Changes in the surfaces of adsorbed para-nitrophenol on HDTMA organoclay—The XRD and TG study
Graphical abstract
Surfaces of a sodium montmorillonite from Neimeng, China were modified through intercalation with the cationic surfactant hexadecyltrimethylammonium bromide labeled HDTMAB (CH3(CH2)15(CH3)3N+Br−). Different configurations of surfactants within montmorillonite interlayer are proposed based on d(001) basal spacings. Changes in these configurations are enforced when para-nitrophenol is adsorbed.
Introduction
Smectites are widely 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], [2], [3], [4], [5]. Among the swelling clays, the most common dioctahedral smectite is montmorillonite, which has two siloxane tetrahedral sheets sandwiching an aluminum octahedral sheet. Due to 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], [7], [8]. However such a difficulty can be overcome by ion exchange of the inorganic cations with organic cations.
Organomontmorillonites are synthesized by intercalating cationic surfactants such as quaternary ammonium compounds into the interlayer space through ion exchange [9], [10], [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 then 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 effect the applications of the organoclay. In particular the hydrophobic nature of the organoclay implies the material can be used as a filter material for water purification.
The objective of this research is to use organoclays to adsorb para-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. This study seeks to show the changes in surface properties of the organoclay with and without the adsorption of pnp.
Section snippets
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 meq/100 g. The para-nitrophenol and HDTMAB used were of analytical grade chemical reagents. The aqueous solubility of para-nitrophenol is . The surfactant used was hexadecyltrimethylammonium bromide labeled
Powder X-ray diffraction analysis
With the cation exchange of the sodium ion for the cationic surfactant, expansion of the montmorillonite layers occurs. This expansion is readily measured by X-ray diffraction. Fig. 1a shows the XRD patterns of montmorillonite and surfactant intercalated montmorillonite hybrids and at different surfactant concentrations (0.5, 0.7, 1.5 and 2.5CEC). Fig. 1b shows the XRD patterns of montmorillonite and surfactant intercalated montmorillonite hybrids with adsorbed para-nitrophenol and at different
Conclusions
Water purification is of extreme importance in many parts of the world, including Australia and China. Many of the worlds water ways and water sources are polluted or contaminated with a range of chemicals including pesticides and herbicides. In this work, we have used para-nitrophenol as a test chemical to design and test an organoclay for the removal of pnp from an aqueous medium. This work has shown that the pnp intercalates the organoclay and displaces the surfactant molecules or rearranges
Acknowledgements
This work was funded by National Natural Science Foundation of China (Grant No. 40372029) and Natural Science Foundation of Guangdong Province (Grants No. 030471 and 05103410). The Inorganic Materials Research Program, Queensland University of Technology, is gratefully acknowledged for infra-structural support.
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