Ferrate(VI) pre-treatment and subsequent electrochemical advanced oxidation processes: Recycling iron for enhancing oxidation of organic pollutants

Pharmaceutical residues as emerging pollutants constitute an issue of growing concern owing to their continuous accumulation in the aquatic environment. The removal of such compounds from water requires the development of efficient technologies as well as a better understanding of degradation mechanisms since expensive treatment strategies are necessary for the total mineralization of these compounds. This paper focuses on the potential of combining ferrate (FeVI) oxidation with either anodic oxidation (AO) or electro-Fenton (EF) processes, in comparison with standalone processes, for the degradation of a model pharmaceutical compound (paracetamol). Particularly, recycling iron from FeVI oxidation as catalyst for the EF process is reported for the first time. A particular emphasis is done on identification/quantification of by-products, comparison of degradation pathways and evolution of acute toxicity during the treatment in order to provide novel insights on the possibility to use selective or non-selective oxidation processes for the removal of emerging organic pollutants in a cost-effective way. The most promising treatment strategy appeared to be the use of ferrate as a pre-oxidation step and further as iron source for the electro-Fenton process. Fast degradation kinetics, low aromatic by-products formation and strong decrease of the acute toxicity of the treated solutions highlighted a promising potential of this combined process for transformation of the persistent organic pollutant into harmless/biodegradable by-products.