Inorganic Coagulant’s Coagulation Characteristics And Coagulation Mechanism Of Typical Dyes

The textile printing and dyeing industry uses a variety of synthetic dyes for dyeing and discharges a large amount of colored wastewater.This wastewater contains a variety of synthetic dyes with complex structures,which makes it difficult to achieve good decolorization effects with conventional biological treatment methods.Coagulation is an economical and effective treatment method.Because there are so many types of dyes and coagulants,in the current research on the removal of dyes from coagulants,the basic coagulation data of different coagulants for different dyes are insufficient,and most of them are to explore the effect of coagulant dosage and solution pH on coagulation.And there is no further comprehensive and in-depth study on the influence of other influencing factors on the coagulation effect and the characteristics of flocs formed after coagulation and the coagulation mechanism.In order to understand the coagulation characteristics and mechanism of traditional inorganic coagulants for azo dyes and anthraquinone dyes,this article selects AlCl_3,FeCl₃,MgCl₂ as coagulants,direct orange S（DO）,direct yellow 11（DY）,direct black 19（DB）,Acid Blue 40（AB）,and Reactive Blue 4（RB）were taken as the removal objects for coagulation simulation experiments.By exploring the effects of coagulant concentration,dye initial pH,and hydraulic conditions on the coagulation effect,combining Zeta potential,changing the coagulant dosing sequence,FT-IR and XRD and other characterization methods to explore the coagulation mechanism of azo dyes C and anthraquinone dyes.With a view to enriching and perfecting the"base database"of dye removal by coagulation method,it provides a theoretical basis for selecting suitable coagulants and compounding coagulants in a targeted manner.The main conclusions of the experiment are as follows:1.AlCl₃,FeCl₃ and MgCl₂ have high coagulation effects on azo dyes（mainly the three direct dyes selected）at low dosage of coagulant;FeCl3 has the best coagulation effect on anthraquinone dyes.The decolorization rates of Acid Blue 40 and Reactive Blue 4 are up to93%and 97%,respectively,and AlCl3 has the worst coagulation effect on anthraquinone dyes.The best decolorization rate of the three azo dyes is greater than that of the two anthraquinone azo dyes.To compare the coagulation effect,for dye wastewater containing both azo and anthraquinones,Fe Cl3 can be preferentially selected for coagulation treatment.2.The structure of the dye has a certain influence on the coagulation effect of the coagulant.The more amino functional groups in the azo dyes,the larger the molecular weight of the dye,and the longer the molecular chain of the dye is easier to remove by coagulation;the pyrazine functional groups in anthraquinone dyes have little effect on the coagulation efficiency of FeCl₃,but for AlCl₃ and AlCl₃ The presence of MgCl₂and pyrazine functional groups respectively play a role in inhibiting and enhancing the coagulation efficiency.In contrast,FeCl₃ has a high removal rate for anthraquinone dyes and azo dyes,while AlCl₃ and MgCl₂ are more suitable for coagulation decolorization of azo dyes.3.The best initial dye pH for different coagulants acting on different dyes is different.Compared with anthraquinone dyes,the optimum pH range of the three traditional inorganic coagulants for azo dyes is wider.The optimal hydraulic conditions for azo dyes and anthraquinone dyes are also different.The optimum fast mixing time for anthraquinone dyes is 10s,azo dyes require 30 or 60s;anthraquinone dyes require the most The suitable slow stirring speed is 30 r/min,which is less than 50 r/min for azo dyes.4.The coagulation mechanism of inorganic coagulant and azo dyes:charge neutralization is the basis of floc formation,not the dominant mechanism of coagulation;chemical combination is the key to floc formation.AlCl₃ chemically combines the final hydrolyzate with the sulfonic acid group on the direct dye molecule to remove the dye;FeCl₃is the instant hydrolyzed product and the final hydrolyzed product together to play a coagulation effect,and chemically combines with the dye molecule,and MgCl₂ hydrolyzes to produce magnesium hydroxide precipitation.The dye molecules are chemically adsorbed on the surface of the precipitation and removed from the water.For anthraquinone dyes,AlCl₃and FeCl₃ are both instantaneous hydrolysates and final hydrolysates that together play a coagulation effect to remove the dyes through physical adsorption;the final hydrolysis of MgCl₂ produces Mg（OH）₂ precipitation,and the dyes are removed by adsorption on the surface of the precipitation.