Shiqiang Yan

Enhanced adsorption removal of methyl orange from aqueous solution by nanostructured proton-containing δ-MnO2

Two nanostructured proton-containing δ-MnO2 (H-δ-MnO2) materials were synthesized through proton exchange for K-containing δ-MnO2 (K-δ-MnO2) nanosheets and nanoparticles prepared by the hydrothermal homogeneous precipitation method and solid-state reaction. Energy-dispersive spectroscopy (EDS) measurements showed that the K+ cations were successfully leached away and replaced with H+ ions. The role of K+ cations in the adsorption process was first explored by replacing the K+ ions in the δ-MnO2 with H+ ions and the adsorption capacities for organic pollutants were investigated using methyl orange (MO) as a model. Experimental results indicated that the H-δ-MnO2 materials with a similar morphology and crystal structure showed excellent adsorption capacities of 375 and 427 mg g−1 for MO, because both of the nanostructured H-δ-MnO2 materials have much larger Brunauer–Emmett–Teller (BET) surface areas than that of K-δ-MnO2. The remarkable adsorption capacities of dye onto H-δ-MnO2 materials indicate that K+ cations in the layered δ-MnO2 structure have an unfavorable influence on the adsorption process. Moreover, physical adsorption mechanisms including electrostatic interaction play a dominant role in the adsorption mechanism between MO and adsorbents. This finding suggests that the proton-exchange process may be a useful method to improve the adsorption affinity of dye contaminants on δ-MnO2 and nanostructured proton-containing δ-MnO2 samples are good candidates for efficient MO removal from wastewater.

Synthesis, characterization and application of amino-functionalized multi-walled carbon nanotubes for effective fast removal of methyl orange from aqueous solution

Amino-functionalized multi-walled carbon nanotubes (NH2-MWCNTs) for the removal of methyl orange (MO) from aqueous solution were prepared for the first time using 1,6-hexanediamine by a simple one-pot process at 198 °C for 8 h. The resulting materials were characterized by different techniques, such as TEM, FTIR, XPS, Raman, elemental analysis and BET surface area measurement. Experimental results indicated that the amount of 1,6-hexanediamine bound on MWCNTs was estimated to be around 5 wt% and the materials showed an excellent adsorption capacity (qmax = 185.53 mg g−1). The adsorption equilibrium could be reached within 10 min. Both Langmuir and Freundlich models showed a better fit with experimental data than the Temkin model and the adsorption kinetics could be accurately described by the pseudo-second-order model. The overall rate process was jointly controlled by intra-particle diffusion and external mass transfer. Moreover, the thermodynamic parameters indicated that the adsorption was spontaneous and exothermic. Results of this work are of great importance for environmental applications of amino-functionalized multi-walled carbon nanotubes as promising dye adsorbents in wastewater treatment.

Adsorption-photocatalytic degradation of methyl orange over a facile one-step hydrothermally synthesized TiO2/ZnO–NH2–RGO nanocomposite

A novel TiO2/ZnO–NH2–reduced graphene oxide (TZ-a-RGO) nanocomposite was successfully prepared using a facile one-step hydrothermal method. The TZ-a-RGO was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) surface area analysis and UV-vis absorption spectrophotometry to investigate its structural features. The TZ-a-RGO was used as a catalyst to remove methyl orange (MO) from wastewater, and the results indicated that this catalytic system has a good performance in terms of removal of MO. The adsorption experiments of the TZ-a-RGO followed the pseudo-second-order kinetic model, and the adsorption isotherms were accurately represented by the Langmuir model. The degradation of methyl orange (MO) by TZ-a-RGO fitted well with the Langmuir–Hinshelwood model, and MO removal was obtained through a synergistic effect of adsorption and photocatalysis. The photocatalytic rate of MO over the composites was as high as 8.2 and 3.2 times that over commercial P25 (Degussa) and TiO2/ZnO, respectively. The potential photocatalytic mechanism for the TZ-a-RGO nanocomposite under UV was discussed.

