Study On Adsorption And Release Of Pb And Cd By Sediment In Hengyang Section Of Xiangjiang River

The adsorption characteristics of Pb2+and Cd2+on sediment collected from theHengyang section of Xiang River were studied through experiments. The affectingfactors such as mud to water ratio, overlying water pH, temperature, the initialconcentration of Pb2+and Cd2+and sediment particle size on adsorption wereanalyzed. And the sediment adsorption kinetics and thermodynamics of Pb2+and Cd2+were discussed. In addition, by using artificial pollution sediment, the releaseaffecting factors such as mud to water ratio, overlying water pH, temperature, humicacid content were analyzed. And the he release kinetics of the process also wasdiscussed.The adsorption quantity of Pb2+and Cd2+increased with elevating pH, elevatingtemperature, decreasing sediment particle size, increasing the initial concentration ofPb2+and Cd2+, decreasing mud to water ratio, but the adsorption rate was reduced inthe last two cases. The adsorption process can be divided into a rapid membranediffusion phase at particle surface in the first30min and a late slow particle diffusionprocess at particle internal. The adsorption kinetics all fitted well to thepseudo-second order equation and the adsorption isotherms of Pb2+and Cd2+meet theLangmuir and Freundlich equation well, but Langmuir equation was better for Pb2+and it’s opposite for Cd2+. The thermodynamic analysis demonstrated that theadsorption process is an endothermic reaction and higher temperature can enhance theadsorption reaction as H＞0; the adsorption process is spontaneous and relies onphysical and chemical adsorption together as G＜0and between-27.20to-10.47kJ/mol; the adsorption entropy increases as S＞0.The release quantity and release rate of Pb and Cd from pollution sedimentdecreased with elevating pH, elevating temperature, increasing mud to water ratio.Humic acid had a complex effect on the release, generally was a certain role inpromoting. The release process can be divided into a rapid membrane diffusion phaseat particle surface in the first30min and a late slow particle diffusion process atparticle internal. The release kinetics all fitted well to the pseudo-second order equation. then slowly for the inside of a particle diffusion process; Whole process isvery good with quasi level2dynamic model to describe, so are the Elovich equationand double constant rate equation, which illustrates release reaction is a complicatedheterogeneous spread process.