Sucgang, Mark Anthony N. 03/31/2016
BS ChE 5 MA2
Adsorption Isotherm Of Acetic Acid Solutions On Activated Carbon
Sucgang, Mark Anthony N.Abuan, Rebecca Grace A. Agudo, Anna Leslie C. Balicha, Eloisa Myra H.
ABSTRACT
The purpose of the experiment was to study the adsorption isotherm of acetic acid solutions on activated carbon. To observe if this system will obey the Freundlich or Langmuir equation, different concentrations of acetic acid were prepared and equilibrated with 1 gram of activated carbon. After shaking for a 10-20 minutes it was set aside undisturbed for a week in a place where the relatively constant temperature is 24 C. A sample from each solution was then titrated with a standardized sodium hydroxide solution. The amount of equilibrium concentration and weight of acetic acid adsorbed was then calculated. From the data gathered, the Freundlich’s and Langmuir’s linear equation were used to observe if they obey the said equations. Comparing the values of r from Freundlich equation of 0.9572 and 0.9734 from those of Langmuir equation having r values of 0.9504 and 0.9523, the system obeys both equation but Freundlich equation is more favored. Thus, Freundlich isotherm is better in describing the adsorption of acetic acid on activated carbon.
Keywords: acetic acid, activated carbon, adsorption, isotherm, titration INTRODUCTION
Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface. Adsorption is a process in which molecules from gas (or liquid) phase land on, interact with and attach to solid surfaces. It is a separation process in which some materials, (adsorbate) is concentrated from a bulk vapor or liquid has on to the surface of a porous solid (adsorbent). The reverse process of adsorption, i.e. the process in which adsorbed molecules escape from solid surfaces, is called desorption. Adsorption appears to be good for the treatment of effluents. The first thing for an efficient adsorption process is the search for a low cost adsorbent with high adsorption capacity and second it should be biodegradable. There are many examples of adsorbents, some of which are activated carbon (adsorbs organics), silica gel (adsorbs moisture), activated alumina (adsorbs moisture), zeolites & molecular sieves and synthetic resins. The commonly used adsorbing agent is the activated carbon, in this experiment activate carbon will be used to study about the adsorption isotherms of acetic acid solutions on activated carbon. All activated carbon may look the same to the naked eye, but pore size and structure makes a huge difference on how well impurities are adsorbed. Adsorption of gases by solids depends on the nature of adsorbent, nature of gas being adsorbed, area of adsorbent, temperature and pressure of the gas. Surface adsorption to a solid falls into two broad categories; Physisorption or Physical adsorption and Chemisorption or Chemical adsorption. If the force of attraction existing between adsorbate and adsorbent are Vander Waal’s forces, the adsorption is called physical adsorption. It is also known as Vander Waal’s adsorption. In the physical adsorption, the force of attraction between the adsorbate and the adsorbent are very weak, therefore this type of adsorption can be easily reversed by heating or be decreasing the pressure. While if the force attraction existing between adsorbate and adsorbent are almost same strength as chemical bonds, the adsorption is called chemical adsorption. It is also known as Langmuir adsorption. In the chemisorption the force of attraction is very strong, therefore adsorption cannot be easily reversed. Chemisorption process is best described by the Langmuir isotherm.
The amount of substance that can be adsorbed on activated carbon depends on the nature of the substance and its concentration, the surface structure of the activated carbon and the temperature of the solution. For a treatment system with a
specific type of carbon and a known substance, there is a relationship between the amount of adsorbed matter per unit of weight of carbon and the equilibrium concentration in the solution, when temperature is constant. This relationship is called isotherm.
Adsorption isotherm is the relation between the amount of substance adsorbed by an adsorbent and the equilibrium pressure or concentration at constant temperature. The shape of isotherm can be described in various mathematical ways. Some of the well-known is the Langmuir and Freundlich isotherm.
