METAL-INDUCED SULFATE ADSORPTION BY SOILS: III. APPLICATION OF LANGMUIR EQUATIONS1

Abstract

The one- and two-surface Langmuir equations were evaluated in studies of the effect of metal type, valence, and concentration on SO42− adsorption by four diverse soils, two from Iowa (dominated by permanent charge) and one each from Chile and Costa Rica (two highly weathered soils with variable charge). The adsorption parameters, Xm and k, calculated using the "one-surface" Langmuir equation, showed that SO42− adsorption by the soils in the presence of trivalent metals was relatively higher than in the presence of mono- or divalent metals. When SO42− and metal were added at equivalent concentrations, the Xm values varied from 14.9 mmolckg−1 for Rathbun soil when Cs+ was the metal ion to 303 mmolckg−1 for Osorno soil when In3+ was the metal ion. The corresponding k values were 0.109 and 0.040, respectively. The Xm values for SO42− adsorption by the soils when the metals were added at a constant concentration (12.0 mmolcL−1) were, in general, less than those when SO42− and its metal ion were added at equivalent concentrations. At the constant initial metal concentration, the Xm values ranged from 10.0 mmolckg−1 for Rathbun soil when K+ or Cs+ was the metal ion to 93.7 mmolckg−1 for Osorno soil when In3+ was the metal ion. The corresponding k values were 0.163 (K+), 0.224 (Cs+), and 1.28 (In3+). The calculated parameters (Xml, k1, Xm2, and k2) of the "two-surface" Langmuir equation for the SO42− adsorption data of the two Iowa soils with predominant permanent charge showed that this equation gave one line instead of two when SO42− and metals were added at equivalent concentrations, suggesting that, in the range of SO42− concentration used, one type of adsorption site or mechanism was involved. The two-surface Langmuir equation adequately described SO42− adsorption data when the initial metal concentration was constant. The estimated adsorption maximum (Xm1 + Xm2) values by the "two-surface" Langmuir equation were always greater than those estimated by the "one-surface" Langmuir equation when the metal ions were added at a constant concentration. For all the SO42− adsorption data, two bonding constants were found, suggesting the presence of two different adsorption sites or different mechanisms, each with a different bonding energy. The results provide evidence that SO42− adsorption by soils is caused by more than one mechanism and that the associated metal ion significantly affects SO42− adsorption, regardless of the mechanism involved.