Fig 1.
Schematic view of crossflow electroultrafiltration with an electrically conductive, cathodic membrane.
As can be seen, unlike electroultrafiltration processes with electrodes on each side of the membrane, the external electric field terminates with the poly(vinyl-alcohol)-carbon nanotube layer, which is upstream of the semipermeable, polymeric membrane.
Table 1.
Physicochemical properties of model proteins used in ultrafiltration experiments.
Fig 2.
Experimental crossflow EUF system flow diagram.
Solid lines represent solution tubing; dashed lines represent electrical wiring.
Fig 3.
Carbon nanotube-polymer thin film deposition and electric potential significantly affect the steady state permeate flux and rates of permeate flux decline.
Normalized permeate flux during UF/EUF of single protein and binary protein solutions at TMP of 1 psi with different cathodic potentials (vs. Ag/AgCl) applied to the PS-35 and PVA-CNT membranes: A) single protein solution of 0.1 g/L HEL (n = 3), B) single protein solution of 0.1 g/L αLA (PS-35, n = 3; PVA-CNT, n = 2), and C) binary protein solution containing 0.1 g/L HEL and 0.1 g/L αLA (PS-35, n = 3; PVA-CNT, n = 2). Error bars represent the standard error of the weighted mean. Electrostatic interactions between the proteins and the carboxylated multiwalled CNTs significantly affect the normalized steady state permeate flux during single protein UF and the rate of flux decline during binary protein UF. The abrupt changes in permeate flux during single protein EUF suggest the application of an external electric potential influences the extent of protein adsorption on the PVA-CNT layer.
Fig 4.
Temporary enhancement in protein sieving with an applied potential is seen during electroultrafiltration of single protein solutions.
Observed sieving coefficient and mass flux during UF/EUF of single protein solutions at TMP of 1 psi at different cathodic potentials (vs. Ag/AgCl) applied to the PS-35 and PVA-CNT membranes: A) and C) single protein solution of 0.1 g/L HEL (n = 3), B) and D) single protein solution of 0.1 g/L αLA (PS-35, n = 3; PVA-CNT, n = 2). Error bars represent the standard error of the weighted mean. Application of an external electric potential results in temporary improvement in sieving and mass flux during both single protein of HEL and αLA.
Fig 5.
Temporary enhancement in selectivity is seen during initial stage of binary protein electroultrafiltration.
Observed sieving coefficient and mass flux during UF/EUF of binary protein solutions at TMP of 1 psi at different cathodic potentials (vs. Ag/AgCl) applied to the PS-35 and PVA-CNT membranes (PS-35, n = 3; PVA-CNT, n = 2): A) and C) Comparison of membranes with (PVA-CNT) and without (PS-35; control) the deposited conductive thin film. B) and D) Comparison of cathodic potentials (vs. Ag/AgCl). Deposition of the PVA-CNT layer on the PS-35 membrane results in temporary separation of binary protein solutions containing species of differing net charges with further enhancement in selectivity upon application of an external electric potential.
Table 2.
Zeta potential measurements of ultrafiltration membranes after protein ultrafiltration and electroultrafiltration at pH 7.4 and 4 mM ionic strength with the membrane functioning as the cathode.
Fig 6.
PVA-CNT layer makes a significant contribution to the global zeta potential due to fouling at the thin film surface.
Schematic of streaming potential setup for measurement of the global zeta potential across the membrane pore walls of the (1) PS-35 support layer, (2) PS-35 skin layer, and (3) PVA-CNT layer (not to scale) under three mechanisms of fouling and concentration polarization above the surface of the PVA-CNT layer: A) minimal fouling and negligible additional hydraulic resistance due to concentration polarization, B) significant concentration polarization, C) extensive fouling. Under A), the PS-35 support layer dominates the contribution to the global zeta potential relative to the other layers due to the significant pressure drop across the thick support layer. Under B), while proteins transported through the membrane give rise to filtrate with a specific permeate concentration (Cp), proteins retained by the PVA-CNT layer result in an elevated wall concentration (Cw) relative to the bulk concentration (Cb). Concentration polarization across the boundary layer (δ) results in significant contributions to the global zeta potential due to the additional hydraulic resistance. Conversely, under C), the PVA-CNT layer fouled with adsorbed protein plays a significant role in the contribution to the global zeta potential due to the reduced effective porosity and pore size and increased layer thickness.
Fig 7.
Protein adsorption and aggregation develop above the PVA-CNT membrane surfaces following crossflow EUF.
Fouling above the PVA-CNT membrane surfaces following protein crossflow EUF (0.1 g/L of each protein component; 555 s-1 crossflow shear rate; 1 psi TMP; 9.33 h duration; and 0 V and -4.6 V potentials (vs. Ag/AgCl) applied to the PVA-CNT membrane). Images were obtained by SEM. Application of an external electric potential results in increased fouling for single protein HEL EUF and decreased fouling for single protein αLA EUF due to electrostatic interactions between the charged protein species and the negatively charged PVA-CNT layer. Application of an electric potential during binary protein EUF of αLA and HEL results in increased fouling due to a combination of protein-protein and protein-PVA-CNT layer electrostatic interactions. The observed multilayer protein adsorption leads to pore narrowing and clogging.