【作者】 徐江；

【导师】 徐新华；

【作者基本信息】 浙江大学 ， 环境工程， 2014， 博士

【摘要】 利用纳米级零价铁(NZVI)技术可以有效去除2,4-二氯苯酚(2,4-DCP),将其还原脱氯转化为苯酚,达到降低毒性和可生物降解的目的。但是,NZVI颗粒由于磁力和范德华力的作用而容易发生团聚,反应之后生成的含铁氢氧化物和氧化物会在NZVI表面形成钝化层,致使其反应活性迅速下降；而且由于颗粒极小,反应后的纳米材料难以回收,如果排放存在一定的环境风险。故本研究通过制备以下纳米级稳定化NZVI体系来克服这些缺点。10-20nm多壁纳米碳管(MWCNTs)可以很好地负载Pd/Fe纳米颗粒,有效去除2,4-DCP。由于π-π键的作用,MWCNTs对苯酚(P)、邻氯酚(o-CP)、对氯酚(p-CP)和2,4-DCP的吸附能力随着苯环上的-C1基团的增多而逐渐增大,反应1min内的吸附去除率分别达到19.7%,60.5%,72.0%和95.1%。当MWCNTs作为载体负载NZVI之后,可以有效抑制NZVI的团聚和钝化现象,尽管NZVI占据部分表面会减弱MWCNTs的吸附能力,但在反应1min内目标污染物2,4-DCP的去除率仍旧达到50%左右,并且随着时间推移逐渐增加至100%,中间脱氯产物(o-CP和p-CP)的浓度在反应过程中始终维持较低水平(<1mg L-1),最终脱氯产物(P)被逐渐释放返回液相中,通过MWCNTs对有机物的不同吸附能力,以及对2,4-DCP的吸附、脱氯以及P的脱附这一过程,达到污染物的降解目的。同时选用较大管径的60-100nmMWCNTs作为对比,两种规格的MWCNTs对酚的吸附能力都遵循2,4-DCP>p-CP> o-CP>P的顺序,通过批量吸附实验探明了MWCNTs对四种有机物的吸附动力学和吸附等温线模型,并系统地考察了两种规格的MWCNTs-Pd/Fe体系对2,4-DCP的去除效果。60-100nm和10-20nmMWCNTs负载的Pd/Fe体系都可以有效去除2,4-DCP,当钯化率为0.20wt%时,反应1min的去除率分别达到41.8%和53.2%,反应5h后逐渐增加到94.6%和77.7%。相比之下,60-100nmMWCNTs负载的Pd/Fe体系的还原脱氯作用较10-20nm MWCNTs强,而吸附作用较弱。60-100nmMWCNTs的吸附和Pd/Fe纳米颗粒的还原作用的结合效果较好。纳米级Fe3O4-Pd/Fe体系比单独Pd/Fe体系具有更高的2,4-DCP还原脱氯效率。Fe3O4的存在使其在外加磁场的作用下为纳米材料的回收提供可能,并且可以显著增强2,4-DCP的脱氯效果；当体系中存在3g L-1Pd/Fe时,Pd/Fe采和Fe3O4的最佳质量比是3.0：2.0,反应5h内可以去除76.4%的2,4-DCP,明显高于同样条件下3.0：0.0(35.8%),3.0：1.0(58.0%),3.0：2.5(66.3%)和3.0：3.0(7.4%)质量比时的去除率。随着钯化率增加,2,4-DCP的脱氯和P的生成速率明显加快,当钯化率为0.20wt%时2,4-DCP去除率最高,反应5h达到98.2%。另外,体系中共存的阴阳离子、天然有机物也会影响2,4-DCP的去除效果。Fe3O4-Pd/Fe体系具有很稳定的催化活性和可循环性。纳米级MWCNTs、Fe3O4和Pd/Fe组成的复合体系可以有效地协同去除2,4-DCP,反应1min,5h和72h后,2,4-DCP的去除率分别达到54.2%,92.3%和100%。探讨了MWCNTs-Fe3O4-Pd/Fe体系对2,4-DCP, p-CP,2-CP和P的吸附动力学,热力学和等温线模型。MWCNTs存在的体系均对污染物具有吸附选择性,遵循目标污染物(2,4-DCP)>中间脱氯产物(p-CP和o-CP)>最终脱氯产物(P)的吸附顺序。MWCNTs-Fe3O4-Pd/Fe体系可以实现快速吸附-逐步脱氯-最终释放的过程。反应72h后,由于吸附能力差,超过82.7%的P脱附并返回液相中,使得MWCNTs-Fe3O4-Pd/Fe颗粒表面的大部分活性位点得到恢复。在5个连续实验中,MWCNTs-Fe3O4-Pd/Fe体系都保持了很高的反应活性,并且反应后的纳米材料可以达到预期的磁性分离回收效果。而另外相关的纳米体系(MWCNTs, MWCNTs-Fe3O4, MWCNTs-Fe3O4-Fe, Pd/Fe, Fe3O4-Pd/Fe和MWCNTs-Pd/Fe)较MWCNTs-Fe3O4-Pd/Fe复合体系在吸附能力、脱氯效果及可回收性等方面均有不同程度的下降。更多还原

