Investigation Of Phosphate And Phosphite Adsorption By A Polyethersulfone-type Affinity Membrane

In this research, reagents including polyethersulfone(PES), chloroacetyl chloride,and diethylenetriamine were employed to fabricate a modified PES-type affinity membrane for the removal of phosphate and phosphite from the aqueous solution. The fabricated affinity membrane was characterized by the techniques of scanning electron microscopy(SEM), energy dispersive X-ray spectrometry(EDS), Fourier transform infrared spectroscopy(FTIR), X-ray photoelectron spectrum(XPS), and nuclear magnetic resonance spectroscopy(NMR).Influences of pH, contact time, temperature and initial phosphate concentration, as well as the coexistent Ni(II), Ca(II), Mg(II), Cl–, and SO42– on the phosphate and phosphite adsorption were evaluated. The adsorption kinetics and the adsorption isotherms of this membrane toward phosphate and phosphite were investigated. In addition, the insight into the phosphate and phosphite uptake at an atomic-scale was revealed by the density functional theory(DFT) calculations. The results indicated that the presence of Ca(II) and Mg(II) increased the uptake of phosphate, and the accelerating role of Ca(II)was stronger than that of Mg(II); conversely, the coexistent Cl– and SO42– showed a disturbance on the uptake of phosphate, and followed the order: SO42- > Cl-. Lagergren second-order and Langmuir models were suitable for describing the adsorption kinetics and adsorption isotherms, and the phosphate adsorption onto the membrane is a spontaneous and endothermic process. In addition, the research suggested that the coexistence of Ni(II) and Ca(II) showed a negative effect on the adsorption of phosphite onto the affinity membrane following the trend: Ca(II) > Ni(II); the presence of Cl– and SO42– disturbed the membrane toward the uptake of phosphite following the order of Cl- <SO42, Lagergren second-order model is competent for describing the adsorption kinetic of the membrane, and the adsorption isotherms can be well described by Langmuir model.The reused property of the modified PES-type affinity membrane is favourable, and this will be the advance significance for expanding its application.In addition, in this research, DFT calculations were applied to evaluate the mechanisms of phosphate and phosphite adsorption by the DETA-PES affinity membraneand influences of the coexistent Cl–, SO42–, Ca(II) and Mg(II). The results indicated that the affinity of the protonated DETA-PES membrane to phosphate and phosphite is stronger than that to Cl– and SO42–, due to the maximum charge transfer between phosphate or phosphite and the(DETA-PES-Hn)+ chain, and the more negative adsorption energy and Gibbs free energy of adsorption for the formed(DETA-PES-Hn)+-H2PO4–(H2PO3–) complex; the coexistent Ca(II) and Mg(II) can be coordinated by the affinity membrane, because of the large amount of charge transfer, the noticeably negative adsorption energy and Gibbs free energy of adsorption for the[M(II)(DETA-PES-Hn)]3+-H2PO4–(H2PO3–) complexes.