Cd removal from aqueous solution using magnetic biochar derived from maize straw and its recycle

China is a large agricultural country whose output of crop straw is more than 800 million tons, but the effective resource utilization technology of straw is still lacking. Heavy metal pollution in water is also one of the major environmental problems at present. Adsorption methods are widely used in removing heavy metals from water environment due to the simple operation, no need to add a large amount of chemical agents, and low energy consumption. The preparation of biochar from maize straw provides a resource utilization approach for agricultural waste. Besides, maize straw biochar can be used as a new type of environmentally friendly material for the treatment and repair of polluted water. Based on the premise of resource utilization of crop straw wastes in farmland ecosystems, the objective of this study was to remove the heavy metal cadmium (Cd) from water using biochar and realize the recycling of biochar. The original maize straw biochar (MSB) and magnetic maize straw biochar (MMSB) that cracked at 500 ℃ were used as experimental materials for this purpose. Based on the characterization of the original biochar and the magnetic biochar, the adsorption and removal effects of the 2 kinds of biochar on Cd(II) in polluted water and the possibility of recycling biochar were explored. The results showed that the adsorption of Cd(II) by biochar increased first and then decreased with the increase of the pH value of the solution, and the adsorption of Cd(II) by MSB and MMSB reached the maximum at the pH value of 5. The maximum adsorption capacities were 27.52 and 33.45 mg/g, respectively. When MSB and MMSB were added at 1.4 and 0.8 g/L, the removal rates of Cd(II) were up to 85.15% and 95.48% respectively, and the difference was significant (P<0.05). Langmuir equation could better simulate the isothermal adsorption behavior, the fitting coefficients of MSB and MMSB were both over 0.970 0, and the maximum adsorption capacities at equilibrium reached 26.03 and 43.45 mg/g respectively, which approached the actual values. The kinetic data had a higher fitting degree with the second-order kinetics equation. The theoretical values of equilibrium adsorption capacity of MSB and MMSB were 13.42 and 24.31 mg/g respectively, which had only differences of 2.26% and 3.84% with the actual values respectively. The maximum desorption rates of MSB and MMSB were 41.10% and 29.88%, respectively, and the desorption rates of MMSB were significantly lower than that of MSB. The adsorption process of Cd(II) by biochar was a combination of various mechanisms, mainly chemical mechanism, and the adsorption mechanism mainly includes electrostatic adsorption, ion exchange, surface complexation and cation-π action. The enhancement of adsorption efficiency and fixation ability of magnetic biochar on Cd(II) might be related to its higher pH value, larger specific surface area, and more polar oxygen-containing functional groups. In addition, in the case of external magnetic fields, magnetic biochar could be recovered and reused by magnetic action. The research results are of great significance to promote the resource utilization of agricultural waste and the advancement of heavy metal purification technology in the water environment. © 2018, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.