John F. Scamehorn

Use of Micellar-Enhanced Ultrafiltration to Remove Dissolved Organics from Aqueous Streams

Abstract Traditional ultrafiltration is ineffective at removing dissolved low molecular weight organics from water. In micellar-enhanced ultrafiltration (MEUF), surfactant is added to the water at concentrations well above the critical micelle concentration. Almost all of the organic originally dissolved (the solute) solubilizes in the micelles formed by the surfactant. The solution then passes through an ultrafiltration membrane with pores small enough to block micelle passage. The permeate contains (at most) only the unsolubilized solute and the surfactant monomer, both at very low concentrations. In this work, the criteria for selecting a surfactant are considered and MEUF is tested on an aqueous stream containing 4-tert-butyl-phenol with hexadecylpyridinium chloride as the surfactant. At high surfactant concentrations (0.25 M) in the retentate, rejections decrease, probably owing to the formation of n-mers (e.g., dimers, trimers, etc.) which are able to pass through the pores along with some solubiliz...

Concentration polarization effects in the use of micellar-enhanced ultrafiltration to remove dissolved organic pollutants from wastewater

Abstract Micellar-enhanced ultrafiltration (MEUF) is used to remove 4-tert-butylphenol (TBP) from aqueous solution, a separation for which traditional ultrafiltration is ineffactive. A micelle—forming surfactant is added to the solution. The micelles solubilize a high fraction of the TBP. The stream is then forced through an ultrafilter. Overall rejection of TBP was greater than 99%. under all conditions studied and did not decrease with increasing pressure drop. Micelles were completely rejected by membranes with pore size 10 000 Dalton MWCO and below. Concentration polarization affects MEUF fluxes under conditions of interest. Bel polarization theory does not completely explain MEUF flux behavior. Selection of optimum operating parameters in MEUF application are discussed.

Removal of divalent metal cations and their mixtures from aqueous streams using micellar-enhanced ultrafiltration

ABSTRACT Micellar-enhanced ultrafiltration (MEUF) is a novel membrane-based separation technique that can be used to remove multivalent metal cations from aqueous streams. In this technique an anionic surfactant is added to the aqueous stream containing the metal cations to be removed. The surfactant forms highly charged aggregates called micelles onto which the metal cations adsorb or bind. The aqueous stream is then passed through an ultrafiltration membrane with pores small enough to block the passage of the micelles and adsorbed metal cations. In this study, MEUF has been shown to remove divalent cadmium, zinc, copper, and calcium ions and their mixtures with rejections of at least 96%. A previously developed equilibrium binding model describes the results successfully. Under reasonable conditions the flux rates are not substantially below that of pure water, indicating the feasibility of MEUF for industrial application.

Anionic and cationic surfactant recovery from water using a multistage foam fractionator

Surfactants can be present at low concentrations in effluent wastewater from various industrial operations. Also, the increasing use of surfactant-based separations results in surfactants in water generated by these separations. The surfactant concentration must sometimes be reduced in order to meet environmental standards in discharging these waters to the environment. Also, recovery of the surfactant for reuse is sometimes economical and desirable. Foam fractionation has been shown to be an effective method of removing anionic or cationic surfactants from water in a single stage in previous works. In this study, the recovery of a cationic surfactant (cetylpyridinium chloride, CPC) and an anionic surfactant (sodium dodecylsulfate, SDS) from water by multistage foam fractionation in a bubble-cap trayed column was investigated with one to four stages operated in steady-state mode for surfactant concentrations less than the critical micelle concentration (CMC). In a previous study of a single-stage foam fractionator, CPC was shown to be effectively removed from water, and in agreement with this study. In this study, multiple trays are investigated. Enrichment ratios as high as 120.23 were observed and increased with decreasing superficial air flow rate, increasing foam height of the top tray, increasing feed liquid flow rate, decreasing feed surfactant concentration, and increasing number of stages. The fractional surfactant removal can be as high as 100% and increases with decreasing air flow rate, increasing foam height per tray, increasing feed liquid flow rate, increasing feed surfactant concentration, and increasing number of stages. Scale-up of foam fractionation for recovery or removal of surfactant from water to a multi-tray column was successful.

Precipitation phenomena in mixtures of anionic and cationic surfactants in aqueous solutions

Abstract The precipitation phase boundary for mixtures of sodium dodecyl sulfate (NaDS) and dodecylpyridinium chloride (DPCl) is determined over a wide range of concentrations. The phase boundary is composed of a monomer—precipitate equilibrium curve, where no micelles exist in solution, and two branches (one NaDS-rich and one DPCl-rich) where monomer, micelles, and precipitate exist in equilibrium. A model is developed to predict the precipitation boundary by combining regular solution theory, to calculate monomer—micelle equilibrium, with a solubility product relationship between surfactant monomer concentrations, to calculate monomer—precipitate equilibrium. Results from the model are shown to work well, except in regions where coacervate formation occurs. An empirical modification to the model is used to account for coacervate formation so that all experimental phase boundaries can be described quite well. The model can also predict the amount of precipitate that will form in any NaDS—DPCl mixture and the results are shown to agree well with experimental measurements.

