a column test for leaching of organochlorines from soil by amphiphilic nonionic nanopolymers

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This article was downloaded by: [University of California Santa Cruz] On: 08 October 2014, At: 18:56 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Journal of Environmental Science and Health, Part B: Pesticides, Food Contaminants, and Agricultural Wastes Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesb20 A column test for leaching of organochlorines from soil by amphiphilic nonionic nanopolymers Benjalak Karnchanasest a b & Darryl W. Hawker c a Institute of Environmental Research , Chulalongkorn University , Bangkok, Thailand b Center of Excellence for Environmental and Hazardous Waste and Management , Chulalongkorn University , Bangkok, Thailand c School of Environment , Griffith University , Nathan, Qld, Australia Published online: 24 May 2011. To cite this article: Benjalak Karnchanasest & Darryl W. Hawker (2011) A column test for leaching of organochlorines from soil by amphiphilic nonionic nanopolymers, Journal of Environmental Science and Health, Part B: Pesticides, Food Contaminants, and Agricultural Wastes, 46:5, 411-418 To link to this article: http://dx.doi.org/10.1080/03601234.2011.572508 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions

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This article was downloaded by: [University of California Santa Cruz]On: 08 October 2014, At: 18:56Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

Journal of Environmental Science and Health, Part B:Pesticides, Food Contaminants, and Agricultural WastesPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/lesb20

A column test for leaching of organochlorines from soilby amphiphilic nonionic nanopolymersBenjalak Karnchanasest a b & Darryl W. Hawker ca Institute of Environmental Research , Chulalongkorn University , Bangkok, Thailandb Center of Excellence for Environmental and Hazardous Waste and Management ,Chulalongkorn University , Bangkok, Thailandc School of Environment , Griffith University , Nathan, Qld, AustraliaPublished online: 24 May 2011.

To cite this article: Benjalak Karnchanasest & Darryl W. Hawker (2011) A column test for leaching of organochlorinesfrom soil by amphiphilic nonionic nanopolymers, Journal of Environmental Science and Health, Part B: Pesticides, FoodContaminants, and Agricultural Wastes, 46:5, 411-418

To link to this article: http://dx.doi.org/10.1080/03601234.2011.572508

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of theContent. Any opinions and views expressed in this publication are the opinions and views of the authors, andare not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon andshould be independently verified with primary sources of information. Taylor and Francis shall not be liable forany losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoeveror howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use ofthe Content.

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Journal of Environmental Science and Health Part B (2011) 46, 411–418Copyright C© Taylor & Francis Group, LLCISSN: 0360-1234 (Print); 1532-4109 (Online)DOI: 10.1080/03601234.2011.572508

A column test for leaching of organochlorines from soil byamphiphilic nonionic nanopolymers

BENJALAK KARNCHANASEST1,2 and DARRYL W. HAWKER3

1Institute of Environmental Research, Chulalongkorn University, Bangkok, Thailand2Center of Excellence for Environmental and Hazardous Waste and Management, Chulalongkorn University, Bangkok, Thailand3School of Environment, Griffith University, Nathan, Qld, Australia

Amphiphilic nonionic cross-linked nanopolymers (NPs) were synthesized to examine removal of five organochlorines (OCs), namelylindane, heptachlor, aldrin, dieldrin, endrin, and DDT, from a range of Thai agricultural soils. The synthesized NP particles hadpolarity characteristics similar to those of nonionic surfactant micelles and were largely in the size range of 55–155 nm. This workaimed to determine the optimal conditions for leaching of OC contaminated soil with NPs and also to investigate the role andinfluence of soil properties on this leaching. An investigation of the concentrations of aqueous dispersions of these particles foundthat a concentration of 10 g L−1. was found most effective in leaching the OCs from a column of spiked soil. The optimal contacttime that allowed a NP dispersion and spiked soil to reach equilibrium was 48 h. The results indicated influencing factors for OCremoval and soil remediation were properties both of the soil and the compounds themselves. Soil organic carbon (SOC) contentand soil texture played an important role on the sorption as well as compound hydrophobicity expressed as log KOW values. Theremoval efficiency was found to be in the range of 85.2–92.8 % for all soil samples and in the order of DDT < aldrin < heptachlor <

dieldrin < endrin < lindane regardless of soil type. This order is inversely related to the log KOC values of these compounds. For OCcompounds with a similar molecular structure, removal efficiency was related to molecular weight (MW).

