Journal of Colloid and Interface Science 274 (2004) 613–624 www.elsevier.com/locate/jcis Dodecyl sulfate–hydrotalcite nanocomposites for trapping chlorinated organic pollutants in water Hongting Zhao ∗ and Kathryn L. Nagy 1 Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA Received 28 August 2003; accepted 15 March 2004 Available online 24 April 2004 Abstract A series of hybrid organic–inorganic nanocomposite materials was synthesized by three different procedures using sodium dodecyl sulfate (DDS) and magnesium–aluminum layered double hydroxide (Mg/Al LDH with a Mg/Al molar ratio of 2 to 5). Both the pH of the exchange medium (6.5 to 10) and the Mg/Al molar ratio of the LDH affected the basal spacing, the content of DDS retained and the orientation of the DDS chains within the interlamellar space. For LDH with higher charge density (Mg/Al = 2 and 3), DDS molecules likely formed a perpen- dicular monolayer within the LDH interlayer and the solution pH had little effect on the basal spacing, with a mean and standard deviation of 25.5 ± 0.4 Å. However, for LDH with lower charge density (Mg/Al = 4 and 5), DDS molecules more likely formed an interpenetrating bilayer, and the basal spacing significantly increased with increasing pH, with a mean and standard deviation of 32.7 ± 5.2 Å. Sorption of trichloroethylene and tetrachloroethylene by DDS-LDH varied with synthesis conditions, LDH type and DDS configuration in the interlayer. DDS-Mg 3 Al-LDH had the highest affinity for both trichloroethylene and tetrachloroethylene in water, either comparable to or as much as four times higher than other clay-derived sorbents, followed by DDS-Mg 4 Al-LDH and DDS-Mg 5 Al-LDH. DDS-Mg 2 Al-LDH had the lowest sorption affinity although the highest amount of DDS. The pH of the exchange solution also affected the amount of DDS retained by the LDH as well as the sorption efficiency. Mg 3 Al-LDH has a charge equivalent area of 32.2 Å 2 /charge, which allows the formation of optimal DDS configuration within its interlayer, thus resulting in the highest affinity for the chlorinated compounds. The DDS-Mg/Al-LDHs can be easily synthesized either ex situ or in situ at low temperature, indicating the feasibility of practical applications. The results obtained by controlling the synthesis procedure suggest that different arrangements of DDS molecules in the LDH interlayers can be obtained and optimized for the sorption of specific sorbates. 2004 Elsevier Inc. All rights reserved. Keywords: Layered double hydroxide; Hydrotalcite; Dodecyl sulfate; Anionic surfactant; Sorption; Organic pollutants; Trichloroethylene; Tetrachloroethylene; Intercalation; Organoclay 1. Introduction Organic contamination of soil and water is a worldwide issue [1]. Removal of organic pollutants from the environ- ment or immobilization in situ are major goals of wastewater treatment and cleanup efforts. In the past decade, great effort has been devoted to developing new sorbents and stabilizers for hydrophobic organic compounds (HOCs) by the pillar- ing of inorganic layered compounds with polynuclear com- plex ions or bulky organic molecules [2,3]. Organic cation- * Corresponding author. Present address: Sandia National Laboratories, MS 0779, Albuquerque, NM 87111, USA. Fax: +1-505-844-2348. E-mail address: [email protected] (H. Zhao). 1 Present address: Department of Earth and Environmental Sciences, University of Illinois at Chicago (MC-186), Chicago, IL 60607, USA. exchanged 2:1 clay minerals (organoclays) have been stud- ied extensively for this purpose [4–8]. Layered double hydroxides (LDHs), or hydrotalcite-like compounds, have been investigated mainly because of their potential industrial uses such as catalysts, ion-exchangers, optical hosts, ceramic precursors, and antacids [9,10]. LDHs with various intercalated anions are used for catalysts and separation supports [11–13]. LDHs are antitypes of 2:1 clay minerals with positively charged metal oxide/hydroxide sheets compensated by anions in the interlayers. LDHs can be readily synthesized under laboratory conditions, with eas- ily controllable chemical composition of the layer and inter- layer domains [14,15]. The general formula of an LDH [9] is given as M 1−x M III x (OH) 2 ξ + A n− ξ/n ·zH 2 O, 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.03.055
Transcript
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lfatehangeon of then-tionting
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Journal of Colloid and Interface Science 274 (2004) 613–624www.elsevier.com/locate/jcis
Dodecyl sulfate–hydrotalcite nanocomposites for trapping chlorinaorganic pollutants in water
Hongting Zhao∗ and Kathryn L. Nagy1
Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA
Received 28 August 2003; accepted 15 March 2004
Available online 24 April 2004
Abstract
A series of hybrid organic–inorganic nanocomposite materials was synthesized by three different procedures using sodium dodecyl su(DDS) and magnesium–aluminum layered double hydroxide (Mg/Al LDH with a Mg/Al molar ratio of 2 to 5). Both the pH of the excmedium (6.5 to 10) and the Mg/Al molar ratio of the LDH affected the basal spacing, the content of DDS retained and the orientatiDDS chains within the interlamellar space. For LDH with higher charge density (Mg/Al= 2 and 3), DDS molecules likely formed a perpedicular monolayer within the LDH interlayer and the solution pH hadlittle effect on the basal spacing, with a mean and standard deviaof 25.5± 0.4 Å. However, for LDH with lower charge density (Mg/Al= 4 and 5), DDS molecules more likely formed an interpenetrabilayer, and the basal spacing significantly increased withincreasing pH, with a mean and standard deviation of 32.7± 5.2 Å. Sorption oftrichloroethylene and tetrachloroethylene by DDS-LDH varied with synthesis conditions, LDH type and DDS configuration in the inDDS-Mg3Al-LDH had the highest affinity for both trichloroethylene and tetrachloroethylene in water, either comparable to or as mucfour times higher than other clay-derived sorbents, followed by DDS-Mg4Al-LDH and DDS-Mg5Al-LDH. DDS-Mg2Al-LDH had the lowestsorption affinity although the highest amount of DDS. The pH of the exchange solution also affected the amount of DDS retained byas well as the sorption efficiency. Mg3Al-LDH has a charge equivalent area of 32.2 Å2/charge, which allows the formation of optimal DDconfiguration within its interlayer, thus resulting in the highest affinity for the chlorinated compounds. The DDS-Mg/Al-LDHs can be easilysynthesized either ex situ or in situ at low temperature, indicating the feasibility of practical applications. The results obtained by cothe synthesis procedure suggest that different arrangements of DDS molecules in the LDH interlayers can be obtained and optimsorption of specific sorbates. 2004 Elsevier Inc. All rights reserved.
