Abstract
Membranes for membrane distillation (MD) are mostly made of polymeric and ceramic materials. We demonstrate here that the laterally-compressed, vertically-aligned CNTs (VACNT) obtainable from a CNT forest are an excellent membrane material for vacuum membrane distillation (VMD). The VACNT structure provides interstices between CNTs for extracting vaporized water molecules, while efficiently filtering the impurity salts. The VACNT membrane is shown to deliver excellent performance when tested for the desalination of 3.5 wt% NaCl water solution, as exemplified by the permeability of 68 LMH (liter per square meter per hour) achieved at the salt rejection of over 99.8% at 65 °C. We also demonstrate that the VACNT membrane performance can be maintained with time with the aid of a simple cleaning procedure, which bodes well for a long lifetime of the membrane for VMD application.
Highlights
Carbon nanotubes (CNTs) have been a workhorse for many applications in diverse elds including mechanics, electronics, photonics, and others.[1,2,3] carbon nanotubes (CNTs) have been utilized for water treatments
The vertical CNT forest was grown on a Fe-coated Si wafer by the water-assisted CVD12 in which the vapor removes amorphous carbon to help extend the catalyst life, thereby enabling the ‘super-growth’ of CNTs to the millimeter scale ($1.2 mm in average in this work; Fig. 1a)
Connected to the vacuum outlet through the cold trap, the membrane assembly is dipped into the 3.5 wt% NaCl solution ( ‘feed water’ herea er), which is heated to a controlled temperature and circulated. This heated feed water can readily penetrate through the vertically-aligned CNT (VACNT) membrane, which is facilitated by good wetting of the pores between CNTs, as discussed in detail later
Summary
Carbon nanotubes (CNTs) have been a workhorse for many applications in diverse elds including mechanics, electronics, photonics, and others.[1,2,3] CNTs have been utilized for water treatments. The as-grown VACNT structure can be mechanically compressed[10] for the VMD membrane.
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