纳米Ni/Fe及超声辅助纳米Ni/Fe对水中氯代有机物脱氯研究
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摘要
本文用液相化学还原法制备了纳米Fe及纳米Ni/Fe双金属合金粉末,通过TEM、AAS、BET-N_2、XRD和XPS等表征手段,测定了纳米Fe和纳米Ni/Fe颗粒的形状和大小,Ni/Fe中Ni、Fe等组分含量,BET比表面积及其物相结构(晶体结构),表面组成及表面电子能态。
本文研究内容:1)采用所制备的纳米Ni/Fe双金属微粒对水中具有代表性的小分子氯代脂肪烃三氯乙烯(TCE)和氯酚类如4-氯酚(4-CP),2,4-二氯酚(2,4-DCP)及常规下难以生物降解的五氯酚(PCP)等进行了催化还原脱氯研究;2)为了克服在反应过程中Ni/Fe双金属表面生成氢氧化物覆盖的弱点,引用了超声波技术加以辅助;3)利用超声辐照对上述氯代有机物进行单纯超声空化降解。
上述实验考察了纳米Ni/Fe中Ni含量、纳米Ni/Fe的投加量、初始pH值、反应温度、反应物初始浓度、超声波输出功率等因素对处理效果和反应速率的影响,并通过中间产物和最终产物分析,研究了上述氯代有机物在纳米Ni/Fe双金属作用下催化还原脱氯及在超声空化作用下降解的反应机理。获得了反应过程中各反应物和生成物的浓度变化规律,并计算了准一级表观速率常数和表观反应活化能。
实验结果表明:
1.纳米Fe及纳米Ni/Fe颗粒的平均粒径在30 nm左右,通过调节Fe~(2+)和Ni~(2+)的浓度配比,可制备出不同Ni含量的Ni/Fe双金属纳米粒子。Ni含量为5wt%的Ni/Fe纳米颗粒,其BET比表面积为22.8 m~2g~(-1)。XPS结果表明,纳米Ni/Fe表面有一定程度的氧化,其表面Ni浓度高于本体Ni浓度。纳米Fe及纳米Ni/Fe的XRD谱图上只在20约为44.5°处有一极为弥散的宽衍射峰,表明其为非晶态合金结构,此种物相结构具有较高的加氢催化活性和选择性。
2.纳米Ni/Fe双金属对水中TCE、4-CP、2,4-DCP及PCP均具较好的加氢脱氯效能,在初始pH值为6.5、Ni含量5wt%、Ni/Fe投加量为0.4g/L的条件下,60min内TCE的脱氯率达72%,4-CP在此条件下的脱氯率60min达87%,2,4-DCP的脱氯率为85.6%,PCP的脱氯反应较难,在初始pH值为5.4,Ni含量为10%,Ni/Fe投加量为2.5g/L及超声辅助下,PCP的去除率仅为16.7%。
3.超声辐照对TCE有较好的降解效果,辐照60min氯离子回收率(与初始浓度有关)达45%,而用所选的超声源对氯酚类物质进行超声辐照却没有明显的降解效果。超声辅助纳米Ni/Fe对TCE处理效果高于二者单独作用处理效果的加和,表明二者具有协同效应。在选定的实验条件下反应30min,单独超声辐照、单独纳米Ni/Fe及二者协同作用下,TCE的脱氯率分别为6%、26%和45%。超声辅助纳米Ni/Fe处理PCP时,二者的协同作用极为显著。产生这种协同效应的原因:由于超声波的空化效应可产生极强烈的冲击波和微射流以及震动和搅拌,可以去除双金属表面形成的钝化层,强化界面间的化学反应和传递过程,促进反应表面的更新和活化,因此可大大提高反应活性。
4.影响纳米Ni/Fe双金属对上述氯代有机物催化还原脱氯效果的因素有:初始pH值、Ni含量、Ni/Fe投加量及反应温度等。实验结果表明,较低初始pH值、较高Ni含量、较大的Ni/Fe投加量均有利于脱氯反应;在一定温度范围内,相对较高的反应温度有利于脱氯反应的进行;在实验所考察的初始浓度范围内,处理效果与其初始浓度关系较小。4-CP与纳米Ni/Fe反应的表观活化能为21.1kJ/mol,2,4-DCP与纳米Ni/Fe反应的表观活化能为58.2kJ/mol。
5.超声辐照降解水中TCE反应速率的主要影响因素有:超声波的输出功率、饱和气体的种类、TCE的初始浓度及自由基清除剂等。TCE降解率随初始浓度增加而减小,呈现准一级反应动力学特性;增加超声波输出功率可以明显提高TCE的降解率,在60min内TCE的去除率可达90%以上;空化气体对TCE降解率的影响顺序为Ar＞O_2～空气;自由基清除剂的加入对TCE的降解效果稍有影响,表明TCE的超声降解主要是在空化泡内以及空化泡的气-液界面内的高温热解,同时在超声降解TCE过程中也可能伴随着自由基氧化作用。
