Sunlight-activated carbon nanotubes for anionic dye removal: characterization, efficiency, and biological assessment against some aquaculture and human pathogenic bacteria

Ammonia-functionalized multi-walled carbon nanotubes (MWCNTs/NH₃) were synthesized and evaluated for the sunlight-enhanced removal of Acid Red 73 (AR73) dye, as well as to assess its potential application against aquaculture and human pathogenic bacteria. Characterization of FTIR, SEM, and EDX confirmed a porous structure (70–110 nm diameters). FTIR confirmed functional groups (O-H, N-H, C ≡ C, C = C) critical for dye adsorption. SEM revealed a porous, entangled MWCNT structure with high surface area (70–110 nm tube diameters). EDX showed 88.10% carbon and 10.59% oxygen, with trace Si, Ca, and Fe. TGA indicated ammonia decomposition (100–250 °C) and CNT oxidation (300–900 °C). Batch experiments demonstrated rapid removal (97% within 30 min) and equilibrium at 180 min. Optimal conditions included pH 5 (99.16% efficiency) and 0.02 g/L dosage (100% removal), though adsorption capacity decreased at higher doses. The Langmuir isotherm (R²=0.939) revealed monolayer adsorption with a high capacity (312.5 mg/g), while pseudo-second-order kinetics (R²=0.999) indicated chemisorption dominance. Electrostatic interactions between protonated NH₃⁺ and anionic dye, hydrogen bonding, and π-π stacking were key mechanisms. Sunlight further enhanced removal via sunlight-enhanced adsorption. Despite limited antibacterial activity (low sensitivity for Staphylococcus aureus and Vibrio fluvialis ), the composite showed no broad-spectrum efficacy. This study highlights ammonia-functionalized MWCNT as a sustainable, energy-efficient adsorbent for high-capacity dye removal, leveraging solar energy to mitigate industrial wastewater pollution.