Photocatalytic mineralization of codeine by UV-A/TiO2--Kinetics, intermediates, and pathways.

This study investigated the photocatalytic degradation of codeine by UV-irradiated TiO2. The degradation kinetics was determined under varied conditions including the TiO2 loading, codeine concentration, and pH. Codeine and several reaction intermediates including morphine were identified and tracked during degradation using HPLC/MS-MS technique, along with TOC and IC measurements. Specifically, removal of 100 µg/L of spike codeine was complete in 3 min by contact with a 0.1 g/L suspension of TiO2 under UV irradiation at pH 7. The degradation kinetics of codeine was first-order with respect to both the catalyst TiO2 and the reactant codeine, with enhanced reaction rates with increasing pH up to pH 9. Mineralization of codeine was possible upon prolonged contact; near complete mineralization of 10 mg/L of codeine was achieved in 90 min with 0.1 g/L TiO2 under irradiation at pH 5, during which the organic nitrogen was converted to NH3-N (74%) and NO3-N (22%). Based on the identified intermediates, two degradation pathways were proposed of which one involved ipso-substitution followed by cleavage of the aromatic ring and another involved repeated hydroxylation of the codeine molecule followed by its fragmentation.

Photocatalytic degradation of methamphetamine by UV/TiO2 - kinetics, intermediates, and products.

Methamphetamine (MAT) is a prescription drug and often a substance of abuse. It is found in WWTP influents and effluents as well as surface waters in many regions, elevating concerns about their potential impact. MAT is not effectively removed by conventional processes of domestic wastewater treatment plants (WWTPs). To contemplate advanced treatment, this study evaluates the feasibility of eliminating MAT by UV-illuminated TiO2, a potential retrofit to existing UV disinfection units. The degradation kinetics and mechanism of MAT by TiO2 under low-wattage UV illumination (9 W with maximum output at 365 nm) were investigated. Experimental parameters were varied including the TiO2 loading, MAT concentration, and pH. During treatment, MAT and its intermediates were tracked by HPLC-MS/MS, along with TOC and IC measurements to determine the mineralization extent. In contact with 0.1 g/L of TiO2 under illumination at pH 7, an entire spike amount of 100 µg/L of MAT was removed from deionized water after 3 min and 76 µg/L of MAT was removed from the secondary wastewater effluent after 30 min. The degradation of MAT followed an apparent first-order kinetics. Near complete mineralization of MAT from 10 mg/L was achieved in 180 min with 0.1 g/L of TiO2 at pH 5, by which the organic nitrogen was converted to NH4(+) and NO3(-). Based on identified intermediates, two degradation pathways were deduced that involved cleavage of the side chain as well as hydroxylation of the MAT compound. The photocatalytic UV/TiO2 process shows promise in arresting the release of MAT and its intermediate derivatives into the water environment.