Znd. Eng. Chem. Res. 1994,33,1612-1619
1612
Mass-Transfer Model of Mercury Removal from Water via Microemulsion Liquid Membranes Karen Larson, Bhavani Raghuraman, and John Wiencek' Department of Chemical and Biochemical Engineering, Rutgers University, Piscataway, New Jersey 08855
A diffusion/reaction model is developed for mercury extraction using a microemulsion liquid membrane. The model incorporates the uniqueness of the mercury-oleic acid chemistry by including the role of oxygen in the surfactant on equilibrium extraction and stripping. Features which distinguish microemulsions from coarse emulsions are also taken into account. The difference is manifested primarily in the emulsion macrodrop size. The Sauter mean diameter determined from photographs ranges from 0.014 to 0.017 mm compared to 0.5-1 mm for coarse emulsion systems. Experimentally determined equilibrium constants and mass-transfer coefficient for the mercuryoleic acid system are used in the model equations. Model simulations show the expected effects of pH and equilibrium constants on extraction kinetics and interior concentration profiles. The model accurately predicts both the initial extraction kinetics and final mercury extraction equilibrium. The good agreement between theory and experiment suggests that the mechanism of extraction using microemulsions is very similar to that of coarse emulsions once the appropriate physical parameters which distinguish microemulsions from coarse emulsions have been incorporated. This is the first model to describe a carrier-mediated microemulsion extraction. RECENINO PHASE
Introduction Aqueous streams contaminated with heavy metals, especially mercury, are a serious environmental problem. In addition to contaminated water, the leaching of heavy metals from landfills to nearby groundwater is a cause for growing concern. Discarded batteries are a major source of mercury in landfills. Traditional methods of removing heavy metals from water (e.g., precipitation) result in contaminated sludges that have to be landfilled, which can lead to groundwater contamination. Electrochemical recovery is a promising method of recycling metals; however plating efficiencies are low for dilute waste streams. Emulsion liquid membranes (ELMS) have been successfully utilized to treat aqueous streams contaminated with heavy metal ions like copper, zinc, cadmium, nickel, mercury, lead, and chromium (Gu et al., 1986; Fuller and Li, 1984; Weiss and Grigoriev, 1982; Boyadzhiev and Bezenshek, 1983; Kitagawa et al., 1977; Izatt et ai., 1987). ELMS, first invented by Li (1968), 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. Figure 1 is a schematic representation of an emulsion liquid membrane extraction of mercury(I1). Combining the extraction and stripping processes removes equilibrium limitations and reduces metal concentrations in the feed to very low levels. Demulsification by application of highvoltage electric fields has proved to be the most efficient means of recovering the internal aqueous phase from the ELM (Draxler et al., 1988). Heavy metals concentrated in the receiving phase can be recovered by electroplating or crystallization and the oil phase recycled. The emulsions described above (also called coarse emulsions) are formulated by inputting mechanical energy and over a period of time can separate, partly or wholly, into their constituent
* To
whom correspondence should be addressed. E-mail:
[email protected]. 0888-5885/94/2633-1612$04.50/0
(LowpH to strip)
-IMCRODROPS
+
I \
FEEDPRASE (High p H to Extract)
Figure 1. Schematic representation of mercury ion extraction with an emulsion liquid membrane. Mercury(I1) is transported to the emulsion/feed phase interface and reacts with the complexing agent (CH)to form a solublemercury complex (HgC2).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.
