Adsorption of p-Nitrophenol on Mono-, Di-, and Trialkyl Surfactant

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Adsorption of p-Nitrophenol on Mono-, Di-, and Trialkyl Surfactant-Intercalated Organoclays: A Comparative Study Qin Zhou,†,‡,§ Hongping He,*,†,‡ Ray L. Frost,*,‡ and Yunfei Xi‡,| Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China, Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland UniVersity of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia, Graduate UniVersity of Chinese Academy of Sciences, Beijing 100039, China, and Centre for EnVironmental Risk Assessment and Remediation, UniVersity of South Australia, Mawson Lakes, SA, 5095, Australia ReceiVed: December 4, 2006; In Final Form: March 4, 2007

Organoclays were obtained by the cationic exchange of mono-, di-, and trialkyl chain surfactants (hexadecyltrimethylammonium bromide, dimethyldioctadecylammonium bromide, methyltrioctadecylammonium bromide) for sodium ions in an aqueous solution with Na-montmorillonite (Na-Mt). The characterization of organoclays with and without adsorbed p-nitrophenol was determined by X-ray diffraction, transmission electron microscopy, and thermal analysis. Differences in the surfaces and in the interlayer of the mono-, di-, and trialkyl chain organoclays resulted in differences in the adsorption efficiency for p-nitrophenol with tri > di > mono . Na-Mt.

Introduction Smectites are extensively used in a wide range of applications as a result of their high cation exchange capacity, swelling capacity, high surface areas, and consequential strong adsorption capacities.1-5 Among the swelling clays, the most common dioctahedral smectite is montmorillonite, which has two siloxane tetrahedral sheets sandwiching an aluminum octahedral sheet. Because of an isomorphic substitution within the layers (for example, Al3+ replaced by Mg2+ or Fe2+ in the octahedral sheet; Si4+ replaced by Al3+ in the tetrahedral sheet), the clay layer is negatively charged, which is counterbalanced by the exchangeable cations such as Na+, Ca2+ in the interlayer. The hydration of inorganic cations on the exchange sites causes the clay mineral surfaces to be hydrophilic. Thus, natural clays are ineffective sorbents for organic compounds.6-8 However, such a difficulty can be overcome by ion exchange of the inorganic cations with organic cations. Organo-montmorillonites are synthesized by intercalating cationic surfactants such as quaternary ammonium compounds into the interlayer space through ion exchange.9-11 When using long-chain alkyl ammonium cations, hydrophobic partition medium within the clay interlayer can form and function analogously to a bulk organic phase. The interlayer height of clay before modification is relatively small, and the intergallery environment is hydrophilic. Intercalation of cationic surfactant not only changes the surface properties from hydrophilic to hydrophobic but also greatly increases the basal spacing of the layers. Such surface property changes affect the applications of the organoclay. The objective of this research is to use organoclays to adsorb p-nitrophenol (pnp) from an aqueous solution. We wish to use pnp as a test molecule to see if the organoclay will be effective in removing the pnp from water and to compare the adsorption †

Chinese Academy of Sciences. Queensland University of Technology. § Graduate University of Chinese Academy of Sciences. | University of South Australia. ‡

