A magnetic macro-porous biochar sphere as vehicle for the activation and removal of heavy metals from contaminated agricultural soil

Remediation of contaminated agricultural soils focused on fixing heavy metals in soils to reduce their bioavailability. However, with the influence of pH, redox potential, and microorganism in the soils, heavy metals were likely to be released from a fixed state. The study aims to investigate magnetic porous biochar spheres and their roles in in situ sorption and removal of heavy metal from the contaminated agricultural soil. The magnetic biochar spheres were prepared using alginate and FeCl3 through a one-step gelation to obtain alginate-Fe3+ gel and subsequently pyrolyzed. Results of Ultraviolet–visible spectrophotometer, X-ray diffraction and Fourier transform infrared spectrometer showed the decomposition of alginate and the formation of Fe3O4 and FeCl2 hydrates nanoparticles on the spheres when the spheres were pyrolyzed at 300 °C. Scanning electron microscope image showed that the CFe300 sphere has a macro-porous structure. The CFe300 sphere exhibited good adsorption capacity of hexavalent chromium from aqueous solution. The CFe300 sphere decreased the bioavailable and total cadmium (II) concentrations from 1.55 ± 0.08 to 0.32 ± 0.04 mg/kg and from 2.81 ± 0.02 to 1.39 ± 0.06 mg/kg, respectively. Moreover, the CFe300 sphere decreased bioavailable and total arsenate (V) concentrations from 1.26 ± 0.003 to 0.85 ± 0.01 mg/kg and from 60.23 ± 0.39 to 27.34 ± 0.94 mg/kg, respectively. Subsequently, addition of water into the soil induced the spheres to leave the soil and float on the surface of soil/water mixture. Thus, the magnetic biochar spheres can be much easier to separate from the soil using magnets.