Colloidal Cd Mobilization during the Dissolution of Cd-Birnessite by Organic Matter: Role of Mineral Transformation and Organic Molecule Fractionation

Cadmium (Cd) immobilized on manganese oxides (MnOx) in soils can undergo remobilization through reductive dissolution by natural organic matter (OM). However, the dominant Cd species released in this process remain incompletely understood, particularly under different pH conditions. Herein, we investigated dynamic Cd speciation during the anaerobic reduction of Cd-bearing birnessite by Suwannee River Fulvic Acid (SRFA), revealing a progressive dominance of colloidal Cd throughout the experiment. This trend was amplified at higher pH, as evidenced by its ultimate contributions of 36.6%, 57.8%, and 77.1% to total dissolved Cd at pH 4.5, 6.0, and 8.0, respectively. Moreover, two distinct colloidal Cd fractions were identified by asymmetrical flow field-flow fractionation, extended X-ray absorption fine structure, and Fourier transform ion cyclotron resonance mass spectrometry. The first fraction (1 kDa to 50 nm) carried 27.7–45.6% dissolved Cd via complexation with SRFA derivatives, exhibiting a lower oxidation state, higher susceptibility to reoxidation, and increased Cd release risk. The second fraction (200–450 nm) carried 8.9–31.6% dissolved Cd and evolved from organic aggregates into MnOx-OM colloids. These composite colloids could enhance colloidal Cd stability and potential mobility, especially under pH 6.0 and 8.0, due to stable crystalline manganite/hausmannite formation and enriched oxidized OM. We highlight that the released colloidal Cd during MnOx reductive dissolution under varying pH could exhibit variable mobility, reactivity, and environmental risk.