired value using HN03 and NaOH, as required. The mercury concentration in the aqueous phase was measured using a Perkin-Elmer Model 3030 Atomic Absorption Spectrophotometer. The mercury concentration in the recovered organic phase was determined by performing two back-extractions on the samples of the organic phase with 6 N H,SO, and then analyzing the aqueous phases. The water content of the emulsion was measured using a Mettler Karl Fischer Titrator. An Olympus AH-2 photomicroscope was used to obtain photomicrographs of the emulsion samples mounted on slides. 2.1. Emulsion preparation The coarse emulsion formulation had 8 1 wt% of 0.345 M oleic acid (extracting agent ) in 90 : 10 wt% tetradecane: mineral oil (solvent), 4 wt% ECA 5025 (surfactant ) and 15 wt% 6 N HzS04

(stripping reagent) and was prepared by blending the components in a high speed mixer for 2 min. The water content of this emulsion was 11.5 wt%. Spent mercury-rich coarse emulsion needed for demulsification experiments was obtained by contacting 5 1 of 253 ppm Hg solution at pH 2.8 with 500 ml of coarse emulsion in a stirred contactor at 350 rpm. After 30 min of contacting, the phases were allowed to settle and the emulsion portion was removed. The mercury concentration of the aqueous phase was 4.6 ppm. The water content of the spent emulsion was 32.4 wt%. The increase in the aqueous content of the internal phase was due to swelling caused by the osmotic pressure difference between the internal phase and feed phase. Microemulsions were prepared by first adding the surfactant, DNP-8 (typically 3 to 10 wt%) to the ion exchanger/solvent (0.345 M oleic acid/ tetradecane) solution. This solution was then added to the aqueous phase containing the stripping reagent (6 N H2S04). During addition, the contents spontaneously emulsified (coarse emulsion). The emulsion was allowed to equilibrate overnight at ambient temperature. At equilibrium, a clear upper phase microemulsion could be observed along with an oil-in-water coarse emulsion in the lower phase. The water content of a microemulsion formulated with 10 wt% surfactant was measured to be 10 wt%. Spent mercury-rich microemulsion for chemical demulsification experiments was obtained by dispersing 500 ml of the microemulsion in 5 1 of aqueous mercury solution (typical concentration was 65275 ppm) for 30 min in a stirred contactor. The exact initial feed phase concentration for mercury and the microemulsion formulation for each experiment are given along with the results for that experiment in the next section. 2.2. Demulsification experiments Experiments to test the feasibility of electrostatic coalescence were performed using fresh coarse emulsions and microemulsions as well as mercury-rich (spent) coarse emulsions and microemulsions. A Hipotronics AC Dielectric Test Set Model 730- 1 served as the power supply. The

K. Larson et al. /Journal of Membrane Science 91(1994) 231-248

maximum voltage rating for the unit is 30 kV. As a safety precaution, the power shutoff was set at a maximum current of 3 mA. Since emulsions are primarily non-conducting, currents generated during a high voltage experiment are generally much less than 1 mA. The ground electrode was 1/ 16 Mo.d. stainless steel tubing while the insulated high voltage electrode was PVC-coated hook-up wire (Newark Electronics; Catalog No. 3782205 WA) having a nominal o.d. of 0.25”. The PVC-coating thickness was H 1 mm. In order to make the surface of this electrode hydrophobic, the wire was coated with heat shrink teflon (Newark Electronics; Catalog No. 37N299). The ground and high voltage electrodes were inserted into a specially constructed plexiglass demulsification &I, as shown schematically, in Fig. 2. The high voltage components, as well as the demulsification cell, were placed in a chemical

235

fume hood. All metal parts in the hood were grounded. Demulsification experiments with coarse emulsions were performed at ambient temperature; consequently, the recirculation system which was capable of heating the emulsion phase was not used. In a typical experiment with coarse emulsions, the demulsification cell was filled with 98 ml of 2 N H2S04. This concentration is approximately equivalent to that of the internal phase of the water-swollen emulsion. A sulfuric acid solution was used as the aqueous phase so that the equilibrium between mercury in the membrane phase of the emulsion and that in the internal phase would not be affected during the demulsification process. Other workers have also used the aqueous component of the emulsion in demulsification cells [ 17 1. The emulsion phase ( 150 ml) was then added to the cell. With these volumes, the spacing between the aqueous/

High Voltage Electrode Recirc. Pump

1 High Voltage Source

s

Heat Exchange Coil Aqueous

Thermostated

Bath

c

4

Demulsification -

CeU

(Emulsion volume with recirc. loop: 315 ml Aqueous volume: 1OOml)

Fig. 2. Schematic representation of demulsification cell.

