So I watched an old ep of Shark Tank and I saw this guy:
Sea water contains about 0.1-2 mg/tonne of gold dissolved in water (average 1 mg/tonne). But considering the amount of seawater available, it is a really huge goldmine! Theoretically fine, but problems were practical (which prevented profitable extraction till now). This can however become possible with the old electrolysis technique, with the only difference that the voltage difference between the electrodes must be maintained slightly less than the minimum potential difference required for electrolysis of water (yes, there is a minimum pot. difference, say 1.48 volts, below which water won't be hydrolyzed. But since gold lies below hydrogen in electrochemical series, it will get deposited on the cathode!). Since it is impractical to pump millions of gallons of water, it is more practical to move the electrodes over vast regions of oceans. This process can be made much more profitable by another simple process (which I explain later).
With a slight modification, the propellers of ships can be designed to form the electrodes! Each of the 3 blades will be a stack of 3 blades (like a sandwich) with the sandwiched blade maintained +ve and the other two forming cathode (of course they won't be touching each other. There will be a gap of a few cm between each blade, supported by rubber/cork). The tilt of each blades will be much less than conventional propeller, so that it makes much more revolution per advancement, and hence scan the volume of water more effectively. It is practical to make each blade 1.7 meter in length, so that cross sectional area of circle formed on revolution of blades will be 10 meter square. This will scan 10 tonne of water per 1 meter moved by the ship. Considering that efficiency of extraction is only 0.1 mg/tonne, it comes to 1 mg/meter of distance covered (or 1 gram per k.m. or 1 k.g gold per 1000 k.m.) So, this may not be profitable if ship is designed only for gold hunt. But it can be a real bonus for commercial ships which has to cover thousands of k.m. anyway.
This process can be made much more profitable by another simple process.
Consider this practical concept: there are 3 primary ways of separating U235 from U238. Forget the diffusion & centrifugal processes. 3rd method: You shine a laser light of exactly matching wavelength to selectively excite U235 (it is easy nowadays, since we wave Cu vapor laser & dye-lasers for fine tuning)
https://en.wikipedia.org/wiki/Atomic_vapor_laser_isotope_separation
PLAN: Just like a Sodium vapor lamp, or Copper vapor lamp/laser (
https://en.wikipedia.org/wiki/Copper_vapor_laser) it is possible to make a gold vapor lamp, which will selectively excite Au ions, thus requiring even lesser voltage (I believe it will be lesser than the critical voltage which starts breaking H2O into Hydrogen & oxygen). This process/step is critical because as the concentration of ions (Au in this case) start decreasing, the voltage required to extract starts increasing. But selective excitation should help a lot. And besides, it will help to dissociate (charge) neutral covalently bonded gold-monohydroxide.
[Let me explain and elaborate about the gold vapor light. It can be built using same technique as that of a copper vapor lamp. Actually, copper vapor laser is one of the few lasers that can be home built!
Since using pure gold vapor lamp is difficult to construct, because of the extremely high temperature, necessary to create gold vapor, therefore, gold halides, like gold chloride or gold bromide or gold iodide may be substituted, since they form vapors at much lower temperatures]
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