Effect of ageing on desorption of lead and cadmium from sediments: Kinetics and desorption-resistance

In this study, the effect of ageing on sorption isotherms and desorption behaviors (kinetics and desorption-resistance) of lead (Pb) and cadmium (Cd) in natural sediments was investigated. Several sorption models such as Freundlich, Langmuir and Dubinin- Radushkevich models were fitted to the sorption data. The sorption affinity and sorption capacity of the heavy metals onto sediments increased with cation exchange capacity (CEC) and BET surface area (ABET). The sorption affinity of Pb was higher than that of Cd in all sediments at all ageing time scales (1, 30 and 100 d). Four differentmodels: the one-sitemass transfer model (OSMTM), the pseudofirst- order kinetic model (PFOKM), the pseudo-second-order kinetic model (PSOKM) and the two compartment first-order kinetic model (TCFOKM) were used to analyze desorption kinetics. All models predicted that the sorbed amount (qe,s ) at the apparent desorption equilibria increased as the cation exchange capacity (CEC) and BET surface area (A _BET) of the sediments increased. However, the fast desorption fractions ( f1,d ) decreased with increasing CEC, ABET and organic carbon content ( f∞). Sequential desorption experiments were conducted to investigate the effect of ageing on desorption-resistance and a biphasic desorption model was fitted to the data. The biphasic desorption model parameters indicate that the maximum capacity of desorption resistant fraction (q^irr_max) of Pb was higher than that of Cd and the q ^irr_max increased with CEC, A_BET and ageing time for both Pb and Cd. Sequential extraction analysis revealed that the transformation of heavy metals in more mobile fraction into less mobile fractions was the main reason for the increase in desorption-resistance.