ARTICLE pubs.acs.org/IECR
Adsorption of Pentachlorophenol from Aqueous Solution on Dodecylbenzenesulfonate Modified NickelTitanium Layered Double Hydroxide Nanocomposites Zhiyue Gao, Bing Du, Guoyan Zhang, Yan Gao, Zejiang Li, Hui Zhang,* and Xue Duan State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, P. O. Box 98, Beijing 100029, China
bS Supporting Information ABSTRACT: Dodecylbenzenesulfonate (DBS) modified NiRTi layered double hydroxides (LDHs) with different Ni/Ti molar ratios were prepared as hydrophobic organics sorbent by one-step co-precipitation method. The basal spacing of DBSNiRTi (d003 = 2.993.54 nm) is significantly increased compared with cyanate-intecalated Ni5TiLDH (0.73 nm) due to the intercalation of surfactant anions between the LDH layers. The greatly enhanced contact angle (123.5144.2°) for water and the reduced surface area (1.723.25 m2/g) of DBSNiRTi nanocomposites demonstrate their strong hydrophobic property. The highest sorption capacity for pentachlorophenol (PCP) of DBSNi5Ti among all samples is intimately related to its stacking model of the interlayer hydrophobic moiety and organic carbon content. The linear model well-fits for PCP sorption isotherms on DBSNiRTi (R2 > 0.96), implying a partitioning sorption process. Along with the effect of medium pH and temperature on PCP sorptiondesorption study on DBSNi5Ti, an adsolubilization mechanism, an exothermic sorption nature, and a significant hysteresis phenomenon during sorptiondesorption process are revealed.
1. INTRODUCTION Pentachlorophenol (PCP), with considerable chemical stability, is one of the “Priority Pollutants”.1 As a polar, hydrophobic, and ionizable weak acid organic compound, PCP was at one time worldwide used as an insecticide, herbicide, wood preservative, disinfectant, and ingredient in antifouling paints.2,3 However, anaerobic biodegradation of chlorophenols is relatively slow; thus, PCP is preserved in water and the soil sediments for a very long time,4 leading to great harm not only on the ecosystem but also on human health through inhalation, adsorption through the skin, and consumption of contaminated food and water due to their toxicity, poor biodegradability, carcinogenic, and recalcitrant properties.2 Nowadays, to reduce the impact of PCP on the environment and human health, the search for alternative sorbents has been proposed to remove PCP from wastewater and soil sediments.512 Although adsorption with activated carbon has been one of the most efficient processes for PCP removal from contaminated water,5,6 activated carbon is expensive not only for its market price but also due to the cost of the regeneration process,7 resulting in a renewed interest in the search for alternative sorbents such as fungal biomass,8 soil,911 and hexadecyltrimethylammonium-modified montmorillonite (HDTMAclay).12 However, the sorption of PCP by highquality, low-cost, and facile synthesized layered double hydroxide (LDH) materials has not been studied so far. LDH materials, also known as hydrotalcite-like compounds, have the general formula [MII1xMIIIx(OH)2]xþ(Anx/n) 3 mH2O, in which MII and MIII denote cations such as Mg2þ, Ni2þ, Mn2þ, Zn2þ and Al3þ, Cr3þ, Fe3þ, respectively, An is the interlayer exchangeable inorganic or organic anions, x is the layer charge density (x = MII/(MII þ MIII)), and m is the number of r 2011 American Chemical Society
interlayer water for structural stabilization function.1315 LDH materials and their derivates can be used as effective sorbents to remove a variety of organic and inorganic anionic pollutants from solutions due to their high anion-exchange capacity and flexible interlayer space.13,14 The inorganic LDH materials have favorable sorption property for inorganic anions16 and ionizable organic compounds containing hydrophilic groups,17 but unfavorable for hydrophobic ionizable organic compounds (HIOCs).18 Previous studies indicated that an organic molecule can be inserted into the interlayer region of the LDH materials, leading to an organo-LDH.19 When anionic surfactants are inserted between the LDH layers, the interlayer surface can be transformed from hydrophilic to hydrophobic, and this transformation can effectively enhance LDH materials’ affinity for hydrophobic compounds, making surfactant modified LDH alternative hydrophobic organic sorbents.2022 Zhao and Nagy21 reported an obviously enhanced sorption for hydrophobic tri- and tetrachloroethylene over dodecyl sulfate modified MgAl LDH. The inorganic LDH containing titanium has been synthesized recently and attracted increasing attention23,24 in terms of the good performance in the transformation of organic molecules in liquid-phase reactions25 and the photocatalysis property of Ticontaining LDH-derived materials.26 In spite of some investigations on the sorption of hydrophobic organics over surfactant modified LDH,27 these Ti-containing LDH materials are seldom
Received: August 23, 2010 Accepted: March 18, 2011 Revised: February 8, 2011 Published: April 01, 2011 5334
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Industrial & Engineering Chemistry Research used to adsorb HIOCs and the corresponding sorption process and mechanism are less understood. In this work, dodecylbenzenesulfonate (DBS) modified NiTi LDH with varied Ni/Ti molar ratios (R) were facile prepared by a one-step co-precipitation method as hydrophobic organics sorbent and compared with dodecyl sulfate modified LDH and cyanate intercalated LDH materials. The sorption desorption properties of PCP on as-prepared DBSNiRTi nanocomposites were studied for the first time by batch equilibrium method. The LDH layer charge density, interlamellar anionic configurations, surface properties, medium pH, and temperature are shown to affect the sorbent efficiency.
