Located in southeastern Senegal, 700 kilometers from Dakar, Sabodala lies within the Mako greenstone belt, which forms the western part of the Birimian gold province. From an administrative standpoint, the study area is located in the municipality of Sabodala, which is part of the district of Sabodala, the department of Saraya, and the region of Kédougou [23] ( Figure 1 ).

Tailings from the Sabodala mine consist mainly of easily erodible, metal-rich, unconsolidated fine materials of various colorations stored in the tailings pond. Tailings samples are collected from the tailings pond with a pre-cleaned shovel to a depth of between 0 and 20 cm. At each point, a 0.5 to 1 kg soil sample was collected. A total of ten (10) samples were taken. Collected tailings are placed in Ziploc bags and stored at 4˚C until analysis, with a specific code. For each sampling point, the geographical coordinates were determined using GPS. The sampling map shows the spatial distribution of the samples ( Figure 2 ).

Aerobic bacterial populations were assessed indirectly in mine tailings. This aerobic microbial activity was assessed by a classical qualitative method, using the protocol presented in Zagury et al. (1997) [20] and modified according to Ouangrawa (2007) [21] . In this test, iron- or sulfur-oxidizing bacteria were monitored

by pH measurements, and the evolution of ORP over a given period. Qualitative assessment in shake flasks consists of a biotic test and an abiotic or control test. In each test, 2.25 g of dry mine tailings are mixed in an Erlenmeyer flask containing 150 ml of demineralized water and a substrate appropriate for the target population ( Table 1 ). The initial pH of each sample was adjusted to an optimum value using 1M H₂SO₄ or 1M NaOH, depending on the bacterial population. After incubation on a rotary shaker, electrochemical parameters (pH and oxidation-reduction potential (ORP)) were monitored in parallel with a control. The control is autoclaved at 121˚C for 25 minutes and by adding sodium azide. The control allowed us to distinguish between chemical and biological processes. Incubation on a rotary shaker (250 rpm) was carried out for 24 days at 25˚C. pH and ORP values were measured every 2 - 3 days until stabilization, for approximately one month as described by [24] . The experimental protocol is described in Figures 3-6 .

The Shapiro-Wilk test is used to assess the normality of the data distribution, while the Bartlett test is used to check the homogeneity of variances between groups. When the data meet the conditions of normality (p ≥ 0.05 for the Shapiro-Wilk test) and homogeneity of variance (p ≥ 0.05 for the Bartlett test), an analysis of variance (ANOVA) is applied to compare variations in pH and oxidation-reduction potential (ORP) based on the days and different experimental media. If the ANOVA test indicates a significant difference (p < 0.05), a post-hoc test such as Tukey’s test is used to make pairwise comparisons between the days. If the data do not meet normality (p < 0.05 for the Shapiro-Wilk test) or homogeneity of variance (p < 0.05 for the Bartlett test), the non-parametric Kruskal-Wallis test is used to compare variations in pH and ORP as a function of time and media, offering a suitable alternative for non-normal distributions.

The activities of iron-oxidising bacteria on sulphide mine tailings are based on a decrease in pH and an increase in oxidation-reduction potential (ORP) over time in the reaction medium. To study acidophilic bacteria, the initial pH was rigorously adjusted to 3.5 to create conditions conducive to their development. In the 10 samples, the temporal changes in pH are almost identical. As an example, Figure 7 shows the change curves for samples 2 and 6. The pH curves for both biotic and abiotic environments show a slight increase over the first few days, followed by stabilisation at around pH = 4.5 for around five days ( Figure 7 ). This increase could be explained by the reactivity of the carbonates (calcite and ferrodolomite) present in the tailings [25] [26] . These carbonate minerals are likely to react with the acid in the environment, increasing the pH by neutralisation mechanisms. After this initial phase, a slight drop in pH was recorded until the end of the experiment. This acidity of the medium can be attributed to the chemical oxidation of iron (II) to iron (III), followed by the precipitation of iron (III) to form iron hydroxides (Fe(OH)₃) (Equation (3)) [27] . This phenomenon was corroborated by the appearance of a reddish-orange suspension in certain samples a few days after the start of the experiment. This colour change reflects the formation of iron (III) hydroxides, supporting the hypothesis that oxidation reactions caused the acidification [27] .

