Lead (Pb) solubility is commonly limited by dissolution–precipitation reactions of secondary mineral phases in contaminated soils and water. In the research described here, Pb solubility and free Pb²⁺ ion activities were measured following the precipitation of Pb minerals from aqueous solutions containing sulfate or carbonate in a 1:5 mole ratio in the absence and presence of phosphate over the pH range 4.0–9.0. Using X-ray diffraction and Fourier-transform infrared spectroscopic analysis, we identified anglesite formed in sulfate-containing solutions at low pH. At higher pH, Pb carbonate and carbonate-sulfate minerals, hydrocerussite and leadhillite, were formed in preference to anglesite. Precipitates formed in the Pb-carbonate systems over the pH range of 6 to 9 were composed of cerussite and hydrocerussite, with the latter favored only at the highest pH investigated. The addition of phosphate into the Pb-sulfate and Pb-carbonate systems resulted in the precipitation of Pb₃(PO₄)₂ and structurally related pyromorphite minerals and prevented Pb sulfate and carbonate mineral formation. Phosphate increased the efficiency of Pb removal from solution and decreased free Pb²⁺ ion activity, causing over 99.9% of Pb to be precipitated. Free Pb²⁺ ion activities measured using the ion-selective electrode revealed lower values than predicted from thermodynamic constants, indicating that the precipitated minerals may have lower K_SP values than generally reported in thermodynamic databases. Conversely, dissolved Pb was frequently greater than predicted based on a speciation model using accepted thermodynamic constants for Pb ion-pair formation in solution. The tendency of the thermodynamic models to underestimate Pb solubility while overestimating free Pb²⁺ activity in these systems, at least in the higher pH range, indicates that soluble Pb ion-pair formation constants and K_SP values need correction in the models.

Biochar addition to soils is a promising strategy for mitigating cadmium (Cd) mobilization and carbon emission, but how biochar-to-soil interaction enabling a synergy between these two goals at redox heterointerface remains unclear. Herein, we conducted three types of paddy soil incubations with phosphorus/iron-doped biochar to explore the underlying factors and processes controlling Cd and carbon transformation under redox conditions. Upon flooding, lower soil redox potential resulted in soluble and extractable Cd transformed into Fe/Mn-bound fraction, coinciding with elevated CO₂ and CH₄ fluxes. During subsequent drainage, soil pH decrease caused associated Cd transformed back into exchangeable fraction, coupled with cumulative CO₂ dropped. Both porewater and sequential extraction results revealed that the remobilization of Cd and carbon during redox fluctuations is largely related to Fe/Mn (hydr)oxide-induced effects. Microscopic and spectroscopic techniques determined that the organo-mineral (e.g., aliphatic C and Fe–O/Si–O groups) interactions are of crucial importance in influencing Cd and carbon distribution patterns on soil microaggregates. Further sequencing and correlation analyses vertified that this biochar facilitated simultaneous Cd and carbon retention via altering soil biogeochemistry, especially redox-controlled abiotic and microbial transformation processes. Overall, these findings shed light on the interactive effects of Cd and carbon mitigation with biochar amendment for redox paddy environments. Graphical Abstract