Ionic Transport Regimes for Nanoscale Transport towards the Development of Low Energy Water Desalination Membranes
- PDF / 206,782 Bytes
- 4 Pages / 595.22 x 842 pts (A4) Page_size
- 1 Downloads / 331 Views
1169-Q02-07
Ionic Transport Regimes for Nanoscale Transport towards the Development of Low Energy Water Desalination Membranes Sankha Banerjee, Daisy Fung, Shaurya Prakash *1 1 Department of Mechanical Engineering The Ohio State University, Columbus, OH 43220 * Corresponding Author, E-mail: [email protected] ABSTRACT New materials, methods, and membranes are being developed for applications in water purification. One of the model systems that can be used for fundamental studies in nanoscale transport phenomena for new membrane technologies are nanocapillary array membranes (NCAMs). Toward developing more efficient membranes for water desalination, parameters such as the concentration polarization region which are influenced by the unstirred layers, surface properties (e.g., surface charge and surface energy) of the nanocapillaries, and the electric double layer (EDL) which mediates transport across NCAMs must be better understood. In this paper, a series of parametric experiments that were conducted to better understand the relative importance of unstirred layers with respect to the transport across nanocapillaries are described. Bulk salt concentration and potential drop across the NCAMs, were varied in a systematic manner to determine the influence EDL thickness and electromigration on transport regimes for ionic permeation across NCAMs. Based on previously developed methodologies, the experiments reported here were conducted in a permeation cell with an NCAM separating two reservoirs containing potassium phosphate buffer with a concentration range from 200 µM-10 mM. Methylene blue (MB) is used as an organic marker and the transport is quantified by tracking MB concentration in each reservoir with UV/VIS spectroscopy. INTRODUCTION Recent studies have highlighted the need for developing new technologies concerning desalination of saltwater to meet the increasing demand for freshwater [1, 2]. New energy efficient nanofluidic devices and membranes are being developed for such applications [3, 7]. For developing better devices and membranes, fundamental understanding of ionic transport in nanoscale systems should be further enhanced. This paper describes the identification of transport regimes in nanoscale transport through membranes containing nanocapillaries. Two main parameters influence transport at the nanoscale, (i) the unstirred layers adjacent to the membranes, and (ii) the electric double layer inside the pores [4, 5]. Unstirred layers (USLs) are analogous to concentration and hydrodynamic boundary layers and are located at the membrane-bulk solution interface. Transport through the USLs is dominated by diffusion and can be influenced by external stirring and applied perturbations to the USLs [5]. Equation (1) describes the use of concentration gradients in setting-up a governing equation for determining the USL thickness. Cb − C 0
δ
=
∂C ∂y
(1) y =0
where δ is the thickness of the unstirred layer, Cb is the bulk concentration, C0 is the ∂C is the flux of ionic species moving inwards to concentration a
Data Loading...
