Synthesis Of Flower-like Ferrihydrite And Its Catalytic Degradation Of Azo Dyes

Azo dyes are widely used in the printing and dyeing industry due to the simple preparation procedure,low cost,outstanding performance.Under certain conditions,most azo dyes could be decomposed to form more than 20 kinds of aromatic amines with teratogenicity,mutagenicity and carcinogenicity,all of which is harmful to human health.So,the treatment of dye wastewater has been the hot topic in environmental research.Our reach group has been engaging in researching the synthesis of iron oxides in solution and its application in wastewater treatment.In earlier research,we prepared pure ferrihydrite using Fe³⁺ solution as raw material with three synthesis methods.The photocatalytic degradation mechanism of organic pollutants such as yellow 10 and phenol red in ferrihydrite/H₂O₂ system was discussed.Based on the previous studies and the above analysis,a novel flower-like ferrihydrite was prepared using ferrous ions as raw material and citrate as additive.In this paper,azo dye orange II and naphthol blue black were used as the target pollutants to investigate their degradation process and mechanism in the presence of flower-like ferrihydrite and Na₂S₂O₈ under visible light irradiation.The result of this paper can provide a new theoretical basis and technical support for the effective treatment of dye wastewater.So,it has important theoretical and practical value.The main research contents and research achievement of this paper are as follows:?1?Ferrous sulfate used as the raw material and citric acid as the additive,flower-like ferrihydrite was prepared by air oxidation method.The influence of species of iron salts,different oxidants and different additives on the formation of flower-like ferrihydrite were studied.Besides,we preliminary speculated the possible growth mechanism of flower-like ferrihydrite.The result shows that the species of ferrous salts has no effect on the formation of flower-like ferrihydrite.Flower-like ferrihydrite could be formed both in the presence of H₂O₂ and air.However,perfect flower-like ferrihydrite couldn't be obtained when using Na₂S₂O₈ or NaClO as additives.When the citric acid is replaced by sodium acetate and succinic acid,rod-shaped ?-FeOOH was formed.When the citric acid is replaced by glucose and sucrose,although the product is ferrihydrite,it is not flower-like morphology.Besides,flower-like ferrihydrite wasn't also formed when using sodium tartrate and sodium dodecyl sulfonate as additives.Based on the experimental results and the structure of the additive,the formation mechanism of flower-like ferrihydrite is that three carboxyl groups of citric acid exhibit a large affinity to Fe?II?ions which makes ferrihydrite nanoparticles aggregate and orientated growth around in three carboxyl groups to form a flower structure.?2?Ferrihydrite was used as catalyst and orange II as the target pollutant to investigate the degradation process and degradation mechanism of orange II in the presence of Na₂S₂O₈ under visible light irradiation.Simultaneously,the reusability of ferrihydrite was explored.The results indicate that the system has higher degradation efficiency for azo dye orange IIand the degradation rate of 80 ppm orange II can reach 98.43% after 6 h.The TOC results indicate a complete mineralization rate of 83.2%.Firstly,Orange II in the system was degraded to form the amino acid and 1-amino-2-naphthol.Then,sulfanilic acid molecules were futher oxidized to generate small molecules,and ultimately be completely mineralized.1-amino-2-naphthol is oxidized to ninhydrin,which is then converted to formic acid and acetic by a series of oxidative degradation steps and finally conveted to CO₂.The result of 6 cycles shows that the degradation rate of orange II was 98%,indicating the as-perpared ferrihydrite has a good reusability.?3?Ferrihydrite was used as catalyst and naphthol blue black as the target pollutant to investigate the degradation process and degradation mechanism of naphthol blue black in the presence of Na₂S₂O₈.The results indicate that the degradation rate was 98.26% at 120 min for 80 ppm azo dye naphthol blue black.62% of naphthol blue black were fully mineralized and the remaining 38% was degraded into small molecules containing nitrobenzene.In addition,the degradation process of binary mixture of orange II and naphthol blue black was explored.Ferrihydrite/Na₂S₂O₈ system also exhibits good degradation efficiency and the degradation rate of the two dyes was above 95% after 6 h.