effect of the three different surfactants-organoclays. This study seeks to show the changes in surface properties of the organoclay with and without the adsorption of pnp. Experimental Methods Materials. Montmorillonite (Na0.053Ca0.176Mg0.1‚nH2O)[Al1.58Fe0.03Mg0.39][Si3.77Al0.23]O10(OH)2 used was primarily Ca-Mt from Neimeng, China. The montmorillonite was cation exchanged with sodium ions by repeated reaction with sodium carbonate. Its cation exchange capacity (CEC) is 90.8 mequiv/ 100 g. The pnp and surfactants (HDTMAB, DDOAB, MTOAB) used were of analytical grade chemical reagents. The aqueous solubility of p-nitrophenol is 1.6 × 104 mg/L at 25 °C. The surfactants used were hexadecyltrimethylammonium bromide labeled HDTMAB [CH3(CH2)15] NBr (CH3)3, dimethyldioctadecylammonium bromide labeled DDOAB [CH3(CH2)17]2NBr(CH3)2, and methyltrioctadecylammonium bromide labeled MTOAB [CH3(CH2)17]3NBr(CH3). Preparation of the Organoclay. The synthesis of organoclay hybrids was undertaken by the following procedure. Pure CaMt was added into a 0.2 M Na2CO3 solution and was stirred for 3 h with 800 rpm. Drops of HCl were then added into the suspension to dissolve the CO32-. The suspension was washed several times with deionized water until chloride free and was dried at 110 °C. Such treated montmorillonite is designated as Na-Mt. The clarifying surfactant solution was obtained when certain amounts of surfactants were added into hot distilled water. Then special amounts of Na-Mt were added into the abovementioned solution and the mixtures were stirred slightly to avoid the yield of spume in an 80 °C water bath for 2 h. The water/Na-Mt mass ratio is 10. Then the suspension was subsequently washed with distilled water four times. The moist solid material was dried at 60 °C and was ground with a mortar. The obtained cationic surfactant-exchanged montmorillonites were labeled as 0.5CEC-S, 0.7CEC-S, 1.5CEC-S, 2.5CEC-S, 0.5CEC-D, 0.7CEC-D, 1.5CEC-D, 2.5CEC-D, and 1.5CEC-T,

10.1021/jp0683364 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/27/2007

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Zhou et al.

respectively. The symbols S, D, and T signify HDTMAB, DDOAB, and MTOAB, respectively. Adsorption of the p-Nitrophenol on the Organoclay. A total of 0.2 g of different type of montmorillonites were combined with 30 mL of different concentrations of pnitrophenol solution whose initial pH value is about 5.0 in 50 mL Erlenmeyer flasks with glass caps. The flasks were shaken for 6 h at 25 °C on a shaker at 150 rpm. After being centrifuged at 3500 rpm for 10 min, the pnp concentration in the aqueous phase was determined by a UV-260 spectrophotometer at 317 nm, the detection limits being 0.05 mg/L. The pnp removal rate on the organoclays was calculated by the following equation: R ) (C0 - Ct)/C0 × 100% where R is the pnp removal rate, C0 is the initial concentration, and Ct is the equilibrium concentration. The losses of the pnp by both photochemical decomposition and volatilization were found to be negligible during adsorption.12 Characterization Methods Thermogravimetric Analysis (TGA). Thermogravimetric analyses of the surfactant-modified montmorillonite hybrids were obtained using a TA Instruments Inc. Q500 high-resolution TGA operating at ramp 5 °C/min with resolution 6.0 °C from room temperature to 1000 °C in a high-purity flowing nitrogen atmosphere (60 cm3/min). Approximately 50 mg of finely ground sample was heated in an open platinum crucible. X-ray Diffraction (XRD). The Na-Mt and organoclays were pressed in stainless steel sample holders. XRD patterns were recorded using Cu KR radiation (n ) 1.5418 Å) on a Philips PANalytical X′ Pert PRO diffractometer operating at 40 kV and 40 mA with 0.25° divergence slit, 0.5° antiscattter slit, between 1.5 and 20° (2θ) at a step size of 0.0167°. For XRD at low-angle section, it was between 1 and 5° (2θ) at a step size of 0.0167° with variable divergence slit and 0.125° antiscatter slit. Results and Discussion Powder XRD Analysis. The XRD patterns of Na-Mt with and without adsorbed pnp, 0.5 single alkyl organoclay with and without pnp, 0.5 double alkyl organoclay with and without pnp, and 0.6 triple alkyl organoclay with and without pnp are shown in Figure 1. The XRD patterns for the same materials at 0.7CEC and 1.5CEC are shown in Figures 2 and 3. Figure 4 shows the variation of the d(001) spacing with CEC concentration and surfactant with and without pnp. Upon intercalation of the surfactant molecules into the montmorillonite, the diffraction peaks are often sharper than those of the original clay, thus indicating a better more well defined arrangement of the surfactant molecules within the clay structure. It is possible that where intense d(001) peaks are observed, secondary peaks of much lower intensity at about half the d(001) spacing are observed. The basal spacing for Na-Mt is 1.24 nm. This basal spacing increases to 1.49 nm upon adsorption of the pnp. The difference between the basal spacing and thickness of a single clay layer (0.96 mm) is 0.53 nm, which corresponds to the size of the pnp molecule. This expansion of the clay layers to 1.49 nm upon adsorption of the pnp provides evidence that the pnp is adsorbed between the clay layers. When the organoclay is synthesized with the single alkyl chain surfactant (HDTMAB) and the double alkyl chain surfactant (DDOAB), the basal spacing of 1.49 nm is preserved. Even after adsorption of the pnp on the 0.5CEC-S organoclay, the expansion remains at 1.49 nm. The XRD pattern of the 0.5CEC-D organoclay does show additional expansions

