Molecular-level understanding of malic acid retention mechanisms in ternary kaolinite-Fe(III)-malic acid systems: The importance of Fe speciation

International audienceMultivalent cation interaction and mineral association play a vital role in organic carbon (OC) stability in tropical soils. However, the retention mechanisms of OC in mineral-cation-OC systems remain largely unclear at the molecular level. Of all OC forms, low molecular weight organic acids (LMWOAs) are readily biodegradable. Given the general acidic conditions and dominance of kaolinite in tropical soils, we investigated the retention mechanisms of malic acid (MA) in kaolinite-Fe(III)-MA systems with various Fe/MA molar ratios at pH similar to 3.5 using Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. Our results indicated the formed ferrihydrite mainly contributed to the enhanced MA retention through adsorption and/or coprecipitation at high Fe/MA molar ratios in kaolinite-Fe(III)-MA systems relative to kaolinite-MA systems. With the decrease of Fe/MA molar ratios, ternary complexation of MA with kaolinite via a tetrahedral Fe bridge occurred in kaolinite-Fe(III)-MA systems. This study highlighted the significant impact of Fe speciation on MA retention mechanisms in kaolinite-Fe(III)-MA systems under acidic conditions, and clearly showed the impact of ferrihydrite-induced adsorption/coprecipitation and Fe-bridged ternary complexation on MA retention in ternary kaolinite-Fe(III)MA systems, which will enhance our understanding of the dynamics of MA and other LMWOAs in tropical soils