Chromium(VI) reduction in aqueous medium by means of catalytic membrane reactors

Chromium(VI) reduction in aqueous medium by means of catalytic membrane reactorsA novel method for catalytic reduction of chromium(VI) to chromium(III) in model and real water is proposed in this work. Hydrogen was used as a reducer. Three different catalytic membrane reactors were prepared and employed in the studied reaction. The catalytic membrane reactors were prepared using commercial corundum hollow fiber membranes. The active phases, palladium, cerium oxide, iron oxide or combinations of them, were incorporated by incipient wetness impregnation of the corresponding water-soluble salts followed by calcination and reduction. The catalytic tests were performed in semi-batch mode at ambient conditions. A mass flow controller was used to supply hydrogen to one end of the membrane whilst the other end was kept closed. The reactors were submerged into a vessel containing the chromate solution. The concentration of the chromate was continuously monitored following a standard analytical method. The initial chromate concentration was varied between 0.5 and 18 mg L-1. In order to completely reduce the Cr(VI) to Cr(III), it was necessary to decrease the pH of the solution to 3. In the end of the tests the final solutions were neutralized to pH 8 and filtered. The content of chromium in the final solutions and in the precipitates was determined by inductively coupled plasma (ICP) technique. It was demonstrated that for the studied reaction the presence of palladium is essential. In all experiments, with synthetic and real water adjusted to pH 3, the catalytic membrane reactors containing palladium (0.3-0.9 w/w%) effectively reduced the chromium to levels below 0.03 mg L-1 without losing activity in repetitive runs. The results from ICP analyses demonstrated that the chromium can be completely eliminated from the water after neutralization and filtration

Heterogeneous catalytic oxidation of phenol by in situ generated hydrogen peroxide applying novel catalytic membrane reactors

10.1016/j.cej.2014.09.064This work presents a novel method for oxidation of organic matter in water solutions based on catalytic membrane reactors. The oxidant, hydrogen peroxide, is generated directly in the bulk of the liquid investigated. Commercial symmetric alumina hollow fibers have been used as a starting material thereafter introducing the active phases. It has been proven that two different catalysts are necessary in order to complete the overall reaction, as well as to generate hydrogen peroxide and a heterogeneous Fenton process. Palladium has been used for the hydrogen peroxide generation and a second active phase, transitional metal oxides or homogeneous Fe2+, has been used for the hydroxyl radical generation. An additional method for specific Pd loading to the reaction zone based on sputtering technique has been developed. All prepared catalytic membrane reactors (CMRs) are capable of generating hydrogen peroxide in amounts comparable to CMRs reported in the literature. The catalytic membrane reactors prepared by Pd impregnation show very high activity and stability in phenol oxidation reaching 40% of the generated H2O2 usage in the oxidation reaction. Despite the very high activity of the catalytic membrane reactors obtained by Pd sputtering in H2O2 production they suffer very fast deactivation. Specific reactivation including a calcination step has been found to be appropriate for the recovery of their activity. Additional experiments give new insights for better understanding of Pd deactivation especially when the metal particles are of nanometer sizes