Figure 1. XRD patterns of montmorillonite with and without adsorbed pnp, 0.5CEC-single alkyl chain with and without adsorbed pnp, 0.5CEC-double alkyl chain with and without adsorbed pnp, and 0.6CEC-triple alkyl chain with and without adsorbed pnp.

at 1.94 and 3.35 nm. These expansions are because of the arrangement of the surfactant molecules within the clay layers. This variation is illustrated in Figure 4. For the 3.35 nm expansion, it is proposed that the DDOAB molecules are stacking within the clay layers. For the 0.5CEC double alkyl organoclay upon adsorption of pnp, the basal spacings of 1.83 and 1.47 nm are observed. It is noted that the large d(001) spacing of 3.35 nm is no longer observed. This suggests that the pnp molecules are affecting the stacking arrangements of the surfactant molecules within the clay interlayer. A similar phenomenon is observed for the trialkyl organoclay. The 0.6CEC-T organoclay shows basal spacings of 1.18, 1.69, and 3.71 nm. The slight increase in spacing between the dialkyl and trialkyl organoclays (3.71 compared with 3.35 nm) is attributed to the increase in size of the surfactant molecule. Upon adsorption of the pnp on the 0.6CEC-T organoclay, basal spacings of 1.61 and 1.27 nm are observed. Again the adsorption of the pnp affects the arrangement of the surfactant molecules in the organoclay interlayer. The 0.7CEC-S organoclay shows an increase in d(001) spacing from 1.49 to 1.78 nm, which upon adsorption of the pnp decreases to 1.49 nm (Figure 2). The 0.7CEC-D organoclay has two basal spacings of 1.44 and 3.13 nm, which upon adsorption of the pnp decreases to 1.86 and 1.43 nm. For the

Adsorption of pnp on Surfactant-Intercalated Organoclays

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1.0CEC-T organoclay, three spacings at 1.23, 1.83, and 3.78 nm are observed. Upon adsorption of the pnp, the values of 1.00, 1.33, and 1.86 nm are obtained. For both the 0.7CEC-D and the 1.0CEC-T organoclays, the adsorption of the pnp causes a disruption of the arrangement of the surfactant molecules. This change in d-spacings may be observed in Figure 4. It is proposed that the pnp molecules bond to the siloxane surfaces of the montmorillonite thus preventing the stacking of the surfactant molecules. The adsorption behavior of 1.5CEC organoclays appears different to that at concentrations less than 1.0CEC. The clay basal spacing is 1.24 nm, which expands to 1.49 nm upon adsorption of the pnp. Upon formation of the 1.5CEC-S organoclay, d(001) spacings of 1.71 and 2.71 nm are obtained. Where upon adsorption of the pnp, spacings of 1.06, 1.56, and 3.11 nm are obtained. It is apparent that the adsorption of the pnp has caused rearrangement of the surfactant molecules. For the 1.5CEC-D organoclay, basal spacings of 1.18, 1.80, and 3.59 nm are observed, which upon adsorption of the pnp cause further expansion of these basal spacings to 1.34, 1.99, and 3.91 nm. The increase in the basal spacings is attributed to the adsorption of the pnp molecules on the surfactant molecules. The 1.5CEC-T organoclay has d(001) spacings of 1.32, 1.81, 2.61, and 4.96 nm. Upon adsorption of the pnp molecules,

