Characteristics of Modified Biochars and Their Immobilization Effect on Cu and Cd in Polluted Farmland Soil Around Smelter

Heavy metals in farmland soil are one of the most hazardous pollutants in the environment, owing to their universality and irreversibility. Modified biochar has been widely used in the adsorption and immobilization of heavy metals in soil, and its applicability is mainly determined by the types of heavy metals, pollution levels, and soil environmental conditions. Soil pollution is gradually becoming more complex and diversified, and heavy metal pollutants mostly occur in the form of compound pollution. However, most studies have focused on single heavy metal pollutant or the addition of heavy metal to soil. This study used rice straw as a raw material to prepare biochar, and modified it with K3PO4, KMnO4, and NaOH. The physicochemical and structural characteristics of the modified biochars were detected using a BET accelerated surface area and porosimetry system, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and the biochars were then analyzed for the availability and forms of Cd and Cu in soils contaminated with heavy metals in the mining area. The results showed that the surface roughness of the modified biochar increased to different degrees with increases in specific surface area and pore volume, with the NaOH modified biochar showing the most significant increases from 4.96 m2·g-1 to 60.79 m2·g-1, and from 0.02 cm3·g-1 to 0.12 cm3·g-1, respectively. The pore diameter changed in the opposite direction. The absorption peaks of the functional groups of the modified biochar were all changed, with K3PO4 modified biochar exhibiting the greatest degree of change. The addition of biochar significantly improved the soil pH value (P<0.05), and the pH value of the soil treated with K3PO4 modified biochar exhibited the largest increase. With an application of 20.5% K3PO4 modified biochar, the availability of Cu and Cd in the soil was significantly reduced, by 75.44% and 67.70%, respectively. The immobilization efficiency of Cu was much higher than that of Cd. The best immobilization efficiency of Cu and Cd in soil was achieved with K3PO4 modified biochar. With an addition of 2% K3PO4 modified biochar, the immobilization efficiency of Cu and Cd was 61.06% and 4.12%, respectively. In summary, K3PO4 modified biochar had a better immobilization effect on both Cu and Cd in compound contaminated soil.