Effective Elimination of Contaminant Antibiotics Using High-Surface-Area Magnetic-Functionalized Graphene Nanocomposites Developed from Plastic Waste.

The presence of pharmaceutical residues in aquatic environments represents a risk for the equilibrium of the ecosystem and may seriously affect human safety itself in the long term. To address this issue, we have synthesized functional materials based on highly-reduced graphene oxide (HRGO), sulfonated graphene (SG), and magnetic sulfonated graphene (MSG). The method of synthesis adopted is simple and inexpensive and makes use of plastic bottle waste as the raw material. We have tested the fabricated materials for their adsorption efficiency against two model antibiotics in aqueous solutions, namely Garamycin and Ampicillin. Our tests involved the optimization of different experimental parameters of the adsorption process, such as starting antibiotic concentration, amount of adsorbent, and time. Finally, we characterized the effect of the antibiotic adsorption process on common living organisms, namely Escherichia coli DH5α (E. coli DH5α) bacteria. The results obtained demonstrate the efficiency of the method in addressing the issue of the emergence of antibiotic-resistant bacteria, which will help in preventing changes in the ecosystem.


=
(1) where VNaOH and CNaOH are the volume and concentration of NaOH solution used in the titration and W is the dry weight of SG or MSG samples. The EW for SG and MSG was found to be 0.7 and 0.37 g mol -1 .
For Zeta potential measurements, 0.05 g of HRGO, SG, or MSG was added into 10 mL of 1M NaCl solutions and the suspensions were sonicated until fully dispersed. The pH of the suspensions was adjusted from pH 3 to 9 using 0.1 M NaOH or HCl, and the Zeta potential was then determined using a Malvern Nanosizer Zeta potential.

Efficiency Verification of Garamycin and Ampicillin Adsorption Processes
To verify the efficiency of Garamycin and Ampicillin adsorption processes on HRGO, SG, and MSG, an experiment using E. coli DH5α, which has sensitivity to antibiotic concentration ≥50 ppm, was conducted. The experiment measured the optical density (OD) of culture absorbance at 600 nm after 24 h incubation at 37 ˚C. The experiment conditions were 500 mg L -1 antibiotic as initial concentration, a dose of adsorbent of 2 mg mL -1 , solution pH was adjusted at 5.5, and inoculated with E. coli DH5α were identical and used at the same time in the tubes to evaluate the material difference.

Kinetics of adsorption process
Kinetics study was important as it describes the uptake rate of adsorbate. The rate and mechanism of the antibiotics adsorption process on adsorbent could be elucidated based on kinetic studies. In order to elucidate the adsorption kinetics, the pseudo-first-order and pseudo second-order models were applied.
where k1 and k2 are pseudo-first-order and pseudo-second-order adsorption rate constants, respectively. The pseudo-first-order kinetic model is more suitable for low concentration of solute. It can be written in the following form: where qeis the amount of dye adsorbed at saturationper gram of adsorbent (mg g -1 ), qt is the amount of dye adsorbed at time t per gram of adsorbent (mg g -1 ), and k1 (min −1 ) is the rate constant of the pseudo first-order adsorption. While, the pseudo-second-order kinetic model is dependent on the solute amount adsorbed on the surface of adsorbent and the adsorbed amount at equilibrium.

Adsorption Isotherms
The antibiotics sorption capacity of the prepared material at different initial concentrations at equilibrium can be illustrated by the adsorption isotherms. Adsorption isotherms describe how the adsorbate interacts with adsorbents and give a thorough understanding of the nature of interaction. Several isotherm equations have been developed and employed for such analysis and the two important isotherms were applied.

Langmuir Isotherm Model
Langmuir's isotherm was used for monolayer adsorption on a surface containing a finite number of identified sites with negligible interaction between adsorbed molecules and assumes uniform energies of adsorption on the surface. In addition the maximum adsorption depends on the saturation level of monolayer. The Langmuir isotherm is represented by the following linear equation: where qe is the solid-phase antibiotic concentration in equilibrium with the liquid-phase concentration Ce expressed in mole L −1 , qm is the maximum monolayer adsorption capacity (mg g −1 ), and KL is an equilibrium constant (L mol −1 ). A straight line with slope of 1/qm and intercept of 1/KLqm is obtained when Ce/qe is plotted against Ce. The separation factor (RL) is a dimensionless constant which is an essential characteristic of the Langmuir model. The equation of RL is expressed as: where Co (mg L −1 ) is the highest studied initial antibiotic concentration, (Co = 900 mg L −1 ).RL indicates if the isotherm is unfavorable when RL>1, linear at RL=1, favorable at 0<RL<1, or irreversible at RL=0.

Freundlich Isotherm Model
Adsorbents that follow the Freundlich isotherm equation are assumed to have a heterogeneous surface consisting of sites with different adsorption potentials, and each type of site is assumed to adsorb molecules, as in the Langmuir equation: where Kf is constant (function of energy of adsorption and temperature) and n is a constant related to adsorption intensity, by plotting lnqe versus ln Ce which gave a straight line with slope of 1/n and intercept of lnKf. The magnitude of the "n" shows an indication of the favorability of adsorption.         As illustrated in Tables S7, S8 and S9 of model validations, the agreement between the obtained and estimated removal efficiency showed that using response surface method to design the experiments can be considered as an effective choice in the optimization of process parameters besides its uses as an experimental design and statistical analysis.