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Applications and Misapplications of the Langmuir Equation to Soil Adsorption Phenomena¹

ABSTRACT

When the development of the Langmuir adsorption equation is critically examined, it is evident that the equation soil scientists have been using [C/x/m = C/k + I/kb, where C is concentration of adsorbate, x/m is the amount adsorbed per unit weight adsorbent, k is the adsorption maximum, and b is a constant] is in the wrong form. This error is of no great importance when the equation is merely used to obtain a calculated adsorption maximum for comparison to other adsorbent properties. However, it does become important when attempts are made to understand adsorption dynamics and bonding strengths. The commonly reported curvilinear nature of the C/x/m vs. C plots is simply the result of not considering the effect of desorbed ions in the equilibrium solution, rather than being due to multiple adsorption mechanisms. When the equation is corrected by considering desorbed ions, the isotherm becomes linear. In addition, the constant, b, of the Langmuir equation is not simply related to the bonding energy of the adsorbed ion, but to the ratio of adsorbed and desorbed ion bonding energies.

¹ Joint publication of the New Hampshire Agric. Exp. St. (Scientific Contribution no. 856) and the Pennsylvania Agric. Exp. St. (Journal Pap. no. 5238).

² Associate Professor of Soil Chemistry, Inst. of Natural and Environ. Resour., Univ. of New Hampshire, Durham, NH 03824, and Professor of Soil Chemistry, Dep. of Agronomy, Pennsylvania State Univ., University Park, PA 16802.

Received for publication February 18, 1977. Accepted for publication June 24, 1977.

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