| 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. |
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