**4. Conclusions**

The results obtained in this study demonstrated that QCS is a useful biosorbent for the bioremediation of aqueous solutions contaminated with Ni2<sup>+</sup> and containing inorganic impurities. The effect of background electrolytes varied depending on the type and concentration of the electrolyte. The biosorption of Ni2<sup>+</sup> was not affected by a NaCl ionic strength of 0.2 mM but was affected by higher ionic strengths. In addition, the background electrolytes (KCl, NaNO3, Na2SO4, CaCl2, MgSO4, and MgCl2) had no effect on the biosorption of the heavy metal at a concentration of 0.2 mM. However, at concentrations of 2 and 20 mM, the divalent Ca2+ and Mg<sup>2</sup>+ cations affected the biosorption of Ni2<sup>+</sup> significantly, whereas the monovalent Na<sup>+</sup> and K<sup>+</sup> cations affected the biosorption of Ni2<sup>+</sup> only slightly. The kinetic experimental data were well represented using the Elovich, fractional power, and pseudo-second order models. In order to determine the full potential of QCS as a commercial biosorbent, Ni2<sup>+</sup> biosorption from industrial wastewater will be evaluated in a future research study.

**Author Contributions:** Conceptualization, E.C.-U.; methodology, E.A.-G., G.M.C.-C., E.C.-U.; software, E.A.-G.; validation, E.A.-G.; formal analysis, E.A.-G., G.M.C.-C., E.C.-U.; investigation, E.A.-G.; resources, E.C.-U.; writing—original draft preparation, E.A.-G., G.M.C.-C., E.C.-U.; writing—review and editing, E.A.-G., G.M.C.-C., E.C.-U.; visualization, E.A.-G.; supervision, E.C.-U.; project administration, E.C.-U.; funding acquisition, E.C.-U. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Instituto Politécnico Nacional, Secretaría de Investigación y Posgrado, project number: SIP20201439.

**Acknowledgments:** E.C.-U. and G.M.C.-C. hold grants from EDI-IPN, COFAA-IPN, and SNI-CONACYT. E.A.-G. holds a gran<sup>t</sup> from SNI-CONACYT.

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