spacings of 1.35, 1.82, and 2.55 nm are observed. The large d-spacing of 4.96 nm is no longer observed. Transmisson Electron Microscopy (TEM). The TEM images of selected organoclays with and without adsorbed pnp are shown in Figure 5. The image of 1.5CEC-S organoclay shows spacings of 1.69 and 2.73 nm in good agreement with the values obtained from XRD where values of 1.71 and 2.71 nm were obtained. Spacings of 1.08, 1.61, and 3.13 nm were observed for 1.5CEC-S-4000. These values may be compared with the values of 1.06, 1.56, and 3.11 nm obtained by XRD. The adsorption of pnp results in smaller basal spacings. This suggests the pnp alters the structural arrangements of the surfactant molecules in the clay interlayer. For the 1.5CEC-D-4000 (Figure 5), spacings of 1.02, 1.36, 2.0, and 3.96 nm are found. These values may be compared with the basal spacings obtained by XRD of 1.00, 1.34, 1.99, and 3.91 nm. Excellent agreement is obtained between the values obtained by the two techniques. The basal spacings for 1.5CECT-4000 are 1.07, 1.82, 1.35, and 2.58 nm. These values are in excellent agreement with the values of 2.55, 1.82, 1.35, and 1.08 nm. Thermogravimetric Analysis. Valuable information can be obtained from the thermal analysis patterns of the organoclays. The thermal decomposition temperatures and stability of the


Zhou et al.

Figure 4. Variation of the d(001) spacing with surfactant and surfactant loading.

Figure 5. TEM images of 1.5CEC-S, 1.5CEC-S-4000, 1.5CEC-D4000, and 1.5CEC-T-4000.

organoclays and the organoclays with adsorbed pnp can be obtained. The differential thermogravimetric analysis (DTG) patterns of Na-Mt with and without adsorbed pnp, 0.5 single alkyl organoclay with and without pnp, 0.5 double alkyl organoclay with and without pnp, and 0.6 triple alkyl organoclay with and without pnp are shown in Figure 6. The DTG patterns

for the same materials at 0.7CEC and 1.5CEC concentrations are shown in Figures 7 and 8. The Na-Mt shows two thermal decomposition steps at 485 and 617 °C. The latter temperature is ascribed to the dehydroxylation of the clay and the former is attributed to the loss of OH units from the ends of the clay layers. Upon adsorption of pnp, four thermal decomposition steps are observed at 179, 285, 625, and 797 °C. p-Nitrophenol sublimes at 131 °C. The 285 °C decomposition step is attributed to pnp intercalated with the surfactant molecules in the clay interlayer. p-Nitrophenol sublimes to a vapor at 131 °C. When adsorbed on the Na-Mt, 3.85% is lost at 179 °C and 2.0% mass loss is found at 285 °C. This mass loss is attributed to some pnp being intercalated between the clay layers. The dehydroxylation of the montmorillonite now occurs at the higher temperature of 625 °C. This provides evidence for the chemical reaction of the pnp and the siloxane surfaces of the montmorillonite. It is noted that a thermal decomposition at 797 °C is observed, but it is not known what is the reason for this higher temperature decomposition. The HDTMAB (CH3(CH2)15(CH3)3N+Br-) decomposes or is combusted at 202 °C. For the 0.5CEC-S, a DTG peak is observed at 85 °C attributed to the loss of adsorbed water. A second peak is observed at 117 °C and is assigned to the loss of bound water. Two DTG peaks are observed at 317 and 365 °C and are attributed to the combustion and to the loss of the surfactant molecule from between the clay sheets. It is proposed that the reason for the two peaks is due to different structural arrangements of the surfactant molecules in the interlayer gallery. The DTG peak for 0.5CEC-S at 602 °C is as before assigned to the dehydroxylation of the montmorillonite. It is noted that there is a significant shift to lower temperatures of the dehydroxylation of the organoclay. This shift is attributed to the penetration of the methyl groups of the surfactant into the siloxane layer. This may provide a mechanism for the removal of the OH units as water vapor. The thermal decomposition of 0.5CEC-S with adsorbed pnp is very different to that of the organoclay. Two thermal decomposition steps not observed in the decomposition of the 0.5CEC-S are found at 192 and 278 °C with mass losses of 4.45% and 4.1%. These mass loss steps are ascribed to pnp adsorbed on the organoclay. The reason for two mass loss steps is attributed to the desorption


