XPS and two-dimensional FTIR correlation analysis on the binding characteristics of humic acid onto kaolinite surface

• Published in: The Science of the total environment 2020-07, Vol.724, p.138154-138154, Article 138154
• Main Authors: Chen, Hongfeng, Li, Qi, Wang, Mingxia, Ji, Daobin, Tan, Wenfeng
• Format: Article
• Language: English
• Subjects: 2D-FTIR-CoS; Binding mechanism; Humic acid; Kaolinite; XPS
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2020.138154

The stabilization and preservation of soil organic matter have been attributed to the strong reactive sites of mineral surfaces that cause physical isolation and chemical stabilization due to the organic-mineral interface. However, much of the micro-scale knowledge about interactions between organic ligands and minerals largely remains at the qualitative level, and neglects the heterogeneity of functional groups of organic matter. Here, we report the use of molecular-scale technologies of two-dimensional FTIR Correlation Spectroscopy (2D-FTIR-CoS) and X-ray Photoelectron Spectroscopy (XPS) to directly measure the binding processes of humic acid (JGHA) groups onto kaolinite surface. The spectroscopy results showed that the carboxylate groups, aliphatic OH and aromatic structure participate in the binding of JGHA on kaolinite surface. The carboxylic and phenolic hydroxyl interact with kaolinite surface through the interfacial COAl/Si bonds. Kaolinite prefers to adsorb C-groups at pH 4.0 and O-groups at pH 8.0. The interaction of COO– group at 1566 cm−1 of JGHA leads to the formation of inner-sphere complex first and then outer-sphere complex with increasing contact time. The interaction of COOH group at 1261 cm−1 with the AlOH2+ of kaolinite was could be ascribed to ligand exchange and/or electrostatic attraction, whose contribution was evaluated to be 13.90%, 7.65% and 0% at pH 4.0, 6.0 and 8.0, respectively. These results of molecular binding provide quantitative mechanistic insights into organic-mineral interactions and expound the effect of functional groups of HA on binding mechanisms, and thus bring important clues for better understanding the mobility and transformation of land‑carbon including mineral-bound carbon. [Display omitted] •2D-FTIR-CoS and XPS were used to characterize the complexation of JGHA-kaolinite.•The COOH interact with AlOH2+ to form inner-sphere and then outer-sphere complex with contact time.•Kaolinite prefers to adsorb C-groups at low pH and O-groups at high pH.•The contribution of binding mechanisms to form JGHA-kaolinite complex is quantified.