Comparative adsorption of tetracyclines on biochars and stevensite: Looking for for the most effective adsorbent

Tetracyclines are one of the most widely used class of veterinary and human antibiotics. The conventional treatment of wastewater based on activated sludge is not effective to remove antibiotics and their residues are still biologically active, which represents a problem in terms of bacterial resistance. The main objective of this work is to assess ability of stevensite and two biochars to adsorb three tetracycline antibiotics from water. Batch adsorption experiments were carried out to test the ability of these materials to adsorb tetracyclines. Then desorption experiments were performed to determine the adsorption strength on stevensite. In order to elucidate the adsorption mechanism of tetracyclines on stevensite, cation exchange analysis and spectroscopic analyses by IR and XRD were performed. The adsorption of tetracyclines on stevensite was tested on continuous system with water artificially contaminated. Finally, the designed filter was validated with tetracyclines spiked wastewater. The two biochars and stevensite were able to adsorb between 60 and 100% of the tetracyclines present in the batch system. Stevensite was the material with the highest tetracyclines removal capacity (around 100% at low concentrations of tetracyclines). Biochars showed less affinity for tetracyclines adsorption (70%). Tetracyclines desorption from stevensite reached values lower than 10% for low tetracyclines concentrations. The IR spectroscopy suggested that cation exchange is the main mechanism of tetracyclines adsorption on clay and also proved the role of amide and amine groups in this adsorption. The cation exchange mechanism was confirmed by displacement of Ca and Mg from stevensite. A continuous wastewater flow through a system composed by stevensite leaved this system with no tetracyclines, indicating water purification by tetracyclines adsorption in clay.