Organic contamination of soil and water is a worldwiissue [1]. Removal of organic pollutants from the enviroment or immobilization in situ are major goals of wastewatreatment and cleanup efforts. In the past decade, greathas been devoted to developing new sorbents and stabifor hydrophobic organic compounds (HOCs) by the pilling of inorganic layered compounds with polynuclear coplex ions or bulky organic molecules [2,3]. Organic catio
* Corresponding author. Present address: Sandia National LaboratorieMS 0779, Albuquerque, NM 87111, USA. Fax: +1-505-844-2348.
E-mail address: [email protected] (H. Zhao).1 Present address: Department of Earth and Environmental Science
University of Illinois at Chicago (MC-186), Chicago, IL 60607, USA.
0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2004.03.055
ts
exchanged 2:1 clay minerals (organoclays) have beenied extensively for this purpose [4–8].
Layered double hydroxides (LDHs), or hydrotalcite-licompounds, have been investigated mainly because of thepotential industrial uses such as catalysts, ion-exchanoptical hosts, ceramic precursors, and antacids [9,10]. Lwith various intercalated anions are used for catalysts aseparation supports [11–13]. LDHs are antitypes ofclay minerals with positively charged metal oxide/hydroxsheets compensated by anions in the interlayers. LDHsbe readily synthesized under laboratory conditions, with eily controllable chemical composition of the layer and intlayer domains [14,15]. The general formula of an LDHis given as[M1−xMIII
614 H. Zhao, K.L. Nagy / Journal of Colloid and Interface Science 274 (2004) 613–624
s
ts
DHintionr cano-nic
24–DHin-ur-
tioncid-
heithpli-,30]idwasatoganded ar astedt fosig-nge
re-de-for
ruc-alsfor-
s asd theacy.Hur-Cs
rep-queons,-
l-iumne-en-,mLnderthe
iousex-
gture
astic
andr un-atuse.
blingtheced0–
atwere
ctionwith-
nde-s
the.0)
e-at
orde-
d asucon-
60–f
ion
where M is Li+ or a divalent cation such as Ca2+, Mg2+,Ni2+, Co2+, Zn2+, Mn2+ or Cu2+, MIII is a trivalent cationsuch as Al3+, Cr3+, Co3+, Ni3+, Mn3+, Fe3+, V3+ orGa3+, and A is an interlayer anion such as Cl−, NO−
3 ,ClO−
4 , CO2−3 , or SO2−
4 . The M2+/M3+ ratio generally rangefrom 1 to 5 [9].
Positively charged LDH structures can sorb contaminansuch as phenols [16], radioactive131I [17], chromate [18],and selenate/selenite [19]. However, inorganic parent Lcrystallites have low sorption affinity for nonionic HOCswater, because LDHs are hydrophilic [20–23]. Intercalaof organic anions, such as surfactants, into the interlayealter LDH surface properties from hydrophilic to hydrophbic and result in enhanced sorption capacity for nonioorganic pollutants [24]. Recently, a few reports [20,21,30] have been published on the use of organic-modifiedLfor removing organic pollutants from water and resultsdicate the further potential of these materials for that ppose. For example, previous study [25] found that sorpof large guest molecules, e.g., pyrene, by an organic aexchanged Li/Al-LDH could be regulated by varying tchain length of the carboxylic acids or type of anion wLDH. This sieving effect has been demonstrated in its apcation as a stationary phase in gas chromatography [29The selectivity of a hydrophobic LDH toward organic liqumixtures has also been reported [24], and this materialsuggested for use as a stationary phase in gas chromraphy or as self-assembled films [29,30] in membraneseparation design. Organohydrotalcites have been studipesticide sorbents [27,28] for use in decontamination oslow-release sources. You et al. [23] recently investigadodecylbenzenesulfonate-exchanged LDH as a sorbentrichloroethylene and tetrachloroethylene; however, nonificant difference in sorption capacity was found amothe products derived from Mg/Al LDH with different chargdensity.
Despite numerous investigations of the intercalationactions of organic anionic species in LDHs [11,29–38],tailed understanding of the resulting sorptive propertiesHOCs is still lacking. Also, the ease of synthesis and sttural controllability would make such LDH-based materiadvantageous for practical applications, including in situmation.
Therefore, to optimize the use of LDH-based materialsorbents for HOCs in water, it is necessary to understanfactors and synthetic conditions that determine their efficWe have investigated the pillaring reactions of Mg/Al-LDof varying layer charge with DDS, a commonly studied sfactant, and the use of these phases for sorption of HOTrichloroethylene and trichloroethylene were chosen asresentative HOCs due to their common occurrence in aous solutions [1]. Layer charge density, synthesis conditiand interlamellar structure are shown to affect sorbent efficiency.
