Yafei Zhao

Study on the adsorption of Neutral Red from aqueous solution onto halloysite nanotubes

Halloysite nanotubes (HNTs), a low-cost available clay mineral, were tested for the ability to remove cationic dye, Neutral Red (NR), from aqueous solution. Natural HNTs used as adsorbent in this work were initially characterized by XRD, FT-IR, TEM and BET. The effect of adsorbent dose, initial pH, temperature, initial concentration and contact time were investigated. Adsorption increased with increase in adsorbent dose, initial pH, temperature and initial concentration. The equilibrium data were well described by both the Langmuir and Freundlich isotherm models. The maximum adsorption capacity was 54.85, 59.24 and 65.45mg/g at 298, 308 and 318K, respectively. Batch kinetic experiments showed that the adsorption followed pseudo-second-order kinetic model with correlation coefficients greater than 0.999. Thermodynamic parameters of DeltaG(0), DeltaH(0) and DeltaS(0) indicated the adsorption process was spontaneous and endothermic. The results above confirmed that HNTs had the potential to be utilized as low-cost and relatively effective adsorbent for cationic dyes removal.

Preparation of highly ordered cubic NaA zeolite from halloysite mineral for adsorption of ammonium ions

Well-ordered cubic NaA zeolite was first synthesized using natural halloysite mineral with nanotubular structure as source material by hydro-thermal method. SEM and HRTEM images indicate that the synthesized NaA zeolite is cubic-shaped crystal with planar surface, well-defined edges and symmetrical and uniform pore channels. The adsorption behavior of ammonium ions (NH(4)(+)) from aqueous solution onto NaA zeolite was investigated as a function of parameters such as equilibrium time, pH, initial NH(4)(+) concentration, temperature and competitive cations. The Langmuir and Freundlich adsorption models were applied to describe the equilibrium isotherms. A maximum adsorption capacity of 44.3 mg g(-1) of NH(4)(+) was achieved. The regeneration and reusable ability of this adsorbent was evaluated, and the results indicated that the recovered adsorbent could be used again for NH(4)(+) removal with nearly constant adsorption capacity. Thermodynamic parameters such as change in free energy (DeltaG(0)), enthalpy (DeltaH(0)) and entropy (DeltaS(0)) were also determined, which indicated that the adsorption was a spontaneous and exothermic process at ambient conditions. Compared with other adsorbents, the as-synthesized NaA zeolite displays a faster adsorption rate and higher adsorption capacity, which implies potential application for removing NH(4)(+) pollutants from wastewaters.

Ultrafine MnO2 nanoparticles decorated on graphene oxide as a highly efficient and recyclable catalyst for aerobic oxidation of benzyl alcohol

The highly active and selective aerobic oxidation of aromatic alcohols over earth-abundant, inexpensive and recyclable catalysts is highly desirable. We fabricated herein MnO2/graphene oxide (GO) composites by a facile in-situ growth approach at room temperature and used them in selective aerobic oxidation of benzyl alcohol to benzaldehyde. TEM, XRD, FTIR, XPS and N2 adsorption/desorption analysis were employed to systematically investigate the morphology, particle size, structure and surface properties of the catalysts. The 96.8% benzyl alcohol conversion and 100% benzaldehyde selectivity over the MnO2/GO (10/100) catalyst with well dispersive ultrafine MnO2 nanoparticles (ca. 3nm) can be obtained within 3h under 383K. Simultaneously, no appreciable loss of activity and selectivity occurred after recycling use up to six times. Due to their significant low cost, excellent catalytic performance, the MnO2/GO composites have huge application prospect in organic synthesis.

