Kinetic and equilibrium studies on the zinc adsorption–desorption characteristics of some promising biochars in aqueous solutions

This study investigates the adsorption and desorption of zinc by different biochars from aqueous solutions. Biohars from two feedstocks, including apple wood (WB) and rice straw (RB), were prepared at two pyrolysis temperatures, 300 and 600 °C (WB300, WB600, RB300 and RB600, respectively). Adsorption studies were performed at different initial pH (not adjusted and adjusted pH at 4 and 6) and ionic strength levels (0.01, 0.03, 0.1 M KNO 3 ) in the batch mode. Desorption experiments were conducted under all the above-mentioned conditions but without pH adjustment, at five separate stages. The adsorption of Zn by biochars reached the near steady state within 24 hours with the best fit to Elovich model for WB600, RB300 and RB600 while the pseudo-second-order model provided a better fit to WB300. Maximum sorption was 4.3, 16.4, 17.9 and 33.3 mg g -1 , on average, for WB300, WB600, RB300 and RB600, respectively. The removal efficiency of Zn by RB600 was higher (85%) than other biochars, and the adsorption data of RB600 were fitted better by the Langmuir model than by Freundlich and Dubinin–Radushkevich models. Zn adsorption increasing the initial pH of solutions from 4 to 6 increased the Zn adsorption in RB600, RB300 and WB600; however, the maximum sorption in WB300 was detected at pH 4. The rise in solution ionic strength from 0.01 M to 0.1 M dropped the Zn adsorption capacity in all the studied biochars. Findings suggested that rice straw-derived biochars showed a better performance than woody biochars in Zn sorption and retention and can serve as economical and efficient absorbents for Zn removal from aqueous solutions. Graphical abstract