.
-
s
r
.
-
2. Materials and methods
2.1. Synthesis of Mg/Al LDH
A series of Mg/Al LDH solids was prepared by simutaneously adding dropwise an aqueous solution of sodhydroxide (2.0 M) and a mixed aqueous solution of magsium chloride and aluminum chloride (total metal conctration of 1 M with a Mg2+/Al3+ molar ratio of 2.0, 3.04.0, or 5.0) into a large plastic beaker containing 100deionized water which had been boiled and stored unitrogen before use. Nitrogen was bubbled throughouttitration process to minimize dissolved carbonate. Prevstudies showed that carbonate anions are often difficult tochange in Mg/Al, Ca/Al, and Ni/Al hydroxides [9]. Durinthe titration, the temperature and pH of the reaction mixwere maintained at 23±1◦C and 10±0.2, respectively. Thesuspension was stirred for three hours, transferred to plbottles, and placed in an oven at 65◦C for 4 days. After cool-ing to room temperature, the suspension was centrifugedthe precipitate washed extensively using deionized watetil free of chloride (AgNO3 test). The products were dried65◦C, ground, and stored in polyethylene bottles before
2.2. DDS-LDH preparation
Sodium dodecyl sulfate (DDS) (>98% CH3(CH2)11OSO3Na, Aldrich) was incorporated into LDH interlayers viathree different procedures, each conducted under bubnitrogen to minimize dissolved carbonate. In the first,ex situ exchange route, a known mass of LDH was plainto a plastic beaker and allowed to react with DDS (450 mL 0.1 M DDS solution per 1 g LDH) for 2 days65◦C. The supernatant was decanted, and the solidsreacted again with a fresh solution for 2 days at 65◦C. Thesuspensions were shaken occasionally during the reaperiods. The organic derivatives were washed 6 timesdeionized water (∼200 mL) and one time with a 1:1 water/ethanol mixture, dried at 65◦C, ground, and stored ipolyethylene bottles before use. Ex situ products werenoted as DDS-MgxAl-LDH(ex). The second procedure wasimilar to the first except that DDS was introduced intoLDH interlayer at different pH values (6.5, 8.5, and 10at a DDS:LDH ratio of 12 mmol/g. The mixture of LDHand water (∼1 g/20 mL) was homogenized by stirring bfore adding the DDS solution. The pH was maintained6.5± 0.2, 8.5± 0.2, or 10± 0.2 by addition of either 0.1 NNaOH or 0.1 N HCl. After stirring at room temperature f3 days, the products were centrifuged and treated asscribed in the first method. These products are denoteDDS-MgxAl-LDH(pH). The third method was an in sitsynthesis: separate solutions of dissolved metals (totalcentration of 1 M with a molar Mg2+/Al3+ value of 2, 3,4, and 5) and 2 M NaOH were added dropwise into130 mL of 0.1 M DDS solution to give a DDS:LDH ratio o12 mmol/g at room temperature. Preliminary investigat
H. Zhao, K.L. Nagy / Journal of Colloid and Interface Science 274 (2004) 613–624 615
re-ed
rredmeDDS
the-lin-
06)c-DSc0%Hnsosi-tely
ec-micl-µmpho-tron
uili--thinon-th anith
ral-
oor-
DHlar
se
the, in-
esispac-lt ofged
rsatedto
rom
re-thee inwredareor
entseris
indicated that when the molar ratio of DDS:LDH was<1,some LDH coexisted with the DDS-LDH complex, asvealed by XRD. Therefore, the DDS:LDH molar ratio uswas>1.5. The pH was maintained at 10± 0.2. After stirringfor one day at room temperature, the mixture was transfeto an oven at 65◦C for 3 days, and then subjected to the saprocedures as above. These products were denoted asMgxAl-LDH(in).
2.3. Instrumental
Organic carbon analysis was performed on the synsized products with a high temperature combustion/alkaity titration method using elemental analyzer (Model 11(Carlo Erba Strumentazione, Milan, Italy). X-ray diffration (XRD) analyses were carried out using a Scintag X2000 diffractometer with CuKα radiation. Electrophoretimobilities were measured using a Malvern Zetasizer 300(Malvern, England) with solid concentrations of 0.005(w/w) and 0.01 M NaCl. pH was adjusted using NaOand HCl to values ranging from 4 to 12 and solutiowere equilibrated for 24 h before measurement. Comptional analyses were conducted by dissolving approxima40 mg of calcined (600◦C for 5 h) sample in 100 mL 20%(v/v) HNO3 and assaying by ICP atomic emission sptroscopy. Samples were imaged using TappingMode atoforce microscopy (AFM) with a Digital Instruments Mutimode Nanoscope IIIa, silicon (TESP) tips, and a 15range scanner. Scanning electron microscope (SEM)tomicrographs were obtained using an ISI scanning elecmicroscope.
2.4. Sorption isotherms for trichloroethylene andtetrachloroethylene
Sorption isotherms were determined using batch eqbration at 25± 1 ◦C [39]. Investigation of the sorption kinetics over 48 h showed that equilibrium was reached wi16 h. Trichloroethylene and tetrachloroethylene (HOC) ccentrations were quantified to within±3% using a HewletPackard 5980 Series II gas chromatograph equipped witelectron capture detector and an HP-5 capillary column wHe carrier gas. Controls without solids were run in palel for each concentration of organic investigated. The HOCrecovery was on average∼93%, indicating a small loss tvessel walls and/or volatilization. The data were not crected for this loss.
