## 3 Results and Discussion
### Removal Efficiency
Results of X-ray absorption fine structure (XAFS) spectroscopy analysis showed that below pH 4.4, metal binds mostly to phosphoryl ligands available in the _Bacillus subtilis_ wall, whereas at more alkaline pH, adsorption to carboxyl groups becomes more important. At pH 7.8, activation of a different binding site was observed [46]. Presumably, the tested strain may also produce alkaline complexing agents as metabolites from the catabolism of proteins [47]. These metabolites may dissolve Mn by the formation of soluble metal complexion, which could easily bind to the bacteria cell surface. The increased Mn(II) as well as Cd(II) and Zn(II) adsorption onto bacteria with increasing pH has already been observed by other authors [42; 43; 48]. Most probably, the phenomena described above may be the cause of differences observed by us in the biosorption of Mn (Figure 4a), associated with a self-mediated change in pH. Moreover, when soil at a concentration of 6% (_w/v_) was used, a decrease in surface tension (Figure 4b) was observed, which takes place when bacteria produce biosurfactants. It was speculated that this phenomenon was linked with high Mn extraction efficiency (81%) obtained on the third day of the experiment. Biosurfactants are metal-complexing agents that may increase the hydrophobicity of the bacterial cell surface, allowing hydrophobic substrates to associate more easily with bacterial cells [49; 50]. According to the literature, biosurfactants are effective in remediation of metal-contaminated environments, especially under alkaline conditions [51]. This was confirmed by biosurfactant synthesizing _Burkholderia_ sp. _Z-90_, which produces alkaline conditions during heavy metal (Zn, Pb, Mn, Cd, Cu, and As) removal from soil, indicating that this strain facilitated heavy metals removal via increasing pH. Nevertheless, apart from soil concentration and pH, the change of the leachate surface tension value was increased during the bioleaching process (Figure 4b), likely due to an interaction between Mn in the soil and the biosurfactant. The same results were obtained by [PERSON] et al., [52] who concluded that it was caused by pre-modification the soil surface properties by the biosurfactant, due to the formation of a metal complex on the soil particles. Other authors also observed the significant contribution of biosurfactants in the removal of heavy metals-contaminated soils. For instance, [PERSON] and [PERSON] [53] reported that _Bacillus subtilis_ producing lipopeptide biosurfactants removed a significant amount of metals: Zinc (32.07%), Cadmium (44.2%), Copper (26.2%) Cobalt (35.4%), Nickel (32.2%), and Lead (40.3%). [PERSON] and [PERSON] [54], demonstrated that biosurfactants have the ability to remove the large amount of heavy metals from soil sediments. The removal percentage were up to 27% of Ni, 13% of Zn, and 37% of Cu, when the biosurfactants were used in a continuous flow configuration. Presence of rhamnids, supported recovery of Cd\({}^{2+}\) from a soil component--kaolin [55]. In another study, dihammolipid selectively removed bioavailable and bound forms of Pb (88%) and Cd (92%) from contaminated soil [56].