Mystical Material Might Help Solve Global Energy Problems.

Ateam led by Guangshan Zhu of Northeast Normal University in China has cleverly developed a porous material that can extract uranyl species from the sea.[1] This type of work is becoming exceedingly important as there is an overwhelming amount of data that make the existence of climate change undeniable. For instance, since 1998, we have experienced 10 of the hottest years on record.[2] In addition to heat-waves,[3] Arctic glaciers are melting[4] and oceans are rising,[5] and we are experiencing extreme weather patterns globally.[2] The potentially dire consequences that currently face humanity necessitate our conversion from fossil fuels toward sustainable, cleaner energy sources.[2] Although science aims to develop a diverse portfolio of prospective energy sources, nuclear power is an important avenue to explore; however, for nuclear energy production to become feasible long-term and on larger scales, a sustainable supply of uranium is essential.

Uranium-235 is a naturally occurring fissile isotope that is used in nuclear power production. Unfortunately, current uranium reserves on land, estimated to be ∼4.85 million tons, will likely be exhausted in the coming decades.[5] This limited, diminishing supply is driving efforts to develop new technologies able to harvest uranyl species (UO22+) directly from the sea. While it is estimated that the sea contains approximately 4.5 billion tons of this commodity,[6] an amount able to help supply the world with energy, uranyl extraction is extremely challenging. This difficulty stems from low concentrations that are less than 3 μg per liter. To put this in perspective, over 300 000 L of seawater would need to be filtered to extract a single gram of uranyl. Further, we must also consider that the targeted species are found among large quantities of other inorganic species, such as vanadyl,[7] which complicate things further.

To mitigate the selectivity issue, Zhu et al. have designed a porous aromatic framework referred to as MISS-PAF-1, which is constructed from predefined organic building units.[1] (See Figure .) Such porous organic and hybrid frameworks are an emergent topic in separation science that is popularized due to the easy chemical modification of the organic struts. Using simple synthetic techniques, various chemical functionality can be readily appended to the internal pore surface that is tailored for a specific use.[8] This designability, in combination with high internal surface areas, promotes a wide range of applications such as gas and liquid separations, small-molecule storage, and catalysis.

Figure 1

The surface of PAF-1 is decorated with complexing moieties able to selectively extract uranyl species from the sea.

The unique aspect of the work published in this issue of ACS Central Science is how the authors engineer selective moieties onto the internal pore surface of PAF-1.[1] The traditional approach used by others is to first chemically graft uranyl complexing moieties inside the pores. While the authors demonstrate that this method lends to materials that can take up large quantities of uranyl species, they lack selectivity in the presence of common inorganic interferents such as Li+, Na+, K+, Ca2+, Mg2+, and VO3–. The lack of selectivity is due to a random distribution of the grafted functional groups throughout the framework, a shortfall that will certainly compromise a material’s efficiency in uranyl extraction from seawater. In an effort to overcome the limitation, the authors cleverly developed a molecular coordination template strategy. In other words, they encoded very specific, directional interactions for uranyl ions inside the porous framework. To do this, they first mix uranyl ions with salicylaldoxime molecules to create a uranyl coordination complex having a predetermined configuration of oxime fragments. Next, the complex was grafted onto the internal surface of the PAF-1 template, and subsequently the uranyl species were released via treatment with sodium bicarbonate. The resulting modified material offers suitable anchors able to achieve a desired local bonding geometry that is specific to the uranyl ion. Using a number of characterization techniques the authors determine that the high selectivity is indeed due to the tailor-made binding configuration, which includes two salicylaldoxime molecules per uranyl species.

The resulting material, MISS-PAF-1, has a BET surface area of 412 m2/g and offers unprecedented selectivity for uranyl over the aforementioned common inorganic interferents. The selectivity factors, which are greater than 100, are far superior to materials prepared using the traditional approach. In fact, MISS-PAF-1 can extract 99.97% of uranyl from a 5 ppm solution, reducing concentrations below 1.6 ppb in less than 120 min. It should be noted that 1.6 ppb is below the aforementioned concentration of uranyl in seawater, which provoked the authors to think about extraction directly from the sea. The material also exhibits a maximum adsorption capacity of 253 mg of uranyl per gram of adsorbent, and the calculated distribution coefficient is 1.4 × 107 mL/g, also illustrating an extremely high affinity toward the targeted species. In addition to these already strong points, MISS-PAF-1 has been regenerated 10 times with minimal to no loss in extraction capacity. Last and most impressively, the authors soaked the material in the sea for 56 days. Afterward, it was found to contain 5.79 mg of uranyl per gram of adsorbent. This value is already 4 times higher than other reported materials tested under similar conditions.[9]

The scientific advancement made by Zhu et al. is related to the precision with which the authors are able to design a material for a targeted species. As such, this work provokes the question: what will futuristic mining activities look like? One can think of many other high-value commodities that are likewise found in inconceivable places like the sewage or the sea. Should capacities and extraction rates be boosted, work of this kind could aid in the development of a number of new, energetically efficient, and environmentally mindful extraction processes.