Discussion Examples for Adsorption Isotherms | Activated Carbon Adsorption and Concentration Changes

What Is Adsorption?

Adsorption is a phenomenon in which molecules or ions in a gas or liquid gather at and are retained on a solid surface.
The uptake of dyes, organic substances, ions, and other species from a solution onto the surface of activated carbon is also adsorption.
Adsorption is a phenomenon that occurs at the solid surface and is distinguished from absorption, in which a substance is taken into the entire interior of a solid.

Adsorption tends to occur more readily as the surface area increases.
Activated carbon has numerous pores and an extremely large specific surface area, so it readily adsorbs substances from solution.
In adsorption experiments, the amount of substance adsorbed onto activated carbon is determined from the difference in concentration before and after adsorption.

Main Items to Include in the Results

In the results of an adsorption isotherm experiment, organize the type and mass of adsorbent, type of adsorbate, initial concentration, equilibrium concentration, solution volume, contact time, temperature, measurement method, adsorption capacity, adsorption percentage, and adsorption-isotherm graph.
Adsorption capacity is often determined from the difference between the initial concentration and equilibrium concentration.

Main Items to Include in the Results

• Type of adsorbent
• Mass of activated carbon
• Particle size of activated carbon
• Type of adsorbate
• Initial concentration
• Equilibrium concentration
• Amount of concentration decrease
• Solution volume
• Contact time
• Stirring conditions
• Measurement temperature
• Filtration or centrifugation conditions
• Absorbance or titration value
• Adsorption capacity
• Adsorption percentage
• Shape of the adsorption isotherm
• Relationship with the Langmuir or Freundlich equation
• Causes of error and points for improvement

Reference Experimental Values and Calculation Examples for Adsorption Isotherm Experiments

Here, using a dye adsorption experiment with activated carbon, the initial concentration, equilibrium concentration, and adsorption capacity are organized, and the process of summarizing the results as an adsorption isotherm is confirmed using reference experimental values.

An adsorption isotherm represents the relationship, at a constant temperature, between the equilibrium concentration of a substance remaining in solution and the amount adsorbed onto the adsorbent.
By performing experiments with different initial concentrations, it is possible to investigate how much dye activated carbon can adsorb.

Reference Experimental Conditions

Example of a Calibration Curve

Suppose that a calibration curve has been prepared using standard solutions in order to determine the methylene blue concentration from absorbance.
In this reference example, the concentration is determined using the following calibration curve.

Absorbance = 0.0385 × Concentration

Therefore, the equation for determining concentration from absorbance is as follows.

Concentration = Absorbance ÷ 0.0385

Measurement Results When the Initial Concentration Is Varied

The following is an example in which the same amount of activated carbon is added to aqueous methylene blue solutions with different initial concentrations, and after sufficient shaking, the samples are filtered and the absorbance of the supernatant is measured.

Example Calculation of Equilibrium Concentration

For Sample D, suppose the absorbance at equilibrium was 0.616.
Using the calibration curve, the equilibrium concentration can be determined as follows.

Equilibrium concentration Ce = 0.616 ÷ 0.0385 = 16.0 mg/L

Therefore, the solution with an initial concentration of 40.0 mg/L decreased to 16.0 mg/L after contact with activated carbon.

Example Calculation of Removal Percentage

The removal percentage expresses, as a percentage, how much of the adsorbate was removed by adsorption relative to the initial concentration.

Removal percentage (%) = (C0 − Ce) ÷ C0 × 100

For Sample D, because the initial concentration is 40.0 mg/L and the equilibrium concentration is 16.0 mg/L, the removal percentage is calculated as follows.

Removal percentage = (40.0 − 16.0) ÷ 40.0 × 100 = 60.0%

From this result, 60.0% of the dye initially present in Sample D is considered to have been removed by the activated carbon.

Example Calculation of Equilibrium Adsorption Capacity

Equilibrium adsorption capacity represents the amount of substance adsorbed per 1 g of adsorbent.
If the solution volume is V and the amount of activated carbon is m, it can be determined by the following equation.

qe = (C0 − Ce) × V ÷ m

In this reference example, the solution volume V is 100 mL = 0.100 L and the amount of activated carbon m is 0.10 g.
The equilibrium adsorption capacity of Sample D can be calculated as follows.

qe = (40.0 − 16.0) mg/L × 0.100 L ÷ 0.10 g

qe = 24.0 mg/g

Therefore, in Sample D, 24.0 mg of methylene blue was adsorbed per 1 g of activated carbon.

