# Enhanced Methyl _Tert_-Butyl Ether Removal by Mixed Consortium: Performance and Adaptability
## 3 Result and Discussion
### The Mixture of Three Pure Cultures Effectively Removed MTBE in the Presence of Heavy Metals  
The co-contamination of organic and inorganic (such as MTBE and heavy metals) has been detected all over the world. Heavy metals are toxic to microorganisms, thus disturbing the biodegradation system. Exploring the interaction between bacteria and metals contained in gasoline on MTBE removal may develop a practical method for in-site remediation of MTBE contaminated sites. The capabilities of strains to degrade MTBE in the presence of metal ions (Fe\({}^{2+}\), Mn\({}^{2+}\), Pb\({}^{2+}\), Zn\({}^{2+}\), Cu\({}^{2+}\), and Cd\({}^{2+}\)) detected in gasoline-impacted soils and water was investigated. The growth response of strains to six metal ions with different concentrations in a MSMmedium containing MTBE as carbon source is presented in Figure 4a. As can be seen, the maximum biomass varied in the range of 0.1045-0.183 and 0.0755-0.143 for metal concentrations of 1 and 20 mg/L, respectively. At a metal concentration of 1 mg/L, the growth of strains exhibited similarity and there was no substantial change in the biomass of three tested metal ions (Pb\({}^{2+}\), Cu\({}^{2+}\) and Cd\({}^{2+}\)). In comparison, Zn\({}^{2+}\) caused a sharp decrease in biomass, indicating that strains grew well in the presence of metal ions other than Zn\({}^{2+}\) at concentration below 1 mg/L. Furthermore, the biomass decreased with increasing metal concentration, while the biomass treated with Fe\({}^{2+}\) hardly changed. The growth of strains was inhibited by replacing essential ions or blocking the important functional group with the introduction of metal ions except for Mn\({}^{2+}\) and Fe\({}^{2+}\) at concentration up to 20 mg/L [40]. Under two test concentrations, the strains treated with Fe\({}^{2+}\) and Mn\({}^{2+}\) grew better, while Zn\({}^{2+}\) had a stronger inhibitory effect on the growth of strains. Several interactions were observed during MTBE biodegradation, including enhancement and inhibition (Figure 4b). The introduction of Mn\({}^{2+}\) at a concentration of 1 mg/L promoted the growth of strains and consequently offered a stimulating effect on MTBE biodegradation, while suppressing the removal at a high metal concentration (20 mg/L). Similarly, the removal of MTBE was enhanced at a low concentration (1 mg/L) of Fe\({}^{2+}\), and the addition of Fe\({}^{2+}\) had no obvious inhibitory effect until a concentration of 20 mg/L was reached. However, other metal ions did not cause any stimulation. For Pb\({}^{2+}\), Cu\({}^{2+}\) and Cd\({}^{2+}\), the removal of MTBE changed little at a metal concentration of 1 mg/L, but was inhibited as the metal concentration increased to 20 mg/L. It was found that Zn\({}^{2+}\) was detrimental to MTBE biodegradation, and its introduction restrained MTBE removal even at a concentration of 1 mg/L. Additionally, the inhibition was getting severe at a concentration of 20 mg/L; that is, the degradation efficiency was negatively correlated with the concentration of Zn\({}^{2+}\). As shown in Figure 4b, the system was upset at 20 mg/L for all metal ions tested, and the MTBE degradation efficiency decreased constantly in the following order: Zn\({}^{2+}\) > Pb\({}^{2+}\) > Cu\({}^{2+}\) > Cd\({}^{2+}\) > Mn\({}^{2+}\) > Fe\({}^{2+}\). Among then, Zn\({}^{2+}\) had the strongest inhibitory effect and was the only metal ion that showed inhibition at 1 mg/L. The four metal ions (Zn\({}^{2+}\), Pb\({}^{2+}\), Cu\({}^{2+}\), Cd\({}^{2+}\)) induced inhibition and showed a dose-dependent response, and the other Fe\({}^{2+}\) and Mn\({}^{2+}\) exhibited weaker negative effects. Figure 4: Effect of metal ions on degrading bacteria. The (**a**) maximum cell density of strains and (**b**) the yield of MTBE degradation with various metal ions, the concentrations of metals were controlled at 1 mg/L (dotted line) and 20 mg/L (solid line), respectively. The results in this study are consistent with reports published previously. [PERSON] et al.