The chemical reaction for a monolayer adsorption can be represented as follows:
A + S ↔ AS
where AS represents a solute molecule bound to a surface site on S. The equilibrium constant
Kads for this reaction is given by:
Kads=
[
AS]
[
A
]
[
S ]
equation [1.1] [A] denotes the concentration of A, while the other two terms [S] and [AS] are two-dimensional analogs of concentration and are expressed in units such as mol/cm2. The principle of chemical equilibrium holds with these terms. The complete form of the Langmuir isotherm considers (Eq.1.1) in terms of surface coverage q which is defined as the fraction of the adsorption sites to which a solute molecule has become attached. An expression for the fraction of the surface with unattached sites is therefore (1 - q). Given these definitions, we can rewrite the term [AS]/[S] as[
AS ]
[
S]
=
θ
1−θ
equation [1.2] Now we express [A] as C and rewrite (Eq. 1) as:Kads=
θ
C(1−θ)
equation [1.3] Rearranging, we obtain the final form of the Langmuir adsorption isotherm:θ=
Kads C
Figure 1.1 General Form of Langmuir Isotherms
If we define Y as the amount of adsorption in units of moles adsorbate per mass adsorbant, and Ymax and the maximal adsorption, then:
θ=
Y
Ymax
equation [1.5] and the isotherm can be expressed as:C
Y
=
1
KadsYmax
+
C
Ymax
equation [1.6]This is the form of the isotherm we will use for our Charcoal-Acetic Acid system.
At lower concentrations, an alternate isotherm developed by Herbert F. Freundlich frequently describes the data better. The Freundlich Isotherm is:
Y =k C
ln equation [1.7]where the Freundlich parameters k and n are empirically determined. A plot of log Y vs. log C allows for a determination of these parameters.
DESIGN AND METHODOLOGY A. Preparation of Solutions
First 1000 mL of a 0.100N sodium hydroxide (NaOH) solution was prepared. To prepare the solution, about four grams of sodium hydroxide pellets were weighed in the analytical balance. The pellets were then dissolved in a small quantity of water and were transferred to a 1000 mL volumetric flask. Sufficient water was added until the mark to make up 1000 mL of the solution. The solution was mixed thoroughly and was transferred to a reagent bottle and was labeled.
Next 500 mL of 0.500N acetic acid (HAc) solution was prepared. To prepare the solution, about 18 mL of glacial acetic acid was pipetted carefully into a 500 mL volumetric flask and enough water was added to reach the mark. The solution was mixed thoroughly and was transferred to a reagent bottle and was labeled.
The titrant solution, which was the NaOH solution, was standardized using potassium phthalate (KHP). The amount needed for standardizing the solution was weighed using the analytical balance and was placed in an Erlenmeyer flask. This amount of KHP was dissolved with a small amount of water. A few drops of phenolphthalein were added to the flask and was titrated with the NaOH solution. A second standardization was done. The normality of the titrant solution was determined up to four digits.
B. Preparation of Adsorption Solutions
For the preparation of the adsorption solutions, 100 mL each of N/4, N/8, N/16, N/32 and N/64 HAc solutions. The volume of N/2 or 0.500N HAc solution required in the preparation of these solution was computed. The computed volumes were pipetted into different 100 mL volumetric flasks and enough water was added to these flasks until the mark. These solutions were mixed thoroughly and 80 mL of these solutions were transferred to properly labeled Erlenmeyer flasks and corks were placed as stoppers. A second set of these solutions were made for the second trial.
Approximately one gram of activated carbon was weighed using an analytical balance and was transferred
to an Erlenmeyer flask containing the prepared solution. This was done to the other Erlenmeyer flasks. The weight of the activated carbon placed in each erlenmeyer flask was carefully recorded. The contents of the twelve flasks were mixed thoroughly for about 10 to 20 minutes, and were then left in a place where the temperature remained relatively constant. The prepared solutions were left for a week to let the solution adsorbed by activated carbon reach equilibrium.
C. Determination of the Equilibrium Concentrations of the Solutions
After a week, the solutions were carefully removed from their safekeeping place as not to disturb the solution. For each of the solution, 10 mL of the clear liquid was pipetted into a clean Erlenmeyer flask for titration. Small amount water was added to the walls of the flasks. A few drops of phenolphthalein were placed in the solutions as an indicator and were titrated with the standardized NaOH solution. A second titration was done with the second set of prepared solutions. The concentration of the original concentration was calculated with N/2 equal to 0.5000N acetic acid solution. After the titration the equilibrium concentration of the each acetic acid solutions were calculated using the formula:
NHAcVHAc = NNaOHVNaOH equation [2.1]
where NHAc is the equilibrium concentration of the acetic acid
solution, VHAc is the volume of acetic acid used, NNaOH is the
concentration of the standardized NaOH, and VNaOH is the
volume of the standardized NaOH used. The weight of the acetic acid at equilibrium was also calculated using the formula:
wt. of HAc =
(N)(MW)(L solution)
f
equation[2.2]
where N is the calculated normality of the equilibrium solution, MW is the molar mass of acetic acid, L solution is the volume of acetic acid solution equilibrated with about 1 gram of activated carbon, and f is the equivalence per mole of a substance. By deducting the weight of acetic acid in the equilibrium solution from that in the original solution, the
weight of acetic acid adsorbed can be obtained. After which the ratio of the weight of the acetic adsorbed per gram of activated carbon was also calculated.
