Removal of phenol from aqueous solution using reduced graphene oxide as adsorbent: isotherm, kinetic, and thermodynamic studies.

This work focuses on the batch adsorption study of phenol from an aqueous solution. Here, reduced graphene oxide (RGO) is used as an adsorbent. To synthesize reduced graphene oxide from graphene oxide, hydrazine monohydrate is used as a reducing agent. The synthesized samples were characterized using SEM, EDX, XRD, FTIR, BET surface area analyzer, RAMAN spectra, and zeta potential. The effects of solution pH, adsorption time, temperature, adsorbent dosage, and initial phenol concentration on adsorption characteristics were systematically studied. The optimized adsorption parameters were 0.4 g/L of adsorbent dosage, pH of 8.0, adsorption time 75 min, and temperature of 30 °C. The adsorption isotherm data follows the Langmuir isotherm model, and the maximum adsorption capacity (q) was 602.41 mg/g. The kinetic data of the adsorption follows the pseudo-second-order kinetic model. The Boyd model confirmed that film diffusion was the rate-limiting step in the adsorption process. The thermodynamic study of phenol adsorption using RGO confirms the endothermic nature of the process. The negative values of Gibb's free energy (ΔG) confirm that the process was spontaneous. The positive value of change in entropy (ΔS = 346.885 J/K) suggests that the randomness was increased at the solution/solid interface. The most important feature of this adsorbent was it could be easily and efficiently regenerated from phenol-loaded adsorbent with a negligible effect on removal efficiency. This study evidenced an effective use of RGO as an adsorbent for phenol removal.