Adsorption–Desorption Behavior of Copper at Contaminated Levels in Red Soils from China

Adsorption–desorption of copper (Cu 2+ ) at contaminated levels in two red soils was investigated. The red soil derived from the Quaternary red earths (clayey, kaolinitic thermic plinthite Aquult) (REQ) adsorbed more Cu 2+ than the red soil developed on the Arenaceous rock (clayey, mixed siliceous thermic typic Dystrochrept) (RAR). The maximum adsorption values ( M A ) that are obtained from the simple Langmuir model were 25.90 and 20.17 mmol Cu 2+ kg −1 soil, respectively, for REQ and RAR. Adsorption of Cu 2+ decreased soil pH, by 0.8 unit for the REQ soil and 0.6 unit for the RAR soil at the highest loadings. The number of protons released per Cu 2+ adsorbed increased sigmoidally with increasing initial Cu 2+ concentration for the RAR soil, but the relationship was almost linear for the REQ soil. The RAR soil released about 2.57 moles of proton per mole of Cu 2+ adsorbed at the highest Cu 2+ loading and the corresponding value for the REQ soil was 1.12. The distribution coefficient ( K d ) decreased exponentially with increasing Cu 2+ loading. Most of the adsorbed Cu 2+ in the soils was readily desorbed in the NH 4 Ac. After five successive extractions with 1 mol L −1 NH 4 Ac (pH 5.0), 61 to 95% of the total adsorbed Cu 2+ in the RAR soil was desorbed and the corresponding value for the REQ soil was 85 to 92%, indicating that the RAR soil had a greater affinity for Cu 2+ than the REQ soil at low levels of adsorbed Cu 2+