Removing nitrates from water using freezing

This small study was prompted by a visitor to my website. He asked whether it would be possible to reduce the nitrate concentration in water by turning part of the water into ice. I promised him I'd find out, and promises must be kept.

The test method is very simple: into a one-litre rectangular clear plastic container, I poured about 800 mL of tap water with a known nitrate concentration (119 mg/L). I placed the container in the freezer compartment and kept it there until roughly a third of the water had turned to ice. I then took the container out of the freezer and separated the ice from the unfrozen water. After waiting for the ice to turn back into liquid at room temperature, I again measured the nitrate concentration in it, using a reasonably precise spectrometric method. Besides the nitrate concentration, I also measured the specific electrical conductivity (SEC) of both water samples. As a reminder, SEC is proportional to the concentration of dissolved salts in water. The results of this small study are given in the table.

Water quality indicator, its units	Initial (unfrozen) water	Water obtained from melted ice	Decrease, %
Specific electrical conductivity (SEC), μS/cm	570	112	80,3
Nitrate concentration, mg/L	119	22,5	81,1
Salt concentration, mg/L*	285	56	80,3

*This is an approximate salt concentration. It was obtained by multiplying SEC by 0,5.

What did this small study show? It showed that this method can significantly reduce the concentration of various dissolved salts (including nitrates) in any water. An equally important question is: how cost-effective is this method? Unfortunately, I don't have an answer to that.

For those interested in meltwater, I recommend reading the article on my website titled “Miraculous waters (silver, living and dead, meltwater, and others)”.