Again, take note that MTCaHCO and MTCa must be in kilograms and V must be in

cubic meters. [Ca ]me„ is then in milligram equivalents per liter.

10.13.3 Concentration of Mg2+

The concentration of the magnesium ion in the treated water comes from the magnesium from the magnesium bicarbonate not precipitated, and the magnesium from the noncarbonate hardness of magnesium not precipitated.

As the treated water is recarbonated, the 16 mg/L of Mg(OH)2 as CaCO3 from the limit of technology converts to the bicarbonate form,

Equation (10.7) has been written as

T.hardi asCaCO,

Thus, the 16 mg/L of Mg(OH)2 as CaCO3 is equivalent to 16/50 = 0.32 meq/L of Mg2+ = 0.32 meq/L of HCO—.

The possibility exists that the 16 mg/L of Mg(OH)2 as CaCO3 will react in the presence of excess bicarbonates to form magnesium carbonate; however, the Ksp of magnesium carbonate is around 10—5 compared to that of the Ksp of magnesium

hydroxide, which is 9(10 ). Thus, Mg(OH)2 will stay as Mg(OH)2 until recarbonation.

Recall that MMgHCO (= f MgHœ3MTMgHco3) mass of magnesium bicarbonate from the carbonate hardness of magnesium was precipitated. Again, let all this mass be measured in kilograms. Also, let [Mg(HCO3)2]mgnot, in mg/L, be the concentration of magnesium bicarbonate not precipitated. Thus,

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