3. Results
3.1. Parent LDH solid characterization
Representative XRD patterns (Fig. 1) of the parent Lmaterials are similar to published patterns [10]. The moMg/Al ratios of the final products are fairly close to tho
of the reactants (Table 1). Inorganic carbon contents ofprepared products were measured to be less than 0.2%dicating slight carbonate contamination during the synthprocedure. With decreasing Al substitution, the basal sing gradually increased from 7.74 up to 8.22 Å as a resuweaker coulombic attraction between the positively charlayers and the negatively charged interlayer anion Cl− [40].Conversely, increasing the proportion of Al in the layeincreased unit layer charge, charge density, and estimanion exchange capacity (AEC) [41]. LDH was reportedhave high anion exchange capacity generally ranging f200 to 500 meq/100 g [42].
3.2. DDS-LDH solids characterization
3.2.1. DDS contentThe organic carbon contents (O.C. (%, w/w)) in the p
pared products (Table 2) are relatively consistent withdecrease in Mg/Al ratio and the corresponding increasAEC. All of the DDS-LDH products prepared in situ shosimilar elemental Mg and Al composition to those prepaex situ. However, their DDS contents and basal spacingslightly different from those prepared in situ (Table 2). Fexample, for LDH with higher charge density (Mg/Al= 2and 3), products prepared in situ have higher DDS contthan those prepared ex situ. In contrast, for LDH with lowcharge density (Mg/Al= 4 and 5), the opposite tendencyobserved.
616 H. Zhao, K.L. Nagy / Journal of Colloid and Interface Science 274 (2004) 613–624
ge (%)
Table 1Properties of the synthesized Mg1−xAlx -LDH
a Calculated based upon chemical analysis.b a = 2d100.c Area of a Mg1−xAlx (OH)2 octahedral unit= √
3(a2/2).d Anion exchange capacity (AEC) was estimated assuming 2H2O per unit cell [41].e Numbers in parentheses are data for in situ synthesizedDDS-Mg/Al LDH products. Theproducts were calcined at 600◦C for 6 h before undergoing
elemental analysis.
Table 2Total organic carbon content (O.C., %) and basal spacings (d003, Å) of DDS-Mg/Al-LDH products synthesized via different methods
a Numbers in parentheses are data for the calculated DDS weight percentage(%) in the products and the corresponding estimated exchange percentabased upon AEC.
DS
fatees
elyrs ofl
obic
nteriaedsures
ar-ntsy 72ly
DS
enthe
n,m 7DDS
dpre-. 1)at-seden-l
asalred
trol
ight
of
at
10
Based upon estimated AEC, the maximum possible Dcontent is 55.4, 49.3, 44.6, and 41.1% for Mg2Al-LDH,Mg3Al-LDH, Mg 4Al-LDH, and Mg5Al-LDH, respectively.Thus, exchange of interlayer chloride by dodecyl sulproceeds to∼100% of theoretically estimated AEC valufor the ex situ DDS-LDHs, with the exception of Mg5Al-LDH(ex) which proceeds up to∼116%.
Overall, both ex situ and in situ products have relativhigh DDS saturation (∼100%), close to that required foelectroneutrality. Samples that contained DDS in excesthat needed to satisfy the layer charge likely had neutraNa–DDS on internal and external surfaces via hydrophattraction [11].
Because previous studies have shown that OH− has highaffinity for LDH [43,44], we investigated DDS intercalatioreactions at pH 6.5, 8.5, and 10 and assessed the masorptivity for HOCs. Low pH values were not investigatbecause the LDH lattice can withstand only short expoto mildly acidic solutions (pH∼4.5) [45] or else dissolverapidly (pH< 4) [46].
All samples synthesized as a function of pH for a nrow range of the ratio DDS:LDH have lower DDS contethan those exchanged ex situ and in situ (Table 2). Onlto 92% of the estimated AEC wascompensated, presumabdue to competition with OH−, the starting molar DDS:LDHratio, and shorter reaction time. In fact, the content of Dretained by Mg2Al-LDH and Mg5Al-LDH generally de-creased with increasing pH. However, for Mg3Al-LDH and
l
Mg4Al-LDH, the content of DDS first decreased and thincreased to values at pH 10.0 that were higher thanconcentrations in the otherLDH samples. For comparisoPavan et al. [36] also reported that an increase in pH (froto 9) resulted in a decrease in the amount of adsorbedby Mg2Al-CO3 LDH.
3.2.2. Basal spacingIntercalation of DDS into the LDH interlayer resulte
in increased basal spacing, similar to those reportedviously [9,11,33,35]. Representative XRD patterns (Figof the DDS-LDH materials are similar to published pterns [10]. The basal spacing (Table 2) generally increawith increase in Mg/Al ratio and decrease in charge dsity. However, DDS-Mg2Al LDH had a slightly higher basaspacing (25.9 ± 0.3 Å) than DDS-Mg3Al LDH (25.0 ±0.4 Å). Also, all products prepared in situ have greater bspacing (ranging from 24.3 to 29.2 Å) than those prepaex situ (ranging from 25.1 to 31.2 Å), suggesting a conby the synthesis conditions.
The pH values of the exchange medium had only sleffects on the basal spacings of Mg2Al-LDH and Mg3Al-LDH (25.5 ± 0.4 Å), but significant effects on thoseMg4Al-LDH and Mg5Al-LDH, ranging from 27 up to 40 Å(32.7± 5.2 Å) (Table 2). Clearfield et al. [11] reported thDDS intercalation in synthetic Ni4Al-LDH was significantlyaffected by pH. They found that varying pH from 5 tocaused the basal spacing of the DDS-Ni4Al-LDH to range
H. Zhao, K.L. Nagy / Journal of Colloid and Interface Science 274 (2004) 613–624 617
Fig. 2. AFM image of parent Mg3Al-LDH.
