Use of Amphiphilic Polymer Nanoparticles as a Nano-Absorbent for Enhancing Efficiency of Micelle-enhanced Ultrafiltration Process

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Use of Amphiphilic Polymer Nanoparticles as a Nano-Absorbent for Enhancing Efficiency of Micelle-enhanced Ultrafiltration

Process

Jin-Kie Shim^†, In-Sook Park, and Ju-Young Kim*^,†

Environment and Energy Division, Korea Institute of Industrial Technology, Chonan 330-825, Korea

*Department of Advanced Materials Engineering, Kangwon National University, Kangwon 245-711, Korea Received February 28, 2007; Accepted August 30, 2007

Abstract: A new ultrafiltration process using amphiphilic polymer nanoparticles was suggested for removal of dissolved hydrophobic pollutant from water. Amphiphilic polymer nanoparticles having micelle-like structure was synthesized using amphiphilic urethane acrylate anionomer (UAA) chains having polytetramethylene oxide-based hydrophobic segment and hydrophilic carboxylic acid group at the same backbone. UAA chain were mixed with water and formed nanoparticles dispersed at water, which was converted to crosslinked amphiphilic polyurethane (CAP) nanoparticles via soap-free emulsion polymerization process. CAP nanoparticles could solubilize phenanthrene dissolved in water and exhibit interfacial activity like surfactant do. At the same ultrafiltration condition, CAP nanoparticles showed higher rejection % compared to anionic surfactant, sodium do- decyl sulfate (SDS), Consequently, CAP nanoparticles could remove 95 % of phenanthrene from water via ultrafiltration process even at low concentration (30∼1000 mg/L).

Keywords: amphiphilic polymer, nanoparticles, micelle-enhanced ultrafiltration, phenanthrene

Introduction

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The contamination of surface water and groundwater by hydrophobic organic compounds is worldwide environmental problem. Organic compounds can release from the incineration of solid wastes, forest fires, the in- complete combustion of fossil fuels, and improper dis- posal of gasoline, oil, and solvents. These compounds contain various hydrophobic organic pollutants such as benzene, toluene, trichloroethylene (TCE), polycyclic ar- omatic hydrocarbon (PAH), etc, which can contaminate a large volume of soil and water by sorption onto soil and dissolving in water [1-4].

Most of these organic pollutants have low solubility in water (ppm and below), but remain toxic even at these low concentrations, so they can contaminate a very large volume of water. Conventional separation processes such as electrolyte dialysis, reverse osmosis, activated carbon adsorption, solvent extraction, and distillation have been

†To whom all correspondence should be addressed.

(e-mail: [email protected], [email protected])

used for removal of low molecular organics from wastewater [5-7]. These processes do not appear to be an at- tractive route on a large scale. Supercritical water oxida- tion, ozonation, and irradiation with UV right are recently developed processes, which are only feasible economically if feed stream is concentrated. Usual ultrafiltration process cannot be applied efficiently since the membrane pore size is not small enough to reject metal ions and small organics, resulting in low flux. Reverse osmosis process can be used for this purpose but need high costs. So, micelle-enhanced ultrafiltration (MEUF) is recently developed to remove dissolved organics [8-14] and/or heavy metal [15-19] present in small or trace quantities from aqueous solutions.

Scamehron and coworkers proposed this MEUF process to separate and remove dissolved organic pollutants from water by aid of surfactant micelles via UF process [20,21]. That is, at higher than a certain concentration level (critical micelles concentration, cmc), micelles formed by nano-sized aggregation of surfactant mono- mers solubilize organic pollutants and then separated by UF membrane. In this system, however, the fatal draw-

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back is leakage of surfactant especially at a low concentration below cmc, which becomes a secondary pollution.

