Carbonate accelerates while humic acid inhibits ferrihydrite transformation in soil: kinetics and mechanisms of As and Pb sequestration under fluctuating redox conditions

Understanding how common soil constituents regulate iron oxide transformation is essential for predicting the stability of co-occurring Pb and As under dynamic conditions. Here, a decoupling and comparative strategy was used to isolate carbonate-versus humic acid (HA)-driven controls on the transformation of Pb-bearing, As-bearing, and Pb-As co-bearing ferrihydrite across six well-defined systems. An evidence chain combining kinetic experiments, spectroscopic and mineralogical analyses, and anaerobic-aerobic soil incubation linked interfacial chemistry with contaminant fate. The results showed that the transformation pathway and products of ferrihydrite were primarily dictated by the mode of surface complexation, with carbonate and HA playing distinct roles. Carbonate ions associated with ferrihydrite via weak outer-sphere and bidentate inner-sphere complexation, accelerated dissolution-recrystallization, and directed transformation toward goethite. This process simultaneously promoted the re-immobilization of Pb and As via adsorption, lattice incorporation, and PbCO3 precipitation. In contrast, HA formed strong inner-sphere complexes; Pb further induced surface fractionation of phenolic-/carboxylic-rich HA components and promoted stable cyclic complexation, enhancing interfacial passivation and stabilizing Pb and As predominantly via edge-/surface-associated adsorption. Soil redox incubation corroborated net stabilization under fluctuating conditions, with available Pb decreasing 71% from 104 to 29.8 mg/kg and available As 79% from 61.0 to 12.6 mg/kg, significantly outperforming the control. We conclude that carbonate functions as a transformation promoter and re-immobilization facilitator, whereas HA acts as a transformation inhibitor and surface stabilizer. These findings provide valuable insights for predicting and managing risks of Pb and As in complex, redox-fluctuating environments.