Oxidative treatment of emerging micropollutants and viral pathogens by potassium permanganate and ferrate: Kinetics and mechanisms.

摘要

Providing clean drinking water is a primary challenge of this century. The ubiquitous occurrence of pharmaceutically active compounds (PhACs), including antibiotics, anticonvulsants, painkillers, estrogenic hormones, lipid regulators, beta-blockers, antihistamines, X-ray contrast media, etc., in drinking water sources has been reported in recent years. The presence of these contaminants, although at low concentrations, raises public concerns about potential adverse effects on aquatic ecology and human health. Another emerging concern in drinking water safety is the formation of toxic disinfection by-products (DBPs) when treating water with conventional disinfectants (i.e., free chlorine), and thus alternative disinfectants and disinfection processes are sought to control DBPs formation while still providing a sufficient barrier to pathogens. Chemical oxidation processes involving permanganate [MnO4-, Mn(VII)] and ferrate [FeO42-, Fe(VI)] salts are promising technologies for treatment of many PhACs. Permanganate is already widely used in water treatment facilities (e.g., for treatment of taste and odor compounds, soluble iron(II) and manganese(II)), while ferrate is an emerging water treatment oxidant that also has potential for use as an alternative disinfectant. This study investigates the oxidative transformation of PhACs using permanganate and ferrate and the use of ferrate for inactivation of a surrogate viral pathogen, MS2 bacteriophage.;Survey tests show that permanganate and ferrate are both selective oxidants that target compounds with specific electron-rich moieties, including olefin, phenol, amine, cyclopropyl, thioether, and alkyne groups. Detailed kinetics studies were undertaken to characterize Mn(VII) oxidation of five representative PhACs that exhibit moderate to high reactivity (carbamazepine, CBZ; ciprofloxacin, CPR; lincomycin, LCM; trimethoprim, TMP; and 17alpha-ethinylestradiol, EE2), Fe(VI) oxidation of one representative PhAC (CBZ), and Fe(VI) inactivation of MS2 phage (Fe(VI) reactions with other PhACs were not conducted because recent literature reports addressed the topic). The Mn(VII) and Fe(VI) reactions examined with PhAC and MS2 phage were found to follow generalized second-order rate laws, first-order in oxidant concentration and first-order in target contaminant concentration. The temperature dependence of reaction rate constants was found to follow the Arrhenius equation. Changing of solution pH had varying effects on reaction rates, attributed to change in electron density on the target reactive groups upon protonation/deprotonation. The effects of pH on reaction rates were quantitatively described by kinetic models considering parallel reactions between different individual contaminant species and individual oxidant species. For Mn(VII) reactions, removal of PhACs in drinking water utility source waters was generally well predicted by kinetic models that include temperature, KMnO4 dosage, pH, and source water oxidant demand as input parameters.;A large number of reaction products from Mn(VII) oxidation of CBZ, CPR, LCM, TMP, and EE2 and Fe(VI) oxidation of CBZ were identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Structures of reaction products were proposed based on MS spectral data along with information collected from proton nuclear magnetic resonance (1H-NMR), chromatographic retention time, and reported literature on Mn(VII) reactions with specific organic functional groups. Mn(VII) and Fe(VI) rapidly oxidize CBZ by electrophilic attack at the olefinic group on the central heterocyclic ring. Mn(VII) oxidation of CPR was found to occur primary on the tertiary aromatic amine group on the piperazine ring, with minor reactions on the aliphatic amine and the cyclopropyl group. LCM was oxidized by Mn(VII) through the aliphatic amine group on the pyrrolidine ring and thioether group attached to the pyranose ring. TMP oxidation by Mn(VII) was proposed to occur at the C=C bonds on the pyrimidine ring and the bridging methylene group. EE2 oxidation by Mn(VII) resulted in several types of products, including dehydrogenated EE2, hydroxylated EE2, phenolic ring cleavage products, and products with structural modifications on the ethynyl group.;Although little mineralization of PhAC solutions was observed after Mn(VII) treatment, results from bioassay tests of three antibiotics show that the antibacterial activity was effectively removed upon reaction with Mn(VII), demonstrating that incomplete oxidation of PhACs during Mn(VII) treatment will likely be sufficient to eliminate the pharmaceutical activity of impacted source waters.;Overall, results show that reactions with Mn(VII) likely contribute to the fate of many PhACs in water treatment plants that currently use Mn(VII), and the kinetic model developed in this study can be used to predict the extent of PhAC removal by Mn(VII) treatment. For water contaminated with highly Mn(VII)-reactive PhACs (e.g., carbamazepine, estradiol), specific application of Mn(VII) may be warranted. Results suggest Fe(VI) may be a useful disinfecting agent, but more work is needed to characterize its activity and mode of inactivating with other pathogens of concern.