Study of Cadmium Desorption Kinetics from Riverine Sediments

Abstract

In natural streams, the majority of heavy metal ions are generally associated ‎with ‎sediment particles. Under some environmental conditions, these metal ions may release from ‎the ‎sediment particles.‎ In such conditions, the desorption rate of heavy metals is very important for decision-makers of ‎water quality assessment. In this study, the effect of cadmium desorption ‎from the river bed ‎sediments has been ‎experimentally investigated. Artificially contaminated sediments were used ‎for performing batch desorption ‎experiments. The experiments were conducted by adding 1 gr of contaminated ‎sediment (D50 = 0.53 mm) with a known concentration and shaking until observing a ‎roughly ‎constant cadmium concentration in the solution. It was concluded that the cadmium ions were strongly bond to the river bed sediment; meanwhile, at the ‎equilibrium time, up to about 7 to 29 percent of cadmium ions were ‎released from the‎ artificially contaminated sediments. The experiments were followed by ‎two agitation rates of 100 and 200 rpm. It was revealed that by increasing the flow turbulence, the amount ‎of desorbed cadmium is slightly increased. Besides, the desorption kinetics was evaluated using eight models of Zero-, first-, second-, third-order, ‎parabolic diffusion, double parabolic diffusion, two constant rate, and simple Elovich. The results of the evaluation showed that simple Elovich (with R2 = 0.991), double parabolic diffusion (with R2 = 0.9882), two constant rate (with R2 = 0.983) and parabolic diffusion models (with R2 = 0.846) have respectively the best performance in calculation of Cd desorption rate from the sediments.