2. EXPERIMENTAL SECTION 2.1. Preparation of NiTi LDH. A typical synthetic procedure is as follows: solution A (70 mL) containing 1.0 mL of TiCl4 solution (the solution was prepared by TiCl4 and HCl with volume ratio of 1/1; thereinto TiCl4 is 0.004 mol) and 0.02 mol of Ni(NO3)2 3 6H2O with Ni/Ti ratio of 5 and solution B (150 mL of 0.5 M NaOH) were simultaneously added stepwise into solution C (125 mL of 0.072 M DBS solution) under vigorous stirring at 35 °C until a final pH 7.8. Nitrogen was bubbled throughout the co-precipitation procedures to minimize the CO32 contamination from atmosphere. The resulting slurry was aged at 80 °C for 24 h, centrifuged and washed extensively with decarbonated deionized water and then with anhydrous ethanol, and finally dried at 60 °C for 24 h giving a product DBSNi5Ti. The DBSNiRTi series with varied Ni/Ti ratios (R = 3, 4, 6) can be obtained using the same procedure by varying Ni2þ dosage. For comparison, a dodecyl sulfate (DS) modified Ni5Ti LDH (DSNi5Ti) was prepared via a similar step but with DS substituted for DBS, and a cyanate (CNO) intercalated Ni5Ti LDH (CNONi5Ti) was prepared as follows: 0.5 mL of TiCl4 solution (TiCl4 and HCl with volume ratio of 1/1; thereinto TiCl4 is 0.002 mol), 0.01 mol of Ni(NO3)2 3 6H2O, and 6.5 g of urea were dissolved in 100 mL of decarbonated deionized water under vigorous stirring. The resultant was stirred for 24 h at a refluxing temperature (373 K in aqueous solution) and then filtered. The filter cake was washed twice with deionized water and once with anhydrous ethanol, and finally dried overnight at 60 °C. 2.2. Characterization. Powder X-ray diffraction (XRD) data were recorded on a Shimadzu XRD-6000 diffractometer using Cu KR radiation (λ = 1.542 Å; 2θ, 270°; 40 kV; 30 mA). FT-IR spectra were obtained on a Bruker Vector 22 spectrometer using a standard KBr disk method (sample/KBr = 1/100) from 4000 to 400 cm1 with a resolution of 2 cm1. Contact angle (CA) was obtained through the sessile drop method using JC2000A contact angle/interface tensile measurer. Elemental analysis for metal and S elements was performed on a Shimadzu ICPS-7500 model inductively coupled plasma emission spectrometer (ICPES). Scanning electron microscopy (SEM) images were taken on a Hitachi S-4700. X-ray photoelectron spectroscopy (XPS) analysis was performed on a VG ESCALAB 250. The pressure in the analysis chamber during the experiments was 2 109 Pa. Spectra were obtained using a standard Al KR source (hν = 1486.6 eV). UVvis absorption spectra were recorded on a Shimadzu UV-2501 PC UVvis spectrophotometer. The PCP concentrations in solution were determined by HPLC on a Shimadzu LC-10AT VP instrument with UVvis detector SPD-10A VP at λmax = 218 nm. The mobile phase was 2%
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HAC (acetic acid)/CH3OH (methanol) (10/90) with a flow rate of 0.9 mL min1. The PCP concentration was quantified with an external standard method. The point of zero charge (PZC) for the DBSNi5Ti was measured by a batch equilibrium method proposed by Milonjic.5,6 Accordingly, the samples of DBSNi5Ti (0.1 g) were shaken in PVC vials for 24 h with 40 mL of 0.1 M NaCl at different pH values. Initial pH values were obtained by adding an amount of NaOH or HCl solution (0.1 M), keeping the ionic strength constant. The amount of H or OH ions adsorbed by DBSNi5Ti was calculated from the difference between the initial and the final concentration of H or OH ions. Experimental results of the pHPZC determination are given as pH values of filtered solutions after equilibration (pHf) with DBSNi5Ti as a function of initial pH values (pHi) of solution (Supporting Information Figure S1). It can be seen that PZC of DBSNi5Ti, in NaCl aqueous solutions, is at pH 8.0. 2.3. Batch Sorption and Desorption Experiments. PCP, a pesticide with a molecular weight of 266.5 g/mol, solubility in water of 18 mg/L at 25 °C,28 was used as a model contaminant in this study. All sorption isotherms were determined by the batch equilibrium method as previously described.10,29 In detail, PCP was dissolved in methanol to form a 100 mg/L stock solution. Before use, the stock solution was diluted to a set concentration with an electrolyte matrix containing 1 mM CaCl2, 0.1 mM MgCl2, and 0.5 mM Na2B4O7 3 10H2O. Formaldehyde was added at a concentration of 0.75 g 3 L1 formaldehyde to minimize aerobic biodegradation during batch experiments.29 The prepared nanocomposite (30 mg) were mixed with 30 mL of variable PCP concentrations (1.040.0 mg/L) in polycarbonate centrifuge tubes, and the tubes were sealed and shaken at constant temperature (25 °C) for 24 h in the dark (exposure of PCP solution to light was minimized to avoid photodegradation during experiments, and thus the observed change in PCP concentration in the aqueous phase during the sorption/desorption processes was considered to be due solely to sorption on the sorbents). An incubation period of 24 h was chosen on the basis of a preliminary kinetic test, which was conducted for 50 h, and the data revealed a concentration plateau after 24 h (Supporting Information Figure S2). The mixture was centrifuged by 3000 rpm for 5 min, and the supernatant was filtrated with 0.22 μm Millipore filter to completely filter the LDH particles remaining in suspension and then offered for analysis of aqueous-phase PCP concentrations by HPLC. The control experiments were also performed under identical conditions, except no sorbent was added and the PCP loss in control was less than 4%. The mass of PCP sorbed at equilibrium (Qe) was calculated by performing a mass balance on PCP by the equation of Qe/(mg/g LDH) = V(C0 Ct)/W and removal efficiency (RE) by RE/% = [(C0 Ct)/C0] 100, where V is the solution volume (L), Co the initial PCP concentration in solution, Ct the equilibrium PCP concentration in solution at time t (mg/L), and W the weight of sorbent added (g). Effects of medium pH and temperature on sorption isotherms of PCP on DBSNi5Ti were performed identically to other sorption studies. The pH effect studies were conducted at 25 °C over a range of pH 512 (the pH was adjusted by either dilute HCl or NaOH solution). Temperature effect studies were conducted at pH 7 with a constant-temperature water bath at 15, 25, and 35 °C, respectively. A cycle of sorption and desorption consisting of sorption followed by a single-dilution desorption step at varied medium pH (range of pH 512) values was conducted to investigate the sorptiondesorption hysteresis. Sorption was performed as 5335
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Figure 1. XRD patterns (left) and FT-IR (right) spectra of DBSNi3Ti (A), DBSNi4Ti (B), DBSNi5Ti (C), DBSNi6Ti(D), DSNi5Ti (E), CNONi5Ti (F), and pure DBS and DS.
Table 1. Structural Parameter and Physicochemical Properties of the LDH Intercalates with DBS, CNO, and DS as Interlayer Anions, Respectively samples
d003 (nm)
d006 (nm)
d110 (nm)
DBSNi3Ti
3.17
DBSNi4Ti
3.28
1.54
0.1542
DBSNi5Ti DBSNi6Ti
3.54 2.99
1.55 1.52
0.1546
DSNi5Ti
2.89
CNONi5Ti
0.73
0.36
0.1549
Ni/Ti a
2x b
An % c
CA d (deg)
SA e (m2/g)
2.58
0.56
50.1 (59.4)
141.3
3.25
3.36
0.46
54.9 (56.1)
142.5
2.87
4.15 5.00
0.38 0.34
53.9 (53.5) 49.2 (49.4)
144.2 123.5
1.72 2.56
4.62
0.36
28.2
37.7
5.56
0.24
17.7
0
7.30 124.10
Based on ICP data. b 2x stands for the layer charge density of Ti4þ-containing LDH, in which x = Ti4þ/(Ni2þ þ Ti4þ). c The weight percentage of interlayer DBS anion upon ICP data of S element, and value in parentheses derived from the LDH general formula. d The shortened form of contact angle. e The shortened form of specific surface area. a
described previously.10 After the sorption assays, desorption was initiated at 25 °C by withdrawing about 95% volume of the supernatant and replenishing the samples with the same volume fresh PCP-free background electrolyte solution with corresponding pH values (actual amounts were determined by weight), and the tubes were sealed and shaken at 100 rpm for 24 h in the dark. The period desorption of 24 h was chosen after a preliminary desorption kinetic test. At the end of the desorption period, the tubes were centrifuged at 3000 rpm for 5 min and the supernatant was filtrated with Millipore filter and offered for analysis of solution-phase PCP concentration by HPLC. The desorption procedure was repeated until the PCP concentration in the supernatant was below the detection limit.
