Simultaneous enhancement of soil properties along with water-holding and restriction of Pb-Cd mobility in a soil-plant system by the addition of a phosphorus-modified biochar to the soil.

Soil quality deterioration and heavy metal contamination have greatly limited soil productivity in mining areas. As soil is a complex system with various properties and interactions, it is imperative to conduct a comprehensive investigation to understand the amendment's mechanisms at work in the soil in mining areas as well as effective ways to address its deteriorating quality. In this study, a potassium dihydrogen phosphate-modified maize straw-cow dung biochar (PBC) was applied as a soil amendment. Various physicochemical properties of the soil including organic matter, total nitrogen, available phosphorus, and pore characteristics were analyzed. This study also assessed soil-saturated water content and soil moisture characteristic curve. Lettuce biomass was measured and changes in various speciation of Pb and Cd in the soil, and the accumulation of Pb and Cd in lettuce were examined. Results showed that the addition of PBC increased soil organic matter, total nitrogen, and available phosphorus while reducing soil bulk density, it also increased soil porosity, saturated water content, and capillary water capacity. Soil structure analysis using CT scanning revealed that 3% PBC increased the macrospores volume fraction while 5% PBC made the pores more uniform. Lettuce biomass increased by 53.3%. 5% PBC resulted in a 56.79% and 38.30% reduction in Pb and a 44.56% and 16.60% reduction in Cd in roots and shoots of lettuce respectively. PBC facilitated the transformation of Pb and Cd from unstable fractions to stable fractions through complexation and precipitation. Overall, the addition of PBC effectively improved soil nutrients, porosity, and water-holding capacity, promoted plant growth, immobilized Pb and Cd, as well as reduced the bioavailability in contaminated-soil from mining areas. This study provides an effective strategy and a new perspective for the remediation of Pb-Cd-contaminated soils.

Sulfoaluminate cement-modified straw biochar as a soil amendment to inhibit Pb-Cd mobility in the soil-romaine lettuce system.

Biochar is widely used to remediate soil polluted by potentially toxic elements (PTEs), while the effect of a new type of biochar, obtained from modified cement material, on the mobility of Pb and Cd in the soil-plant system is still unknown. In this study, soils doped with sulfoaluminate cement modified biochar (SBC) were characterized using a series of approaches including FTIR, XRD, and XPS, and combined with pot experiments to explore its synergistic effects on the speciation transformation, accumulation, and mobility of both Pb and Cd in a soil-romaine lettuce system in heavily contaminated soils containing 500 mg·kg-1-Pb and 3 mg·kg-1-Cd. The results showed that SBC effectively immobilized Pb and Cd in the soil and that this was achieved through cation exchange, complexation, and gel encapsulation. Moreover, SBC also changed the soil physicochemical properties and indirectly affected the speciation transformation of Pb and Cd. FTIR and XRD analyses revealed that the groups such as -OH, -COOH, SO42-, and SiO32-introduced by SBC stimulated the conversion from the soluble to the residual state of Pb. XPS analysis indicated that, the deviation of the C-O-C, C-OOH, and O-CO peak and the increased in area suggested that organic groups in the SBC were engaged in the immobilization mechanism of Pb and Cd. The transformation of residual Cd in other extractable fractions might be due to either enhanced soil reducibility or competitive adsorption with Pb. In 5% SBC soil, Pb was reduced by 27.69% and 64.84%, and Cd was reduced by 20.45% and 35.87% for shoots and roots of romaine lettuce, respectively. SBC showed a significantly positive correlation with SOM, while SOM showed a highly significantly negative correlation with both Pb and Cd in the roots. In summary, SBC can be strongly recommended as a green amendment to remediate Pb-Cd contaminated soil and to inhibit the mobility to plant.