Degradation of oxytetracycline and characterization of byproducts generated by Fenton or photo-Fenton like processes after adsorption on natural and iron(III)-modified montmorillonite clays

Abstract

The combination of adsorption with advanced oxidation is presented as an alternative to remove antibiotics in water, particularly using clays as cheap adsorbents. However, little is known about the evolution of the reaction in a heterogeneous phase, the formation of byproducts and their persistence. In this work oxytetracycline (OTC), raw (MMT) and iron modified montmorillonite (Fe-MMT), and Fenton (F) or photo-Fenton (phF) like reactions were employed as model antibiotic, adsorbents, and oxidation treatments, respectively. Homogeneous F and phF experiments for OTC degradation were also performed. Adsorption experiments demonstrated that incorporation of Fe(III) to MMT increased OTC adsorption at both studied pHs (3.0 and 7.0), due to the formation of OTC-Fe(III) complexes. MMT or Fe-MMT containing adsorbed OTC (OTC-MMT or OTC-Fe-MMT) exposed to F or phF like processes at pH 3.0 or 7.0 released solely OTC oxidation byproducts. Mineralization was only observed for OTC-Fe-MMT at both pH values, after a maximum TOC was attained, being more marked for phF than for F like processes. The structures of transformation products were proposed based on HPLC-MSn analysis of withdrawn samples from solution. Similar intermediates were identified in homogeneous and heterogeneous oxidation processes although the total number was lower for the latter. OTC degradation occurred by several pathways including oxidations by hydrogen abstraction and hydroxyl addition, as well as ring contractions and losses of water, carbonyl and carboxamide moieties. The performance and outcomes of phF like process applied to OTC-Fe-MMT at initial pH 3.0 and 7.0 were comparable to homogeneous phF at pH 3.0.