Kinetic Study of Adsorption Processes in Solution An Undergraduate Physical Chemistry Experiment Julio Casado, Francisco Salvador, and Santiago R i n c h Universldad de Salamanca, Departamento de Quimica Fisica, E 37008-Salamanca. Spain Since their discovery in the serond half of the 18th century rhe analvriral and industrial avulicatinnsoiadsorntion have been widely employed, some b i them being extraordinarily relevant; chromatography, techniques based on ion exchange, and heterogeneous catalysis are important examples. One consequence of this has been the rebirth of surface chemistry such that to the classical aspects of this field, covered in books such as those of Adamson ( I ) or Adam (21, have been added research works which were possible thanks to the use of new working techniques: Electron Spectroscopy for Chemical Analysis (ESCA), Auger Electron Spectroscopy (AES), Low Energy Electron Diffraction (LEED), etc., all of which provide hitherto unavailable information a t the molecular level. However, despite the fact that the adsorption phenomenon owes its particular fundamentals to thermodvnamics. and kinetics, there are very few, if any, books o; papers dealing with the practical aspects of phvsical chemistrv that include kinetic-type experiments onthis phenomenonapart from the classical study of adsorption equilibrium (adsorption isotherms) ( 3 , 4 ) . Notwithstanding this, the growing importance of environmental chemistry, in general, and of ecokinetics (a term proposed to refer to the environmental applications of chemical kinetics) in oarticular.. suenests a nromisine future for studies used as thk basis for the tertiar; metho& of the treatment of environmental media such as water and air (5). Of such tertiary methods, the most promising for the e l i k nation of dissolved oreanic matter is that of adsorption onto the activated support such as activated carbon (6). The present work proposes a very simple kinetic method for the monitoring of adsorption processes which may be organized as a practical laboratory class and may be of use in research into chemical kinetics. Apart from the simplicity of the method, the authors also believe that it presents clearcut advantages over the static methods generally used in adsorption studies; for example, the possibility of working with a large number of kinetic data taken over an extended time range during the process studied, which is not easy to achieve when using static techniques. u-
Theoretical Foundations of the Model In general a physicochemical process of adsorption may be represented as
where A = adsorbate in solution; S = free active sites of the surface of the adwrbel~t:A - S = occupted active sites, and k . and k d are the ratoroettirientsof the adsorption and deinrvtion processes, respectively. One very simple kinetic equation that can be used to fit the experimental data in order to calculate the rate coefficient and reaction order is --=
k,,,[Ajm[S]~
dt
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(2)
where -(d[A])ldt = the disappearance rate of the adsorbate from the bulk of the solution, k,, = experimental rate coef, are the reaction orders with respect to ficient, and w and 9 the adsorbate and adsorbent. The model may be further simplified if an excess working concentration of adsorbent is emnloved. Under such condi~~-~~~~ tions, the equilibrium (1)may be Eonsidered as shifted to the right ~
ha
A+S+A-S
(3)
and kinetic eqn. (2) becomes
where
If the volume of the solution remains constant, [S]may be considered as proportional t o the mass of the adsorbent (M), such that the previous equation becomes
Description ot Method The process proposed as the object of study in this work is the adsorption of a dye, crystal violet (CV) onto activated carbon (Merck, grain size 2.5-mm diameter), previously washed in boiling water and dried at 150 'C. The kinetic monitoring of the reaction is carried out spectrophotometrically, measuring the dye concentration a t 590 nm with a conventional colorimeter. Apparatus The kinetic data are ohtained hv averv,sim~le . flow svstem. The renrtim flask, submerged in n thmnorulrred hnth of 1 I, rs equrpped wrth a xtrrrer (a small external motor wrth a Teflm rod placed in the glass rude whrch passes thnmgh the lrd of the flask: the stirring blade is surrounded by a protective mesh in order to avoid the damaging of the adsorbent). A peristaltic pump pushes the solution (throughaglass tube of 1.5-mm diameter) from the reaction flaskthrough the colorimeter fitted with a flow cuvette (10-mm optical pathway), after which it returns the solution to the reaction flask. The access ofthe dve to C is achrcved through a hypodermic the cuvetre of I ~ cdorrmeter needle uhich reache, the very buttom of the
