The kinetics of heavy metals immobilization by modified halloysite

Halloysite is a clay mineral, belonging to the kaolinite group. Its structure is composed of stacked 1:1 layers built from octahedral (alumina) and tetrahedral (silica) sheets, linked through hydrogen bonds formed between oxygen atoms of tetrahedral sheet and inner surface OH groups of octahedral sheet. Due to the fact that Poland has several kaolin deposits, it is important to undertake research concerning possible application after appropriate modification (Matusik & Bajda 2013, Matusik & Kłapyta 2013, Matusik et al. 2013). Beyond harmful properties, the decisive factor in the selection of heavy metals was their high prevalence in the environment. The purpose of the research was to analyze the kinetics of heavy metals immobilization by natural and modified halloysite. The mineral (H) used in the study came from Polish deposit located in Dunino near Legnica, which is owned by the Intermark company. The sample, apart from halloysite, which exhibits a tubular morphology, contains kaolinite forming plates. The modification procedure involved two following steps. Firstly, material was intercalated with dimethyl sulfoxide (DMSO) by mixing 12.5 g of mineral with 90 mL DMSO and 10 mL H2O (HDMSO). The second step involved grafting process, in which the HDMSO was refluxed with 150 ml of diethanolamine (DEA) for 24 h at 180°C under argon flow. Afterwards, the final sample (HD) was washed with isopropanol and subsequently with water to remove DMSO remnants and the excess of DEA. The materials were characterized using XRD and IR methods. The materials sorption affinity towards Pb(II), Zn(II), Cd(II) and Cu(II) was investigated. The experiment was carried out for a mixture of all four metals (multi-element system) at equal 1 mmol/L concentration at initial pH 5.2. The material either H or HDEA (1 g) was mixed with 50 mL of solution (20 g/L - solid/solution ratio). The suspension aliqouts were collected after 0.5, 1, 1.5, 2, 3, 4, 5, 6, 8, 10, 15, 20, 30 min, filtered immediately, and further analyzed using AAS method.The XRD patterns showed that the modification did not alter all halloysite layers. The 10.4 Å peak of the new complex, and the 7.3 Å peak of raw mineral were observed. The IR spectra of H sample in 3700-3600 cm-1 region revealed four distinct bands attributed to different vibration modes of inner surface and inner hydroxyls. After DMSO intercalation new bands at 3540 cm-1 and 3503 cm-1 were noticed connected to interlayer hydrogen bonding S=O...HO between DMSO molecules and OH hydroxyls of the octahedral sheet. The bands at 3022 cm-1, 2936 cm-1, and 2918 cm-1, were due to C-H stretching vibrations of DMSO methyl groups. The IR spectra after DEA grafting, showed the disappearance of bands related to DMSO molecules indicating their removal. The adsorption behavior of tested heavy metals onto raw and modified halloysite differs. In the experiment with H sample the equilibrium was achieved almost immediately. On the other hand, in the case of HD sample the sorption increased gradually and the equilibrium was reached after about 30 min. The relatively slowest uptake was particularly noticeable for Cu(II) and Pb(II). The sorption on raw halloysite may take place through surface complexation and/or ion-exchange. The sorption on natural halloysite was found to follow the sequence Pb(II) > Zn(II) > Cu(II) ≈ Cd(II), which reflects the cations hydrolysis behavior. After 30 minutes the sorption capacity for H sample reached 15 mmol Pb/kg, 6 mmol Zn/kg, 1.5 mmol Cu/kg, 1 mmol Cd/kg. The final pH decreased to 4.2 confirming the protons release characteristic for surface complexation mechanism. The sorption on modified halloysite was found to be the following: Cu(II) > Pb(II) > Zn(II) ≈ Cd(II). The sorption reached an equilibrium equal to: 25 mmol Cu/kg, 20 mmol Pb/kg, 10 mmol Zn/kg and 8 mmol Cd/kg. It is worth to mention that 16-fold increase for Cu(II) and 8-fold increase for Cd(II) were noticed. In particular the Cu(II) sorption increase is due to formation of Cu(II)-DEA complexes. The final pH of solution increased to 5.3 due to competitive sorption of protons to amine nitrogen of the DEA.