Preparation Of Graphitic Carbon Nitride-based Photocatalysts And Performance Investigation Of Visible Light Degradation Of Dyeing Wastewater

Dyeing wastewater is difficult to be effectively treated due to its complex chemical composition,high chroma,strong toxicity,and difficult degradation.When it is arbitrarily discharged into nature,it not only seriously pollutes water resources and the ecological environment,but also threatens the survival and health of human beings.Therefore,there is an urgent need to develop new technologies to efficiently treat dyeing wastewater.As a green way to completely decompose pollutants by using solar energy,photocatalytic technology is considered to have broad application prospects in the field of dyeing wastewater treatment.Photocatalyst is the key for photocatalytic technology,and its development has always been the focus of research.Graphitic carbon nitride g-C₃N₄（CN）is considered to be a potential photocatalytic material due to its abundant raw material sources,good visible light response,good chemical stability,and non-toxicity.It has become a research hotspot in the field of photocatalytic degradation of dyeing wastewater.However,CN has drawbacks such as small surface area and excessively fast recombination of photogenerated electron-hole pairs,resulting in unsatisfactory photocatalytic activity.Based on the above,in this dissertation,melamine was used as the main raw material,and CN-based composites with higher photocatalytic degradation performance were prepared by screening synthesis condition,transition metal doping,semiconductor compounding,and porous biochar modification.The crystal structure,microscopic morphology,and optical features of the composites were characterized.The photodegradation performances,recycling stability,and degradation mechanism of rhodamine B（Rh B）and methyl orange（MO）were studied.Finally,the degradation paths of the two dyes during photodegradation were analyzed in detail,and the photodegradation of real dyeing wastewater was also explored,which provided a way for the effective treatment of industrial dyeing wastewater.The main research contents and results are as follows:（1）A series of CN-X photocatalysts were prepared by thermal polycondensation.The results showed that,with the increase of calcination temperature,CN-X shifted to a higher crystallinity,more stable graphene-like structure,larger surface area,and more excellent optical properties,which improved its photodegradation performance to dyes.When the calcination temperature was 700°C,CN-700 sample showed the best photocatalytic activity with the degradation rates of Rh B and MO reaching 99.11%and98.81%,respectively,after 30 min and 4 h of visible light irradiation.In addition,CN-700 exhibited excellent recycling stability for the photodegradation of both dyes after 6recycles use.Finally,it was found that·O₂^-radical and·OH radical played a major role in the system by analyzing the photodegradation mechanism.（2）A series of Fe-doped（Fe-CN）,Ni-doped（Ni-CN）,and Fe-Ni co-doped（Fe-Ni-CN）photocatalysts were synthesized by the one-pot method.The results showed that Fe and Ni existed in the form of Fe³⁺and Ni²⁺after doping,and they were incorporated into the CN structure through the formation of Fe-N and Ni-N coordination bonds,forming a metal ion doping energy level,which was beneficial to the separation of photogenerated electrons-hole pairs.Compared with CN,the doped photocatalysts displayed effectively enhanced dye degradation performance,and the photocatalytic activity of co-doped Fe-Ni-CN was further improved compared with Fe or Ni doping.Fe-Ni-CN-2 showed the best photocatalytic activity with the degradation rates of Rh B and MO reaching 99.53%and 99.28%,respectively,after 20 min and 3 h of visible light irradiation.Finally,it was found that·O₂^-radical and·OH radical still played a major role in the system by exploring the mechanism of Fe-Ni-CN-2 in the photodegradation of dyes.（3）Ag₃PO₄-CN composites were prepared by the co-precipitation method.Compared with CN and Ag₃PO₄,the photodegradation performances of Ag₃PO₄-CN in both Rh B and MO were greatly enhanced after composition,which was attributed to the unique heterojunction caused by the combination of the well-matched energy band structure between CN and Ag₃PO₄.With the increase of the mass of Ag₃PO₄,the activity of Ag₃PO₄-CN on photodegradation dyes showed a trend of decrease after a prior increase.When the mass fraction of Ag₃PO₄ was 30%,Ag₃PO₄-CN-30 showed the best photodegradation performance after 12.5 min and 2 h of visible light irradiation,which degradation rates of Rh B and MO were up to 97.95%and 98.34%,respectively.Additionally,Ag₃PO₄-CN-30 exhibited excellent recycling stability for dyes degradation.Finally,it was found that h⁺radical and·O₂^-radical played a major role in the photodegradation process by analyzing the degradation mechanism of Ag₃PO₄-CN-30.（4）CC-CN composites were prepared by the solvothermal method after porous corncob carbon（CC）modification.The adsorption performance of the modified CC-CN for dyes was greatly improved,and with the increase of the amount of CC,the adsorption rates of the two dyes increased first and then decreased.When CC-CN-3 was used as the adsorbent,the adsorption rates for Rh B and MO reached 88.97%and 93.04%after 30min and 4 h of shading reaction,respectively,showing the best adsorption performance.The whole dye adsorption process conformed to the Langmuir model and pseudo-second-order kinetics,and belonged to an endothermic reaction.When CC-CN-3 was used as the photocatalyst,the photodegradation performance of the two dyes was improved,and the whole photodegradation process conformed to pseudo-first-order kinetics.The above indicated that the CC-CN-3 composite displayed both high adsorption performance and high photocatalytic performance for the removal of dyes.（5）At the end of the dissertation,the degradation paths and the intermediates of Rh B and MO dyes in the photodegradation process were discussed.During the photodegradation process of Rh B and MO,the related chromophores were first reacted with active radicals,causing the color of the reaction solution to become lighter,and then the bond-breaking oxidation process occurred.Rh B was mainly involved in the two reactions of deethylation and hydroxylation,and MO mainly involved the removal of sulfonic acid group and dimethylamino group and the cleavage of azo double bond.Then,the two dyes underwent the ring-opening cleavage of the benzene ring through the continuous attack of hydroxyl radicals,and then underwent deep oxidation to generate small molecular organic acids,which finally oxidized to generate CO₂ and H₂O.Furthermore,the as-prepared photocatalysts were used in the photodegradation experiment of real dyeing wastewater,and it was found that all photocatalysts could effectively degrade real dyeing wastewater.The degradation rate of Ag₃PO₄-CN-30reached 53.42%when photodegradation for 4 h,which reduced the COD value decreased from 1326 mg/L to 546 mg/L,which has reference value for the practical application of photocatalytic degradation of real dyeing wastewater.