Optimization and Mechanism of Cadmium Removal Using Biochar Obtained from Wetland Emergent Plant Zizania caduciflora

Wetland emergent plants are considered a good source of biochar materials, which have proven to be an effective alternative to activated carbon for water treatment and soil amendment purposes. In this work, physicochemical properties of biochar made from the root, stem, and leaf of Zizania caduciflora (ZCR, ZCS, and ZCL) under the same pyrolysis condition were compared to evaluate their ability on adsorbing cadmium (Cd) in aqueous solutions. Batch experiments exhibited that the maximum adsorption capacities of Cd(II) by different organs were in the order of ZCL (59.8 mg/g) > ZCS (40.6 mg/g) > ZCR (36.2 mg/g), which have shown some advantages in relevant scientific researches. The adsorption trends at different temperatures were determined and fitted using Langmuir and Freundlich models. The best-fitted adsorption isotherm was found to be the Langmuir isotherm. Two different kinetic models, pseudo-first-order and pseudo-second-order, were used to fit the kinetics data, with the latter best describing the experimental data (R-2 > 0.989). Statistical analyses showed that ionic strength played an important role in controlling the sorption of Cd(II) onto the ZCR, ZCS, and ZCL. Concerning the mechanism, the adsorption of cadmium on biochar is recognized to be a chemical process as surface complexation and metal ions exchange, while it also exhibits some physisorption features due to the porous and defective structure.