Polymeric ligand exchange: A new approach toward enhanced separation of environmental contaminants

Separation of various industrially and environmentally important trace contaminants from water and wastewater represents a critical environmental issue as well as a technical challenge. Currently, physico-chemical processes such as coagulation, precipitation, and adsorption methods are employed to comply with the increasingly stringent regulations. However, conventional processes necessitate expensive chemical additions and produce large amounts of sludge, which needs to be further disposed. It is also particularly difficult to separate the target trace species using conventional sorbents due to the strong competition from other co-present species such as sulfate, chloride, nitrate, bicarbonate and dissolved organic matter.; This study reports a new approach, the polymeric ligand exchange process, toward enhancing the removal of various trace contaminants that possess salient ligand characteristics. A new class of sorbents, referred to as Polymeric Ligand Exchangers (PLE) was developed by immobilizing copper(II) ions onto specialty chelating resins, which possess extremely high affinity toward the copper(II) ions. The immobilized copper(II) ions function as new anion exchange sites. Experimental results validate that the tailored functional groups can sorb anionic ligands highly selectively due to concurrent electrostatic interactions and Lewis acid-base interactions. Compared to currently used commercial ion exchange resins, the phosphate-sulfate separation factors of the PLEs are more than two orders of magnitude higher and the removal capacities of the PLEs toward phosphate, chromate, and oxalate are more than 6 to 12 times higher. The PLEs are amenable to efficient regeneration using 6% NaCl at a pH of 4.5. They perform well under real wastewater conditions and can be reused for hundreds of cycles of fixed-bed column runs without significant capacity drop or physical deterioration.; Experimental evidence also indicates that the PLEs conform to standard ion exchange stoichiometry and that the sorption isotherm can be described by the Langmuir isotherm. Due to the high charge density of copper(II) ions, the pH on the PLEs' sorption sites is found to be higher than that in the bulk solution phase. A method based on the Donnan potential model was proposed to quantify this pH difference.; Kinetically, the intra-particle diffusion was identified as the rate-limiting step for the PLEs. The effective intra-particle diffusivity with respect to the target species is severely retarded by the high affinity of the PLEs. Resin swelling tests support the critical role of free water molecules in the polymer phase in affecting the intra-particle diffusivity.; The phosphate breakthrough curves conform to the constant pattern profile and can be predicted analytically using the linear driving force model.; Experimental data verified the existence of the linear free energy relationship between PLEs' separation factors and the solution copper-ligand stability constants.