Using decrease in salinity to release phsophates and nitrates from substrate?

[Cody Ray]

If I have done my research correctly, lowering the salinity of water will cause the release of phosphates and nitrates from the substrate. I have 2 questions about this:

How fast will nitrate and phosphate be release into the water? Can the water reach a satruration point, where phosphate and nitrate can no longer enter into the water?

Also, once this occurs, how long will it take (once the salinity is raised back to it's original reading) for both nitrate and phosphate to stop being released into the water?

I want to use this to help rejuvinate sand that has been in a system for a long period of time (there seem to be many problems with DSB after about 4 years, which to me would indicate that the sand has reached it's limit to absorb nitrate and/or phosphate).

What I would like to do is hook up a bucket of sand to my system and allow it to absorb nitrate and phosphate. Every 2 years I would like to disconnect the bucket from the system and remove the nitrates and phosphates.

 

[Randy Holmes-Farley]

If I have done my research correctly, lowering the salinity of water will cause the release of phosphates and nitrates from the substrate.

I don't believe that to be true. Phosphate will bind to calcium carbonate (CaCO3) substrates in fresh and salt water, and nitrate does not bind to CaCO3. Reducing the pH will help desorb phosphate, but may also dissolve the substrate.

Organic detritus will not release phosphate or nitrate as a consequence of reducing salinity.