## 3 Results and Discussion
### Wastewater Physico-Chemical Parameters  
The efficiency of wastewater treatment with yeast cultures and biosurfactants could be influenced by several parameters, such as temperature, pH, conductivity, dissolved oxygen, etc. [56; 57; 58; 59]. Temperature is an important parameter which affects the metabolic processes of yeast, while pH could influence heavy metal solubility and yeast cell surface charge, thereby impacting the biosorption capacity of the yeast cells. Additionally, the conductivity, reflecting the ionic content of the wastewater, can influence the treatment efficiency by indicating potential competition between the ions and the heavy metals for the yeast binding sites. On the other hand, yeasts use dissolved oxygen to effectively metabolize and reduce heavy metals in wastewater. Table 2 presents the evolution of pH and conductivity of synthetic wastewater during the experiments. Temperature was kept quasi-constant at about 26 \(\pm\) 1 \({}^{\circ}\)C. The samples were aerated daily using a wood air diffuser for about 15 min in order to increase the amount of dissolved oxygen at about 5 mg/L (the initial amount of dissolved oxygen was 7.5 \(\pm\) 0.5 mg/L). A general increase in the conductivity of the samples was observed during the experiments for both yeast strains, with the maximum conductivity values being observed in \begin{table}
\begin{tabular}{l l l l l l l l} \hline \hline \multirow{2}{*}{**Time (h)**} & \multicolumn{6}{c}{_Rhodotorula mucilaginousa_** CMGB-G1 (P1)**} \\ \cline{2-9}  & **Parameters** & _Sw_ & _Pb_ & _Cd_ & _Bio Y_ & _Bio Y + Pb_ & _Bio Y + Cd_ \\ \hline \multirow{2}{*}{0} & Conductivity (\(\mu\)S-cm\({}^{-1}\)) & 1078 & 1040 & 1075 & 1264 & 1262 & 1180 \\ \cline{2-9}  & pH & 7.23 & 7.12 & 7.11 & 7.34 & 7.34 & 7.32 \\ \hline \multirow{2}{*}{24} & Conductivity (\(\mu\)S-cm\({}^{-1}\)) & 1094 & 1070 & 1115 & 1286 & 1280 & 1270 \\ \cline{2-9}  & pH & 6.86 & 6.44 & 6.68 & 6.34 & 6.49 & 7.06 \\ \hline \multirow{2}{*}{48} & Conductivity (\(\mu\)S-cm\({}^{-1}\)) & 1855 & 1900 & 1889 & 2234 & 2205 & 2265 \\ \cline{2-9}  & pH & 6.31 & 6.26 & 6.95 & 6.60 & 6.34 & 6.95 \\ \hline \multirow{2}{*}{96} & Conductivity (\(\mu\)S-cm\({}^{-1}\)) & 1866 & 1899 & 1720 & 2266 & 2273 & 2099 \\ \cline{2-9}  & pH & 6.84 & 6.14 & 7.31 & 7.21 & 7.41 & 7.36 \\ \hline \multirow{2}{*}{120} & Conductivity (\(\mu\)S-cm\({}^{-1}\)) & 1906 & 1961 & 1746 & 2135 & 2177 & 2019 \\ \cline{2-9}  & pH & 7.35 & 6.91 & 7.49 & 7.83 & 7.95 & 7.84 \\ \hline \multicolumn{9}{c}{_Rhodotorula mucilaginousa_** CMGB188 (P2)**} \\ \hline \multirow{2}{*}{0} & Conductivity (\(\mu\)S-cm\({}^{-1}\)) & 1072 & 1076 & 1072 & 1260 & 1261 & 1237 \\ \cline{2-9}  & pH & 7.19 & 7.11 & 7.11 & 7.34 & 7.34 & 7.33 \\ \hline \multirow{2}{*}{24} & Conductivity (\(\mu\)S-cm\({}^{-1}\)) & 1090 & 1086 & 1100 & 1286 & 1280 & 1282 \\ \cline{2-9}  & pH & 6.75 & 6.5 & 6.77 & 6.43 & 6.43 & 7.24 \\ \hline \multirow{2}{*}{48} & Conductivity (\(\mu\)S-cm\({}^{-1}\)) & 1868 & 1858 & 1864 & 2230 & 2220 & 2267 \\ \cline{2-9}  & pH & 6.26 & 6.22 & 6.79 & 6.45 & 6.37 & 7.01 \\ \hline \multirow{2}{*}{96} & Conductivity (\(\mu\)S-cm\({}^{-1}\)) & 1879 & 1877 & 1832 & 2267 & 2280 & 2179 \\ \cline{2-9}  & pH & 6.31 & 6.12 & 7.38 & 7.32 & 7.44 & 7.3 \\ \hline \multirow{2}{*}{120} & Conductivity (\(\mu\)S-cm\({}^{-1}\)) & 1936 & 1926 & 1750 & 2156 & 2197 & 2145 \\ \cline{2-9}  & pH & 7.07 & 6.96 & 7.39 & 7.82 & 7.92 & 7.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Variations in the physico-chemical parameters of inoculated synthetic wastewater. the case of samples with biosurfactants and heavy metals. The growth of yeasts involves the release of ions, such as ammonium ions derived from the assimilation of nitrogen, which contribute to the increase in the ionic strength of the water and, consequently, to the overall conductivity of the water [60; 61].