(a) (b)
Fig. 3. SEM images of (a) parent Mg3Al-LDH and (b) DDS-Mg3Al-LDH(ex) (short bar= 100 nm).
edgh a
n-in
hy-datare-gy.he
ped
usly
am-Hs
a-to
eandf the
from 25.9 to 42 Å, respectively, and the amount of retainDDS to decrease. They reported a basal spacing as hi47.6 Å for Ni4Al-LDH when pH was higher than 10.
3.2.3. MorphologyAFM analysis (Fig. 2) indicated that untreated LDH co
sisted of thin, hexagonal, plate-like crystals 0.2–0.5 µmsize. In contrast, DDS-LDH particles agglomerated viadrophobic interactions and were not easily imageable (not shown). The SEM images (Fig. 3) indicated that DDStention dramatically changed the LDH particle morpholoThe DDS-Mg3Al-LDH particles are agglomerated and tsurfaces are more diffuse and not as sharp as that of Mg3Al-LDH, suggesting that there might be excess DDS dra
sover the particles on outer surfaces, as observed previoby others [34–36].
This was further confirmedby electrophoretic mobility(EM) measurements (data not shown) on these two sples. Within the pH range of 5–11, all the synthesized LDhad positive EM while DDS-Mg/Al-LDHs(ex) had negtive EM. (The isoelectric point of LDHs was reportedbe >11.0 [47,48].) For example, Mg2Al-LDH had an EMin 0.01 M NaCl of+3.5 µm s−1/V cm−1 in (at pH∼5.02),while DDS-Mg2Al-LDH(ex) had an EM of approximately−1.7 µm s−1/V cm−1 (at pH∼5.57). This indicates that thinteraction of DDS with LDH caused the charge reversalmay be a result of sorption onto the external surfaces oLDH, in agreement with previous results [36,38].
618 H. Zhao, K.L. Nagy / Journal of Colloid and Interface Science 274 (2004) 613–624
e at
rva-es,96arti-ted,
eenat-
bed
e at
atioi-
om-lid-, thetter
ef-he
s anol.as-ing
ed
Fig. 4. Sorption isotherms for trichloroethylene and tetrachloroethylen25± 1 ◦C by DDS-LDH materials synthesized ex situ.
3.3. HOC sorption by DDS-LDH products
Despite some data sets that visually show slight cuture, all sorption isotherms can be fit with straight linwith correlation coefficients typically in the range of 0.to 0.99 (Figs. 4–6). Linear isotherms suggest that a ptioning process controlled the sorption [49]. As expecuntreated LDH has low affinityfor both trichloroethyleneand tetrachloroethylene,due to the hydrophilicity of theLDH surfaces. Linear sorption of HOCs commonly has bobserved on surfactant-modified clays and soil organic mter [50–53]. This linear sorption behavior can be descrisimply by
Qs = KdCe,
whereQs is the amount of solute sorbed (mg/kg), Ce is theequilibrium aqueous solute concentration (mg/L), and Kd
Fig. 5. Sorption isotherms for trichloroethylene and tetrachloroethylen25± 1 ◦C by DDS-LDH materials synthesized in situ.
is the sorption coefficient, which corresponds to the rof the amount (mg/kg) of the sorbed chemical to its equlibrium aqueous concentration (mg/L). Higher Kd valuescorrespond to a greater degree of sorption. For direct cparison and reflecting the dominant influence of the sophase organic carbon on the sorption of organic solutessorption coefficient often is normalized on an organic-mabasis [5],
Kom = Kd/foc,
whereKom is the organic matter-normalized partition coficient andfoc is the fractional organic carbon content of tsorbent (Tables 3 and 4).
By using relative sorptivity (Kom/Kow) [6], the sorptiveefficiency of the organic matter in DDS-LDH materials apartitioning medium can be compared with that of octaThe relative sorptivity for various DDS-LDH products wplotted in Fig. 7. Only theKom/Kow values for some DDSMg3Al-LDH products were greater than unity, suggestthat, in these products, DDS-derived organic matter form
H. Zhao, K.L. Nagy / Journal of Colloid and Interface Science 274 (2004) 613–624 619
nd
ation
t
Fig. 6. Sorption isotherms for tetrachloroethylene at 25± 1◦C by DDS-LDH materials synthesized as a function of solution pH. (The data were grouped alabeled for clarity.)
Table 3Selected parameters for the sorption of trichloroethylene and tetrachloroethylene by DDS-Mg/Al-LDH
a logKow = the log of partition coefficient value between octanol and water [21].b Aqueous solubility data were from Zhao and Vance [21] and references therein.c Kd = sorption coefficient between the DDS-Mg/Al-LDH and water. For direct comparison, all isotherm data were evaluated by linear regression equ
(R > 0.96).d logKom = the log value of organic-matter-normalized sorption coefficient.Kom = Kd[100/(%O.C. × foc)], wherefoc equals to the molecular weigh
of the DDS divided by the weight of C in the DDS (≈1.84).
l ise
rlysis
for
the
ra-–9
yr-ra-
a much better partitioning medium than octanol. Octanoconsidered to be relativelyhydrophilic as compared with thC12 alkyl groups of DDS [6].
