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Halloysite-based material with improved cation sorption properties

Wścisło A., Matusik J.

Geology, Geophysics and Environment

2012

Vol. 38, no. 4

553--554

Halloysite is 1:1 layered, dioctahedral phyllosilicate. It belongs to kaolinite subgroup of kaolinite-serpentine group. Halloysite is often defined as a hydrated phase of kaolinite. Two types of this mineral are distinguished: hydrated 10 A-halloysite, with water molecules in the interlayer space and dehydrated 7 A-halloysite. Widely conducted research on modified kaolin group minerals, either intercalates or grafted compounds with selected organic molecules opened many possibilities for using this layered aluminosilicate in industry and environmental protection. One of the possible applications is the sorption of heavy metals. For the raw halloysite the sorption exclusively takes place on the particles surface. The interlayer space of the mineral is not accessible for cations. Thus the research goal was to provide the access of cations to the interlayer using organic modifications with selected aminoalcohols (Letaief & Detellier 2007). Moreover, the sorption properties of the new material towards selected cations were examined. Dehydrated type of halloysite from Polish deposit Dunino located near Legnica, which is still exploited was used for the experiments. Halloysite-dimethyl sulfoxide (DMSO) complex was used as a precursor. For this purpose 12.5 g of halloysite was reacted with a solution containing 90 mL of DMSO and 10 mL of H2O at room temperature for 7 days. Afterwards the sample was centrifuged and dried at 65°C for 24 h (HDMSO sample). In the next step, the HDMSO was reacted with diethanolamine (DEOA) or triethanolamine (TEOA) at 180°C under argon for 24 h. Finally, it was washed with isopropanol (HTEOA-I sample) and subsequently with water (HTEOA-W sample). The samples were examined by X-ray diffraction (XRD), infrared spectroscopy (IR) and CHNS elemental analysis. The XRD diffraction patterns of HDMSO with D001=11.3 A confirmed the intercalation of halloysite with DMSO. The D001 peaks for the HTEOA-I and HTEOA-W samples were found at 11.3 A and 10.8 A, respectively. In turn, the dml peaks for the HDEOA-I and HDEOA-W were found at 11.3 A and 10.8 A, respectively. The dml decrease after water washing was due to removal of the DMSO remnants and intercalated however not grafted aminoalcohol molecules from the interlayer space. The difference between the d001 values for the HDEOA-W and HTEOA-W results from the size of the organic molecules. The IR bands related to hydrogen bonds between DMSO and kaolinite OH groups were observed at 3539 cm-1 and 3504 cm-1. The formation of HDEOA and HTEOA was confirmed by the disappearance of peaks characteristic for the HDMSO and the presence of C-H stretching vibrations in the 3000-2800 cm-1 region. As the structure of the obtained materials was resistant to water washing they can be named as grafted compounds (Letaief & Detellier 2007, Matusik et al. 2012). The CHNS analysis allowed to calculate the theoretical sorption capacity of the materials (HDEOA: 264 mmol/kg, HTEOA: 355 mmol/kg) assuming that the center, which attracts cations was connected with nitrogen electron pair of aminoalcohols. The materials were tested towards lead sorption. The equilibrium experiments were carried out in the Pb concentration range 0.005-5.0 mmol/L at pH 5. The materials were shaken in Pb solutions (20 g/L ratio) for 24 hours at room temperature. The concentration of Pb was measured using atomic absorption spectroscopy method (AAS). Sorption of Pb on unmodified halloysite reached ~37 mmol/kg for its highest concentration and is relatively high as for the minerals from kaolinite group. However, the sorption for the halloysite modified with aminoalcohols is significantly higher. It was equal to ~57 mmol/kg for the HTEOA and ~62 mmol/kg for the HDEOA. It seems that the structure of molecules determines the sorption capacity. The TEOA has three alkyl chains linked to the nitrogen and the DEOA has two chains. This may cause reduced availability of nitrogen for cations due to steric effects. Therefore, the HDEOA complex achieves higher sorption values. The pH after sorption on the modified halloysite increases rapidly in contrast to raw halloysite which is probably due to adsorption of protons to nitrogen and their competition with lead. This affects the sorption capacity and will be the subject of further study.