High-performance removal of phenol from aqueous solutions using EG- and PEG-functionalized biochar: equilibrium, kinetic and thermodynamic study with optimization by response surface methodology (RSM)

Phenol and its derivatives threaten human health and reduce ecosystem functions if these pollutants are released into the water. Adsorption of phenol and derivatives onto adsorbents has been considered as one of the easy operation, cost-effective as well as highly effective methods for water purification. In this work, four different biochar-based magnetic adsorbents including biochar/ZnFe2O4 (BC/ZnFe2O4), oxidized biochar/ZnFe2O4 (OX-BC/ZnFe2O4), ethylene glycol-functionalized biochar/ZnFe2O4 (EG-BC/ZnFe2O4) and polyethylene glycol-functionalized biochar/ZnFe2O4 (PEG-BC/ZnFe2O4) were successfully fabricated and used for adsorption of phenol from aqueous medium. The maximum adsorption capacity for phenol removal in the presence of BC/ZnFe2O4, OX-BC/ZnFe2O4, EG-BC/ZnFe2O4 and PEG-BC/ZnFe2O4 adsorbents was obtained 66.3, 84.8, 344.8 and 588.3 mg/g, respectively. The addition of ethylene glycol and polyethylene glycol functional groups into biochar matrix can enhance the surface area of biochar and create more active sites on the biochar surface as adsorbents. Moreover, the addition of ZnFe2O4 nanoparticles can help the synthesized adsorbents separate so easily from aqueous media. Also, the presence of ZnFe2O4 nanoparticles can increase hydrogen bonding and this can enhance removal efficiency. To optimize removal parameter such as mixture pH, contact time and dosage of adsorbent, the response surface methodology-Box-Behnken design (RSM-BBD) was employed. The optimal pH for all of the adsorbents was 7, while the optimal time as well as the optimal dosage of adsorbent was obtained 170, 60, 30 and 25 min as well as 0.7, 0.5, 0.4 and 0.15 g/L in the presence of BC/ZnFe2O4, OX-BC/ZnFe2O4, EG-BC/ZnFe2O4 and PEG-BC/ZnFe2O4 adsorbents, respectively. Kinetically, the empirical results of adsorption fitted with the second-order model and all the adsorbents followed the Langmuir isotherm model. Thermodynamic study exhibited that the adsorption of phenol on the all adsorbents was spontaneous. Also, enthalpy and entropy of all the adsorbents were positive. The obtained results showed that all of the synthesized adsorbents had good stability after six consecutive runs.