Relationship Between Initial Concentration, Removal Percentage, and Adsorption Capacity

As the initial concentration increases, the equilibrium adsorption capacity increases.
On the other hand, the removal percentage decreases from 80.0% to 42.0%.

The higher the initial concentration, the greater the amount of dye present in the solution, so the total amount adsorbed onto the activated carbon increases.
However, because the adsorption sites on the activated carbon are limited, at higher concentrations not all of the dye can be removed and the removal percentage tends to decrease.

Organization as an Adsorption Isotherm

In an adsorption isotherm, the equilibrium concentration Ce is plotted on the horizontal axis and the equilibrium adsorption capacity qe on the vertical axis.
In this reference example, qe increases as Ce increases, but the increase becomes somewhat more gradual in the high-concentration region.

When Considering Langmuir-Type Adsorption

In Langmuir-type adsorption, the adsorbent surface is considered to have a limited number of adsorption sites, and the adsorption capacity approaches a maximum value at high concentrations.

In this reference example, the equilibrium adsorption capacity increases as the initial concentration increases, but the increase becomes smaller in the high-concentration region.
This can be explained by the adsorption sites on the activated-carbon surface gradually becoming occupied and the system approaching saturation.

When Considering Freundlich-Type Adsorption

In Freundlich-type adsorption, the adsorbent surface is considered to be nonuniform, with sites that are easy to adsorb onto and sites that are more difficult to adsorb onto.
It can be interpreted that adsorption occurs preferentially at easily adsorbed sites at low concentrations and then extends to more difficult sites as the concentration increases.

In this reference example as well, the equilibrium adsorption capacity increases as the equilibrium concentration increases, so it can be considered that dye molecules were adsorbed onto various adsorption sites on the activated-carbon surface.

Example of How to Write the Results

The initial concentration was varied over the range from 5.0 to 100.0 mg/L, and the amount of methylene blue adsorbed by activated carbon was determined.
At an initial concentration of 5.0 mg/L, the equilibrium concentration was 1.0 mg/L, the removal percentage was 80.0%, and the equilibrium adsorption capacity was 4.0 mg/g.
On the other hand, at an initial concentration of 100.0 mg/L, the equilibrium concentration was 58.0 mg/L, the removal percentage was 42.0%, and the equilibrium adsorption capacity was 42.0 mg/g.

As the initial concentration increased, the equilibrium adsorption capacity increased, but the removal percentage decreased.
This was considered to result from the fact that increasing the amount of dye in the solution increased the total amount adsorbed onto the activated carbon, while the number of adsorption sites on the activated carbon was limited and therefore the dye could not be sufficiently removed in the high-concentration region.

Points for Connecting the Results to the Discussion

In a discussion of an adsorption isotherm experiment, it is important to note that the removal percentage and adsorption capacity do not necessarily change in the same manner when the concentration changes.
Under high-concentration conditions, the adsorption capacity may be large even though the removal percentage is low.

• Did the equilibrium adsorption capacity increase as the initial concentration increased?
• Did the removal percentage decrease as the initial concentration increased?
• Did the increase in adsorption capacity become more gradual in the high-concentration region, showing a tendency to approach saturation?
• Can the number of adsorption sites on the activated-carbon surface be considered limited?
• Which explanation, Langmuir-type or Freundlich-type, better fits the results?
• Could insufficient filtration or contamination by fine activated-carbon particles have affected the absorbance?
• Was the contact time sufficient for adsorption to approach equilibrium?

Example Discussion

In this experiment, the equilibrium adsorption capacity qe per 1 g of activated carbon increased as the initial concentration increased.
At an initial concentration of 5.0 mg/L, qe was 4.0 mg/g, whereas at an initial concentration of 100.0 mg/L, it was 42.0 mg/g.
This was considered to result from the greater number of dye molecules present in the solution at higher initial concentrations, increasing the opportunity for contact with the activated-carbon surface.

On the other hand, the removal percentage decreased as the initial concentration increased.
Under low-concentration conditions, the number of dye molecules was small relative to the adsorption sites on the activated carbon, so a large proportion of the dye was removed.
However, under high-concentration conditions, the amount of dye was large and the adsorption sites on the activated-carbon surface approached saturation, so more dye remained in the solution and the removal percentage was considered to have decreased.