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Figure 6. DTG patterns of montmorillonite with and without adsorbed pnp, 0.5CEC-single alkyl chain, 0.5CEC-double alkyl chain, 0.6CECtriple alkyl chain with adsorbed pnp, pnp, HDTMAB, DDOAB, and MTOAB.


of pnp and the removal of pnp from between the clay layers. The mass loss at 368 °C of 2.66% is the loss of the surfactant. Two additional mass loss steps at 417 and 466 °C are observed for the 0.5CEC-S with adsorbed pnp. The observation of these two mass loss steps confirms changes in the structure of the surfactant within the clay layers, and the pnp is reacting with the clay surfaces. The DDOAB shows thermal decomposition steps at 168, 245, and 305 °C attributed to combustion of the surfactant molecules (Figure 6). The 0.5CEC organoclay differential thermal analysis pattern shows peaks at 590, 389, and 300 °C. A slight decrease in the dehydroxylation of the clay is observed (590 compared with 617 °C). The peaks at 389 and 300 °C are higher than is observed for the surfactant on its own. This suggests that the surfactant molecule is bonded to the clay surfaces and more heat is required to remove the surfactant molecules from the clay surfaces. The DTG pattern for the 0.5CEC-D-4000 (organoclay with adsorbed pnp) is quite different to that of the 0.5CEC-D organoclay. Three decomposition temperatures are observed at 189, 268, and 382 °C. Interestingly, the dehydroxylation peak at 590 °C is not found. This suggests the reaction of the clay with pnp occurs through the OH units of the montmorillonite and the dehydroxylation temperature is reduced to 382 °C. The two peaks at 286 and 189 °C are attributed to

the combustion of the surfactant and as no DTG peaks attributable to the pnp are observed, it is proposed that the pnp is removed simultaneously with the surfactant. The 0.6CEC-T organoclay shows decomposition steps at 306, 345, and 585 °C. The first two steps are attributed to the loss of surfactant from within the interlayer. The fact that these temperatures are higher than that for the surfactant show the surfactant molecules are bonded to the montmorillonite siloxane layer. This proposal is supported by the decrease in the dehydroxylation temperature from 617 to 585 °C. It is also noted that the thermal decomposition of the end groups are no longer observed. This indicates that chemical bonding of the surfactant molecules to the OH units at the end of the clay layers occurs. Upon adsorption of the pnp, thermal decomposition steps are observed at 196, 289, and 345 °C (Figure 6, 0.6CEC-T-4000). Again the adsorption of pnp has affected the stability of the surfactant molecules and lowered the temperature of the dehydroxylation of the montmorillonite. A similar DTG pattern is observed for 0.7CEC-S-4000 as for 0.5CEC-S-4000. Two thermal decomposition steps are observed at 188 and 268 °C. These are attributed to the loss of adsorbed pnp. The thermal decomposition step at 370 °C is the same as for 0.5CEC-S. Two additional thermal decomposition steps are observed at 408 and 455 °C and are attributed to the



