Catalytic Dechlorination Of The Chlorinated Endocrine Disruptor 2,4-D By Pd/Fe And Improvement Of Its Reactivity

Removing the chlorinated endocrine disruptors in water by using Pd/Fe is an effective way with great application prospect. But the easily passivation of Pd/Fe particles surface and the tendency to agglomerate of Pd/Fe nanoparticles decrease their reactivity. Preventing the formation of passivation layer in the Pd/Fe surface using complexing agent and keeping Pd/Fe nanoparticles dispersivity by loading or stablization can maintain their high reactivity.In this paper, we investigated the effects of different stabilizers such as PAA, guar gum and Fe3O4 on Pd/Fe nanoparticles stability and reactivity, studied the effects of experiment parameters on 2,4-D catalytic dechlorination using Fe3O4 stablized Pd/Fe nanoparticles and optimized the experimental conditions. The dispersibility of Pd/Fe nanoparticles was observed using TEM and the dechlorination process of 2,4-D was analyzed using HPLC. On the other hand, we enhanced the catalytic dechlorination of 2,4-D by continuously dropping of EDTA to the Pd/Fe system, inferred the accelerated mechanism by examing concentration variation of Pd, EDTA and 2,4-D in Zero-valent iron system. The effects of experiment parameters on catalytic dechlorination capability of Pd/Fe system were also investigated by HPLC. The results showed as follows:1. The stabilizers dispersed the Pd/Fe nanoparticles based on different mechanism, Fe3O4-stabilized Pd/Fe nanoparticles showed better dispersibility.2. The stabilizers changed the capability of Pd/Fe nanoparticles in catalytic dechlorination of 2,4-D. PAA and guar gum less enhanced the reactivity or even inhibited it. Fe3O4 enhanced catalytic dechlorination by stabilizing the Pd/Fe nanoparticles and composed of primary batteries with Fe.3. The catalytic dechlorination of 2,4-D by Fe3O4 stabilized Pd/Fe nanoparticles was a pseudo first order reaction. The apparent activation energy 64.00 kJ/mol and pre-exponential factor 4.79×109 min-1 was obtained in the temperature of 20-30 ℃ in this research. The intermediate of the reaction was 2-CPA with the final organic product PA. Larger Fe3O4 dosage, more Pd loading and lower initial pH could speed up the dechlorination rate. The appropriate experimental parameters for 2,4-D removal were:CFe 1 g/L, Fe3O/FO 4:1, Pd loading 0.5%,200 rpm, pH 7.8 and 25 ℃, within 210 min, the removal of 20 mg/L 2,4-D achieved up to 93.5%。4. The addition of EDTA to the Pd/Fe complexed the Fe2+(and possible Fe3+) generated from catalytic dechlorination of 2,4-D, preventd or slowed down the formation of passivation layer, and accelerated the reaction rate. But bulk EDTA competitively adsorpted with 2,4-D, inhibited the adsorption and catalytic dechlorination of 2,4-D. Continuously dropping of EDTA prevented bulk presence of free EDTA, reduced the impacts of competitive adsorption.5. The catalytic dechlorination of 2,4-D in the Pd/Fe system of continuously dropping of EDTA was a pseudo first order reaction. EDTA enhanced the capability of catalytic dechlorination more than 300%, with 2-CPA and trace 4-CPA as intermediates, PA as the final organic product. More Pd loading and lower initial pH could speed up the dechlorination rate. The appropriate experimental parameters for 2,4-D removal were:the EDTA flow rate 20 mL/h, CEDTA 25 mM, Pd loading 0.05%,200 rpm, pH 4.2 and 30 ℃, within 210 min, the removal of 20 mg/L 2,4-D achieved up to 100%.