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emulsion interface and the high voltage electrode was ~1.5 ’ . The voltages used in these experiments ranged from 10 to 20 kV. As demulsification proceeded, the level of the aqueous phase was observed to rise. When the level reached a point -0.5” below the high voltage wire, the power was shut off, and aqueous phase was removed from the cell (usually N 30 ml was removed). About 30 ml of emulsion was added to the cell to maintain the same initial volume of organic phase. The power supply was turned back on. This batchwise demulsification procedure was continued until the organic phase was free of water. Samples of the emulsion were taken periodically over the course of the experiments for water content and microscopic analyses. At the end of the experiment, samples of the organic and aqueous phases were taken for mercury analysis. Demulsification experiments using microemulsions at room temperature followed the same procedure. For experiments at higher temperatures, the emulsion and aqueous phases were first heated to the desired temperature in a separate temperature-controlled water bath. The aqueous phase (98 ml) was first added to the cell, followed by 3 15 ml of microemulsion (the emulsion phase holdup volume in the cell and heating loop). The emulsion phase temperature in the cell was maintained by recirculation with a peristaltic pump through a copper coil immersed in a thermostated bath. When the cell temperature reached steady state, the power supply was turned on, as described earlier, and the batchwise procedure was followed. In addition to electrostatic coalescence as a potential demulsification process, the effectiveness of butanol addition and temperature on demulsification rate were also evaluated. In these experiments, 150 ml of spent emulsion was placed into a glass jar and heated to the desired temperature in a thermostated bath. Butanol was added volumetrically, and the jar was sealed and placed back in the bath. Periodically, a small sample of the organic phase was removed for water content and microscopic analyses. At the end of the demulsification experiment, samples of the aqueous and organic phases were taken for mercury analysis. Electrostatic coalescence ex-

periments with butanol addition to the emulsion phase were also carried out. 2.3. Recycleof the organicphase Organic phases recovered from demulsification experiments were used to reformulate microemulsions, and their efficiency for mercury extraction was investigated. Organic phase samples recovered by chemical demulsification using butanol had to be treated to remove the butanol before being used to form microemulsions. Butanol was removed by vacuum distillation at N 50 mmHg and a bath temperature of 50’ C using a rotating evaporator (Buchi) . The samples were distilled until liquid ceased to condense in the collection trap. A few milliliters of liquid was collected in the trap from each distillation. Trace amounts of butanol remained in the organic phase as detected by the odor. Leakage of the internal phase stripping reagent, sulfuric acid, into the feed phase was measured by analyzing the sulfate concentration in the feed phase by ion chromatography (Dionex 4500i). 3. Results and discussion 3. I. Demulsificationof coarseemulsions At an applied voltage of 10 kV, the spent mercury-rich coarse emulsion with a starting water content of 32.4 wt% showed immediate coalescence in the emulsion phase and sedimentation of the internal phase into the lower aqueous phase could be observed. The physical appearance of the emulsion during the process visibly changed. The initial appearance is characterized by a milky white color that is completely opaque. After application of 10 kV for N 10 min, the water content decreased to 8.6 wt%. The emulsion has a yellow color (ECA 5025 is yellow) and different layers in the emulsion phase are evident. A thin white band of emulsion lies at the water/emulsion interface. Between that band and the high voltage electrode, the emulsion is yellow and “relatively” clear. Above the high voltage electrode, the emulsion is just slightly more opaque

K. Larson et al. /Journal of Membrane Science 91(1994) 231-248

and somewhat lighter in color. When the water content of the emulsion is reduced to 1.18 wt% the entire organic phase is yellow. The remaining opacity is probably due to the presence of very small droplets of water remaining in the organic phase, as well as the presence of some solids which settled out much later. This experiment was repeated with a new batch of swollen coarse emulsion. The demulsification kinetics for the two experiments are illustrated in Fig. 3. The demulsification rate drops off quite markedly after N 15 min of applied voltage. The decrease in demulsification kinetics can be understood by considering the force acting on the water droplets under an applied electric field [Eq. ( 1) 1. During the demulsification experiments, the applied electric field was kept constant. As a result of coalescence, the radius of the water droplets increased until they were large enough to settle. As the water content of the emulsion decreased, the distance between drop lets increased. Since the force acting on the water droplets is inversely proportional to d4, demulsification rate should decrease as the distance between droplets increases because of a de-

0

P

4)

al

al

la,

12l

Time (min)

Fig. 3. Demulsification kinetics of Hg-rich coarse emulsion: ( 0 ) experiment 3-72, ( A ) experiment 3-75. The two curves represent two demulsification experiments of the same coarse emulsion. Emulsification rate is very fast in the initial stages of the experiments but drops off markedly as the water content decreases. This decrease in rate is attributed to the decrease in the driving force for demulsitication as the distance between water droplets increases.