3. RESULTS AND DISCUSSION 3.1. Structure Analysis. The LDH structures were characterized by their X-ray diffraction patterns and, in particular, by their basal reflections, as shown in Figure 1(left). As expected, the pattern of CNONi5Ti, with d003 ∼ 0.73 nm (Table 1), agrees with reported values for LDHcyanate.24 The DBSNiRTi samples exhibit comparatively ordered layered structures with basal spacings d003 = 2.993.54 nm (Table 1), which are a little
larger than that of DSNi5Ti (2.89 nm) while remarkably larger than that of CNONi5Ti (0.73 nm), revealing greatly extended interlayer regions mainly due to the intercalation of DBS anions with larger organic moiety than that of DS and even much larger than CNO molecules between the layers. The broad scatterings at 2θ ∼ 19° corresponding to a spacing of 0.40.5 nm in surfactant modified LDH, absent in CNONi5Ti, were assigned to scattering by the hydrocarbon chains similar to the previous report.30 Furthermore, with increasing Ni/Ti ratios, i.e., decreased layered charge density, the d003 is increased orderly as DBSNi3Ti (3.17 nm) < DBSNi4Ti (3.28 nm) < DBSNi5Ti (3.54 nm) due to the sequentially weakened hostguest interactions.13,14 While the contrarily decreased d003 of DBSNi6Ti (2.99 nm) might be mostly due to its poor crystallinity of the LDH phase upon its high Ni/Ti ratio, resulting in the possible formation of few amorphous compounds such as Ni(OH)2.13 The ICP data show that the DBS weight percentage is obviously lower than the theoretical value in DBSNi3Ti, implying the possible nitrate co-intercalated due to the relatively high layer charge density of DBSNi3Ti, while other DBSNiRTi samples with relatively low LDH layer charge densities present approximately single DBS intercalation, though their Ni/Ti ratios are lower than those in the starting solution. The XPS 5336
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Industrial & Engineering Chemistry Research data confirm that the titanium in intercalates exists solely as Ti4þ species. Combining with the XRD reflections’ relative intensity and symmetry (Figure 1(left)C) and the d003 values, it is concluded that DBSNi5Ti has a highly ordered interlamellar with larger d spacing. As for the slightly larger d003 of all DBSNiRTi samples than previously reported DBSMg3Al LDH (2.68 nm),20 it can be ascribed to the varied DBS interlayer arrangement probably originating from the varied layer charge density and layer metal component. The FTIR spectra (Figure 1(right)) provide more information on interlayer species. For DBSNiRTi samples and DSNi5Ti, the common broad bands at ∼3500 cm1 are ascribed to the ν(OH) of the LDH layers and interlayer water molecules,13 though the maximum absorbance of DBSNi6Ti near 3640 cm1 indicates the presence of hydroxide. The more characteristic bands are those due to typical CH stretchings (νas(CH3) at 29572960 cm1, νas(CH2) at 29262929 cm1, and νs(CH2) at 28552858 cm1)20,30,31 and CH2 scissoring (14641468 cm1) approaching those previously reported.30 Furthermore, the νas(SdO) and νs(SdO) in DBSNiRTi downshift to 1165 and 1038 cm1, respectively, compared with those in pure NaDBS at 1191 and 1044 cm1, and similarly those in DSNi5Ti downshift to 1194 and 1067 cm1, compared with those in pure NaDS at 1225 and 1087 cm1, indicating a geometric disturbance of the SO3 functional groups.30,32 A downshift in frequency corresponds to a weakening of the SdO bond strength due to the formation of a hydrogen bond between the OH groups of the LDH layer and the SO3 groups via a path SdO 3 3 3 HOM (M = Ni or Ti). Meanwhile for DBSNiRTi, the bands at 1215 and 1130 cm1 can be assigned to the SO3 group, one at 1009 cm1 to a CH aromatic in-plane bend, one at 832 cm1 to a CH aromatic outof-plane bend, and one at 1602 cm1 to a CdC aromatic stretch,30,33 while a broad one at ∼1385 cm1 for DBSNi3Ti implies the co-intercalated NO3 in this sample. It can be envisioned that the presence of aliphatic chains and aromatic rings in DBSNiRTi samples makes them potential candidates for the sorption of organic phenol compounds such as PCP molecules via interactions among the hydrophobic groups. 3.2. Interlayer Configurations. The interlayer spacings of DBSNiRTi samples distribute about four values 3.17, 3.28, 3.54, and 2.99 nm, correlating with the orientation of the alkyl chains of DBS incorporated between the LDH layers. Based on the perpendicular monolayer orientation of interlayer DBS anions, the theoretically derived d-spacing for DBSNiRTi is ca. 2.69 nm;34 thus, the interlayer space occupied by DBS would be approximately 2.21 nm, considering the LDH layer thickness of ∼0.48 nm. The height of the benzenesulfonate group is 0.51 nm involving a three-point attachment of the SO3 group to the LDH layer resulting in a perpendicular position of benzene ring. The van der Waals distance between the tail of a DBS anion and its next neighboring hydroxide layer is ∼0.14 nm, and the height of alkyl chain of DBS is ∼1.53 nm. All of the surfactant chains should stand upright with all chains in all-trans conformation, considering that half of the positive charges are neutralized from each side of the layer,21 and consequently the interlayer anion stacking configuration of as-prepared surfactant modified LDH is tentatively presented in Figure 2. The DBSNi5Ti with the largest d003 (3.54 nm) probably corresponds to an antiparallel perpendicular monolayer arrangement (Figure 2C). The DBSNi3Ti and DBSNi4Ti with slightly smaller d003 values consist of two antiparallel tilted interpenetrating monolayers
