February 20, 2017

Stanford researchers improve extraction of uranium from seawater

In the recent work, Stanford researchers improved on all three variables related to extracting uranium from seawater: capacity, rate and reuse. Their key advance was to create a conductive hybrid fiber incorporating carbon and amidoxime. By sending pulses of electricity down the fiber, they altered the properties of the hybrid fiber so that more uranyl ions could be collected.

There is about ten million tons of uranium reserves on land. There is much more uranium on land than those reserves but it would be less economical to go for inferior deposits. It had already been shown that uranium could be obtained from seawater at about four times the current cost of the good land based uranium resources. There is about 3 to 4 billion tons of uranium in the worlds oceans. A fiber based extraction process would involve making the equivalent of fishing nets of uranium extracting fiber and putting them into very strong ocean currents that would move a lot of water through the fiber. The uranium in the water (about one grain per liter) would then be pulled from the ocean. The nets of uranium would fill and then get reeled into the boat or platform for processing.

Scientists have long known that uranium dissolved in seawater combines chemically with oxygen to form uranyl ions with a positive charge. Extracting these uranyl ions involves dipping plastic fibers containing a compound called amidoxime into seawater. The uranyl ions essentially stick to the amidoxime. When the strands become saturated, the plastic is chemically treated to free the uranyl, which then has to be refined for use in reactors just like ore from a mine.

How practical this approach is depends on three main variables: how much uranyl sticks to the fibers; how quickly ions can be captured; and how many times the fibers can be reused.

In recent tests by the time the standard amidoxime fiber had become saturated, Stanford’s amidoxime-carbon hybrid fibers had already adsorbed 9 times as much uranyl and were still not saturated. What’s more, the electrified fiber captured three times as much uranyl during an 11-hour test using seawater from Half Moon Bay, about an hour from Stanford, and had three times the useful lifespan of the standard amidoxime.

“We have a lot of work to do still but these are big steps toward practicality,” Cui said.

In the real sea water tests, the voltage used was 5 V. Based on the flow system result. Using a flow rate of 6 mL/min (0.1 cm/s), the extracted mass for 4 L was 1. 62 µg. The daily extracted U mass would be ~3.5 µg. This corresponds to U price of ~$1.9/g. The target price of U extraction from sea water is $0.2-0.3/g. This is not an unreachable price, considering that more optimization work can be done to decrease the cost of HW-ACE method. Further optimization can be done to increase the uptake of U by increasing the electrode surface area, designing better operation system, decreasing the operation voltage and also increase the coulombic efficiency (suppress the unwanted byproducts). If from optimization, the applied voltage can be reduced to less than 2 Volts, then the energy consumption would decrease 5 times and the price will be $0.4/g which is very close to the target price. The material cost from additional activated carbon is equivalent to only 7.6% of the electricity cost which does not play a vital role in the final cost.

The benefit of using HW-ACE method which could reduce the cost is that the extraction capacity is much larger than physicochemical adsorption. Therefore, the need for recycle is reduced which could save some cost from regeneration.

In summary, the HW-ACE showed advantages in extracting U from sea water like high capacity, faster kinetics and so on. Further effort is needed to keep optimizing the system to reduce the power consumption, increase the coulombic efficiency and increase the extraction kinetics in order to make its cost competitive.

Schematics of physicochemical and HW-ACE extraction.

HW-ACE uranium extraction performance in simulated sea water. Uranium extraction from sea water using HW-ACE method comparing to physicochemical method with initial uranium concentration of a, ~100 ppb, b, ~1.0 ppm, c, ~10 ppm, d, ~300 ppm, e, ~600 ppm and f, ~1000 ppm. Simulated sea water was made using deionized water as solvent.

Nature Energy - A half-wave rectified alternating current electrochemical method for uranium extraction from seawater

In total there is hundreds of times more uranium in sea water than on land, but extracting it for use in nuclear power generation is challenging due to its low concentration (∼3 ppb) and the high salinity background. Current approaches based on sorbent materials are limited due to their surface-based physicochemical adsorption nature. Here we use a half-wave rectified alternating current electrochemical (HW-ACE) method for uranium extraction from sea water based on an amidoxime-functionalized carbon electrode. The amidoxime functionalization enables surface specific binding to uranyl ions, while the electric field can migrate the ions to the electrode and induce electrodeposition of uranium compounds, forming charge-neutral species. Extraction is not limited by the electrode surface area, and the alternating manner of the applied voltage prevents unwanted cations from blocking the active sites and avoids water splitting. The HW-ACE method achieved a ninefold higher uranium extraction capacity (1,932 mg g−1) without saturation and fourfold faster kinetics than conventional physicochemical methods using uranium-spiked sea water.

SOURCES- Stanford, Nature Energy

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