Use of organo-montmorillonite for the nitrate retention in water: influence of alkyl length of loaded surfactants

Abstract

The removal of NO₃− from water was performed and compared among four organo-montmorillonites (OMt) with quaternary amine surfactants of different chain lengths [octadecyl trimethyl-(ODTMA), hexadecyl trimethyl-(HDTMA), tetradecyl trimethyl-(TDTMA) and dodecyl trimethyl-ammonium (DDTMA)] and loadings [100%, 200%, 400% of the Mt cation exchange capacity (CEC)]. The maximum adsorption of NO₃− was attained by the long chain length surfactants (64 and 100 mg NO₃− ions per gram of OMt, for HDTMA and ODTMA, respectively) with initial loading of 400% with respect to the CEC of Mt. The short chain length surfactants (DDTMA and TDTMA) did not show adsorption of NO₃− except for TDTMA 400 sample. The NO₃− adsorption produced a slight expansion of the interlayer thickness of the OMt samples loaded with long-chain surfactants that was assigned to the entrance of NO₃− by a synergic effect with these surfactants. The decrease of the negative zeta potential, found for OMt samples relative to that of Mt sample, was attributed to the surfactant chain length rather than to the actual surfactant loaded amounts, which allowed attaining positive zeta potential values for the OMt loaded with long-chain surfactants. The NO₃− adsorption on OMt samples caused a slight decrease in the positive zeta potential values, reflecting the external surface coating by NO₃−. For long-chain molecules, this behaviour also indicated the formation of greater interactions such as ion pairs between the positive polar group of the surfactant and NO₃− ions.