pH-Dependent preferential adsorption and stability of humic substances on goethite: The dual role of aromatic and aliphatic moieties.

Preferential adsorption occurs during interaction between humic substances (HS) and metal (hydr)oxides, however, the preferential adsorption and thermal stability mechanism remains poorly understood. Here we show that solution pH can modify the configuration of humic acid (HA) and fulvic acid (FA), and thereby altering their fractionation and thermal stability in relation to goethite. At pH 5-7, particles with a molar mass of 3.5-15 kDa were preferentially adsorbed for both fulvic acid (FA) and humic acid (HA). These particles enriched in aromatic moieties and carboxylic groups (-COOH), which enhanced the stability of HS by facilitating the formation of inner-sphere complexes. Conversely, at pH levels exceeding 9.2, large particles (> 50 kDa) and those rich in aliphatic moieties were preferentially adsorbed for both HA and FA, attributed to the increased significance of hydrophobic interactions. However, this stabilization effect on HS was limited under these conditions. FA particles exhibit a lower degree of chemical heterogeneity compared to HA particles. For FA, the variations in aromatic structures (-COOH) and aliphatic moieties were coupled, following a consistent trend in preferential adsorption as pH changed. In contrast, HA demonstrated an opposite pH dependency, with aromatic (-COOH) and aliphatic moieties displaying divergent patterns of preferential adsorption. These innovative discoveries clarify the behavior of organic matter in various environmental conditions and its interactions with minerals surfaces, thereby establishing a critical theoretical foundation for understanding the sequestration of soil organic carbon, emphasizing the dual roles of aromatic and aliphatic moieties in the preferential adsorption and stability of humic substances on goethite.