Investigation of Water Sterilization by Solar-Light-Driven Bi2WO6-based Photocatalysts

概要

Water contamination caused by pathogenic microorganisms such as Escherichia coli (E. coli) and Enterococcus sp. has been considered as a major concern all over the world, which results in millions of people affected and even death. Although traditional methods including ozonation, ultraviolet radiation and chlorination have been demonstrated to be effective for water disinfection, the various drawbacks of high operational cost, regrowth of resistant bacteria and generation of toxic byproducts limit their practical application. Recently, Bi2WO6 based photocatalytic technology has emerged as a promising method for water disinfection due to its high effectiveness and great potential to utilize sunlight. In the previous study, our lab successfully synthesized solar-light-driven heterostructures Ag/Ag2O/BiPO4/Bi2WO6 (Ag/P/BWO) with excellent performance for organic degradation for the first time. Compared to the traditional heterostructures, construction of Z-scheme photocatalytic system could further enhance the charge-separation ability and redox reaction capacity, in which the ineffectual e- and h+ were recombined, reserving effective e- and h+ with high redox properties. g-C3N4, which has attracted a lot of attention for photocatalysis application due to its facile synthesis and prominent optical properties, was reported to possess matched band structure with Bi2WO6 and BiPO4 to construct Z-scheme system. Therefore, introducing g-C3N4 (CN) to Ag/P/BWO might further enhance its photocatalytic activity via developing Z-scheme system with Ag-Ag2O as electron mediator. Up to date, there is limited information about the application of these two Bi2WO6 based photocatalysts in water disinfection. Therefore, the objective of this study is to firstly investigate the water sterilization ability of Ag/P/BWO, then introduce CN to Ag/P/BWO to construct Z-scheme system CN-Ag/P/BWO for water disinfection, in hope of providing more useful information for environmental water disinfection.

Firstly, the photocatalytic disinfection activity of Ag/P/BWO composite was investigated by the inactivation of E. coli under simulated solar light. The results showed Ag/P/BWO composite exhibited remarkable disinfection efficiency, which could completely inactivate 2.5 ×10 7 cfu/mL E. coli only within 20 min with optimal catalyst loading of 0.5 g/L. Then, the inactivation mechanism was systematically investigated. The results indicated the major roles of photogenerated active species including·O 2- and h+ in bactericidal process. At the early stage of disinfection, the antioxidant system was initiated and increased enzyme activities to fight oxidative attack. With accumulation of oxidative damage, the activities of antioxidant enzymes were suppressed, which resulted in deterioration of defense system and led to subsequent damage of cell membrane, causing intracellular component leakage (K+, proteins and nucleic acid) and final collapse of bacterial cells. Besides, Ag/P/BWO composite exhibited high adaptability for water disinfection at a wide range of environmental factors including light intensity, temperature, pH and humic acid. The slight release of Ag+ (< 0.1 mg/L) and high stability of photocatalysts during successive disinfection experiments implied its great potential for application.

Then, CN was introduced to Ag/P/BWO to construct Z-scheme composite CN-Ag/P/BWO for water disinfection. The results showed that Z-scheme composite CN-Ag/P/BWO was successfully constructed for the first time and the CN introduction significantly extended the light absorption ability, enhanced charge carries separation/transfer capacity and improved surface wettability of CN-Ag/P/BWO. In addition, the photocatalytic disinfection activity was explored by the inactivation of recalcitrant gram-positive bacteria, Enterococcus sp. under simulated solar light. The results indicated the novel composite CN-Ag/P/BWO showed excellent disinfection performance toward Enterococcus sp. (1.5×10 7 cfu/mL), with complete inactivation only within 60 min. Besides, the disinfection mechanism toward Enterococcus sp. was systematically investigated based on the bactericidal process, major active species and charge transfer pathway. The results demonstrated the deteriorated antioxidant system was responsible for the damaged cell membrane integrity and energy system (ATP synthesis), which further led to the leakage of intracellular components (protein and nucleic acid) and final collapse of bacteria. Moreover, the photogenerated h+, ·O 2-, and ·OH played cr ucial roles in Enterococcus sp. inactivation process and the Z-scheme transfer pathway of CN-Ag/P/BWO enabled these active species with strong oxidizing ability. Furthermore, CN-Ag/P/BWO performed well for different environmental factors and real lake water disinfection. The high stability further confirmed its practicability for water disinfection.

Therefore, this work provided effective solar-light-driven Bi2WO6 based photocatalysts for water disinfection, which could contribute to mitigate the risk of waterborne diseases and also improve the public health and environment.