## 3 Results and Discussion
### Effect Analysis of Exogenous Substances on PAHs Degradation
#### 3.3.2 Heavy Metal Ions  
Sensitivity Analysis  
Heavy metals are common components in oily sludge. In the process of microbial treatment of oily sludge, many heavy metals become protein-precipitant and cause microbial death. Some of microorganisms exhibit a biphasic response to a number of heavy metals (Cu\({}^{2+}\), Zn\({}^{2+}\)); an appropriate amount of heavy metals contribute to the growth of microorganisms, but they kill microorganisms if excessive [51]. Other metals have toxic effects (Pb\({}^{2+}\)) at a low concentrations. Previous experiments ignored the disadvantage of heavy metals. Therefore, it is necessary to explore the sensitivity and resistance of P&A bacterial consortium to heavy metal particles. The bacteriostatic circles produced by the P&A bacterial consortium are shown in Figure 7, and the average diameter of the bacteriostatic circles are shown in Table 4.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{
\begin{tabular}{c} **Compound** \\ **Concentration** \\ **(g/L)** \\ \end{tabular} } & \multicolumn{6}{c}{**Average Diameter of Bacteriostatic Circles (mm)**} \\ \cline{2-9}  & \multicolumn{3}{c}{**CuSO\({}_{4}\) 5H\({}_{2}\)O**} & \multicolumn{3}{c}{**ZnSO\({}_{4}\) 7H\({}_{2}\)O**} & \multicolumn{3}{c}{**Pb(NO\({}_{3}\))\({}_{2}\)**} \\ \cline{2-9}  & **1-1** & **1-6** & **P\&A** & **1-1** & **1-6** & **P\&A** & **1-1** & **1-6** & **P\&A** \\ \hline
**0.1** & — & — & — & — & — & — & — & — & — \\
**0.5** & — & 7 & 7 & — & — & — & — & — & — \\
**1** & — & 8 & 13 & — & — & — & — & — & — \\
**5** & — & 9 & 16 & — & 10 & 5 & — & — & 7 \\
**10** & 15 & 16 & 22 & 8 & 12 & 10 & 8 & — & 10 \\
**20** & 30 & 30 & 30 & 16 & 14 & 19 & 11 & 8 & 13 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Average diameter of bacteriostatic circles produced by DSTP.  
DSTP is lied tightly on the surface of LB solid medium, which was inoculated with different bacterium. The heavy metal ions in the paper diffuse in agar, and the concentration of heavy metal ions decreases logarithmically with the extension of diffusion distance. The bacterium forms a transparent bacteriostatic circle around the paper. The size of bacteriostatic circle can reflect the sensitivity of bacteria to heavy metal ions. Additionally, it has a negative correlation with the MIC, that is, the larger the diameter of bacteriostatic circle, the higher the antibacterial activity [52]. It can be seen from Figure 7 and Table 4 that, with the increase of the concentration of heavy metal ions, the diameters of the bacteriostatic circles are also increased. When the concentrations of compounds are higher than 0.5, 5, and 5 g/L, respectively (i.e., the concentrations of Cu\({}^{2+}\), Zn\({}^{2+}\), and Pb\({}^{2+}\) are higher than 2.002, 17.388, and 15.096 mM, respectively), the growth of bacterium was inhibited.  
Resistance Analysis  
The negative effects of compound concentrations on the growth of P&A bacterial consortium are shown in Figure 8.  
The MIC of P&A bacterial consortium to different compounds were 0.5 g/L (Cu\({}^{2+}\) 2.002 mM), 5 g/L (Zn\({}^{2+}\) 17.388 mM), and 3.125 g/L (Pb\({}^{2+}\) 9.435 mM), respectively, which are higher than the reported contents of Cu\({}^{2+}\), Zn\({}^{2+}\), and Pb\({}^{2+}\) in oil sludge [53]. It indicates that P&A bacterial consortium can tolerate high concentrations of Cu\({}^{2+}\), Zn\({}^{2+}\), and Pb\({}^{2+}\), and grow normally in oil sludge.  
Figure 7: Bacteriostatic circles produced by different DSTP. (**a**) Bacteriostatic circles produced by CuSO\({}_{4}\).5H\({}_{2}\)O DSTP. (**b**) Bacteriostatic circles produced by ZnSO\({}_{4}\).7H\({}_{2}\)O DSTP. (**c**) Bacteriostatic circles produced by Pb(NO\({}_{3}\))\({}_{2}\) DSTP.