phases. A microemulsion, on the other hand, forms spontaneously when the aqueous and organic phases are brought in contact in the presence of a surfactant. Because it is in thermodynamic equilibrium, microemulsion systems are very stable. The internal droplet size of the coarse emulsion is dependent on the efficiency of the mixing process and is typically in the 0.1-10-pm range. Droplet sizes in microemulsions are much smaller and are in the range of 100 A. Microemulsions, because of their higher 0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 6, 1994 1613 stability, offer potential advantages when used as liquid membranes over coarse emulsion liquid membranes (Wiencek and Qutubuddin, 1989). Microemulsions have been used to remove acetic acid from water (Wiencek and Qutubuddin, 1988), copper ions from water by incorporating benzoylacetone (Wiencek and Qutubuddin, 1992),and mercuric ion from water by incorporating oleic acid (Larson and Wiencek, 1992). The goal of this work is to model extraction rates of mercury from water via microemulsion liquid membranes. The liquid membrane extraction typically involves mass transfer of metal (mercury) in the feed to the emulsion interface, reaction of the metal with the organiccomplexing agent (oleicacid), and diffusion of the metal complex across the globule with simultaneous stripping at the inner interface. In order to successfully model the liquid membrane extraction, the equilibrium and kinetics of the metal-complexing agent reaction must first be characterized. Larson and Wiencek (1992) developed a model to predict equilibrium for this system. This model also incorporates the effect of surfactant and modifiers that are present in microemulsion formulations and that could affect the equilibrium. Mercury stripping from an oleic acid organic phase using a 6 N solution of sulfuric acid was also characterized and modeled. A microemulsion which incorporated oleic acid as the cation exchanger and 6 N sulfuric acid as the internal phase reduced the aqueous solution mercury content from 460 to 0.25 ppm in a single contacting. This extraction represented a 32-fold improvement in extraction over the equilibrium limited solvent extraction case. The second phase of this research studied extraction kinetics for the mercury-oleic acid system and concluded that the reaction is instantaneous and that mass-transfer resistances are controlling (Larson and Wiencek, 1993). The present study is the final phase of the project where an overall diffusion/reaction model is developed for microemulsion extraction of mercury. Existing liquid membrane extraction models for coarse emulsions are briefly discussed below. These models help provide the basis for the model which describes mercury extraction using microemulsion liquid membranes. Emulsion liquid membrane separations can be classified as type I transport (extraction of soluble organics such as phenol, cresol, amines, and organic acids from water) or type I1 transport (carrier-mediated transport for extraction of heavy metals from wastewater). In type I transport, the internal phase contains a solute which reacts irreversibly with the diffusing species, thus maximizing the concentration gradient across the membrane phase. In type I1 transport, an ion exchange reagent incorporated in the membrane phase complexes with the diffusing species to transport it across the membrane to the internal phase wherein the decomplexation occurs and the diffusing species is immobilized. The earliest models used a simple approach in which the mass-transfer rate is assumed to be directly proportional to the average solute concentration difference between the continuous and the internal reagent phases (Cahn and Li, 1974; Boyadzhiev et al., 1977). However the effective permeation rate constant was found to vary with time. The spherical shell approach assumes that the emulsion globule is a hollow sphere and that the rate is limited by diffusion of the solute througha spherical shell of the membrane phase of some defined thickness to the interior aqueous phase (Matulevicius and Li, 1975; Hochhauser and Cussler, 1975; Ho and Li, 1992). This approach does not consider the physical structure of the emulsion, and analysis of the diffusion/reaction process occurring within the emulsion globule during a separation is not possible. The first model to consider the physical geometry was the advancing front model (Ho et al., 1982).
Here the extracted solute diffuses through the organic (membrane) phase to a reaction front where it reacts instantaneously and irreversibly with the internal phase. As the internal phase is consumed during the course of an extraction,the reaction front advances toward the globule center. At a radius greater than that of the reaction front, the internal phase is completely consumed. When the radius is less than that of the reaction front, the internal phase is unreacted. A sharp boundary or front separates regions of spent and fresh internal phase. The assumptions made were (1) the reaction with the internal phase is instantaneous and irreversible, (2) the presence of surfactants in the organic phase suppresses internal droplet circulation, (3) globule size distribution is monodisperse and the average size of the globules is described in terms of the Sauter mean diameter, (4) there is no coalescence and redispersion of the emulsion globules, and (5) internal droplets are treated as a continuum because of the high water content of emulsions and hence the concentration within the globule is described in terms of an average local concentration. External mass-transfer resistances and leakage were assumed to be negligible. Agreement between experiment and model is quite good considering that there are no adjustable parameters. However, the model is compared to data taken at initial extraction times. At longer extraction times, the model overpredicts the extraction rate. The authors attribute the difference to emulsion leakage during the experiment. The advancing front model has been modified by various workers to account for external-phase mass-transfer resistance (Stroeve and Varanasi, 1984;Fales and Stroeve, 1984), reaction reversibility in the internal phase (Teramot0 et al., 1983; Bunge and Noble, 19841, finite reaction kinetics in the internal phase (Janakiraman, 1985), and leakage of the internal phase into the external phase (Borwankaret al., 1988). Lorbach et al. (1986)and Lorbach and Marr (1987) developed a simplified model with fewer parameters than used by Teramoto et al. (1983) for type I1 facilitated transport. They assumed the pH change in the external phase to be negligible because of the large volume ratio of external feed phase to membrane phase normally used. They also assumed the same diffusivity for the free carrier and the carrier-metal complex.
