Binding mechanisms and phosphorus adsorption performance of soil mineral-BDOM complexes

Dissolved organic matter from biochar (BDOM) inevitably integrates into the geochemical cycle throughout the remediation process. However, the binding mechanisms of BDOM onto various soil minerals, as well as the efficacy of phosphorus removal, remain inadequately understood. In this study, the formation and mechanism of montmorillonite-BDOM, ferrihydrite-BDOM, and goethite-BDOM complexes (referred to as MB, FB, and GB, respectively) were investigated and their structural morphology was analyzed using characterization methods such as BET, SEM, XRD, and FT-IR. Results indicated that FB had the largest specific surface area (SSA, 225.63 m 2 /g) and its adsorption was primarily governed by ligand exchange (58.51–76.47 %), whereas van der Waals forces predominated in the adsorption by montmorillonite (60.69–67.22 %). Batch tests showed that the adsorption properties of phosphorus were influenced by SSA and porosity of the complexes, which were FB > ferrihydrite > goethite > GB > MB > montmorillonite. The R 2 of pseudo-second-order kinetics (0.802–0.994) and Freundlich adsorption isotherm (0.970–0.980) for minerals and complexes to phosphorus fit better compared to pseudo-first-order kinetics (0.632–0.979) and Langmuir isotherm (0.798–0.966). The effect of environmental conditions on adsorption was also studied, showing that a slightly lower pH favored minerals and complexes other than montmorillonite and MB, with citric acid promoting FB adsorption by 18.68 % (16.34 mg/g) over the control, whereas oxalic acid was more favorable for MB adsorption, with a 41.16 % increase (3.60 mg/g). This study underscores the importance of the mineral adsorption-binding BDOM process and provides valuable insights into the transport and transformation of phosphorus in the natural environment.