ADSORPTION OF TOXIC BIVALENT CATIONS Pb(II) AND Cd(II) ONTO HYBRID GEOPOLYMERS FROM ENVIRONMENTAL SAMPLES

1. Introduction Geopolymers are amorphous aluminosilicates characterized by a structure like tectosilicates, obtained through a polycondensation reaction of an aluminosilicate material, such as metakaolin as well as wastes deriving from thermochemical processes as ashes, in a strongly alkaline solution. By adding a silicone oligomeric resin through a crosslink reaction approach, a hybrid material with good mechanical properties as well as high fire resistance is obtained. This material has been used to remove from soils and water contaminated2 metals as Pb(II), Cd(II), extremely dangerous to public health and the environment due to bioaccumulation. 2. Results and Discussion In the present work, the adsorption of toxic metals by using a hybrid geopolymer, obtained by co-reticulation reaction of metakaolin based geopolymer and polydimethylsiloxane1 (10% by weight) at 25 °C in NaClO4 as ionic medium has been studied. By potentiometric measurements it has been deduced that the solid presents superficial acid-base properties, (due to silanol and/or aluminolic groups) with the following surface acid constants : >X− + H+ ⇆ >XH log KX = 3.1 ± 0.1 >TOH + H+ ⇆ >TOH2+ log KB = 5.2 ± 0.2 >TOH ⇆ >TO− + H+ log KA = −10.1 ± 0.2 The solid shows a point of zero charge (PZC) of 6.9 ± 0.2. 3. Conclusions Kinetic measurements show that adsorption process follows the pseudo-second order kinetic model. From the adsorption isotherms the data collected are compatible with Langmuir's model. By the percentage of cadmium ions adsorbed on the solid sites involved, it is observed that for pH < 5.0 in 0.1 M NaClO4 are involved the sites with fixed charge (X), as the pH increases, the acid−base sites are involved.