Phosphorus-modified biochar from salvia miltiorrhiza dregs: synthesis, characterization, and dual-functional synergy for heavy metal immobilization and soil fertility augmentation

Abstract Phosphorus-modified biochar (3K-BC) was prepared from Salvia miltiorrhiza dregs via phosphate modification and applied for lead (Pb) and cadmium (Cd) removal from aqueous solutions and soil. The successful incorporation of orthophosphate into the biochar was confirmed by 31 P NMR and other analytical techniques. Solution-based experiments showed that 3K-BC followed pseudo-second-order kinetics and best fitted the Langmuir isotherm model, indicating multiple chemical adsorption processes. The maximum adsorption capacities for Pb and Cd were 361.82 and 123.03 mg g −1 , respectively. The adsorption mechanism involved physical adsorption, precipitation, complexation with oxygen-containing functional groups, and cation exchange. Soil experiments demonstrated that 3K-BC significantly reduced heavy metal (HM) bioavailability, with greater reductions at higher application rates. Speciation analysis revealed a decrease in the acid-soluble fraction of Pb and Cd, while their stable, residual forms increased, effectively reducing metal mobility. Further confirmation was obtained through pot experiments, which showed that the application of 3K-BC increased the yield of Ligusticum chuanxiong by 61% and promoted its growth. In addition, the concentration of ligustilide increased from 24.24 to 29.63 mg g −1 . Under the condition of HM pollution, the total effective components of Ligusticum chuanxiong in the experimental group increased by 22.1%. This study provides a simple and effective strategy for remediating Pb- and Cd-contaminated water and soil while simultaneously recycling agricultural waste, thereby serving a dual purpose. Highlights Phosphorus-modified biochar (3K-BC) adsorbs 361.82 mg g −1 Pb and 123.03 mg g −1 Cd. 3K-BC cuts Pb/Cd bioavailability by 93%, stabilizing metals and reducing ecological risk. 3K-BC boosts Ligusticum chuanxiong yield by 61% while limiting heavy metal uptake. Converting herbal residues into biochar supports pollution control and soil fertility. Graphical Abstract