RESULTS AND DISCUSSION
Room Temperature: 24°C Pressure: 24.96 in Hg
TRIAL 1
TABLE 3.1 Original Concentration of Acetic Acid Solution and Weight of Acetic Acid in the Original Solution Original
Concentration
Volume of Solution
Weight of HAc in the Original Solution Volume of HAc in Titration Volume of NaOH Used N/2 0.4841 N 80 mL 2.3237 g 10 mL 45.7 mL N/4 0.2421 N 80 mL 1.1621 g 10 mL 22.5 mL N/8 0.1210 N 80 mL 0.5808 g 10 mL 10.8 mL N/16 0.0605 N 80 mL 0.2904 g 20 mL 9.9 mL N/32 0.0303 N 80 mL 0.1454 g 20 mL 4.3 mL N/64 0.0151 N 80 mL 0.0725 g 40 mL 4.7 mL
TABLE 3.2 Equilibrium Concentrations, Weight of Acetic Acid Adsorbed per gram Activated Carbon Equilibrium Concentration Weight of HAc at Equilibrium Weight of HAc Adsorbed Weight of Activated Carbon
Weight of HAc per gram of Activated Carbon N/2 0.4515 N 2.1672 g 0.1565 g 1.0 g 0.1565 g HAc/ g AC N/4 0.2223 N 1.0670 g 0.0951 g 1.0 g 0.0951 g HAc/ g AC N/8 0.1067 N 0.5112 g 0.0686 g 1.0 g 0.0686 g HAc/ g AC N/16 0.0489 N 0.2347 g 0.0557 g 1.0 g 0.0557 g HAc/ g AC N/32 0.0212 N 0.1018 g 0.0436 g 1.0 g 0.0436 g HAc/ g AC N/64 0.0116 N 0.0557 g 0.0168 g 1.0 g 0.0168 g HAc/ g AC Freundlich constants: k = 0.2323 n = 1.9262 r = 0.9572 Langmuir constants: a = 1.4449 b = 7.8429 r = 0.9504 TRIAL 2
TABLE 3.3 Original Concentration of Acetic Acid Solution and Weight of Acetic Acid in the Original Solution Original
Concentration
Volume of Solution
Weight of HAc in the Original Solution Volume of HAc in Titration Volume of NaOH Used N/2 0.4841 N 80 mL 2.3237 g 10 mL 46 mL N/4 0.2421 N 80 mL 1.1621 g 10 mL 22.7 mL N/8 0.1210 N 80 mL 0.5808 g 10 mL 10.8 mL N/16 0.0605 N 80 mL 0.2904 g 20 mL 10 mL N/32 0.0303 N 80 mL 0.1454 g 20 mL 4.3 mL N/64 0.0151 N 80 mL 0.0725 g 40 mL 4.3 mL
TABLE 3.4 Equilibrium Concentrations, Weight of Acetic Acid Adsorbed per gram Activated Carbon Equilibrium Concentration Weight of HAc at Equilibrium Weight of HAc Adsorbed Weight of Activated Carbon
Weight of HAc per gram of Activated Carbon N/2 0.4545 N 2.1816 g 0.1421 g 1.0 g 0.1421 g HAc/ g AC N/4 0.2243 N 1.0766 g 0.0855 g 1.0 g 0.0855 g HAc/ g AC N/8 0.1067 N 0.5122 g 0.0686 g 1.0 g 0.0686 g HAc/ g AC N/16 0.0494 N 0.2371 g 0.0533 g 1.0 g 0.0533 g HAc/ g AC N/32 0.0212 N 0.1018 g 0.0436 g 1.0 g 0.0436 g HAc/ g AC N/64 0.0106 N 0.0509 g 0.0216 g 1.0 g 0.0216 g HAc/ g AC
Freundlich constants: k = 0.1880 n = 2.2883 r = 0.9734 Langmuir constants: a = 1.5913 b = 10.1175 r = 0.9523
The experiment was done with two trials. Tables 3.1 and 3.3 shows the original concentration of the acetic acid solution obtained by titrating it with the standardized NaOH solution. The volume of solution was the volume of the acetic acid in different concentrations equilibrated with about 1 gram of activated carbon. The weight of acetic acid on the original solution was obtained after the titration with the respective volumes of acetic acid solution in different concentrations and the corresponding volume of the standardized NaOH solution used. When all the necessary data for the original solution found in tables 3.1 and 3.3 are obtained and recorded, each solution in different concentrations were equilibrated with about 1 gram of activated carbon.
