Adsorption-Desorption Characteristics of Cadmium in Variable Charge Soils

Abstract

Cadmium (Cd) has received considerable attention because of its association with various human health problems. The behavior of adsorption-desorption of Cd at contaminated levels in two variable charge soils were investigated. The red soil (RAR) developed on the Arenaceous rock (clayey, mixed siliceous thermic typic Dystrochrept) adsorbed more Cd^{2 +} than the red soil (REQ) derived from the Quaternary red earths (clayey, kaolinitic thermic plinthite Aquult). The characteristics of Cd adsorption could be described by the Freundlich equation (r ² = 0.997 and 0.989, respectively, for the RAR and REQ) and the simple Langmuir adsorption equation (r ² = 0.985 and 0.977, respectively, for the RAR and REQ). The maximum adsorption values (X _m ) that were obtained from the simple Langmuir model were 36.23 mmol Cd^{2 +} kg^{− 1} soil and 31.15 mmol Cd^{2 +} kg^{− 1} soil, respectively for the RAR and REQ. Adsorption of Cd^{2 +} decreased soil pH by 1.28 unit for the RAR soil and 1.23 unit for the REQ soil at the highest loading. The distribution coefficient (k _d ) of Cd in the soil decreased exponentially with increasing Cd^{2 +} loading. The adsorption of cadmium in the two variable charge soils was characterized by a rapid process that lasted approximately 15 min, followed by a slower but longer period. 85.5% and 79.4% of the added Cd were adsorbed within two hours by the RAR and REQ soil, respectively. More Cd^{2 +} was adsorbed at 10^ˆC than at 25^ˆC or 40^ˆC. After five successive desorptions with 0.01 mol L⁻¹ NaNO₃ solution, 53.3% of the total adsorbed Cd^{2 +} in the RAR soil was desorbed and the corresponding value of the REQ soil was 46.5%, indicating that the RAR soil had a lower affinity for Cd^{2 +} than the REQ soil at the same Cd^{2 +} loading.

Keywords:

• Adsorption-desorption characteristics
• Cadmium
• Soil pH
• Temperature effect
• Variable charges soils