| Nanoscale zero-valent iron(nZVI)is an extremely promising environmental remediation nanomaterial.Because of its strong reducibility,it can reductively remove a series of pollutants,including halogenated alkanes,halogenated aromatic hydrocarbons,non-metallic inorganic substances,and heavy metal ions.However,the industrial grade nZVI used in the current market has usually a large size with an uneven distribution,thus greatly reducing its application performance.In this context,a method for continuous and rapid preparation of nZVI with small size and narrow distribution in a rotating packed bed(RPB)reactor was proposed in this thesis,and the influences of various experimental factors on the particle size and dispersibility were explored.In addition,its application in the removal of heavy metal ions was also studied.The main contents were as follows:(1)nZVI was prepared in a conventional stirred tank reactor(STR)using sodium borohydride reduction method.The influences of experimental factors such as the reaction solvent,the type and amount of surfactant,the molar ratio of reactants,the reaction temperature,the ratio of alcohol to water in the solvent,and the concentration of Fe2+ on the particle size and particle dispersity were investigated.The obtained optimal preparation conditions in the beaker are as follows:ethanol-water mixed solution was used as solvent,the volume ratio of ethanol to water was 1:1,CMC was used as surfactant,its dosage was 30 wt.%,the molar ratio of FeSO4 to NaBH4 was 1:3,the concentration of Fe2+ was 0.05 M and the reaction temperature was 3 0℃.nZVI obtained under the optimal preparation comditions showed a core-shell structure with particle size ranging from 10 to 25 nm and an average particle size of approximately 17.0 nm.XRD and XPS results showed that the core of nZVI was zero-valent iron coated with a layer of iron oxide.FT-IR results indicated that CMC was successfully grafted onto the surface of nZVI.Grafting negatively charged CMC molecules can inhibit the growth of nano iron particles,and relieve the agglomeration resulted from its own magnetism.So small and well-dispersed nZVI particles can be obtained.(2)Based on the optimal preparation conditions in the STR,nZVI was prepared in the RPB using high-gravity technology.The effects of the type and amount of surfactant,the rotating speed and feed flow rate of RPB on the particle size was investigated.The obtained optimal preparation conditions in the RPB are as follows:PVP was used as surfactant and its dosage was 10 wt.%,rotating speed was 2000 rpm,feed flow rate was 555 mL/min,other parameters followed the optimal conditions in the beaker.The average particle size of nZVI prepared in RPB was 13.7 nm,which was smaller than that prepared in the STR,and the particle size distribution was narrower.Besides,nZVI was still core-shell structured because of its surface oxidation.(3)The application of nZVI for Cr(Ⅵ)removal was studied using diphenylcarbazide spectrophotometry.The results showed that Cr(Ⅵ)removal efficiency increased with the increase of nZVI dosage,dereased with the increase of initial pH,and increased with the increase of reaction temperature.Kinetic studies showed that the reaction process of Cr(Ⅵ)reduction by nZVI well accorded with the pseudo-first-order reaction kinetics model.The reaction rate constant also increased with the increase of nZVI dosage,dereased with the increase of initial pH,and increased with the increase of reaction temperature.Thermodynamic studies illustrated that the reaction process was spontaneous.The process absorbed heat and had increased entropy.Compared with STR-nZVI,RPB-nZVI had higher Cr(Ⅵ)removal efficiency and faster reaction rate due to its smaller particle size.RPB-nZVI could completely remove Cr(Ⅵ)after 10 min,while the removal efficiency was only 80.3%for STR-nZVI,and it only reached 92.5%after 60 min.Moreover,the reaction rate constant of RPB-nZVI was 0.6332 min-1,which was 17.4 times than that of STR-nZVI. |
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