Researchers from Pacific Northwest National Laboratory have found that reducing graphene oxide (GO) membranes with ultraviolet (UV) light alters the oxygen functional groups on the GO surface. This modification results in a different, chromatography-like separation mechanism that is selective for charge rather than size.
Image credit: Chemical Engineering Journal
Developing efficient, selective, and scalable separations for critical materials, including lithium and magnesium, is essential to meeting the increasing demands for clean energy technologies and alleviating challenges with domestic supply chains. GO membranes have shown promise for separating ions from mixed solutions based on size.
While previous work used GO membranes for size-based separations, UV light reduction expands the potential uses of GO membranes by altering the separation mechanism. This modification approach allows researchers to tune the functionality of GO membranes via a straightforward method that uses no harsh or specialty chemicals.
Larger, doubly charged cations, such as calcium, move through the membranes faster than smaller, singly charged cations such as lithium. The smaller lithium cations permeate more slowly through the UV-rGO membranes than larger cations, such as calcium and magnesium, resulting in a 3- to 4-fold improvement in the separation selectivity between these representative cations.
UV exposure selectively removed hydroxyl (–OH) groups from the GO basal planes, leading to enhanced interactions of metal cations with functional groups located at the edges of GO. This resulted in a lower ratio of free mobile lithium in solution compared to calcium cations.
A separation mechanism analogous to chromatography is proposed for UV-rGO, emphasizing the crucial role of different oxygen groups on GO in controlling selective ion transport through GO membranes.