Study On Catalytic Debromination Of2,2’,4,4’-Tetrabromodiphenyl Ether By Nanoscale Pd/Fe Bimetallic Particles

Polybrominated diphenyl ethers (PBDEs) recognized as a new class of environmentalpersistent toxic contaminants have been distributed widely in the world and some of itshomologues have bioaccumulation and biomagnification. It is known that PBDEs can breakthe balance of thyroid hormone in animal and human body, and have carcinogenic potential.Numerous degradation technologies such as microbial degradation, photolytic degradation,and reduction by zero valent metals, have been explored but fall short due to limiteddegradation, long reaction times, slow debromination, and more toxic products. Therefore, it’surgent to explore a fast and thorough degradation technology for PBDEs.Base on the effects of catalytic reduction and adsorption of nanoscale Pd/Fe, as well as itshigh efficiency in the field of halogenated organic compounds degradation, nanoscale Pd/Fecatalytic reduction was used in this study for the treatment of PBDEs. A reaction system ofnanoscale Pd/Fe-methanol/water-2,2’,4,4’-Tetrabromodiphenylether (BDE-47) wasconducted. First, synthesizing Pd/Fe nanoparticles by chemical reduction and characterized byX-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electronmicroscopy (SEM) and specific surface area analysis (BET).Then, batch experiments wereperformed to investigate the degradation efficiency of Pd/Fe nanoparticals and the influentialfactors. On this basis, orthogonal experiments were designed to determine the optimumreaction conditions and the importance order of influential factors. Finally, the reactionmechanisms and kinetics of nanoscale Pd/Fe catalytic reduction were deeply investigated;meanwhile, possible debromination pathways of BDE-47were also speculated.The results of characterization showed that the nanoscale Pd/Fe were spherical with adiameter in the range of50~100nm and a BET surface area of35.23m2g-1. The crystalstructure of iron was a regular α-Fe crystalline state and Fe3O4may exist. Pd was dispersed oniron surface in amorphous form. Elements on the nanoscale Pd/Fe surface were Fe, Pd, C, Oand B。Batch experimental results indicated that the reduction efficiencies of BDE-47bynanoscale Fe and nanoscale Pd/Fe under the same reaction conditions were39.83%and100%after60min. The reaction activity of Pd/Fe nanoparticles initially improved and then impaired with the increasing loading of Pd. The BDE-47removal rate was decreased as the ratio ofmethanol-water increased and it was positively correlated with reaction temperature over therange of25℃~40℃, but negatively correlated with initial concentration of BDE-47.Moreover,increasing the dosage of Pd/Fe nanoparticles could accelerate the reduction rate. Acid oralkalescence (pH) was more favorable for BDE-47reduction. The results of orthogonalexperiments showed that the order of importance was methanol-water ratio>reaction time>Pd/Fe nanoparticles dosage> reaction temperature> initial pH. It was also found that BDE-47with a concentration of10mg L-1could be completely removed under the optimum reactionconditions (a methanol-water ratio of50:50, a Pd/Fe nanoparticles dosage of3g L-1, a pH of5,a temperature of35℃) after45min.The catalytic reduction of BDE-47by nanoscale Pd/Fe was a catalytic hydrogenationprocess: BDE-47was adsorbed on the surface of nanoscale Pd/Fe, H2produced by thereaction of Fe and H2O was adsorbed and transformed into highly reactive H*by Pd, then, H*and BDE-47with a high concentration on the surface of nanoscale Pd/Fe reacted. The valuesof pH slightly increased and ORP slightly decreased during the experiments, the concentrationof Br-was increased. Kinetics analysis indicated that BDE-47removal by nanoscale Pd/Fefollowed pseudo-first-order kinetics. The removal rate of BDE-47increases with the increaseof Pd loading and reaction temperature. In acidic and alkaline conditions, the reaction rates ofBDE-47reduction were faster than that in neutral condition. At the reaction temperature of298~313K, the activated energy of the reaction is147.95kJ·mol-1, indicating that BDE-47reduction was controlled by chemical reaction process. The reduction of BDE-47was mainlya stepwise debromination reaction process which was dominated by the pathway fromn-bromo-to (n-1)-bromodiphenyl ethers, although simultaneous multistep debrominationseemed to exist. The p-bromines were more susceptible than ortho-bromines and twobromines on the same phenyl ring were more sensitive to debrominate simultaneously. Thebrominated intermediates of BDE-47reduction by nanoscale Pd/Fe contained two tri-BDEisomers, three di-BDE isomers and two mono-BDE isomers. The bromine atoms in BDE-47were completely removed after90min, generating diphenyl oxide as the final product.