Even at higher than cmc, surfactants cannot be 100 % re- jected and separated by UF membrane because of break- age of surfactant micelles. So, a new type of micelle-like nanoparticle should be developed to overcome these drawbacks and highly improve efficiency of MEUF process. Micelle-like nanoparticles could be prepared using amphiphilic molecules such as surfactant, and amphiphilic block copolymers. The CMC of amphiphilic block copolymers is extremely low and their dispersion efficiency is retained even at extremely high dilution, and so amphiphilic block copolymers can be used as an alternative material for surfactant [22]. However, since these polymers are very expensive and can be obtained only by extremely difficult synthetic processes, their practical ap- plications are limited.

In this study, we present the synthesis of micelle-like nanoparticles using amphiphilic oligomer chain, urethane acrylate nonionomer (UAA) to use it as a new type of functional nano-absorbent for hydrophobic pollutants dissolved at water. Also, we present a new MEUF process whose separation efficiency is highly enhanced by amphiphilic polymer nanoparticles, which can solubilize low molecular weight hydrophobic pollutants like surfactant micelles do at conventional MEUF process. Since amphiphilic urethane acrylate anionomer (UAA) chains have hydrophobic polyteramethylene oxide-based segment and anionic hydrophilic segment at the same backbone. UAA chains could form nanoparticles dispersed at water by just mixing with water, which were converted to crosslinked amphiphilic polymer (CAP) nanoparticles dispersed at water through crosslinking polymerization process [23]. We first presented solubilization performance and interfacial activity of amphiphilic CAP nanoparticles. Also, retention recovery of CAP and separation efficiency of UF using amphiphilic polymer nanoparticles were examined and compared it with that of conventional MEUF at the same condition.

Experimental

Materials

The mixed cellulose esters (MCE) ultrafiltration flat membranes (catalog no. VSWP04700) purchased from Millipore were used. The pore size of MCE membrane is a 0.025 µm and the wettability of that is hydrophilic. We have used phenanthrene as a model hydrophobic pollutant. Among hydrophobic organic pollutants, PAHs such as pyrene, naphthalene, and phenanthrene, are of special interest because they are strongly sorbed to soils or sediments. PAH are also receiving increasing attention because of their toxicity (highly carcinogenic) and their

continuous release in the environment through human ac- tivities associated with combustion and petroleum pro- duction [24-28]. The aqueous solubility of phenanthrene is reported to be 1.29 mg/L, and its octanol-water parti- tion coefficient is 3700. Radio-labeled phenanthrene was purchased from Sigma Chemical Co., USA (9-¹⁴C, 13.1 µCi/µmol). Sodium dodecy sulfate (SDS, Formula weight 288.38), anionic surfactant used in this study was obtained from Aldrich Chemical Co., USA.

In the synthesis of amphiphilic UAA precursor chains, poly(tetramethylene glycol) (PTMG, Mw = 1000, Al- drich Chemical Co., U.S.A.), 2,4-toluene diisocyanate (TDI, Aldrich Chemical Co., U.S.A), 2-hydroxyethyl methacrylate (2-HEMA, Aldrich Chemical Co., U.S.A), and dimethylol propionic acid (DMPA, Aldrich Chemi- cal Co., U.S.A) were used as received. Potassium persul- fate (KPS, Wako Pure Chemicals Co., Japan) and 2,2- azobisiso-butyronitrile (AIBN, Aldrich Chemical Co., U.S.A) was recrystallized from distilled deionized (DDI) water and absolute ethanol, respectively. N-methyl-2- pyrrolidone (NMP, Aldrich Chemical Co., U.S.A) was used as a solvent for DMPA and as viscosity thinner of the synthesized precursors.

