Desalination facilities are almost all now using Reverse Osmosis to get drinkable water from seawater. High pressure forces water molecules through the R/O membranes which results in the water not passing through becoming much saltier. This reject water is about half the seawater entering the facility. ..It gets sent back into the sea.
Were the sodium removed from it, the reject water would become suitable for irrigation, which would nearly double the output of useful water from a reverse osmosis facility. Such facilities are expensive, so doubling the useful amount of water from one would improve its financial aspect. Note that this may be an add-on to an existing R/O facility.
Enter Potassium Bicarbonate
The key to this is the low solubility of sodium bicarbonate, which will drop out when sufficient potassium bicarbonate is added, leaving a solution of potassium chloride (KCl). It is likely to be about 12% by weight, which is too high and needs to be reduced. ..One way is letting plants take the potassium ions out of the water and store them in their leaves. A preliminary step may be to mix in water already low in potassium ions and so lower their concentration in the water passing by the plants.
A good candidate for such removal is water hyacinth (Eichornia crassipes), which floats and grows rapidly. However, it is generally thought a weed difficult to control, so caution might dictate some other plant be selected. Whichever plant it is, flowing water slowly past them in a lengthy canal should result in the exit water being low in potassium and thus well-suited for irrigation use.
Upon being lifted from the canal, these plants may be sun dried and then compacted into pellets or wafers, to be marketed as an organic fertiliser. They would contain plenty of nutrients, in particular nitrogen and potassium. ..But it is the leaves which mostly do, and the dried leaves may be separated from the rest — by pin milling and air sorting, as example — to become the fertiliser, while the rest is otherwise made use of.
Making Potassium Bicarbonate
From ordinary potash fertiliser (KCl) can come the potassium bicarbonate needed. [1].. The first step is mixing the dry KCl, more likely a slurry of it, with a strong solution of sodium acetate, and then using an alcohol (ethanol works) to extract potassium acetate leaving NaCl behind. [2].. Next: gas the alcoholic extract with CO2 to drop out potassium carbonate, which is not soluble in alcohol. Neither is potassium bicarbonate, and when water is present also, that is what gets dropt out.
About half the potassium reacts this way, while the rest gets an acetic acid molecule somewhat firmly attached. This rather surprising result was disclosed in an old British patent [521 202]. There, heat was used to drive off the acid itself. Here, instead, contact with sodium bicarbonate will provide the sodium acetate wanted in the first step.