| Microfiltration membranes (0.1–0.5 μm pore size) have been functionalized with polyamino acids for the purpose of metal sorption. Understanding how the metal is sorbed was the focus of this dissertation. Various observations through experimentation were combined with a first principles model describing the electrostatic field in the membrane to gain insight into the fundamental behavior of the sorbents. These membranes represent a departure from more traditional ion exchange materials where capacity is a strong function of the internal surface area. Rather than placement of functional groups on the surface, the functional groups are located in the membrane pore on polymer chains attached to the internal surface. Metal sorption then takes place by interaction with the immobilized polyamino acid chains by ion exchange, chelation, and counterion condensation, all in convective flow. Helix-forming ability adds selectivity in the case of carboxylated polyamino acids, while studies with thiol-containing ligands, such as polycysteine, made metal sorption at very high background ionic strength possible. The mass transfer advantages of this arrangement are apparent, as transport processes are by convection rather than diffusion.; Metal sorption occurs by three mechanisms, namely ion exchange, chelation, and counterion condensation. The latter plays an important role in these materials since the immobilized charged chains produce a strong electrostatic field that, when integrated with respect to the pore radius, can be utilized to calculate an equilibrium metal concentration profile in the membrane pore. When this model was combined with a material balance on the metal sorbed membrane, reasonable values for the single adjustable parameter in the model were obtained. The model predicted the potential well (formed in the membrane by the attached polyamino acids) that was responsible for the high metal sorption capacities in excess of typical capacities for traditional ion-exchange resins. Further understanding of the sorption mechanisms was derived from charge balances on the functionalized membranes. Finally, the polyamino acid functionalized microfiltration membranes were examined for applicability to various environmental and hybrid processes. |