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with angles of 65 and 79°, respectively, between the alkyl chain and the LDH layer (Figure 2A,B). DBSNi6Ti with the smallest d003 value possibly corresponds to an antiparallel perpendicular interpenetrating monolayer arrangement (Figure 2D). This antiparallel arrangement greatly reduces the repulsion between the anions since the negative poles are far apart;35 meanwhile the phenyl alkyl chains within the LDH interlayer can effectively provide a hydrophobic interaction region. As for DSNi5Ti (d003 = 2.88 nm), a perpendicular bilayer arrangement with DS anions overlap to some degree is recommended.21 3.3. Surface Properties and the Morphology. Contact angles for water of DBSNiRTi samples lie in 123.5144.2° (Table 1 and Supporting Information Figure S3), demonstrating considerable hydrophobic property upon the intercalation of DBS anions between the LDH layers. While the CA of DSNi5Ti (37.7°; Table 1) is much smaller than that of DBSNiRTi, probably due to the lack of a strong hydrophobic benzene ring in DS compared with DBS. As for CNONi5Ti, CA = 0° shows strong hydrophilic surface property similar to that previously reported on CO3MgAlLDH.20 The SEM images (Figure 3) indicate that the intercalation of DBS between the LDH layers dramatically alter the morphology of LDH nanoparticles. The DBSNiRTi samples and DSNi5Ti are obviously agglomerated, and their surfaces are more diffuse due to the hydrophobic interaction of the interlayer alkyl chains and not as sharp as that of CNONi5Ti with clear platelike morphology (Figure 3), similar to that previously reported.33 The BrunauerEmmett Teller (BET) data show that the specific surface areas of DBSNiRTi samples ( DBSNi 4 Ti (706) > DBSNi6Ti (509) > DBSNi3Ti (323), and all far greater than those on the DSNi5Ti (172) and CNONi5Ti (35) ascribing to the significantly enhanced hydrophobicity of DBSNiRTi (see Table 1). It is noted that the Kd value is increased as the Ni/ Ti ratio increases from 3 to 5, while deceased at Ni/:Ti ratio of 6. This can be related to the largest interlayer space (3.54 nm) and relatively lower DBS content of DBSNi5Ti. These results imply that the PCP sorption on the DBSNiRTi samples is probably 5338
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Figure 4. Equilibrium adsorption isotherms for PCP at 25°C and pH 7 on intercalated materials DBSNi3Ti, DBSNi4Ti, DBSNi5Ti, DBSNi6Ti, DSNi5Ti, and CNONi5Ti. (Data dots refer to the experimental result; solid lines refer to the linear fitting result.)
Table 2. Constants of the Linear Regression Model Based on PCP Adsorption Isotherms at Varied Media pH Values and Temperatures
Figure 3. SEM images of DBSNi5Ti (A), DSNi5Ti (B), and CNONi5Ti (C).
related to the interlayer DBS arrangement of DBSNiRTi nanocomposites. The larger layer charge densities of DBSNi3Ti (0.56) and DBSNi4Ti (0.46) than that of DBSNi5Ti (0.38) (Table 1) lead to high DBS content with relatively dense interlayer stacking of the former, in which DBS chains are more likely aggregated in bundles rather than evenly distributed within the interlayer region, and consequently produce strong hydrophobic attraction among the intercalated anions. Contrarily, the DBSNi5Ti with larger d003 more likely possesses a loose interlayer DBS stacking mode and thus leads to its highest PCP sorption due to its easier adsolubilization of PCP into the loose interlayer hydrophobic region.38 As for DBSNi6Ti, the low PCP sorption degree is mainly due to its smallest interlayer space and DBS content. This is the first study on the sorption of PCP from aqueous solution onto the organo-LDH materials. The obtained equilibrium PCP sorption capacities of the four samples are all much higher than the values of PCP on soil (Kd = 0.44) reported by DiVincenzo and Sparks11 and on HDTMAclay (Kd = 0.02) by Stapleton et al.,12 probably due to the large interlayer distance and high hydrophobicity of DBS modified NiTi LDH.
samples
T (°C)
pH
Kd a
R2
Kom b 215
DBSNi3Ti
25
7
323
0.984
DBSNi4Ti
25
7
706
0.981
471
DBSNi5Ti
15
7
2281
0.999
1521
25
5
358
0.957
239
25
7
1286
0.983
857
25
8
3409
0.965
2273
25 25
9 12
2403 429
0.993 0.992
1602 286
35
7
974
0.994
649
DBSNi6Ti
25
7
509
0.987
339
DSNi5Ti
25
7
172
0.985
99
CNONi5Ti
25
7
35
0.882
-
a
Kd = partition coefficient with units of milliliters per kilogram. For direct comparison, all isotherm data were evaluated by the linear regression equation (R2 > 0.96). b Kom = the value of organic-matternormalized sorption coefficient. Kom = Kd/foc, where foc equals the molecular weight of DBS (DS) divided by the weight of C in DBS ≈ 1.50 (DS ≈ 1.73).
For direct comparison and as a reflection of the dominant influence of the solid-phase organic carbon on the sorption of organic solutes, the sorption coefficient often is normalized on an organic matter basis,21,34,39 Kom ¼ Kd =foc
ð2Þ
where Kom is the organic-matter-normalized partition coefficient and foc is the fractional organic carbon content of the sorbent (Table 2). Compared to DBSNiRTi with higher Kom (2152273), the lower sorption capacity of DSNi5Ti (Kom = 99) for PCP can be mostly ascribed to the feeblish hydrophobicity of DSNi5Ti due to its lower organic carbon content. The sorption isotherm of PCP onto CNONi5Ti was better 5339
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Figure 6. Equilibrium adsorption isotherms of DBSNi5Ti for PCP at varied medium temperatures.
PCP species in solution and the surface of the DBSNi5Ti nanoparticles. PCP T PCP þ Hþ
Figure 5. (A) Equilibrium adsorption isotherms of DBSNi5Ti for PCP at 25 °C under different medium pH values (inset is the measured maximum PCP removal efficiency at varied pH) and (B) speciation diagram of PCP at 25 °C.
