Enhancement of E. coli inactivation by photosensitized erythrosine-based solar disinfection under weakly acidic conditions

Cost-effective disinfection technology is urgently needed in poor rural areas. Erythrosine (ERY)-based solar disinfection (SODIS) provides a promising solution because of its effective inactivation of viruses and gram-positive bacteria at low cost. However, the poor gram-negative bacteria (G − , e.g., Escherichia coli ) inactivation of photosensitized ERY inhibits its application. Herein, for the first time, the protonation of ERY was found to greatly enhance its G − inactivation, and 99.99999% (7.0 log) of E. coli were completely inactivated within only 30 s using 2.5 mg/L ERY under 200 mW/cm 2 visible light irradiation. The inactivation rate constant ( k ) reached 17.5 min −1 at pH 4.0, which was 4730 times higher than that at pH 7.0. At a lower pH, more severe cell wall and genomic DNA damage was observed. A linear correlation between k and monoanionic ERY (HE − ) content was obtained, indicating that HE − rather than dianionic ERY (E 2− ) participated in the inactivation at pH 5.0–7.0, which was further explained by the higher production of reactive oxygen species and bacterial adsorption of HE − than E 2− . Both 1 O 2 and O 2 − • dominated bacterial inactivation, contributing 56.8% and 43.2%, respectively. O 2 − • but not 1 O 2 caused ERY photobleaching. OH • was not involved in either inactivation or photobleaching. Humic acid and salts (NaCl, Na 2 SO 4 , CaCl 2 , and MgCl 2 ) slightly inhibited inactivation, while NaHCO 3 accelerated inactivation. Complete inactivation (99.9999%) of E. coli was achieved within ∼30 min at pH 5.0 in ERY-based SODIS with good adaptation to various water matrices and weather (sunny or partly cloudy). This work will help to promote the application of ERY-based disinfection especially for SODIS in poor rural areas.