Adsorption properties of tetracycline onto graphene oxide: equilibrium, kinetic and thermodynamic studies.

Graphene oxide (GO) nanoparticle is a high potential effective absorbent. Tetracycline (TC) is a broad-spectrum antibiotic produced, indicated for use against many bacterial infections. In the present research, a systematic study of the adsorption and release process of tetracycline on GO was performed by varying pH, sorption time and temperature. The results of our studies showed that tetracycline strongly loads on the GO surface via π-π interaction and cation-π bonding. Investigation of TC adsorption kinetics showed that the equilibrium was reached within 15 min following the pseudo-second-order model with observed rate constants of k2 = 0.2742-0.5362 g/mg min (at different temperatures). The sorption data has interpreted by the Langmuir model with the maximum adsorption of 323 mg/g (298 K). The mean energy of adsorption was determined 1.83 kJ/mol (298 K) based on the Dubinin-Radushkevich (D-R) adsorption isotherm. Moreover, the thermodynamic parameters such as ΔH°, ΔS° and ΔG° values for the adsorption were estimated which indicated the endothermic and spontaneous nature of the sorption process. The electrochemistry approved an ideal reaction for the adsorption under electrodic process. Simulation of GO and TC was done by LAMMPS. Force studies in z direction showed that tetracycline comes close to GO sheet by C8 direction. Then it goes far and turns and again comes close from amine group to the GO sheet

Variation of resultant force in the z direction <i>vs.</i> time.

<p>Variation of resultant force in the z direction <i>vs.</i> time.</p

Langmuir, Freundlich and D–R constants and correlation coefficients of TC adsorption on GO at different temperatures.

*<p>Dubinin–Radushkevich.</p

The pseudo-first-order (a) and the pseudo-second-order (b) kinetics model for adsorption of tetracycline on GO suspension (20.0 mg/L), pH = 3.6, T = 298, 308, 318 K.

<p>The pseudo-first-order (a) and the pseudo-second-order (b) kinetics model for adsorption of tetracycline on GO suspension (20.0 mg/L), pH = 3.6, T = 298, 308, 318 K.</p

Structure of graphene oxide (a), Structure of tetracycline and pKa values (b).

<p>Structure of graphene oxide (a), Structure of tetracycline and pKa values (b).</p

FTIR spectra of free TC, free GO and TC after adsorption on GO (GO-TC).

<p>FTIR spectra of free TC, free GO and TC after adsorption on GO (GO-TC).</p

Isotherm of TC (6.0–180.0 mg/L) adsorption on GO (20.0 mg/L) at different temperatures (298, 303, 308 K).

<p>Isotherm of TC (6.0–180.0 mg/L) adsorption on GO (20.0 mg/L) at different temperatures (298, 303, 308 K).</p

Kinetic parameters for TC adsorption on GO at different temperatures.

<p>Kinetic parameters for TC adsorption on GO at different temperatures.</p

Effect of pH on the adsorption efficiency of TC (10.0–100.0 mg/L) on GO (20.0 mg/L); Temp. 25±0.1.

<p>Effect of pH on the adsorption efficiency of TC (10.0–100.0 mg/L) on GO (20.0 mg/L); Temp. 25±0.1.</p

Thermodynamic parameters for the adsorption of TC on GO.

<p>Thermodynamic parameters for the adsorption of TC on GO.</p