Concept model of the formation process of humic acid–kaolin complexes deduced by trichloroethylene sorption experiments and various characterizations

• Interactions between humic acid (HA) and kaolin during the formation of their complexes were confirmed.
• Ca2+ could enhance HA and kaolin self-aggregations and impact HA-kaolin interactions.
• HA would first patch onto charged kaolin surface sites and filled the pores, and subsequently form a multilayer loading on kaolin.

To explore the interactions between soil organic matter and minerals, humic acid (HA, as organic matter), kaolin (as a mineral component) and Ca2+ (as metal ions) were used to prepare HA-kaolin and Ca-HA-kaolin complexes. These complexes were used in trichloroethylene (TCE) sorption experiments and various characterizations. Interactions between HA and kaolin during the formation of their complexes were confirmed by the obvious differences between the Qe (experimental sorbed TCE) and Qe_p (predicted sorbed TCE) values of all detected samples. The partition coefficient kd obtained for the different samples indicated that both the organic content (fom) and Ca2+ could significantly impact the interactions. Based on experimental results and various characterizations, a concept model was developed. In the absence of Ca2+, HA molecules first patched onto charged sites of kaolin surfaces, filling the pores. Subsequently, as the HA content increased and the first HA layer reached saturation, an outer layer of HA began to form, compressing the inner HA layer. As HA loading continued, the second layer reached saturation, such that an outer-third layer began to form, compressing the inner layers. In the presence of Ca2+, which not only can promote kaolin self-aggregation but can also boost HA attachment to kaolin, HA molecules were first surrounded by kaolin. Subsequently, first and second layers formed (with inner layer compression) via the same process as described above in the absence of Ca2+, except that the second layer continued to load rather than reach saturation, within the investigated conditions, because of enhanced HA aggregation caused by Ca2+.

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