Results shown in Tables 3 and 4 and Fig. 7 cleaindicate that the sorption efficiency varies with synthemethod and product composition. The DDS-Mg3Al-LDHsample prepared at pH 10 has much higher sorptivitytrichloroethylene than the other DDS-Mg3Al-LDH samples,
all of which sorb both HOCs to a greater extent thanother materials. DDS-Mg2Al-LDH always shows the low-est sorption affinity for both trichloroethylene and tetchloroethylene within each synthesis series, as much as 5times lower than the DDS-Mg3Al-LDH.
Sorption of trichloroethylene and tetrachloroethylene bDDS-LDH does not follow a trend with increasing oganic content, but rather varies with the DDS configu
620 H. Zhao, K.L. Nagy / Journal of Colloid and Interface Science 274 (2004) 613–624
-
m-
uite
tra-ding
oste,the
ertra-
ef-
sal
exyean
ter-e of
ro-rp-andthat
dl-rous
hile
yl-nged
-salyl--
Alttered to
Åecyl8 Å
aayer
allt
side
-
ulesDStedo
Table 4Sorption coefficients (Kd) and corresponding logKom values for the sorption of tetrachloroethylene by DDS-Mg/Al-LDH synthesized at various pHvalues
a Numbers in parentheses are logKom values.b DDS-Mg3Al-LDH(pH 10) had aKd value of 280 and logKom of 2.79
for trichloroethylene.
Fig. 7. Relative sorptivity (Kom/Kow) of various DDS-LDH products.
tion, synthesis conditions and LDH composition. For exaples, DDS-Mg4-Al-LDH and DDS-Mg5Al-LDH have sim-ilar intermediate sorption affinities. DDS-Mg2Al-LDH andDDS-Mg3Al-LDH have similar surfactant loadings andd-spacings (e.g., for those synthesized at pH 10), but qdifferent affinities for HOCs.
With the exception of DDS-Mg3Al-LDH, the in situsynthesized DDS-LDHs sorbed trichloroethylene and techloroethylene more strongly than those of corresponproducts synthesized ex situ. DDS-Mg3Al-LDH synthesizedex situ, but exchanged as a function of pH, shows the msorption for both trichloroethylene and tetrachloroethylensuggesting that the synthetic conditions must affectmolecular configuration of DDS within the LDH interlayand thereby the resulting sorption properties. For techloroethylene, with the exception of DDS-Mg2Al-LDH, all
DDS-Mg/Al(pH) products have relatively higher sorptiveficiency than those prepared ex situ.
The synthetic medium pH affected not only the baspacing of both DDS-Mg4Al-LDH and DDS-Mg5Al-LDH,but also the DDS-loading as compared to that from thesitu/in situ syntheses. Values of logKom decreased slightlwith increasing pH of the exchange medium with a mand standard deviation of 3.16± 0.04. The increase of pHgenerally resulted in less DDS retained and a larger inlayer thickness, which may have caused the lower uptakHOCs.
Overall, sorption of trichloroethylene and tetrachloethylene by DDS-LDHs is equal to or greater than sotion by organoclays, surfactant-templated materials,other clay-derived materials. Previous studies showedHDTMA-exchanged Wyoming smectite has a logKom valueof 2.30 for trichloroethylene [52], and HDTMA-modifiezeolite has a logKom value of 2.82 for tetrachloroethyene [53]. As-synthesized surfactant-templated mesopomaterials (MCM-41) have a logKom value of 2.40 for tri-chloroethylene and 2.99 for tetrachloroethylene [36], wβ-cyclodextrin intercalated Mg/Al LDH has a logKom valueof 2.65 for trichloroethylene and 2.89 for tetrachloroethene [20,21]. However, dodecylbenzenesulfonate-exchaLDHs were reported to have high logKom value for trichlo-roethylene (2.68± 0.04) and tetrachloroethylene (3.28±0.05) [23].
4. Discussion
The results show that reaction between DDS and Mg/AlLDHs yields products with varying DDS contents, baspacing and different sorption capacities for trichloroethene and tetrachloroethylene.The differences in HOC sorption must arise from the DDS configuration within Mg/LDH interlayer and be related to charge density. To beelucidate the possible reasons, specific parameters nebe considered.
We first considered the LDH layer thickness of 4.78[54] and the van der Waals end-to-end length of dodsulfate, estimated from the chemical structure, of 20.[55]. Based on products with a basal spacing(d003) in theneighborhood of 26 Å, such as DDS-Mg2Al-LDH and DDS-Mg3Al-LDH, we deduce that DDS molecules likely formperpendicular monolayer. For the perpendicular monolstructures, the DDS chains should stand upright withchains in an all-trans conformation [56], considering thahalf of the positive charges are neutralized from eachof the layer.
For products with a basal spacing>26 Å, such as DDSMg4Al-LDH and DDS-Mg5Al-LDH, a perpendicular bi-layer arrangement is probable in which the DDS molecoverlap to some degree (Fig. 2) [9,11]. However, the Dchains could be either partially overlapped as illustrain Fig. 8 or they could be tilted with only partial or n
H. Zhao, K.L. Nagy / Journal of Colloid and Interface Science 274 (2004) 613–624 621
(a) (b) (c)
Fig. 8. Schematic arrangement of DDS molecules within the LDH interlayer (a) perpendicular monolayer (∼26 Å); (b) and (c) interpenetrating bilayer (>26 Å)(not to scale).
bor-eldinge-
g fo
gu-on-
decydxylicp
thelention
a-medti-
n-
om-les.
as
t
icu-ng.
d
iona
aceareatedndin-
ur ifin-
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e ofn isnfig-
andn
ellacentle-n-drge
ase
as
hatrm-
asreenly
overlap [11], resulting in open spaces between neighing chains. Using chemical graphics modeling, Clearfiet al. [11] calculated that for products with a basal spacof 36.4 Å, the DDS chains might form a bilayer arrangment with a tilt angle of 49.4◦ in which there is no DDSoverlap. Therefore, the range of observed basal spacinDDS-Mg4Al-LDH and DDS-Mg5Al-LDH could result fromdifferent angles of tilt or overlap (Fig. 2).
