Study On Preparation Of Cellulose-stabilized Pd/Fe Nanoparticles And Catalytic Debromination Of 2,2',4,4'-Tetrabromodiphenyl Ether

As an emergingnew type of persistent organic pollutants,polybrominated diphenyl ethers?PBDEs?have caused great risk to the ecological environment and human health.2,2',4,4'-tetrabromodiphenyl ether?BDE47?has attracted more and more attention due to its high frequency detected in environment and high toxicity among all of the PBDEs congeners.Thus,there is an urgent need to develop a safe,efficient and environmentally friendly technology for BDE47 degradation.In recent years,Pd/Fe nanoparticles?Pd/Fe NPs?have aroused a considerable interest due to their large specific surface areas and high reactivity.However,their application is limited because of agglomeration and oxidation.In order to solve this problem,polyanionic cellulose?PAC?,hydroxyethyl cellulose?HEC?and hydroxypropyl methyl cellulose?HPMC?were have been added during the preparation process of Pd/Fe NPs.In order to select the better cellulose as a stabilizer,a series of characterization methods and debromination reactivity tests for BDE47 were carried out.In this study,an efficient BDE47 catalytic reduction system was constructed by using polyanionic cellulose stabilized Pd/Fe NPs?PAC-Pd/Fe NPs?.Effects of PAC dosage,Pd loading,PAC-Pd/Fe NPs dosage,temperature,initial pH,initial concentration of BDE47,humic acid,DO,common anions,et al.on BDE47 removal were also investigated by batch experiments,and established the reaction kinetics model of BDE47 degradation.Further,the effects of different surfactants on the solubilization and catalytic reduction of BDE47 were also analyzed.Finally,the products and degradation mechanism of BDE47 by PAC-Pd/Fe NPs were also investigated.The main results of this study are as follows:?1?The addition of cellulose can reduce the particle size and increase the specific surface area of the nanoparticles.The average particle size and specific surface area of B-Pd/Fe NPs were 132.5±18.9 nm and 32.3±1.8 m² g^-1,respectively.And the average particle size of HEC-Pd/Fe,HPMC-Pd/Fe and PAC-Pd/Fe NPs were 37.2±9.2,35±8.1 and 27±5.2 nm,their specific surface areaswere 42.2±2.8,43.8±3.2 and 58.6±4.5 m² g^-1,respectively.There were HEC,HPMC and PAC molecules on the surfaces of C-Pd/Fe NPs,respectively.PAC molecules were bound with Fe⁰ by both carboxyl monodentate and intermolecular hydrogen bonding,while HEC and HPMC were only bound onto nanoparticle surfaces by intermolecular hydrogen bonding.Both Fe⁰ and Fe oxides were found on the surfaces of B-Pd/Fe and C-Pd/Fe NPs,but the addition of cellulose could improve the anti-oxidation properties of NPs,and PAC exhibited much better performance than HPMC and HEC.PAC could improve the dispersion stability of Pd/Fe NPs in aqueous solution through electrostaticrepulsion and steric hindrance forces.The dispersion stability of HEC-Pd/Fe and HPMC-Pd/Fe NPs was inferior to PAC,because HEC and HPMC only offered steric hindrance.The addition of cellulose can improve removal performance for BDE47,the reaction activity of PAC-Pd/Fe NPs was the highest,followed by HPMC-Pd/Fe and HEC-Pd/Fe NPs,B-Pd/Fe NPs was the worst.?2?The removal rate of BDE47 had an increased trend firstly and then decreased with the increase of PAC dosage,and the optimum dosage of PAC was 1 g L^-1.Pd loading could significantly enhance the BDE47 removal rate,and the optimum Pd loading was 0.3%.The removal performance of BDE47 increased with the increases of PAC-Pd/Fe NPs dosage,but high dosage would cause the decrease ofunit removal capability?URC?of PAC-Pd/Fe NPs.The best removal performance for BDE47 could be achieved at weak acidic pH.The removal efficiency of BDE47 decreased with the increase of initial BDE47 concentration.The removal performance of BDE47 under anaerobic conditions was better than that of under aerobic condition.The degradation of BDE47 could be inhibited in the presence of NO₃^-,while Cland HCO₃^-had insignificant effect on the removal of BDE47.?3?When the concentration of CTAB,SDS,Brij35 and TX-100 were higher than that of their critical micelle concentration?cmc?,the strong solubilization effect for BDE47 was obvious observed.The solubilization of CTAB,SDS,Brij35 and TX-100 on BDE47 was in accord with the firstorder kinetic model,and the solubilization kinetic coefficients were0.07978,0.09822,0.09189 and 0.2704 h-1,respectively.The amount of CTAB,SDS and Brij35 adsorbed on PAC-Pd/Fe NPs were increased with the increases of surfactant concentration,and the adsorption amount tended to be stable when the surfactant concentration reached cmc,respectively.But for TX-100,the adsorption capacity continued to increase when its concentration was higher than cmc,and remained stable at around 1.5 g L^-1.The removal rate of BDE47 was very low in pure water system.The addition of surfactant could improve the apparent solubility of BDE47,and then increased the removal rate of BDE47.However,excessive surfactant could cause the BDE47 difficult to move out from the hydrophobic micellar core,which caused the low removal rate for BDE47.?4?The catalytic reduction for BDE47 by PAC-Pd/Fe NPs was a debromination reaction.BDE47 was degraded mainly by stepwise debromination.The para-Br could be easier removed when compared to ortho-Br.The major main pathway of catalytic reductive debromination of BDE47 was: "BDE47?BDE17?BDE4?BDE1?DE".The release of Brand Fe²⁺ in the system accorded with the first order kinetic mode,and the concentration of Brand Fe²⁺ in the system reached to 0.61 mg L^-1 and 2.7 mg L^-1 after reaction for 10 min,respectively.At the initial stage of the reaction,the pH in the system increased rapidly from5.0 to 7.8,and then gradually stabilized at about around 8.5.The ORP quickly decreased from+20 mV to-449 mV at the early stage of the reaction,and eventually stabilized at around-520 m V.According to the results of EDS and XPS,Fe⁰ on the surface of nanoparticles was oxidized in the process of reductive bromination of BDE47.The degradation of BDE47 by PAC-Pd/Fe NPs was a catalytic hydrogenation reaction,in which Pd was the only active site,while H transfer was the main mechanism of the reaction.