Improved copper sorption on grafted kaolinites of different structural order

Kaolin group minerals exhibit relatively low sorption capacity as the migration of ions and their sorption in the interlayer space is not possible. The ions attraction is limited to the particles faces and edges through surface complexation and ion-exchange mechanisms. The ongoing research on functionalized kaolinites enabled to synthesize new nanomaterials with possible applications in industry and environmental protection. The modification procedures mainly involve structure alteration by intercalation and/or grafting processes. It is worth to underline that kaolinite 1:1 layer has exposed inner surface hydroxyls which are susceptible for reactions with selected organic molecules and as a results new materials with interesting properties may be obtained. Heavy metals in excessive amount are toxic and may lead to serious health problems. Thus, the purification of heavy metal contaminated aqueous solutions is of environmental importance. The aim of the study was to examine the sorption properties of kaolinites grafted with selected aminoalcohols towards Cu(II). For the experiments, two types of Polish kaolinites were chosen: well ordered type from Maria III deposit (M) (located about 20 km SW from Bolesławiec) and poorly ordered type from Jaroszów deposit (J) (located about 10 km NE from Strzegom). The modification consisted of (i) a preparation of kaolinite-dimethyl sulfoxide intercalate (MDMSO and JDMSO) and (ii) its further grafting with diethanolamine (DEA) or triethanolamine (TEA) (Letaief & Detellier 2007). The synthesized MDEA, MTEA, JDEA and JTEA samples were examined using XRD, IR and CHNS methods. The Cu(II) sorption equilibrium experiments were performed for the 0.005-10.0 mmol/L concentration range at room temperature (initial pH 5). The suspensions (20 g/L) were shaken for 24 hours. Afterwards, the Cu(II) concentration was measured using AAS method. The d001 reflections for the MDEA, MTEA, JDEA and JTEA increased from 7.2 Å (M and J samples) to 10.2 Å, 10.8 Å, 10.1 Å and 11.0 Å, respectively which confirmed the formation of grafted compounds. The presence of organic molecules resulted in an appearance of C-H stretching bands (2800-3000 cm-1) in the IR spectra. Moreover, changes in the O-H stretching region (3600-3700 cm-1) were also noticed due to interaction of aminoalcohols with kaolinites hydroxyls. An assumption was made that the sorption of cations will take place by the nitrogen lone electron pair of the grafted DEA or TEA. Thus, the theoretical sorption capacity associated with nitrogen was calculated on the basis of CHNS analysis and was the following: 184 mmol/kg (MDEA), 223 mmol/kg (MTEA), 122 mmol/kg (JDEA) and 323 mmol/kg (JTEA). The highest Cu(II) sorption was observed for the JTEA sample: 119 mmol/kg, while for the J and JDEA samples it reached 65 mmol/kg and 88 mmol/kg, respectively. The sorption improvement was less pronounced for materials based on the M sample. Both for the pure M sample and the MTEA sample, the sorption capacity was equal to ∼62 mmol/kg, while for the MDEA sample it was higher and reached 72 mmol/kg. It can be concluded that the performed modifications have improved the kaolinites sorption capacity. The improvement was due to cations attraction by the nitrogen lone electron pairs after their migration into the interlayer space. Worth emphasizing is that the Cu(II) ions readily form Cu-aminoalcohol complexes in aqueous solutions (Karadag et al. 2001). As a result of such mechanism, the pH value has increased, which is attributed to competitive protons sorption. The adsorption energy, estimated on the basis of Dubinin-Radushkevich equation, for all the reactions was in the 10-14 kJ/mol range. This indicates that the adsorption energy corresponds to energies characteristic for the ion exchange reactions (Debnath & Ghosh 2007). In most cases the sorption isotherms followed the Langmuir model.