Multilayer Adsorption Performance For Dyes On Coal Tar-based Porous Carbons

Adsorption method is widely used in the treatment of dye wastewater because it can remove the dye molecles from the wastewater under mild conditions with simple operation.The removal of dyes from wastewater is commonly carried out by activated carbons,owing to their high surface area for accommodating large organic molecules.It is more effective in increasing the adsorption capacity of activated carbons by utilization of existing pores than that of optimizing the structure of activated carbons.In this paper,the coal tar carbon?CTC?was prepared by the template method,and the modified porous carbon which is named NC was obtained by soaking with ethylenediamine.The physical structure and surface chemistry character of the as-made carbons were tested by nitrogen adsorption-desorption,field emission scanning electron microscopy?FESEM?,transmission electron microscopy?TEM?and XPS.The acid dyes?Congo red and Acid fuchsin?and the basic dye?Methylene blue?were adsorbed onto the as-made carbons by multilayer adsorption,which refers to the layer-by-layer sequential uptake of positive and negative charged dyes onto the activated carbons.The static and fixed-bed adsorption processes were carried out to explore the feasibility and superiority of multilayer dye adsorption.The main results were as follows:?1?The CTCs were prepared from coal tar using CaO as template coupled with KOH activation by conventional heating,which were irregular block structure with containing rich mesopores.The carbon materials with different structure were prepared by adjusting the ratio of activator/carbon source and final termination temperature.When the ratio of coal tar,CaO and KOH was 1:2:4 with the final temperature of 900 ^oC,the porous carbon which is named CTC_4-900 shows high specific surface area of 3101 m²/g.The CTCs exhibited the excellent first layer adsorption performance for Congo red,owing to their high surface area and abundant adsorption activation sites.The CTC_4-900 had the highest adsorption capacity for Congo red with the saturation adsorption capacity of 3885 mg/g.The adsorption isotherm of Congo red on CTCs can be described by Langmuir adsorption isotherm model.Methylene blue was adsorbed on CTCs-CR as the second layer.The CTC_4-900-CR had the highest adsorption capacity for Methylene blue with the saturation adsorption capacity of 931 mg/g.The adsorption isotherm of Methylene blue on CTCs-CR can be described by Langmuir adsorption isotherm model.Congo red was adsorbed on CTCs-CR-MB as the third layer.The CTC_4-900-CR-MB had the highest adsorption capacity for Congo red with the saturation adsorption capacity of1001 mg/g.Compared with the second layer,the R² of the Freundlich model was increased,suggesting a significant heterogeneity of the adsorption sites.?2?The modified carbons,which were named NC₂ and NC₃,were prepared from CTC_4-900 by soaking different concentrations of ethylenediamine.The mesopores volumeand and specific surface area of the modified carbons were reduced.The NC₃shows the specific surface area of 1934 m²/g.The surface chemical properties of NCs were changed due to the decrease of oxygen functional groups and the increase of nitrogen functional groups,which was beneficial to the adsorption of acid dyes.Acid fuchsin was first adsorbed onto NCs.The isotherm results show the NC₃ had the highest adsorption capacity for Acid fuchsin with the saturation adsorption capacity of2486 mg/g,which can be described by Freundlich adsorption isotherm model.Methylene blue was adsorbed on NCs-AF as the second layer.For CTC_4-900-900 containing more available adsorption sites than those of NCs,the CTC_4-900-CR had the highest second layer adsorption capacity for Methylene blue,with the saturation adsorption capacity of 481 mg/g,and the isotherm can be described by Langmuir model.Acid fuchsin was adsorbed on NCs-AF-MB as the third layer.The third layer adsorption was a direct function of the charge density of the second layer.Thus CTC_4-900-CR-MB had the highest adsorption capacity for Acid fuchsin with the saturation adsorption capacity of 635 mg/g,and the isotherm can be described by Freundlich model.?3?The fixed-bed multilayer dye adsorption was carried out to study the dynamic adsorption process using granular carbon ZLC_4-900 and ZLNC₃ as fillers materials.The porous carbon structures for CTC_4-900 and NC₃ changed less during the granulation process.The multilayer adsorption results show the adsorption column which was saturated with the acid dye also had considerable adsorption capacity for basic dye in the second layer,and the adsorption column which was saturated with basic dye in the second layer can still adsorb a certain amount of acid dye in third layer,which proved the feasibility of multilayer dye adsorption.The fixed-bed adsorption capacity for dyes was affected by the experimental conditions.Increasing the initial concentration of the adsorbate and the carbon filling amount,and reducing the influent flow rate were beneficial to improve the fixed-bed adsorption capacity.In this paper,the optimum adsorption conditions for the best adsorption capacity are:the initial concentration of Congo red and Acid fuchsin was 500 mg/L and 300 mg/L,respectively.The influent flow rate was 1.27 mL/min,and the filling amount of carbon was 0.05 g.