Mechanistic Study Of Cr(Ⅵ) Removal By Sodium Alginate Derived Composite Materials

High concentrated chromium containing wastewater was widely produced from various industries.Cr(VI)is highly toxic and can easily migrated in the environment,thus the efficient removal of Cr(VI)in water is of great importance to the ecological environment.The adsorption method has the advantages of simple operation and low cost,while the photocatalytic reduction method is green and highly efficient.The synergistic application of these two methods to remove Cr(VI)in water can provide a new pathway for the treatment of Cr(VI)in industrial wastewater.Sodium alginate(SA)is a natural biomass material,which is abundant in nature and environmental friendly.It can form gel by cross-linking and curing,with good removal effect on heavy metal ions in water.Therefore,it has been used by many scientific researchers to study the removal of Cr(VI).However,the removal efficiency of metal ions by single SA gel is relatively limited due to its own physical and chemical properties.Therefore,this study first explored the adsorption characteristics of SA for Cr(VI)after physical and chemical modification.Secondly,the modified SA was physically blended with graphene oxide(GO)to explore its adsorption and photocatalytic properties for Cr(VI)under different light irradiation conditions.Finally,the modified SA was crosslinked with different crosslinking agents to explore its adsorption and photocatalytic properties for Cr(VI).The specific research results are as follows:(1)SA/PEI: SA/PEI was characterized by FT-IR,Zeta potential,SEM and XPS.A large number of amino groups presented on SA/PEI surface after chemical modification,which were protonated under acidic conditions.The protonated functional groups can electrostatically attract negatively charged Cr(VI)species in water.In addition,the carbonyl,amine and imine groups on SA/PEI can reduce Cr(VI)to Cr(III).According to the model fitting results of adsorption kinetics and isotherm,the adsorption process conformed to the pseudo-first-order kinetic model or pseudo-second-order kinetic model and Langmuir model.According to thermodynamic calculation,the adsorption reaction is a spontaneous endothermic process.Finally,the adsorption equilibrium of Cr(VI)with the concentration of 100mg/L was achieved within 12 h according to the adsorption experiment results.(2)GSPC-20: According to the calculation of UV-vis DRS,bandgap width and valence band,it is known that GSPC-20 contains C=C and C=O groups.Under UV irradiation,it led to π-π* and n-π* electron transitions,which is conducive to the generation of photoelectron that reduced Cr(VI).Since the modified SA can adsorb Cr(VI)electrostatically under acidic conditions,the removal mechanism of Cr(VI)by GSPC-20 under UV irradiation is electrostatic adsorption and photocatalytic reduction.According to the fitting of adsorption kinetics and isotherm,the adsorption process conformed to pseudo-second-order kinetics and Freundlich isothermal model.The photocatalytic kinetics confirmed the quasi-second-order kinetics,and the photocatalytic rate decreased with the increase of Cr(VI)concentration.The cyclic experiments showed that GSPC-20 had good stability,and the removal efficiencies of Cr(VI)after five adsorption and photocatalytic cycles were 67.72 and 79.96%,respectively.In addition,the removal rates of Cr(VI)with a concentration of 100mg/L under dark and ultraviolet conditions were 77.12 %(14 h)and 100 %(220 min),respectively.(3)SPC and SPF: SPC and SPF were characterized and calculated by UV-vis DRS,photocurrent transient response,EIS,band gap width and valence band.It was known that SPF was conducive to the absorption of UV light,enhanced photocurrent intensity,and inhibited the recombination of electrons and holes.According to DFT,XPS and Zeta potential analysis,the removal mechanisms of Cr(VI)by SPC and SPF under dark conditions were electrostatic adsorption and chemical reduction.The removal mechanisms of Cr(VI)by SPC and SPF under UV irradiation were electrostatic adsorption and chemical reduction and electrostatic adsorption and photocatalytic reduction,respectively.The kinetic fitting of adsorption and photocatalytic process showed that the adsorption process of SPC and SPF on Cr(VI)was more in line with the pseudo-second-order kinetics,and the photocatalytic process was more in line with the pseudo-second-order kinetics,and the photocatalytic rate decreased with the increase of Cr(VI)concentration.Finally,the removal rates of Cr(VI)in the wastewater containing Cr(VI)from an electroplating plant by SPC and SPF were 72.95 % and 79.98 %,respectively,after five photocatalytic cycles.Under UV irradiation,100 mg/L Cr(VI)could be completely removed by SPC and SPF within 120 and 45 min,respectively.