loss of surfactant molecules and pnp. It is proposed that the pnp causes the structural rearrangement of the surfactant molecules with the clay layers. The thermal decomposition steps for the 0.7CEC-D organoclay are similar to that for the 0.5CEC-D organoclay with DTG peaks observed at 293, 396, and 590 °C. The latter is due to the montmorillonite and the first two steps to the loss of the surfactant. For the 0.7CEC-D-4000 organoclay (Figure 7) with adsorbed pnp, DTG peaks are observed at 193, 283, and 385 °C. No peak is found at 590 °C. This suggests that the pnp results in the lowering of the clay dehydroxylation temperature. The reaction of pnp with the siloxane surface provides a mechanism for the montmorillonite dehydroxylation. The surfactant MTOAB thermally decomposes at 163, 274, and 323 °C. The montmorillonite shows two decomposition steps at 485 and 617 °C, which are attributed to the dehydroxylation of end OH and inner OH units. The 1.0CEC-T organoclay shows decomposition steps at 294, 342, and 587 °C. The first two steps are attributed to the loss of surfactant from within the interlayer. The fact that these temperatures are higher than that for the surfactant show the surfactant molecules are bonded to the montmorillonite siloxane layer. This proposal is supported by the decrease in the dehydroxylation temperature from 617 to 587 °C. It is also noted that the thermal decomposition of the

Zhou et al. end groups is no longer observed. This indicates that chemical bonding of the surfactant molecules to the OH units at the end of the clay layers occurs. For the 1.0CEC-T-4000, decomposition steps at 172, 287, and 348 °C are observed. It is noted that the thermal decomposition steps of the surfactant shift from 163, 274, and 323 °C to 287 and 348 °C. When the surfactant loading is increased above 1.0CEC, the thermal decomposition patterns resemble that of the organoclay without pnp adsorption (Figure 8). The 1.5CEC-S-4000 with adsorbed pnp shows three decomposition steps at 187, 222, and 227 °C. The dehydroxylation of the organoclay occurs at 387 °C. In Figure 8, the higher temperature steps at around 607 °C are not shown in the 1.5CEC-S DTG curves. The reason for this is that these DTG peaks are hidden in the background. The intense DTG peaks at 222 °C (1.5CEC) are so large that smaller peaks are buried in the background. The temperatures for the loss of pnp are significantly greater than that of pure pnp. This shows that the pnp is strongly bonded to the organoclay. For the 1.5CEC-D organoclay without adsorbed pnp, DTG peaks are observed at 229, 341, 392, and 590 °C. The first three peaks are ascribed to the combustion of the surfactant and the latter to the dehydroxylation of the montmorillonite. It is noted that the three temperatures (229, 341, 392 °C) are significantly higher than for the DTG peaks of the surfactant (168, 245, and 305 °C). This difference in temperatures is attributed to the bonding of the surfactant molecules to the montmorillonite surfaces. Heating the organoclay to higher temperatures is required before the surfactant molecules are removed. For the 1.5CEC-D-4000, thermal decomposition steps are observed at 195, 232, 288, 328, and 409 °C. The DTG peak at 195 °C is ascribed to loss of pnp through sublimation. The next three thermal decomposition steps are ascribed to the loss of surfactant, which occurs at significantly higher temperatures than the pure surfactant. The thermal decomposition step at 409 °C is attributed to the dehydroxylation of the montmorillonitic clay. At the 1.5CEC-T organoclay, thermal decomposition steps of 182, 284, 302, 342, 415, and 593 °C are found. The first three decomposition steps are assigned to the loss of MTOAB, which is adsorbed on the external surfaces of the montmorillonite clay. The decomposition steps at 342 and 415 °C are assigned to the intercalated surfactant within the montmorillonite layers. The decomposition step at 593 °C is due to the dehydroxylation of the montmorillonite clay. Upon adsorption of the pnp on the 1.5CEC-T-4000 organoclay, decomposition steps are observed at 156, 191, 248, 288, and 342 °C. The first decomposition step may be attributed to the sublimation of the p-nitrophenol. The next three decomposition steps are ascribed to the loss of the surfactant adsorbed on the external surfaces of the organoclay. The decomposition step at 342 °C is assigned to the loss of the intercalated surfactant. It is noted that no dehydroxylation temperature of 593 °C is observed. It is concluded that the bonding of the pnp to the montmorillonite lowers this dehydroxylation temperature. Thus, it is proposed that the pnp penetrates the siloxane layer of the clay and bonds through the ditrigonal space of the siloxane hexagonal units to the inner OH units. Adsorption of p-Nitrophenol and Its Removal by Organoclay. The purpose of synthesizing organoclays is to test their usefulness at the adsorption of pnp from water. Figure 9 shows the percent removal of pnp from water as a function of the surfactant and its concentration. The Na-Mt does remove the pnp to a 5.9% level, which although significant is ineffective.


