| In recent years, infectious disease epidemics, such as those of the Ebola virus, Middle East respiratory syndrome coronavirus（MERS-Co V） and severe acute respiratory syndrome（SARS）, have attracted increasing attention worldwide. The outbreak and transmission of fulminating infectious diseases will strongly threaten human safety and bring risks to the public health. The inactivation of indoor biological contaminants has been shown to play an important role in preventing the transmission of pathogens and ensuring human safety. Therefore, a high-efficiency sterilization technique is required to effectively decontaminate indoor environments.Compared with liquid sterilization chemicals, fumigating decontaminants feature several advantages such as a high efficiency, large disinfection volume, good penetration ability and simple operation and have been widely employed to sterilize enclosed spaces. Chlorine dioxide（Cl O2） gas is with strong oxidation, little toxic, no carcinogenic and teratogenic. As an emerging fumigating decontaminant, Cl O2 gas has advantages of efficiency, safety, and environment friendly. Previous studies in China and abroad mainly focused on the evaluation of disinfection effect in different enclosed spaces. However, there have been little studies to elucidate the inactivation tendency, kinetics and mechanism by Cl O2 gas. The influence tendency of important factors on disinfection effect is still not well understood. Theoretic support for predicting disinfection effect and designing effective treatment conditions is lacking. Therefore, it is necessary to establish inactivation kinetics models by Cl O2 gas in an enclosed space and to reveal the influence of important factors. Besides, the on-line detection of treatment conditions is also an important technology of the disinfection by Cl O2 gas because of the huge influence of key factors on disinfection efficiency. It makes sense to solve the problem that is the on-line detection of treatment conditions under corrosion environment.In this study, both inactivation kinetics models and the predictive model using response surface methodology（RSM） were built based on the experiment platform and method for indoor disinfection by Cl O2 gas. The influence tendency and mechanism of key factors such as gas concentration, relative humidity（RH）, and exposure time were determined to provide the theoretic support for the disinfection applications by Cl O2 gas. The detection technology of treatment conditions by Cl O2 gas were studied based on tunable diode laser absorption spectroscopy（TDLAS）. The temperature and humidity detection system for disinfection environment was established to monitor the temperature and RH on line, accurately, and steadily. These contents above would provide the theoretic and technical support for the application and popularization of disinfection technology and equipment by Cl O2 gas.The experiment platform for indoor disinfection by Cl O2 gas was built according to the practical necessity of disinfection experiments. The platform had good gas tightness and uniformity of Cl O2 gas. It could also simulate real enclosed spaces, monitor treatment conditions, and continuously produce Cl O2 gas. The experiment method for indoor disinfection by Cl O2 gas was also proposed in this study. This method chose typical microorganisms as biological indicators, standardized the preparation of biological indicators, the process of disinfection experiments and the method of microbial enumeration. The research on the experiment platform and method can provide standard enclosed spaces, desired treatment conditions, safe operation and normative experiment methods as the hardware basis in this study.The inactivation kinetics models of Cl O2 gas in an enclosed space were built in this study. Firstly, the inactivation of Bacillus subtilis subsp. niger spores and Staphylococcus albus were conducted by Cl O2 gas under different gas concentrations. The inactivation kinetics models were established based on the first-order model and classical nonlinear models such as Hom model, Weibull model, and I-Q model. The results showed that the Weibull model had simpler equation, more meaningful parameters, and better goodness-of-fit compared with other models. The gas concentration was significantly and positively correlated with the inactivation of microorganisms. The survival curves of B. subtilis subsp. niger spores contained a downward concavity（shoulder） while the curves of S. albus contained a upward concavity（tailing）, indicating a different inactivation tendency between vegetative bacteria and spores. Spores were more resistant to the Cl O2 gas and required longer treatment times to effective inactivation compared to vegetative bacteria.Then the inactivation of B. subtilis subsp. niger spores and S. albus were conducted by Cl O2 gas under different RH conditions. A new model, the Weibull-H model, as well as the the first-order model were established to reveal the inactivation tendency and kinetics of Cl O2 gas. The results showed