237

creased driving force. This qualitative description of the demulsification process and its effect on kinetics is consistent with the photomicrographs of the samples taken at different stages of demulsification, Fig. 4. The mercury-rich coarse emulsion at t = 0 consists of many closely spaced yet discrete internal phase droplets quite polydisperse in size. At a magnification of -2000, the size of the droplets is fn the micron range. Fig. 4b shows the same emulsion after the water content has been reduced to 9.2 wt%. The internal phase droplets are roughly the same size as those of Fig. 4a, but are spaced much further apart. With only 4.5 wt% water remaining in the emulsion (Fig. 4c), the distance between drop lets is much greater. At the end of the experiment, the water content of the emulsion is reduced to 0.98 wt%, and as shown in Fig. 4d, no internal phase droplets are visible. Table 1 summarizes the material balance calculations based on the measured mercury concentrations in the recovered aqueous and organic phases from both these experiments (3-72 and 3-75). The results of the experiments are very consistent with one another. The mercury content of the internal phase is in the range of 2600 ppm. This represents a lo-fold concentration of mercury compared with the 250 ppm concentration in the initial feed phase. The final volume of the internal phase, taking into account swelling, is 140 ml. For these two experiments, 57-64% of the recovered mercury has accumulated in the internal phase. The concentration of mercury in the recovered organic phase is 638 ppm in the first experiment and 502 ppm in the second. This represents 31 to 36% of the recovered mercury. The relatively high concentration of mercury remaining in the organic phase could be due to the effect of dilution of the internal stripping reagent with water on stripping equilibrium. A weaker stripping solution would favor mercury : oleic acid complex equilibrium (i.e., mercury in the organic phase). Clearly, the effect of emulsion swelling is to reduce the efficiency of the liquid membrane process. The mercury material balance is 58.9 and 55.1%, respectively, for the two experiments. The lack of material balance may be attributed to the formation of some mercury

238

K. Larson et al. /Journal of Membrane Science 91(1994) 231-248

Fig. 4. Photomicrographs of Hg-rich coarse emulsion at various stages of demulsification by electrostatic coalescence. (a) At t=O, H20= 32.3 wt%. The emulsion consists of many closely spaced yet discrete internal phase droplets. (b) Hz0=9.2 wt%. The internal phase droplets are still about the same size but spaced much further apart. The increase in distance between the droplets is consistent with the observed slowing ofthe demulsification rate shown in Fig. 3. (c) HzO=4.5 wt%. (d) H,O=0.98 wtl. No water droplets are evident. 2000 x magnification. Droplet sizes range from 1 to 20 pm.

precipitate in the organic phase, as noted earlier. The amount of mercury in the solids could not be quantified. To determine if mercury in the solids was the cause of the low material balance, a coarse emulsion was formulated with 10 wt% decanol. Decanal helps to solubilize the mercury : oleic acid complex in the organic phase [ 111. The emulsion formulation for this experiment consisted of: 0.345 M oleic acid in 5 : 10: 85 wt% mineral oil : decanol :tetradecane, 4.7 wt% ECA 5025 surfactant and 15.3 wt% 6 N H2S04. The water content of the emulsion was 10.6 wt%. Spent coarse emulsion with 52.6 wt% water was obtained after contacting with a 107.7 ppm mer-

cury solution. The mercury-rich emulsion was demulsified to a final water content of 0.88 wt% at an applied electric field strength of 10 kV. The material balance calculations for this experiment are summarized in Table 1 under experiment 4-2. Mercury in the internal phase of this emulsion is 1000 ppm, a concentration factor of 10. The concentration in the oil phase (after two back-extractions with 6 N H,SO,) is 485.9 ppm. No solids were evident in these samples. The final material balance for this experiment is 120%. Although unrealistically high, the result supports the hypothesis that the lack of material balance in the two earlier experiments is attributed to

K. Larson et al. /Journal of Membrane Science 91 (I 994) 231-248 Table 1 Mercury material balance from demulsified

239

coarse emulsion

Experiment

3-72

3-75

4-2

Membranesolvent composition Mineral oil-tetradecane-decanol

1O-90-0

1O-90-0

10-85-S

Start Hg in feed ( ppm ) Hg in feed (mg )

250 1250

250 1250

End Hg in internal (ppm) Hg in internal (mg) Hg in organic ( ppm ) Hg in organic (mg) Hg in spent aqueous (mg) Hg miscellaneous (mg)

2518 423 638 269.1 22.9 20.4

2631 442 502 212 22.9 11.1

1000 399 486.9 215.3 25.1 5.6

Total recovered Hg (mg)

736

688.3

645.6

58.9

Material balance (% )

some mercury precipitation tion step.

