Assessment of different iron species as activators of S2O82- and HSO5- for inactivation of wild bacteria strains

Abstract

This work assesses the potential activation of peroxymonosulfate (PMS, HSO5 −) and persulfate (PS, S2O8 2−) by different homogeneous iron species, FeSO4 and Fe(III)-citrate and heterogeneous Fe2O3 and nano zero valent iron particles (nZVI), to generate sulfate radicals for inactivation of wild bacteria strains (Escherichia coli and Enterococcus sp.) spiked in simulated wastewater effluents. The effectiveness of each iron source as activator was studied in combination with UV-A radiation at natural pH of water (≈ 7.2) using millimolar doses of oxidants (0.5 mM) at different oxidant:iron molar ratio (1:1 and 1:2). In all cases, the addition of iron enhanced the efficiency of the treatments compared to those carried out without it (PMS/UV-A and PS/UV-A); however, significant differences were evidenced depending on the type of bacteria, oxidant and iron specie. For instance, Escherichia coli was reduced (5-log) below the limit of detection (1 CFU/mL) within 2–15 min by activation of PMS and PS with all the iron species evaluated By contrast, under analogous operating conditions the efficiency was notably lower for Enterococcus sp. In this case, it is worth mentioning the notable efficiency through the use of Fe(III)-citrate, which allowed to attained a 3.50-log removal of Enterococcus sp. after 90 min, in contrast to the low reduction (1.8-log) attained with FeSO4. On the other hand, heterogeneous iron species showed an efficiency for bacteria inactivation comparable in most cases to that of homogeneous iron sources. Taking into consideration the potential advantages of heterogeneous catalysts, as for example the recovery and reuse of the catalyst thus reducing the operating costs, the obtained results are encouraging. For instance, E.coli population was removed below the detection limit (5-log) in less than 5 min with both Fe2O3 and ZVI, whereas a 2-log decrease of Enterococcus sp. was attained after 90 min of treatment, even without PMS. Finally, it was assessed the recovery and re-use of ZVI particles. According to X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) analysis, the reaction conditions induced a progressive fading of the core-shell structure of nZVI particles, with a decrease of the efficiency for bacteria inactivation in subsequent uses. As an example, E.coli is reduced in 5-log in 15 min in the first use, and 4.70-log in 90 min with the third use of ZVI.