Lanthanum phosphate, lanthanum carbonate, etc

[Aqua Lab Rat]

I've been reading through old posts on here about removing phosphate with lanthanum, and there are lots of posts, and lots of answering by Matt. But I'm a little unclear about some things:

Does the lanthanum react simultaneously with carbonate and phosphate, or does the it react with carbonate and then react again to form lanthanum phosphate? I'd read somewhere that lanthanum carbonate is unstable. I'd assume it is simultaneous but faster with carbonate, sort of a "rabbit and the hare" race.

How much does the carbonate alkalinity drop from adding the lanthanum?

I assume that both combinations cause cloudy water by coming out of solution.

I want to make sure I understand because I'm trying to write a short presentation about phosphates in pools and don't want to misinform anyone.

 

[JoyfulNoise]

The reaction pathways are simultaneous - lanthanum will react with both carbonate anions and phosphate anions. I have not looked at the detailed reaction equilibrium constants to see how the concentrations vary. But, lanthanum carbonate is not stable and, in the presence of excess phosphate, it will slowly convert from carbonate to phosphate. This is why you ideally want to wait 24-48 hours AFTER adding the lanthanum chloride before you backwash or clean the filter - you want to make sure the lanthanum has had time to fully react. It also takes time for the cloudiness to clear as the precipitate formed is fairly small in size.

Remember, you are dosing to remove a few thousand ppb per 10,000 gallons typically so any temporary reduction in carbonate is going to be minuscule compared to the overall carbonate alkalinity.

 

[Aqua Lab Rat]

OK, thank you Matt. I'm glad TFP has knowledgeable people like yourself to help the rest of us along. I've learned a lot on this forum.

BTW, it has been an unusually cool Spring here in Minnesota. Even had overnight frost on the grass Monday night. But highs in the 80s forecasted for next week.

 

[JoyfulNoise]

BTW, it has been an unusually cool Spring here in Minnesota. Even had overnight frost on the grass Monday night. But highs in the 80s forecasted for next week.

Two words -

Polar Vortex