Characterization of KOH modified biochars from different pyrolysis temperatures and enhanced adsorption of antibiotics

Poplar biochars from pyrolysis at temperatures of 300, 500 and 700 °C were modified with KOH and characterized by elemental analysis, Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), and nitrogen adsorption–desorption isotherm studies. Adsorption experiments were carried out on the modified biochars to remove tetracycline (TC) from aqueous solution. The results showed that KOH modification could increase or decrease TC adsorption onto biochars depending on the different pyrolysis temperatures. Maximum adsorption capacities (qe,m) of TC in modified biochar from a low pyrolysis temperature of 300 °C increased up to 21.17 mg g−1 relative to 4.30 mg g−1 in unmodified biochar of 300 °C (final TC concentrations were 8.83 and 25.70 mg L−1, respectively). In contrast, qe,m decreased from 7.37 and 11.63 mg L−1 to 4.97 and 7.13 mg L−1 in biochars from higher pyrolysis temperatures of 500 and 700 °C with and without modification, respectively, even with an increase in SBET. The adsorption ability of biochar can remain over a wider range of pH in modified biochar relative to unmodified biochar. Further analysis indicated that there was a strong linear regression relationship between qe,m and total functional oxygen groups using Bohem titration (n = 6, R2 = 0.84), whereas no significant relationship was observed between qe,m and SBET in this experiment. The result suggested that KOH modification of biochar from a low pyrolysis temperature can enhance TC adsorption and can be used over a wide range of pH, which may be a good choice for disposal of organic pollutants in aqueous solution.