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Yes, it is possible to extract gold from seawater in theory, but it's really not practical to do so. It is estimated that there is, in total, about eight million tons of gold dissolved in the world’s oceans. While this is a lot, the oceans are huge, and the actual concentration of gold in seawater is only about 0.0000000006%. To put it another way, there is between 0.1 and 2.0 mg per ton, depending on location, making it uneconomical to extract using any current technology.
The Sea as a Source of Minerals
Since the sea is fed by rivers that flow over land containing metals, minerals and ores, a great variety of these can be found in seawater in the form of soluble compounds. This is added to by material from hydrothermal vents — fractures in geologically active areas of the ocean floor through which hot, mineral-rich water flows into the ocean. The concentration of different elements in the oceans depends not only on their abundance in the Earth’s rocks, but in their reactivity and solubility.
By far the most common metals in seawater are sodium, magnesium, calcium and potassium, in that order. These are all very common, in combined form, in the Earth’s crust, and they can form a variety of compounds that are very soluble in water. Gold, aside from being a rare element, is also very unreactive, and does not easily form compounds that can find their way into the ocean by dissolving in water. For these reasons, the concentration of this metal in seawater is extremely low.
Gold was first detected in seawater in 1872 by the British chemist S. Sonstadt, and ever since, a number of people have promoted the idea of extracting the precious metal from the oceans. One of the most notable was the German chemist Fritz Haber, co-inventor of the Haber-Bosch process, who spent a portion of his career attempting to devise a practical method of obtaining the metal from seawater to pay for Germany’s post-World War I debt. When it became clear just how low the concentration of gold actually was, he abandoned his efforts. Despite a number of hoaxes and scams, serious interest in the subject continues, as reserves of gold on the Earth’s surface diminish.
Due to the extreme dilution of gold in seawater, there is currently no economically viable method of obtaining the metal from this source. Vast amounts of seawater would have to be evaporated to concentrate it sufficiently to allow it to be recovered by any conventional process. This in itself would require a lot of energy, with more power and raw materials being consumed in the actual extraction process. The cost would amount to far more than the value of the gold obtained.
The only elements that are currently produced commercially from seawater are the non-metals chlorine and bromine. In the latter case, most is obtained from more concentrated non-marine brine deposits, but some is produced in Israel from the very salty waters of the Dead Sea. The concentrations of these elements in seawater are, however, enormously greater than that of gold.
In the past, iodine has been obtained indirectly from the sea by processing seaweed, which concentrates the element. A similar form of biological extraction could conceivably form the basis of a future means of gold production. Although seaweed does not concentrate the metal, some other organisms, such as certain types of bacteria, may do so. It is therefore just possible that organisms of some sort, either natural or genetically engineered, could be used to recover gold from the sea, but this is highly speculative and may never become viable.