The impact of hydrophobic coating on the performance of carbon nanotube bucky-paper membranes in membrane distillation

Abstract

Self-supporting bucky-papers membranes were processed from chemical vapour deposition grown carbon nanotubes and coated with a thin layer of poly(tetra-fluoro-ethylene) (PTFE) to enhance their hydrophobicity and improve their mechanical stability over time, without drastically changing their average pore size and porosity. Contact angles and bubble point pressure were respectively increased by 23% and 28%. After coating the membranes were tested in a direct contact membrane distillation setup at various feed temperatures with 35 g/L NaCl feed solution and coated samples exhibited improved lifespan and water vapour permeability at salt rejection higher than 99%. The bucky-paper membranes were also compared with commercial PTFE membranes of similar properties.

Introduction

The engineering of carbon nanotube (CNT) based membranes has been investigated over the past 10 years as a mean to overcome global water stress and shortage [1], [2], [3]. Several approaches have been undertaken to benefit from the unique CNT properties and fabricate membranes exhibiting enhanced permeability and selectivity [4], [5], [6], [7] compared to existing membranes used in reverse osmosis [8], [9], [10], nanofiltration [11], ultrafiltration [12], pervaporation [13], [14], [15] or capacitive deionisation [16]. Selective ion transport across aligned CNT composites [17] was demonstrated and enhanced flux across pristine CNT composite membrane [13], [18] were encouraging steps to further pursue their use in separation. An alternative low energy route to desalinate water is Direct Contact Membrane Distillation (DCMD) where a membrane is used as a separation barrier between two streams of water at different temperature. The hot saline solution contacts the membrane on one side and water vapour, generated from the vapour pressure gradient across the air gap is transported across the membrane before being condensed on the cold side.

Suitable membranes for DCMD [19] need to be porous, hydrophobic and thin to facilitate the water vapour permeation while preventing direct bridges across the membrane air-gap [20]. Bucky-papers (BP) [21], [22], non-woven of entangled CNTs, exhibit high porosity and hydrophobicity [23] and previous work showed that CNT composite Bucky-papers (BP) could be successfully engineered and used as membranes for water desalination by direct contact membrane distillation [24], [25].

In this paper CNT BPs were coated with poly(tetra-fluoro-ethylene) by sputtering. Extensive properties characterisation, such as contact angle, pore size distribution and porosity, were performed on the membranes which were tested in a DCMD rig for desalination of saline solution. The composite membranes were compared with non coated BP and with commercial PTFE membranes.