Keywords: Organochlorines; nanopolymers; column leaching; contaminated soil; remediation.

Introduction

Organochlorine compounds (OCs) are trace components ofsome agricultural soil and organic amended soils. They areregarded as hazardous due to their potential for bioaccu-mulation, environmental persistence and toxicity. Althoughtheir use has been banned or restricted under treaties suchas the Stockholm Convention,[1] they are still routinelyfound in the environment, particularly agricultural soils.[2]

Furthermore, as Ozkoc et al.[3] point out, such compoundsmay still be used illegally in some regions, contributing totheir observed levels.

Generally, hydrophobic organic contaminants (HOCs)such as OCs exhibit, by their very nature, extensive sorp-tion to soil organic matter and therefore relatively smallleached concentrations are available for soil microorgan-isms to biodegrade.[4] In addition, the rate of desorptionis also likely to be small. This means for example thatsorbed OCs can act as a long-term contamination source

Address correspondence to Dr. Benjalak Karnchanasest, In-stitute of Environmental Research, Chulalongkorn University,Bangkok, Thailand; E-mail: [email protected] September 13, 2010.

for groundwater.[5] Remediation of soil and sediment ma-terial containing OCs can be problematic because of slowkinetics. It has been shown that equilibrium of OCs be-tween sorbent particles and water can take on the order ofyears.[6−8]

As a result, soil remediation by conventional pump-and-treat methods involving extraction and treatment of con-taminated water have had little or only qualified success.The time required and the volume of water required toflush the contaminated sorbent could become so great asto be prohibitive.[9]

Another widely used method for the removal ofHOCs from contaminated soil is soil washing withsurfactants.[10,11] The technique involves surfactantmolecules desorbing HOCs from soil through HOCdissolution in surfactant micelles. Surfactant micelles havebeen shown to enhance the desorption rate of HOCs fromsoil and thus reduce the time required for remediation com-pared to simply washing with water.[8] HOC partitioninginto the hydrophobic interior of micelles enhances the ap-parent solubility of the HOC.[12] However, this procedureis only applicable when the surfactant concentration ishigher than the critical micelle concentration (CMC).[13] Inaddition, micelles are generally not stable and most

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Table 1. Relevant physical and chemical properties of soil.

Composition

Soil Sample Texture % Sand % Silt % Clay pH CEC(cmol kg−1) % Soil Organic Carbon

I Loam 40.2 35.6 24.2 4.87 4.8 0.92II Clay loam 41.8 26.0 32.2 4.84 7.1 1.35III Clay 21.8 25.8 52.4 5.20 14.6 1.66IV Clay 13.8 21.8 64.4 5.42 11.3 1.84V Clay loam 43.8 27.8 28.4 6.71 18.9 2.51

surfactants can themselves be sorbed by the soil, resultingin a lower concentration of mobile HOCs.[14] Kim et al.note also that at surfactant concentrations above theCMC, contaminant emulsions may form that are difficultto extract from soils.[15]

In order to address the shortcomings of surfactant-enhanced remediation techniques, amphiphilic cross-linkedpolyurethane nanopolymers (NPs) have been investigatedfor remediation of soils contaminated with polycyclicaromatic hydrocarbons (PAHs).[5,15,16] These NPs can besynthesized to possess properties similar to those of surfac-tant micelles, with a hydrophobic interior and hydrophilicexterior.[15] Some are self-emulsifiable or self-organizing,with colloidal dispersions of NPs formed by simple stirring.The polymeric phase can then be cross-linked. Whereassurfactant-based micelles can break down and lose sur-factant molecules through sorption to the soil, these NPsremain structurally intact due to the cross-linking. Further-more, variation in ratios of reactant monomers can result inNPs with different properties such as particle size, mobilityin soil and extent of cross-linking.[5,17]

