Green phytoextracts as natural photosensitizers in LED-based photodynamic disinfection of multidrug-resistant bacteria in wastewater effluent

Abstract

The photodynamic treatment (PDT) process is a promising technology to control emerging pollutants and antimicrobial resistance problems in the water environment. The reactive oxygen species (ROS) produced when natural and nontoxic photosensitizers (PS) are exposed to light are the key functional components of the PDT process that can effectively achieve microbial inactivation with minimal negative impact. This study examined the application of green phytoextracts from two plants, Chamaecyparis obtusa and Moringa oleifera, as natural photosensitizers for the white light-emitting diode (LED) based photodynamic disinfection of multidrug-resistant (MDR) and total coliforms (TC) from secondary effluent in full-scale municipal wastewater treatment plants. Gas chromatography–mass spectrometry and Fourier transform infrared spectroscopy showed that the phytoextracts contained 57 compounds, particularly aromatic and phenolic hydroxyl compounds. The phytoextracts showed an excellent activity as a PS compared to the intrinsic antibacterial effect. Under a light intensity of 17 mW/cm2, the complete inactivation (6.55 Log CFU/mL) was observed at an irradiation time of 100 min for Escherichia coli ART-2 and 80 min for Staphylococcus aureus, meaning that E. coli was resistant. The light intensity was an important factor influencing photodynamic disinfection. For the complete disinfection of TC satisfying the water reclamation regulation, the irradiation time requirement was 20 min under a light intensity of 80 mW/cm2. During the photodynamic reaction, a significant amount of ROS was generated from the phytoextracts as the light irradiation time was increased. The major ROS was singlet oxygen (1O2, Type II) during the initial 40 min of reaction time and hydroxyl radical (•OH, Type I) after 40 min until complete inactivation.