J . S o p o u š e k ú l o h a
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1. Interphase adsorption
Adsorption is a process where particles (sorbent) presented in the gas phase or
solution above the solid phase (adsorbent) accumulate on the interface as a result
of an existence of surface attraction forces. The total number of sorbent active sites on
the adsorbate surface is limited. The fraction of the active sites θ𝑖𝑖 occupied by sorbent 𝑖𝑖
is ranging from 0 to 1 and depends on the temperature, pressure and phase
composition above the adsorbent. At a constant temperature, the adsorption is
described by the so-called "adsorption isotherm". Adsorption plays an important role in
numerous chemical and physical processes. Examples include chemical reactions on
surfaces of catalysts or substance accumulation on the surfaces of porous materials.
1.a. Determination of parameters of adsorption isotherm in methylene blue –
water - activated carbon system
A model example of the physical sorption of the dissolved substance in liquid on
solid phase surface is adsorption of dyes on activated carbon by the action of Van
der Waals's attractive forces. The adsorption isotherm is in this case the dependence of
the amount of the adsorbed dye na on its equilibrium concentration c in the solution.
When adsorbing the methylene blue dye on activated carbon, the equilibrium between
carbon surface and solution is established after 24 hours. We can obtain values that will
allow getting the isotherm to be practically identical to the true equilibrium after 30
minutes of shaking if we choose the constant adsorbent weights.
The theoretical description of adsorption uses different adsorption models that leads to
mathematically different descriptions. The most famous are:
Langmuir isotherm:
( )c
c
nn aa
ω
ω
+
⋅=
1
.max, (1.1.)
where 𝑛𝑛a,max is the maximum amount of the adsorbed substance (θ = 1), ω is the socalled
adsorption parameter equal to the reciprocal value of concentration at which the
half degree of coverage (θ = 0.5) is reached, ie 𝑛𝑛a = 𝑛𝑛a,max /2 .
Freundlich isotherm:
( )n k ca
n
= ⋅
1
(1.2.)
where k and n are empirical parameters specific for different adsorption systems.
Temkin isotherm:
( )n k k ca = ⋅1 2ln (1.3.)
where k1 a k2 are empirical parameters of Temkin isotherm for adsorption system.
TASK: Measure the amount of methylene blue adsorbed on activated carbon at
different dye concentration in solution at constant temperature (ie the
experimental adsorption isotherm). Determine the parameters of Langmuir, Freundlich
and Temkin isotherms for this adsorption system. Obtain the best fit of the experimental
results using theoretical isotherms in one common graph. Decide what theoretical
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ú l o h a J . S o p o u š e k
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5.a-b
isotherm describes the best the sorption of methylene blue on activated carbon. Use the
sum of quadrates of deviation between experimental and fitted values as criterion what
isotherm fits the best the experiment.
LABORATORY AIDS AND CHEMICALS: Spectrophotometer, laboratory scales,
laboratory shaker, 6 flasks (250 cm3
), 6 funnels and funnel stand, 6 Erlenmayer
flasks, 1 graduated flask (50 cm3
), 1 graduated flask (25 cm3
), scale pipettes (1, 5, 10 a
25 cm3
), weighing boat, spoon, filter paper, activated carbon, stock solution 2 mg/cm3
methylene blue (CAS 122965-43-9).
INSTRUCTIONS: Pour up 6 flasks (250ml), each to final volume 50ml by 7.5; 8; 9;
10.5; 12; 14 cm3
of stock solution of methylene blue and pure water. Add the
weight 90 mg of the activated carbon into each flask (discuss the weight with the
superior). Close flasks with stoppers and shake for 30 min on the laboratory shaker to
obtain the adsorption equilibrium.
Use a dry filter for adsorbent separation. Remove the first portion of the filtrate (about
20 cm3
) until the filter paper is saturated with dye then collect only the remaining part of
the dye filtrate. Use the final part of the dye filtrate to spectrophotometrically
determination the dye concentration.
