| People increasingly pay more attention to water pollution, and effective treatment of wastewater has a positive impact on both human health and environment protection. Natural mineral materials and their modified products have been used as adsorbent in many studies. Halloysite nanotubes (HNTs), a cost-effective and readily available tubular natural mineral, are a perfect starting material for the preparation of adsorbents. Thus, in this study, we synthesized different types of zeolites from HNTs and prepared new adsorbents from HNTs using different modified methods. At the same time, the adsorption properties of zeolites and modified HNTs for ammonium, dyes and heavy metals were also investigated. These researches have provided new research thoughts for the deep processing of HNTs and their application in environmental area. The main research contents are listed as follows:(1) Synthesizing zeolite Y, zeolite A and zeolite X from HNTsZeolite A, zeolite X and low-silica zeolite Y were synthesized from natural nanotubular halloysite mineral by hydrothermal method. In the synthesis process of zeolite Y, halloysite consisted of SiO2 and Al2O3 was used as starting material with adding supplementary silica and alumina sources. In the synthesis process of zeolite A and zeolite X, halloysite was the only starting material and without adding other chemicals. The products were characterized by XRD, FTIR, SEM, TEM and HRTEM, and the results showed that the synthesized zeolites were pure form, single phase and highly crystalline.(2) Investigating the adsorption properties of the as-synthesized zeolite Y, zeolite A and zeolite X for ammoniumAmmonium adsorption properties of the as-synthesized zeolite Y, zeolite A and zeolite X were studied using batch experiments and results revealed that its adsorption properties were strongly dependent on contact time, adsorbent dosage, pH, ionic strength, temperature and initial concentration. The equilibrium data fit well with the Langmuir isotherm and the Freundlich isotherm. Kinetic studies showed that the adsorption followed pseudo-second-order model. Thermodynamic parameters such as change in free energy (ΔG0), enthalpy (ΔH0) and entropy (ΔS0) were also determined, which indicated that the adsorption of ammonium on the three zeolites was spontaneous and exothermic at ambient conditions. The synthesized zeolites can be regenerated and re-used for ammonium adsorption. Due to their low cost, high adsorption capacity and fast adsorption rate, the zeolites synthesized from halloysite have the potential to be utilized for cost-effective removal of ammonium from wastewater.(3) Investigating the adsorption behavior of HNTs for Neutral Red and chemically activated halloysite nanotubes for Methylene BlueHNTs, a low-cost available clay mineral, were tested for the ability to remove cationic dye, Neutral Red (NR), from aqueous solution. The adsorption behavior of Methylene Blue (MB) from aqueous solutions onto chemically activated halloysite nanotubes (C-HNTs) was investigated. C-HNTs were obtained by treatment of raw HNTs with hydrochloric acid followed by sodium chloride at room temperature. The effect of adsorbent dose, initial pH, temperature, initial concentration and contact time were investigated. Adsorption increases with increase in adsorbent dose, initial pH, temperature and initial concentration. The equilibrium data were well described by both Langmuir and Freundlich isotherm models. The maximum adsorption capacity for NR was 65.45 mg·g-1 and the adsorption capacity for MB was 103.63 mg·g-1 at 318 K, respectively. Batch kinetic experiments showed that both adsorption followed pseudo-second-order kinetic model. Thermodynamic parameters ofΔG0,ΔH0 andΔS0 indicated the adsorption processes were spontaneous and endothermic. The adsorption mechanism of HNTs for NR and C-HNTs for MB was also explored. The results above confirmed that HNTs and C-HNTs had the potential to be utilized as low-cost and relatively effective adsorbent for cationic dyes removal.(4) Modifying HNTs with chitosan and investigating the adsorption properties of modified HNTs for Cr(Ⅵ)A new biosorbent nanocomposite (CS/HNTs) was developed by dispersing chitosan, a naturally and abundantly available biopolymer, onto halloysite nanotubes (HNTs). Equilibrium and kinetic adsorption characteristics of chromium (Ⅵ) on the biosorbent were studied. The effect of pH, agitation time, initial concentration and adsorbent dose on the extent of adsorption were investigated. The experimental data were fitted to Langmuir and Freundlich adsorption isotherms. Kinetic data were fitted to pseudo-second-order and intraparticle diffusion models. The maximum monolayer adsorption capacity of CS/HNTs as obtained from Langmuir adsorption isotherm was found to be 14.49 mg·g-1 for Cr(Ⅵ). The experimental results demonstrated that CS/HNTs could be used for the removal of Cr(Ⅵ) ions from aqueous medium through adsorption.From what has been discussed, pure form, single phase and highly crystalline zeolites were synthesized from halloysite mineral. The as-synthesized zeolites have high adsorption capacity for ammonium from aqueous solutions. In addition, the modified HNTs have high adsorption capacity for dyes and Cr(Ⅵ). Moreover, HNTs are readily available and cost effective, so HNTs can be used as a prospect material in environmental area.
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