Simultaneous removal of dissolved organics and divalent metal cations from water using micellar-enhanced ultrafiltration

Abstract Micellar-enhanced ultrafiltration (MEUF) is a separation process using surfactants and membranes, which can remove dissolved organic solutes or multivalent ions from water with high rejections. In MEUF, a surfactant is added to the aqueous stream at concentrations such that the vast majority of the surfactant molecules are present in micelles. The surfactant has a charge opposite that of the multivalent ion being removed. The organic solute will tend to solubilize within the micelle and the multivalent counterion will tend to bind on the micelle surface due to electrostatic attraction. The solution is then treated by ultrafiltration with membrane pore sizes small enough to block the micelles. Although effective separations of organic solutes or multivalent ions have been demonstrated, the simultaneous removal of organics and multivalent ions has not been studied previously. In this work, MEUF has been applied to mixtures containing phenol or o -cresol, and simultaneously, Zn 2+ and/or Ni 2+ , using an anionic surfactant. The results demonstrate that removal of organic solute is not significantly affected by the presence of the metal and vice versa. Because the predominant mechanisms of removal of metal ions and organic solutes are totally different, this result is reasonable from theoretical considerations. The ability of MEUF to simultaneously remove organic and multivalent metal (or metal complex) solutes makes it economically attractive.

Water softening using polyelectrolyte-enhanced ultrafiltration

Abstract The use of polyelectrolyte-enhanced ultrafiltration (PEUF) for water softening has been studied at several temperatures in the presence and absence of added salt. It is shown that in the absence of added salt. PEUF is highly effective in the rejection of up to 99.7% of hardness ions from an aqueous stream. At a low concentration of added salt. PEUF is effective in the removal of hardness. As salt concentration increases, however, hardness rejection decreases dramatically. The experimental results have been effectively modeled using an ion-binding model based on Oosawas two-phase approximation theory.

Thermodynamics of mixed micelle formation

Abstract A new method of calculating the composition of mixed micelles in equilibrium with monomer of known composition is proposed. This technique applies the Gibbs-Duhem equation to be mixed micelle, which is treated as a pseudophase. The method needs only CMC data as a function of monomer composition, but is limited to binary surfactant systems. The proposed methodology is applied to cationic/cationic, cationic/nonionic, and anionic/nonionic surfactant pairs. The calculated monomer-micelle equilibrium is found to be very similar to that from ideal solution theory for the cationic/cationic system and to the much-used regular solution theory for the nonideal systems.

Precipitation of mixtures of anionic and cationic surfactants: II. Effect of surfactant structure, temperature, and pH

Abstract Precipitation phase boundaries for sodium alkyl sulfate/dodecylpyridium chloride were measured over a wide range of surfactant concentrations as a function of pH, temperature, and anionic surfactant alkyl chain length. Increasing temperature and decreasing surfactant alkyl chain length generally tend to decrease the tendency to precipitate. A previously developed model for predicting anionic/cationic surfactant precipitation phase boundaries described the experimental results very well except in some high surfactant concentration regions where coacervate and gels formed. This model uses a simple solubility product relationship, with regular solution theory, to describe mixed micelle formation. The environment of the alkyl chains is more favorable in the precipitate than in the micelles, from measured thermodynamic properties.

Economic feasibility study of polyelectrolyte-enhanced ultrafiltration (PEUF) for water softening

Abstract In polyelectrolyte-enhanced ultrafiltration (PEUF), a water-soluble anionic polyelectrolyte (in this study sodium polystyrene sulfonate or PSS) is added to hard water. The calcium and magnesium bind to the polymer which has a high enough molecular weight to be rejected by an ultrafiltration membrane. The permeate is softened water. Economically, the PSS needs to be recovered from the retentate for reuse. Three methods of recovery developed in this study were addition of NaCl, Na2CO3 or HCl to PEUF to regenerate PSS. Of the three PEUF processes considered, NaCl/PEUF as compared to Na2CO3/PEUF and HCl/PEUF provided the best scheme for the water softening process. PEUF is shown in this study to be competitive with lime softening at low flow rates. The PEUF process is more expensive than ion exchange for a stream containing only hardness ions. However, PEUF becomes nearly comparable with ion exchange for a stream containing hardness ions as well as bacteria, viruses and pyrogen. The cost comparisons are based on fully continuous operations and include treatment of waste streams from each process.