Previous work has shown that NP particles were onlyweakly sorbed onto sandy soils, due to ionic or polar func-tional groups on the NP surface.[15] Colloidal NP disper-sions extracted up to 98 % of the sorbate and in columnexperiments, fewer pore volumes of a NP dispersion wererequired to achieve a given level of remediation comparedto surfactant solutions.[15] However, attention has hithertobeen focused on the remediation of PAH contaminatedsoils where variation in soil characteristics was limited. Ithas been mentioned previously that OC contaminated soilsare still found and are potentially hazardous. It is of inter-est then to investigate the efficacy of NPs in remediation ofOC contaminated soils with varying characteristics. Thus,the aims of this work were to (1) determine the optimalconditions for leaching of OCs from soil with NPs and (2)investigate the role and influence of different soil propertieson this removal.

Materials and methods

Soil

Five different soil surface samples (10–15 cm) were col-lected from agricultural soils in Udon Thani province,

northeastern Thailand. Each soil sample was separatelymixed, sieved through a 0.5 mm sieve and stored in air-tight,clean glass containers at room temperature. Each samplethen was analyzed for pH (soil:water=1:1), texture, cationexchange capacity (CEC) (ammonium saturation and dis-tillation) and soil organic carbon (SOC) content (Walkley-Black method). Characteristics of these soils (denoted assoils I to V), as determined by these methods, are summa-rized in Table 1. Analysis for native OCs showed levels tobe below the detection limit.

Chemicals

Poly(tetramethylene glycol) (PTMG) and 2,4-toluene di-isocyanate (TDI) were used as received from AldrichChemical Co. 2-Hydroxyethyl methacrylate (HEMA) andpoly(ethylene glycol) (PEG) were purchased from Flukaand N,N dimethylacetamide (DMAc) and potassium per-sulfate (KPS) from Acros. The solvents n-hexane and di-ethyl ether were obtained from J.T. Baker. Anhydroussodium sulfate (Na2SO4) was obtained from the MerckChemical Co. The Florisil used was Pesticide Residue grade(60/1000 mesh) also from Merck Chemical Co. Sodiumazide (NaN3) was used as received from Aldrich ChemicalCo.

Test compounds

Organochlorines (OCs) selected for this study werealdrin (1, 2, 3, 4, 10, 10-Hexachloro-1, 4, 4a, 5, 8,8a-hexahydro-1, 4:5, 8-dimethanonaphthalene), dieldrin((1aR,2R,2aS,3S,6R,6aR,7S,7aS)-3,4,5,6,9,9-hexachloro-1a,2,2a,3,6,6a,7,7a-octahydro-2,7:3,6-dimethanonaphtho[2,3-b]oxirene), endrin (1aR,2S,2aS,3S,6R,6aR,7R,7aS)-3,4,5,6,9,9-hexachloro-1a,2,2a,3,6,6a,7,7a-octahydro-2, 7:63,-dimethanonaphtho[2,3-b]oxirene), lindane (γ -hexachlorocyclohexane or benzenehexachloride(BHC)), heptachlor (1,4,5,6,7,8,8-Heptachloro-3a,4,7,7a-tetrahydro-4,7-methano-1H-indene), and DDT (1,1,1-trichloro-2,2-di(4-chlorophenyl)ethane). As Poolpak etal. note, all these OCs have been imported and appliedto agricultural land in Thailand.[18] The compound2,4,5,6-Tetrachloro-m-xylene was used as an internal stan-dard. All these compounds were obtained from Sigma-Aldrich.

