Kinetics and mechanism of copper release from selected agricultural calcareous soils of northern Iran

Abstract

Availability of soil copper (Cu) to plants is dependent on the release into the soil solution of Cu from the solid phases. To ascertain the pattern of Cu release and the soil characteristics affecting it, the kinetics of Cu release by diethylenetriaminepentaacetic acid (DTPA) from 14 calcareous soils of northern Iran was investigated using six kinetic models. The release pattern was generally characterised by an initial fast reaction followed by a slower reaction that continued up to 192 h. The kinetic data were best described by the power function equation as evidenced by relatively high values of the coefficient of determination (R2) and relatively low values of the standard error of the estimate. The Cu release pattern based on the parabolic diffusion equation suggested the possible involvement of two different mechanisms. Conformity of the shell progressive particle diffusion equation to kinetic data may indicate that the release rate is controlled by diffusion of Cu and/or DTPA molecules through a reacted soil particle or aggregate rather than diffusion through the film surrounding soil particles. Rate constants for the power function equation (a and b), the parabolic diffusion equation (k′p), and the initial release rate (ab) were closely associated with cation exchange capacity (CEC). From results reported herein, CEC seems to be the most important influencing factor in controlling Cu release from calcareous soils of northern Iran.