|Fig. 1: Structure of uranyl ion, left, and predicted state of uranyl in the ocean, coordinated to carbonate ions, right|
Nuclear power provides a technologically feasible, carbon free alternative to conventional fossil fuel power plants. Although there are many designs for modern nuclear fission plants, the majority require a uranium fuel source. Terrestrial uranium reserves are relatively small, and while estimates vary, are expected to be depleted within the next two centuries. [1,2] If nuclear fission is to remain a viable form of power, other sources must be developed.
The majority of the earth’s uranium is not found in easily minable terrestrial ore, but is instead dissolved in the ocean. Uranium concentration in seawater has been shown to be consistent in various locations around the world, even at depths up to 5000 feet, at 3 parts per billion (ppb). [1,2] If this concentration remains constant at deeper depths, with a total of 1.3 × 1022 liters of seawater, we can approximate a supply of 4 billion tons of uranium in the ocean, trillions of dollars of fuel. This can be compared to current estimates of only 20 million tons of uranium reserves on land.
Uranium found in seawater has the same isotopic ratio of U-235 to U-238 as terrestrial ores, meaning it will require further enrichment upon collection, but this process is well developed and should not be a barrier for use. [2,3] Due to the oxidizing environment of the earth's oceans, uranium is found in the uranium (VI) oxidation state, as a uranyl ion seen in Fig. 1. In addition, scientists predict that 97% of the uranium will be bound to counter ions such as carbonates and acetates, increasing the stability of the ion in solution and hindering possible methods of extraction, although the small concentration prevents direct detection of this state. In addition, uranium concentration is about eight orders of magnitude smaller than other major ions, such as sodium, magnesium and calcium, requiring very strong selectivity and affinity for extraction. [1-3] Finally, the amount of uranium in each liter of water provides a small amount of energy; it is only enough power to pump that liter 17 m uphill, so care must be taken to ensure the energy budget is still favorable. 
There are currently several extraction methods in development. Methods for adsorption of the uranyl ion through binding to a chemical ligand have been widely developed, tested in the ocean, and are considered the most promising of current possible methods. [2,4,5] Other ideas include the use of biological bacteria or algae to concentrate the uranium for easier extraction, and nanomembrane filtering. [1,2,6] Each of these methods has a similar process of concentrating the dilute uranium into a more easily extractable form, but require the processing of large amounts of seawater.
Using a chemical adsorbent to accumulate uranium from the seawater is a widely developed and promising method. The uranyl ion, due to its unique linear shape and additional oxygen atoms, allows for the use of specific ligands tailored towards these properties. Modern adsorbents include hydrous titanium hydroxide, amidoxime capped polymers, and synthetic organic ligands that can be attached to a stable support. [2-4] These have been selected from a wide variety of possible absorbents due to their high affinity and selectivity, their stability and ability to function in seawater conditions, and reusability, however research towards better ligands continues even today.  These adsorbents then must come into contact with a large amount of seawater, slowly accumulating and capturing uranium. After a lengthy accumulation time, the uranium can then be extracted. A sample extraction from titanium hydroxide involves soaking in sodium bicarbonate and dilute nitric acid, a relatively simple procedure.  Current benchmark standards include 30% adsorption efficiency, and greater than 90% desorption efficiency. [2,3,5]
Recently, a larger scale test of these adsorption methods was undergone by a Japanese team of researchers. They lowered a cage of 350kg of braided amidoxime polymer into the ocean for 240 days, and extracted greater than 1 kg of "yellow cake" uranium, the product needed for further enrichment. While more tests must be done, the researchers claim such a system could extract uranium at a cost of $200-300/kg uranium, two to three times the current cost at $100-150/kg.  It has been argued that fuel is a small proportion of the cost of a nuclear power plant, so this increase will not drastically affect the cost of electricity.  However, more tests are required to determine the reusability of such systems, and problems with scaling up must be overcome, including ensuring that large amounts of non-depleted uranium sources of seawater continuously come into contact with such large cages, without the use of energy for pumping.
Two other possible methods proposed both by scientists and throughout the blogosphere include the use of biological accumulation, or nanomembrane filtration. It has been shown that algae and bacteria can be bioengineered to accumulate uranium.  Specific algae could therefore be induced to bloom in a specific area of the ocean, trapping uranium. The algae could then be collected and processed to provide uranium in a more concentrated form for extraction. Nanomembranes are currently used to filter water for purification purposes. If specific sized pores could be generated for the uranyl ion, these membranes might be used to filter water and capture the uranium ions. Further work must be done on these methods before cost estimates and feasibility are believable.
The basic science behind extracting uranium from seawater has been demonstrated as feasible, but is unlikely to be developed without government intervention until the lower cost uranium ores are depleted. The engineering problem of ensuring that fresh seawater reaches the processing points, of either adsorbants, biological species, or nanomembranes, requires new innovations for a solution. While some possibilities have been proposed, including attaching the adsorbants to drifting icebergs, or building buoyant processing plants that are pushed around by waves, prototypes must be developed.  Overall, the extraction of uranium from seawater is a promising source of $700 trillion worth of uranium, which will be quickly developed for extraction once lower cost sources of uranium have been depleted.
© Craig Gorin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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