When viewed as an adsorption isotherm, the equilibrium adsorption capacity qe increased as the equilibrium concentration Ce increased, but the increase became somewhat more gradual in the high-concentration region.
This was considered to result from the adsorption sites on the activated-carbon surface gradually becoming occupied and the adsorption approaching saturation.
Therefore, these results can be readily explained by the concept of saturation adsorption such as the Langmuir model.

However, the activated-carbon surface is not uniform, and adsorption sites with different affinities are considered to exist because of differences in pore structure and surface functional groups.
Therefore, the results can also be interpreted as adsorption onto a heterogeneous surface, as in the Freundlich model.
In an actual report, it is important to plot the measured points and examine which adsorption-isotherm equation the trend more closely resembles.

Summary

In an adsorption isotherm experiment, the adsorption characteristics of activated carbon can be evaluated by varying the initial concentration and determining the equilibrium concentration and equilibrium adsorption capacity.

In this reference example, the equilibrium adsorption capacity increased as the initial concentration increased, but the removal percentage decreased.
In a report, organizing the relationships among concentration, adsorption capacity, and removal percentage and relating them to saturation of adsorption sites and heterogeneity of the activated-carbon surface makes the discussion easier.

Why Activated Carbon Is Used as an Adsorbent

Activated carbon is a porous material composed mainly of carbon.
Because it contains numerous fine pores and has an extremely large surface area, it can efficiently adsorb substances from solution.
It is often used particularly for removal of organic substances, dyes, odor components, hydrophobic substances, and similar materials.

The adsorption capacity of activated carbon changes depending on pore diameter, specific surface area, surface functional groups, particle size, pretreatment conditions, and other factors.
Even for the same mass of activated carbon, the adsorption capacity changes if the surface area or pore structure differs.
Therefore, when discussing activated-carbon adsorption results, it is useful to consider not only that “adsorption occurred because activated carbon was added” but also the roles of surface structure and pores.

What Can Be Learned from Concentration Changes?

When the solution concentration decreases after activated carbon is added, the decrease mainly corresponds to the amount of substance adsorbed onto the activated carbon.
The larger the difference between the initial concentration and equilibrium concentration, the greater the amount of adsorbate removed from the solution.
By calculating the adsorption capacity, it is possible to evaluate how much substance was adsorbed per 1 g of activated carbon.

However, a decrease in concentration is not necessarily caused entirely by adsorption.
Sample loss during filtration, adsorption onto the container, decomposition, precipitation, measurement errors, and other factors may also cause concentration decreases.
Therefore, performing blank tests and control experiments makes it possible to more accurately judge changes caused by activated-carbon adsorption.

How to Determine Adsorption Capacity

Adsorption capacity is determined from the concentration difference before and after adsorption, the solution volume, and the mass of the adsorbent.
If the initial concentration is C0, equilibrium concentration is Ce, solution volume is V, and adsorbent mass is m, the adsorption capacity qe per unit mass is expressed as follows.
It is important to use consistent units for concentration and volume.

qe = (C0 – Ce)V / m

C0: initial concentration, Ce: equilibrium concentration, V: solution volume, m: adsorbent mass

A larger qe indicates that more substance was adsorbed per 1 g of activated carbon.
When comparing adsorption capacities, the mass of activated carbon and solution volume must be standardized.
In addition, the unit of adsorption capacity, such as mg/g or mol/g, must be clearly stated.

Discussion of Adsorption Percentage

Adsorption percentage is the proportion of the adsorbate initially present in the solution that was removed by activated carbon.
It can be determined from the initial concentration and equilibrium concentration.
A higher adsorption percentage means that activated carbon had a greater removal effect.

Adsorption percentage = (C0 – Ce) / C0 × 100

However, adsorption percentage and adsorption capacity do not mean the same thing.
At low concentrations, the adsorption percentage may be high even though the absolute adsorption capacity is small.
At high concentrations, the adsorption capacity may be large, but the adsorption percentage may decrease because the activated-carbon surface approaches saturation.
It is important to distinguish between these two indicators in the discussion.

What Is Adsorption Equilibrium?

Adsorption equilibrium is the state in which the rate at which adsorbate in the solution is adsorbed onto the activated-carbon surface and the rate at which it desorbs from the surface back into the solution are balanced.
Adsorption does not continue indefinitely in one direction, but approaches an equilibrium state after a certain period.
The solution concentration at this time is called the equilibrium concentration Ce.

