Photocatalytic degradation of trihalomethanes and haloacetonitriles on graphitic carbon nitride under visible light irradiation.

Trihalomethanes (THMs) and haloacetonitriles (HANs), most common disinfection by-products in drinking water, pose adverse environmental impacts and potential risks to human health. There is a pressing need to develop innovative, economically feasible, and environmentally benign processes to control these persistent contaminants. In this paper, visible-light-responsive graphitic carbon nitride (g-C3N4) samples were synthesized to degrade the THMs and HANs and the photocatalytic degradation mechanism was explored. The results indicated that a carbon-doped g-C3N4 with an optimum dopant content (MCB0.07) displayed the best photocatalytic activity for the total trihalomethanes (TTHM) and total haloacetonitriles (THAN), with the reaction rate constant of 11.6 and 10.4 (10-3 min-1), respectively. MCB0.07 demonstrated a high THMs and HANs removal efficiency under visible light irradiation and could be reused. According to scavenger tests of the selected reactive species and X-ray photoelectron spectroscopy, holes play a dominant role for both THMs and HANs degradation on the MCB0.07. The degradation of HANs by holes proceeded mainly through breakage of the CC bond in the CCN group. The THMs degradation was achieved through hydrogen abstraction or/and dehalogenation. The brominated-THMs/HANs were more photosensitive than their chlorinated analogous and were less stable than bromo-chloro-THMs/HANs. This study sheds light on the mechanism of the photocatalytic degradation of THMs and HANs under visible light irradiation by carbon-doped g-C3N4. Furthermore, it could provide insights for engineering applications and contaminant control in drinking water purification.

Enhanced inactivation of antibiotic-resistant bacteria isolated from secondary effluents by g-C3N4 photocatalysis.

The extensive use of antibiotics has resulted in the development of antibiotic-resistant bacteria (ARB), which may not be completely removed by traditional wastewater treatment processes. More effective approaches to disinfection are needed to prevent the release of ARB into the surface water. The metal-free photocatalyst graphitic carbon nitride (g-C3N4) has aroused great interest as a possible agent for water and wastewater treatment, due to its low cytotoxicity and photoactivity with visible light. In this study, the efficacy of g-C3N4 was assessed as a possible means to enhance ARB inactivation by irradiation. ARB were isolated and purified from secondary effluents in 4 municipal wastewater treatment plants. Of these, 4 typical multi-drug ARB isolates, belonging to Enterobacteriaceae, were selected for irradiation experiments. Inactivation was seen to increase with irradiation time. At 60 min, the inactivation of the 4 ARB isolates by light at > 300 nm and > 400 nm was in the range of 0.25-0.39 log and 0.16-0.19 log, respectively. The use of g-C3N4-mediated photocatalysis at the same wavelengths significantly enhanced that to 0.64-1.26 log and 0.31-0.41 log, respectively. The antibiotic susceptibility of the ARB isolates remained unchanged either prior to or after irradiation and was independent of photon fluence, reaction time, and the presence of g-C3N4. This study establishes a baseline for understanding the effectiveness of g-C3N4 photocatalysis on inactivation of ARB in wastewaters and lays the foundation for further improvement in the use of photocatalysis for wastewater treatment.

[Formation of Brominated Disinfection By-products in Low Temperature Multi-effect Distillation (LT-MED) Process for Seawater Desalination].

Changes in water quality and brominated disinfection by-products (Br-DBPs) during a low temperature multi-effect distillation (LT-MED) process for seawater desalination were investigated. The concentrations of bromide ion and specific ultraviolet absorbance (SUVA) in the seawater (i.e. the influent of LT-MED) were 54.6 mg·L-1 and 1.7 L·(mg·m)-1, respectively. The tryptophan-like aromatic protein, fulvic acid-like and soluble microbial by-product-like organics dominated the fluorescent dissolved organic matter (DOM) in the seawater. After the NaClO pre-chlorination in the LT-MED process, the concentrations of DBPs in the seawater were significantly increased, especially Br-DBPs, and Bromoform(CHBr3) accounted for 100% of total trihalomethanes (THMs), Bromoacetic acid (C2H3BrO2) and dibromoacetic acid (C2H2Br2O2) accounted for 31.9% and 68.1%, respectively of total haloacetic acids (HAAs), while 4-Bromophenol (C6H5BrO) accounted for 100% of total halogenated phenols (HPs). The formation of THMs, HAAs and HPs was not detected in the finishing water produced by the LT-MED desalination process, but these substances were retained in the concentrated brine, of which THMs, HAAs and HPs were 56.9, 35.0 and 0.1 µg·L-1.