during the extrac-

3.2. Demulsification of microemulsions Heat treatment: Fresh microemulsion separated overnight into two distinct phases when kept at 55 “C. However, a spent mercury-rich microemulsion formulated with 5 wt% DNP-8 surfactant and containing 58.7 wt% water required 10 days at 5 5 ’ C for complete separation of the phases. As seen from the results here, the drawback of heat treatment alone is slow demulsification kinetics. For this reason, heat treatment in conjunction with some other technique which improves coalescence kinetics is required. Electrostatic coalescence: Table 2 reports re-

120

55.1

sults of experiments where a lo- 15 kV electric field was applied simultaneously with heat treatment on various microemulsions. Unlike the coarse emulsion demulsification, no rapid coalescence was observed. A fresh microemulsion (with a water content of 9.9 wt%) showed some initial coalescence at 50 ’ C after which the emulsion was actually observed to absorb some water. The final water content ( 13.9 wt%) is observed to be higher than the initial content. Two reasons for the lack of demulsification were hypothesized: ( 1) The radius of the internal phase droplets is too small. JZq.( 1) states that the driving force for coalescence of water drop lets under an applied electric field is proportional to r6. One of the features which distinguishes microemulsions from coarse emulsions

Table 2 Electrical demulsification of microemulsions: initial microemulsion is formulated by equilibrating 10 wt”h DNP-8 surfactant in tetradecane with a 6 N sulfuric acid solution

Fresh microemulsion Water-swollen microemulsion Mercury-rich microemulsion

107.7 538.5

0.345 Moleic acid and 4.7 or

Temperature (“C)

DNP-8 (wt%)

Starting water content (wt%)

Final water content (wt% I

50 60 50

10 10 4.7

9.9 40.0 60

13.9 17.9 14.5

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K. Larson et al. /Journal of Membrane Science 91(1994) 231-248

is the small internal phase drop size ( 100 8, versus 5 pm). This represents a 10” reduction in driving force. (2) The surfactant provides a stabilizing effect on the emulsion. Coarse emulsions contain only 3-4 wt% surfactant while this microemulsion is formulated with 10 wt% surfactant. An increase in surfactant concentration is known to hinder coalescence [ 17 1. Hypothesis 1 was tested by using a water-swollen microemulsion (with a water content of 40 wt%) obtained by contacting a fresh microemulsion (water content of 9.9 wt%) with water in a stirred contactor for 30 min at a treat ratio (ratio of feed volume to microemulsion volume) of 10. The swelling of the emulsion is most likely attributed to the osmotic pressure difference between the aqueous phase and the sulfuric acid internal phase. Swelling may cause the internal phase droplets to become larger. If such is the case, then demulsification of a spent, waterswollen microemulsion should be more effective than demulsification of a fresh microemulsion which has very small internal phase droplets. At an applied voltage of 15 kV and a temperature of 60°C some sedimentation of internal phase was observed initially. After a period of N 10 min, the water content was reduced to N 20 wt%. The system then exhibited some interesting periodic behavior. In the first step the water/emulsion interface became extremely turbulent, almost as if it were boiling. During this time, the emulsion swelled (with water). The interface then became quiescent, followed in the final stage by some demulsification. However, based on a mark noting the original location of the interface, the amount of water demulsified during this period did not exceed the amount of water taken up by the emulsion during the swelling period. This unusual behavior had a periodicity of about 15 min. The reason for this periodic behavior is not understood. After several of the swelling-demulsification episodes, the power was turned off. The final water content of the emulsion was 17.9 wt%. The results of this experiment support the hypothesis that the small droplet radius of a fresh microemulsion contributes to the slow demulsification kinetics when subjected to an electric field. When the droplet size increases due to

swelling, it is possible to demulsify the excess water by electrostatic coalescence; however, once the water content approaches that of the equilibrated (fresh) microemulsion, demulsification stops. To check the effect of surfactant concentration on demulsitication, microemulsions with surfactant concentrations in the 3.8-10 wt% range were formulated and tested for efficiency in mercury extraction and leakage of the internal phase. The water content of the microemulsion decreases from 9.9 wt% for 10 wt% surfactant formulation to 6.91 wt% for 3.8 wt% surfactant formulation and this lowers the efficiency of mercury extraction at low surfactant concentrations. Decreasing the surfactant concentration from 10 to 3.8 wt% results in an increase in the aqueous phase raffinate mercury concentration from 1.18 to 2.26 ppm compared to an initial feed phase concentration of 500 ppm. Leakage of the internal phase is not substantial and does not exhibit any trend with surfactant concentration, thus substantiating the hypothesis that lower extraction efficiency is attributed to lower internal phase water content. The main differences between the extraction samples were qualitative. For example, some of the microemulsion samples became quite “sticky” and turbid during the mercury extraction experiment and the surfactant stuck to the test tube walls. The lowest surfactant concentration tested that did not display any adverse effects was the formulation containing 4.7 wt% DNP-8. This concentration allows for a direct comparison with a coarse emulsion. This microemulsion had a starting water content of 4.8 wt%. The spent mercury-rich microemulsion was obtained by dispersing the emulsion in 70.2 ppm mercury solution for 30 min at a treat ratio of 10. The water content of the swollen microemulsion was 60 wt%. Demulsification at lo- 15 kV and 50’ C showed some initial coalescence which resulted in a reduction of the water content to 48.8 wt% in 10 min after which the demulsitication rate slowed considerably. The final water content could not be reduced below 14.5 wt%. The remaining emulsion was placed in an oven at 55 ‘C for 10 days, where, finally, separate oil and water phases were recovered.

