Preparation Of Rare-earth Doped Magnetic Biopolymer Composite Adsorbents And Their Adsorption Properties For Direct Dyes

With the rapid development of modern industry,especially the rapid development of dye production and printing and dyeing industry,the discharge of dye wastewater is increasing day by day.If it is not treated or not treated properly,dye wastewater will not only harm the environment,but also threaten the life safety of human beings.How to effectively treat dye wastewater has become the research focus of environmental treatment workers.Sodium alginate?SA?,as a rich natural marine resource,has not only rich-COOH and-OH active groups in its molecular chain,but also the characteristics of biocompatible,non-toxic and harmless,which makes it widely to be used in wastewater treatment.As a kind of solid waste discharged from coal-fired power plant,fly ash?FA?has a high utilization value in dye wastewater treatment because of its adsorption performance.In this study,two kinds of rare-earth doped magnetic biopolymer composite adsorbents were synthesized on the basis of the team's previous research.One kind of magnetic alginate gel microspheres?magnetic SA/La?was synthesized by cross-linking reaction between SA and lanthanide ions?La?III??.Another kind of yttrium doped magnetic chitosan/fly ash composite material?magnetic Y/CTS/FA?was prepared by loading a small amount of chitosan?Chitosan,CTS?,Fe₃O₄ particles and rare-earth Y?III?ions using FA as substrate.The adsorption materials were characterized by SEM-EDS,XRD,FT-IR,VSM,specific surface area and pore size analysis.Direct red brown RN?DRB RN?and direct dark brown 2M?DDB 2M?dyes were used as the adsorption objects of magnetic SA/La,and direct red brown RN?DRB RN?and direct lake blue 5B?DLB 5B?were used as the adsorption objects dye as the adsorption object of magnetic Y/CTS/FA,the preparation conditions,adsorption conditions,adsorption kinetics,thermodynamics and isothermal adsorption of each kind of adsorption materials were studied respectively,and the adsorption mechanism was discussed.The specific research results are as follows:?1?By combining the dye adsorption experiments,the optimum preparation conditions of magnetic SA/La adsorbent were obtained.At room temperature,4%SA solution was mixed with Fe₃O₄ at a mass ratio of 10:2 to form a magnetic SA solution,and then dropped into 3.5%La?NO?₃ solution of80 mL,the uniform-size gel microspheres were formed after the reaction of2h,and the target product was obtained by washing and drying.The magnetic SA/La gel microsphere is a black brown particle with"cauliflower"shape.The saturation magnetization?Ms?is 11.19 emu/g,which belongs to the magnetic adsorption material with mesoporous structure.?2?The adsorption conditions of magnetic SA/La for two direct dyes of DRB RN and DDB 2M were studied.The optimum results are:when the initial concentration of dye solution is 3000 mg/L,the dosage of adsorbent is0.10 g,and the natural pH value of the dye solution is 8.65 and 8.60,respectively,the adsorption time is 120 min at 298 K,the adsorption capacities of two dyes onto magnetic SA/La gel microspheres can reach 678mg/g and 688 mg/g respectively.?3?The results of adsorption kinetics,isotherm adsorption and thermodynamics of magnetic SA/La for DRB RN and DDB 2M show that:the adsorption process at different temperatures can be described by pseudo second-order adsorption kinetics equation,both surface diffusion and particle internal diffusion control the adsorption rate;The isotherm adsorption data better conform to Freundlich model;The adsorption behavior of DRB RN and DDB 2M by magnetic SA/La can also be well described by D-R isotherm adsorption model.The calculation results of thermodynamic parameters??G<0,?H<0 and?S>0?indicate that the two adsorption reactions are spontaneous exothermic nature with increasing degree of disorder.Combined with FT-IR and UV-vis analysis of adsorbents before and after dye adsorption,the main adsorption between adsorbates and adsorbent is electrostatic adsorption,hydrogen bonding and ion exchange.?4?The optimized preparation conditions of magnetic Y/CTS/FA composite materials were studied by adsorption experiments.The mass ratio of FA,Y???ion,CTS and Fe₃O₄ was 10:2:2:2.According to the mass ratio,dissolved a certain mass of CTS in 5%glacial acetic acid solution of 9.8 mL,and FA was added and stirred for 120 min to form CTS/FA solution,and then20g/L Y?NO₃?₃ solution with a certain volume was added into CTS/FA solution and kept reaction for 30 min.And continuing to add a certain mass of Fe₃O₄ particles into mixed liquid and was dispersed by ultrasound for a certain time,the reaction mixture was dried,grinded and sieved to obtain the magnetic Y/CTS/FA composite.The characterization test shows that:compared with FA,the surface morphology of magnetic Y/CTS/FA composite changed greatly,the specific surface area and pore volume decreased,and the saturation magnetization was 6.92 emu/g.The adsorption capacities of DRB RN and DLB 5B are 16.55 and 43.64 times of FA,respectively.?5?The optimum conditions for the adsorption of DRB RN and DLB 5B dyes on the magnetic Y/CTS/FA composite are as follows:when the initial concentrations of dye solution with natural pH value?8.65 and 9.0respectively?are 5000 mg/L and 2300 mg/L,the dosages of adsorbent are0.075 g and 0.10 g respectively,the adsorption time is 60 min at 298 K,the adsorption capacities of magnetic Y/CTS/FA to DRB RN and DLB 5B can reach 1575 mg/g and 533 mg/g,respectively.?6?The results of adsorption kinetics,isothermal adsorption,thermodynamics and adsorption mechanism of DRB RN and DLB 5B dyes by magnetic Y/CTS/FA composite are as follows:the adsorption process of DRB RN and DLB 5B dyes by the adsorbent can be described by pseudo second-order adsorption kinetic model.Particle diffusion is the main speed control step in the adsorption processes.The isotherm data of DRB RN dye adsorbed by the adsorbent are all better in accordance with Freundlich model.The adsorption isotherm data of DLB 5B dye by magnetic Y/CTS/FA is better in accordance with Langmuir model.The adsorption behavior of DRB RN and DLB 5B by magnetic Y/CTS/FA composite can also be well described by D-R isotherm adsorption model.According to the analysis results of FT-IR,UV-vis and XPS patterns of magnetic Y/CTS/FA before and after dye adsorption,the adsorption includes electrostatic interaction,hydrogen bonding,ion exchange and coordination.In addition,the results of adsorption thermodynamics of the two dyes on the magnetic Y/CTS/FA composite??G<0,?H<0,?S>0?shows that the two adsorption reactions are spontaneous exothermic reactions with increasing disorder.The two kinds of adsorption materials,magnetic SA/La and magnetic Y/CTS/FA,have low cost and good adsorption effect.They can be used for the efficient removal of direct dyes in high concentration dye wastewater,and the adsorption materials have a wide range of pH value application,can be separated from water under the effect of external magnetic force,effectively avoid the occurrence of secondary pollution.This provides a feasible way for the preparation of new environmentally friendly and efficient adsorbents,and it is expected to have a broad development prospect in the field of water environment protection.