The tetracycline is an amphoteric molecule with multiple ionizable functional groups and formed a series of species at different pH levels. In fact, TET is hydrophilic and the predominate TET species are TETH3+ at pH < 3.3, TETH20 at 3.3 < pH < 7.7, TETH− at 7.7 < pH < 9.7, and TET2− at pH > 9.7 (Huang et al., 2016a). The TET becomes more negatively charged as pH is increased, which may make its sorption behavior to biochar-based materials more complicated. The zeta potentials of the biochar-based
