Adsorption of harmful dyes and antimicrobial studies utilizing recyclable ZnO, its composites with conventionally used activated carbon, and waste orange peel as a greener approach

In this work, zinc oxide (ZnO) nanoparticles and its composites with activated carbon (AC) and orange peel (OP) were synthesized, characterized through scientific techniques such as FESEM, EDX, HRTEM, BET, FTIR, XRD etc, and used as adsorbents to treat methylene blue (MB) and methyl orange (MO) in wastewater. Different process parameters (like change in pH value, amount of adsorbent, concentration of dye, temperature, and contact time) were considered to examine the dye removal efficiency of ZnO and its composites. Maximum uptake capacity for MB/MO removal obtained was found to be 175.7/110.68 mg/g (ZnO), 180.03/107.97 mg/g (ZnO@AC) and 205.38/166.25 mg/g (ZnO@OP). Furthermore, thermodynamic, kinetic, and isothermic variables were calculated to get a clear vision about the mechanism involved during adsorption of both the dyes. The reusability examination of all adsorbents suggested continued reuse of adsorbents even after five cycles. Considering true adsorption performance, partition coefficient values were calculated for all adsorbents, e.g., 37.86/13, 41.1/12.45, and 73.61/32.48 mg.g^-1.μM^-1 in case of ZnO, ZnO@AC, and ZnO@OP for removal of MB/MO dyes, respectively. Furthermore, the zone of inhibition was estimated by performing antimicrobial experiments against Bacillus subtilis (ZnO@OP 19 mm ˃ ZnO & ZnO@AC 14 mm), Staphylococcus aureus (ZnO@OP 18 mm ˃ ZnO 17 mm ˃ ZnO@AC 12 mm), Pseudomonas aeruginosa (ZnO@OP 20 mm ˃ ZnO 16 mm ˃ ZnO@AC 13 mm), and Escherichia coli (ZnO@OP 19 mm ˃ ZnO 15 mm ˃ ZnO@AC 12 mm) respectively. Also, real water studies confirmed their promising future as adsorbents for real field applications.