Mitigation and degradation of natural organic matters (NOMs) during ferrate(VI) application for drinking water treatment

Ferrate(VI), as an alternative for pre-oxidation in drinking water treatment, has recently captured renewed interest. However, the knowledge in ferrate(VI) chemistry remains largely undeveloped. The information regarding ferrate(VI) reactions with natural organic matters (NOMs), an important water matrix component affecting water treatment, is highly limited. In this study, bench scale tests were performed to study ferrate(VI) decay and reactions with NOMs in a typical surface water matrix. Results showed that ferrate(VI) decay exhibited a pseudo 2nd-order reaction pattern (kobs = 15.2–1.6 mM⁻¹ min⁻¹ and 36.3–4.0 mM⁻¹ min⁻¹ with 1.0–7.0 mg/L Fe(VI) at initial pH 7.8 and 5.8, respectively), suggesting that self-decomposition is principally responsible for ferrate(VI) consumption. Ferrate(VI) tended to attacked electron-rich moieties in NOM molecules, but had limited capability to mineralize NOMs. Consequently, ferrate(VI) effectively reduced UV254 and specific UV absorbance (SUVA254), but poorly removed dissolved organic carbon (DOC). Generally, lower pH and higher ferrate(VI) dose favored the NOM destruction. Fe(VI) (3.0 mg/L) could remove 16% of initial DOC (4.43 mg/L), 56% of initial UV254 (0.063 cm⁻¹), and 48% of initial SUVA254 (0.033 cm⁻¹ (mg/L)⁻¹) at pH 5.80. Further organics analyses indicate that ferrate(VI) readily degraded hydrophobic and transphilic NOM fractions, but scarcely decomposed hydrophilic fraction. Fluorescence excitation-emission matrix (EEM) and fluorescence regional integration (FRI) analyses revealed that ferrate(VI) preferentially reacted with fulvic-like (region III) and humic-like (region V) substances and certain aromatic proteins (region II), difficultly decomposed soluble microbial byproducts (region IV), and rarely oxidized aromatic proteins in region I.