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Leaching with amphiphilic nonionic nanopolymers 413

Nanopolymers (NPs)

The NPs were synthesized after the methods of Tungitti-plakorn et al.[5] to be amphiphilic and have a hydrophobicinterior region but a hydrophilic surface in order to pro-mote NP mobility in the soil. The monomers TDI: PTMG:HEMA: PEG were used in the molar ratio of 2:1:1:1 forthe synthesis as detailed below.[5]

Synthesis of poly(ethylene glycol)-modified urethaneacrylate (PUMA) precursor chains

TDI (6.954 g) was mixed with 20.002 g PTMG ina reactor, stirred at 35 ◦C for 30 min, and stirredagain for 5 h at 45 ◦C. The reactor was cooled to20◦C, 2.603 g HEMA was then added while stirring for30 min followed by stirring for a further 3 h at 45 ◦C.Finally, 40.00 g PEG in DMAc was introduced and themixture temperature was increased to 65 ◦C with stirringfor 12 h. The synthesis was carried out under high purity(99.99 %) nitrogen gas to avoid the introduction of anyoxygen into the system. The HEMA adds a reactive vinylfunctionality to the molecules which can be utilized forlater cross-linking purposes while the PEG ultimately be-comes the hydrophilic chains extending from the exterior ofthe NPs.[19] Synthesis of the PUMA precursor chains wasconfirmed using Fourier Transform Infrared Spectrometry.The end point was taken as the disappearance of the ab-sorbance of the isocyanate functional group (NCO) at awavenumber of 2270 cm−1.

Formation of a colloidal dispersion of NPs

Deionized water (DI) was added dropwise to a mixture ofthe PUMA precursor chains at 35 ◦C with vigorous stirringuntil a dispersion of NPs was formed.

Cross-linking reaction

The KPS was used a radical initiator for this process. At 65◦C, 5% KPS solution was added to the dispersion preparedabove in a round bottom flask equipped with magneticstirrer and flushed with high purity nitrogen gas and stirredfor 8 h. The dispersions of cross-linked NPs obtained werefiltered through a 0.45 µm size binder-free glass microfiberfilter (Whatman GF/C) and then the pH adjusted from 2 to7 using NaOH. The size of the NP particles was measuredusing a laser particle size distribution analyzer (MalvernMastersizer S, Malvern Instruments, UK).

NP dispersions were then prepared in 6 working concen-trations (5, 10, 15, 20, 25, 30 g L−1 in DI water) for use inleaching and recovery experiments.

Experimental procedures

Extraction of spiked OCs from soil

For assessment of the extent of OC removal by NPs, deter-mination of concentrations of the spiked test compounds

in soil was necessary. The soil (30 g) was extracted with150 mL hexane in a soxhlet extractor for 2 h. The cooledextracts were filtered through a GF/C filter and the fil-trate was reduced in volume by a rotary evaporator to 5mL and then to 2 mL by a nitrogen stream. The soil ex-tract was then subject to cleanup and quantification stepsas described below.

Extraction of ocs from aqueous solution

During this work, spiked soil in contact with NP solu-tions in columns was eluted with water to leach the OCpesticides. Elution ceased after no more OC was foundin the eluent. For OC analysis of this eluant, because ofpotential trace interferences from the soil, it was first fil-tered through a GF/C filter extracted by the traditionalliquid-liquid method with 2 portions of 30 mL hexane ina separating funnel and the two layers allowed to sepa-rate. The combined hexane phase was dried over anhydrousNa2SO4, reduced in volume to 2 mL and afterwards usedfor gas chromatography-electron capture detection (GC-ECD) analysis.

Cleanup

The soil extracts were fed drop by drop onto a glass columncontaining Florisil (10 × 2 cm) with a layer of Na2SO4 atthe bottom. A solution of 15 % diethyl ether in hexane wasused to elute the Florisil column and the eluant was thenreduced in volume to 2 ml for quantification of the OCs inthe solution.

Quantification

The concentrates were quantified using a HP 6890 GCequipped with a HP 7683 autosampler, capillary column(HP-5 30m × 0.32 mm ID × 0.25µm) and ECD. The op-timal temperature program was as follows: a 120 ◦C initialcolumn temperature ramped at 10 ◦C min−1 to 250 ◦C andheld for 3 min, and ramped again at 50 ◦C min−1 to 325 ◦Cwhich was held for 3 min. Helium was used as the carriergas (20 mL min−1) while nitrogen was used as the makeupgas (60 mL min−1). The split ratio was 5:1 and injectionvolume was 1 µL. Identification of OCs was achieved fromtheir retention time as compared to those of pure authen-tic standards. The retention times (min) ranged from 5.29(tetrachloroxylene), 6.17 (lindane), 8.34 (heptachlor), 9.02(aldrin), 11.08 (dieldrin), 11.46 (endrin), 12.57 (DDT). TheOC amounts were interpolated from calibration curves thatplotted the OC/internal standard peak ratios against con-centrations of standards.

