The Research on Characteristic Curves and Changes in Gibbs Free Energy in Liquid/Solid Adsorption System Between Vermiculite and Pb2+ Ion
|School||Central South University of Forestry Science and Technology|
|Keywords||vermiculite Pb2+ water rinse acid rinse characteristic curves Gibbsfree energy adsorption capacity critical limit of ion adsorption density|
Experiments of both equilibrium and kinetic adsorptions of Pb2+, Cd2+and Zn2+ions from aqueous solutions onto water rinsed vermiculite samples were conducted to investigate the relationship between reaction components in liquid/solid ion adsorption systems. The changes in the Gibb’s free energy△G and chemical potential△μ of the tested adsorption reactions were also analyzed with purpose to understand the nature of the ion adsorption process and to find solutions for determining the important parameters needed for adsorption system design in its pratical application. Main results obtained from this study are summarized as follows:(1) It was confirmed by the Pb2+ion adsorption tests that there are two potential adsorption limits subject to, respectively, the system adsorption capacity Bo and the initial ion concentration A0. The predictions of adsorption density in equilibrium adsorption and kinetic adsorption using the developed models agreeed well with the experimental data obtained from the tested samples, indicating that the presented models based on the principle of the four mutually related essential components can be applied to describe the adsorption of Pb2+ions onto water rinsed vermiculite.(2) The three adsorbate/adsorbent ratios, x (the equilibrium adsorption density, the ratio of ion adsorption quantity per unit volume Q to adsorbent concentration W), y (the ratio of initial adsorbate concentration A0to W), and z (the ratio of equilibrium ion concentration in liquid phase to W) were found to be related with unique values in the tested range. The plots of x-z, x/y and x/p versus y were determined as the characteristic curves for the examined Pb2+ion adsorption systems. The characteristic curves eliminated both the adsorbent and ion concentration effects and thus can be used for design of ion adsorption systems in practice.(3) The calculated values of the change in the Gibbs free energy△G and the change in the chemical potential△μ for both equilibrium and dynamic reactions were negative at all tested points, which confirms the spontaneous nature of the conducted adsorption reactions under the tested conditions. The absolute values of△G was found to be positively related to the initial quantities of the reactants (namely, A0and B0). In contrast the change in the chemical potential△u was found to be independent of both A0and B0and it had a unique value at a given y. Thus the U-y curve (the plot of the change in chemical potential of the adsorption reaction versus y) can also be reagared as a characteristic curve of the ion adsorption reaction. The nature of the presence of an upper limit of the U-y curve provided an effective way for determination of p values. It was observed in the tests that y=p at the upper limit of the U-y curve. The tested results gave support to the proposed hypothesis that the ion adsorption reaction is basically a process in which the ions move between the liquid and solid phase without essential changes in their chemical nature. In reactions where there were no formations of new substances, the change in the standard chemical potential of the reactions should be approximately zero and the equilibrium coefficient of the adsorption reaction K should be close to1. The adsorption quantities can thus be determined by the simplified four adsorption component model given K=1.(4) The water rinsed samples adsorbed significantly higher amount of the tested ions than the acid rinsed samples, which indicates that the water rinsed swelling vermiculite can be used as a buffer filler in a wetland system. The p value of the swelling vermiculite sample for Pb2+, Cd2+and Zn2+ions was found to be54,12and8.8mg·g-1, respectively, the xc value of that was found to be1.4,0.23and0.21mg·g-1and the optimal reaction time (tc) of that was determined as0.5h,2hand2h, respectively.