| Chlorinated organic compounds (COCs) have been introduced into environmentthrough various channels and become persistent organic pollutant (POPs) which areendangering human health and ecological environment, while been used in every area ofindusty as very important chemical materials. Advanced reduction technology, such aszero-valent iron (ZVI) technology, provides a new way for dechlorination anddetoxification of COCs. ZVI is a cheap, non-toxic and harmless technology, which canreduce and dechlorinate COCs at room temperature and atmospheric pressure, and isalso promising zero carbon emissions. After the introduction of Pd the catalyst andNano-materials technology, the ZVI reduction system appears much betterdechlorination efficiency, and now to be supported as nano-scale catalytic reducingagent is the main trend of the future development of ZVI. The film-supported bimetallicNPs technology is still having several problems to deal with: the modification methodsof PVDF films are defective; the catalytic reductive system is not very effective todechlorinate COCs with smaller molecular and less chlorines; and there aren’t anysystematic studies about the mechanism of dechlorination by film-supported Pd/Fe NPs.In this study, Pd/Fe bimetallic nano-particles were supported by noble modifiedpoly(vinylidene fluoride)(PVDF) films, which were prepared by three-step hydrophilicmodification, and used to dechlorinate COCs.First of all, PVDF films were modified to have hydrophilicity and PVDF-g-AAfilms were produced by three modification steps:(1) alkaline treatment to defluorinatethe original PVDF film;(2) nucleophilic addition to hydrophilize the PVDF film; and (3)grafting acrylic acid to change the hydrophilization extent of the modified support film.The reaction parameters of the “alkaline treatment” step were optimized by using anorthogonal test, and the results were: the concentration of KOH was0.5mol/L, theconcentration of KMnO4was2wt.%, reaction temperature was40oC, and reaction timewas15min. The optimal concentration of acrylic acid was experimentally testified as20wt.%by testing Fe loading content of films and dechlorination of monochloroaceticacid (MCAA). Secondly, based on the “alkaline treatment” step, PVDF films weremodified by grafting polyacrylic acid (PAA), and PVDF-g-PAA-1films andPVDF-g-PAA-2films were prepared by grafting method and in situ polymerizationmethod, respectively. The optimal PAA concentration for PVDF-g-PAA-1preparationwas30wt.%, which was testified by testing Fe loading content of films and MCAAdechlorination; and the reaction parameters of the in situ polymerization method wereoptimized by orthogonal experiment, and the results were: the concentration of acrylicacid was30wt.%, the dosage of benzoyl peroxide (BPO) was50mg, reactiontemperature was80oC, and reaction time was3h. PVDF films, PVDF-g-AA films, PVDF-g-PAA-1films, PVDF-g-PAA-2films, andPd/Fe nano-particles (NPs) supported by three kinds of modified PVDF films werecharacterized by SEM, EDS, XPS, XRD, FT-IR, BET, and contact angle analysis toidentify morphology, the composition and valence of surface elements, surface groups,the specific surface area, and hydrophilicity. The results suggest that these three kinds ofmodified films had been hydrophilised (PVDF-g-PAA-2> PVDF-g-PAA-1>PVDF-g-AA); and Pd/Fe bimetallic NPs were immobilized in the support films with adiameter about50nm, which had better dispersion and smaller aggregation tendency.Effects of Pd loading, NPs addition, initial concentration of the contaminant, initialpH value of reaction system, and reaction temperature on dechlorination efficiency ofMCAA and2,4-dichlorophenol (DCP) were investigated. In the MCAA dechlorinationsystem, the optimal Pd loading contents of Pd-Fe/PVDF-g-AA, Pd-Fe/PVDF-g-PAA-1,and Pd-Fe/PVDF-g-PAA-2were1.202wt.%,1.197wt.%, and1.213wt.%, respectively;and in the DCP dechlorination system, the optimal Pd loading contents were0.505wt.%,0.513wt.%, and0.576wt.%, respectively. Dechlorination efficiency of both MCAA andDCP increased with the increase of Pd loading content when Pd loading was below theoptimal content, while decreased when Pd loading was beyond the optimal content.Increasing NPs addition or reaction temperature resulted in the increase ofdechlorination efficiency, whereas increasing initial concentration of MCAA or DCPcaused the decrease of dechlorination efficiency. The dechlorination system byfilm-supported Pd/Fe NPs were more tolerant in pH change than the system by freesuspended Pd/Fe NPs.Film-supported Pd/Fe NPs had better dechlorination efficiency of MCAA or DCPthan both film-supported ZVI NPs and free suspended Pd/Fe NPs. The catalysis of Pdwas testified by calculating the activation energy of the dechlorination reaction. Thehydrogen produced by the ZVI corrosion could dissociate into highly reducinghydrogen atoms on the surface of Pd NPs, which promotes the indirect reductionreaction. On the other side, the support film doesn’t only play the role of loading NPs,but also could promote the dechlorination by increasing the dispersion of Pd/Fe NPs anddecreasing the excessive accumulation of hydrogen on the surface of Pd/Fe NPs.Among three kinds of film-supported Pd/Fe NPs, Pd-Fe/PVDF-g-PAA-2appeared thebest catalytic reductive efficiency. Moreover, The stability of Pd-Fe/PVDF-g-PAA-2were preliminarily studied by a reusing batch experiment where above90%dechlorination efficiency of MCAA was accomplished for7times. The good chelationfor Fe2+of PAA on the PVDF-g-PAA-2would avoid the secondary pollution caused bythe dissolved iron and prevent the formation of iron oxide passivation layer, whichcould avoid the deactivation of immobilized Pd/Fe NPs. |
PDF Full Text Request