Adsorption Characteristics of Cadmium on Particle-Sized Aggregates from Black Soil
A batch method was used to investigate the thermodynamics and kinetics of cadmium adsorption on different particle-sized aggregates derived from black soil. The results showed that the Cd2 * capacity on different particle-sized fractions increased with increasing concentration in the adsorption equilibrium, which decreased in the order of clay > silt > bulk soil > coarse sand > fine sand. The adsorption isotherm of Cd2* on different organic-mineral aggregates could significantly conform to the Langmuir and Freundlich equations, while linear equations were not feasible or suitable. When the size of the particle increased, its sorption amount and intensity decreased. With regard to the contents of the fractions in the soils, the contribution of each fraction decreased in the order of silt > fine sand > clay > coarse sand. The amount of Cd2* adsorbed by clay,silt and fine sand exceeded 99% . The results indicated that the adsorption of Cd2* to the aggregates could be divided into two stages: a fast reaction in the first 20 minutes,and a slow reaction later. The amount of capacity by the fast reaction exceeded 90% of the maximum adsorption capacity. The adsorption process for Cd2 * on the four fractions was found to follow pseudo second-order adsorption kinetics, which indicated that the adsorption is mainly a chemical process, and the adsorption speed decreased with increase of particle size.
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