fitted by the Langmuir model with R2 > 0.98, implying a homogenously monolayer distribution sorption on the hydrophilic surface of CNONi5Ti. The LDH surface area (Table 1) and sorption coefficient (Table 2) are not significantly correlated, suggesting that organic compound sorption is unrelated to the surface area instead of the partitioning sorption process involving adsolubilization of organic compounds into an interlayer three-dimensional organic phase rather than on the external surface of the DBSNiRTi nanocomposites.38,40 It may be deduced that the main interaction between DBSNiRTi and PCP can be possibly attributed to the hydrophobic effect. 3.5. Effect of pH on PCP Sorption. Because PCP is a hydrophobic ionizable organic compound (eq 3, pKa = 4.75) that can exist as either the protonated or the deprotonated species upon the solution pH,12 the pH of solution would greatly affect the hydrophobicity and solubility of PCP.11,41 The determination of the effect of the solution pH on the sorption of PCP (Figure 5A) indicates that the PCP sorption on DBSNi5 Ti nanoparticles is increased with increasing pH when pH e 8 (Kd varied from 358 to 3409), but contrarily reduced at pH 9 (Kd = 2403) and at pH 12 (Kd = 429), which may be related to the interaction between the
ðpKa ¼ 4:75Þ
ð3Þ
Considering the determined PZC ca. 8.0 of DBSNi5Ti (Supporting Information Figure S1), the surface of DBSNi5Ti nanoparticles is positively charged at pH 5 and 7 and nearly zero charged at pH 8, but negatively charged at pH 9 and 12. Upon the dissociation equilibrium of PCP (Figure 5B), the nonionized form of PCP species (uncharged) represent more than 40% of all PCP species when the solution pH < 5, whereas more than 95% presented as the ionized form PCP species (negatively charged) when the pH > 6.5 The present sorption results seem to imply the dominance of hydrophobic force along with hydrogen bond and electrostatic interaction. At pH 8, the uncharged LDH nanoparticles are more likely to adsolubilize the hydrophobic PCP anions into its hydrophobilized interlayer via predominant hydrophobic force, despite possible surface binding such as a hydrogen bond.36 Meanwhile, the neutral LDH nanoparticles may closely attach together through the hydrophobilized surface, leading to a hydrophobic network, which may also strengthen the PCP sorption upon distribution sorption mechanism. Therefore, the maximum PCP sorption is obtained at pH 8. At pH 7, the LDH nanoparticles may repel each other due to their positively charged surfaces and meanwhile may attract PCP anions via electrostatic force, thus properly facilitating the formation of surface hydrogen bond between the PCP anions and the LDH layers, particularly on the external sites at the entrance of the interlayer spaces due to its directional and more localized nature, which may obstruct PCP from entering into the hydrophobic interlamellar and thus result in relatively lower PCP sorption. At pH 9, the negatively charged LDH surface may repel the PCP anions compared with the uncharged (pH 8) and positively charged (pH 7) LDH nanoparticles, and consequently results in less surface hydrogen bond and relatively dominant hydrophobic effect. Accordingly, it is reasonable to obtain the order of Kd (pH 8) > Kd (pH 9) > Kd (pH 7). As for considerably low PCP sorption at pH 5, it can be related to two aspects. On one hand, almost equal ionized and nonionized PCP species may form a surface hydrogen bond with the LDH layer at low pH and, on the other hand, PCP has limited solubility at acidic pH, both of which may depress the PCP sorption in terms of the partitioning 5340
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Figure 7. Desorption isotherms of PCP from PCP-adsorbed DBSNi5Ti at different medium pH values at 25 °C.
sorption mechanism. Then the minimum PCP sorption at pH 12 can be reasonably ascribed to strong electrostatic repulsion between PCP anions and completely negative LDH particles, preventing PCP from being adsolubilized into the LDH interlayer regions by hydrophobic force. These results imply that the sorption of HIOCs on the surfactant modified LDH nanocomposites is mainly determined by the hydrophobic effect between the HIOCs and the hydrophobilized LDH nanocomposites. 3.6. Effect of Temperature on PCP Sorption. Figure 6 depicts the equilibrium sorption isotherms of PCP over DBS Ni5Ti at 15, 25, and 35 °C. A comparison of the experimental data with the sorption model fitting results show that the linear equation represents the best fit of sorption data (R2 > 0.98). And the equilibrium sorption capacity increases with decreasing temperature, together with the sorption coefficient Kom decreased from 1521 to 649 with increasing temperature (Table 2), suggesting that the interaction between PCP and DBSLDH nanoparticles is exothermic in nature. Thermodynamic parameters such as Gibbs free energy (ΔG°), standard enthalpy change
(ΔH°), and standard entropy change (ΔS°) were determined using the following equations and van’t Hoff plot:11 ln Kd ¼
ΔH° ΔS° þ RT R
ΔG° ¼ RT ln Kd ¼ ΔH° TΔS°
ð4Þ ð5Þ
where R is the universal gas constant (8.314 J/(mol 3 K)) and T is temperature (K). The values of ΔH° (7.57 kcal/mol) and ΔS° (10.93 kcal/K) were derived respectively from the slope and intercept of the van’t Hoff plot (inset in Figure 6). The negative value of ΔG° (4.41 kcal/mol) indicates that the PCP removal by DBSLDH nanocomposites is a spontaneous process,11 implying that the hydrophobic force was strong enough to break the potential and drive the adsolubilization of PCP into the hydrophobic interlayer region of the DBSLDH nanocomposite. 3.7. Desorption Isotherm of PCP Sorbed DBSNi5Ti at Varied Medium pH Values. The desorption procedure was repeated for three runs, and the corresponding PCP desorption 5341
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Table 3. Sorption and Desorption Isotherm Parameters and Hysteresis Indices for PCP on DBSNi5Ti in Different pH Media (25 °C)a pH 5 sorption
desorption
HI
pH 7
pH 8
pH 9
pH 12
c
0.358 (0.034)
1.286 (0.067)
3.409 (0.228)
2.403 (0.117)
0.429 (0.011)
ab R
1.370 (0.500)c 0.9783
0.700 (0.575) 0.9915
3.898 (1.273) 0.9823
1.061 (0.764) 0.9929
0.885 (0.173) 0.9960
Kd
1.702 (0.217)
3.710 (0.210)
15.285 (0.872)
10.594 (0.561)
1.454 (0.212)
a
0.442 (0.534)
0.246 (0.494)
14.259 (1.565)
1.391 (0.793)
1.943 (0.756)
Kd
b
R
0.9543
0.9905
0.9920
0.9917
0.9509
Ce1
0.27 (0.760.68)
1.21 (0.512.39)
2.91 (0.29- 9.69)
5.86 (2.53-21.88)
0.07 (1.754.35)
Ce2
0.11 (0.491.19)
1.50 (0.922.31)
4.59 (1.8015.49)
4.29 (2.756.90)
3.87 (1.177.94)
a
pH = 5, Ce1 = 1 mg/L, Ce2 = 1.5 mg/L; pH = 7, Ce1 = 1.5 mg/L, Ce2 = 3 mg/L; pH = 8, Ce1 = 0.5 mg/L, Ce2 = 2 mg/L; pH = 9, Ce1 = 1 mg/L, Ce2 = 2 mg/ L; pH = 12, Ce1 = 1 mg/L, Ce2 = 4 mg/L. b Constant of linear isotherms for sorption or desorption (Qe = KdCe þ a). Here the unit of Kd is milliliter per gram. c (...) = parameter standard deviations.
