Kinetics of copper desorption from highly calcareous soils

Desorption of copper (Cu) is an important factor in determining Cu availability in calcareous soils. Kinetics of native and added Cu desorption by DTPA (diethylene-triaminepentaacetic-acid) from 15 highly calcareous soils of southern Iran were studied in a laboratory experiment. Our results showed that two constantrate, Elovich, simple Elovich, and parabolic-diffusion equations were the best-fitted equations among eight kinetic models used. The copper desorption pattern based on the parabolic-diffusion equation revealed that the rate of native Cu desorption was higher in the first 2h followed by a slower release rate, which suggests that two different mechanisms are involved. The trend may describe why the DTPA soil test has been considerably successful in predicting Cu availability in calcareous soils. Stepwise multiple regression equations indicated that CCE (calcium carbonate equivalent), CEC (cation exchange capacity), and clay content are the most important soil characteristics that predict the rate constants of the kinetic models. Mean extractant recovery percentage (ERP) of the soils was only 20%, which indicated that after 20 days, DTPA extracted only one-fifth of added Cu. Regression equations indicated that as soil OM (organic matter) content increased, the value of ERP decreased. From results reported herein it seems that CCE, CEC, and clay are the most important factors controlling Cu release from highly calcareous soils of southern Iran. However, the initial soil Cu desorption rate is probably controlled by CEC.