Confirmation of OC identity

A gas chromatograph coupled with a mass selective detec-tor (Agilent 5975C Series) equipped with a 30 m × 0.32 mmID × 0.25µm HP-5 capillary column was used to confirm

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414 Karnchanasest and Hawker

OC identity. The instrument was connected to a personalcomputer capable of peak detection and integration. Themass selective detector was operated in the electron impactmode at 70 eV. Transfer line temperature was 280 ◦C, ionsource temperature of 230 ◦C and quadrupole temperature180 ◦C. The identity of an OC was confirmed by com-parison of the mass spectral base peak and fragmentationpattern. Mass spectra of OCs in water and soil extractsmatched well with those in the OC database. The opera-tional conditions for the GC were the same as describedabove for the GC-ECD.

Soil pore volume and porosity

A 2.5 × 10 cm glass column was filled with soil (30 g) pre-treated with a 0.02 % NaN3 in 5mM CaSO4 solution toprevent microbial growth. The volume of soil in the columnwas measured. The soil in the column was saturated withDI water, weighed then the water drained out and the soilweighed again. Soil pore volume (PV) is equivalent to thatof the drained DI water. The soil pore volume to soil columnvolume multiplied by 100 gave the soil porosity, which forall soil samples was found to be in the range of 50–65 %.

Column leaching

Glass columns were used in order to reduce OC adsorptionto the column surface. The working concentration of OCsin soil was 2 mg kg−1 based on average concentrations ofthese test compounds found in Thai agricultural soils.[20,21]

The soil column was pre-eluted with 0.02% NaN3 in 5mMCaSO4 solution to reduce the occurrence of colloidal soildispersions and inhibit microbial activity.

Soils were spiked by adding solutions of the OCs in avolatile organic solvent (hexane/acetone) to the soil usinga syringe. The solvent was allowed to evaporate and the soilthoroughly mixed. A 30 g soil sample was spiked with OCsin this way to obtain a concentration of 2 mg kg−1 for eachOC, then the soil packed into a column. This was repeateduntil 9 columns had been prepared. A 5 g L−1 NP dispersion(approximately 1PV) was then added to each column so thatit just covered the soil surface and the contents allowed tostand for time periods of 3, 6, 12, 24, 36, 48, 72, 96 and120 h with the 9 different columns. Following these timeperiods, the columns were upwardly eluted with DI waterat 20 mL h−1. The eluant was then taken for extractionand quantification as described above. Elution was haltedwhen no OC was found in collected eluant (approximately300 ml). The leaching was repeated, but with 10, 15, 20, 25and 30 g L−1 NP dispersions. This whole process was thenrepeated for the other 4 soil samples.

NP recovery

The 6 NP concentrations employed in the leaching experi-ments described above were also employed in recovery ex-periments involving 30 g soil in a column under the same

conditions as for the leaching experiments but without theOC spike. Following pre-elution with NaN3 solution, andelution with DI water at 20 mL h−1, the eluant was collectedevery PV, dried at 50 ◦C and determined for NP residue.Collecting ceased when no NP was found in the eluant.

Results and discussion

Characteristics of synthesized NPs

The cross-linked amphiphilic PUMA particles preparedhave hydrophilic poly(ethylene oxide) chains anchored tothe particle surface as pendant chains. These cause the NPparticles to be stable and uncharged. The chains have hy-droxyl end groups and it has been proposed that they extenda sufficient distance from the particle surface to prevent anyVan der Waals attraction amongst the particles.[5] Thus,these NPs are not affected by high ionic strength solutionsand consequent processes such as double layer compres-sion. This is an important consideration when dealing witheluant from soils. In producing the dispersion of the PUMAprecursor chains in DI water, the precursor chains self-organize to form nano-sized particles with polarity char-acteristics similar to nonionic surfactant micelles. The finalstep of cross-linking of the chains prevents their disaggre-gation occurring during interaction with the soil matrix(Table 1).[17]