6.超声辅助纳米Ni/Fe双金属处理PCP的主要影响因素有:初始pH值、Ni含量、镍/铁投加量、超声波输出功率等。结果表明,纳米Ni/Fe双金属粒子对PCP具有很好的脱氯效果,在最佳条件下,反应30 min内脱氯率达46%,当有超声波辅助时,PCP的脱氯率在相同反应时间内可高达96%。初始pH值对脱氯效果有非常显著的影响,当从酸性变为中性时,脱氯率显著降低。此外,较高的Ni含量、较大的Ni/Fe投加量及较大的超声波输出功率均有利于PCP的脱氯反应。
7.利用GC/FID和GC/MS分析了TCE脱氯产物,结果表明,在纳米Ni/Fe的作用下,TCE的脱氯产物为乙烷、丁烷和丁烯等,没有检测到氯代中间产物,可推测该反应是发生在双金属表面的加氢脱氯。超声辐照处理TCE时,除氯离子以外没能检测到任何其他降解产物,可认为其彻底矿化了。由HPLC和GC/MS对氯代酚类脱氯产物的检测结果表明:4-CP的脱氯产物只有苯酚和氯离子,脱氯过程中没有生成其他氯代产物。2,4-DCP的脱氯中间产物有2-CP和4-CP,且在反应过程中,两种单氯酚的量一直很小,表明中间产物没有发生积累;实验过程中检测到PCP的各种脱氯中间产物,但在反应的最初阶段即有苯酚生成,表明2,4-DCP和PCP在Ni/Fe双金属表面既发生逐级脱氯又发生两个或以上的氯原子同时脱去,反应的最终产物只有苯酚和氯离子。
The Ni/Fe bimetallic particles have been prepared using sodium borohydride (NaBH_4) in laboratory as the reductant to reduce Ni~(2+) and Fe~(2+) 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 Ni~(2+) and Fe~(2+) in aqueous solution. The Ni/Fe bimetallic nanoparticles with nickel content 5 wt% has a BET surface area of 22.8 m~2/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 > O_2～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.
本文研究内容:1)采用所制备的纳米Ni/Fe双金属微粒对水中具有代表性的小分子氯代脂肪烃三氯乙烯(TCE)和氯酚类如4-氯酚(4-CP),2,4-二氯酚(2,4-DCP)及常规下难以生物降解的五氯酚(PCP)等进行了催化还原脱氯研究;2)为了克服在反应过程中Ni/Fe双金属表面生成氢氧化物覆盖的弱点,引用了超声波技术加以辅助;3)利用超声辐照对上述氯代有机物进行单纯超声空化降解。
上述实验考察了纳米Ni/Fe中Ni含量、纳米Ni/Fe的投加量、初始pH值、反应温度、反应物初始浓度、超声波输出功率等因素对处理效果和反应速率的影响,并通过中间产物和最终产物分析,研究了上述氯代有机物在纳米Ni/Fe双金属作用下催化还原脱氯及在超声空化作用下降解的反应机理。获得了反应过程中各反应物和生成物的浓度变化规律,并计算了准一级表观速率常数和表观反应活化能。
实验结果表明:
1.纳米Fe及纳米Ni/Fe颗粒的平均粒径在30 nm左右,通过调节Fe~(2+)和Ni~(2+)的浓度配比,可制备出不同Ni含量的Ni/Fe双金属纳米粒子。Ni含量为5wt%的Ni/Fe纳米颗粒,其BET比表面积为22.8 m~2g~(-1)。XPS结果表明,纳米Ni/Fe表面有一定程度的氧化,其表面Ni浓度高于本体Ni浓度。纳米Fe及纳米Ni/Fe的XRD谱图上只在20约为44.5°处有一极为弥散的宽衍射峰,表明其为非晶态合金结构,此种物相结构具有较高的加氢催化活性和选择性。
2.纳米Ni/Fe双金属对水中TCE、4-CP、2,4-DCP及PCP均具较好的加氢脱氯效能,在初始pH值为6.5、Ni含量5wt%、Ni/Fe投加量为0.4g/L的条件下,60min内TCE的脱氯率达72%,4-CP在此条件下的脱氯率60min达87%,2,4-DCP的脱氯率为85.6%,PCP的脱氯反应较难,在初始pH值为5.4,Ni含量为10%,Ni/Fe投加量为2.5g/L及超声辅助下,PCP的去除率仅为16.7%。