K. Larson et al. /Journal of Membrane Science 91(1994) 231-248

This demulsification experiment was also monitored microscopically, as shown in Fig. 5. The differences between a mercury-rich coarse emulsion and microemulsion are immediately apparent. Fig. 5a is a photomicrograph of the initial mercury-rich emulsion containing 60 wt% water. The internal phase droplets are substantially smaller than those of the coarse emulsion, although much larger than 100 A. Groups of droplets appear as aggregates rather than discrete droplets. As the water content is reduced to 48 (Fig. 5b), 19.7 (Fig. SC) and. 14.5 wt% (Fig. 5d), the droplets remain as aggregated clumps. The only difference between the photomicrographs is that at the lower water concentrations, the distance between aggregates is larger.

241

Based on the results of these experiments as well as the photographs of the emulsions, electrostatic coalescence of microemulsions is concluded to be an ineffkient technique because of the small size of the internal phase droplets. The water content of a swollen microemulsion cannot be reduced below lo- 15 wt%. Decreasing the surfactant concentration to the levels used in coarse emulsions does not improve the degree of demulsification. The amount of surfactant does not control the demulsifkation process, rather it is the type of surfactant and the way in which it interacts with the components within the emulsion that is controlling. Kizling and Stenius [ 19 ] have shown for microemulsion systems that above a certain minimum surfactant concentra-

Fig. 5. Photomicrographs of Hg-rich microemulsion at various stages of demulsification by electrostatic coalescence. (a) Initial mercury-rich microemulsion. Hz0 = 60 wt%. Internal phase droplets are much smaller than those of a coarse emulsion. Droplets are highly aggregated. (b) HsO= 48 w-t%. (c) HzO= 19.7 wt%. (d) Final sample from electrostatic coalescence experiment. Hz0 = 14.5 wt%. During demulsification, droplets remain highly aggregated but the distance between the aggregates increases. The temperature was maintained at 50°C for the experiment.

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tion, the number of water molecules solubilized per ethylene oxide group is constant. This minimum surfactant concentration corresponds to one mole of water per mole of ethylene oxide. A decrease in surfactant concentration (as long as it is above the minimum concentration for micelle formation) results in a decrease in the number of micelles. The micelle structure and droplet size, however, are not affected. Consequently, if the demulsilication kinetics are solely limited by droplet size, decreasing surfactant concentration would not be expected to improve the kinetics since the droplet size is unaffected. The difference in droplet size between the coarse emulsion and microemulsion is clearly evident from a comparison of the photomicrographs of the two types of emulsions, Evidently, electrostatic coalescence in combination with temperature does not sufficiently affect the surfactant/ water interactions in a microemulsion to the degree required for water to coalesce and sediment. Chemical demulsification: Fig. 6 summarizes the results for demulsification with butanol addition. The histogram summarizes the effect of butanol concentration and temperature on the extent of demulsification after 5.5 h of treatment. Without the use of butanol, no demulsilication occurs. In fact, as discussed earlier, a sample of the microemulsion (5 wt% DNP-8) placed