【Abstract】 2,4-dichlorophenol (2,4-DCP) could be effectively removed by nano-scale zero valent iron (NZVI), and dechlorinated to phenol (P), which was of low toxicity and more biodegradable. However, NZVI particles would be aggregated due to magnetic and van der Waals force, and new generated iron hydroxide and oxide would form a passivation layer on the NZVI surface, then the reactivity would decline. Moreover, nano materials were difficult to retrieve as the particles were too small, which had environmental risk. In order to overcome these shortages, nano-scale stabilized NZVI systems were prepared:Nanoscale Pd/Fe particles were well supported by10-20nm multi-walled carbon nanotubes (MWCNTs), which were used to remove2,4-DCP. The adsorption capacity of MWCNTs was found to be increased with the increasing amount of chlorine atoms, and the removal rate of P,2-chlorophenol (2-CP),4-chlorophenol (4-CP) and2,4-DCP reached19.7%,60.5%,72.0%and95.1%in a short period of1min by MWCNTs, respectively, which probably due to π-π interaction. The adsorption kinetics and adsorption isotherm were discussed. MWCNTs as a supporter, was effective for avoiding the agglomeration of nZVI. Furthermore, the speedy removal efficiency of the initial substances (2,4-DCP) reached about50%in1min, and over the time continued to rise to100%, remaining low concentrations (<1mg L-1) of the intermediate substances (o-CP, p-CP), and gradual release of the final substance (P) from MWCNTs-Pd/Fe composites during the whole process, proposed a novel method for in situ remediation technology.60-100nm MWCNTs was also introduced, and a systematic investigation of two specified MWCNTs-Pd/Fe nanocomposites to remove2.4-DCP is presented. Both MWCNTs-Pd/Fe nanocomposites showed excellent adsorption efficiencies for phenols and followed the sequential order;2,4-DCP> p-CP> o-CP> P. Batch adsorption experiments including kinetics and isotherm were also intensively investigated. Significantly high2,4-DCP removal was observed after1min when it reached to49.7%and53.2%, then continuously increased up to95.2%and77.7%after5h at0.20wt%Pd loading by60-100nm and10-20nm MWCNTs-Pd/Fe nanocomposites, respectively. However, stronger dechlorination and weaker adsorption was found in60-100nm MWCNTs-Pd/Fe nanocomposites. Moreover, an integrated approach of physical adsorption by60-100nm MWCNTs and chemical reduction by Pd/Fe nanoparticles to remove2,4-DCP was successfully achieved.The nano-scale Pd/Fe-Fe3O4nanocomposites showed higher dechlorination efficiency of2,4-DCP rather than bare Pd/Fe nanoparticles in the batch dechlorination experiments. Fe3O4provided a convenient way to recycle the nanocomposites with an external magnetic field and significantly enhanced2,4-DCP dechlorination.3.0:2.0nFe3O4was the optimal mass ratio of Pd/Fe:Fe3O4in the presence of3g L-1Pd/Fe nanoparticles in our system, removing76.4%2,4-DCP within5h in the aqueous environment. This was much higher than3.0:0.0(35.8%),3.0:1.0(58.0%),3.0:2.5(66.3%), and3.0:3.0(7.4%) mass ratio of Pd/Fe:Fe3O4employed under the same conditions. Efficiencies of dechlorination and phenol formations were increased significantly when the amount of Pd increased, whereas the highest2,4-DCP removal efficiency was observed98.2%at0.20wt%Pd loading. Moreover, co-existed ions and nature organic material would affect the2,4-DCP removal. The nanocomposites showed stable catalytic activity, and promising to recycle during the process.2,4-DCP was effectively removed by the synergetic system consists of nano-scale MWCNTs, Fe3O4and Pd/Fe. As high as54.2%,92.3%, and100%of2,4-DCP was simultaneously adsorbed and dechlorinated after1min,5h, and72h, respectively. The adsorption kinetics, thermodynamics, and isotherms of2,4-DCP, p-CP,2-CP, and P were discussed. Notably, an adsorption priority order of initial contaminant (2,4-DCP)> intermediate dechlorinated product (p-CP and o-CP)> final dechlorinated product (P) was observed in the presence of MWCNTs. Rapid adsorption, gradual dechlorination, and final desorption were achieved by MWCNTs-Fe3O4-Pd/Fe system. Over82.7%of P was desorbed and released to aqueous phase after72h due to its low adsorption capacity leaving majority of active sites available on the surface of MWCNTs-Fe3O4-Pd/Fe. MWCNTs-Fe3O4-Pd/Fe system maintained high activity in continuous five runs experiment, and exceptional retrievability was revealed via external magnetic separation after reaction. Other relative system (MWCNTs, MWCNTs-Fe3O4, MWCNTs-Fe3O4-Fe, Pd/Fe, Fe3O4-Pd/Fe, and MWCNTs-Pd/Fe) was found to have one or two shortcomings of lower adsorption, weaker dechlorination, and poorer retrievability.更多还原