Decolorization of Methylene Blue Using Chitosan, Aspergillus caespitosus, and a Fungal Biomass–Chitosan Composite: Toxicity Assessment using Daphnia magna

Methylene blue (MB), a cationic dye, is widely utilized in the paper, textiles, pharmaceutical, and ink production industries. The direct discharge of dyes into aquatic environments can cause significant harm to the ecosystem. MB exhibits remarkable stability, rendering its natural decomposition extremely challenging exceedingly difficult. Therefore, it is essential to eradicate the dye and protect the environment from its contamination. This work evaluated the efficacy of removing MB utilizing chitosan, fungal biomass, and a fungal biomass–chitosan composite as biosorbents, along with the subsequent ecotoxicity assessment. The fungal isolate was obtained from water and identified as Aspergillus caespitosus (PV449101) using morphological and molecular features. The decolorization process was investigated under optimized conditions, including pH, MB dye concentration, incubation times, and biosorbent dosage. The results indicated that fungal biomass attained 95.58% decolorization of MB at pH 8 within 60 minutes, while chitosan obtained 91.23% at pH 8 and the composite reached 94.18% at pH 10, equilibrating in 120 and 30 minutes, respectively. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) were employed to assess the biosorbent properties prior to and following the sorption process. This study demonstrated also a considerable reduction in dye zoo-toxicity, as measured by Daphnia magna. Acute toxicity was assessed using LC₅₀ and calculated toxicity unit (TU) values. Untreated MB was highly toxic. Chitosan reduced toxicity from Class IV to Class III, while fungal biomass achieved Class I status. Notably, the chitosan–biomass composite demonstrated superior detoxification (LC₅₀: 1345 ppm; TU: 0.07), reclassifying MB as slightly toxic. This study presents a novel approach by using A. caespitosus and its biomass-chitosan composite for the first time to remove MB from aqueous solutions. Our results proved high decolorization efficiency, combined with eco-friendly and cost-effective properties, and their promising potential in wastewater treatment.