Figure 8. XRD (A) and FT-IR (B) spectra of PCP-adsorbed DBSNi5Ti compared with PCP and DBSNi5Ti.
isotherms at pH 7 and 8 with relatively high sorption capacity (Supporting Information Figure S5) show almost overlapped data. Obviously under these conditions, it is difficult for the PCP transferred from the DBSNi5Ti into the solution until a new equilibrium is reached. Figure 7 indicates that, within the concentration range studied, the PCP desorption isotherms data at varied pH values are well-fitted by the linear regression model (R2 > 0.98). It is clearly noted that the PCP desorption isotherms are hysteretic compared to sorption isotherms. The hysteresis index (HI) can be calculated as follows:41 Qed Qes hysteresis index ¼ ð6Þ Qes T , Ce where the superscripts s and d refer to the solid-phase solute concentrations for the single-cycle sorption and desorption experiments, respectively, and the bracket subscripts T and C specify conditions held constant, i.e., temperature and residual solution-phase concentration. Desorption hysteresis indices at 25 °C and two different concentration levels were calculated for DBSNi5Ti in varied pH media using linear sorption and desorption parameters, and the results are listed in Table 3. To evaluate relative errors in the calculated HI values, the range of each was computed on the basis of one-standard deviation (σ) values of the corresponding linear model parameters, Kd and
Figure 9. UVvis spectra of PCP-adsorbed DBS-Ni5Ti compared with PCP, DBS, and DBSNi5Ti.
a. The lower HI value in each case was calculated using eq 6 and expressions of Qde = (Kd 1σ)Ce þ (a 1σ) and Qse = (Kd 1σ)Ce þ (a þ 1σ) and the higher value calculated with the same equation but employing expressions of Qde = (Kd þ 1σ)Ce þ (a þ 1σ) and Qse = (Kd 1σ)Ce þ (a 1σ); here, Qde and Qse were 5342
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Figure 10. Adsorption mechanism model of DBSNiRTi for PCP.
computed respectively using desorption and sorption isotherm parameters and their respective standard deviations. A positive HI value indicates that sorptiondesorption hysteresis is statistically significant on a one-standard deviation basis. The calculated HI values and their ranges for the DBSNi5Ti at four pH media are summarized in Table 3. It is evident that the majority of the calculated HI values except the one at pH 5 at lower concentration level are significantly greater than zero and that the lower range (1σ) calculated HI value is nonnegative for at least two of the three levels of Ce in each case. This clearly indicates the occurrence of sorptiondesorption hysteresis for PCP on DBSNi5Ti. The entrapment of PCP molecules within the highly hydrophobilized organic interlayer region of DBS Ni5Ti may contribute significantly to sorptiondesorption hysteresis similar to that previously reported by Huang et al.42,43 As a polar hydrophobic organic compound, PCP is adsolubilized into the hydrophobilized interlayer domain of DBSNi5Ti and strongly interacted with organic matter DBS anions and therefore led to an extremely slow desorption or irreversible sorption.43 This phenomenon is quite similar to the PCP sorption on sediments intimately related to their organic matter content4 but very different from that on active carbon due to significantly various properties.5 The present findings indicate that the PCP can be strongly immobilized in as-prepared DBSNiRTi nanocomposites, and the toxicity of organic pollutants sorbed in DBSNiRTi would be lower because of hysteresis during the sorptiondesorption processes, and thus enable these nanocomposites an effective sorbent. 3.8. Adsorption Mechanism. Following equilibrium sorption, the PCP sorbed DBSNi5Ti was recovered and submitted for XRD and IR analyses. It can be seen from Figure 8A that the (003) peak in PCP sorbed DBSNi5Ti can be clearly seen at almost the same 2θ angle (d003 = 3.20 nm) as pristine DBSNi5Ti but obviously weakened, implying the poor crystallinity after sorption of PCP. This phenomenon indicates that PCP molecules might be adsolubilized into the hydrophobic interlayer domain of DBSNi5Ti instead of between the LDH layer and interlayer DBS anion. Meanwhile, there are no peaks ascribed to PCP observed in the pattern of PCP sorbed DBSNi5Ti, indicating no PCP crystal formed on the surface of DBSNi5Ti nanoparticles. The IR spectra (Figure 8B) show that the strong and sharp CH scissoring at 1464 cm1 of DBSNi5Ti is largely depressed in the PCP sorbed DBSNi5Ti sample though its position is unchanged, while the relatively weak but sharp CdC aromatic stretchings at 1409 and 1379 cm1 in DBSNi5Ti become relatively strong but broad at 1404 and 1378 cm1 for
the PCP sorbed DBSNi5Ti sample. These changes can be tentatively attributed to the strong sorbatesorbent interactions involving ππ interaction occurring between the phenyl rings of both PCP and DBSNi5Ti, given that for PCP (Figure 8B) the band at 1418 and 1382 cm1 can be ascribed to the CdC aromatic stretching,44 via the adsolubilization mechanism with PCP entrapped in the hydrophobic interlayer region of DBSNi5Ti. As for the changes related to the sulfonate group (1215, 1167, and 1130 cm1), it can be attributed to the hydrogen bondings between the phenol hydroxyl oxygen anions and/or sulfonate group and the LDH layer considering the bands at 1220 and 1194 cm1 due to the CO stretching in PCP.44 The UVvis absorption spectra (Figure 9) furthermore indicate that both the PCP sorbed DBSNi5Ti and the DBSNi5Ti show absorption at ∼223 nm due to πfπ* transition because of the existence of the alkylbenzene, while, for PCP, a clear band at 218 nm due to πfπ* transition can be ascribed to the substitution of strong-draw electron groups Cl on the aromatic ring. A careful examination for the spectra lines suggests that the absorption band at 223 nm for PCP adsorbed DBSNi5Ti (labeled with an asterisk (*) in Figure 9) slightly downshifts, indicating the interaction of PCP anions with DBS within the interlayer region of DBSNi5Ti. More evidently, the absorption at 247 nm merely for both PCP adsorbed DBSNi5Ti and PCP directly confirm the existence of PCP in the PCP sorbed sample. Therefore, it can be deduced that the synergetic effect of a strong adsolubilization mechanism involving the ππ interaction and hydrogen bond between PCP and DBSNi5Ti and slight electrostatic attraction afford the high sorption ability of the present DBSmodified NiTi LDH materials. On the basis of the above analyses, a possible adsolubilization mechanism model is tentatively proposed in Figure 10.
