Tri-protonated ferrate: A novel electro-synthesised pre-oxidant/coagulant for NOM and DBP precursors removal in drinking water production

Abstract

The generation of tri-protonated form of ferrate (H3FeVIO4+) in acidic media offers unique opportunities for drinking water treatment. Compared to the more conventional deprotonated ferrate form (FeVIO42−), H3FeVIO4+ is an extremely strong oxidant rendering it both an effective pre-oxidant and coagulant for water treatment applications. In this work, the treatment performance of H3FeVIO4+ at removing natural organic matter (NOM) and disinfection by-product (DBP) precursors was evaluated and compared against FeVIO42− and the most commonly-used iron-based coagulant: ferric chloride (FeCl3) when treating the water from two drinking water reservoirs in Australia. Results demonstrate that FeVIO42− and H3FeVIO4+ react with organic matter in intrinsically different ways, influencing the removal of dissolved organic carbon (DOC) and the formation potentials of carbonaceous-DBPs upon subsequent disinfection with chlorine or chloramine. H3FeVIO4+ dosing achieved the highest DOC removal observed: 77 % for water from site A and 60 % for water from B, both significantly superior to FeVIO42− (water A: 22 % and water B:18 %) and FeCl3 (water A: 40 % and water B: 41 %) dosing. Moreover, H3FeVIO4+ pre-oxidation of water A and subsequent chlorination reduced trihalomethanes (THMs), iodo-trihalomethanes (I-THMs), haloketones (HKs) and chloral hydrate (CH) precursors by 81 %, 92 %, 53 % and 61 %, respectively, whereas for water B the THMs, I-THMs, HKs and CH precursors were reduced by 60 %, 52 %, 20 % and 58 %, respectively, heavily outperforming the performance of FeVIO42− and FeCl3 in the removal of all measured DBPs. Finally, the formation of bromate was not observed when either ferrate(VI) forms were applied, thus rendering H3FeVIO4+ an interesting alternative pre-oxidant/coagulant, even for high bromide-containing waters (such as water B: 240 μg L−1).