Sustainable Activated Carbons from Agricultural Residues Dedicated to Antibiotic Removal by Adsorption

The objectives of this study are to convert at laboratory scale agricultural residues into activated carbons (AC) with specific properties, to characterize them and to test them in adsorption reactor for tetracycline removal, a common antibiotic. Two new ACs were produced by direct activation with steam from beet pulp (BP-H2O) and peanut hulls (PH-H2O) in environmental friendly conditions. BP-H2O and PH-H2O present carbon content ranged between 78% and 91%, similar BET surface areas (821 and 829 m(2).g(-1) respectively) and pH(PZC) values (9.8). Their porosities are different: PH-H2O is mainly microporous (84%) with 0.403 cm(3).g(-1) of total porous volume, whereas BP-H2O develops a mesoporous volume of 0.361 cm(3).g-(1) representing 50% of the total porous volume. Two other commercial granular AC carbons (GAC1 and GAC2) were also characterised and used for comparison. The adsorption study is conducted in batch reactors. Two parts can be observed from kinetic decay curves: a very fast concentration decrease during the first 12 h, followed by a slow adsorption. An optimal contact time of 120 h is also deduced from these curves. It is shown as well that adsorption decreases with an increase of pH, indicating that the form preferentially adsorbed is probably the zwitterion form of the tetracycline. From equilibrium isotherms data, two adsorption models have been used: Langmuir and Freundlich. Both of them lead to a very good description of the experimental data. Maximum adsorption capacities deduced from the Langmuir equation follow the sequence: GAC2 (817 mg.g(-1))>BP-H2O (288 mg.g(-1))>GAC1 (133 mg.g(-1))>PH-H2O (28 mg.g(-1)). In real spring waters spiked with TC (tetracyclines), adsorption isotherms show that the maximum adsorption capacity of BP-H2O is slightly increased to 309 mg.g(-1) while it is decreased by one third to 550 mg.g(-1) in the case of GAC2. This study demonstrates that the production of AC from agricultural residues, at lab-scale, is feasible and leads to genuine activated carbons with different intrinsic properties.