Mechanisms of virus control during iron electrocoagulation – Microfiltration of surface water

Results from a laboratory-scale study evaluating virus control by a hybrid iron electrocoagulation – microfiltration process revealed only 1.0–1.5 log MS2 bacteriophage reduction even at relatively high iron dosages (∼13 mg/L as Fe) for natural surface water containing moderate natural organic matter (NOM) concentrations (4.5 mg/L dissolved organic carbon, DOC). In contrast, much greater reductions were measured (6.5-log at pH 6.4 and 4-log at pH 7.5) at similar iron dosages for synthetic water that was devoid of NOM. Quantitative agreement with Faraday’s law with 2-electron transfer and speciation with phenanthroline demonstrated electrochemical generation of soluble ferrous iron. Near quantitative extraction of viruses by dissolving flocs formed in synthetic water provided direct evidence of their removal by sorption and enmeshment onto iron hydroxide flocs. In contrast, only approximately 1% of the viruses were associated with the flocs formed in natural water consistent with the measured poor removals. 1–2 logs of virus inactivation were also observed in the electrochemical cell for synthetic water (no NOM) but not for surface water (4.5 mg/L DOC). Sweep flocculation was the dominant destabilization mechanism since the ζ potential did not reach zero even when 6-log virus reductions were achieved. Charge neutralization only played a secondary role since ζ potential → 0 with increasing iron electrocoagulant dosage. Importantly, virus removal from synthetic water decreased when Suwanee River Humic Acid was added. Therefore, NOM present in natural waters appears to reduce the effectiveness of iron electrocoagulation pretreatment to microfiltration for virus control by complexing ferrous ions. This inhibits (i) Fe2+ oxidation, precipitation, and virus destabilization and (ii) virus inactivation through reactive oxygen species intermediates or by direct interactions with Fe2+ ions.

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► Iron electrocoagulation is not effective for virus control from surface water.
► NOM complexation of Fe2+ inhibits virus removal/inactivation from natural water.
► Viruses are inactivated in the electrochemical cell only in the absence of NOM.
► Cake layer of iron hydroxide flocs enhances virus removal (dynamic membrane).
► Data from synthetic waters cannot be extrapolated to natural waters containing NOM.