Phosphate Absorption Rates in Aragonite

[Dan_P]

How long did you allow it to adsorb? Was it stirring? How did you do the desorption? Just filter off the fluid and replace with fresh salt water? I usually waited overnight. Don't know if that was needed or not.

The adsorption was extremely fast with 2 ppm PO4 and was undetectable by two hours. Because I worked at small scale, I did the adsorption study with a series of adding 2 ppm Instant Ocean, decanting the spent solution, and adding fresh medium, etc. The last add gave a 2 hour adsorption value 0.56 ppm and a 4 hour value of 0.14 ppm. So, not quite at equilibrium, making the 3 mg/g a close estimate. Twenty fours for the last measurement would have been better.

Mixing was important for achieving rapid adsorption. I used an orbital shaker to swirl the Instant Ocean. Too much swirling though moved the particles and caused rust colored water. Even though the water was clear and colorless at 140 rpm, all testing was performed on 0.45 micron filtered samples.

Desorption was studied by decanting the medium, wicking out excess water with a small piece of paper. This was done to avoid breaking up the solid. The small amount of residual water introduced a tiny error. The lack of desorption may not be time related. I saw a desorption curve for GFO and at the low equilibrium PO4 concentration that I was working at, very little PO4 desorbed. Just a case of not having the analytical sensitivity to detect desorption. I would expect to detect it at higher PO4 equilibrium concentrations.

 

[Randy Holmes-Farley Verified Community Expert]

The adsorption was extremely fast with 2 ppm PO4 and was undetectable by two hours. Because I worked at small scale, I did the adsorption study with a series of adding 2 ppm Instant Ocean, decanting the spent solution, and adding fresh medium, etc. The last add gave a 2 hour adsorption value 0.56 ppm and a 4 hour value of 0.14 ppm. So, not quite at equilibrium, making the 3 mg/g a close estimate. Twenty fours for the last measurement would have been better.

Mixing was important for achieving rapid adsorption. I used an orbital shaker to swirl the Instant Ocean. Too much swirling though moved the particles and caused rust colored water. Even though the water was clear and colorless at 140 rpm, all testing was performed on 0.45 micron filtered samples.

Desorption was studied by decanting the medium, wicking out excess water with a small piece of paper. This was done to avoid breaking up the solid. The small amount of residual water introduced a tiny error. The lack of desorption may not be time related. I saw a desorption curve for GFO and at the low equilibrium PO4 concentration that I was working at, very little PO4 desorbed. Just a case of not having the analytical sensitivity to detect desorption. I would expect to detect it at higher PO4 equilibrium concentrations.

Thanks, Dan. :)