Matériaux composites inorgano-organiques à base de smectite obtenus par co-intercalation de la thiouree et des ions cetyltrimethylammonium (CTA+) : synthèse, caractérisations et applications électroanalytiques

Abstract

A Cameroonian smectite clay collected from the Sabga locality (North West of Cameroon) was first treated [Sa(Na)]and then modified by co-intercalation of cetyltrimethylammonium ions (CTA+ ) and thiourea in one step synthesis approach, aiming at improving its capacity to adsorb heavy metals ions. The organoclay obtained after modification was denoted by Sa(CTAx,T) where x, the fraction of the cation exchange capacity (CEC) of Sa(Na), which corresponding to the number of moles of CTA + used during the synthesis and T, the number of mole of thiourea. The amount of thiourea T was held constant at 7.5 CEC during synthesis, while the quantity of CTA+ ions x was varied (0; 0.25, 0.5; 0.75 and 1 CEC). The obtained organoclays were firstly characterized using several techniques such as XRD, BET, XPS, EDX and UV analyses, elemental analysis and electrochemical analysis and then, to modify the surface of a glassy carbon electrode (GCE). The modified electrode was exploited for the detection either Pb(II) and Cd(II) ions in trace amount when organoclay film Sa(CTAx ,T) was used or electrochemical detection an organic micropollutant, paranitrophenol (PNP) when organoclay film Sa(CTAx ), obtained by intercalation of CTA+ ion alone was used. The results showed that the modification was successfully made. The intercalation of organic compounds lead to an expansion of the basal spacing of the Sa(Na) clay, a decrease of its specific surface area and volume of micropore which an increase of the amount of CTA+ ions inserted in interfoliar space. The characterization by cyclic voltammetry, carried to study the electrochemical behavior of [Fe(CN)6 ]3- and [Ru(NH3 ) 6 ]3+ ions complexes on organoclay electrode indicated that, the surface charge of modified clays of type Sa(CTAx ,T) depends on the pH of assay medium. Modified clays of type Sa(CTAx ) showed permanent overall positive surface charge which increases with the amount of CTA+ inserted. The GCE/Sa(CTA0.25,T) electrode is likely most sensitive for detection of Pb2+ ions alone with current peak 8 times more intense than the one recorded on GCE/Sa(Na). After optimization of some parameters related to the detection and accumulation steps, a calibration line was plotted between 1x10-8 and 1x10 -7 M for Pb2+ and between 1x10-7 and 1x10 -6 M for the Cd 2+ . Based on the background noise signal (SN/3) ratio the estimated limit of detection for Pb(II) was 0.029 and 0.42 nM individual and simultaneous detection respectively and 1.27 nM for Cd(II) is simultaneous detection. Considering the strong affinity of Sa(CTA0.25 , T) sensor with Pb2+ ions, a new and more reliable method in determining the concentration of Cd 2+ ions in a medium also containing Pb 2+ and which takes into account the interference amongst currents peaks was established. The GCE modified by organoclay film of type Sa(CTA x ) was used for detection of traces of PNP and GCE/Sa(CTA1.0 ) electrode proved to be the most sensitive to the detection of PNP. After optimization of several electroanalytical parameters related to accumulation and detection steps, a PNP calibration line was plotted in concentration range from 0.4 to 5 μmol/L, allowing the obtention of the detection limit equal to 3.75 x 10-8 M