Αποικοδόμηση υδάτινων έμμονων ρυπαντών και ρυπαντών προτεραιότητας μέσω της ενεργοποίησης του υπερμονοθειικού καλίου από διάφορες πηγές σιδήρου (Fe2+, Fe3+, ZVI)

Abstract

This project studied the removal of persistent organic pollutants from water with novel Sulfate Radical-based Advanced Oxidation Processes (SR-AOPs). The model contaminants included caffeine (stimulant), ibuprofen (analgesic), and diuron (herbicide) and were chosen because of structural differentiations and susceptibility of their functional groups to the radical species formed. Chemical oxidation was achieved through electron transfer activation of peroxymonosulfate (PMS), from different iron sources including the ferrous ion (Fe2+), the ferric ion (Fe3+) and zero-valent iron (ZVI). The in-situ generated radical species mainly included sulfate radicals (SO4-), which have high redox potential for electron abstraction (2.5-3.1 V). However, other radical species can also be formed through side reactions such hydroxyl radicals (HO) and peroxymonosulfate radicals (SO5-). The obtained results of this study were compared with a previous study by Rodríguez et al. (2012), where all the previously mentioned iron sources where used for persulfate (PS) activation instead of PMS. To enable comparison, all the experimental conditions were kept the same as in Rodríguez et al. (2012), including initial contaminant concentration (0,1mM), temperature (T= 20ºC), initial solution pH (pH= 3.5), and estimation of the added oxidant concentrations for each model contaminant. Samples were taken at specific time intervals for the determination of the remaining contaminant concentration (HPLC-PDA), and the remaining oxidant concentration (Tiamo-system). Initially, the objective was to find the optimum molar ratio between Fe2+ /PMS (1:100, 1:10, 1:5 and 1:1) that results in the highest contaminant removal. Among the tested molar ratios (Fe2+/PMS=1:100, 1:10, 1:5 and 1:1), Fe2+/PMS=1:5 was found to be the optimum one and was further tested for the remaining iron sources. Among the iron sources tested, ZVI was the most efficient and reactive form because of the slow and continuous release of Fe(II) and electrons that favors the production of a greater percentage SO4-. The reaction between ZVI and PMS takes place on the outer surface of the ZVI particles, where the diameter of each ZVI particle used, determines the release of Fe(II). Controlled Fe(II) release, minimizes undesirable quenching (consumption) of SO4- by the Fe(II), making ZVI the preferred method for PMS activation. Overall, the contaminant with the highest removal percentage, irrespectively of the oxidizing system used, was ibuprofen because of its aromatic ring and lack of electron withdrawal groups in its structure. The least degradable was caffeine, due to heteroatoms (N) in its structure that has reduced second order kinetic constants. The diuron appeared to have a significant decrease in its concentration due to dechloronization that takes places when radicals attack. To the best of our knowledge, this is the first study that compared the efficiency of the iron/PMS oxidation system activated with different iron sources for the removal of a mixture of persistent organic compounds under the same experimental conditions.