Effects of biochar-supported nano-hydroxyapatite on cadmium availability and pepper growth in contaminated soils

The remediation of cadmium (Cd)-contaminated soil using biochar (BC) derived from agricultural and forestry waste has gained significant attention due to its ability to convert active soil Cd components to stable forms, reduce bioavailability, decrease Cd absorption by pepper plants, and enhance the nutritional benefits of soil. However, there is limited research on the effects of different passivating agents on soil Cd during various growth stages (seedling, flowering, and maturity) of peppers. In this study, we investigated the cyclic changes in soil physicochemical properties, Cd chemical forms, and their effects on pepper growth by applying different biochar-supported nano-hydroxyapatite. Our results revealed a decreasing trend in the physical and chemical indicators of soil during the flowering stage, following an initial peak. Notably, in the mature stage, the application of nBC3 at an 11 % mass ratio significantly reduced soil Cd content by 57.6 % and fixed it by 77 %. This treatment also increased soil Cd by 48.1 % compared with the control (CK, without any treatment) and reduced its accumulation in the pepper plant by 36.6 %. pH, organic matter, and phosphorus were identified as the main factors influencing Cd fixation in the soil. These findings showed that the in situ application of nBC3 composite material throughout the entire cultivation cycle effectively remediated Cd-contaminated soil and enhanced the quality of agricultural products. This study provides valuable insights into the effects of passivating agents on soil Cd dynamics and offers a theoretical basis for practical remediation strategies.