Efficiency of magnesium ions removal from water in processes of its mitigation

Significant concentrations of calcium and magnesium ions in natural waters force the majority of domestic drinking and energy waters to undergo preliminary softening. Therefore, mitigation technologies are becoming particularly acute today, and research in this field is increasing every year. Since water hardness is determined by the total content of calcium and magnesium ions, these elements are the focus of such research. 
Traditionally, it is believed that calcium ions are the first to be removed, and magnesium ions are less likely to form a solid phase. However, the effectiveness of softening technology depends equally on both cations. Therefore, sufficient attention should also be paid to magnesium ions. Sodium phosphate is considered a promising reagent in this direction. Its use in the processes of removing calcium ions allows to ensure the residual hardness of water at the level of 0.1 - 0.2 mg-eq/dm3 in a wide range of temperatures and hydrogen index. Detailed studies of the use of sodium phosphate in the processes of removing magnesium ions showed its insufficient efficiency. 
The effectiveness of soda-sodium softening allows to ensure at pH 11.0 - 11.5 the residual hardness of water at the level of 0.4 - 0.6 mg-eq/dm3. But the need to adjust the hydrogen index and the high consumption of reagents make this technology unsuitable for widespread use. The determining factor in water softening processes using phosphate ions is the ratio between the concentrations of phosphate ions and magnesium ions K = [PO43-, mg-eq] / [Mg2+, mg-eq]. Taking into account the strict requirements of current regulatory documents for the content of phosphates in treated waters, it is desirable to carry out the treatment with stoichiometric ratios of reagents for a more complete reaction between the components. The advantage of sodium phosphate as a reagent for removing magnesium ions can be considered the fact that in the pH range of 3.16 - 10.07 at K = 1, the residual hardness ranges from 1.8 to 3.4 mg-eq/dm3. At the same time, the minimum value of the residual stiffness was recorded at the level of 0.75 mg-eq/dm3 at pH 10.07 and K = 2. As the pH decreases, a stable decrease in efficiency is observed, although it is not very significant. Thus, during the transition from an alkaline to an acidic medium, the residual concentration of magnesium ions increases by a factor of 2, regardless of the value of the coefficient K. Similar trends persist in the case of a change in the initial hardness of water. The biggest difference is observed at values ​​of K ≤ 1, which is explained by the deficiency of phosphate anions and the impossibility of forming a solid phase of stoichiometric composition. However, even with a stoichiometric ratio of reactants (K = 1), the residual hardness of treated water is quite significant and ranges from 2.5 to 3 mg-eq/dm3. A further increase in the dose of sodium phosphate allows to slightly reduce the residual hardness of the treated water. Thus, the minimum residual hardness of treated water at K = 2.0 is fixed at the level of 0.7 mg-eq/dm3 with an initial hardness of 22.9 mg-eq/dm3. An increase in the residual hardness of the treated water with a decrease in its initial hardness was also noted. This situation is caused, in our opinion, by a significant increase in the mass of the solid phase and an increase in pH with an increase in the dose of sodium phosphate. 
A significant advantage of sodium phosphate as a precipitator of calcium ions is the fact that the temperature of the water practically does not affect the efficiency of the process in a wide range of temperatures. This trend is also characteristic of magnesium ions. In the temperature range of 5 - 70 °C, the softening efficiency remains stable. Moreover, the solid phase is formed immediately after draining the solutions. If we take into account the high efficiency of phosphates in removing calcium ions, then in general, the use of phosphates for softening water can be permissible, provided that the formed solid phase is completely separated from the water. Detailed studies of the efficiency of separation of magnesium phosphate particles by settling or filtering are as important as the processes of solid phase formation.