Synthesis of Amphiphilic Urethane Acrylate Aniono- mers (UAA)

Amphiphilic UAA precursor chains were synthesized through an established three-step process. Synthesis of UAA chains was carried out using a 500 mL 4-neck ves- sel with a stirrer, a thermometer, a reflux condenser and an inlet system for nitrogen gas. The preparation method is illustrated in previous papers in details [24,29]. To varying the hydrophilic/hydrophobic balance of UAA chains, the molar ratio of PTMG/DMPA was changed from 6/4, 5/5, and 2/8 in the synthesis formulation, where the molar ratio of NCO/OH was fixed as 1.5/1. Symbol UAA64 represents UAA precursor chain synthesized at the 6/4 molar ratio of PTMG/DMPA. The hydrophilicity of UAA chains increases in the order of UAA64, UAA55, and UAA28. Proposed chemical structure of UAA and recipe for synthesis of UAA chains is shown in Figure 1 and Table 1, respectively. The polystyrene equivalent molecular weight of synthesized UAA chains is a 3750∼6700 weight average molecular weight with a polydispersity of 1.93∼2.01.

Synthesis of Crosslinked Amphiphilic Polymer (CAP) Nanoparticles

To be used in UF process without blocking membrane pores, UAA precursor chains needed to be prepared in nano-sized particles to allow them to flow through soil bed and maximize the contact area. UAA chains were first emulsified through the phase inversion emulsifica- tion process without using any external surfactant, be-

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Table 1. Recipe for Synthesis of UAN Chains and Their Molecular Weight

UAA chain Molar ratio of PTMG/DMPA/TD/2‐HEMA Mw (g/mol)

UAA 64 0.6 / 0.4 / 1.5 /1 6700

UAA 55 0.5 / 0.5 / 1.5 /1 5670

UAA 28 0.2 / 0.8 / 1.5 /1 3750

Table 2. Recipe for synthesis of CAP Nanoparticles and Their Particles Size

CAP nanoparticles Ingredient

Particle Size (nm)

UAA Water KPS

CAP64 UAA 64 (12 g) 88 g 0.05 g 61.20

CAP55 UAA 55 (12 g) 88 g 0.05 g 58.32

CAP28 UAA 28 (12 g) 88 g 0.05 g 32.40

Figure 1. Schematic presentation of UAA chains and CAP nanoparticles.

cause a surfactant may affect the adsorption of CAP nanoparticles onto soil. 90 g of DDI water are slowly dropped into the homogeneous mixture of UAA, triethyl- amine (TEA) as neutralizing reagents and AIBN. After the formation of UAA nanoparticles dispersed at water, crosslinking polymerization of UAA nanoparticles was carried out at 80 ^oC for 5 h with stirring at 200 rpm to ob- tain CAP nanoparticles. Recipe for this emulsion polymerization is shown in Table 2. Postulated microstructure of CAP is represented in Figure 1. The CAP nanoparticle synthesized with UAA 64, UAA55, and UAA28 chain is named as CAP 64, CAP 55, and CAP 28 nanoparticle, respectively. As a result, the sizes of the

obtained CAP measured using dynamic light scattering were in the range of 32∼61 nm.

After completion of polymerization, the CAP emulsions were poured into 100 mL beaker containing CaCl₂ aqueous solution to aggregate CAP nanoparticles. After col- lecting via filtration, and aggregated CAP nanoparticles were immersed at acetone/water mixture to remove un- reacted materials and initiators, etc. and dried for 24 h in a vacuum oven. The conversion from UAA chains to crosslinked CAP nanoparticles was determined using fol- lowing equation:

Conversion (wt%) = (W_d/W_t × TSC) × 100 %

Where, W_d and W_t is collected sample weight (g) and sample weight after washing and drying (g), respectively.

TSC is theoretical solid content value per gram of collected sample at 100 % conversion. Conversions for UAA chains were in the range of 92∼95 %.

Surface Tension Test

Surface tension of SDS solution, CAP emulsions were determined with a Model 20 surface tensiometer (Fisher Scientific.). This instrument operates on the du Nöuy principle, in which a platinum ring is suspended from a torsion balance. Each aqueous sample was tested with the tensiometer at controlled temperature (25 ^oC) until at least three consistent surface tension reading were obtained. Between each reading, the ring was cleaned with acetone and heated to redness in a gas flame.