Facile synthesis of Fe3O4/hierarchical-Mn3O4/graphene oxide as a synergistic catalyst for activation of peroxymonosulfate for degradation of organic pollutants

Highly effective Fe3O4/Mn3O4/reduced graphene oxide (rGO) hybrids were synthesized as a heterogeneous catalyst for the degradation of organic dyes in aqueous solution using sulfate radical-based advanced oxidation processes. The physicochemical properties of the composite were characterized by several techniques, such as X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectra (FTIR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Brunauer–Emmett–Teller (BET) theory. The effects of different parameters on the catalytic activity of Fe3O4/Mn3O4/rGO, including initial Methylene Blue (MB) concentration, peroxymonosulfate (PMS) concentration, catalyst dosage, pH value, and temperature were assessed. Typically, 98.8% removal of 50 mg L−1 of MB and 68.3% reduction of TOC could be achieved in 30 min under the following conditions: temperature 25 °C, 100 mg L−1 of catalyst, and PMS dosage of 0.3 g L−1, showing a significant enhancement of the catalytic activity of the catalyst in the degradation of organic pollutants in aqueous solution compared with Fe3O4/rGO and Mn3O4/rGO. The catalyst exhibited high stability and good reusability according to three successive repeated reactions. Based on the radical experiments, the catalytic activity of Fe3O4/Mn3O4/rGO hybrids for degradation of MB is closely related with the amount of the sulfate and hydroxyl radicals generated from PMS. The excellent catalytic performance of the Fe3O4/Mn3O4/rGO is mainly attributed to the synergistic effects of Fe3O4, Mn3O4, rGO, and Oxone.

A multifunctional magnetic core–shell fibrous silica sensing probe for highly sensitive detection and removal of Zn2+ from aqueous solution

In the present work, a multifunctional core–shell magnetic fibrous silica sensing probe AQ-Fe3O4@SiO2@KCC-1 was prepared for the detection, adsorption and removal of Zn2+. The core was composed of superparamagnetic Fe3O4 nanodots, while the shell consisted of fibrous silica molecular sieve KCC-1 and was decorated by a quinoline derived probe. The multifunctional inorganic–organic hybrid material was characterized by transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, N2 adsorption–desorption, thermogravimetric analysis, and vibrating sample magnetometry. Its sensing performance towards Zn2+ was then discussed in detail. The experimental results suggested that this functional material exhibited enhanced fluorescence in the presence of Zn2+ ions and a high selectivity for Zn2+ over the competing metal ions in aqueous solution. The binding ratio of the AQ-Fe3O4@SiO2@KCC-1–Zn2+ complexes was determined from the Job plot to be 1:1. The association constant was calculated to be 1.85 × 105 M−1, with a detection limit of 1.08 × 10−7 M. Furthermore, the adsorption process of Zn2+ on the magnetic fibrous silica composite AQ-Fe3O4@SiO2@KCC-1 was well described by the Langmuir isotherm equation and the equilibrium adsorption capacity is 157.2327 mg g−1. These results indicate that these multifunctional magnetic fibrous silica sensing materials may find potential and favorable applications for simple detection and efficient removal of Zn2+ in toxicological and environmental fields.

The structure–property relationship of manganese oxides: highly efficient removal of methyl orange from aqueous solution

Manganese oxides of various crystal structures (α-, β-, γ- and δ-MnO2, Mn2O3, Mn3O4 and amorphous) were synthesized by facile methods. The adsorption capacities for methyl orange of these materials were investigated and the resulting materials were characterized by different techniques, such as SEM, XRD and BET surface area measurements. The adsorption capacities were strongly dependent on the crystallographic structures and morphologies, and followed the order of A-MnO2 > Mn2O3 > Mn3O4 > α-MnO2 nanowires > β-MnO2 > γ-MnO2 > α-MnO2 nanotubes >δ-MnO2, while the adsorption properties could be greatly improved by increasing the surface area and pore properties of the adsorbents. A-MnO2 was found to be the most effective adsorbent among the other materials and the adsorption process was systematically investigated. The adsorption kinetics data closely followed the pseudo-second-order kinetic model and the results obtained from the intraparticle diffusion model indicated that the overall process was jointly influenced by external mass transfer and intra-particle diffusion. The maximum adsorption capacity determined from the Langmuir isotherm model was 1488.7 mg g−1. Moreover, thermodynamic analyses revealed that the adsorption of MO onto A-MnO2 was spontaneous and exothermic, and the physical adsorption mechanisms including electrostatic interactions played a dominant role in the adsorption process between MO and A-MnO2. These combined results indicated that A-MnO2 is an efficient adsorbent for the removal of MO from wastewater.

