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Precipitation of inorganic (metals) in a water treatment system?

Please help and explain to me clearly......a wastewater contains various cations such as Na, K, Mg, Ca, Ba, Fe, Cu, Zn, and Al; the pH of the wastewater is approx. 6.0

To that wastewater is added H3PO4 (phosphoric acid) in an approximate dose of .00034 gals of H3PO4 per gallon of wastewater.

I'm less concerned about the stoichiometry and believe I can figure that out, but I did notice in the lab that the greatest mass and volume of precipitate was aluminum, then zinc etc.

Help me understand specifically why the dissociated phosphate ion would prefer to bond with cations like Al and Zn. I think I know why but help me better understand.

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Why do certain phosphate salts precipitate more preferentially than others?

First of all, we would not expect phosphate to be helpful in precipitating sodium ions or potassium ions? How come? Sodium phosphate and potassium phosphate (as are practically all sodium or potassium salts) are soluble in water. Therefore, they won't precipitate with the addition of phosphate.

But, what about the rest? All of the other salts (magnesium phosphate, calcium phosphate, etc.) are called "slightly soluble salts." Sometimes, you will hear them called "slightly soluble electrolytes." Same thing. When we deal with such compounds it is important to recall the Ksp, the solubility product constant. The Ksp ...

Solution Summary

Please help and explain to me clearly......a wastewater contains various cations such as Na, K, Mg, Ca, Ba, Fe, Cu, Zn, and Al; the pH of the wastewater is approx. 6.0

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