If the concentration is measured before equilibrium is reached, the adsorption capacity may be underestimated.
This is because when the contact time is too short, the adsorbate has not moved sufficiently to the activated-carbon surface or into the pores.
To create an adsorption isotherm, each concentration condition must be allowed sufficient contact time and measured under conditions close to equilibrium.

What Is an Adsorption Isotherm?

An adsorption isotherm is a graph showing the relationship between equilibrium concentration Ce and adsorption capacity qe at a constant temperature.
It is called an “isotherm” because the measurement is performed at a constant temperature.
By examining an adsorption isotherm, it is possible to determine how the adsorption capacity changes from low to high concentrations.

In general, adsorption capacity increases as equilibrium concentration increases.
However, because the number of adsorption sites on the adsorbent surface is limited, the increase in adsorption capacity becomes more gradual once the concentration becomes sufficiently high.
From this shape, it is possible to discuss whether the adsorption is close to monolayer adsorption or adsorption onto a heterogeneous surface.

Concept of the Langmuir Adsorption Isotherm Equation

The Langmuir adsorption isotherm equation is a model that assumes that the adsorbent surface has a fixed number of adsorption sites and that one molecule is adsorbed at each site.
In other words, monolayer adsorption is assumed.
At low concentrations, many adsorption sites are vacant, so the adsorption capacity increases readily, whereas at high concentrations the adsorption sites become occupied and the adsorption capacity approaches a maximum value.

In Langmuir-type adsorption, the adsorption isotherm initially increases steeply and then approaches saturation.
Although it is an idealized model assuming a uniform activated-carbon surface and identical adsorption sites, it is useful for considering the maximum adsorption capacity.
When experimental results level off at high concentration, the Langmuir concept can be applied.

qe = qmaxKCe / (1 + KCe)

qmax: maximum adsorption capacity, K: constant related to adsorption strength

Concept of the Freundlich Adsorption Isotherm Equation

The Freundlich adsorption isotherm equation is an empirical equation representing adsorption onto a heterogeneous surface.
The activated-carbon surface is not completely uniform, and pore sizes, surface functional groups, and adsorption sites with different affinities are present.
Therefore, the Freundlich equation is often used for actual activated-carbon adsorption.

In the Freundlich equation, adsorption capacity qe is considered proportional to a power of the equilibrium concentration Ce.
A linear relationship may be obtained when a log-log plot is made.
If the experimental results do not show clear Langmuir-type saturation and adsorption capacity continues to increase with concentration, the Freundlich model can be examined.

qe = KCe1/n

log qe = log K + (1/n)log Ce

Difference Between Langmuir-Type and Freundlich-Type Adsorption

The Langmuir model assumes uniform adsorption sites, monolayer adsorption, and an approach to a maximum adsorption capacity.
In contrast, the Freundlich model is an empirical equation that assumes a heterogeneous adsorbent surface with differences in ease of adsorption among sites.
Because porous materials such as activated carbon have heterogeneous surfaces, the results may sometimes be closer to Freundlich-type behavior.

If the experimental results clearly approach saturation at high concentration, the Langmuir model can be examined, while if the log-log plot shows good linearity, the Freundlich model can be considered.
However, the results cannot always be completely explained by only one of the two models.
Depending on the experimental range and measurement error, the results may appear to resemble both.

Effect of Initial Concentration

The higher the initial concentration, the more adsorbate molecules are present in the solution and the greater the driving force for movement toward the activated-carbon surface.
Therefore, adsorption capacity tends to increase as the initial concentration increases.
At low concentrations, adsorption sites are sufficiently available, so the adsorption percentage may become high.

However, at higher concentrations, adsorption sites on the activated-carbon surface gradually become occupied and the increase in adsorption capacity becomes more gradual.
At still higher concentrations, the adsorption sites approach saturation and additional adsorbate tends to remain in the solution.
Therefore, adsorption capacity and adsorption percentage must be discussed separately.

Effect of the Amount of Activated Carbon

Increasing the amount of activated carbon increases the available adsorption surface area and number of adsorption sites, so more adsorbate is removed from the solution.
Therefore, for a solution with the same initial concentration, increasing the amount of activated carbon tends to lower the equilibrium concentration and increase the adsorption percentage.

However, the adsorption capacity qe per unit mass does not necessarily increase when the amount of activated carbon is increased.
If too much activated carbon is used, the adsorbate may not be distributed sufficiently among all adsorption sites and the adsorption capacity per 1 g of activated carbon may appear smaller.
When considering the effect of adsorbent amount, total removal and adsorption capacity per unit mass must be distinguished.