in a 55 “C oven required 10 days for complete separation of the phases. For a given temperature, increasing the concentration of BuOH results in a decrease in the water content of the emulsion. Increasing the temperature for a given BuOH concentration also decreases the water content of the emulsion. Simultaneous application of electric fields did not change the efficiency of chemical demulsification in any way. Addition of butanol shifts the phase behavior of the system. It could also be concentrating at the interface of the internal phase droplets and weakening the binding forces of the emulsion structure as suggested by Lissant [ 121. High temperature reduces the viscosity of the system as well as shifts the equilibrium towards a lower water content. The overall effect is an enhancement of the coalescence process. This coalescence process is evident in the photomicrographs of Fig. 7 which show the progress of demulsilication with 3 ~01% BuOH at 60°C. The surfactant concentration is 5 wt%. The water concentration in the samples is 24.5, 10, and 3.5 wt%, respectively. Unlike the microemulsions shown in Fig. 5, the internal phase droplets in these photomicrographs are substantially larger. As the water content decreases, the distance between the aggregates of the large internal phase droplets increases. The size of the internal phase droplets also increases with time. At 3.5 wt% water, the internal phase droplets are no longer visible. The kinetics of this chemical demulsilication process were also studied. The water concentration in the emulsion was normalized with respect to the oil phase concentration. Therefore, D, represents the ratio: g H,O/g oil. It follows from the results presented in Fig. 6 that the demulsification rate takes on the following form: $-k(T)f{[BuOH],[Surfactant],D}

Fig. 6. Demulsification of a mercury-rich microemulsion with butanol. Treatment time: 5.5 h. Initial surfactant concentration: 5 wtl. Demulsification is drastically improved with the addition of butanol. For a constant butanol concentration, an increase in temperature also improves demulsitication

Non-linear regression of the data indicates that for the majority of the experiments (six out of nine), demulsification rate is second order in normalized water concentration. The non-linear

K. Larson et al. /Journal of Membrane Science 91(1994) 231-248

243

regression constants as well as regression constants for a forced second-order fit for each experiment are summarized in Table 3. Second-order plots of the demulsification data at 50°C for a 5 wt% DNP-8 mercury-rich microemulsion are illustrated in Fig. 8. The demulsification rate increases with increasing BuOH concentration; it increases with increasing temperature; and it decreases with increasing surfactant concentration. Second-order demulsification kinetics implies that the rate-limiting step is droplet-droplet encounters of the internal phase droplets. Flocculation is typically a second-order process which depends upon the rate of the droplet encounters

WI.

Fig. 7. Photomicrographs of mercury-rich microemulsions at various stages of demulsification with butanol. Addition of 3 ~01% BuOH and maintaining the emulsion temperature at 60°C effectively demulsifies the microemulsion. Butanol addition causes the internal phase droplets to grow in size and this helps the coalescence process. (a) Hz0 = 24.5 wt%. (b ) HzO=lOwt%. (c) H,O=3.5wt%.

Material balance calculations for the chemical demulsification experiments are summarized in Table 4. Reproducibility between the samples is quite good. For example, the only difference between experiments 15-4 and 15-5 is the amount of butanol used for demulsification since the mercury-rich microemulsion came from the same mercury extraction experiment. Likewise, demulsification experiments 17-5 and 17-6 used mercury-rich microemulsion from the same mercury extraction. Lack of closure on the material balance, however, is attributed to volatilization losses. Experiments 15-4 and 15-5 utilized a microemulsion containing 5 wt% DNP-8 surfactant. The concentration of mercury in the internal phase is 100 ppm which represents only a 2-fold concentration. The microemulsion used in experiments 17-5 and 17-6 contained 10 wt% DNP-8. Mercury concentration in the internal phase is N 230 ppm, a 4-fold concentration effect, The major disadvantage of the microemulsion system is the high degree of swelling which results in a dilution of mercury in the internal phase. The mercury concentration factor in the internal phase of coarse emulsion samples is roughly 1O-fold (Table 1). Those emulsions after 30 min of contact have about half the degree of swelling compared to the microemulsion samples. Swelling of a microemulsion can be minimized by reducing the contact time during extraction. 95% of the mercury extraction is complete within the first 5 min of extraction [ 111 and hence a 30 min contact-

244

K. Larson et al. /Journal of Membrane Science 91 (I 994) 231-248

Table 3 Regression constants regression

for demulsification

Temperature (“C)

BuOH (vol%)

kinetics of microemulsions

DNP-8 (wt%)

by butanol addition:

2nd Order Krate

second-order

tit versus non-linear

Non-linear regression R

Krate

Order

R

2.64 2.33 2.01 1.16

0.981 0.997 0.997 0.988

1 1 1

22

0 2 3

5 5 5

0 0.05536 0.1506

50

0 2 3 4 5

5 5 5 5 5

0.06131 0.2808 1.3511 23.45 196.23

1 0.983 0.996 0.998 0.972

60

0 2 3

5 5 5

0.00208 0.5506 3.802

0.053 0.882 0.988

0.046 241 3.79

2.2 7.51 1.93

0.85 0.977 0.994

50

4 5 5

10 10 10

0.888 0.999 0.927

0.31 18.63 9.83

0.479 1.97 1.25

1 0.999 0.988

4.285 19.41 53.3

0.3891 2.199 23.59 32.32

1/ 10 to l/5 also helps reduce swelling. For the same 30 min contact time, a spent microemulsion from a 1 / 5 treat ratio extraction has a water content of 30 wt%.

o

Q

20

4

,5

I ~J

.’

