Fundamental Research On The Formation Mechanism And Transformation Principle Of The Reactive Oxygen Species In Sub-molten Salt Media Using The Second-order Nonlinear Optical Spectroscopy And Molecular Simulation

The current utilization of amphoteric metal resources suffers from relatively low efficiency but high environmental contamination during the extraction process.Aiming to solve these obstacles,a sub-molten salt(SMS)oxidative process has been developed with high extraction efficiency and environmental friendliness by the Institute of Process Engineering,Chinese Academy of Sciences.The SMS oxidation system provides favorable chemical environment that allows for the presence of considerable reactive oxygen species(ROS).The latter plays an important role on directly transferring amphoteric metal minerals with extremely high conversion rate(approaching to 100%).However,the exact oxidation mechanism is still unrevealed in particular with the reaction mechanism of those ROS in SMS system.In this dissertation,an qualitative-quantitative measurement of ROS in SMS system has been first established by using second-order nonlinear sum-frequency generation(SFG)technique,which can characterize the operando molecular configuration of ROS especially with the configuration of hydrogen-bond(H-bond)at the interface.SFG can detect the asymmetric molecular vibration at the interface of high sensitivity,and therefore,the higher concentration of the ROS in SMS system,the more parallel-type of the H-bond configuration with ROS at the interface,thus generating more symmetric H-bond vibration and reduced SFG response.On the basis of SFG detection,we have revealed the quantitative relationship between the molecular configuration of ROS and its concentration in SMS system.Furthermore,in order to better explain the results of SFG,the molecular/quantum simulation has been applied to identify the vibration characteristics at the interface.On the combination of SFG-based quantitative analysis and molecular/quantum simulation,the conversion mechanism of ROS and the regulation principle of ROS in SMS system is proposed,which provide insights to further support the development of SMS system.The progress achieved in this dissertation are as follows:(1)The formation mechanism of ROS in SMS system is revealed by quantum simulation as well as the effect of numbers of water molecules,alkali species and alkali concentration on ROS generation.The result indicates that oxygen molecules first reacted with hydroxyl ions to form the intermediate ions of O32-.The latter consequently reacted with O2-ions to form HO2-ions,which was identified as the main ROS species existed in SMS system.The formation of intermediate ions of O32-was found to effectively lower the activation energy of HO2-.With respect of alkali species,potassium hydroxide generated less formation energy of HO2-ions than that of sodium hydroxide,suggesting more appropriate environment for the formation of ROS in SMS system.(2)In-situ SFG-based detection method of ROS has been established based on identifying the hydrogen bond configuration at the interface.SFG spectroscopy in combination with molecular simulation was used the first time to study the molecular structure and vibration principle of the ROS in the SMS media from molecular level.The SFG wavelength of 2935 cm-1 was confirmed to be the fingerprint peak of H02-species in the SMS media.The adsorption and rearrangement of HO2-ions could generate symmetrical vibration at the interface,leading to the decrease in the HO2-characteristic peak located at 2935 cm-1.Therefore,the decrease of SFG intensity at 2935 cm-1 was concluded to represent the increase of HO2-ion concentration.And based on the above analysis,quantitative detection of HO2-ions could be achieved by comparing the change of SFG signal at 2935 cm-1.(3)Regulation principle of ROS in SMS system has been proposed based on the self-established SFG-based quantitative measurement of ROS.It was suggested that the fine bubbles could significantly facilitate the formation of HO2-ions in SMS system.Especially,when the size of oxygen bubbles was reduced to 0.5 ?m,the concentration of HO2-ions was five times higher than that of traditional oxygen bubbles,and fifty times higher than that with traditional nitrogen bubbles.Further studies showed that the ROS concentration in potassium hydroxide medium was higher than that in sodium hydroxide medium.