*3.5. Proposed Mechanism for Cd2*<sup>+</sup> *Removal*

Based on the findings of this work, we proposed the possible mechanism of *T. fasciculatum* and *T. longibrachiatum* response to Cd2<sup>+</sup> stress (Figure 5). The bioaccumulation of Cd2<sup>+</sup> in *T. fasciculatum* and *T. longibrachiatum* is a cumulative result of three pathways. First the Cd2<sup>+</sup> get adsorbed over the fungal outer membrane [52]. It has been shown in the FTIR results that various functional groups available on the fungal surface showed significant changes in their peak positions, indicating the chemical adsorption of Cd2<sup>+</sup> over the membrane. Then, the ions are engulfed in the fungal cytoplasm where they are assimilated and sequestrated by the cellular reactions [53]. In addition, the fungi can release exopolysaccharides (high-molecular-weight polymers composed of sugar residues) to entrap or adsorb the extracellular Cd2<sup>+</sup> ions [54].

**Figure 5.** Pictorial illustration of Cd2<sup>+</sup> removal by *T. fasciculatum* and *T. longibrachiatum.*

### **4. Conclusions**

In summary, 25 sludge, industrial effluents and sewage samples were collected to isolate fungal isolates and screened to their tolerance to Cd2<sup>+</sup> in PDA medium containing Cd2<sup>+</sup> from 25 to 75 mg/L. There was a decline in the number of fungi for their tolerance to Cd2<sup>+</sup> with the increment in concentration of Cd2<sup>+</sup> from 25 to 75 mg/L. The removal of Cd2<sup>+</sup> by highly effective fungi *T. fasciculatum* and *T. longibrachiatum* was dependent on various experimental parameters such as temperature, initial metal ion concentrations, pH, incubation time, inoculum size, etc. Under optimized conditions, i.e., pH 5.0, inoculum size (2.5% for *T. fasciculatum* and 2.0% for *T. longibrachiatum*), incubation time (120 h), initial Cd2<sup>+</sup> concentration (20 mg/L), and temperature (30 ◦C), the maximum removal of Cd2<sup>+</sup> was 67.10% for *T. fasciculatum* and 76.25% for *T. longibrachiatum*. The observed data were well fitted with the Langmuir and Freundlich isotherm models. The SEM examinations confirmed the morphological changes in response to the Cd2<sup>+</sup> aggregation in *T. fasciculatum* and *T. longibrachiatum*. Both the fungal isolate exhibited Cd2<sup>+</sup> binding on the cell wall surface as a mechanism of metal tolerance. The FTIR studies confirmed the presence of various functional groups such as –CH, –OH, -C=O and –NH in the binding of Cd2<sup>+</sup> ions with the fungal biomass, i.e., *T. fasciculatum* and *T. longibrachiatum*. The observed results revealed the potential biosorption tendency of *T. fasciculatum* and *T. longibrachiatum* to remove Cd2<sup>+</sup> from wastewater.

**Author Contributions:** Conceptualization, R.K. (Raman Kumar), N.S. and A.U.; methodology, R.K. (Raman Kumar), N.S. and A.U.; software, R.K. (Raman Kumar), N.S., P.S., A.U., R.K. (Rajeev Kumar), P.K.J., F.A.A., A.A.A. and N.A.-Z.; validation, R.K. (Raman Kumar), P.S., A.U., R.K. (Rajeev Kumar), P.K.J., F.A.A., A.A.A., and N.A.-Z.; formal analysis, R.K. (Raman Kumar), N.S. and A.U.; the investigation, R.K. (Raman Kumar), and A.U.; resources, R.K. (Raman Kumar), P.S., A.U., R.K. (Rajeev Kumar), N.S.,P.K.J., F.A.A., A.A.A., and N.A.-Z.; data curation, R.K. (Raman Kumar), P.S., A.U., R.K. (Rajeev Kumar), N.S., P.K.J., F.A.A., A.A.A., and N.A.-Z.; writing—original draft preparation, R.K. (Raman Kumar) and A.U.; writing—review and editing, R.K. (Raman Kumar) and A.U.; visualization, R.K. (Raman Kumar), P.S., A.U., R.K. (Rajeev Kumar), N.S., P.K.J., F.A.A., A.A.A. and N.A.-Z.; supervision, R.K. (Raman Kumar); N.S. project administration, R.K. (Raman Kumar), N.S. and A.U. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through the research group No. RGP-1441-305.

**Acknowledgments:** The authors are thankful to Haryana State Council of Science and Technology, Panchkula, Haryana for providing financial support for carrying out research work (Grant No. HSCST/R&D/2018/209R). The authors are thankful to Maharishi Markandeshwar (Deemed to be University), Mullana, India for providing the lab facilities. The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through the research group No. RGP-1441-305.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


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