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Adsorption from Solution An Experiment To Illustrate the Langmuir Adsorption Isotherm Davld G. Duff, Sheina M. C. Ross, and D. Huw Vaughan Paisley College of Technology, High Street. Paisley, Renfrewshire PA1 2BE. Scotland The study of adsorption is important for an understanding of such processes as heterogeneous catalysis, chromatographic analysis, and the dyeing of textiles. However, there are few simple and easily performed experiments available to illustrate the quantitative aspects of adsorption.' The purpose of the present paper is to describe a simple experiment to illustrate the ahs&ption process from solution and its auantitative treatment using the L a n m u i r adsorption isotherm. Also, the constants of the ~ a n i m u i radsorption isotherm, for example, the limiting amount adsorbed by the adsorbent, may be evaluated, and this in turn allows an approximate evaluation of its specific surface area. Furthermore. the use of sand as an adsorbent in this experiment has an important potential environmental application. Effluent from plants in the dyeing industry contains highly colored species as well as appreciable quantities of materials with a biological oxygen demand (BOD) and suspended solids. Water pollution regulations now require treatment of these wastes prior to discharge. Whereas hiological treatment processes are generally efficient for BOD and suspended solids removal, they are largely ineffective for removing color from the waste. The discharge of highly colored waste is not only aesthetically displeasing hut also hinders light penetration and may in consequence upset hiological processes in a river. In addition the dyes may be toxic to some oreanisms and hence cause the destruction of aquatic commu~ities.The most commonly used adsorbent for treatment of textile effluents is activated charcoal, but this is relatively expensive. However, sand is a cheap and commerciallv available material that mav have potential in the treatment of effluents. Experlrnenlal Procedure The adsorbent used in the experiment was ordinary hrown "Builder's sand", which was first sieved to remove particles of diameter 2 1 mm. The sand was then washed several times with copious amounts of water and finally dried in an oven at 100 "C.
Recommended Volumes of SoMlon* Experiment No.
Vol. of solution A (mL)
Vol. of solution B(mL)
1
25 40 60 80 100 150
-
2 3 4 5
6 7
-
n
25 A"
A rommercially available sample of the chloride salt of the basic dye Malachite Green ICI 42000) was used to prepare two stock dve solutions in 25% methanol-water IVA'I. -~ . . . The first stock skution (solution A) had anabsorbance of -2.0 and the second (solution B) an absorbance of -1.0. when of the dye (610 nm) and in a 1.0-em cell. measured a t the in The value for the molar absorptivity for the dye a t ,A, this solution was found to be 6.0 X 10' dm3 mol-' cm-', and this gives a value for the concentration of solution A of 3.3 X ~
~~
in,,
. . --. -.-... .
Accurately weighed samples of -1.0 a of the sand were placed in eight grate 250-mL conicalflasks, and the volumes of the two solutions indicated in the table were added to the respective flasks. The flasks were then stoppered with rubber stoppers wrapped in metal foil and shaken \.igorously ona mechanicalshaker for -2.3 h. At theend of this time the absorbances of the solutions in the flasks were measured a t I Dunlcz, 8. L. J. Chem. Educ. 1961,38,357-358; Dandy, A. J. J. Chem. Educ. 1964, 41, 47; Hanson, J. C.; Stafford, F. E. J. Chem. Educ. 1965,42,88-91; Brina, R.; De Banisti, A. J. Chem. Educ. 1987, 64,175-176.
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Figwe 1. Adsorption isotherm fa Malachite (+em on brown sand; x = grams of dye adswbed by 1kg of sand. C = equilibrium concernration of dye. (g L-'. 0 = recommended ~olutions,X = other solutions. the ,A, of the dye. Some of the solutions, especially those of smaller total volume, were slightly cloudy a t the end of the 2.5 h and were centrifuged prior to measuring their ahsorbance. I t was found that this method whereby a constant weight of sand and a variable total volume of dye solution is used gives reproducible results. In addition the experimental conditions shown in the table are found to give final absorhance values that can he measured conveniently using a l-cm cell, while the final equilibrium conditions cover a wide range, thus presenting the student with a convincing set of results with which to test the Langmuir adsorption isotherm. Treatment of Results
Many solids are ahle to adsorb solutes from solution, and the relationship between the amount adsorbed and the concentration of solute (adsorbate) in solution a t equilihrium is often described bv the Lanemuir adsomtion isotherm. which is derived in.most advanced phgsicaichemistry text: books. For adsorption of solutes from solution the isotherm may be written
where x represents the amount of solute adsorbed (in moles or grams) per unit mass of adsorbent (usually 1 kg) and c is the concentration of soluce in the solution that is in eouilibrium with the adsorhent. x, is the limiting amount of adsorbate that can he taken up by unit mass of adsorbent, and K is a constant. Both K and x, are constant for the particular system being studied and for agiven temperature. A plot of x against c shows that initially x increases as c increases, hut then x tends to a limit, x,, as c becomes large. The applicability of the Langmuir adsorption isotherm to a particular system is usually tested by rearranging eq 1in the form:
Thus a plot of l/x aeainst l/c should he linear with a eradient llx,K and interce;t on the l l x axis of llx,. The initial numher of moles, or erams. of solute in solution is determined from the volume and initial concentration of the stock solution. Measurement of the absorhance of the solution when equilibrium is reached yields the equilibrium concentration c and the final numher of moles, or grams, of solute in solution. The difference between the initial and 818
Journal of Chemical Education
Figure 2. Langmuir plot of liw data in Figure 1. final numher of moles. or erams. of solute vields the amount of solute adsorbed for'thz particular weiiht of adsorhent. A plot of x aeainst c for Malachite Green adsorbed on brown sand is s&wn in Figure 1, which includes, in addition to the eight recommended solutions. several others of differ. ent initial conditions to illustrate that the system appears to conform to the Langmuir adsorption isotherm over a wide range of concentrations. Figure 2 shows the linear plot of the data in Figure 1; points corresponding to very low and very high values of c are omitted from this latter plot as these are generally inherently inaccurate. Calculation of the Speclllc Surtace Area of the Solid
I t is a relatively simple extension of the above experiment to determine the approximate specific area, S, of the sdsorbent using the value of x, obtained from the linear plot in Figure 2. Thus: where N is Avoeadro's numher and "a" is the area of the dve molecule. In this case the value of "a" for Malachite Green can be taken as equal to 2.25 nm', which is the value calculated for the flatwise adsor~tionof Crystal Violet (CI 42555).a closelv related mo~ecule.~ Popie13 has pointed out that in experiments of this type care must be taken in interpreting the results to allow for adsorption of the solvent, aggregation of the dye, and the porosity of the solid adsorhent. In this particular case the experimental conditions are such that a dilute methanolic solution of dye is used with a relatively nonporous solid so the complications arising from these three factors can be neglected. We have found that this simple experiment illustrating the Langmuir adsorption isotherm can he extended to other solutes and adsorhents. Thus we have obtained satisfactory results using other basic dyes such as Crystal Violet and other adsorbents such as white "Morar" sand and sawdust (pine wood).
Giles. C. H. In Adsorption from Solution at the SolidLiquid hterface; Parfiit, G. D.; Rochester, C. H., Eds.: Academic: 1983: Chapter 7; Giles. C. H.: Agnihotri, V. G.: Mclver, N. J. Colloid interface Sci. 1975, 50. 24-31; Giles. C. H.; D'Silva. A. P. Trans. Faraday Soc. 1969,65,2516-2528. Popiel, W. J. J. Chem. Educ. 1966, 43, 415-418.
