Functionalized Cellulose Nanofiber Membranes for Heavy Metals Removal from Impaired Water

In this project, high-performance, nanofiber-based ion-exchange membranes were prepared for removal of heavy metals from impaired waters. Our hypothesis was that such membranes would have high metal ion capacities resulting from large surface area-to-volume ratios and high water permeability relative to barrier-type membranes such as reverse osmosis and nanofiltration. The nanofiber membranes were produced by electrospinning cellulose acetate nanofiber mats, which were strengthened by thermal-mechanical annealing and then converted to regenerated cellulose nanofiber mats. Subsequent surface modification transformed the membranes into ion-exchange membranes for removal of trace metal ions in water.

Cellulose acetate was selected as the starting material because it has been used widely to electrospin fibers due to simple processing. The electrospinning formulation and conditions yielded fiber mats having a fiber diameter in submicron range and a thickness range of 160 to 330 μm. Following annealing, the nanofiber mats were converted to regenerated cellulose, which is a low-fouling material that can be modified using convenient surface chemistry to introduce the ion-exchange ligands.

Ion exchange using carboxylate anion ligands is a potential strategy to capture cadmium and nickel ions found in industrial wastewaters. Thus, the regenerated cellulose nanofiber mats were modified by grafting poly(itaconic acid) (dicarboxylic acid side groups) and poly(acrylic acid) (monocarboxylic acid side groups) to the surfaces of individual nanofibers that had been modified with poly(glycidyl methacrylate).The success of poly(itaconic acid) and poly(acrylic acid) grafting was confirmed by attenuated total reflectance Fourier-transform infrared spectroscopy.

Direct-flow filtration measurements showed that the nanofiber membranes have water permeabilities of 400-1600 L/m²/bar/h, much higher than reverse osmosis or nanofiltration membranes. The single-component ion-exchange isotherms were measured at constant pH for cadmium, nickel, and calcium ions. Competitive sorption measurements were made using environmentally relevant concentrations of these ions to determine the selectivity of the membranes for cadmium ions. Experimental isotherms for Cd-Ca and Cd-Ni were compared to model predictions. The multi-component isotherms for Cd-Ca and Cd-Ni systems were modeled using the competitive Langmuir model with Langmuir parameters obtained by fitting single-component isotherms.

Results show that the polymer-modified nanofiber membranes have outstanding permeability and high selectivity for heavy metals over a common competing anion present at much higher concentration. The nanofiber membrane adsorbers bind cadmium ions with static capacities that are similar to and, in some cases, superior to commercial ion-exchange resins and activated carbon.