Effect mechanism of biochar's zeta potential on farmland soil's cadmium immobilization

In situ passivation of heavy metals by biochar mainly focuses on the effect of biochar's pH, surface oxygen-containing functional groups (OCFGs), and ash content. In this paper, starting with the measurement of biochar's electrical properties under different pyrolysis atmospheres and temperatures, the changes in the zeta potential of biochar and the consequent effects on cadmium immobilization in soil are studied. The results show that the zeta potential of biochar from the pyrolysis of high temperature (800 °C) is higher than that of biochar at low temperatures, so its electronegativity is weaker than that of biochar at low temperatures, but the protective effect on wheat is stronger than that of biochar obtained at low temperatures. The zeta potential of biochar obtained under a CO₂ atmosphere was higher than that of biochar prepared under a N₂ atmosphere, so its protective effect on wheat was stronger than that of biochar under N₂. The reason is that biochar particles with a high zeta potential and weak electronegativity have higher cohesion and are better at in situ passivation of Cd in soils. Namely, biochar obtained at high pyrolysis temperatures (800 °C) and prepared under a CO₂ atmosphere has better effect on Cd immobilization.