Heterogeneous Fenton-like degradation of organic pollutants in petroleum refinery wastewater by copper-type layered double hydroxides

Abstract

A series of Ni(2-x)Cu(x)Al-LDH layered ternary double hydroxides with x:0.0; 0.5; 1.5; and 2.0 were synthesized as catalysts to degrade aromatic and aliphatic organic compounds present in petroleum refinery wastewaters by means of a Fenton-type reaction and with hydrogen peroxide as a free radical generator. Samples obtained by coprecipitation at constant pH with a molar ratio R:(Ni2++Cu2+)/Al3+ equal to 2.0 were characterized by PXRD, FT-IR, ESEM-EDS, ICP-OES, TGA and N2 physisorption.The results of the oxidation reaction showed that catalytic activity varied inversely with the Ni2+/Cu2+ ratio, and activity was maximum for x:2.0. The catalyst can remove 74.8% of TOC, and the aromatic compounds can be completely oxidized by H2O2 after 90 min at 60°C under pH: pHPZC conditions, a less excessive dosage of H2O2 (H2O2/COD:5), and a reaction medium temperature of 60°C. However, this catalysis presents phases other than LDH, such as malachite and gibbsite.For lower Ni2+/Cu2+ ratios, the highly dispersed MO6 octahedra and the Jahn-Teller distortion effect facilitate electron transfer from Ni and thus increase the percentage of Cu+, which can react with H2O2 to produce strong hydroxyl radicals. In the degradation process of aromatic organic compounds catalyzed by Ni(2-x)Cu(x)Al-LDH, hydrogen peroxide can first adsorb on the surface of the catalyst and then get an electron from Cu+ ions to produce Cu2+, HO• and OH− ions. HO• can then react with the aromatic compounds to produce intermediates, organic acids, and alkanes. The presence of multivalent Cu, Ni and Al cations in LDH can build a closed electron cycle and rapidly regenerate Cu+ species.The catalytic oxidation process had first-order kinetics with an activation energy of 44.803 kJ mol−1 for the TOC disappearance reaction.