Mechanism of negative surface charge formation on biochar and its effect on the fixation of soil Cd

The studies of the mechanism of Cd fixation by biochar have mainly focused on the pore size, pH, and oxygen-containing functional groups, and few researches have paid close attention to the effect of the negative charge in biochar surface. In this paper, biochar was produced in the CO₂ atmosphere at different pyrolysis temperatures, and the influence of the pyrolysis temperature on biochar surface charge were explored. The cause of the negative charge on the biochar surface has been analysed, and the optimal preparing temperature for the biochar with the best effect of cadmium immobilization from soil has been found. The results show that with the increasing temperature from 300 to 700 °C, the negative surface charge on biochar surface gradually decreases, while the fixed amount of Cd increases. The factors affecting the surface charge of biochar are ash content, pH, oxygen-containing functional groups, polar groups, and hydrogen bonds. Among them, the pH, oxygen-containing functional groups and polar groups have positive effects on the surface negative charge, whereas the hydrogen bond has a negative effect. The determinant of surface charge is the hydroxyl group, the content of hydroxyl group decreases as increasing temperature, resulting in a decrease in surface negative charge.