黄棕壤不同粒级组分对镉的吸附动力学与热力学研究

采用一次平衡法研究了黄棕壤不同粒级组分(粘粒≤2 μm、粉粒2～20 μm、细砂粒20～200 μm、粗砂粒200～2000 μm)对镉的吸附动力学与热力学,并采用拉格朗日假一级动力学方程、假二级动力学方程、颗粒内扩散模型对试验数据进行拟合.结果表明,2种温度下各粒级组分对镉的吸附均可分为快反应和慢反应2个阶段,0～15 min内为快反应阶段,吸附量达到饱和吸附量的95%以上,此后为慢反应阶段;随着温度由25℃升高到45℃,各组分对镉的饱和吸附量增加了4.86%～25.3%;各组分对镉的吸附动力学符合拉格朗日假二级动力学方程,吸附过程以化学吸附为主;二级动力学吸附速率常数表明,随着各组分粒级增大,吸附速率降低;在试验温度范围内随着温度升高,吸附速率加快;吸附过程的限速步骤为颗粒间扩散;各粒级组分对镉的吸附为吸热反应,反应能自发进行.

Kinetics and thermodynamics of Cd (II) adsorption onto particle-sized fractions of yellow-brown soil

A batch method was used to investigate the kinetics and thermodynamics of cadmium adsorption onto different particle-sized fractions (2 000 - 200 microm, 200 - 20 microm, 20 - 2 microm and 2 - 0 microm, which were named coarse sand, fine sand, silt and clay respectively according to international system) derived from yellow brown soil by using a novel technology combined with wet sieving, sedimentation-siphoning and centrifugation method. The results showed that the reaction of adsorption can be divided into two types: a fast reaction in the first 15 min and a slow reaction in the later reaction course. The amounts of Cd2+ adsorbed by the fast reaction exceed 95% of the adsorption capacity. As the temperature increased from 25 degrees C to 45 degrees C, the adsorption capacities of the four fractions increase by 4.86%-25.3%. First-order rate equation and pseudo second-order rate equations were applied to express adsorption kinetics. Adsorption processes for Cd2+ onto the four fractions were found to follow pseudo-second order type adsorption kinetics. The pseudo second-order rate constants exhibited that adsorption speed to reach equilibrium decreased with the increase of particle size. Intra-particle diffusion might be the major rate-limiting step. Thermodynamic parameters including deltaH0, deltaS0 and deltaG0 were also calculated from graphical interpretation of the experimental data. Positive deltaH0 values indicated the adsorption processes to be endothermic. Negative deltaG0 values implied that adsorption reaction was a spontaneous process.