# _Penicillin_ spp. XK10, Fungi with Potential to Repair Cadmium and Antimony Pollution
## 4 Discussion  
In this study, the tolerance XK10 to Cd in solid medium was greater than 5 mM and to Sb was greater than 7 mM. In liquid medium, XK10 could grow normally even when the concentration of Sb reached 20 mM, indicating that XK10 has a strong tolerance to Sb. Under normal conditions, heavy metals might poison the cells of soil microorganisms, slow their growth and cause microbial death. However, over time, some microorganisms may adapt to the heavy metals to survive and gradually become the dominant population in the soil [14; 35]. In the presence of high concentrations of heavy metals, some microorganisms with a certain tolerance to heavy metals survive, and some reduce toxicity through biotransformation or metabolic activities. The tolerance of microorganisms to heavy metals can be determined by the minimum inhibitory concentration (MIC). MIC refers to the minimum \begin{table}
\begin{tabular}{c c c} \hline \hline
**The Name of the Strain** & **Cd(II) Single Adsorption** & **Sb(III) Single Adsorption** \\  & **(XK10)** & **(XK10)** \\ \hline Langmuir model \(K_{L}\) & 0.2153 & 0.0325 \\ \(q_{max}\) & 0.28 & 4.81 \\ \(R^{2}\) & 0.3307 & 0.5081 \\ Freundlich model \(K_{F}\) & 0.0674 & 0.6593 \\ \(n\) & 2.1329 & 2.7155 \\ \(R^{2}\) & 0.4265 & 0.6387 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of isotherm model on biosorption of Cd(II) and Sb(III) by XK10. concentration of heavy metals in which a single microbial colony cannot grow. Its determination can be divided into liquid culture and solid culture, and the MIC values under different culture conditions are very different [36]. For example, _Penicillium chrysogenum_ was significantly less tolerant of the heavy metal cadmium when grown on solid media than on liquid media [37]. In general, the higher the tolerance of the fungus, the greater its potential for application for environmental remediation. Currently, _Penicillium_ has been shown to be tolerant to a variety of heavy metals [15]. The results of the analysis of the minimum inhibitory growth concentrations of different microorganisms for Cd and Sb showed that XK10 was more tolerant to both Cd and Sb than most microorganisms (Table 2). Therefore, it has a strong application potentiality. The initial Cd concentration in the solution affected the growth and adsorption efficiency of the strain. The removal of Cd by XK10 was not high at low concentrations and gradually increased with increasing concentrations, probably due to the higher initial Cd concentration providing the driving force to overcome the mass transfer resistance of the biosorption between the biosorption media [33]. The best removal of Sb by XK10 was achieved at an Sb concentration of 1 mM. The removal of Sb by XK10 decreased significantly with the increasing Sb concentration. It decreased to 0% at an Sb concentration of 4 mM. When the Sb concentration was too high, the repulsion between heavy metal ions in the solution increased, the removal sites on the cell membrane were saturated and some of the bacteria were lysed or autolyzed, thus leading to poor removal [41]. Solution pH can affect biosorption by influencing the activity of adsorption sites on the surface of the strain and the morphology of heavy metal ions [42,43]. At an initial pH of 2-4, the adsorption rates of XK10 for both Cd and Sb increased significantly and reached the maximum adsorption rate when the pH was 4. Poor protonation or ionization of functional groups at low pH leads to weak complexation stability of metal ions [44]. When the initial pH increases, the medium pH of XK10 increases (medium pH \(<\) 5), the negative charge on the surface of the biosorbent increases, the adsorption effect is enhanced, and positively charged Cd\({}^{2+}\) and Sb(OH)\({}^{2+}\) are adsorbed and the adsorption rate increases [45]. At an initial pH of 4-6, XK10 showed a decreasing trend in the adsorption rates of both Cd and Sb.