We report the mechanism study of pentachlorophenol adsorption to montmorillonite. This was done by (a) pentachlorophenol adsorprion to alkyl chain varied n-alkyltrimethylammonium montmorillonite and (b) pentachlorophenol adsorption to only interlayer-site exchanged n-alkyltrimethylammonium montmorillonite.
Montmorillonite is a 2:1 type layer, smectite group, dioctahedral clay and general approximate formula of montmorillonite is as follows;
$M^{n+}_{x/n}[Al_{4_0-x}Mg_x](Si_{8.0})O_{20}(OH)_4-yH_2O$
[octahedral](tetrahedral)site
5 series of the n-alkyltrimethylammonium montmorillonite(n = 1, 4, 8, 12, 16) were synthesized. To prepare not commercially available butyl-trimethylammonium salt and octyl-trimethylammonium salt, butyltrimethyl ammonium iodide and octyl trimethylammonium iodide were synthesized from its precursors. Use of Thermal Gravimetric Analysis and Differential Thermal Gravimetry were suggested and verified to distingush basal-surface site and interlayer site. Applying this TGA/DTG method, only interlayer site exchanged montmorillonite was synthesized with 40% cation exchange capacity of butyl-, octyl-, dodecyl-trimethyl ammonium cations. Every synthesized organo-montmorilonites were also characterized by powder X-ray diffraction method.
The adsorption of pentachlorphenol to organo montmorillonite shows anomalous effect as alkyl chain of ammonium ion increase. When alkyl chain increase from 1 to 4, adsorption ability was slightly increased. But alkyl chain increase to 8, a sudden decrease of adsorption ability was observed. As alkyl chain length increase further to 12 and 16, however, continuous increase of adsorption ability was observed. By contrast, with only inter layer site exchanged organomontmorillonite, there were continuous decrease of adsorption ability as alkyl chain length increases from 4 to 12. From this results, we suggest the model of hydrophobic interaction as follows. 1) Adsorption occurs primarily in interlayer space and which is decreased as alkyl chain of alkylammonium ions increase because alkyl chain itself fill the interlayer space 2) When alkyl chain length of alkylammonium ion has carbon number 8, there is no more interlaye space and no partitioning medium 3) As alkylchain length increase upto 12 or 16, hemimicelle is formed on the basal surface of montmorillonite and it behaves as partitioning medium to uptake hydrophobic material and there is little space in interlayer site to adsorb.
Understanding of hydrophobic interaction between organo-montmorillonite with hydrophobic organic pollutant is expected to apply for designing of advanced sorbent to remove toxic hydrophobic organic pollutant in industrial wastewater