Study On Preparation Of Molybdenum-based Adsorbent And Its Adsorption Performance For Cr(Ⅵ)

As a toxic heavy metal,chromium exists in environmental wastewater mainly in two valence states of trivalent chromium ion Cr(Ⅲ)and hexavalent chromium ion Cr(Ⅵ).Cr(Ⅲ)has relatively low toxicity and is an important element for normal operation of organisms.It is generally mainly adsorbed on the surface of solid substances.However,Cr(Ⅵ)has relatively strong toxicity,teratogenicity,mutagenicity and carcinogenicity,as well as relatively strong migration,enrichment and oxidation capacity.These properties make Cr(Ⅵ)more dangerous and difficult to handle.Therefore,it is imperative to remove Cr(Ⅵ)from wastewater or reduce it to Cr(Ⅲ)before it is released to the environment.Adsorption method has been widely used for the treatment of heavy metals in wastewater due to its advantages of high efficiency,low cost,easy operation and the reuse of adsorbed metals.Therefore,most scholars have focused on the research and development of new high-efficiency adsorbents,while molybdenum-based materials have attracted much attention due to their excellent physical and chemical properties.In this study,three different adsorbents for Cr(Ⅵ)were prepared based on three molybdenum-based materials:molybdenum trioxide,molybdenum dioxide and molybdenum carbide.These adsorbents are amino-molybdenum trioxide(NH2-MoO3),cobalt oxide/molybdenum dioxide/molybdenum carbide(CoO/MoO2/Mo2C)and ferric hydroxide/molybdenum carbide(FeOOH/Mo-Mo2C).The three materials all showed good adsorption performance for Cr(Ⅵ)in water.Among them,CoO/MoO2/Mo2C and FeOOH/Mo-Mo2C showed strong selective adsorption for Cr(Ⅵ),which was superior to most reported adsorbents for Cr(Ⅵ).(1)MoO3 nanorods were prepared by a simple hydrothermal method,and then aminated by organosilane and toluene to form amino-molybdenum trioxide(NH2-MoO3)adsorbent.Compared with untreated molybdenum trioxide,the adsorption capacity of NH2-MoO3 for Cr(Ⅵ)was significantly enhanced,showing a good adsorption prospect.The structures and morphologies were characterized by scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS)and X-ray diffraction(XRD)patterns.These results demonstrated that we had successfully modified molybdenum trioxide.The detailed adsorption mechanism of molybdenum trioxide was studied systematically from the aspects of adsorbent type,pH effect,initial concentration,kinetics,isothermal model and cyclic regeneration.It was found that the kinetic process of adsorption of Cr(Ⅵ)was more in line with the quasi-two-stage model.The kinetic fitting results showed that the two-stage model(R2=0.985)was more appropriate than the first-stage model(R2=0.939)and was affected by multiple steps.The optimal pH of the material was 2.The thermodynamic fitting results showed that the Langmuir model could better describe the adsorption process of Cr(Ⅵ)on NH2-MoO3.The qmax calculated by the Langmuir model was 84.03 mg·g-1,which was very close to the experimental value of 80.41 mg·g-1.It was speculated that the possible adsorption mechanisms include electrostatic interaction,reduction and chelation.(2)The precursor of cobalt oxide/molybdenum dioxide/molybdenum carbide(CoO/MoO2/Mo2C)adsorbent was prepared by one-step hydrothermal method,and then the CoO/MoO2/Mo2C adsorbent was synthesized by high-temperature calcination.The characterization by SEM,TEM,high power lens(HRTEM),X-ray diffraction(XRD)and X-ray photoelectron spectroscopy(XPS)showed that the cobalt oxide/molybdenum dioxide/molybdenum carbide(CoO/MoO2/Mo2C)adsorbent with high specific surface area was successfully prepared.The results of simulated Cr(Ⅵ)adsorption experiments showed that the combination of the three materials enhanced the adsorption performance of the monomer materials and reached the maximum adsorption capacity at the initial concentration of 200 ppm.The fitting result of the quasi-two-stage kinetic model for the adsorption process was better.The second-stage kinetic regression coefficient(R2=0.999)was much larger than the first-stage kinetic regression coefficient(R2=0.555).The calculated qmax based on the quasi-two-stage kinetic model was 270.3 mg·g-1,which was very close to the experimental result of 263.9 mg·g-1,indicating that the adsorption was controlled by the chemical mechanism.The regression coefficient value of Langmuir isothermal line(R2=0.942)was higher than that of Freundlich isothermal line(R2=0.708),suggesting that the adsorption was more in line with the Langmuir model,and it was single-molecule layer adsorption.(3)The monomer materials of ferric hydroxide and molybdenum-molybdenum carbide were prepared by simple chemical method and high-temperature calcination method,respectively.Then the FeOOH was supported on Mo-Mo2C by impregnation method to synthesize the adsorbent of FeOOH/molybdenum-molybdenum carbide(FeOOH/Mo-Mo2C).The adsorbent had an adsorption capacity up to 377.07 mg·g-1.The adsorption performance of this adsorbent was obviously superior to that of FeOOH and Mo-Mo2C monomers.The degradation experiment was carried out using simulated chromium-containing wastewater.The experiment proved that the adsorbent had good adsorption performance under acidic condition,and its zero point of charge was 4.75.The kinetics and isothermal line model fitting showed that the adsorption process was also more in line with the two-stage kinetics model and Langmuir model,indicating that the adsorption was single molecular layer chemisorption,and the adsorption amount of the material decreased from 377.07 mg·g-1 to 327.51 mg·g-1 after being recycled for five times,indicating that the regeneration performance was good,and the adsorption capacity for Cr(Ⅵ)was better than that of most materials.