Fabricating roughened surfaces on halloysite nanotubes via alkali etching for deposition of high-efficiency Pt nanocatalysts

Technological applications of heterogeneous nanocatalysts on supports have generally relied on the surface or interface properties of supports. Herein, we report a facile approach to fabricate roughened surfaces on halloysite nanotubes (HNTs) through etching the wall of HNTs in a molten-salt system. SEM, TEM, XRD, FTIR, AFM and N2 adsorption/desorption analysis are employed to systematically investigate the morphology, structure and surface properties. The results suggest that the roughness of HNTs surface has been significantly increased and defects are formed on the tube wall without structural damage. Subsequently, the roughened halloysite nanotubes (RHNTs) are used as supports to prepare heterogeneous nanocatalyst. Pt nanoparticles with a uniform size can be homogeneously deposited onto the RHNTs surfaces via a one-step hydrothermal reduction. The as-prepared Pt@RHNTs catalyst exhibits remarkably improved activity and selectivity for the hydrogenation of cinnamaldehyde towards cinnamyl alcohol compared with the pristine halloysite support. Furthermore, Pt@RHNTs catalyst shows rapid catalytic rates in hydrogenation reactions and excellent leaching resistance in cycle uses.

Ammonium removal from aqueous solution by zeolite X synthesized from halloysite mineral

Zeolite X was successfully synthesized from natural halloysite mineral by hydrothermal method. The synthesized zeolite X was characterized by XRD, SEM, TEM and HRTEM. The characterization indicated that zeolite X had high crystallinity together with symmetrical and uniform pore channels. Ammonium (NH₄+) adsorption properties of zeolite X were studied using batch experiments. The results revealed that high initial concentration and low temperature favored NH₄+ adsorption on zeolite X. Both Langmuir and Freundlich isotherms fit well with the equilibrium data. Kinetic studies showed that the adsorption followed pseudo-second-order model. Intra-particle diffusion analysis demonstrated that NH₄+ diffused quickly among the particles at the initial 20 min of the adsorption process, and then the diffusion slowed down and stabilized. Thermodynamic parameters such as change in free energy (ΔG⁰), enthalpy (ΔH⁰) and entropy (ΔS⁰) indicated that the adsorption was spontaneous and exothermic at ambient conditions. The reusable ability of zeolite X was also evaluated. Due to its low cost, high adsorption capacity and fast adsorption rate, zeolite X synthesized from halloysite could be used as an effective and environmental-friendly adsorbent for NH₄+ removal.

Removal of Ammonium from Wastewater by Pure Form Low-Silica Zeolite Y Synthesized from Halloysite Mineral

Pure form, single phase, and highly crystalline low-silica zeolite Y was synthesized from natural nanotubular halloysite mineral by the hydrothermal method. In the synthesis process, the halloysite consisted of SiO2 and Al2O3 was used as starting material with adding supplementary silica and alumina sources. Ammonium adsorption properties of the as-synthesized zeolite Y were studied using batch experiments and the 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 compared with the Freundlich isotherm. Kinetic studies showed that the adsorption followed the pseudo-second-order model. Thermodynamic parameters such as change in free energy (ΔG 0), enthalpy (ΔH 0), and entropy (ΔS 0) were also determined, which indicated that the adsorption of ammonium on zeolite Y was a spontaneous and exothermic process at ambient conditions. Due to its low cost, high adsorption capacity and fast adsorption rate, the zeolite Y synthesized from halloysite has the potential to be utilized for the cost-effective removal of ammonium from wastewater.

Tuning the performance of Pt–Ni alloy/reduced graphene oxide catalysts for 4-nitrophenol reduction

An environmentally benign and economic reaction system with an effective catalyst for 4-nitrophenol reduction is highly desirable. Here, we synthesized reduced graphene oxide (RGO) supported Pt–Ni alloy catalysts with different atomic ratios of Pt and Ni, investigated their morphology, size, dispersity, structure and elemental valence, and studied their catalytic activity in order to tune their performance for 4-nitrophenol reduction. It is worth pointing out that the RGO support can efficiently avoid the aggregation of Pt–Ni alloy nanoparticles, and the most dispersed and smallest Pt–Ni particles on RGO can be obtained when the atomic ratio of Pt to Ni is 1 : 9. The Pt–Ni/RGO (1 : 9) nanocatalyst also shows a higher catalytic activity toward the conversion of 4-NP to 4-AP with a catalytic rate constant of 0.3700 min−1 than Pt–Ni/RGO (1 : 3) and Pt–Ni/RGO (1 : 25), and much higher than that of Pt/RGO, Ni/RGO and bare Pt–Ni owing to the well-defined composition, small particle size (10 nm), good dispersion, synergistic effect between Pt and Ni, and electron transfer between RGO and Pt–Ni alloy nanoparticles. In addition, the catalyst possesses good stability and recyclability for the catalytic reduction reaction. The Pt–Ni/RGO nanocatalyst, with well-defined composition, small particle size, uniform dispersity, high catalytic rate, and recyclability, should be an ideal catalyst for specific applications in liquid phase reactions.

