IR spectra in the studies of anion sorption on natural sorbents

Abstract

This work presents the results of FT-IR spectroscopic studies of anions – chromates, phosphates, arsenates, sulfates and nitrates – sorbed from aqueous solutions (different concentrations of anions) on natural sorbents. The sorption has been conducted on zeolite (clinoptilolite) and smectite (mixtures of clay minerals containing mainly montmorillonite and kaolinite) which have been separated from natural Polish deposit. The Na-forms of sorbents were exchanged with hexadecyltrimethylammonium cations (HDTMA +). Cation exchange capacities (CEC) of clinoptilolite and smectite were measured. Their values are 23 mmol/100 g and 41 mmol/100 g respectively. The used initial inputs of HDTMA correspond to 100% (1.0) and 200% (2.0) CEC of the minerals. Organo-modified sorbents were subsequently used to immobilization mentioned anions. It was proven that those anions’ sorption causes changes in IR spectra of the HDTMA-zeolite and smectite. These alterations are dependent on the kind of anions that were sorbed. In both cases, variations are due to bands corresponding to the characteristic Si–O(Si,Al) vibrations (occurring in alumino- and silicooxygen tetrahedra building spatial framework of zeolite or layer structure of smectite), OH group vibrations and alkylammonium surfactant vibrations have been observed. Systematic changes in the spectra connected with the anion concentration in the initial solution have been revealed. The amounts of sorbed CrO 4 2 - , AsO 4 3 - , PO 4 3 - , SO 4 2 - , and NO 3 - ions were calculated from the difference between their concentrations in solutions before (initial concentration) and after (equilibrium concentration) sorption experiments. Concentrations of chromates were determined by colorimetry using the biphenylocarbazide method. For phosphates and arsenates molybdenum blue method, turbidimetric method for sulfates and hydrazine reduction method for nitrates was used.