3.3. Recycle of the organic phase

,,’ 4vol%BuOH :

Fig. 8. Second-order demulsifkation kinetics at T= 50°C and 5 wt% DNP-8. D=g H,O/g oil. The demulsification rate increases nearly 200-fold with an increase in BuOH concentration at this temperature. The second-order plot is a good tit of the data and is consistent with the process of flocculation of the internal phase droplets as the rate-determining step.

ing period is not desired, since it has a negative impact on the efficiency of back-extraction into the internal phase. Increasing the treat ratio from

The organic phase recovered from the microemulsion sample that was placed in a 55 ‘C oven for 10 days was equilibrated with 6 N H2S04 to form a new microemulsion. The original microemulsion had been prepared with 4.5 wt% DNP8 and contained 4.8 wt% water at equilibrium. The recovered organic phase, however, did not form a microemulsion because no water was solubilized. This is most likely due to surfactant degradation as a result of prolonged exposure to heat. To determine if this is the case, another sample of recovered organic phase was separated into two parts. An additional 5 wt% DNP-8 was added to one sample and 7.5 wt% DNP-8 was added to the second sample. Microemulsions with a water content of 7.77 and 10.7 wt% were formed upon equilibration with 6 NH$O+ This

K. Larson et al. /Journal of Membrane Science 91 (I 994) 231-248

245

Table 4 Mercury material balance for chemically demulsified microemulsion Experiment DNP-8 surfactant (wt%) BuOH (vol%), T (“C) H20, start of extraction (wt%) H20, end of extraction (wt%)

15-4 5 4, 50 7.6 62.4

15-5 5 5, 50 7.6 62.4

17-S 10 4,60 8.7 53.5

17-6 10 5, 60 8.7 53.5

Start

Hg in feed (ppm) Hg in feed (mg )

57.9 289.5

57.9 289.5

57.3 286.3

57.3 286.3

101.2 52.9 436 183 1.04 236.9

100 52.2 426.6 179.2 1.04 232.4

229.9 81.4 375.9 156.4 0.239 238

229.9 81.4 346 143.9 0.239 225.5

81.8

80.3

83.1

78.8

End

Hg in internal ( ppm ) Hg in internal (mg) Hg in organic ( ppm ) Hg in organic (mg) Hg in spent aqueous (mg) Total recovered Hg (mg) Material balance (% )

confirmed the fact that failure to form a microemulsion was due to surfactant degradation upon prolonged exposure to heat. The organic phase recovered from the butanol demulsification experiment 17-5 (see Table 4) was used in a second set of recycle experiments after removing the butanol by vacuum distillation. The treated organic phase on equilibration with 6 iV H2S04 formed a microemulsion with a water content of 5.7 wt%. No additional oleic acid or DNP-8 was added to the recovered organic phase. The water content is substantially lower than that typically measured for microemulsions with 10 wt% DNP-8 and 6 N H2S04 (in the 10 wt% range). It is possible that some of the surfactant and/or oleic acid was degraded during either the demulsification or distillation steps. The kinetics of mercury extraction using this microemulsion is shown in Fig. 9. The leakage of the internal stripping phase into the outer aqueous feed phase is a measure of the membrane stability. An increase in the sulfate concentration in the feed phase during extraction is a direct measure of the leakage of the internal stripping reagent, sulfuric acid, into the feed phase. Fig. 10 shows the sulfate leakage rate for this extraction where the sulfate concentration in

the feed phase was measured by ion chromatography. The final feed phase mercury concentration is lower than that of the solvent extraction control (thus internal phase stripping is occurring); however, the extraction is not as efficient as with the fresh microemulsion. The low extraction efficiency is probably due to the lower internal phase water content (5.7 wt%) compared to a fresh microemulsion ( 10 wt% water), as well as to leakage which is twice that of the fresh microemulsion. Leakage from the recycled microemulsion is probably due to the presence of some residual butanol in the organic phase of the microemulsion. Several possible explanations for the lower extraction efficiency of microemulsions formulated from recycled organic phase have been hypothesized and include: surfactant or oleic acid degradation during demulsification; and traces of butanol in the organic phase. Both of these phenomena result in a “weak” liquid membrane that leaks internal phase. Data on sulfate leakage rates and the stabilizing effect of added surfactant support the hypothesis. Another possibility is that the presence of mercury in the recovered organic phase may contribute to reduced extraction effrciency. To test this hypothesis, an experiment was