J. Phys. Chem. C, Vol. 111, No. 20, 2007 7493 (e) The effective removal of pnp from water would require multiple adsorptions. Conclusions Water purification is of extreme importance in many parts of the world, including Australia and China. Many of the worlds waterways and water sources are polluted or contaminated with a range of chemicals including pesticides and herbicides. In this work, we have used pnp as a test chemical to design and test organoclays with one, two, and three alkyl chains for the effective removal of pnp from potable water. This work has shown that the pnp intercalates the organoclay and displaces the surfactant molecules and rearranges the structural arrangement of the surfactant molecules within the organoclay interlayer. It was found that the effective removal of pnp from an aqueous medium is a function of the surfactant molecule and its concentration. The greater the CEC value and the greater the number of alkyl chains in the surfactant molecule, the greater the percentage of the pnp that is removed. Acknowledgment. This work was funded by Chinese Academy of Sciences (Grant No. Kzcx2-yw-112) and Natural Science Foundation of Guangdong Province (Grant No. 030471 and 05103410). The Inorganic Materials Research Program, Queensland University of Technology, is gratefully acknowledged for infrastructural support. References and Notes

Figure 9. Percent removal of pnp as a function of surfactant and surfactant loading.

Figure 9 shows (a) The percent removal of pnp is a function of the CEC concentration. (b) There is little difference in the percent removal with the number of alkyl groups of the surfactant. (c) This difference increases at the higher CEC levels. (d) The trialkyl surfactant is slightly more effective at removing the pnp.

(1) Alther, G. R. Contam. Soils 2003, 8, 189-200. (2) Alther, G. R. Spec. Publ. - R. Soc. Chem. 2000, 259, 277-288. (3) Breen, C.; Watson, R.; Madejova, J.; Komadel, P.; Klapyta, Z. Langmuir 1997, 13, 6473-6479. (4) Dentel, S. K.; Bottero, J. Y.; Khatib, K.; Demougeot, H.; Duguet, J. P.; Anselme, C. Water Res. 1995, 29, 1273-80. (5) He, H. P.; Guo, J. G.; Xie, X. D.; Peng, J. L. EnViron. Int. 2001, 26, 347-352. (6) Chaiko, D. Preparation of Organoclays with Improved Dispersibility from Smectites and Kaolin Clays by Coating Clays with Water-Soluble polymer. (University of Chicago, U.S.A.). Application: WO WO, 2002; pp 24. (7) Kim, J. H.; Shin, W. S.; Kim, Y. H.; Choi, S. J.; Jeon, Y. W.; Song, D. I. Water Sci. Technol. 2003, 47, 59-64. (8) Prost, R.; Yaron, B. Soil Sci. 2001, 166, 880-895. (9) He, H.; Frost, R. L.; Deng, F.; Zhu, J.; Wen, X.; Yuan, P. Clays Clay Miner. 2004, 52, 350-356. (10) Mortland, M. M.; Shaobai, S.; Boyd, S. A. Clays Clay Miner. 1986, 34, 581-5. (11) Soule, N. M.; Burns, S. E. J. Geotech. GeoenViron. Eng. 2001, 127, 363-370. (12) Zhu, L.; Chen, B. EnViron. Sci. Technol. 2000, 34, 2997-3002.

Adsorption of p-Nitrophenol on Mono-, Di-, and Trialkyl Surfactant

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