Filtration Experiments

A dead-end stirred cell filtration system, illustrated at Figure 2, was used to characterize the filtration performance with CAP nanoparticles and SDS solution. Before all filtration experiments, membranes were soaked in de-ionized water for, at least, 24 h prior to use to remove any organic components and compacted at a transmembrane pressure 4 kgf/cm² for 2 h. And then the con-

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Figure 2. Schematic presentation of a dead-end stirred cell filtration system.

stant water flow rate was noted down to calculate membrane permeability at each operating pressure (1, 2, and 3 kgf/cm²).

Experiments were carried out to study the effects of applied pressure, concentration of SDS and CAP nanoparticle on rejection. The concentrations of CAP nanoparticles and SDS solution were analyzed by total organic carbon analyzer (Pharma TOC, Analytik Jena AG Co., Germany). An electronic balance (AR2140, OHAUS Corp., USA) was used to measure permeate mass. The rejection (R) is defined as follows:

_{    }

_

_

where C_feed and C_per are the concentration of solute in feed and permeate solutions, respectively.

Pressure in the cell was maintained by nitrogen gas.

Magnetic stirrer kept the stirred speed constantly at 250

∼300 rpm. All the experiments were conducted at room temperature. ¹⁴C-phenanthrene aqueous solution (activity 0.056∼0.068 Ci/mL) was prepared by a method de- scribed previously [30,31]. 1 mL of ¹⁴C-phenanthrene aqueous solution was added into CAP aqueous solution, and then mixed by magnetic stirrer over 2 h. The concentrations of CAP nanoparticles in the aqueous solution were varied from 30 to 1000 mg/L. The experiments were also tested at ¹⁴C-phenanthrene concentration range of 160∼1000 decay events per minute (DPM) to measure effects of ¹⁴C-phenanthrene concentration on re- jectionrate for the CAP nanoparticles. A sample was prepared by expressing a permeate solution 1 mL into a scintillation vial containing 10 mL of ready safe. The LSC counted the DPM of the radiolabeled hydrophobic pollutant by employing the H-number quench monitor and compensation technique. The ¹⁴C-labeled solution

activity was obtained by dividing the corrected DPM by the solution volume and then converting this value to the equivalent number of moles of phenanthrene per liter of solution by using the conversion factor of 2.22 × 10⁶ DPM/µCi. DPM values were measured by the LSC to at least 95 % confidence level and recorded for subsequent background correction and conversion to phenanthrene concentration units. Background DPM rates were peri- odically measured in scintillation vials containing scintillation cocktail but no sample.

Solubilization of Crystalline Phenanthrene in Surfact- ant or CAP Solutions

Solubility of phenanthrene in surfactant or CAP solutions was determined using mixture of radiolabeled phenanthrene and nonlabeled phenanthrene [32]. Con- centrated phenanthrene solutions (35 g/L) were prepared in methylene chloride. 2 mL of phenanthrene solution was added into 25 mL glass scintillation vial equipped with open-top screw caps and Teflon-backed septa. After evaporation of methylene chloride, SDS or CAP aqueous solutions (10 mL) of various concentrations (143∼ 38000 mg/L) were added into the vials. Since amount of phenanthrene remaining in the vials was much greater than the solubility of phenanthrene in water, the loss of phenanthrene due to evaporation was inconsequential.

The vials were sealed and gently agitated with a rotary tumbler for 1 day. After completion of mixing, 5 mL of supernatant was withdrawn and centrifuged at an accel- eration 15000 × g. 1 mL of sample was transferred into scintillation vials (Poly-Q vial P/N 566740, Beckman Coulter, U.S.A) containing 10 mL of Ecolume cocktail (Ready Safe P/N 141349, Beckman Coulter, U.S.A), and the concentrations of ¹⁴C-phenanthrene in the aqueous phase was measured using a Liquid Scintillation Counter (LSC).

