*Article Rhizopus oryzae***-Mediated Green Synthesis of Magnesium Oxide Nanoparticles (MgO-NPs): A Promising Tool for Antimicrobial, Mosquitocidal Action, and Tanning Effluent Treatment**

**Saad El-Din Hassan 1,\* , Amr Fouda 1,\* , Ebrahim Saied <sup>1</sup> , Mohamed M. S. Farag <sup>1</sup> , Ahmed M. Eid <sup>1</sup> , Mohammed G. Barghoth <sup>1</sup> , Mohamed A. Awad <sup>2</sup> , Mohammed F. Hamza 3,4 and Mohamed F. Awad 5,6**


**Abstract:** The metabolites of the fungal strain *Rhizopus oryaze* were used as a biocatalyst for the green-synthesis of magnesium oxide nanoparticles (MgO-NPs). The production methodology was optimized to attain the maximum productivity as follows: 4 mM of precursor, at pH 8, incubation temperature of 35 ◦C, and reaction time of 36 h between metabolites and precursor. The as-formed MgO-NPs were characterized by UV-Vis spectroscopy, TEM, SEM-EDX, XRD, DLS, FT-IR, and XPS analyses. These analytical techniques proved to gain crystalline, homogenous, and well-dispersed spherical MgO-NPs with an average size of 20.38 ± 9.9 nm. The potentiality of MgO-NPs was doseand time-dependent. The biogenic MgO-NPs was found to be a promising antimicrobial agent against the pathogens including *Staphylococcus aureus*, *Bacillus subtilis*, *Pseudomonas aeruginosa*, *Escherichia coli*, and *Candida albicans* with inhibition zones of 10.6 ± 0.4, 11.5 ± 0.5, 13.7 ± 0.5, 14.3 ± 0.7, and 14.7 ± 0.6 mm, respectively, at 200 µg mL–1. Moreover, MgO-NPs manifested larvicidal and adult repellence activity against *Culex pipiens* at very low concentrations. The highest decolorization percentages of tanning effluents were 95.6 ± 1.6% at 100 µg/ 100 mL after 180 min. At this condition, the physicochemical parameters of tannery effluents, including TSS, TDS, BOD, COD, and conductivity were reduced with percentages of 97.9%, 98.2%, 87.8%, 95.9%, and 97.3%, respectively. Moreover, the chromium ion was adsorbed with percentages of 98.2% at optimum experimental conditions.

**Keywords:** MgO-NPs; optimization; antimicrobial; mosquitocidal and repellence activity; tannery effluents; chromium ion
