Environmental insights from Langmuir adsorption site capacities

Adsorption by soil humic acids (HAs) regulates the bioavailability of nutrients and pollutants, and adsorption studies can lead to useful environmental models. Langmuir's model of reversible adsorption of a gas on a solid surface as it applies to adsorption of a dissolved substance B on insoluble adsorbent A to give insoluble state A·B is examined in terms of the measured site capacity vobsd (mmol solute/g adsorbent) at equilibrium concentration [B], adsorbent site capacity v at infinite concentration [B], and the maximum site capacity of the adsorbent, vmax. The relationship v = Smfvmax = S/〈Mw〉, where S is the number of solute entities per adsorbent site, mf is the mass fraction of adsorption sites and 〈Mw〉 is the adsorbent's mass-average molar mass, is discussed. This relationship is applied to analysis of (1) different adsorption sites on a solid, municipal compost-derived HA and on a proposed HA building block, (2) adsorption of herbicide acifluorfen on a solid HA and (3) adsorption of herbicides alachlor and metachlor on activated charcoals. Practical applications of the relationship are listed.

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► Langmuir model fit of aqueous solute adsorption on a solid humic acid (HA).
► Model generates distinct adsorbent site capacity parameters vobsd, v and vmax.
► Relates v to vmax and the adsorbent mass-average molar mass.
► Applied to adsorption on a proposed HA building block.
► Herbicides adsorption on activated charcoals interpreted.