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How do clay minerals and humic acids in soil hold positive ions?

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chemical structure of cation exchanger — ion-exchange reaction Figure 1: Chemical structure of cation exchanger. The exchangeable ions are marked ⁺. The whole structure is permeated by solvent molecules, usually water (not shown).

Clay minerals and humic acids in soil retain positive ions (cations) primarily through electrostatic attraction to negatively charged sites on their surfaces.123

Clay Minerals

Clay minerals possess a net negative charge due to deficiencies in their positive or negative charge balance, often resulting from ionic substitutions within their crystal structures. These minerals adsorb cations and retain them in an exchangeable state around the outside of their structural units. Additionally, some clay minerals, such as allophane and imogolite, feature surface hydroxyl groups that can dissociate, creating negatively charged sites that strongly attract heavy metal ions. These ions are held by electrostatic forces and can be exchanged for other ions in the soil solution.21

Humic Acids

Humic acids, which are components of soil humus, contain acidic functional groups that dissociate in water. This dissociation leaves the molecules with a net negative charge. These negatively charged sites interact with and sequester metal cations, effectively removing them from further interaction with the surrounding environment. This process is a critical aspect of nutrient cycling and the detoxification of hazardous compounds in soil.43