Mechanisms of soil humic acid adsorption onto montmorillonite and kaolinite

Hongfeng Chen; Luuk K Koopal; Juan Xiong; Marcelo Avena; Wenfeng Tan

doi:10.1016/j.jcis.2017.05.078

Mechanisms of soil humic acid adsorption onto montmorillonite and kaolinite

J Colloid Interface Sci. 2017 Oct 15:504:457-467. doi: 10.1016/j.jcis.2017.05.078. Epub 2017 May 24.

Authors

Hongfeng Chen¹, Luuk K Koopal², Juan Xiong¹, Marcelo Avena³, Wenfeng Tan⁴

Affiliations

¹ Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtze River), Ministry of Agriculture, College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, PR China.
² Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtze River), Ministry of Agriculture, College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, PR China; Physical Chemistry and Soft Matter, Wageningen University and Research, Stippeneng 4 (Helix), 6708 WE Wageningen, The Netherlands.
³ INQUISUR, Departamento de Química, Universidad Nacional del Sur (UNS)-CONICET, Av. Alem 1253, B8000CPB Bahía Blanca, Argentina.
⁴ Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtze River), Ministry of Agriculture, College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, PR China; State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences and the Ministry of Water Resources, Yangling, Shaanxi Province 712100, PR China. Electronic address: wenfeng.tan@hotmail.com.

PMID: 28600939
DOI: 10.1016/j.jcis.2017.05.078

Abstract

To explore the adsorption mechanisms of a soil humic acid (HA) on purified kaolinite and montmorillonite, a combination of adsorption measurements, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and isothermal titration calorimetry (ITC) was employed at pH 4.0, 6.0 and 8.0. The adsorption affinities and plateaus of HA on the two clays increased with decreasing pH, indicating the importance of electrostatic interaction. The effects were more significant for kaolinite than for montmorillonite. The substantial adsorption at pH 8.0 indicated hydrophobic interaction and/or H-bonding also played a role. The ATR-FTIR results at pH 8.0 showed that the Si-O groups located at basal faces of the two clays were involved in the adsorption process. For kaolinite, at pH 4.0 and 6.0, HA adsorption occurred via OH groups on the edge faces and basal octahedral faces (both positively charged), plus some adsorption at Si-O group. The exothermic molar adsorption enthalpy decreased relatively dramatically with adsorption up to adsorption values of 0.7μmol/g on montmorillonite and 1.0μmol/g on kaolinite, but the decrease was attenuated at higher adsorption. The high exothermic molar enthalpy of HA binding to the clays was ascribed to ligand exchange and electrostatic binding, which are enthalpy-driven. At high adsorption values, JGHA adsorption by hydrophobic attraction and H-bonding also occurs.

Keywords: Adsorption isotherm; Attenuated total reflectance Fourier transform infrared (ATR FTIR); Clay basal plate; Clay edge; Humic acid; Isothermal titration calorimetry (ITC); Kaolinite; Molar Gibbs energy/enthalpy/entropy of adsorption; Montmorillonite.