Results and Discussion

Enhanced Solubilization of Hydrophobic Organic Pollutant

Amphiphilic molecules, surfactant, can increase solubility of hydrophobic organic compounds (HOC) in water phase because the hydrophobic core of surfactant micelles can accommodate a certain amount of lipophilic organic compound. This solubilization capability of surfactant makes itself useful materials for soil-washing and wastewater treatment process. So, the enhanced solubility of a certain HOC by a surfactant can be used as an index for evaluating a surfactant for soil-washing process and wastewater treatment process of sorbed and solubi- lized HOC [33-36]. Especially, in MEUF process, surfactant molecules added at aqueous phase should solubilize

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(a) (b)

Figure 3. Schematic presentation of MEUF filtration mechanism containing (a) surfactant and (b) CAP nanoparticles.

Figure 4. Enhanced solubility of phenanthrene at aqueous phase in the presence of SDS or CAP nanoparticles: ■ CAP 64, ● CAP 55, ▲ CAP 28, and ▼SDS.

HOC and be separated by ultrafiltration membrane to remove HOC from water, as illustrated at Figure 3(a) [20,21]. That is, solubilization performance of surfactant for HOC is very decisive factor for determining efficiency of a MEUF process. So, to be used at MEUF process instead of a surfactant, CAP nanoparticles should solubilize HOC and be separated by ultrafiltration membrane (See Figure 3(b)). Firstly, solubilizing efficiency of

CAP nanoparticles for HOC (phenanthrene) was eval- uated and compared to that of SDS.

Solubilization efficiency of a surfactant is generally de- scribed as molar solubilization ratio (MSR) or weight solubilization ratio (WSR), but molecular weight of CAP nanoparticles can’t be calculated because those particles were formed by crosslinking polymerization of UAA.

That is, molecular weight of crosslinked polymer can be considered infinite. So, we can not but help describing solubilization efficiency of CAP nanoparticles as enhanced solubility of phenanthrene at water that is ratio of C/Co: C is the concentration of phenanthrene in aqueous solution containing CAP nanoparticles or SDS and Co is the concentration of phenanthrene in pure water phase.

So, C/Co of CAP nanoparticles should be greater than 1 to be a useful material for absorbing and removing phenanthrene dissolved at water. Enhanced solubility (C/Co) of phenanthrene at various concentrations of SDS or CAP nanoparticles at aqueous solutions is given in Figure 4.

C/Co of CAP aqueous solution increased with the increase of concentration of CAP nanoparticles. Even at the lowest concentration (143 mg/L), C/Co of CAP nanoparticles was greater than 1, indicating that CAP nanoparticles can solubilize phenanthrene within their hydrophobic interiors just like the solubilization of phenanthrene in surfactant micelles. At the highest concentration (38000 mg/L), SDS micelles and CAP nanoparticles can solubilize approximately 12 times and 2∼8

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Figure 5. Surface tension for SDS and CAP nanoparticle solutions: ■ CAP64, ● CAP 55, ▲ CAP 28, and ▼ SDS.

times the phenanthrene that an equal amount of pure water will solubilize. At our MEUF process, concentration of SDS and CAP nanoparticles was varied from 200∼ 1000 mg/L. At the concentration region, CAP 64 and SDS showed almost same C/Co of phenanthrene.

CAP 64 nanoparticles show the better enhanced-solubilization performance than CAP 55 and CAP 28 nanoparticles. This difference may be due to the difference of hydrophilicity or hydrophobicity among these nanoparticles. For the synthesis of UAA chains, DMPA in- troduces hydrophilic carboxylic groups to hydrophobic PTMG-based UAA chain backbone, which make water-insoluble urethane acrylate chains dispersible in water and form nanoparticles at water. As the molar ratio of DMPA to PTMG increases in the synthesis of UAA chains, the hydrophilicity of UAA chains increase, resulting in the formation of smaller CAP nanoparticles having higher hydrophilicity. So, UAA 64 chain used in the preparation of CAP 64 nanoparticles was synthesized at the highest molar ratio of PTMG/DMPA, so that CAP 64 nanoparticles have the highest hydrophobicity among CAP nanoparticles, resulting in the highest enhanced solubilization performance of CAP 64.

