Modeling on the Sorption Kinetics of Lead andCadmium onto Natural Sediments

Abstract

In this study, sorption kinetics of lead (Pb) and cadmium (Cd) onto coastal sediments were investigated at pH 5.5 using laboratory batch adsorbers. Four different models: one-site mass transfer model (OSMTM), pseudo-first-order kinetic model (PFOKM), pseudo-second-order kinetic model (PSOKM) and two compartment first-order kinetic model (TCFOKM) were used to analyze the sorption kinetics. As expected from the number of model parameters involved, the three-parameter TCFOKM was better than the two-parameter OSMTM, PFOKM and PSOKM in describing sorption kinetics of Pb and Cd onto sediments. Most sorption of Pb and Cd was rapidly completed within the first three hours, followed by slow sorption in the subsequent period of sorption. All models predicted that the sorbed amount at the apparent sorption (qe,s) equilibria increased as the CEC and surface area of the sediments increased, regardless of initial spiking concentration (C0) and heavy metal and the sediment type. The sorption rate constant (ks, hr-1) in OSMTM also increased as the CEC and BET surface area increased. The rate constant of pseudo-first-order sorption (kp1,s, hr-1) in PFOKM were not correlated with sediment characteristics. The results of PSOKM analysis showed that the rate constant of pseudo-secondorder sorption (kp2,s, g mmol-1 hr-1) and the initial sorption rate (vo,s, mg g-1 hr-1) were not correlated with sediment characteristics. The fast sorption fraction (f1,s) in TCFOKM increased as CEC and BET surface increased regardless of initial aqueous phase concentrations. The sorption rate constant of fast fraction (k1,s=100.1-101.0 hr-1) was much greater than that of slow sorption fraction (k2,s=10-2-10-4 hr-1) respectively.