title: Retention Mechanisms of Citric Acid in Ternary Kaolinite-Fe(III)-Citrate Acid Systems Using Fe K-edge EXAFS and L3,2-edge XANES Spectroscopy.
abstract: 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 ~3.5 using Fe K-edge EXAFS and L3,2-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.
authors.affiliation: Department of Plant and Soil Sciences, Delaware Environmental Institute, University of Delaware, Newark, USA, 19716.
authors.forename: Jianjun
authors.initials: J
authors.lastname: Yang
authors.affiliation: Canadian Light Source Inc., University of Saskatchewan, Saskatoon, Canada, S7N 2V3.
authors.forename: Jian
authors.initials: J
authors.lastname: Wang
authors.affiliation: Department of Plant and Soil Sciences, Delaware Environmental Institute, University of Delaware, Newark, USA, 19716.
authors.forename: Weinan
authors.initials: W
authors.lastname: Pan
authors.affiliation: Canadian Light Source Inc., University of Saskatchewan, Saskatoon, Canada, S7N 2V3.
authors.forename: Tom
authors.initials: T
authors.lastname: Regier
authors.affiliation: Canadian Light Source Inc., University of Saskatchewan, Saskatoon, Canada, S7N 2V3.
authors.forename: Yongfeng
authors.initials: Y
authors.lastname: Hu
authors.affiliation: CNRS, Institute of Ecology and Environment Paris, IEES, UMR (CNRS-INRA-UPMC-UPEC-IRD), Thiverval-Grignon, France, 78850.
authors.forename: Cornelia
authors.initials: C
authors.lastname: Rumpel
authors.affiliation: Global Centre for Environmental Remediation (GCER), University of Newcastle, NSW, 2308, Australia.|Centre for Environmental Risk Assessment and Remediation (CERAR),University of South Australia, Mawson Lakes, Australia, SA 5095.
authors.forename: Nanthi
authors.initials: N
authors.lastname: Bolan
authors.affiliation: Department of Plant and Soil Sciences, Delaware Environmental Institute, University of Delaware, Newark, USA, 19716.
authors.forename: Donald
authors.initials: D
authors.lastname: Sparks