A physical characteristic of particular importance withNPs is particle size. The NP particle size distribution pro-duced in this work is compared with distributions fromothers in Table 2. Compared with Karnchanasest andSantisukkasaem,[16] the particle size distribution is smaller.The molar ratio of the monomer reactants were the same,but the final particle size distribution is likely to be affectedby factors such as temperature, stirring speed and periodof stirring during dispersion formation.[22] To show the ef-fect of varying monomer reactant ratios, Tungittiplakornet al. employed a higher relative proportion of HEMA butless PEG.[5] The former governs the extent of cross-linkingwhile the latter is responsible for the pendant hydrophilicchains. Park et al. have also observed that the average chainlengths and therefore average molecular weights of the PEGemployed can influence NP particle size.[19] They found NPparticle size to be a parabolic function of PEG size.

Characteristics of soil samples

The five soil samples used were categorized as acidic andsoil types were loam, clay loam and clay as seen in Table 1. Itis noteworthy that the soil samples had SOC contents from0.92 to 2.51 %. All soils were found to be free of detectablelevels of the OC test compounds and the internal standard.

Column leaching of OCs

Leaching of OCs from spiked soil using just DI water hasbeen found to be ineffective. Conte et al. for example ob-served that clean up of PAH contaminated soils by water

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Leaching with amphiphilic nonionic nanopolymers 415

Table 2. Size (nm) and % volume of NP particles synthesized in this study compared to those from other work.

This study Karnchanasest and Santisukkasaem[16] Tungittiplakorn et al.[5]

Size (nm) % Volume Size (nm) % Volume Size (nm) % Volume

55 7.44 50 2.66 ⇑65 11.31 60 5.45 60–80 na75 11.87 70 8.28 ⇓85 10.29 80 10.7795 7.93 90 12.07115 5.72 110 11.29135 4.00 130 8.45155 2.76 150 4.88

was not effective, but that surfactants including humic acidachieved removal efficiencies of up to 90 %.[23] PreparedNPs have all the virtues of surfactants such as increas-ing the apparent solubility of contaminants and facilitat-ing their mobility in porous media.[8] However, unlike NPs,surfactants need to be present at concentrations above theirCMC to be effective. They also interact with sorbents suchas soil to a greater extent. It has been found that NPs of thesame type as synthesized in this work have soil/water dis-tribution coefficients orders of magnitude less than thoseof surfactants.[15] The use of NPs has also been reported toincrease the bioavailability of PAHs such as phenanthreneto microbial populations,[8] whereas surfactant micelles caninhibit biotransformation rates of such compounds.[24]

To investigate the effect of NP concentration on removalefficiency for OCs, NP dispersions with concentrationsranging from 5 to 30 g L−1 were added to spiked soil for48 hours, then the column eluted with DI water. The re-sults for lindane with soil V are shown in Figure 1. A NPconcentration of 10 g L−1 is more efficient at removing thelindane and remediating the soil than a 5 g L−1 dispersion.However, there is little difference in the efficiency betweendispersions with concentrations of 10 to 30 g L−1. Resultsfor the other OCs of interest were similar.

Despite surfactants and NP dispersions aiming to in-crease contaminant desorption rates from soil, Tungitti-plakorn et al. found that only approximately 80 % of spikedphenanthrene was removed when a NP dispersion wasadded to the eluant phase as part of the leaching process.[5]

In this process, there were a number of 24 h periods of flow

interruption where the NP dispersion was able to stand incontact with the spiked soil. Although not tested, it was an-ticipated that relatively slow desorption kinetics limited theremoval of phenanthrene and that longer periods of flowinterruption would result in greater removal efficiency.