Model Formulation for Extraction of Mercury with a Microemuhion Liquid Membrane The models discussed so far have been used to describe coarse emulsion separations. Extraction of mercury using a microemulsion falls into the category of type I1 transport because oleic acid is used as an ion exchanger to solubilize mercury in the organic phase of the emulsion. Consequently, the model equations must include not only diffusion of the mercury-oleic acid complex and internalphase stripping, but also the reaction equilibrium of mercury and oleic acid. Additionally, the kinetics of the ion exchange reaction must be taken into account in the rate equations. The modeling approach is similar to that of workers who have modeled Cu2+and Zn2+extraction (Teramoto et al., 1983; Lorbach et al., 1986; Lorbach and Marr, 1987). Although conceptually quite similar, there are some differences between their models and the one that will be presented here. This model will describe an extraction using a microemulsion liquid membrane whereas the other models describe a coarse emulsion liquid membrane extraction. This difference is manifested primarily in the emulsion macrodrop size. The Sauter mean diameter determined from photographs in their experiments ranges from 0.5 X to 1.0 X m in contrast to 1.4 X 10-5-1.7 X lod m measured for this
1614 Ind. Eng. Chem. Res., Vol. 33, No. 6, 1994
system. The smaller droplet size translates into a higher surface area for mass transfer, which implies faster separation kinetics. The chemistry of copper extraction by the acetophenone oxime reagent and zinc extraction by bis(2-ethylhexy1)phosphoricacid (D2EHPA) is different from that of mercury and oleic acid; consequently, this model must be tailored to the uniqueness of mercuryoleic acid interactions. Because of the role that the oxygen content in the surfactant plays on equilibrium extraction of mercury (Larson and Wiencek, 1992), this model must also account for complex dimerization. Finally, copper extraction kinetics using the oxime reagent are slow whereas the kinetics of the mercury-oleic acid reaction are instantaneous. Development of the model equations for batch extraction of mercury in a stirred tank reactor begins with a summary of the pertinent equilibrium reactions (Larson and Wiencek, 1992). Chemical Equations. extraction from aqueous phase:
+
-
Hg2+ 2(HR),
HgR2*2HR+ 2H+
K,, = 0.449 (1)
organic phase dimer formation: 2 HgR2.2HR
oxygen
(HgR2.2HR),
stripping reaction: HgR2.2HR
+ 2H+
-
K,, = 1600 (L/moD2 (2)
Hg2++ 2(=),
K,, = 32.8
(3)
The overbars in the above equations denote organic-phase species. Mathematical Description. External Phase. The equation for the disappearance of mercury from the external feed phase is given by
Globules. The equation of continuity for oleic acid dimer (B) in the membrane phase ( r < R ) is a[Hglint (8) Vm-2vintat r dr at while the equation of continuity for the mercury-oleic acid complex (C) is
+
arc1 -
Vm-
at
CZrepresents the complex dimer that is formed in the organic phase as a result of the presence of oxygen in the surfactant, where
Because the oxygen concentration was typically of the order of 1 kmol/m3 compared to le kmol/m3 for the mercury complex concentration in the organic phase, the oxygen concentration can be taken to be constant over the course of the extraction. Moreover, the surfactant is not expected to be localized in the emulsion because it is needed to stabilize the microdrops throughout the emulsion globule. Consequently, a new equilibrium constant can be defined as follows:
therefore,
arc,]
-= K
at
a ' -[CI2 at
e2
Substituting eq 13 into eq 9 gives where (5) initial condition:
boundary condition:
r=R
boundary conditions: [Hgl,=
[Cls[Hl,2 0.9Ke,[B12
