The Study On Dechlorination Of Chlorinated Organic Compounds In Water By Ni/Fe Nanoparticles And With Assistance Of Ultrasound

The Ni/Fe bimetallic particles have been prepared using sodium borohydride (NaBH₄) in laboratory as the reductant to reduce Ni²⁺ and Fe²⁺ in aqueous solution sinultaeously. The shape, diameter, content of Ni and Fe, BET surface area and crystal structure of the particles have been characterized by TEM, XRD, BET and XPS.The research of this paper involves:(1)Study on the catalytic reductive dechlorination of trichloroethene, 4-chlorophenol, 2,4-dichlorophenol and pentachlorophenol using laboratory-prepared Ni/Fe nanoscale bimetallic particles. Trichloroethene, a typical volatile chlorinated organic compounds (COCs), chlorophenols, soluble and relatively difficult to dechlorinate, and pentachlorophenol, has been used extensively throughout the world as wood preservative and general biocide, and was listed in the priority pollutants the US EPA; (2) To overcome the foible of the hydroxides covered layer on the surface of Ni/Fe bimetallic particles during reaction process, the technigue of coupled ultrasound was employed; (3) Ultrasound was used for ultrasonic dechlorination of the COCs only.The factors that influence the dechlorination efficicncy, such as initial pH value, nickel content in the Ni/Fe particles, metal to solution ratio, initial concentration of the COCs, reaction temperature and output power of ultrasonic irradiation have been investigated. The mechanisms of Ni/Fe bimetallic nanoparticles for catalytic reduction dechlorination of COCs and ultrasonic irradiation COCs have been deduced through the results from GC/MS and HPLC analysis. The pseudo-first order apparent rate constant and apparent activation energy have been calculated from the concentration variation of reacts and products.The results are as following:1. The particles were proved to be nanoscale spherical with an average diameter of about 30 nm. The Ni/Fe bimetallic particles with various nickel contents can be synthesized through adjusting the concentration ratio of Ni²⁺ and Fe²⁺ in aqueous solution. The Ni/Fe bimetallic nanoparticles with nickel content 5 wt% has a BET surface area of 22.8 m²/g. XPS results indicate that the particles surface have been oxided at a certain extent, and its surface concentration of nickel is higher than its total concentration in the Ni/Fe particles. The XRD patterns of the fresh Fe, Ni/Fe nanoparticles and those have been irradiated by ultrasound under acid conditon for 40 min show the only one broad peak around 2θ= 44.5°, indicating that Ni, Fe and B species form amorphous alloy. It is belived that such amorphous alloy structure is favorable for both the activity and selectivity, and even the sulfur resistance in many hydrogenation reactions owing to the stronger synergistic effect between active sites, the more highly unsaturated active sites, and the more homogeneous distribution of these active sites.2. The nanoscale Ni/Fe bimetallic particles exhibit high reactivity for trichloroethylene, 4-chlorophenol, 2,4-dichlorophenol and phentachlorophenol dechlorination. Dechlorination efficiencies of trichloroethylene, 4-chlorophenol, 2,4-dichlorophenol is above 70%, 87% and 86%, respectively, in 60 min at initial pH value 6.5, nickel content of 5 wt% and Ni/Fe metal to solution ratio of 0.4 g/L. However, pentachlorophenol is hardly to dechlorinate at above conditions, the dechlorination efficiency of pentachlorophenol is only to 17% in the same period under the condition of initial pH value 5.4, nickel content of 10 wt%, Ni/Fe metal to solution ratio of 2.5 g/L and assistance of ultrasound.3. The results also indicate that ultrasonic irradiation is very effective for the degradation of trichloroethylene, the recovery of chloride reaches 45% within 60 min (relation to the initial trichloroethylene concentration). However, the degradation efficiencies are scarcely any for the chlorophenols at the same conditions. The dechlorination efficiency of trichloroethylene by using ultrasonic irradiation combined with nanoscale bimetallic Ni/Fe particles is higher than the sum of Ni/Fe particles and ultrasound individually. The dechlorination efficiencies of trichloroethylene in the presence of ultrasonic irradiation only, bimetallic Ni/Fe nanoparticles only, and both ultrasonic irradiation and bimetallic nanoparticles is