Adsorption of tetracycline from water by magnetic biochar from pine bark

Abstract

The occurrence of tetracycline (TC) in water systems ends up in water cycle which leads to unnecessary consumption of the antibiotic by humans and animals. This leads to the weakening of the immune system of the human body. This calls for the removal of the antibiotic from water. This work was performed to evaluate low cost adsorbent, pine bark biochar (PBB), to remove tetracycline from aqueous solution via adsorption pathway. Pine bark biochar was prepared using pine bark from forest waste. The pyrolysis of the raw material was performed in tubular furnace at 600 oC in nitrogen atmosphere. The PBB, hence obtained, was modified via co-precipitation of aqueous ferric and ferrous ion solution to obtain magnetic pine bark biochar (M-PBB). Batch adsorption experiments were conducted to examine the adsorption removal of TC by PBB and M-PBB on the impact of pH, dosage, and temperature. The aqueous concentrations of TC were determined by UV-VIS spectrophotometer. The adsorbents were characterized by SEM/EDX, TGA and pHpzc. The adsorption mechanism was evaluated by fitting widely used Langmuir, Freundlich, Temkin and Dubinin-Radushkevich (D-R) isotherms. Also, the experimental data are analyzed by kinetics models (pseudo first order, pseudo second order, intra-particle diffusion and Elovich) and thermodynamics (Gibs free energy, enthalpy and entropy). Characterization of the materials showed that iron oxide was prevalent in M-PBB. SEM images exhibited coating of iron oxide on the surface of biochar. EDX supported the findings of SEM micrographs. TGA approximated ~27% iron oxide content in the M-PBB. When compared to PBB, M-PBB exhibited higher adsorption capacity for tetracycline (from aqueous solutions). At similar experimental conditions (i.e. tetracycline initial concentration: 25 mg/L, pH 6, and 16 hrs contact time), the equilibrium adsorption capacity (qe) was found to be 2.1 mg/g and 10.5 mg/g for PBB and of M-PBB, respectively. The maximum adsorption capacity (qm) of M-PBB was 15.3 mg/g. The highest adsorption was observed at pH 4-6 owing to ionization of tetracycline with respect to pH. A high correlation co-efficient (R² ≈0.9) of Freundlich isotherm postulated multilayer sorption of tetracycline on M-PBB at pH 6 and 9. The kinetic studies showed the pseudo-second-order was more suitable for indicating adsorption of TC molecules on the surface to be the rate limiting step in the process. Thermodynamic analysis was discovered that adsorption process is favorable, spontaneous and endothermic at studied temperature (293 – 323 K). M-PBB could be a having potential for removal of TC from water as a waste, low cost and easy to removable adsorbent.