Application of a Modified Langmuir Isotherm to Phosphate Sorption by some Acid Soils

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Abstract
Phosphate sorption by 11 acid soils was conducted at 298 K and at a pH of about 6.0 to determine how well the sorption can be described by a modified Langmuir isotherm. The modified Langmuir isotherm, which takes into consideration the electrostatic interaction among the phosphate surface species, described the sorption data reasonably well. The average affinity constant for the soils was 1.82 × 104 L mol−1, and the energy derived from the interaction component averaged 1.42 RT, where R is a gas constant and T is absolute temperature. The affinity constant was significantly affected by the amorphous Al and Fe contents of the soils, but no significant relationship between the interaction energy and the clay, organic matter, and amorphous Al and Fe contents of the soils was observed. Although the classical Langmuir isotherm slightly overestimated or underestimated the affinity constant, depending on which linearized form was used, it considerably underestimated the capacity of the soils to sorb phosphate particularly at higher phosphate concentrations. A simplified equation was developed based on the modified Langmuir isotherm, assuming a constant 1 RT contribution from the interaction component. The equation was as simple as that for the classical Langmuir isotherm but provided a better estimate of the sorption capacity of phosphate by soils than did the latter one.