To investigate this, we added the NP dispersions tocolumns of spiked soil for periods of 3, 6, 12, 24, 36, 48,72, 96 and 120 h before upward elution with DI water.The results for lindane with soil sample V are illustrated inFigure 2. Other OCs showed similar results. Consistent withthe prediction of Tungittiplakorn et al.,[5] time periods up to36 h resulted in enhanced removal efficiency, but for longertime periods, no further efficiency was realized. It shouldbe recognized that the octanol-water partition coefficients(KOW) of lindane and phenanthrene are similar (104.26 and104.35 respectively) according to the KOWWINTM v. 1.67database.[25] Therefore desorption kinetics and times toequilibrium in a soil/NP dispersion system would be ex-pected to be similar for these two compounds.

NP recovery

To be useful as a remediation tool, any NP particles sorbedto soil should be able to be washed off with a minimum vol-ume of water. Kim et al. note that a relatively large amountof water would be needed for surfactants in surfactant-enhanced remediation.[15] This is because of more extensivesorption of the micelles themselves, as well as their tendencyto break up and for individual surfactant molecules to sorbto soil. As mentioned above, NP particles tend to display

Fig. 1. Efficiency of various NP dispersion concentrations for the removal of lindane from soil. NP dispersions had been in contactwith soil sample V (30 g) spiked with OCs for 48 h before leaching by DI water.

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Fig. 2. Optimal time for standing 10 g L−1 NP dispersion in contact with soil sample V (30 g) spiked with OCs to leach lindane withDI water. Other OCs showed similar results.

less affinity for soils compared with micelles and moreover,the particles retain structural integrity due to cross-linking.Figure 3 shows the amount of NP in each PV of DI watereluant for the various concentrations of NPs. As might beexpected, dispersions of lower NP concentration in a col-umn contain a smaller amount of particles and require asmaller volume of water. Based on the experiment using the10 g L−1 NP dispersion, NPs can be effectively completelyflushed out within 20 PVs with a recovery of 94.9 %. OtherNP dispersion concentrations gave recoveries in the rangeof 90.6–92.7 %.

The unrecovered NPs could be trapped in the soil par-ticles. Researchers have described silt and sand particlesin the soil as often being coated with clay particles, ce-mented together by organic matter, mineral oxides andcarbonates.[26] Clay particles themselves when in contactwith water can become agglomerated so that nano-sizedparticles may be retained.

It is difficult to compare this result in terms of PVsrequired to other work with nonionic NP particles sincesituation-specific factors such as eluant flow and soil poros-ity and flow rate are important.[27] One further advantageof NP particles that has been proposed however is thatsince they are generally larger than surfactant micelles,they can be recovered from eluant or wash water by mem-branes with larger pore sizes, offering economic and prac-tical benefits.[27]

Fig. 3. Amount of NP (g) in each PV that was leached from soil(30 g) columns containing various NP dispersion concentrations(g L−1) by DI water at 20 mL min−1 flow rate.

Role and influence of soil properties on OC removal

One of the aims of this work was to investigate the efficacyof OC removal from contaminated soils by NP dispersions.The second was to investigate the role and influence ofsoil properties on this process. Experimental observationsfrom this work were that although discrimination betweenthe different OCs of interest was not great, the extent ofremoval was always in the same order; DDT < aldrin <

heptachlor < dieldrin < endrin < lindane regardless ofsoil. This discrimination may involve aspects of molecularsize and structure that have already been reported to playa role in soil sorption strength for sorbates such as PAHsand PCBs.[28]

In considering remediation efficiency for all OCs of inter-est over all soil types, the order was soil I< soil II < soil V< soil III< soil IV. This order corresponds to the increasingSOC content of the soils, except for soil V. The predomi-nant role of SOC in controlling the sorption of unchargedorganic compounds has been extensively documented.[29]

However, given the anomalous result for soil V it is per-haps useful to focus on other soil characteristics as well,such as sand, clay and silt content. It has reported thatthe SOC in the sand fraction differs from that in clay andsilt.[28] The SOC in sand consists mainly of fresh or slightlydecomposed plant material or debris with a high concentra-tion of carbohydrates and is easily degradable. On the otherhand, the SOC of clay and silt represents a more advanced

Fig. 4. Removal efficiency of lindane by a NP dispersion (10g L−1) in relation to soil composition. All other OCs showedsimilar results.

