Application of electrospun fiber membranes in water purification

Abstract

Electrospun membranes are attractive for the liquid filtration applications especially as microfiltration membranes because they are low in solidity and have open, highly interconnected porous structures. Nevertheless, liquid filtration processes are pressure driven; hence, it is crucial to understand the compressive behaviors of electrospun membranes. Compressive properties of electrospun fiber mats are reported for the first time in this thesis. Membranes of bisphenol-A polysulfone (PSU) and of poly(trimethyl hexamethylene terephthalamide) (PA6(3)T) were electrospun and annealed at a range of temperatures spanning the glass transition temperature of each polymer. The data for applied stress versus solidity of membrane were found to be welldescribed by a power law of the form [sigma]zz =kE([phi]n - [phi]no) where [sigma]-zz is the applied stress and [phi] is solidity, in accord with the analysis of Toll (Polym. Eng Sci., 2004). The values of n range from 3.2 to 6 for PSU and from 8.0 to 20 for PA 6(3)T. The lowest values in each case were exhibited by mats annealed near the glass transition temperature of the fiber material. The higher values of n are attributed to fiber slippage via a mechanism analogous to that of work hardening of metals. The values of kE can vary by an order of magnitude and were difficult to determine precisely, due to the nature of the power law and the inhomogeneity of the mats. The hydraulic permeabilities of electrospun fiber membranes are found to be functions of their compressibilities. Hydraulic permeabilities of electrospun PSU membranes experience a decrease of more than 60% in permeability between 5 and 140 kPa, due to the increase in solidity, attributed to flow-induced compression. This behavior is explained using a simple model based on Darcy's law applied to a compressible, porous medium. Happel's equation is used to model the permeability of the fiber membranes, and Toll's equation is used to model their compressibilities. The permeation model accurately estimates the changes in solidity, and hence the permeability of the electrospun membranes, over a range of pressure differentials. The permeability of commercial phase inversion membrane was higher than those of electrospun membranes at pressures greater than 8 kPa. Microfiltration of emulsions of oil (dodecane) in water using electrospun PA6(3)T membranes was demonstrated. Rejection of the emulsified dodecane decreased from (85 +/- 5) % to (4.3 +/- 0.9) % when the ratio of droplet diameter to fiber diameter (dp/df) decreased from 2.5 +/- 0.4 to 0.57 +/- 0.04, respectively. The normalized flux (relative to the pure water flux) decreased in proportion to the increase in emulsified oil concentration, and decreased with the increase in the total solidity of the membranes. The resistances from the oil were in series with the resistances of the membranes tested. The resistivity of the foulant increased with an increase in the concentration of oil. Foulant deposition models showed that the oil droplets formed a coating that enveloped the fibers. The normalized flux of electrospun membranes was approximately three times higher than that of commercial phase inversion membrane of comparable bubble point diameter, while exhibiting a similar rejection.