Adsorption Investigation Of MA-DTPA/PVDF Chelating Membrane Towards Nickel Ions

With the joint use of the solution blending and phase inversion techniques, themelamine-diethylenetriaminepentaacetic acid/polyvinylidene fluoride （MA-DTPA/PVDF）chelating membrane bearing polyaminecarboxylate groups was prepared for the removalof Ni（Ⅱ） in different forms from the wastewater.The membrane was characterized by SEM, NMR and FTIR techniques. The effectsof the coexistent cations such as Ca（Ⅱ）, NH₄⁺, Fe（ⅡI） and the organic acids includinglactic acid, succinic acid, citric acid on the Ni（Ⅱ） uptake of the membrane were studied.Quantitative adsorption experiments were performed for elucidating the adsorptioncharacteristics regarding the presence of coexistent cations and the organic acids. Theinfluence of Fe（Ⅱ）, Co（Ⅱ）, Cu（Ⅱ） and Zn（Ⅱ） in the ternary composite-plating effluents wasalso taken into account. Furthermore, membrane filtration experiment was also conducted.FTIR and NMR results suggest the polyaminecarboxylate groups have been blended to thechelating membrane successfully and the chelation dominates in the Ni（Ⅱ） adsorption. Thechelating membrane exhibits significant adsorption towards the hydrated Ni（Ⅱ） with theuptake above 0.030mg/cm². The coexistent cations interfere with the Ni（Ⅱ） adsorption inthe order of Ca（Ⅱ）>NH₄⁺>Fe（Ⅲ）. The complexing capability of the organic acid is in theorder of lactic acid <succinic acid <citric acid, which is responsible for the decrease of theNi（Ⅱ） uptake. The batch adsorption results show that the sorption kinetics fit well toLagergren second-order equation and the isotherms can be well described by Langmuirmodel. D-R plots indicate the ion exchange mechanism of the adsorption process. Thethermodynamic parametersΔG0<0、ΔH0<0、ΔS0>0, demonstrate the spontaneous andexothermic nature of adsorption. In the ternary composite-plating wastewater, theinfluence of the coexistent cations on the Ni（Ⅱ） adsorption is in the order ofZn（Ⅱ）<Co（Ⅱ）<Fe（Ⅱ）<Cu（Ⅱ）, reflecting the affinity that the chelating membrane shows forthese cations. The adsorption/desorption experiment indicates that the chelating membranepossesses excellent property of reuse.The reaction mechanism between melamine and DTPA in the organic solvents wasexplained based on the ab initio theory from the molecular level. Density functional theory （DFT） was adopted to explore the adsorption characteristics of the chelating membranetowards Ni（Ⅱ）. The coordination structures of MA-DTPA ligand with Ni（Ⅱ） wereoptimized and the stable configurations were obtained by analyzing the optimizedgeometrical parameters, the complexing energy and charge population. The geometryoptimizations were performed for Ni（Ⅱ）-organic acid complexes and metal-MA-DTPAcomplexes（metal=Ca（Ⅱ）, Fe（Ⅲ）, Fe（Ⅱ）, Co（Ⅱ）, Cu（Ⅱ）, Zn（Ⅱ））. The complexationenergies, Gibbs free energies and charge population have been calculated to determine thecomplexing stability. The stability sequence of the coexistent cations with MA-DTPAligand is Ca（Ⅱ）>NH₄⁺>Fe（Ⅲ）. The chelating group MA-DTPA shows affinity for the 3dtransition metals in the following order: Zn（Ⅱ）<Co（Ⅱ）<Fe（Ⅱ）<Cu（Ⅱ）. The complexingability of organic acids is in the sequence of lactic acid <succinic acid <citric acid. Thestability of Ni（Ⅱ）-organic acids can not be comparable to that of Ni（Ⅱ）-MA-DTPA, whichis consistent with the experimental results.The degradation processes of PVDF membranes in alkaline solutions were studiedusing FTIR, FT-Raman, XPS and density functional theory （DFT） molecular simulations.The degradation mechanism of PVDF membrane in alkaline solutions is proposed on thebasis of comparing the activation energy of the degradation paths. The results indicate thepresence of C=C bonds and carbon of PVDF skeleton exists in the form of CH₂, CF₂, C—OH and C=O. DFT calculation indicates the defluorination process occurs on the treatedPVDF membrane, which is followed by the incorporation of the oxygen-contained groups,such as the hydroxyl and carbonyl groups.