Immobilization of laccase onto porous polyvinyl alcohol/halloysite hybrid beads for dye removal

Laccase was immobilized in polyvinyl alcohol beads containing halloysite nanotubes (PVA/HNTs) to improve the stability and reusability of enzyme. The porous structure of PVA/HNTs beads facilitates the entrapment of enzyme and prevents the leaching of immobilized laccase as well. Halloysite nanotubes act as bridge to connect the adjacent pores, facilitating the electron transfer and enhancing the mechanical properties. PVA/HNTs beads have high laccase immobilization capacity (237.02 mg/g) and activity recovery yield (79.15%), indicating it can be used as potential support for laccase immobilization. Compared with free laccase, the immobilized laccase on hybrid beads exhibits enhanced pH tolerance (even at pH 8.0), good thermal stability (57.5% of the initial activity can be maintained at 75 °C), and excellent storage stability (81.17% of enzyme activity could be retained after storage at 4 °C for 5 weeks compared with that for free enzyme of 60%). Also, the removal efficiency for reactive blue can reach as high as 93.41% in the presence of redox mediator 2,2-azinobis(3-ethylbenzthiazoline-6-sulfonate), in which adsorption and degradation exist simultaneously. The remarkable pH tolerance, thermal and storage stability, and reuse ability imply potential application of porous PVA/HNTs immobilized enzyme in environmental fields.

Efficient removal of methylene blue in aqueous solution by freeze-dried calcium alginate beads

Novel porous calcium alginate beads were prepared via crosslinking of calcium followed by freeze drying for investigating the adsorption performance for methylene blue. These beads possessed reduced shrinkage, highly porous lamellar structure and high specific surface area, and exhibited enhanced adsorption capacity and much faster adsorption rate compared to the non-porous beads obtained with conventional oven drying method. Methylene blue adsorption capacity increased with increasing of initial concentration and pH, while decreased with increasing of temperature. The adsorption process fitted well with the pseudo-second-order kinetic model and the Langmuir isotherm. The maximum adsorption capacity was 961.5 mg g−1 at 298.15 K. After eight successive adsorption-desorption cycles, the adsorption capacity had negligible decrease. Owing to the high adsorption capability, rapid adsorption rate, easy recovery and reusability, the freeze-dried beads imply a prospective, biodegradable and attractive adsorbent for removing contaminants from wastewater.

CHAPTER 12:Halloysite–Dopamine Hybrid Nanotubes to Immobilize Biomacromolecules

Biomacromolecules, such as protein, DNA, and polysaccharide, have been widely employed for bio-catalyzed synthesis/decomposition, anti-cancer therapy, bio-sensors, biofuel cells, and so on. Immobilizing biomacromolecules onto solid supports is often necessary to improve the operational stability, dispersity and recyclability. Halloysite nanotube has been identified as a promising support for biomacromolecule immobilization, while it requires a facile and mild method to firmly attach biomacromolecules onto halloysites. In this chapter, the development and employment of halloysite−dopamine hybrid nanotubes for biomacromolecule immobilization are presented. Firstly, the state-of-the-art of halloysite nanotubes-based biomacromolecule immobilization is briefly reviewed, especially the existing problems. The second part mentions a versatile technique platform called “dopamine chemistry”, which is inspired by the marine mussel’s adhesion protein. Next, some examples in which the technique platform was employed to solve the problems of pristine halloysite nanotubes are encompassed, including the potential applications of halloysite−dopamine hybrid nanotubes for immobilizing other biomacromolecules. Finally, a summary of this chapter as well as the future perspectives regarding halloysite−dopamine hybrid nanotubes are included.