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.
Highlights
Binding structure of Low molecular weight organic acids (LMWOAs) in ternary kaolinite-cation-LMWOAs systems could be different compared to binary kaolinite- LMWOAs systems, deserving close attention
The kaolinite-Fe(III)-citric acid (CA) systems had maximum CA retention at an Fe/CA molar ratio of 10, the highest percentage (195.8%) of increase in CA in the kaolinite-Fe(III)-CA systems relative to the kaolinite-CA systems occurred at an Fe/CA molar ratio of 2 (Table 1)
Within the Fe/CA molar ratios ranging from 0.25 to 0.05, the enhanced amounts of CA retained in the kaolinite-Fe(III)-CA systems relative to the kaolinite-CA systems reached a maximum and displayed insignificant variation around 15.21~16.82 mg g−1 kaolinite
Summary
Binding structure of LMWOAs in ternary kaolinite-cation-LMWOAs systems could be different compared to binary kaolinite- LMWOAs systems, deserving close attention. Compared to humic acid or humus with relatively high aromatic-C23,24, LMWOAs were enriched with aliphaticand carboxylic-C, which probably resulted in the different retention capacity and binding structure of LMWOAs on minerals in ternary mineral-cation-LMWOAs systems. The EXAFS spectra of Al, as a light element, are difficult to measure using soft X-rays for sorption samples, primarily due to the relative low Al content This could account for Al EXAFS data existing only for pure compounds like Al oxides[34]. Based on these reasons, Fe(III) was chosen to investigate the retention and binding mechanisms of LMWOAs in ternary kaolinite-cation-LMWOAs systems. The objectives of this study are: 1) to compare the retention of CA between binary kaolinite-CA and ternary kaolinite-Fe(III)-CA systems; and 2) to reveal the retention mechanisms of CA and their relative contribution in ternary systems with various Fe/CA molar ratios
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