One-step synthesis of magnetic 1,6-hexanediamine-functionalized reduced graphene oxide–zinc ferrite for fast adsorption of Cr(VI)

A one-step solvothermal method was developed to prepare nearly cubic ZnFe2O4 nanoparticles loaded on 1,6-hexanediamine-functionalized reduced graphene oxide (HDA–RGO–ZnFe2O4) for fast removal of Cr(VI). The materials were characterized by transmission electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, Raman, and vibrating sample magnetometry. Experimental results indicated that Cr(VI) adsorption by HDA–RGO–ZnFe2O4 is strongly pH dependent and the adsorption equilibrium could be reached within 12 min. The kinetic adsorption of different initial concentrations can be accurately described by the pseudo-second-order model. The overall rate process was apparently influenced by external mass transfer and intraparticle diffusion. The Langmuir isotherm model is consistent with the experimental data at different temperatures and the maximum adsorption capacity was determined to be 172.4 mg g−1. Moreover, the thermodynamic parameters indicated that the adsorption process was spontaneous and endothermic. The adsorption mechanism and desorption experiments were also studied. After adsorption, the HDA–RGO–ZnFe2O4 could be quickly separated from the media by an external magnetic field and the adsorption capacity can remain up to 82% after five times of usage. The results implied that HDA–RGO–ZnFe2O4 is a promising adsorbent for the removal of Cr(VI).

Introduction of α-MnO2 nanosheets to NH2 graphene to remove Cr6+ from aqueous solutions

The planar structure of the designed α-MnO2–NH2–RGO hybrid was prepared and characterized and used to remove hexavalent chromium ions (Cr6+) from aqueous solutions. The characterization of the novel adsorbent was carried out using the Fourier transform infrared spectrum (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM), and the Brunauer–Emmett–Teller (BET) theory. Cr6+ adsorption efficiency was investigated as a function of the pH of Cr6+ solution, contact time and temperature. The results showed that the adsorption capacity strongly depended on the pH and that the adsorption equilibrium data were best described by the Freundlich isothermal model. The maximum sorption capacity towards Cr6+ was 371 mg g−1. The kinetic adsorption was fitted to the pseudo-second-order kinetics model and it indicated that the adsorption mechanism is physical or chemical sorption on heterogeneous materials. The thermodynamic parameters were obtained, and the results showed that the adsorption process is spontaneous and exothermic. The adsorption capacity of α-MnO2–NH2–RGO can remain as high as 81% after five usage cycles. As a result, these findings propose that α-MnO2–NH2–RGO could be used as an outstanding adsorbent for Cr6+.

One-pot preparation of a MnO2–graphene–carbon nanotube hybrid material for the removal of methyl orange from aqueous solutions

This study presents a MnO2–graphene–carbon nanotube (MnO2–G–CNT) hybrid material synthesized in a simple one-pot reaction process by a chemical method. The resulting materials were characterized by different techniques, such as TEM, XRD, FTIR, XPS, and BET surface area measurement. The adsorption behaviors of methyl orange (MO) onto the MnO2–G–CNT were firstly systematically investigated and experimental results indicated that the material with the highest loading amount of MnO2 showed an excellent adsorption capacity toward MO. The adsorption kinetics could be well described by the pseudo-second-order model and the Freundlich isotherm model showed a better fit with experimental data than the Langmuir model and the maximum adsorption capacity was determined to be qmax = 476.19 mg g−1. The overall rate process was apparently influenced by intra-particle diffusion and external mass transfer. Moreover, the thermodynamic parameters indicated that the adsorption was spontaneous and exothermic and that the physical adsorption mechanisms included electrostatic interaction, which played a dominant role in the adsorption mechanism between MO and the hybrid material.

Synthesis of poly(m-phenylenediamine)/iron oxide/acid oxidized multi-wall carbon nanotubes for removal of hexavalent chromium

Poly(m-phenylenediamine)-coated Fe3O4/o-MWCNTs nanoparticles (PmPD/Fe3O4/o-MWCNTs) were synthesized by one-step chemical oxidation polymerization. The materials were characterized by transmission electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, vibrating sample magnetometry, and Brunauer–Emmett–Teller surface area measurement. Hexavalent chromium adsorption by PmPD/Fe3O4/o-MWCNTs was found to be strongly dependent on the solution pH. The adsorption isotherm of Cr(VI) onto PmPD/Fe3O4/o-MWCNTs fitted the Langmuir isotherm model at different temperatures and the maximum adsorption capacity was determined to be 346 mg g−1. The adsorption process of different initial concentrations can be described by the pseudo-second-order kinetic model. The intraparticle diffusion study revealed that external mass transfer and intraparticle diffusion apparently influenced the overall rate process. The values of thermodynamic parameters indicated that the adsorption process was spontaneous and endothermic. In addition, the adsorption mechanism included both the physical and the chemical adsorption mechanisms. After adsorption, PmPD/Fe3O4/o-MWCNTs could be conveniently separated from the media by an external magnetic, and the adsorption capacity can remain at up to 52% after five times of usage. These results demonstrate that PmPD/Fe3O4/o-MWCNTs are a good candidate for the removal of Cr(VI) from aqueous solution.