about 6%, 26% and 45% accordingly. This resoults indicate that synergic effect occurred when Ni/Fe nanoscale particles and ultrasonic irradiation present simultaneously. This synergic effect is dramatically for dechlorination of pentachlorophenol. The enhancement is attributed to the continuous cleaning and activation of the particles surface by the combined chemical and physical effects of acoustic cavitiation.4. The factors that influence the dechlorination efficicncy of COCs using nanoscale Ni/Fe bimetallic particles include: initial pH value, nickel content in Ni/Fe bimetal, the mass concentration of nanoscale Ni/Fe particles in the reaction mixture and reaction temperature. Lower initial pH value, higher nickel content, higher bimetal to solution ratio and higher reaction temperature favored the dechlorination reaction. No dependence is found between reaction rate and initial concentration of the above COCs. The apparent activation energies of 4-chlorophenol and 2,4-dichlorophenol reaction with nanoscale Ni/Fe particles is 21.1kJ/mol and 58.2 kJ/mol, respectively.5. The factors that influence the degradation of trichloroethylene in aqueous solution using ultrasonic irradiation include: the output power of ultrasound, the kind of cavitating gases, initial concentration of trichloroethylene and the presence of radical scavenger. The degradation efficiency of trichloroethylene decreases with the increase of initial concentration of trichloroethylene in the pseudo-first order kinetics. With increasing ultrasonic power, the degradation efficiency is enhanced obviously, and the degradation rate reaches over 90% within 60 min. The effect of selected cavitating gases on the degradation efficiency is in order Ar > O₂～Air. The presence of radical scavenger has a little effect on the degradation efficiency wich indicates that ultrasonic degradation of trichloroethylene occurs predominantly both at the bobbles and the liquid-gas interfaces of bubbles where it undergoes high-temperature combustion. Moreover, the results suggest that radical oxidation could appear during the process of ultrasonic degradation of TCE.6. The factors that influence the dechlorination efficiency of pentachlorophenol by Ni/Fe bimetallic nanoscale particles with the assistance of ultrasound as below: initial pH value, nickel content in the Ni/Fe particles, the mass concentration of nanoscale Ni/Fe particles in the reaction mixture and output power of ultrasonic irradiation. The results indicate that Ni/Fe nanoscale bimetallic particles are very effective for the dechlorination of pentachlorophenol. Dechlorination efficiency is 46% in 30 min under the optimal condition without assistance of ultrasound, whereas it increase to 96% when ultrasonic irradiation is used. Initial pH value shows apparent effect on the dechlorination. As the pH varies from acidic condition to neutral condition, the dechlorination efficiency decreases dramatically. In addition, the dechlorination efficiency is improved with the increase of Ni/Fe ratio, nickel content in Ni/Fe particles and ultrasonic output power.7. The reaction products of trichloroethylene with Ni/Fe nanoparticles has been determined by using GC/FID and GC/MS. The results indicate that the dechlorinted products of trichloroethylene include ethane, butane and butene. No chlorinated intermediate and final products were found. It can be concluded that the dechlorination of trichloroethylene occurs when adsorbed on the surface of Ni/Fe bimetallic nanopartices, and the hydrogenation of trichloroethylene simultaneity. There is no other degradation products except chlorine when trichloroethylene treated by ultrasonic irradiation, which indicate that trichloroethylene has been mineralized. The dechlorination products of chlorophenols determined by HPLC and GC/MS are as following: phenol and chlorine for 4-chlorophenol, no other species have been found; phenol and less 2-chlorophenol, 4-chlorophenol for 2,4-dichlorophenol during the reaction process, which indicate that most of the dechlorinted intermediates do not turn into solution but maitained on the surface of Ni/Fe particles to dechlorinate sequentially; and for pentachlorophenol, less chlorinated phenols including tetrachlorophenol, trichlorophenol, dichlorophenol, monochlorophenol are formed during the initial reaction,and phenol is determined as sole product in the end of reaction.