Pd Supported On Magnetic Fe₃O₄Nanoparticles For Electro-Fenton Degradation Of Organic Contaminants

Chemical oxidation has been extensively used for wastewater treatment and groundwater remediation. Among all the oxidation processes, electrochemical oxidation is a promising method due to the environment-friendly, opteraion flexibility and controllability. Because of the limitation of electrode materials and adverse reactions of oxygen evolution reaction, the current efficiency of contaminants oxidation is very low. Electro-Fenton (E-Fenton) process has many superiorities to other process. It produces H2O2in situ, avoiding the risk of its transport, storage and handling. Fe2+can be regenerated on the cathode, thus reduces the production of iron sludge. Nonetheless, the efficiency of H2O2production is highly dependent on the types of cathode materials. Effective production of H2O2needs expensive gas diffusion electrodes with low mechanical stability. Moreover, Fe2+addition complicates the operation.Recently, a novel E-Fenton process, termed Pd-based E-Fenton, was developed by our group. It is based on Pd-catalytic production of H2O2from electro-generated H2and O2, which overcomes the dependence on the cathode material. However, it is difficult to recycle the expensive catalysts Pd powder, and Fe2+was added manually in the form of ferrous salts. Both increases the cost. In recent years, Fe3O4magenetic nanoparticles have been widely used as adsorbents for contaminants removal from wastewaters and groundwater and support of catalysts in environmental applications because they are porous and can be easily recycled by magnetic separation. Fe3O4is also able to catalyze H2O2decomposition in Fenton-like processes due to the existence of surface Fe(Ⅱ) and the leaching of soluble Fe2+In this study, a new integrated catalyst of Pd on Fe3O4(Pd/Fe3O4) magnetic nanoprticle is synthesized and characterized. This catalyst can simultaneously catalyze the production of H2O2and its decomposition to-OH radicals by Fe2+. In addition, it can be easily recycled by magnetic separation. The performance and mechanisms of this new catalyst is evaluated for the Pd-based E-Fenton degradation of organic contaminants. The main conclusions were obtained as follows:(1) The as-synthesized Pd/Fe3O4has a porous structure with the high specific surface area (SBET) of72.8m2/g. The flake size is about100nm. Small aggregates in the size of100nm or less are observed on the surface of flakes. Spot analysis by EDX suggests that Pd was loaded in these aggregates. XRD results prove the loading of Pd on the Fe3O4magenetic nanoparticles.(2) Organic contaminants including phenol, rhodamine B and methyleneblue were efficiently degraded by the addition of Pd/Fe3O4into an undivided electrolytic cell under conditions of50mA,1g/L Pd/Fe3O4(5wt%Pd), pH3and20mg/L initial concentration. There was98%of phenol degraded in60min, and100%of rhodamine B and methylene degraded within60and45min, respectively. Electrolysis alone or with addition of1g/L Pd/Fe3O4can not cause any significant degradation of pollutants. The degradation was minimal at initial pH higher than4(k1=0.002min-1), was remarkable at initial pH of3(k1=0.033min-1), but was considerably decreased at initial pH of2(k1=0.011min-1). Phenol degradation significantly increased with the increase in Pd loadings on Fe3O4and electric current but decreased with the increase in initial phenol concentration.(3) Water electrolysis on the Pt anode and cathode produced gaseous O2and H2. Both gases dissolved into the solution, diffused onto the surface of Pd catalyst and were chemisorbed therein. H2O2was then produced on Pd surface by the combination of activated O2and two atomic H. Production of dissolved Fe2+through reductive dissolution of Fe3O4by atomic H. Because of the close contact of Pd and Fe3O4, the atomic H on Pd surface could reduce the surface Fe (Ⅲ) on Fe3O4in addition to combination with activated O2, leading to the production of dissolved Fe2+. As H2O2and dissolved Fe2+were produced in situ,-OH radicals were immediately generated by the conventional Fenton reaction. Thus, organic contaminants were effectively degraded.(4) For10times of repeated treatment, Pd/Fe3O4showed similar performance on contaminant degradation. The main crystal structure and magnetic property of Pd/Fe3O4varied slightly after treatments. Pd/Fe3O4can be easily recycled by magnetic separation. This preliminarily suggests that Pd/FejO4have a certain potential for sustainable use in the Pd-based E-Fenton process.