Degradation Of Emerging Contaminants By Fe- And Mn- Based Oxidation Methods In Aqueous Solution

Abstract

Presence of pharmaceuticals and endocrine disrupting compounds (EDCs) in the environment is of growing concern due to their potential ecological impacts and constant release. Conventional treatment plants are inefficient in removing such compounds, driving the need for innovative treatment methods. Oxidation processes are attractive options because structure alterations by oxidation can lead to reduced biological activity, increased biodegradability and mineralization. Three oxidation treatment methods, Anodic Fenton treatment (AFT), crypomelane-type manganese oxides and biogenic manganese oxides were evaluated using pharmaceutical and EDCs probes. Ciprofloxacin (CIP), a widely used fluoroquinolone, was degraded in the AFT system. Fast removal was observed in minutes. The degradation kinetics were found to deviate from the classical AFT model. This change in kinetic pattern is because speciation distribution changes as the solution pH self-optimizes to acidic conditions, and reaction affinity of the CIP species toward the hydroxyl radical is protonation dependent. Modification was made to the AFT model to account for the pH induced change in reaction affinity. Process condition was optimized at pH 3.2, giving a Fenton reagent delivery ratio of [H2O2]:[Fe2+] between 10 to15. Analysis on structures and evolution pattern of degradation products indicated elimination of the antibiotic activity of the solution after AFT treatment. Cryptomelane is a framework type manganese oxide whose synthetic counterpart has gained wide industry application. Oxidative degradation of CIP by synthetic cryptomelane (OMS-2) was not successful. Doping V or Mo into the framework of OMS-2 increased its oxidative reactivity significantly. 9% Mo doping was found to give the best performance. Structure characterization results indicated that the improvement was mainly due to increased surface area upon doping. Response surface methodology was applied to find the optimal treatment condition: pH 3 and [Oxidant]:[Substrate] molar ratio [GREATER-THAN OR EQUAL TO] 50. Analysis of degradation products suggested that the oxidation mainly takes place at the piperazine ring of CIP. Lower biological activity of the products is expected since the piperazine ring is an important substituent on the quinolone core structure that affects antibiotic potency. Biogenic manganese oxide (BioMnOx) is formed by Leptothrix discophora SS-1. Successful removal of CIP and BPA by BioMnOx can be achieved, but the reaction rates were slower by BioMnOx, compared to that by synthetic MnO2. This is probably due to a larger particle size, smaller surface area and lower average oxidation states of BioMnOx. The bleaching procedure improved the reactivity of BioMnOx significantly. Generally, lower solution pH is favorable for the oxidation reaction. Presence of cations can slow down the oxidation process even more by competing for the reactive sites. Humic acid can affect the oxidative degradation via two countering effects: blocking of surface reactive sites which reduces reactivity and binding inhibitory MnII ion released during the reaction which accelerates the reaction. The BioMnOx system yielded fewer degradation products than the synthetic MnO2 systems as well as different product distributions. The mechanism for the preference of degradation pathways remains unclear. The applicability of BioMnOx as a water treatment technology is still questionable and requires more evaluation.