| Dyes wastewater from textile, printing industry, etc. which contains lots of organics and salinity has been the difficulty of wastewater treatment. Adsorption as an efficient and economical process has been used for the treatment of dye-contaminated water in the last few years. Natural mineral materials (such as kaoline, montmorillonoid, etc.), due to the abundant resources, low costs and high cation exchange capacity, have been widely used for the removal of dyes from wastewater.Halloysite is a kind of clay mineral with a natural nanosized hollow tubular structure. With the development of research on nanomaterial, halloysite nanotubes (HNTs) have attracted widespread interest. However, there are few relevant researches about the removal of dyes by halloysite. On the other hand, the HNTs powder is tiny, and it is so difficult for the powder separated from aqueous solution, or the separation process would consume amount of time and energy which restrained the practical applications of this material. Therefore, this paper used HNTs as the raw material, prepared the modified HNTs, and investigated the adsorption capacity for cationic dyes (methyl violet, MV) and anionic dyes (methyl orange, MO) wastwater. And a kind of magnetic halloysite nano-composites was prepared by chemical precipitation method, could be separated from aqueous solution rapidly with applying a magnetic field. We obtain the following results.(1) The absorption of cationic dyes onto natural HNTs from wastewater had been studied systematically. The morphology and the textural characterization of HNTs were examined by a transmission electron microscope (TEM) and N2 adsorption-desorption at 77 K using a specific surface area analyzer. The characterization results showed that HNTs contained a transparent central area as tubular and had relatively large specific surface area. Then HNTs was used as adsorbent for methyl violet (MV) removal from aqueous solution. The effects of initial concentration, contact time, temperature, initial pH and ionic strength were investigated. The results showed that HNTs exhibited rapid adsorption rate. The adsorption capacity of HNTs almost unchanged when the pH and temperature varied. Kinetic studies showed that the adsorption followed pseudo-second-order model. The equilibrium data fit well with the Langmuir isotherm and the Freundlich isotherm. The maximum monolayer adsorption capacities of HNTs were 105.26 mg/g at 298 K. The regeneration of HNTs was realized by calcinations and the recovered HNTs still had high adsorption capacity for MV. The results above confirmed that HNTs could be used as an effective adsorbent for cationic dyes removal.(2) The absorption of anionic dyes onto modified HNTs from wastewater had been studied systematically. The HNTs were modified with hexadecyl trimethyl ammonium bromide (HDTMA) and the composite named as HDTMA-HNTs. The characterization results showed that quaternary ammonium cations were grafted onto the nanotubes surface successfully and the surface charge of the HDTMA-HNTs was positively charged below pH 12, and it could be used as adsorbents for anionic dyes. Then the HDTMA-HNTs was used as adsorbent for methyl orange (MO) removal from aqueous solution. The results showed that HDTMA-HNTs exhibited much better adsorptive capacity than HNTs with rapid adsorption rate. The adsorption capacity of MO onto HDTMA-HNTs almost unchanged when the pH and temperature varied. Kinetic studies showed that the adsorption fitted well with the pseudo-second-order model and the equilibrium data fitted well with the Langmuir isotherm. The maximum monolayer adsorption capacities of MO onto HDTMA-HNTs were 94.34 mg/g at 298 K. The used HDTMA-HNTs could be regenerated and the recovered adsorbents still had high adsorption capacity for MO. The results above confirmed that HDTMA-HNTs could be used as an effective adsorbent for anionic dyes removal.(3) The magnetic nano-composite Fe3O4-HNTs was prepared by chemical precipitation method. The as-prepared adsorbents were characterized by the TEM, XRD, Vibrating Sample Magnetometer (VSM) and N2 adsorption-desorption isotherms. The characterization results showed that the magnetic nano-composite exhibits paramagnetic, and could be separated from aqueous solution rapidly with applying a magnetic field. Then the Fe3O4-HNTs were used as adsorbent for MV removal from aqueous solution. The results showed that Fe3O4-HNTs exhibited rapid adsoiption rate. The adsorption capacity of Fe3O4-HNTs almost unchanged when the pH, concentration of metal ion and temperature varied. Kinetic studies showed that the adsorption followed pseudo-second-order model. The equilibrium data fitted well with the Langmuir isotherm and the Freundlich isotherm. The maximum monolayer adsorption capacities of Fe3O4-HNTs were 90.09 mg/g for MV at 298 K. The used Fe3O4-HNTs could be regenerated and the recovered adsorbents could be used again for MV removal. The results above confirmed that Fe3O4-HNTs could be used as an effective adsorbent for cationic dyes removal. |
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