Functionalized Cellulose Nanofiber Membranes for Heavy Metals Removal from Impaired Waters

This dissertation describes the development and performance of high-productivity nanofiber membranes for removal of heavy metal ions from impaired waters. Heavy metal pollutants such as cadmium and nickel are present in wastewaters discharged from facilities in various industries. Water purification systems such as ion-exchange processes have been used widely to remove and recover specific metal ion impurities from wastewaters for ensuring that the maximum discharge limits of heavy metals are not exceeded. However, the productivity of an ion exchange resin process is limited by the high pressure drop across the resin bed, which may increase over time due to media deformation and plugging, and because slow pore diffusion of ions within the resin necessitates long residence times to ensure efficient utilization of ion exchange sites. Compared to resin-based ion-exchange media, columns filled with macroporous ion-exchange membranes offer opportunities for much higher volumetric throughput by minimizing the diffusional path length of the target ions to the functional groups. To be competitive with existing ion-exchange resin media, these membranes must have high capacities based on ligand chemistries that are selective for the target heavy metal ion(s). The first objective of this project was to develop new, nanofiber-based macroporous ion-exchange membranes with high surface area-to-volume ratios and ligands that are selective for the removal of heavy metals such as cadmium and nickel from impaired waters. An electrospinning method was used to produce regenerated cellulose based nanofiber membranes. A “grafting to†method was used to incorporate polymeric ligands with carboxyl functional groups on membrane fiber surfaces for achieving high grafting densities and being comparatively faster to “grafting from†methods. The second objective of this project was to evaluate and model the performance of the nanofiber-based ion-exchange membranes for the rapid removal and recovery of heavy metals from water. Membrane permeabilities were measured by direct-flow filtration. The maximum ion-exchange capacities were determined from uptake isotherms at constant pH. Competitive ion-exchange measurements were made to evaluate the membrane selectivities. Langmuir and competitive Langmuir isotherm models were used to describe these equilibrium measurements. Dynamic ion-exchange measurements were done to study the role of flow rate (i.e., residence time) on metal ion binding capacities. The membrane productivities were compared to commercial ion-exchange resin beads and ion-exchange membranes. Findings of the work are described briefly in the paragraphs that follow, which also provide an outline for the dissertation. Chapter 1 provides a review of background material on heavy metals in industrial wastewaters and heavy metals removal processes, and introduces and highlights advantages of a new approach using ion-exchange membrane technology to remove heavy metals from impaired waters. The specific approaches to address the unique challenges and motivating factors of each study system are given in the introduction sections of the respective dissertation chapters. Chapter 2 describes the development and performance of high-productivity cation-exchange nanofiber membranes for cadmium removal from impaired waters. Details are presented on the fabrication of regenerated cellulose nanofiber based membranes using electrospinning, as well as the use of polymer grafting strategies to coat the nanofiber membranes with poly(acrylic acid) (PAA). The membrane performance (i.e. permeability, cadmium ion-exchange capacities, and selectivity) were measured for membranes preparing using different molecular weights of PAA that were grafted from different solvents. Cadmium static ion-exchange capacities were found to be independent of PAA molecular weight for reasons that are described. Due to PAA swelling behavior, solvent quality during grafting was found to be a primary factor to achieve high cadmium ion-exchange capacities. The maximum static cadmium capacities exceeded 160 mg/g, comparable to traditional ion-exchange media. Sodium and calcium ions were used to evaluate competitive ion exchange, and the results showed that membranes were selective for cadmium over these competing ions, thus demonstrating membrane potential for commercial application. Chapter 3 addresses the development, performance evaluation and modeling of polyelectrolyte-modified nanofiber membranes for binary-component ion-exchange. The role of ligand chemistry was studied by comparing performance of nanofiber membranes coated by PAA and poly(itaconic acid) (PIA). Measures included membrane permeabilities and ion-exchange capacities for single-component and binary-component ion-exchange systems. Single-component ion-exchange isotherms were measured for cadmium, nickel, and calcium ions. The higher metal-polymer complex stability of Cd-PIA over Cd-PAA was found to be an impact factor for achieving high Cd binding capacities. The maximum capacities of PIA-modified membranes exceeded 220 mg Cd/g, comparable to traditional, low productivity ion-exchange media. Also, it was discovered that membranes were selective for Cd over Ni and Ca because of different hydration energies and ionization potentials. Competitive ion-exchange measurements of these ions were made to determine the selectivity of the membranes for cadmium ion and compared to model predictions from the competitive Langmuir model using parameters regressed from single-component isotherms. Chapter 4 presents findings on the performance and efficacy of polyelectrolyte-modified nanofiber membranes in the rapid uptake and recover of heavy metals from impaired waters. Performance measurements quantified the dynamic ion-exchange capacity for Cd, the volumetric productivity (i.e., volume water processed per unit time), and recovery of Cd from the modified membranes by regeneration by using ethylenediaminetetraacetic acid regeneration reagent. Ion-exchange capacities were constant over five cycles of binding-regeneration. A relatively lower Cd dynamic binding capacity of PIA-modified membranes than PAA-modified membranes was observed. The dynamic binding capacities of Cd on both types of nanofiber membrane were independent of the linear flow velocity, with a residence time as low as 2 s. Volumetric productivity was high for the nanofiber membranes, and reached 0.55 mg Cd/g/min, while the commercial resin exhibited productivity at 0.035 mg Cd/g/min. Chapter 5 provides closing remarks and offers suggestions for future work. In all, this dissertation demonstrates the potential and challenges associated with development of nanofiber-based ion-exchange membranes with high binding capacity, selectivity and productivity for heavy metals capture and recovery from impaired waters