Synthesis Of Magnetic Iron And Iron/Yttrium-crosslinked Alginate Polymer Gel Sphere Composites And Their Adsorption For Azo Dyes

Adsorption technology has been one of the most efficient convenient and commonly used methods for the treatment of a large number of dye wastewater produced by various industrial processes,however,the research and development of cost-effective and easily separated green adsorption materials are the key to better application of adsorption methods.In the study,two kinds of magnetic iron and iron/yttrium-crosslinked alginate polymer gel sphere composites(Fe₃O₄@SA-Fe and Fe₃O₄@Fe-SA-Y)were synthesized using sodium alginate(SA)as the matrix owing to its non-toxic,degradable and abundant natural marine resources.The Fe₃O₄@SA-Fe and Fe₃O₄@Fe-SA-Y were characterized by a series of technical means,such as SEM-EDS?XRD?BET-BJH?FTIR?VSM and UV-Vis.The preparation conditions and adsorption conditions of two gel composites were optimized by using congo red(CR),direct red 23(DR 23),direct red 13(DR 13)and direct black 19(DB 19),and the adsorptionkinetics,isothermaladsorption,adsorption thermodynamics and adsorption mechanism were investigated,respectively.The results are as follows:(1)The optimized preparation conditions of Fe₃O₄@SA-Fe polymer gel spheres are obtained.The Fe₃O₄solution of 7 g/L mass concentration is added to SA solution of 16 g/L with stirring evenly at298K,and the mixed solution is dropped into Fe Cl₃crosslinking agent solution with mass concentration of 12.5 g/L.After reacting 4h,Fe₃O₄@SA-Fe polymer gel spheres are obtained by washing and drying.The optimized preparation conditions of Fe₃O₄@Fe-SA-Y gel composites are obtained.0.1 g Fe₃O₄is added to 16 g/L of SA solution at 298K.The uniform mixture is dropped into Fe(?)-Y(?)ion 30 g/L mixed solution with mass ratio of m _Fe:m _Y=2:1 and curing reaction is 3h.The target Fe₃O₄@Fe-SA-Y gel ball composite is obtained by washing and drying.(2)Two gel composites were characterized by a series of technical means,such as SEM-EDS?XRD?BET?FTIR?UV-Vis and VSM.The results show that the powder SA is mainly composed of different bulk and flake particles.The Fe₃O₄@SA-Fe and Fe₃O₄@Fe-SA-Y two gel composites are synthesized respectively by using SA.Their morphology is fundamentally changed,and the surface of the gel particles shows cauliflower-like appearance,consisting entirely of folds and grooves.At the same time,the two gel spheres have good magnetic response performance.After adsorption of pollutants,they can be quickly aggregated and separated from the water body under the action of external magnetic field,which effectively prevents the secondary pollution to the environment.The optimized adsorption conditions for Fe₃O₄@SA-Fe polymer gel spheres for CR and DR 23 azo dyes are as follows:the initial concentrations of CR and DR 23 dye solutions with natural p H values(8.0 and 9.0,respectively)are 3800 mg/L and 1400 mg/L respectively,and adsorption time are 30 min.The adsorption capacities of Fe₃O₄@SA-Fe polymers to CR and DR 23 at room temperature can reach 3017 mg/g and 1305 mg/g respectively and the corresponding removal rates are 95.3%and 93.2%.Fe₃O₄@SA-Fe exhibits super-high adsorption performance.The optimized conditions of Fe₃O₄@Fe-SA-Y adsorbing DR 13and DB 19 are as follows:the initial concentrations and natural p H values of DR 13 and DB 19 dye solutions are 3000 mg/L,3600 mg/L,8.0 and 9.0 respectively,and the adsorption time are 60 min at 298K.The adsorption capacities and removal rates of Fe₃O₄@Fe-SA-Y for DR 13 and DB 19 can reach 2415 mg/g,2807 mg/g,96.6%and 93.6%respectively,indicating that Fe₃O₄@Fe-SA-Y shows super strong adsorption removal efficiency.(3)The adsorption kinetics,isothermal adsorption,adsorption thermodynamics and adsorption mechanism for CR and DR 23 azo dyes onto Fe₃O₄@SA-Fe were studied.The results show that the adsorption processes at different temperatures are in accordance with the pseudo-second-order adsorption kinetic equation.The adsorption rates are extremely fast and mainly controlled by intraparticle diffusion.The isothermal adsorption data are better in line with the Langmuir model and the D-R model.The values of average adsorption energy are in the range of 19.3?27.7 k J/mol and 22.7?24.1 k J/mol,indicating the existence of chemisorption and hydrogen bonding between adsorbent and dye molecules.The results of thermodynamic parameters(?G<0??H<0 and?S>0)of the two azo dyes adsorption on the Fe₃O₄@SA-Fe indicate that the adsorption reactions are all spontaneous reactions with exothermic nature and reduced order.The results of adsorption kinetics,isothermal adsorption,adsorption thermodynamics and adsorption mechanism of Fe₃O₄@Fe-SA-Y for DR 13 and DB 19 show that the adsorption processes at different temperatures can be accurately described by a pseudo-second-order kinetic model,and the adsorption rates are mainly controlled by intraparticle diffusion.The isothermal adsorption behavior of DR 13and DB 19 on the adsorbent can be described by Langmuir model and better conform with the D-R model.The values of average adsorption energy in the range of 20.6?18.9 k J/mol and 17.4?14.4 k J/mol,indicating the existence of chemisorption and hydrogen bonding between the composite and dye molecules.The calculation results of thermodynamic parameters(?G<0??H<0 and?S>0)of Fe₃O₄@Fe-SA-Y for two azo dyes indicate that both adsorptionreactions are spontaneous exothermic reactions with increased confusion.(4)The results of FTIR?UV-Vis and XPS characterization for Fe₃O₄@SA-Fe and Fe₃O₄@Fe-SA-Y gel composites before and after adsorption of azo dyes show that the adsorption between the gel composites and dye molecules is mainly accomplished by electrostatic interaction,hydrogen bonding and surface complexation.The Fe₃O₄@SA-Fe and Fe₃O₄@Fe-SA-Y gel composites prepared in this study have low cost,simple operation and good mechanical properties.In addition,two gel composites have the faster adsorption rates,wide application range of p H values and ultra-high adsorption capacities for four azo dyes from high-concentration wastewater respectively,and the biosorbents after adsorption of azo dyes can be quickly separated from the aqueous phase without secondary pollution.This would provides a feasible way to research and development biosecurity and cost-effective adsorbents.The obtained biosorbents will have good potential applications in wastewater treatment.