Adsorption and desorption of Cu(II) and Cd(II) in the tropical soils during pedogenesis in the basalt from Hainan, China

Adsorption and desorption of Cu(II) and Cd(II) in the soils derived from basalts aged 10, 1,330, and 2,290 kiloyears (ky) in the tropical regions of Hainan, China were investigated. X-ray diffraction analysis indicated that the clay minerals were dominated by smectite, halloysite, and kaolinite mixed with montmorillonite in the soil derived from 10 ky-basalt and by kaolinite, halloysite, and gibbsite in the soils derived from 1,330 and 2,290 ky-basalt. Soil cation exchange capacity (CEC) decreased with the increase in basalt age, while the content of soil-free iron oxides decreased for such increase in basalt age. The amount of Cu(II) and Cd(II) adsorbed by the soil derived from 10 ky-basalt was much higher than that adsorbed by the other two soils. A similar trend was observed for the desorption of Cu(II) and Cd(II) pre-adsorbed to these soils in 1 M KNO3. The desorption rate of Cu(II) decreased with the increase in age of basalts. This indicated that the electrostatic adsorption of Cu(II) decreased with the increase in degree of soil development. Similarly, the specific adsorption of Cu(II) increased with increase in degree of soil development. The desorption rate of Cd(II) showed much higher than the Cu(II), which ranged from 87.5% to 92.6%. This indicated that Cd(II) was adsorbed mainly through the electrostatic attraction. Removal of soil organic matter leads to the decrease in Cu(II) adsorption and the increase in Cu(II) desorption. The overall effect of organic matter removal on the adsorption and desorption of Cd(II) and desorption rate of Cd(II) was smaller compared to Cu(II).