Study On The Effect Of Coexisting Contaminants On The Adsorption Of Cd²⁺ By An Ion-imprinted Thiolfunctionalized Polymer

The effective treatment of heavy metals which are typical environmental contaminants has long been a challenge concerned by environmental sciences and other sectors of the society.With the development of social production and living,an increasing number of inorganic and organic pollutants are discharged into the environment,making environmental pollution gradually presents a trend of combined pollution.With the interactions between a varieties of pollutants,the environmental chemical behavior and ecological effects of pollutants under combined pollution system become more and more complex.However,the influence mechanism of combined pollution on the adsorption of heavy metals lacks systematical and profound study.Based on this,this research chose typical heavy metal Cd²⁺ as the target pollutant and investigated the effect of coexisting contaminants such as macromolecular organic compounds,heavy metal ions and low molecular weight organic acids on the adsorption of Cd²⁺ by an ion-imprinted thiol-functionalized polymer synthesized by surface imprinting technique.The adsorption mechanism between Cd²⁺,coexisting contaminants and the adsorbents was analysized.From this research,we hope to provide scientific guidance for the adsorption removal of heavy metal ions under combined pollution.?1?An ion-imprinted thiol-functionalized polymer?IIP-MPTS/SiO2?was synthesized by surface imprinting technique using silica gel as the matrix,Cd²⁺ as the template,3-mercaptopropyltrimethoxysilane?MPTS?as the functional monomer,and epichlorohydrin as the cross-linking agent,and was characterized by methods such as SEM,FT-IR,and XPS.The results showed that the surface of IIP-MPTS/SiO2 was fluffier and more rough compared with the activated silica gel.A porous polymer layer was formed and the mercapto group was successfully immobilized on the surface of ativated silica gel after the ionic imprinting.?2?The UV-spectra of aqueous solutions containing Cd²⁺ and HA showed an enhanced aggregation of HA and an increased number of Cd-conjugated HA molecules with the increased Cd²⁺ concentration,which resulted in the red shift of the absorption peaks.With the increased ionic strength,the absorption bands of UV-spectrum showed the red shift at the increased absorption band intensity.The results of adsorption experiments showed that with the pH increasing from 2.0 to 8.0,the adsorption capacity of IIP-MPTS/SiO2 in respect to single HA and Cd²⁺ decreased and increased,respectively.A synergetic effect of HA on the adsorption capacity was observed,although the adsorption rate in presence of HA decreased.The Cd²⁺ adsorption in both absence and presence of HA showed its kinetics and isotherms fitting well to the pseudo-second order and Langmuir models,respectively.Thermodynamic studies indicated that the adsorption process was spontaneous and endothermic within the experimental temperature range.The spontaneity of adsorption increased and the sorption barrier decreased when HA was present.Combined with FT-IR and XPS analysis,the mechanism of Cd²⁺ sorption promoted with HA was suggested having HA as a ligand bridge simultaneously forming complexes with surface sites of the IIP-MPTS/SiO2 and Cd²⁺,thus binding Cd²⁺ indirectly to the adsorbent and enhancing the adsorption.?3?Coexisting heavy metal ions showed different levels of competitive effect with Cd²⁺ on IIP-MPTS/SiO2.In single and binary solution systems,the competitive adsorption power of coexisting heavy metal ions for Cd²⁺ was in the order of Cu²⁺ > Zn²⁺ > Ni²⁺ while the order was Zn²⁺ > Cu²⁺ > Ni²⁺ in ternary and quaternary systems.In all the multi-component systems,IIPMPTS/SiO2 sorbents showed strong adsorption selectivity for Cd²⁺.In binary systems,the selectivity coefficients of IIP-MPTS/SiO2 sorbents for Cd²⁺ in respect to systems of Cd²⁺/Cu²⁺,Cd²⁺/Ni²⁺,and Cd²⁺/Zn²⁺ were 3.739,5.734 and 4.153,respectively.In multi-component systems,coexisting heavy metal ions had little influence on the adsorption of Cd²⁺,which was mainly attributed to the high selectivity of IIP-MPTS/SiO2 for Cd²⁺ based on the coordinationgeometry.?4?The adsorption isotherms of citric acid,tartaric acid and oxalic acid on IIP-MPTS/SiO2 fitted well to Freundlich isotherm model compared to Langmuir isotherm model,indicating that the adsorption was multilayer and heterogeneous.Under the same equilibrium concentration,the affinity of IIP-MPTS/SiO2 on oxalic acid was the highest,the second was tartaric acid,and citric acid was the lowest.Citric acid and tartaric acid with low concentrations would slightly promote the adsorption of Cd²⁺ on IIP-MPTS/SiO2 and the promotion concentration range and degree with citric acid was greater than tartaric acid.When the concentration of organic acids were higher than certain values,both citric acid and tartaric acid would inhibit the absorption of Cd²⁺ and the inhibiting effect strengthened with increased concentrations of the organic acids.Coexisting oxalic acid showed inhibiting effect on the absorption of Cd²⁺ in all the experimental concentration range.In a word,the inhibitory effect of oxalic acid on Cd²⁺ adsorption was the highest,the second was tartaric acid,and citric acid was the lowest.Combined with the structure properties of these three low molecular weight organic acids and their cumulative complexing stability constants with Cd²⁺,the promotion and inhibition effect of the three organic acids for Cd²⁺ was affected synthetically by the bridge complexation effect,electrostatic effect as well as competitive effect among the organic acids,Cd²⁺ and IIP-MPTS/SiO2 in the ternary compound system.