To evaluate the role of charge density on DDS confiration and thereby on HOC sorption, it is necessary to csider other parameters. The cross-sectional area of dosulfate is estimated to be∼28 Å2 [35] and the estimatecross-sectional areas for long-chain alcohols and carboacids are approximately 23 Å2 [57]. The sulfate headgrouenlarges the cross-sectional area of DDS. Based uponproperties of the synthesized LDHs, the charge equivaarea (Å2/charge), or available area per monovalent anwas calculated to be 24.4, 32.3, 41.7, and 47.6 Å2 for LDHwith Mg:Al ratios of 2:1, 3:1, 4:1, and 5:1, respectively (Tble 1). On the other hand, the molecular diffusion voluof trichloroethylene and tetrachloroethylene was estimateto be 155 and 184 A3, respectively, and both have an esmated critical molecular dimension of 6.7 Å (length)×6.5 Å(width) × 3.7 Å (thickness) [21]. It is reasonable to coclude that for Mg2Al-LDH, the DDS molecule narrowly fitsinto the equivalent area, leaving insufficient space to accmodate trichloroethyleneand tetrachloroethylenemolecuThus, sorption of HOCs by DDS-Mg2Al-LDH likely arisesfrom DDS sorbed on external surfaces of the LDH.
Crepaldi et al. [37] reported a basal spacing as high47 Å for a DDS-Zn2Cr-LDH. Zn2Cr-LDH has a slightlysmaller charge equivalent area (22.7 Å2) compared to thaof Mg2Al-LDH (24.4 Å2). Apparently DDS just fits into thecharge equivalent area, thus forcing it to form a perpendlar bilayer with minimum overlap and a high basal spaciThe difference in charge equivalent area between Mg2Al-LDH and Mg3Al-LDH might also contribute to the observehigher basal spacing of the former over that of the latter.
Compared to Mg2Al-LDH, Mg 3Al-LDH has an availablearea per monovalent anion larger than the cross-sect
r
l
l
area of the DDS molecule, thus leaving more free spfor accommodating HOCs. Assuming DDS moleculesclosely packed, the intermolecular free distance is estimto be ∼1.8 Å, suggesting that both trichloroethylene atetrachloroethylene could not readily penetrate into thetermolecular space. However, uptake of HOCs may occthe DDS molecules are flexible and can move within theterlayer.
For Mg4Al-LDH and Mg5Al-LDH, the available area pemonovalent anion is much larger than required to accomdate DDS. Also, the DDS molecules may not be perpenular to the layer as in Mg2Al-LDH and Mg3Al-LDH. Thiswould result in higher basalspacing, more loosely packeDDS, and more free intermolecular space—as much as10 times as that in Mg2Al-LDH and Mg3Al-LDH. Thoughmore free space would presumably enhance the uptakHOCs, the sorption results indicate that this configurationot as effective as the hypothesized more structured couration in the Mg3Al-LDH interlayer.
A schematic representation of charge distributionresulting DDS configuration within LDH interlayers cabe constructed assuming that Mg2+ and Al3+ cations areregularly distributed in the LDH layer and the unit c(a) parameter represents the distance between two adjcations [37] (Fig. 9). The diagram shows that DDS mocules within the LDH interlayers with lower charge desity (Mg/Al = 4 and 5) likely form a much loosely packearrangement than that in LDH interlayers with high chadensity (Mg/Al= 2 and 3).
A decrease in DDS loading supposedly would increthe interlayer free space, as in Mg2Al-LDH and Mg3Al-LDH, and thus enhance HOC sorption. However, this wnot observed for Mg2Al-LDH. For example, DDS-Mg2Al-LDH(pH 10) has similar tetrachloroethylene sorption to tof DDS-Mg2Al-LDH(ex), while the former has much higheDDS loading (54.3%) than the latter (47.5%). This phenoenon is explained as follows.
Due to hydrophobic attraction among DDS moleculeswell the relatively high charge density of LDH, DDS molikely may be aggregated in bundles rather than be ev
622 H. Zhao, K.L. Nagy / Journal of Colloid and Interface Science 274 (2004) 613–624
Fig. 9. Schematic representation of DDSarrangement as related to Mg/Al-LDH charge density [37] (not to scale).
ethy-
ghtent
S
ter-in
fors
eg-Sularthat
usay
nds.
ard-asal
salaf-tionS-
ly),re-ra-t theorp-less
e for
Fig. 10. Relative sorptivity (Kom/Kow) of DDS-MgxAl-LDH products asa function of estimated DDS exchange percentage (%) (tetrachlorolene—solid line; trichloroethylene—broken line).
distributed within the interlayers [24]. Therefore, althoudifferent synthesis methods may decrease the DDS conas for Mg2Al-LDH, they might not affect the localized DDconfiguration. For DDS-Mg2Al-LDH, if sorption of HOCsoccurs largely in the portion of DDS sorbed to the exnal LDH surface, this would explain the slight increaseuptake of HOCs by DDS-Mg2Al-LDH(in), which has thehighest DDS loading, sorbs more HOC than other Mg2Al-LDH-derived products (Fig. 10).
