| The catalytic dechlorination of chlorinated compounds in water by iron nanoparticles has received increasing interest in recent years. However, the overall reaction rate may become progressively slower due to the precipitation of ferrous hydroxides on the surface of Pd/Fe during reaction that blocks the reactive sites on Pd/Fe and hinders the direct contact between the chlorinated organic compounds and Pd/Fe. Agglomeration of magnetic metal nanoparticles takes place, and then reduces the specific surface area and the interfacial free energy, thereby diminishing particle reactivity. Batch experiments were conducted to investigate the activity of Pd/Fe whose oxides layer on the surface was removed by ethylene diamine tetraacetic acid (EDTA) or sodium citrate (Cit), and of stabilized zero-valent iron (ZVI) nanoparticles for the catalytic dechlorination of chlorinated compounds in water. Nontoxic and biodegradable Carboxymethyl Cellulose (CMC) is used in the synthesis of the nanoscale zero-valent iron as a stabilizer.In the present paper palladium-iron bimetal was used as a reducing catalyst, EDTA and Cit were used to remove oxides on palladium-iron bimetal surface and to improve palladium-iron bimetal reactivity.We studied the effects of EDTA and Cit dosage on the efficiency of 2,4-Dichlorophenoxyacetic Acid (2,4-D) reductive dechlorination. An efficient method of synthesizing CMC-stabilized Pd/Fe nanoparticles was investigated and the effects of experimental parameters on the efficiency of 2,4-D reductive dechlorinated such as initial concentration, the dosage of reductant, Pd/Fe weight-to-volumn concentration, the dosage of CMC, stirring speed, reaction temperature and pH value were studied. Transmission electron microscopy (TEM), ZETA potential instrument and high performance liquid chromatography (HPLC) determination were used to characterize, analyse and infer the mechenisms.. Results showed that:1. Almost 100% dechlorination efficiency for 2,4-D were achieved with lower Pd%(0.10%), 200 r/min and 20 g/L Pd/Fe at ambient temperature. According to the Arrehenius equation, the apparent activation energy 118.60kJ/mol and pre-exponential factor 1.99×1019 min-1 was obtained in this research.2. EDTA and Cit can effectively remove the oxides on the Pd/Fe bimetal surface. Compared with Pd/Fe bimetal, EDTA increased by 23.5% of the reaction efficiency, and Cit increased by 20.1% of the reaction efficiency. The suitable dosage of EDTA and Cit were 1 mmol/L.3. Compared to Pd/Fe bimetal, CMC-stablized nano Pd/Fe particles had a greater electrostatic repulsion, and lower isoelectric point. At lower pH, 2,4-D were more easily absorbed by CMC-stablized nano Pd/Fe particles. At the same conditions, the CMC- stablized nano Pd/Fe removal efficiency of 2,4-D was about 2.8 times than ordinary nano-Pd/Fe, 5.7 times than the Pd/Fe. Pd/Fe bimetal system dechlorination efficiency was well improved.4. In the system of CMC-stablized nano Pd/Fe, the effect factors of 2,4-D removal include CMC addition, Pd/Fe addition, initial 2,4-D concentration, pH value, reaction temperature and the cations concentration. The removal efficiency increased with the increasing of Pd/Fe addition and reaction temperature. Lower pH value could accelerate the rate of 2,4-D removal.5. The mechanism of 2,4-D reduction by Pd/Fe was as following: 2,4-D reacted with atomic H, the product of Fe0 corroding. And 2,4-D was also removed directly by Fe. 2-chlorophenoxyacetic acid (2-CPA) is the intermediate product and phenoxyacetic acid (PA) is the final product in the process of dechlorination.
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