researchers at the university of california riverside (ucr) bourns college of engineering have developed a carbon nanotube-based heating element that can be sued to recover nearly 100 percent of the water from highly concentrated salt solutions and could alleviate water shortages in arid regions.

hot brines used in traditional membrane distillation systems are highly corrosive, making
the heat exchangers and other system elements expensive, and limiting water recovery.
(university of california riverside)

according to a report from the university, previous attempts at desalination have included reverse osmosis, which is not capable of working in high concentration solutions, and hydraulic fracturing.

while those processes are problematic for several reasons, ucr researchers concentrated on membrane distillation, which is a thermal desalination technology that uses heat to drive vapor across a membrane.

the article explained, “however, hot brines are highly corrosive, making the heat exchangers and other system elements expensive in traditional membrane distillation systems. furthermore, because the process relies on the heat capacity of water, single pass recoveries are quite low (less than 10 percent), leading to complicated heat management requirements.”

to improve efficiency, ucr researchers created a self-heating, carbon nanotube-based membrane that restricts the heating of the brine to the membrane’s surface, which reduces the amount of heat needed and increases the water yield to nearly 100 percent.

“the team also investigated how the application of alternating currents to the membrane heating element could prevent degradation of the carbon nanotubes in the saline environment,” the article added.

“specifically, a threshold frequency was identified where electrochemical oxidation of the nanotubes was prevented, allowing the nanotube films to be operated for significant lengths of time with no reduction in performance. the insights provided by this work will allow carbon nanotube-based heating elements to be used in other applications where electrochemical stability of the nanotubes is a concern.”

the research was recently published in nature nanotechnology. the abstract stated:

“water shortages and brine waste management are increasing challenges for coastal and inland regions, with high-salinity brines presenting a particularly challenging problem. these high-salinity waters require the use of thermally driven treatment processes, such as membrane distillation, which suffer from high complexity and cost.

“here, we demonstrate how controlling the frequency of an applied alternating current at high potentials (20?v_pp) to a porous thin-film carbon nanotube (cnt)/polymer composite joule heating element can prevent cnt degradation in ionizable environments such as high-salinity brines.

“by operating at sufficiently high frequencies, these porous thin-films can be directly immersed in highly ionizable environments and used as flow-through heating elements.

“we demonstrate that porous cnt/polymer composites can be used as self-heating membranes to directly heat high-salinity brines at the water/vapour interface of the membrane distillation element, achieving high single-pass recoveries that approach 100%, far exceeding standard membrane distillation recovery limits.”