EFFECT OF HUMIC ACID ON THE KINETICS AND MECHANISM OF COPPER ADSORPTION IN SOIL-SOLUTION SYSTEM

This paper presents observations made on the kinetics and mechanism of Cu partition in soil-solution system under humic and non-humic conditions at pH 3.72, 4.7, 6.85 and 10. It was noted that, equilibrium is reached after 10 hours, and Cu partition constant, KDM, increases with pH under both conditions. KDM values were higher under humic conditions and the presence of humic acid did not affect the attainment of equilibrium. Kinetics of Cu adsorption under both conditions followed pseudo 2nd order kinetic rate law. The initial adsorption rate values suggested that humic acid decreases the amount of Cu adsorbed as pH increase. Intra-particle diffusivity revealed that the adsorption goes through different mass transfer resistance phases as the soil surface change in response to pH under non-humic conditions. And in humic condition, adsorption follows film diffusion controlled rate-limiting mechanism at all pH. Adsorption of Cu goes through three different stages, and humic acid alters the mechanism of Cu transfer to the solid phase at each stage. Humic acid modifies the soil surface and reduce its adsorption capacity and introduce alternate adsorption mechanism for Cu.