Transforming <i>Citrus sinensis</i> Seed Waste Into Modified MnO ₂ Nanoparticles as Photocatalytic and Biological Agent

ABSTRACT Citrus sinensis (CS) seed extract, a rich source of natural antioxidants and phytochemicals, played a crucial role in the green synthesis of nanoparticles by acting as a natural reducing, capping, and stabilizing agent, offering a sustainable alternative to toxic chemical methods. A novel green synthesis of manganese dioxide nanoparticles (MnO 2 ‐NPs) was done using Citrus sinensis seed extract (CSSE), achieving ultra‐stable and uniformly dispersed NPs with an average size of 15.3 nm (DLS) and crystallite size of 15.23 nm (XRD). The antibacterial activity was carried out against B. cereus, S. aureus (gram‐positive), E. coli, and P. aeruginosa (gram‐negative) bacteria, and antifungal activity against P. chrysogenum, A. niger, A. solani , and F. oxysporum using the tube dilution method. The MIC values for antibacterial activity of CSSE and CS‐MnO 2 ‐NPs were observed to be 15 ± 1.56 and 10 ± 0.73 µg/mL against B. cereus . The MIC values for antifungal activity of CSSE and CS‐MnO 2 ‐NPs were observed to be 40 ± 3.17 and 15 ± 1.89 µg/mL against A. solani . The antioxidant potential of CSSE and CS‐MnO 2 ‐NPs showed a good DPPH scavenging activity with an IC 50 value of 192.41, and 182.42 µg/mL. The CS‐MnO 2 ‐NPs achieved up to 99% degradation efficiency of methylene blue dye (7 × 10 −6 M) within 90 min under UV‐visible light at pH 3 and a catalyst dosage of 3 mg. The PZC of the CS‐MnO 2 ‐NPs is found to be 2.5. Post‐treatment water analysis showed reductions in COD (from 3120 to 2080 mg/L), BOD (from 2.9 to 2.6 mg/L), and increased DO (from 10 to 11.9 mg/L), indicating effective pollutant mineralization, which suggests CS‐MnO 2 ‐NPs as a promising material for antimicrobial and wastewater treatment, supporting circular economy practices.