The worst of the precipitants are the ferric salts. Compare Ksp,Fe( OH)3 . = 1.1(10-3D) and Kjp,FepO = 10-219. These two Ksp's produce comparable concentrations of the ferric ion to precipitate either Fe(OH)3 or FePO4. Thus, at high pH conditions, the phosphate ion would have a big competitor in the form of the hydroxide ion. An Fe3+ available in solution is grabbed by the OH- ion to form the ferric hydroxide, leaving less amount of Fe3+ to precipitate ferric phosphate. The ferric salts are, therefore, a poor performer for removing phosphorus. Also, to be effective requires adjusting the pH to the pH of almost mineral acidity of less than 3.
Now, compare Kjp,Al(OH)3 = 10 and Kp aipo4 = 10 for the case of using alum. This situation is similar to the case of the ferric salts. The concentrations of the aluminum ion to precipitate either the aluminum hydroxide or the aluminum phosphate are comparable. This means that the hydroxyl ion is also a competitor in the phosphate precipitation. The process operates poorly at high pH, although it has a better pH range of equal to or less than 5 compared to that of the ferric salts.
In the case of using lime to precipitate phosphorus, we know that calcium hydroxide has a much, much larger Ksp than that of the apatite, the required precipitate. The former has a Ksp of Ksp Ca(OH) = 7.9(10-6). Compare this to that of apatite which is Kspapalite = 10-55 9. Thus, adding lime to the water means that a large amount of the calcium ion is available to precipitate the apatite, Ca5(PO4)3OH(s). Also, no other precipitate competes with this precipitation. Therefore, using lime is the best method for removing phosphorus. Adding a quantity of lime to the point of saturation, Ca(OH)2 has the potential of removing phosphorus "100%." This process must, however, be investigated. In the overall, the process using lime entails adjusting the pH to greater than 7, which is much, much better than the range for the use of either alum or ferric salts.
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