Removal of Cr (VI) by Biochar Derived from Six Kinds of Garden Wastes: Isotherms and Kinetics

Garden waste is one of the main components of urban solid waste which affects the urban environment. In this study, garden waste of Morus alba L. (SS), Ulmus pumila L. (BY), Salix matsudana Koidz (LS), Populus tomentosa (YS), Sophora japonica Linn (GH) and Platycladus orientalis (L.) Franco (CB) was pyrolyzed at 300 °C, 500 °C, 700 °C to obtain different types of biochar, coded as SSB300, SSB500, SSB700, BYB300, etc., which were tested for their Cr (VI) adsorption capacity. The results demonstrated that the removal efficiency of Cr by biochar pyrolyzed from multiple raw materials at different temperatures was variable, and the pH had a great influence on the adsorption capacity and removal efficiency. GHB700 had the best removal efficiency (89.44%) at a pH of 2 of the solution containing Cr (VI). The pseudo second-order kinetics model showed that Cr (VI) adsorption by biochar was chemisorption. The Langmuir model showed that the adsorption capacity of SSB300 was the largest (51.39 mg·g(−1)), BYB500 was 40.91 mg·g(−1), GHB700, CBB700, LSB700, YSB700 were 36.85 mg·g(−1), 36.54 mg·g(−1), 34.53 mg·g(−1) and 32.66 mg·g(−1), respectively. This research, for the first time, used a variety of garden wastes to prepare biochar, and explored the corresponding raw material and pyrolysis temperature for the treatment of Cr (VI). It is hoped to provide a theoretical basis for the research and utilization of garden wastes and the production and application of biochar.