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/cm2 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 (E2-) 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 E2-. Both 1O2 and O2-• dominated bacterial inactivation, contributing 56.8% and 43.2%, respectively. O2-• but not 1O2 caused ERY photobleaching. OH• was not involved in either inactivation or photobleaching. Humic acid and salts (NaCl, Na2SO4, CaCl2, and MgCl2) slightly inhibited inactivation, while NaHCO3 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.