The adsorption of ketoprofen, naproxen, and diclofenac (non-steroidal anti-inflammatory drugs, NSAIDs) on halloysite/carbon nanocomposites and non-modified halloysite were investigated in this work. Halloysite/carbon nanocomposites were obtained through liquid phase impregnation and carbonization using halloysite as the template and saccharose as the carbon precursor. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared spectrometry (FT-IR), and low-temperature nitrogen adsorption method were employed to study the morphological and structural changes of the halloysite/carbon nanocomposites. The effects of contact time, initial concentration of adsorbates, pH of solution, and mass of adsorbent on the adsorption were studied. Adsorption mechanism was found to fit pseudo-second-order and intra-particle diffusion models. The obtained experimental adsorption data were well represented by the Langmuir multi-center adsorption model. Adsorption ability of halloysite/carbon nanocomposites was much higher for all the studied NSAIDs in comparison to non-modified halloysite. Optimized chemical structures of ketoprofen, naproxen, and diclofenac obtained by Density Functional Theory (DFT) calculation showed that charge distributions of these adsorbate molecules and their ions can be helpful to explain the details of adsorption mechanism of NSAIDs on halloysite/carbon nanocomposites.

The kinetics of photocatalytic degradation of aniline, 2-chloroaniline, and 2,6-dichloroaniline in the presence of halloysite-TiO2 and halloysite-Fe2O3 nanocomposites, halloysite containing naturally dispersed TiO2, Fe2O3, commercial TiO2, P25, and α-Fe2O3 photocatalysts, were investigated with two approaches: the Langmuir–Hinshelwood and first-order equations. Adsorption equilibrium constants and adsorption enthalpies, photodegradation rate constants, and activation energies for photocatalytic degradation were calculated for all studied amines photodegradation. The photodegradation mechanism was proposed according to organic intermediates identified by mass spectrometry and electrophoresis methods. Based on experimental results, it can be concluded that after 300 min of irradiation, aniline, 2-chloro-, and 2,6-dichloroaniline were completely degraded in the presence of used photocatalysts. Research results allowed us to conclude that higher adsorption capacity and immobilization of TiO2 and Fe2O3 on the halloysite surface in the case of halloysite-TiO2 and halloysite-Fe2O3 nanocomposites significantly increases photocatalytic activity of these materials in comparison to the commercial photocatalyst: TiO2, Fe2O3, and P25.

The adsorption of chloroxylenol and chlorophene on halloysite-carbon composites was investigated in batch and flow systems. The synthesis of halloysite-carbon composites through two different methods was performed with microcrystalline cellulose as carbon precursor. The obtained halloysite-carbon composites were characterized by SEM/EDS analysis, the low-temperature nitrogen adsorption/desorption methods, and infrared spectrometry (FT-IR). The SEM/EDS analysis and FT-IR spectra confirmed the presence of carbon on the surface of the halloysite. On the basis of the measurement results in the batch system, the two composites with the best adsorption properties for both adsorbates were chosen for measuring the flow system (using the inverse liquid chromatography). Removal efficiency was equal to 92.26 and 81.36%. It was obtained for chloroxylenol on HNT-m 800 and HNT-Zn 500, respectively. For chlorophene, the removal efficiency had the value of 78.79 and 77.87% on HNT-m 800 and HNT-Zn 800, respectively. Adsorption parameters of chloroxylenol and chlorophene were determined with inverse liquid chromatography methods: the adsorption equilibrium constants were determined with the peak division method and the adsorption capacity of the adsorbents was determined with the breakthrough curve method. Maximum adsorption capacity for the adsorption of chloroxylenol on HNT-m 800 was 5.48 mg·g−1 and on HNT-Zn 500 its value was 2.77 mg·g−1. For the adsorption of chlorophene on HNT-m 800 the value was 4.44 mg·g−1 and on HNT-Zn 800–2.5 mg·g−1. Halloysite-carbon composites can be successfully used as effective adsorbents for removing chloroxylenol and chlorophene from solutions in the flow system.

Analysis of surface properties of halloysite-carbon nanocomposites and non-modified halloysite was carried out with surface sensitive X-ray photoelectron spectroscopy (XPS) and inverse gas chromatography (IGC). The XPS spectra were measured in a wide range of the electron binding energy (survey spectra) and in the region of C 1s photoelectron peak (narrow scans). The IGC results show the changes of halloysite surface from basic for pure halloysite to acidic for carbon-halloysite nanocomposites. Halloysite-carbon nanocomposites were used as adsorbents of paracetamol from an aqueous solution. The adsorption mechanism was found to follow the pseudo-second-order and intra-particle diffusion models. The Langmuir multi-center adsorption model described well the obtained experimental data. The presence of carbon increased significantly the adsorption ability of halloysite-carbon nanocomposites for paracetamol in comparison to the non-modified halloysite.

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