,
An interesting opposite phenomenon is observedDDS-Mg3Al-LDH; a reduction in DDS loading enhancethe sorptive efficiency (Fig. 10). Unlike Mg2Al-LDH,Mg3Al-LDH has a lower charge density and a probable rular distribution of DDS in the interlayers. Reduced DDloading, to some degree, could increase the intermolecspace and better accommodate HOCs. Fig. 10 indicatesit is possible to choose an optimal DDS loading in Mg3Al-LDH to achieve the highest sorptive efficiency and thproduce a material that is more cost-effective. This mexplain why DDS-Mg2Al-LDH(pH 10) and DDS-Mg3Al-LDH(pH 10), though having similar surfactant loadings ad-spacings, have quite different sorption affinity for HOC
Unlike DDS-Mg2Al-LDH and DDS-Mg3Al-LDH prod-ucts, which have relatively constant basal spacing regless of the DDS content and synthetic condition, the bspacing for DDS-Mg4Al-LDH and DDS-Mg5Al-LDH prod-ucts vary significantly. As a result, DDS content, baspacing and the DDS configuration would cooperativelyfect the sorption behavior for HOCs and make the sorpbehavior more complicated (Fig. 10). For example, DDMg4Al-LDH(pH 6.5) and DDS-Mg4Al-LDH(pH 10) havequite similar DDS loadings (35.3 and 35.7%, respectivebut quite different basal spacings (27.2 and 36.6 Å,spectively), and the former has higher sorptivity for tetchloroethylene than the latter (Fig. 7). This suggests thaincrease in the basal spacing leads to a reduction in stion, due to the greater intermolecular distance and theeffectiveness as a partition medium. This is also the casDDS-Mg5Al-LDH(pH 8.5) and DDS-Mg5Al-LDH(pH 10).
H. Zhao, K.L. Nagy / Journal of Colloid and Interface Science 274 (2004) 613–624 623
gingtionug-ligh
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On the other hand, for DDS-Mg4Al-LDH(ex) and DDS-Mg4Al-LDH(pH 6.5), which have similar basal spacin(27.2 and 27.8 Å, respectively) and different DDS load(45.6 and 35.3%, respectively), the latter has higher sorpefficiency for tetrachloroethylene than the former. This sgests that, when basal spacing is relatively constant, a sreduction in DDS loading can enhance the HOC uptake.
We expect that an optimal DDSloading is required forhigh sorptive efficiency. Further decrease in DDS loadwould not necessarily result in higher sorption for DDMg3,4,5Al-LDH products. A decrease in DDS content wouresult in a decrease in layercharge compensation necesitating an increase in the content of the charge-balanCl− anion. The presence of Cl− should increase the overahydrophilicity within the LDH interlayer. Therefore, a combination of optimal DDS loading, basal spacing and Dconfiguration is required to achieve maximum sorptive eciency. The LDH with Mg/Al molar ratio of 3:1 accommodates the optimal amount of DDS and forms the most eftive partition medium in its interlamellar domain. The effeof substrate charge density on sorption of HOCs by DDLDHs is evident from this study. Similar mineral-chareffects on HOC sorption have also been observedorganic-cation-exchanged cationic clays (organoclays)6].
The DDS-Mg/Al-LDH materials investigated here maexhibit totally different sorption behaviors for other oganic compounds of different sizes and/or shapes. AlthoDDS-Mg4Al-LDH and DDS-Mg5Al-LDH have lower sorp-tion affinity for trichloroethylene and tetrachloroethyleas compared to DDS-Mg3Al-LDH, their greater interlayerspacing may make them more effective in accommoing larger guest molecules than trichloroethylene and tetrachloroethylene, such as polynuclear aromatic compouSimilarly, DDS-Mg2Al-LDH may be effective for smalleguest molecules. Regulation of the charge density or Mgmolar ratio thus would lead to products with unique propties for various guest organic compounds.
5. Conclusions
A series of organic–inorganic nanocomposite materwere synthesized using sodium dodecyl sulfate (DDS)magnesium–aluminum layered double hydroxide (MgLDH with a Mg/Al molar ratio of 2 to 5. Layer spacing is sufficient for DDS molecules to orient in a probble monolayer or interpenetrating bilayer-type arrangemwithin the LDH interlayers depending on synthesis cditions. Both the pH value of the exchange medium athe Mg/Al molar ratio of the LDH affect the basal spaing and the arrangement of DDS within the LDH intelayer. Sorption of trichloroethylene and tetrachloroethylby DDS-LDH does not follow a simple trend with increaing organic content, but rather varies with probable Dconfiguration, synthesis conditions and LDH compositi
t
.
Generally, the sorption affinity is in the order DDS-Mg3Al-LDH > DDS-Mg4Al-LDH ∼= DDS-Mg5Al-LDH > DDS-Mg2Al-LDH. DDS-Mg3Al-LDH had the highest affinity forboth HOCs in water, either comparable to or as much astimes higher than other clay-derived sorbents; DDS-Mg2Al-LDH had the lowest sorption affinity though it contains thighest amount of DDS and the sorption is believed to ocon edge/external surface area. Mg3Al-LDH has the optimalcharge density for forming an effective partition mediuwith LDH interlayer.
Acknowledgments
The authors acknowledge research support from theDepartment of Defense through the Strategic Environmtal Research and Development Program (SERDP) anpart a student research grant from the Clay Minerals Sety to H.T. Zhao. We also thank G.F. Vance, UniversityWyoming, for kindly allowing use of his gas chromatograpJ.B. Harsh, Washington State University, for the use ofZetasizer, and Dr. R.P. Bontchev, Sandia National Labtories, for helping with the drawing of schematic structumodels. We also would like to thank two reviewers whocomments helped to improve the manuscript.
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