Surface-passivated rGO@CuO/6A5N2TU colloidal heterostructures for efficient removal of ofloxacin from contaminated water through dual-mode complexation: insights into kinetics and adsorption isotherm model study

Irrational use of ofloxacin, a fluoroquinolone-based derivative, imparts harmful effects to the natural habitat and humans; hence, sorption and removal of such compounds can be of high primordial interest. Structural modifications on the edge and basal planes of reduced graphene oxide (rGO) account for its higher adsorption and removal rates for various pharmaceutical and organic contaminants. Thus, herein for the first time, we report the synthesis of reduced graphene oxide–copper oxide nanocomposite (rGO–CuO), decorated with 6-amino-5-nitroso-2-thiouracil (rGO@CuO/6A5N2TU). The heterostructure was fabricated using a combination of “sonochemical” and “hydrothermal” methods, wherein hydrothermally synthesized rGO–CuO nanocomposite was made to react with 6A5N2TU under ultrasonication to produce rGO@CuO/6A5N2TU. The obtained methodology provides sustainability and prevents the use of harsh chemicals during composite formation. Morphological assessment of rGO@CuO/6A5N2TU shows uniform deposition of 6A5N2TU as sharp needles over the rGO sheets at 100 nm scale, while the CuO deposition shows a flower-like arrangement over the surface of rGO, observed at 1 μm scale. The vibrational spectra of rGO@CuO/6A5N2TU confirm the presence of (N–H), (N=O), and (C=O) groups. The low D band and high G band intensities, obtained by Raman scattering, symbolize the restoration of pie network in rGO and a lesser number of surface functional groups. Moreover, XRD analysis shows high crystallinity and the presence of CuO and other oxygenated functional groups over the surface of rGO@CuO/6A5N2TU. Further, the fabricated metal centered heterostructure was explored for efficient removal of ofloxacin from contaminated water specimens. Batch adsorption modeling and modeled isotherm fitting experiments were done to elucidate the behavior of ofloxacin removal. The time-dependent decrease in the λ max value of ofloxacin in the presence of rGO@CuO/6A5N2TU was determined to estimate removal efficiency. Interestingly, we observed complete removal of ofloxacin at concentrations as low as 20 ppm within 120 min, with adsorption capacities ranging up to ~ 85.8 mg/g and a removal efficiency of ~ 86%. Moreover, the kinetic modeling demonstrates that the adsorption follows pseudo-second-order linear kinetics [ R 2 (~ 0.986)]. The ofloxacin adsorption on rGO@CuO/6A5N2TU fits well with the Langmuir adsorption isotherm, representing monolayer adsorption by the heterogenous substrate, i.e., a surface-oriented chemisorption. Further, the calculation of the thermodynamic parameter shows its spontaneous and exothermic nature, i.e., decreasing the system randomness during adsorption. Upon real sample analysis, the rGO@CuO/6A5N2TU is considered as an ideal adsorbing material with recyclability rates obtained up to seven cycles. Overall, we believe rGO@CuO/6A5N2TU acts as an ideal adsorbent material for the removal of antibiotics from water, limiting its pollution. The present system can be tested for the removal of other water-soluble antibiotic samples.