Optimization of hexadecylpyridinium-modified montmorillonite for removal of perchlorate based on adsorption mechanisms

Adsorption of perchlorate (ClO₄^-) onto hexadecylpyridinium-modified montmorillonite (OMt) was investigated by in situ and ex situ methods. For the in situ method, ClO₄^- was initially bound by hexadecylpyridinium (HDPy⁺) in the form of HDPy-ClO₄ which together with the excess HDPy⁺ were simultaneously captured by Mt. The ex situ strategy was performed by adding synthesized OMt into ClO₄^--bearing solution, where conventionally dried OMt (II-OMt) and innovative OMt (I-OMt) without drying and washing procedure were compared as adsorbents. The adsorption capacity of ClO₄^- and amount of HDPy released into equilibrium solution were evaluated, where surfactant release was rarely addressed in previous publications. I-OMt showed relatively high adsorption capacity of ClO₄^- and negligible release of HDPy. The lower adsorption capacity of ClO₄^- onto II-OMt was caused by the washing procedure which removed the unstably anchored HDPy. Inhibition of HDPy release of I-OMt was explained by reconfiguration of HDPy after ClO₄^- adsorption as supported by results of X-ray diffraction and attenuated total reflection Fourier transform infrared spectroscopy. In addition, for anion adsorption onto conventionally synthesized OMt, the difference between the amounts of released counter ion and that of entrapped target anion is generally ignored. Such phenomenon was critically considered in this study, and the results showed that the difference between adsorbed ClO₄^- and released Cl^- decreased with an increase of HDPy loading and was more significant in I-OMt. This is rationalized by the desorption of HDPy⁺ and formation of HDPy-ClO₄ according to the results of ¹³C nuclear magnetic resonance spectra and the adsorption energy evaluated by the Dubinin-Radushkevich model.