Mechanism Of Manganese Recovery From Polluted Water By Ti O₂photocatalysis System And Its Application

As an important metal resource,manganese is widely used in the iron and steel smelting industry.Due to its special physical and chemical properties,it is reused in railway,shipbuilding,machinery and national defense industry,but it also leads to many environmental pollution problems,especially the excessive content of manganese ions in water has attracted great attention all over the world.Excessive intake of water or food containing manganese will increase the damage to the liver and lead to nervous system diseases.It is an arduous task for environmental protection workers to realize the effective protection of the water environment,meet the needs of human water quality,ensure the safety of drinking water,and realize the recovery of heavy metal resources.As an efficient,green and energy-saving pollutant treatment technology,photocatalysis has attracted extensive attention from researchers.In this thesis,a new method of photocatalytic treatment of water polluted by manganese ions is proposed,which can not only remove manganese ions from water,but also realize its recovery and resource application.The main research contents of this thesis are as follows:（1）Removal efficiency,mechanism and energy consumption analysis of manganese ion in water.Firstly,an anodic photocatalysis system of the nano-Ti O₂ array（Ti O₂NTA）has been constructed.The system was operated in the mode of constant current with 370 nm ultraviolet light as the excitation light used source.The experimental results of photocatalytic removal of manganese cations showed that the photocatalytic system（PEC）had a good removal of on Mn ions in water,and the percentage removal after 80 minutes was 91%,which is 4.5 and 1.8times higher than those of the photocatalysis（PC）and electrocatalysis（EC）systems,respectively.The analysis of the influencing factors of the PEC system showed that under the condition of p H=1-10,Mn ions could be effectively removed,at the optimal p H condition of10.93%Mn could be removed in 80 minutes.With the increase of current density,the removal of Mn first increased and then decreased,The optimal current density was 229μA/cm~2.In addition,the effect in the initial concentration of manganese ion on the removal rate showd that under the concentration of 2-200 mg/L,the concentration of residual manganese ions in water could reach the standard of manganese ions in drinking water（0.1mg/L）of China at a prolonging reaction time of 240 minutes.Scanning electron microscopy（SEM）,X-ray photoelectron spectroscopy（XPS）,energy dispersive spectroscopy（EDS）and electrochemical impedance spectroscopy（EIS）were used to analyze the morphology,optical and electrochemical properties of manganese deposited on Ti O₂ NTA at different times,and then to reveal the deposition mechanism of manganese ions.The results show that Mn mainly existed in the forms of Mn₂O₃(Mn³⁺)and Mn O₂(Mn⁴⁺)on the surface of Ti O₂ NTA.With the reaction continuing,the proportion of Mn⁴⁺gradually increased,and finally all the manganese oxides deposited on the surface of the photoelectrode were transformed into Mn O₂.Combined with the quenching experimental results of reactive oxygen species,it has been found that in this system,the oxidation and deposition of manganese ions mainly depended on the photogenerated holes generated on the catalyst surface,and its contribution accounted for more than 70%,In comparison,superoxide radicals inhibited the removal of manganese ions.With the reaction continuing,the contribution rate of holes increased slightly.Finally,the power consumption of the PEC process of Ti O₂NTA and EC process of Ti/Ir O₂-Ta₂O₅ at the same current density were compared.It is found that the power consumption of the PEC process used in this thesis was 0.20 k Wh/m~3（wastewater）,less than 10%of EC of Ti/Ir O₂-Ta₂O₅.（2）Dissolution efficiency,mechanism and energy consumption analysis of the deposited manganese.Firstly,the experimental results of photocatalytic dissolution of deposited manganese showed that the photocatalytic system（PED）had a good dissolution effect on deposition in water,and the percentage dissolution was more than 90%in 80 minutes.however,a high efficiency was observed under acidic conditions（p H=3）,and the dissolution of manganese could not be realized under neutral and alkaline conditions.It is found that light could effectively accelerate the dissolution of deposited manganese,and 100%dissolution could be achieved in 80 minutes,the efficiency of which was 1.3 times higher than that in dark conditions.For the effect of current density,it is found that increasing the dissolution current density could significantly accelerate the dissolution process,and the optimum dissolution current density was 171μA/cm~2.The resultant dissolution efficiency could reach 100%in 40minutes,which were 1.3,1.3 and 2 times of those 43,86,and 129μA/cm~2,respectively.Scanning electron microscopy（SEM）,X-ray photoelectron spectroscopy（XPS）and energy dispersive spectroscopy（EDS）were used to analyze the morphology,optical and electrochemical properties of manganese on Ti O₂ NTA at different times.The results showed that during the dissolution process,with the reaction continuing,Mn in the sediment was mainly observed in the forms of Mn₂O₃,Mn OOH(Mn³⁺)and Mn O₂(Mn⁴⁺)on the surface of Ti O₂ NTA.With the progress of the reaction,the proportion of Mn⁴⁺decreaseed gradually.At40 minutes,all the manganese oxides deposited on the surface of the photoelectrode were transformed into Mn³⁺,and finally into Mn²⁺,so as to complete the dissolution of deposited manganese.Finally,the energy consumption of the dissolution process was calculated,and to be 7.6 k Wh/kg Mn.（3）Resource utilization of deposited manganese oxides and analysis of triclosan degradation mechanism.The photoelectron spectrum of the manganese oxide prepared by the above-mentioned electrolytic method（1.8 e V）and the valence band of manganese oxide were analyzed.It was found that the manganese oxide was prepared on the position of manganese oxide（-0.55 e V）.The degradation experiment of triclosan based on the composite catalyst under visible light irradiation showed that the composite photocatalyst had efficient degradation ability of triclosan.More than 80%degradation could be achieved in 100 minutes,which was 4 times that of Ti O₂ NTA.The results showed that the degradation efficiency decreased with an increase in the initial concentration（82%,65%,58%and 55%,respectively）.For the effect of manganese oxide deposition,triclosan could be effectively degraded when the manganese ion concentration was 20 and 2 mg/L and the Mn deposition time of 20,40 and80 minutes respectively,Under the optimum condition（i.e.,the initial concentration of 20mg/L and deposition time of 20 minutes）,82%of triclosan could be degraded in 100 minutes.Through the quenching experiment of reactive oxygen species,it was found that hydroxyl and superoxide radicals played a major role in the degradation of triclosan,and the contribution rates reached 50%and 40%respectively.Finally,through the analysis of degradation products,8 intermediate products including 2-chlorophenol were found.It was found that the degradation of triclosan was mainly via the oxidation pathway.To sum up,by constructing a photocatalysis system based on Ti O₂ NTA,results of this thesis have demonstrated that efficient removal and recovery of manganese ions in water could be achieved.The deposition and dissolution mechanisms of manganese ions have been discussed,and the resource utilization of deposited manganese oxides has been proposed,which can realize the efficient degradation of triclosan under the condition of photocatalysis.