that the Weibull-H model demonstrated a better fit for the experimental data compared with the first-order model. The RH was also significantly（P<0.05） and positively correlated with the inactivation of microorganisms. There was a nonlinear and rapid improvement in the inactivation efficiency under a high RH condition（70%-90%）. Moreover, the inactivation efficiency of spores exhibited a larger increase in the range from 70% to 90% RH compared with vegetative bacteria.Based on single factor experiments, the predictive model of Cl O2 gas in an enclosed space by RSM was built in order to reveal the interactions among key factors and to predict the disinfection efficiency. A three-factor Box-Behnken experimental design was used to build a second-order polynomial model and to investigate the effects of the gas concentration, RH and exposure time. The results showed that the obtained data were adequately fitted to the second-order polynomial model with an Radj2 of 0.99. The gas concentration, RH and exposure time all significantly（P<0.05） and positively correlated with the inactivation of B. subtilis subsp. niger spores. The interaction between the Cl O2 gas concentration and RH as well as that between the exposure time and RH indicated significant and synergistic effects（P<0.05）. The predictive model provided response surfaces and contour lines under different treatment conditions which could be used to determine effective treatment conditions to reach the sterility assurance level（SAL） and to provide a reference for the disinfection in different enclosed spaces by Cl O2 gas.The application and model validation for indoor disinfection by Cl O2 gas were studied to observe the disinfection effect of Cl O2 gas for typical biological safety equipment as well as the penetrability of Cl O2 gas for the high efficiency particulate air（HEPA） filter, and to verify the accuracy of the inactivation kinetics models and the predictive model established at the same time. The disinfection experiments were conducted for the biosafety cabinet and the HEPA filter unit by the Cl O2 generation technology of two solid components and the portable disinfection equipment. The results showed that the Cl O2 gas had a significant effect for the disinfection of the biosafety cabinet and the HEPA filter unit, and could effectively penetrate through the HEPA filter under both the active circulation and the pressure diffusion. Compared with the pressure diffusion, the active circulation would let Cl O2 gas pass through the HEPA filter more rapidly and obtain a higher disinfection efficiency. The real disinfection effects were in accordance with the predicted results by the models established, and the predictive model was more accurate and had broader applicability compared with the inactivation kinetics models.Moreover, the temperature and humidity detection system for disinfection environment was established based on the TDLAS. The distributed feedback laser（DFB） at 1370 nm was used as the light source. The on-line monitoring gas house was used as the gas absorption cell. The photoelectric detector was used to collect the optical signal containing the information of water-vapor content, and the optical signal was sent to the receiving and managing system for the further treatment. The RH was calculated based on the amplitude of the first-order harmonic signal. The temperature was obtained based on the digital thermometer which was on the outer wall of the gas house. Then the calibration experiment and the performance test were performed. The results showed that the temperature and humidity detection system based on the TDLAS was with good accuracy, could apply to the disinfection environment under different conditions by Cl O2 gas, had better stability and corrosion resistance compared with the electronic thermohygrometer, and provided a new way to monitor the temperature and RH on line, accurately, and steadily for the disinfection or other corrosion environment.In conclusion, this research conducted the disinfection experiments under different treatment conditions in an enclosed space by Cl O2 gas, established the inactivation kinetics models and the predictive model based on the experimental data, accomplished the evaluation of the disinfection effect of Cl O2 gas for the typical biological safety equipment as well as the model validation, and finally built the temperature and humidity detection system based on the TDLAS. The results indicated the nonlinear inactivation behaviors and kinetics of spores and vegetative bacteria by Cl O2 gas, revealed both the individual effect and the interaction of important factors on the disinfection efficiency, provided the predicted treatment conditions for the SAL and a new monitoring method for the disinfection environment. The research achievement will provide a reference to determine effective treatment conditions to reach the SAL, and be of the theoretic and technical support for the application and popularization of disinfection technology and equipment by ClO2 gas. |