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Leaching with amphiphilic nonionic nanopolymers 417

y = -7.43x + 101.57

r2 = 0.82

8182

8384

858687

8889

9091

1 1.5 2 2.5 3log Koc

% R

emov

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Fig. 5. Removal efficiency of OCs in relation to their log KOC val-ues (calculated from KOC = 0.411 KOW).[25] The OCs were spikedin soil sample V and in contact with a 10 g L−1 NP dispersion for48 h before elution with DI water.

stage of decomposition and consists mainly of aromatic andaliphatic structures[28] which are, in general, more resistantto microbial degradation.[30] This suggests that SOC typeinfluences sorption strength and perhaps even desorptionkinetics. Figure 4 shows removal efficiency for lindane withNP dispersions as a function of soil composition. With in-creasing sand and silt content, there is increasing removalefficiency, however the opposite is observed for clay.

High clay proportions in the soil correspond to decreasedOC removal. These findings applied to all test compoundsand may have important practical consequences.

Earlier it was mentioned that descriptors of molecularsize and structure would likely correlate with extent of sorp-tion. A property of interest in this context is hydrophobicityas characterised by KOW. The organic carbon-water parti-tion coefficient (KOC) is related, to a first approximation,to KOW.[29] Figure 5 demonstrates the relationship betweenremoval efficiency and log KOC for the OCs of interest.The result is promising considering that the KOC valuesextend over an order of magnitude and enables predictionof the removal efficiency of chemicals with similar physico-chemical properties using log KOC. The more hydrophobicOCs appear the most difficult to remediate using NP dis-

y = 0.22x + 4.89

r2 = 0.97

81

82

83

84

85

86

87

88

89

350 355 360 365 370 375 380 385MW

% R

emov

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Fig. 6. Removal efficiency of OCs (lindane excluded) in relationto their MW. The OCs were spiked in soil sample V and in contactwith a 10 g L−1 NP dispersion for 48 hours before elution withDI water.

persions. However, it should be kept in mind that there issome variation in KOW and KOC values in the literature.

It may be that simple descriptors such as molecularweight (MW) are adequate for compounds similar to thecyclodiene OCs employed in this work viz. aldrin, dield-rin, endrin and heptachlor. As mentioned previously, theremoval efficiency of lindane is similar, if not better to thatof the other OCs employed. However, this compound pos-sesses the lowest MW of the group. Figure 6 shows that areasonably significant linear relationship (r2 = 0.97) maybe obtained by plotting removal efficiency versus MW forall the OCs except lindane. It should be cautioned thoughthat the MW range is relatively narrow, and the applica-tion of such equations to compounds with a wider MWrange is uncertain. It does not appear to apply to lindanefor example.

Conclusion

The efficiency of OC soil remediation using NP dispersionsis, as judged from column experiments in this work, depen-dent on soil matrix characteristics such as SOC but alsotexture. Overall however, this technique appears most ap-plicable to highly contaminated soil. NP dispersions havenow been shown to be useful in removing OCs and are likelyto be useful with other non-polar organic compounds thathave similar physicochemical properties including PAHsand polychlorinated biphenyls. Removal efficiency also de-pends on the length of contact time between the NP dis-persion and the soil, with 36 h appearing optimal for thematerials used and under the conditions employed. Theconcentration of the NP dispersion itself is also an influ-encing factor, with most efficient removal observed for con-centration greater than or equal to 10 g L−1. Caveats arefirstly that the soil was spiked and sorptive strength maybe different to that of native pollutants. Secondly, resultsmay be different in the field because microbial activity insoil was controlled in this work using a biocide and lastly,results with some soils may be altered due to the presenceof other coexisting available organic compounds that aretaken up by NPs in a similar way to OCs.

Acknowledgments

This study was funded by Chulalongkorn University. Theauthors would like to express their sincere gratitude to DrWarapong Tungittiplakorn for his suggestions and com-ments on the NP synthesis. Special thanks go to Mr.Chaowut Singhkeaw for his assistance in the laboratory.

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