Kinetics Of Reaction Of Heavy Metals In Variable Charge Soils Under Simulated Acid Rain

The kinetic characteristics of Cu²⁺ and Pb²⁺ adsorption in variable soil under simulated acid rain were studied by using flow-stirred method. The main results were summarized as follows:A kinetic reaction device called as flow-stirred reactor was designed to reasonably investigate reaction kinetics of ions with soils, which has more advantages than traditional batch and flow methods, The solutions in reaction chamber could be sufficiently mixed with soil and the rapid ion-exchange reactions could occur immediately. The problem of unstable flowing rate caused by the block age of clay particles had been solved and the effluent could be quantitatively and automatically collected. Thus a large quantity of data could be obtained.The kinetic characteristics of Cu²⁺ and Pb²⁺ reaction on red soil surface were investigated by using flow-stirred device. It showed that the processes of Cu²⁺ and Pb²⁺ adsorption in soils were divided into rapid and slow reactions. Cu²⁺ and Pb²⁺ adsorption fitted first-order kinetics. Well, indicating that the maximal adsorption amount remarkably decreased with the increase of solution acidity, Adsorption rate of Cu²⁺ and Pb²⁺ in fast reaction phase with the rise of temperature, pH value in effluent was notably lower than initial pH 4.3 and pH 5.5 in influent, respectively, while pH in effluent was over initial pH 3.8 and 3.3 in influent, respectively. It showed that the former released H⁺ and the latter consumed H⁺.The energy characteristics of the reaction kinetics of Cu²⁺ and Pb²⁺ showed that b value of parabolic diffusion could be used to explain the apparent diffusion rate. Net reaction activation energy and pseudo-thermodynamic parameters of Cu²⁺ and Pb²⁺ diffusion were calculated by using b value. E_b^* value increased with increase of solution acidity in influent. It reflected that the energy barrier to be overcome would rise up and the rate of diffusion could accordingly decrease. Enthalpy of activation (ΔH^*) had positive value indicating endothermic reaction. Entropy of activation (ΔS^*) had all negative value, which suggested that the system could arrange itself. Activation energy of H⁺ consumption was lower than that of H⁺ release. It could be beneficial to H⁺ adsorption, which led to slowly dissolve minerals which would be the controlling-ratestep.The adsorption of Cu²⁺ and Pb²⁺ was adequately described by an apparent first-order kinetics and parabolic diffusion and Elovich equations. The maximal amount of Cu²⁺ and Pb²⁺ adsorption fitted by the first-order equation and diffusion rate constant (b value) of Cu²⁺ and Pb²⁺ adsorption fitted by parabolic diffusion equation remarkably decreased with the increase of solution acidity. It could be speculated that the rate-controlling step in the adsorption of Cu²⁺ and Pb²⁺ was the diffusion of ions into the intraparticle space of solid phase. The pH values of effluent were close to the pH value of influent solution with the change of time. The pH values of effluent were lowers than that of pH4.3 and pH5.5 of influent and higher that of pH 3.8 and pH3.3. It showed that the former released H⁺ and the latter consumed H⁺. The apparent release amount of H⁺ could be fitted by the first-order and parabolic diffusion equations, The apparent consumption of H⁺ could be well fitted by the first-order and Elovich equations, and the parabolic diffusion could very well describe the kinetic data of pH 3.8 and pH3.3 of influent by according to correlative coefficient (R²) and standard error (SE).