*3.1. pH Changes of the Medium*

The pH of the solutions in the crystallization process of the phosphate phases always decreased in experiments without organic additives and with the addition of nutrient media and bacteria, it either increased or decreased (Table 2). As can be seen from Table 2, the nutrient media and bacteria affected the pH values, which can be explained both by the influence of crystallization processes and bacterial activity. For instance, interaction of the solution with MHB media slightly reduced the pH value of the solution in the case of the minimum concentrations of inorganic components and in the case of maximum concentrations the pH of the solution increased. Addition of *Pseudomonas aeruginosa* bacteria to the MHB medium slightly increased the pH value of the solution (by 0.4), while addition of the same bacteria to the MPB medium increased the pH value of the solution by much more (by 0.6).


**Table 2.** The change in pH of the solutions during the experiment in nutrient media with the addition of bacteria.

*3.2. Model Solutions with Minimum Concentrations of Additional Ions Characteristic of a Healthy Person's Urine Composition*

In syntheses of phosphates with minimum concentrations of inorganic impurities without additives, formation of the following crystalline phases was observed: Brushite (Ca(HРО4)·2Н2О), octacalcium phosphate (Ca8(HPO4)2(PO4)4·5H2O), and whitlockite (Ca9Mg(HPO4)(PO4)6) [27,28]. Brushite formed in synthetic experiments when the initial pH of the solution ranged from 6.46 to 6.86. Octacalcium phosphate was usually observed together with brushite (less often with whitlockite) in the pH range of 6.46 to 6.95. Whitlockite was obtained in the pH range of 6.95 to 7.54.

Addition of MHB medium to the model solution changed the phase composition of the sediment (Figure 3). In the pH range 6.75–7.3, the brushite phase was detected. Brushite also formed after addition of various bacteria to the solution. Moreover, the whitlockite phase was detected in the syntheses that were carried out in the presence of "kl" at a pH of 7.15. In addition, in the experiments with "e" and "ps" bacteria at pH 7.05–7.15, formation of struvite was identified (together with brushite).

Addition of the MPB medium to the model solution also led to changes in the phase composition of the sediment (Figure 3). In this case, brushite was detected at pH ~7.06. Brushite did not crystallize at such a high pH in the experiments without additives. Another difference in the phase composition of the precipitate was the formation of struvite at a pH of 7.26, which is absent in the products of syntheses without additives. The brushite phase was detected in the sediments of all syntheses, which were carried out in the presence of bacteria. Whitlockite was formed only in the synthesis in which the *E. coli* bacteria were present at a pH of 7.07. Struvite was formed in the syntheses with bacteria, except those experiments with the addition of *Staphylococcus aureus*, at a pH of 7.0 or higher. In all the syntheses with bacteria, the formation of apatite was observed at a pH of 6.72 or higher.

**Figure 3.** Phase composition of synthesized products from model solutions with minimum concentrations of additional ions characteristic of a healthy person's urine composition. Legend: ♦—brushite, +—struvite, Δ—whitlockite, •—apatite, -—octacalcium phosphate, ×—no precipitation; *Escherichia coli* —«e», *Klebsiella pneumoniae*—«kl», *Pseudomonas aeruginosa*—«ps», and *Staphylococcus aureus*—«s».
