Langmuir Equations for Sr, Sulfate and Surfactant Adsorptions.

As the Langmuir equation is the basic theoretical adsorption equation, the application indicates physical interpretation for adsorption. The equation agrees with Sr adsorption on kaolinite under NaCl solution condition. The influence of competing ions, Na and H, and the potential at the adsorption site are supposed to be included in the constant, K.

Sulfate adsorbs on an allophanic andisol which has both positive and negative charges. The measured values can be fitted with the two step Langmuir equation.

  θ = θ₁K₁C/(1+K₁C) + θ₂K₂C/(1+K₂C)

    θ₁ +θ₂= 1

where θ is the adsorption ratio at the adsorbate concentration, C, θ₁ and θ₂ are the maximum adsorption ratio at the site 1 (stronger adsorption site) and at the site 2 (weaker adsorption site), K₁ and K₂ are the adsorption constants at the site 1 and at the site 2. This indicates there are stronger adsorption sites and weaker adsorption sites.

An anionic surfactant adsorbs in highly humic soil which has only negative charge by hydrophobic interaction. The adsorption decreases with decreasing ionic strength and increasing pH because the magnitude of potential induced by the negative charge increases. The relation can be explained with the modified Langmuir equation which includes adsorption site potential, ψ, in the equation.

  θ = KCexp(-eψ/kT)/{1+KCexp(-eψ/kT)}

The trends of the calculated adsorption potential and the zeta potential agree well.