## 4 Results and Discussion
### Estimation of biogeochemical Mineral Neoprecipitation
On the right side of Figure 7A,B, at an alkaline pH of 8.5, the supersaturation of the solution towards biogenic carbonates is seen. The influence of uranyl speciation and iron oxides on uranium biogeochemical redox reactions was discussed previously [49]. The presence of calcium in carbonate-bearing solutions promotes the formation of ternary complexes, Ca2 UO2(CO3)3 and CaUO2(CO3)32, which have a profound impact on U biogeochemical chemistry by decreasing both microbial and abiotic reduction rates. This is clear evidence; moreover, note that at pH 8.5 and Eh -400 mV, there is a peak in the formation of goethite or haematite corresponding to a sharp extension of their stability fields in the Eh-pH plots (Table 8). According to the cited authors, \"goethite and haematite act as sorbents of Ca and, as a result, decrease the proportion of less-reduced Ca2 UO2(CO3)3 species, thus increasing the rate of microbial reduction of uranium compared to that in systems with no ferrihydrite\". The calculation of SIs showed oversaturation of the solution of Sample 2 with respect to ningyoite-phosphate U(IV) at all Eh values (Figure 7B). Ningyoite is a rare mineral; however, U(IV)-phosphate minerals may play an important role in U retention in mining-contaminated wetlands and as possible products of microbial U(VI) reduction [50]. Figure 7: _Cont_. In Sample 3, from the well with maximum pollution, the composition of mineral associations during the Eh decrease changed considerably compared to the results in Figure 7A,B. First, in the experiment, even on the 180 th day, an Eh of only -200 mV was achieved. Second, ningyoite CaU(PO4)2-1-2H2O dissolved, which corresponded to a high and permanent supersaturation of the pure solutions. Instead of ningyoite, U(VI) phosphates  
Figure 7: Thermodynamic model calculations of the saturation indexes (SIs) of water from Sample 1 (**A**), Sample 2 (**B**), and Sample 3 (**C**) after biostimulation with whey. Points indicated with \({}^{*}\) and \({}^{**}\) correspond to the values obtained without fixed pH in conditions of organic matter oxidation to CO2. appeared--saleeite Mg(UO\({}_{2}\))\({}_{2}\)(PO\({}_{4}\))\({}_{2}\):10H\({}_{2}\)O or (UO\({}_{2}\))\({}_{3}\)(PO\({}_{4}\))\({}_{2}\):4H\({}_{2}\)O. These solutions are generally in close equilibrium with UO\({}_{2(\mathrm{am})}\) until strong alkaline conditions, which cause a rapid increase in the stability of ion-uranyl carbonate complexes. Although Sample 3 had 30 times more calcium than Sample 2, the solutions were also undersaturated with respect to gypsum and calcite (SI = -1) until the end of the experiment (in the calculations, pH 6.8 and Eh -200 mV). We can assume that calcium in solid precipitates is predominantly in the form of whitlockite and hydroxylapatite phosphates. The point in Figure 7C is very interesting (pH 6.8, Eh -100 mV). Computations showed that here, the solutions became saturated with pyrite and remaining sulphate. At pH > 6.8 and Eh -200 mV, the solutions were highly supersaturated towards pyrite, and gypsum dissolved (only sulphide was present in the solution). In the experiment, this reaction took place on the 180 th day. It is well known that biological processes depend on the redox of electron donor and acceptor pairs [51; 52]. Obviously, with a shift in Eh from oxidizing conditions to anoxic conditions, the reduction of nitrate occurs first (as in our experiment), followed by Mn and then the Fe(OH)\({}_{3}\)/Fe\({}^{2+}\) pair (reduction of Fe(III)). Sulphur reduction should occur after iron reduction, and uranium reduction is highly dependent on the presence of uranium carbonate complexes. In low-CO\({}_{2}\) media (black arrow), the UO\({}_{2}\)\({}^{2+}\)/U\({}^{4+}\) equilibrium lies in the zone of iron reduction, and there is a dramatic shift in the UO\({}_{2}\)(CO\({}_{3}\))\({}^{4-}\)/U\({}^{4+}\) equilibrium in the direction of decreasing Eh to a region close to that of methanogenesis.