## 4 Results and Discussion
### Estimation of biogeochemical Mineral Neoprecipitation
Thermodynamic modelling of the element speciation in sampled in situ solutions allowed us to track the change in valence of a number of elements at different Eh values (Table 2). It should be added that the main calculation was performed at a fixed pH (e.g., 6.8). Microbial denitrification is known to be followed by an increase in pH [45,46]. In our experiments, no visible change in pH was observed; however, such a process cannot be completely excluded with a high content of nitrate. In the calculations, conditions with more alkaline solution values were therefore simulated to show the direction of the possible evolution of the system when the pH changed. At pH 6.8 and Eh \(-\)100 mV (Well 1 and Sample 1), the mobility of uranium can be limited by the deposition of UO\({}_{2\text{(am)}}\) (Figure 7A), but these are the boundary pH-Eh conditions. Even a small amount of oxygen access (shifting to the left along the X-axis) will lead to the failure of pyrite deposition due to the lack of sulphide (black dots) and the preservation of uranium in solution. Note that at the same critical point, the solutions are completely undersaturated towards iron hydroxides (brown dotted line) and uranium phosphate (UO\({}_{2}\))\({}_{3}\)(PO\({}_{4}\))\({}_{2}\)-2-4H\({}_{2}\)O (blue line). Natural background water is undersaturated with respect to calcite and biomineral aragonite at up to Eh \(-\)300 mV (pH 6.8), and the water will thus be vulnerable to acid drainage. Leaching of the system will result in the supersaturation of the solution with hydrocarbonate ions and calcite precipitation. In Sample 2 (Well 2), the processes of nitrate and sulphate removal were quite slow in the experiment (complete reduction on the 20 th and 60 th days, respectively). Nevertheless, we observed an Eh value of \(-\)400 mV on the 15 th day of the experiment. Thus, according to the model calculations, under biota stimulation in Sample 2, the removal of uranium from solution'n may occur primarily due to the precipitation of (UO\({}_{2}\))\({}_{3}\)(PO\({}_{4}\))\({}_{2}\)-2-4H\({}_{2}\)O phosphates and subsequently of U(IV) oxides. The SI lines of these chemical species intersect at \(0/-50\) mV. This means that on the left side of the graph (Eh \(>-50\) mV), phosphates should be precipitated from the solution, and a white precipitate should be formed, while at Eh \(<-50\) mV, a black precipitate should be formed because of pyrite deposition (as in the experiment). However, the uranium content in the white precipitate is below the detection limit;, i.e., these are common calcium phosphates (see Table 7). As in the previous case, leaching of the system will cause the solution to be supersaturated with hydrocarbonate ions and calcite precipitation. Under these conditions, goethite and hydrooethite reach equilibrium, and their dissolution ceases. A recent article on uranium phosphates demonstrated that uranyl-phosphate minerals can maintain extremely low levels of aqueous uranium in groundwater due to their low solubility [47]. Once formed, such minerals are relatively insensitive to the system redox potential and are also more resistant to dissolution than other uranium minerals under oxic conditions outside of their stability fields [48]. However, the pH range and phosphate-to-bicarbonate ratio need to be precisely adjusted to the correct values to achieve phosphate formation, and the exact conditions depend on the uranium and phosphorous starting concentrations. Future experimental work should be performed to understand the connections between solution chemistry and uranium mineral composition under our conditions (Samples 1-3). The solutions were undersaturated with respect to calcite (SI = \(-\)1), although they were in equilibrium with gypsum to Eh -200 mV, i.e., at the beginning of the intensive bacterial reduction of sulphate. As shown above (reactions 2-4), during bacterial reduction, proton uptake reactions occur, and the pH increases.