Retention mechanisms of citric acid in ternary kaolinite-Fe(III)-citrate acid systems using Fe K-edge EXAFS and L-3,2-edge XANES spectroscopy

Jianjun Yang (1) , Jian Wang (2, 3) , Weinan Pan (1) , Tom Regier (2, 3) , Yongfeng Hu (2, 3) , Cornelia Rumpel (4) , Nanthi Bolan (5, 6) , Donald Sparks (1)

1 University of Delaware [Newark]
2 U of S - University of Saskatchewan [Saskatoon, Canada]
3 Canadian Light Source
4 iEES - Institut d'écologie et des sciences de l'environnement de Paris
5 UoN - University of Newcastle
6 University of South Australia

Organic carbon (OC) stability in tropical soils is strongly interlinked with multivalent cation interaction and mineral association. Low molecular weight organic acids (LMWOAs) represent the readily biodegradable OC. Therefore, investigating retention mechanisms of LMWOAs in mineral-cation-LMWOAs systems is critical to understanding soil C cycling. Given the general acidic conditions and dominance of kaolinite in tropical soils, we investigated the retention mechanisms of citric acid (CA) in kaolinite-Fe(III)-CA systems with various Fe/CA molar ratios at pH similar to 3.5 using Fe K-edge EXAFS and L-3,L-2-edge XANES techniques. With Fe/CA molar ratios >2, the formed ferrihydrite mainly contributed to CA retention through adsorption and/or coprecipitation. With Fe/CA molar ratios from 2 to 0.5, ternary complexation of CA to kaolinite via a five-coordinated Fe(III) bridge retained higher CA than ferrihydrite-induced adsorption and/or coprecipitation. With Fe/CA molar ratios <= 0.5, kaolinite-Fe(III)-citrate complexation preferentially occurred, but less CA was retained than via outer-sphere kaolinite-CA complexation. This study highlighted the significant impact of varied Fe/CA molar ratios on CA retention mechanisms in kaolinite-Fe(III)-CA systems under acidic conditions, and clearly showed the important contribution of Fe-bridged ternary complexation on CA retention. These findings will enhance our understanding of the dynamics of CA and other LMWOAs in tropical soils.

Domaines: Sciences du Vivant [q-bio]; Sciences de l'environnement

Jianjun Yang, Jian Wang, Weinan Pan, Tom Regier, Yongfeng Hu, et al.. Retention mechanisms of citric acid in ternary kaolinite-Fe(III)-citrate acid systems using Fe K-edge EXAFS and L-3,2-edge XANES spectroscopy. Scientific Reports, 2016, 6, pp.1-9. ⟨10.1038/srep26127⟩. ⟨hal-02634418⟩