The other tables 3.2 and 3.4 shows the concentration of acetic acid after each solution was equilibrated with about 1 gram of activated carbon. This was obtained by titrating each solution with the standardized NaOH solution and using equation [2.1]. The weight of acetic acid at equilibrium was calculated using equation [2.2]. Then by deducting the weight of the acetic acid in the equilibrated solution from its weight in
the original solution, the weight of acetic adsorbed can be obtained. The respective weights of activated carbon put in each solution was carefully recorded and the ratio of the weight of acetic acid per gram of activated carbon is computed by dividing the exact amount of activated carbon in each concentration in each trial by the calculated weight of acetic acid in their respective concentrations and trials.
The results show that the standardization of the acetic acid solution was precise because the values of the volume of
the NaOH used were almost the same and had small differences. The analytical balance used had accuracy up to the first decimal and the constant weight of all the activated carbon were obtained due to this. It is also evident that the ratio of activated carbon and acetic, and weight of acetic acid in the equilibrated solution were almost the same for each trial
with their respective concentrations except for concentrations N/2 and N/64. This may be due to error in the titration process where the change in color of the indicator was not observed properly, however, the difference is not as high as to consider that one of the trials was not performed properly.
-4.5-4 -3.5-3 -2.5-2 -1.5-1 -0.50
Figure 3.1 Linear Curve of Freundlich Equation for First Trial
y, ln y
Linear (y, ln y)
ln C ln y
0 0.51 1.52 2.53 3.5
Figure 3.2 Linear Curve of Langmuir Equation for First Trial
y, C/y
Linear (y, C/y)
C C/ y -0.78859999999999997 -3.0078 -5 -4 -3 -2 -1 0
Figure 3.3 Linear Curve of Freundlich Equation for Second Trial
y, ln y Linear (y, ln y) ln C ln y 0 0.51 1.52 2.53 3.5
Figure 3.4 Linear Curve of Freundlich Equation for Second Trial
y, C/y
Linear (y, C/y)
C C/ y
From the given data, the Freundlich and Langmuir constants were calculated using linear regression for each trial and with the equations:
Freundlich Equation: ln y = ln k +
1
n
ln C equation [3.1] Langmuir Equation:C
y
=
1
a
+
b
a
C
equation [3.2]The r represents the closeness of the points from the curve. This determines the best fit adsorption isotherm for the experiment. Their values show that Freundlich isotherm is best fit isotherm rather than the Langmuir isotherm for this experiment as seen from a comparison of their curves and points on the figures 3.1, 3.2, 3.3 and 3.4. The relatively constant temperature considered during the period of the experiment was 24°C.
CONCLUSION AND RECOMMENDATION
The adsorption of acetic acid on activated carbon at an approximately constant temperature 24°C obeys the Freundlich equation and Langmuir equation as can be seen by the evaluated r or the closeness of the points to a curve, but the best fit isotherm for the experiment was the Freundlich isotherm which has lesser deviance of points to its curve.
Possible sources of error in this experiment can be accounted mainly in inaccuracy of reading the measurements. Failure to titrate or standardize the solutions also increases the probability of obtaining values that are less likely to obey the above mentioned equations. The preparation, especially the transfer of exact amount of solutions, has a great impact on the concentration the solution will have. Any excess or
insufficiency in the amount required will greatly affect the result. Although instances that the procedure can be altered it is best to do it as instructed for better comparison of results with same level of possible errors.
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