K. Larson et al. /Journal ofMembrane Science 91(1994) 231-248

0.1

,

Fresh uE Extraction , , / , / / , , , , , , , , , ,

0

10

5

25

20

15 Time (min)

30

Fig. 9. Microemulsion extraction of Hg+’ using recovered oil phase from demulsification with BuOH. (0 ) Recycled microemulsion: Hz0 = 5.72 wt%, (0 ) fresh microemulsion with preloaded mercury: Hz0 = 11.3 wt%. Mercury extraction efficiency is somewhat lower for the experiment using recycled organic phase. Reduced efficiency may be partly due to the presence of traces of BuOH in the organic phase which causes the emulsion to leak and partly due to the lower water content. Fresh microemulsion @E) preloaded with mercury to simulate mercury content in recycled organic phase has a somewhat reduced extraction efficiency compared to the control pE, as expected. Fresh control microemulsion was formulated by equilibrating 10 wt% DNP-8 and 0.345 M oleic acid in tetradecane with 6 N sulfuric acid and had 10 wt% water. Volume ratio of feed to microemulsion= 5.

performed to determine the effect of mercury already present in a microemulsion formulation on mercury extraction efftciency. A microemulsion utilizing fresh raw materials was formulated with the following: 0.345 M oleic acid in tetradecane with 10 wt% DNP-8 surfactant. Based on the concentration of mercury typically measured in the recovered organic phase in our experiments, as well as the weight of fresh organic phase prepared for this experiment, an internal phase consisting of 6 NHzS04 and 2208 ppm mercury was equilibrated with the organic phase. Based on equilibrium calculations [ 111, 2 1.8 mg of mercury will equilibrate into the microemulsion which roughly corresponds to the amount of mercury present in the recycled microemulsion. The amount of water in the equilibrated preloaded microemulsion was 11.3 wt%. This microemulsion was used to extract a 533 ppm solution of mercury at a treat ratio of l/5. The extraction kinetics are shown in Fig. 9. The results show that the efficiency of a microemulsion preloaded with mercury is somewhat reduced when compared to a fresh microemulsion (by about a factor of 2), as would be expected. The mercury extraction efficiency of the preloaded microemulsion, however, is still somewhat better than that of the microemulsion prepared from recycled organic phase. 4. Conclusions

80

Recycled uE 60

/

/ / /

/

/

/

P

Demulsification of mercury-rich microemulsion, reuse of the recovered organic phase, and recovery of mercury from the internal phase represent the last important steps in the process for removal of mercury from contaminated water. The demulsification techniques tested were electrostatic coalescence and demulsification with nbutanol. For demulsification of mercury-rich coarse emulsions, electrostatic coalescence is effective at reducing the water content to less than 1 wt%. However, microemulsions could not be efficiently demulsified using electrostatic coalescence even in conjunction with heat treatment. The water content of spent, water-swollen microemulsions can only be reduced to N lo- 15

22;~jL--&/-‘,I:: _~_ 0

5

10

15

20

25

30

Time (min)

Fig. IO. Leakage rate for microemulsions in Fig. 9. Leakage of recycled microemulsion is higher as compared to that of the control microemulsion (PIE). This is most likely due to the presence of trace amounts of BuOH in the organic phase.

K. Larson et al. /Journal of Membrane Science 91(1994) 231-248

wt% water using this technique. This is attributed to the small internal phase droplet size compared to that of coarse emulsions. Smaller droplet size reduces the driving force for coalescence by a factor of t6 ( r= droplet radius). Even with a surfactant content comparable to that of a coarse emulsion, the driving force for demulsiflcation by electrical means is not sufficient to overcome the forces and interactions which stabilize the emulsion. Butanol, however, is an effective demulsifying agent. Demulsification kinetics are proportional to the butanol concentration and temperature and inversely proportional to the surfactant concentration. The rate of demulsification is second order in water concentration which implies that the rate-limiting step is droplet-droplet encounters of the internal phase droplets. Butanol appears to weaken the water-surfactant interactions. This weakening of the thin surfactant film which separates droplets in the aggregates aids the coalescence processes. Photomicrographs show that microemulsions containing butanol have very large internal phase droplets and aggregates, consistent with the hypothesis. The recovered organic phase was successfully recycled in microemulsion formulations. The reduced extraction efficiency with recycled microemulsions is a result of the lower internal phase water content and higher internal phase leakage rate. The higher leakage rate is attributed to some surfactant degradation during the demulsification processing as well as to residual butanol in the organic phase after the vacuum distillation step. 5. Acknowledgments This work has been funded by the Hazardous Substance Management Research Center of New Jersey, Grant PHYS-28 (A NSF Industry/University Cooperative Center and a New Jersey Commission on Science and Technology Advanced Technology Center); New Jersey Water Resources Research Institute (USGS Grant G1577-02 ); and Merck and Company, Inc.

247

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