At the end of the work, clean the cuvettes and laboratory aids with ethanol.
Determination of calibration curve. Take the save dye solution and prepare 50 cm3
by
diluting with water using ratio 1:40. Pour 7 flasks (25 ml) using 7.5; 8; 9; 10.5; 12;
14 cm3
of dilute stock solution of methylene blue and by water up to the mark. Set a
spectrophotometer to measure absorbance at a wavelength of 660nm. If the
absorbance measurement is outside the calibration curve, dilute the dye
solution appropriately and record the dilution factor which will be used to
calculate the equilibrium dye concentration.
DATA ANALYSIS: Each theoretical isotherm (1.1.), (1.2.) and (1.3.) has the form
𝑦𝑦 = 𝑓𝑓 (𝑥𝑥) where independent variable 𝑥𝑥 is the equilibrium concentration 𝑐𝑐 and
the dependent variable 𝑦𝑦 is the weight of the dye sorbent per unit weight of the
adsorbent. There are two experimental adsorption parameters in each of the relations.
Adsorption parameters can be determined for individual isotherm using one of the
following procedures.
Procedure A (Use of the none-linear regression): Make the best fit of the
experimentally measured adsorption isotherm by an assumed non-linear theoretical
adsorption function. Use the appropriate numerical method which can be found in the
"Solver" function in MS-EXCEL software.
Procedure B (Use of the linear regression): Transform the dependencies (1.1.), (1.2.)
and (1.3.) into linearized formulas. The results are:
1
𝑛𝑛𝑎𝑎
=
1
𝑛𝑛a,max
+
1
ω 𝑛𝑛a,max
⋅
1
𝑐𝑐
(1.4.)
ln 𝑛𝑛𝑎𝑎 = ln 𝑘𝑘 +
1
𝑛𝑛
⋅ ln 𝑐𝑐 (1.5.)
𝑛𝑛𝑎𝑎 = 𝑘𝑘1 𝑙𝑙𝑙𝑙 𝑘𝑘2 + 𝑘𝑘1 ln 𝑐𝑐 (1.6.)
where you can find the new couples of coordinates:
1
𝑛𝑛𝑎𝑎
and
1
𝑐𝑐
, ln 𝑛𝑛𝑎𝑎 and ln 𝑐𝑐, 𝑛𝑛𝑎𝑎 and ln 𝑐𝑐.
Plot the graphs using new coordinates. For each dependence, make linear regression
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and find the intercept and slope. Calculate the isotherm parameters from intercept and
slope for each linear dependence.
REPORT: Calibration table 1: concentration of dye standard [mg/cm
3
] and its
absorbance. Calibration graph 1: Dependence of dye standard absorbance on
its concentration. Table 2: for each adsorption experiment: no of dye solution, weight of
activated carbon in [mg], the amount of dye [mg] in the solution before and after
adsorption, solution dye loss in [mg], equilibrium concentration c [mg/cm3
] evaluated
from Calibration graph 1, amount of dye adsorbed per 1 gram of activated carbon [mg
dye / g act. carbon]; values:
1
𝑛𝑛𝑎𝑎
,
1
𝑐𝑐
, ln 𝑛𝑛𝑎𝑎, ln 𝑐𝑐, and 𝑛𝑛𝑎𝑎 . Graph 2-4: linear dependences
(1.4.), (1.5.), and (1.6.). Table 3: adsorption parameters of Langmuir, Freundlich and
Temkin isotherms. Table 4: 10 equilibrium dye concentrations used as input for
calculation 𝑛𝑛𝑎𝑎 for all isotherms (apply parameters given in Table 3). Common graph 5:
Experimental dependence (plotted by symbols) of adsorbed amount [mg dye / g act.
carbon] on the equilibrium concentration c [mg/cm3
] together with the theoretical
adsorption isotherms obtained by best fit (plotted by lines).
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