Materials used at MEUF process should also have interfacial activity to solubilize and absorb hydrophobic pollutants effectively. So, surface tension of SDS and CAP aqueous solutions were examined at various concentrations and presented at Figure 5. Like SDS solution, CAP solutions show the decrease of surface tension with the increase of concentration of CAP in the solution. This indicates that CAP nanoparticles also have interfacial activity like surfactants do. Unlike surfactants exhibiting abrupt decrease in surface tension at certain concen-

Figure 6. Schematic illustration of aqueous pseudophase of SDS and CAP solutions.

tration, CAP nanoparticle solutions show gradual and lin- ear decrease in surface tension with the increase of CAP concentration. This very different result between SDS and CAP solution can be interpreted as due to the difference of aqueous pseudophase between SDS and CAP solution.

For surfactant molecules, at the concentration below cmc surfactant molecules dissolve at water and exist as monomeric molecules. At the concentration equal to or greater than cmc, surfactant molecules form aggregates (micelle) by the association of surfactant molecules.

Surface tension of surfactant solutions shows abrupt change and almost constant values at this cmc. Surfactant micelles are not permanent structure but can break into monomeric surfactant molecules. So, there coexist dissolved surfactant molecules and micelles at aqueous pseudophase of surfactant molecules.

UAA chains have extremely low solubility in water (practically water insoluble), so on contacting with water, UAA chains associate with each other and form nano-aggregates (CAP nanoparticles) at extremely low concentration. That is, UAA chains do not have a certain concentration for the formation of aggregates; as a consequence, CAP solutions did not show abrupt change in surface

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Figure 7. Rejection % of SDS and CAP nanoparticles as a function of concentration at a constant pressure of 2 Kgf/cm² in the absence of phenanthrene (● CAP 28, ▲ CAP55, ■ CAP 64. ▼ SDS solution).

tension with the change of CAP concentration. In addition, nano-aggregated structure of CAP nanoparticles are permanently locked-in by chemical crosslinking polymerization. So, it can be assumed that there are no dissolved and monomeric UAA chains at aqueous pseudophase of CAP nanoparticles, which is schematically de- scribed in Figure 6.

Rejection Ratio of SDS and CAP Noparticles in UF Membrane

To effectively remove dissolved hydrophobic organic pollutants from water via MEUF process, absorbing material should show very high rejection in UF process.

That is, rejection behavior of an absorbing material is most important factor for efficiency of UF process. So, rejection behavior of SDS and CAP nanoparticles using the same UF membrane and at the same condition should be first examined. Figure 7 shows the rejection % of CAP and SDS with various concentrations at a constant pressure of 2 Kgf/cm² through the same UF membrane.

The concentration of SDS and CAP nanoparticles was varied from 200∼1000 mg/L to examine removal efficiency of SDS and CAP nanoparticles. At all concentrations, SDS solution shows very low rejection ratio (below 50 %), but all CAP nanoparticle solutions exhibit higher rejection ratio (almost higher than 90 %). This indicates that the UF membrane used in this study can effectively separate CAP nanoparticles only at our condition.

Figure 8 shows the rejection % of CAP nanoparticles and SDS at different transmembrane pressure, where the

Figure 8. Rejection % of SDS and CAP nanoparticles as a function of transmembrane pressure at a constant concentration of SDS and CAP nanoparticles (200 mg/L) (● CAP 28, ▲ CAP55, ■ CAP 64. ▼ SDS solution).

concentration of CAP nanoparticles and SDS solution is 200 mg/L. Rejection % of CAP nanoparticles is also higher than that of SDS at all transmembrane pressure.

CAP nanoparticles show almost constant rejection % with the increase of applied transmembrane pressure.

