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 means that the salty water not passing through becomes more salty. 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 many nutrients, in particular nitrogen and potassium. ..But it is mainly in the leaves, which dried 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 crystal KCl, more likely a slurry of it, with a strong solution of sodium acetate. Adding an alcohol (ethanol works) partitions this, with alcoholic potassium acetate above a largely NaCl solution.
[2].. Next: gas the alcoholic extract with CO2 to drop out potassium carbonate, which is not soluble in alcohol. Nor is potassium bicarbonate, and when water is present, 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 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.