Fe³⁺ + 3H₂O → Fe (OH)₃ + 3H⁺ (3)

The ANOVA test showed that the pH variations observed in the two media were not statistically significant (p = 0.21). While differences in pH evolution were observed (S6) at the start of the experiment, these variations were not significant according to the ANOVA results. A different behaviour was observed in the case of acid mine tailings, with a greater decrease in pH in the biotic sample than in the control, which stabilised at pH 3.0 [21] . This same behaviour has also been observed elsewhere in the case of bioleaching experiments on marine sediments dredged by Fe and/or S [25] . The small difference between the biotic sample and the control sample shows the absence of ferro-oxidising acidophilic bacteria.

The temporal changes in POR for the identification of iron-reducing bacteria in mining residues from the biotic test are identical to those from the sterilized abiotic test. As an example, Figure 8 shows the temporal changes in POR for samples 2 and 6. The oxidation reactions can be confirmed by the evolution of the redox potential. The tailings suspensions show oxidising behaviour from the start of the test, with an initial Eh value of +200 mV ( Figure 8 ). Both trials follow a similar trend, with a phase of decreasing ORP followed by a phase of stabilisation. However, the initial drop is more noticeable in the abiotic environment than in the biotic environment. The ANOVA test comparing the two trials revealed no statistically significant difference in the evolution of ORPs (p = 0.21). The non-significant increase in redox potential observed afterwards can be attributed mainly to chemical oxidation processes involving the Fe³⁺/Fe²⁺ redox couple [23] . This redox system plays a key role in regulating the oxidation-reduction potential in iron-rich environments. The transition from ferrous iron (Fe²⁺) to ferric iron (Fe³⁺) via chemical oxidation is responsible for these observed variations, as shown in equation (Equation (4)).

4Fe²⁺ + O₂ + 4H⁺ → 4Fe³⁺ + 2H₂O (4)

In all the experiments, the differences observed in pH and oxidation-reduction potential (ORP) values between biotic and abiotic samples were small and statistically insignificant. The absence of significant differences between the two samples suggests a lack of iron-oxidising bacterial activity, which would otherwise have resulted in measurable changes in pH and redox potential. The small variation observed in the pH and ORP values could be due to the use of an autoclave for sterilising the abiotic samples.

In the 10 samples, the temporal changes in pH are almost identical. As an example, Figure 9 shows the change curves for samples 3 and 4. The biotic test initially showed similar pH values to the abiotic environment, with stabilization around neutrality during the first few days of the experiment. The temporary variation between the two trials was not statistically significant, as confirmed by the Kruskal-Wallis test (p = 0.27). The small variation in pH can be explained by geochemical reactivity within the system. The dissolution of carbonates, such as dolomite and calcite, buffers the environment. In the absence of biogeochemical processes or biological activity affecting pH, as in the case of NSO bacteria, a slight oscillation in pH is observed, as noted above. The documented effects of SNO (sulphur and sulphide-oxidising) bacteria have been observed in previous studies [21] to promote acidification of the media, with a drop in pH from 7.0 to 4.0. However, in our study, the absence of significant variations between biotic and abiotic samples shows that biological processes are not the main driver of the acidification observed, and that it is rather the result of chemical and geochemical mechanisms inherent in the system ( Figure 9 ).

The temporal changes in POR for the identification of iron-reducing bacteria in mining residues from the biotic test are identical to those from the sterilized abiotic test. As an example, Figure 10 shows the temporal changes in POR for samples 3 and 10. At the start of the tests, the ORPs of the reaction media were similar. From day 17 for S3 and day 14 for S10, there was a more rapid increase in ORP in the biotic environment than in the abiotic environment. Biotic ORP values reached higher levels (around 80 mV) than in the abiotic environment (around 60 mV) towards the end ( Figure 10 ). This difference is not significant (p = 0.08) with the Kruskal-Wallis test carried out in both media.

The graphs show a non-significant increase in oxidation-reduction potential (ORP) over time in both conditions. These variations can be attributed mainly to abiotic chemical processes. In sulphur-enriched environments, chemical reactions can play a crucial role in the evolution of the ORP. For example, the oxidation of sulphur in the presence of oxygen can lead to an increase in ORP. The general equation for sulphide oxidation can be represented as follows (Equation (5)):

2S + 2H₂O + 3O₂ → 2H₂SO₄ (5)

This reaction leads to the formation of sulphuric acid (H₂SO₄), increasing the acidity of the medium, which can indirectly affect the ORP. Under oxygen conditions, sulphide oxidation is rapid and may be the main reason for the increase in ORP in both systems, even in the absence of significant microbial activity. The variations in ORP could be linked to a difference in the initial chemical composition of the samples with the addition of sodium azide and heating at 120˚ for 25 minutes.