This can be explained as due to no change of morphol- ogy and size of CAP nanoparticles at higher applied pressure. Because CAP nanoparticles have chemically crosslinked structure, they can maintain their morphol- ogy and size at higher applied pressure. However, SDS solution showed a little increase of rejection % at higher applied pressure, which can be explained as due to break- age of surfactant micelles and adsorption onto the membrane surface.

Rejection of Dissolved Phenanthrene Via UF Mem- brane Process Containing CAP Nanoparticles

Since only CAP nanoparticles exhibited acceptable rejection ratio (higher than 90 %) at our UF membrane condition, CAP nanoparticles were used for removal of dissolved phenanthrene via UF membrane process. Fig- ure 9 shows rejection of dissolved phenanthrene via UF membrane process containing CAP nanoparticles with various concentrations (30∼1000 mg/L) of CAP nanoparticles. Over 90 % of dissolved phenanthrene was re- jected by UF membrane at all concentrations. That is, 97

% of dissolved phenanthrene at aqueous phase was separated and removed from aqueous phase using only 30 mg/L of CAP nanoparticles. This concentration is much lower than cmc of SDS (2883 mg/L).

Although CAP 64 showed higher enhanced solubility of

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Figure 9. Rejection % of phenanthrene as a function of concentration CAP nanoparticles at a constant pressure of 2 Kgf/cm² (● CAP 28, ▲ CAP 55, ■ CAP 64).

Figure 10. Rejection % of phenanthrene as a function of transmembrane pressure at a constant concentration of CAP nanoparticles (1000 mg/L) (● CAP 28, ▲ CAP 55, ■ CAP 64).

phenanthrene than CAP 55 and CAP 28 (as presented at Figure 4), 3 types of CAP nanoparticles exhibited almost same rejection % for dissolved phenanthrene. So, it can be thought that all CAP nanoparticles have enough hydrophobicity for absorbing and removing dissolved phenanthrene. Figure 10 shows rejection of dissolved phenanthrene via UF membrane process at constant concentration of CAP nanoparticles (1000 mg/L) with various transmembrane pressures. Like result for rejection of CAP nanoparticles via UF membrane, rejection of phenanthrene did not depend on applied pressure. This result can be also interpreted as due to chemically crosslinked

microstructure of CAP nanoparticles. Unlike surfactant micelles, which easily break down, microstructure of CAP nanoparticles do not break down easily owing to permanently locked-in microstructure. So, CAP nanoparticles showed almost same rejection via UF membranes at higher pressure, as a consequence, CAP nanoparticles could remove almost same amount of dissolved phenanthrene via UF membrane at elevated pressure.

Conclusion

Without using surfactant molecules or amphiphilic block copolymers, a new nano-sized absorbent, that is, amphiphilic polymer nanoparticles (CAP nanoparticles) could be created using amphiphilic oligomer UAA chains. These CAP nanoparticles absorbed hydrophobic molecules (phenanthrene) and showed interfacial activity like surfactant molecules. At the same ultrafiltration condition, CAP nanoparticles could be recovered almost 100

% and separate dissolved phenanthrene at very low concentration (97 % of rejection at 30 mg/L of concentration), whereas SDS molecules could not be recovered and separate. This big difference of rejection and removal efficiency between SDS and CAP nanoparticles is due to microstructural difference between SDS and CAP nanoparticles and bigger size of CAP nanoparticles compared to SDS micelles. That is, chemically crosslinked microstructure of CAP nanoparticles makes it possible to maintain their structure on contacting with membrane and higher applied pressure. In addition, bigger size of CAP nanoparticles caused much higher recovery of CAP nanoparticles. Even though price of UAA precursor chains used for synthesis of CAP nanoparticles is higher than that of commercialized surfactants, much greater rejection and removal efficiency of CAP nanoparticles as compared to the surfactant will make UF process using CAP nanoparticles an economically feasible and attrac- tive process alternative to conventional MEUF process.

Acknowledgments

This subject is supported by Korea Ministry of Envi- ronmental as “The Eco-technopia 21 project”.

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