4. CONCLUSIONS A series of organicinorganic nanocomposites involving dodecylbenzenesulfonate modified NiTi LDH (DBSNiRTi) were synthesized using one-step co-precipitation method as a function of Ni/Ti ratios and compared with dodecyl sulfateLDH (DSNi5Ti) and cyanateLDH (CNONi5Ti). The basal spacings of DBSNiRTi (d003 = 2.993.54 nm) are a little larger than that of DSNi5Ti (2.89 nm) while greatly larger than that of CNONi5Ti (0.73 nm), revealing largely extended interlayer regions in surfactantLDH systems. The interlayer configurations of the DBSNiRTi samples probably involve three anionic stacking models: loosely antiparallel 5343
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Industrial & Engineering Chemistry Research perpendicular monolayer arrangement, antiparallel perpendicular interpenetrating monolayer arrangement, and antiparallel tilted interpenetrating monolayer arrangement. The greatly enhanced hydrophobicity of DBS-NiRTi samples (contact angles, 123.5144.2°) is attributed to the presence of both aromatic CdC bonds and aliphatic chains. All surfactantLDH samples present higher sorption capacities for PCP than that of CNONi5Ti due to the strong hydrophobicity of the former. The sorption isotherms of PCP on DBSNiRTi and DSNi5Ti are well-fitted by the linear model, implying a partitioning sorption process. The highest PCP sorption capacity on DBSNi5Ti can be related to its loosely stacked interlayer anion arrangement more favoring the adsolubilization of PCP in the hydrophobic interlayer regions. An adsolubilization mechanism model is tentatively proposed mainly involving hydrophobic interactions between PCP and the hydrophobilized DBSNiRTi interlayer and hydrogen bonding. The PCP desorption from the PCP adsorbed DBSNi5Ti sample shows a significant desorption hysteresis, implying a slow or irreversible sorption process and thus possibly enabling the immobilization of PCP on DBSNiRTi as an effectively route to avoid the secondary pollution of PCP in the environment. The present study clearly implies the potential of the surfactantNiTiLDH nanocomposites as sorbent to adsorb hydrophobic organic pollutants from aqueous solution. We are currently studying the further photocatalytic oxidation of PCP by calcined DBSNiRTi samples.
’ ASSOCIATED CONTENT
bS
Supporting Information. Figure S1 showing determination of pHPZC of DBS-Ni5Ti in NaCl solutions (pHi/pHf, initial/ final pH value), Figure S2 showing sorption and removal kinetics plots of PCP onto DBS-Ni5Ti at pH 7 at 25 °C, Figure S3 showing contact angles for water of DBS-Ni5Ti, DS-Ni5Ti, and CNO-Ni5Ti, Figure S4 showing (a) the PCP removal efficiency and adsorption capacity as a function of the initial PCP concentrations (C0 = 140 mg/L) at 25 °C and pH 7 and (b) the measured maximum PCP removal efficiencies for PCP at 25 °C and pH 7 on intercalated materials DBS-Ni3Ti, DBS-Ni4Ti, DBS-Ni5Ti, DBS-Ni6Ti, DS-Ni5Ti, and CNO-Ni5Ti, and Figure S5 showing desorption isotherms of PCP from PCP-adsorbed DBS-Ni5Ti in different pH (7 and 8) media at 25 °C. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ8610 64425872. Fax: þ8610 64425385. E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank the National Nature Science Foundation of China (Grant 20776012), the 111 Project (Grant B07004), and the 973 Program (Grant 2011CBA00508) for financial support. ’ REFERENCES (1) Keith, L. H.; Telliard, W. A. Priority pollutants: I—A perspective view. Environ. Sci. Technol. 1979, 13, 416–423.
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(2) Taylor, T. R.; Tucker, T.; Whalen, M. M. Persistent inhibition of human natural killer cell function by ziram and pentachlorophenol. Environ. Toxicol. 2005, 20, 418–424. (3) Cirelli, D. P. Patterns of pentachlorophenol usage in the United States of America—An overview. In Pentachlorophenol, Chemistry, Pharmacology, and Environmental Toxicology; Rao, K. R., Ed.; Plenum Press; New York, 1978; pp 1318. (4) Chen, Y. X.; Chen, H. L.; Xu, Y. T.; Shen, M. W. Irreversible sorption of pentachlorophenol to sediments: Experimental observations. Environ. Int. 2004, 30, 31–37. (5) Diaz-Flores, P. E.; Leyva-Ramos, R.; Guerrero-Coronado, R. M.; Mendoza-Barron, J. Adsorption of pentachlorophenol from aqueous solution onto activated carbon fiber. Ind. Eng. Chem. Res. 2006, 45, 330–336. (6) Babic, B. M.; Milonjic, S. K.; Polovina, M. J.; Kaludierovic, B. V. Point of zero charge and intrinsic equilibrium constants of activated carbon cloth. Carbon 1999, 37, 477–481. (7) García-Mendieta, A.; Solache-Ríos, M.; Olguín, M. T. Comparison of phenol and 4-chlorophenol adsorption in activated carbon with different physical properties. Sep. Sci. Technol. 2003, 38, 2549–2564. (8) Mathialagan, T.; Viraraghavan, T. Biosorption of pentachlorophenol from aqueous solutions by a fungal biomass. Bioresour. Technol. 2009, 100, 549–558. (9) Cea, M.; Seaman, J. C.; Jara, A. A.; Fuentes, B.; Mora, M. L.; Diez, M. C. Adsorption behavior of 2,4-dichlorophenol and pentachlorophenol in an allophanic soil. Chemosphere. 2007, 67, 1354–1360. (10) He, Y.; Xu, J.; Wang, H.; Ma, Z.; Chen, J. Detailed sorption isotherms of pentachlorophenol on soils and its correlation with soil properties. Environ. Res. 2006, 101, 362–372. (11) DiVincenzo, J. P.; Sparks, D. L. Sorption of the neutral and charged forms of pentachlorophenol on soil: evidence for different mechanisms. Arch. Environ. Contam. Toxicol. 2001, 40, 445–450. (12) Stapleton, M. G.; Sparks, D. L.; Dente, S. K. Sorption of pentachlorophenol to HDTMA-clay as a function of ionic strength and pH. Environ. Sci. Technol. 1994, 28, 2330–2335. (13) Cavani, F.; Trifiro, F.; Vaccari, A. Hydrotalcite-type anionic clays: Preparation, properties and application. Catal. Today 1991, 11, 173–301. (14) Li, F.; Duan, X. Applications of layered double hydroxides. Struct. Bonding (Berlin) 2006, 11, 193–223. (15) Pan, D.; Zhang, H.; Zhang, T.; Duan, X. A novel organicinorganic microhybrids containing anticancer agent doxifluridine and layered double hydroxides: Structure and controlled release properties. Chem. Eng. Sci. 2010, 65, 3762–3771. (16) Goh, K. H.; Lim, T. T.; Dong, Z. Enhanced arsenic removal by hydrothermally treated nanocrystalline Mg/Al layered double hydroxide with nitrate intercalation. Environ. Sci. Technol. 