Influence of ageing of milled clay and its composite with TiO2 on the heavy metal adsorption characteristics

The aim of the present study was to investigate the effect of ageing of mechanochemically synthesized clay and its TiO2 composite on the simultaneous removal of Pb(II), Cd(II), Cu(II) and Zn(II) ions from acidic aqueous solutions. The effect of different ageing times on sorption behavior of 1, 2, 10 and 19 h milled clays, as well as the clay composite with 20 wt% of amorphous TiO2 (TiO2,a), was investigated. Ageing of the milled clays has stronger influence on the removal of Zn(II) and Cd(II) than on the removal of Pb(II) and Cu(1) ions. Ageing is particularly pronounced for the samples milled for 1, 2 and 10 h (which lost enhanced sorption properties after 3 months of ageing), and less pronounced for the samples milled longer period of time, 19 h (which retained sorption properties until 6 months). After these periods of time sorption capacities became nearly the same as the sorption capacities of the raw (unmilled) clay. The different responses on the ageing process of milled clays and composite are a consequence of microstructural changes such as recrystallization of montmorillonite phase and decrease in particle sizes. Slower ageing of composite compare to the milled clays can be related to the stabilization effect of TiO2 particles which are dispersed in the clay matrix thus preventing recrystallization of the sample particles. Only a slight tendency towards the formation of agglomerations was noticed after 12 months of ageing. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved