The mineralization of oxalic acid and bio-treated coking wastewater by catalytic ozonation using nickel oxide

Coking wastewater after biological treatment still possesses potential environmental risk and should be mineralized further. This work focused on the mineralization of bio-treated coking wastewater using catalytic ozonation by NiO. First, oxalic acid, the typical by-product of advanced oxidation process (AOPs), was used to test the catalytic performance of NiOs, prepared by modified hydrothermal methods upon addition of different surfactants. This demonstrated that NiO upon addition of hexadecyltrimethylammonium (CTAB) had the best catalytic activity, due to its high concentration surface hydroxyl density and strong stability. Moreover, the best NiO was applied for the catalytic ozonation of bio-treated coking wastewater. Under our experimental conditions, the total organic carbon (TOC) removal reached 100% after 420 min. In addition, the spectroscopic analysis suggested that compounds with conjugated structures could be significantly removed by both ozonation and catalytic ozonation. Some of these substances were transformed into by-products with aliphatic C-C and O=C-O groups such as organic acids that can inhibit further mineralization.