Adsorption and Thermal Stability of Dissolved Organic Matter on Ca- and Mg-exchanged Montmorillonite: Implications for Persistence in Soils and Sediments

Dissolved organic matter (DOM) represents a vital and highly dynamic component of organic matter in the environment. Despite its significance, there is still limited knowledge regarding how different base cations and chemical conditions influence the preservation of complex natural DOM through adsorption. This study aimed to investigate the adsorption of natural DOM onto montmorillonite (Mt) by systematically exploring the effects of exchanging cations (Ca2+ and Mg2+), pH levels (4 and 7), and DOM molecular weight (< 10, 10–100, and > 100 kDa) on the mechanisms and thermal stability of adsorbed DOM. To this end, we conducted a series of adsorption experiments, complemented by spectroscopic and thermal analyses. Our results revealed that lower pH and higher DOM molecular weight enhanced adsorption, with affinity for carbohydrate-like compounds. Notably, changes in d001 spacing and peak broadening indicated intercalation of DOM into the interlayer space of Mt, which was positively correlated with the amount of DOM adsorbed. Conversely, increased adsorption was found to negatively correlate with thermal stability. This was attributed to the prevalence of weaker DOM-DOM interactions occurring at higher DOM weight and lower pH, owing to reduced repulsion among protonated organic molecules, in contrast to stronger electrostatic interactions near the Mt surface that appeared to dominate at lower DOM weight and higher pH conditions. Intriguingly, no clear preference for DOM adsorption on Ca- or Mg-exchanged Mt was observed, suggesting similar efficiencies of these cations in stabilizing DOM through cation-bridging. Our findings provide significant insights into the mechanisms involved in mineral-organic associations and have implications for understanding the persistence of DOM in soils and sediments.