3.超声辐照对TCE有较好的降解效果,辐照60min氯离子回收率(与初始浓度有关)达45%,而用所选的超声源对氯酚类物质进行超声辐照却没有明显的降解效果。超声辅助纳米Ni/Fe对TCE处理效果高于二者单独作用处理效果的加和,表明二者具有协同效应。在选定的实验条件下反应30min,单独超声辐照、单独纳米Ni/Fe及二者协同作用下,TCE的脱氯率分别为6%、26%和45%。超声辅助纳米Ni/Fe处理PCP时,二者的协同作用极为显著。产生这种协同效应的原因:由于超声波的空化效应可产生极强烈的冲击波和微射流以及震动和搅拌,可以去除双金属表面形成的钝化层,强化界面间的化学反应和传递过程,促进反应表面的更新和活化,因此可大大提高反应活性。
4.影响纳米Ni/Fe双金属对上述氯代有机物催化还原脱氯效果的因素有:初始pH值、Ni含量、Ni/Fe投加量及反应温度等。实验结果表明,较低初始pH值、较高Ni含量、较大的Ni/Fe投加量均有利于脱氯反应;在一定温度范围内,相对较高的反应温度有利于脱氯反应的进行;在实验所考察的初始浓度范围内,处理效果与其初始浓度关系较小。4-CP与纳米Ni/Fe反应的表观活化能为21.1kJ/mol,2,4-DCP与纳米Ni/Fe反应的表观活化能为58.2kJ/mol。
5.超声辐照降解水中TCE反应速率的主要影响因素有:超声波的输出功率、饱和气体的种类、TCE的初始浓度及自由基清除剂等。TCE降解率随初始浓度增加而减小,呈现准一级反应动力学特性;增加超声波输出功率可以明显提高TCE的降解率,在60min内TCE的去除率可达90%以上;空化气体对TCE降解率的影响顺序为Ar＞O_2～空气;自由基清除剂的加入对TCE的降解效果稍有影响,表明TCE的超声降解主要是在空化泡内以及空化泡的气-液界面内的高温热解,同时在超声降解TCE过程中也可能伴随着自由基氧化作用。
6.超声辅助纳米Ni/Fe双金属处理PCP的主要影响因素有:初始pH值、Ni含量、镍/铁投加量、超声波输出功率等。结果表明,纳米Ni/Fe双金属粒子对PCP具有很好的脱氯效果,在最佳条件下,反应30 min内脱氯率达46%,当有超声波辅助时,PCP的脱氯率在相同反应时间内可高达96%。初始pH值对脱氯效果有非常显著的影响,当从酸性变为中性时,脱氯率显著降低。此外,较高的Ni含量、较大的Ni/Fe投加量及较大的超声波输出功率均有利于PCP的脱氯反应。
7.利用GC/FID和GC/MS分析了TCE脱氯产物,结果表明,在纳米Ni/Fe的作用下,TCE的脱氯产物为乙烷、丁烷和丁烯等,没有检测到氯代中间产物,可推测该反应是发生在双金属表面的加氢脱氯。超声辐照处理TCE时,除氯离子以外没能检测到任何其他降解产物,可认为其彻底矿化了。由HPLC和GC/MS对氯代酚类脱氯产物的检测结果表明:4-CP的脱氯产物只有苯酚和氯离子,脱氯过程中没有生成其他氯代产物。2,4-DCP的脱氯中间产物有2-CP和4-CP,且在反应过程中,两种单氯酚的量一直很小,表明中间产物没有发生积累;实验过程中检测到PCP的各种脱氯中间产物,但在反应的最初阶段即有苯酚生成,表明2,4-DCP和PCP在Ni/Fe双金属表面既发生逐级脱氯又发生两个或以上的氯原子同时脱去,反应的最终产物只有苯酚和氯离子。
The Ni/Fe bimetallic particles have been prepared using sodium borohydride (NaBH_4) in laboratory as the reductant to reduce Ni~(2+) and Fe~(2+) 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 Ni~(2+) and Fe~(2+) in aqueous solution. The Ni/Fe bimetallic nanoparticles with nickel content 5 wt% has a BET surface area of 22.8 m~2/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 > O_2～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.
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