2009, 43, 2537–2543. (17) Anirudhan, T. S.; Suchithra, P. S. Adsorptive characteristics of tannin removal from aqueous solutions and coir industry effluents using calcined and uncalcined hydrotalcites. Ind. Eng. Chem. Res. 2007, 46, 4606–4613. (18) Villa, M. V.; Sanchez-Martín, M. J.; Sanchez-Camazano, M. Hydrotalcites and organo-hydrotalcites as sorbents for removing pesticides from water. J. Environ. Sci. Health, Part B 1999, 34, 509–525. (19) Newman, S. P.; Jones, W. Synthesis, characterization and applications of layered double hydroxides containing organic guests. New J. Chem. 1998, 105–115. (20) Wang, B.; Zhang, H.; Evans, D. G.; Duan, X. Surface modification of layered double hydroxides and incorporation of hydrophobic organic compounds. Mater. Chem. Phys. 2005, 92, 190–196. (21) Zhao, H. T.; Nagy, K. L. Dodecyl sulfate-hydrotalcite nanocomposites for trapping chlorinated organic pollutants in water. J. Colloid Interface Sci. 2004, 274, 613–624. (22) Klumpp, E.; Contreras-Ortega, C.; Klahre, P.; Tino, F. J.; Yapar, S.; Portillo, C.; Stegen, S.; Queirolo, F.; Schwuger, M. J. Sorption of 2, 4-dichlorophenol on modified hydrotalcites. Colloids Surf., A 2004, 230, 111–116. 5344
dx.doi.org/10.1021/ie101766e |Ind. Eng. Chem. Res. 2011, 50, 5334–5345
Industrial & Engineering Chemistry Research (23) Saber, O.; Tagaya, H. New layered double hydroxide, ZnTi LDH: Preparation and intercalation reactions. J. Inclusion Phenom. Macrocyclic Chem. 2003, 45, 109–116. Saber, O.; Hatano, B.; Tagaya, H. Controlling of the morphology of Co-Ti LDH. Mater. Sci. Eng. 2005, 25, 462–471. (24) Shu, X.; Zhang, W. H.; He, J.; Gao, F. X.; Zhu, Y. X. Formation of Ni-Ti-layered double hydroxides using homogeneous precipitation method. Solid State Sci. 2006, 8, 634–639. (25) Alvaro, M.; Corma, A.; García, H.; Valencia, S. Influence of the solvent on the titanium beta catalyzed oxidation of phenylethylenes without carbon-carbon double bond cleavage. Appl. Catal., A 1995, 128, L7–L11. (26) Shu, X.; He, J.; Chen, D. Tailoring of phase composition and photoresponsive properties of Ti-containing nanocomposites from layered precursor. J. Phys. Chem. C 2008, 112, 4151–4158. (27) Chuang, Y. H.; Tzou, Y. M.; Wang, M. K.; Liu, C. H.; Chiang, P. N. Removal of 2-chlorophenol from aqueous solution by Mg/Al layered double hydroxide (LDH) and modified LDH. Ind. Eng. Chem. Res. 2008, 47, 3813–3819. (28) Kookana, R. S.; Rogers, S. L. Effect of pulp mill effluent disposal on soil. Rev. Environ. Contam. Toxicol. 1995, 142, 13–64. (29) Kan, A. T.; Fu, G. M.; Hunter, M. A.; Tomson, M. B. Irreversible adsorption of naphthalene and tetrachlorobiphenyl to lula and surrogate sediments. Environ. Sci. Technol. 1997, 31, 2176–2185. (30) Xu, Z. P.; Braterman, P. S. High affinity of dodecylbenzene sulfonate for layered double hydroxide and resulting morphological changes. J. Mater. Chem. 2003, 13, 268–273. (31) Bruna, F.; Pavlovic, I.; Celis, R.; Barriga, C.; Cornejo, J.; Ulibarri, M. A. Organohydrotalcites as novel supports for the slow release of the herbicide terbuthylazine. Appl. Clay Sci. 2008, 42, 194–200. (32) Moujahid, E. M.; Inacio, J.; Besse, J. P.; Leroux, F. Adsorption of styrene sulfonate vs. polystyrene sulfonate on layered double hydroxides. Microporous Mesoporous Mater. 2003, 57, 37–46. (33) Zhang, H.; Wen, X.; Wang, Y. X. Synthesis and characterization of sulfate and dodecylbenzenesulfonate intercalated zinc-iron layered double hydroxides by one-step coprecipitation rout. J. Solid State Chem. 2007, 180, 1636–1647. (34) You, Y.; Zhao, H. T.; Vance, G. F. Hybrid organic-inorganic derivatives of layered double hydroxide and dodecylbenzenesulfonate: Preparation and adsorption characteristics. J. Mater. Chem. 2002, 12, 907–912. (35) Xu, Z. P.; Zeng, H. C. Abrupt structural transformation in hydrotalcite-like compounds Mg1xAlx(OH)2(NO3)x 3 nH2O as a continuous function of nitrate anions. J. Phys. Chem. B 2001, 105, 1743–1749. (36) Chiou, C. T.; Peters, L. J.; Freed, V. H. Physical concept of soilwater equilibria for nonionic organic compounds. Science 1979, 206, 831–832. (37) Li, Z. H.; Bowman, R. S. Sorption of perchloroethylene by surfactant-modified zeolite as controlled by surfactant loading. Environ. Sci. Technol. 1998, 32, 2278–2282. (38) Wu, J. Y.; Harwell, J. H.; O’Rear, E. A. Two-dimensional reaction solvents: Surfactant bilayers in the formation of ultrathin films. Langmuir. 1987, 3, 531–537. (39) Boyd, S. A., Jaynes, W. F., Ross, B. S. Immobilization of organic contaminants by organo-clays: Application to soil restoration and hazardous waste contaminants. In Organic Substances and Sediments in Water, Vol. 1; Baker, R. A, Ed.; Lewis: Chelsea, MI, 1991; pp 181200. (40) Tseng, W. Y.; Lin, J. T.; Mou, C. Y.; Cheng, S.; Liu, S. B.; Chu, P. P.; Liu, H. W. Incorporation of C60 in layered double hydroxide. J. Am. Chem. Soc. 1996, 118, 4411–4418. (41) Amiri, F.; Rahman, M. M.; B€ornick, H.; Worch, E. Sorption behaviour of phenols on natural sandy aquifer material during flowthrough column experiments: The effect of pH. Acta Hydrochim. Hydrobiol. 2004, 32, 214–224. (42) Weber, W. J., Jr.; Huang, W. L.; Yu, H. Hysteresis in the sorption and desorption of hydrophobic organic contaminants by soils and sediments 2. Effects of soil organic matter heterogeneity. J. Contam. Hydrol. 1998, 31, 149–165.
ARTICLE
(43) Huang, W. L.; Weber, W. J., Jr. A distributed reactivity model for sorption by soils and sediments. 10. Relationships between desorption, hysteresis, and the chemical characteristics of organic domains. Environ. Sci. Technol. 1997, 31, 2562–2569. (44) Czaplicka, M.; Kaczmarczyk, B. Infrared study of chlorophenols and products of their photodegradation. Talanta 2006, 70, 940–949.
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