Effect of Contact Time

Adsorption does not finish completely at the moment activated carbon is added.
The adsorbate moves from the solution to the external surface of the activated carbon and then diffuses into the pores and becomes adsorbed.
Therefore, if the contact time is short, equilibrium may not have been reached and the adsorption capacity may be measured as smaller.

In general, the concentration difference is large in the initial stage of adsorption and the adsorption rate is high.
As time passes, adsorption sites become occupied and the system approaches equilibrium, so the adsorption rate becomes slower.
When preparing an adsorption isotherm, it is important to set the same contact time for all samples and, if possible, confirm beforehand the time required to reach equilibrium.

Effect of Stirring

Stirring is important for transporting the adsorbate in the solution to the activated-carbon surface.
If stirring is insufficient, the concentration boundary layer around the activated carbon becomes thicker and it becomes more difficult for the adsorbate to move to the surface.
As a result, the adsorption rate may decrease and the adsorption capacity within a fixed period may become smaller.

Appropriate stirring makes the concentration throughout the solution more uniform and promotes mass transfer to the activated-carbon surface.
However, if stirring conditions differ among samples, comparison of adsorption capacities becomes inaccurate.
In addition, excessively strong stirring may break the activated carbon into finer particles, changing the surface area and affecting the results.

Effect of Activated-Carbon Particle Size

The smaller the particle size of activated carbon, the larger the external surface area and the easier it is for the adsorbate to reach the surface.
Therefore, activated carbon with a smaller particle size may show a faster adsorption rate.
In addition, the diffusion distance into the pores becomes shorter, so the time required to reach equilibrium may also become shorter.

On the other hand, smaller particle sizes make filtration more difficult and fine activated-carbon particles may remain in the filtrate.
If fine particles remain, turbidity may affect absorbance measurement or adsorption may continue even after the intended measurement time.
The effect of particle size must be discussed from both the adsorption rate and the separation operation.

Effect of Temperature

Adsorption is affected by temperature.
In general, for physical adsorption, adsorption may become weaker and adsorption capacity may decrease as temperature increases.
This is because increasing the temperature increases molecular thermal motion and makes desorption from the surface easier.

However, increasing the temperature may also increase the diffusion rate in the solution and make the adsorption rate faster.
Adsorption capacity and adsorption rate must be considered separately.
Because adsorption isotherms are measured at a constant temperature, comparison among samples becomes difficult if the temperature differs among them.

Effect of pH

pH affects the charge state of the adsorbate and the charge state of the activated-carbon surface.
If the adsorbate contains acidic or basic functional groups, its ionization state changes with pH and its interaction with activated carbon changes.
Functional groups on the activated-carbon surface may also become charged depending on pH.

If the adsorbate and activated-carbon surface have charges of the same sign, electrostatic repulsion may occur and adsorption may become more difficult.
If the charges have opposite signs, electrostatic attraction may make adsorption easier.
Therefore, pH is an important factor affecting adsorption capacity and the shape of the adsorption isotherm.

Effect of the Molecular Structure of the Adsorbate

Ease of adsorption is also affected by the molecular structure of the adsorbate.
Highly hydrophobic molecules, planar dye molecules, organic substances containing aromatic rings, and similar species may adsorb readily onto the activated-carbon surface.
This is because hydrophobic interactions, π-π interactions, and other interactions are involved.

On the other hand, strongly hydrated ions or substances that are extremely soluble in water may have difficulty moving to the activated-carbon surface.
Large molecules may also have difficulty entering the pores of activated carbon.
Molecular size, polarity, charge, and functional groups of the adsorbate are important factors when discussing differences in adsorption capacity.

Determining Concentration by Absorbance Measurement

In adsorption experiments using dyes, the concentrations before and after adsorption may be determined from absorbance.
A calibration curve is prepared, and the equilibrium concentration is determined by measuring the absorbance of the filtrate.
A higher absorbance indicates a higher dye concentration in the solution, and a decrease in absorbance after activated-carbon treatment indicates that the dye was adsorbed.

However, if fine activated-carbon particles remain in the filtrate, light scattering may cause the absorbance to appear higher.
In addition, accurate concentration determination becomes difficult when the concentration exceeds the range of the calibration curve.
In absorbance measurement, it is important to perform sufficient filtration and measure within the linear range of the calibration curve.

Effect of the Filtration Operation

In an adsorption experiment, activated carbon and solution are brought into contact, after which the activated carbon is removed by filtration or centrifugation and the concentration in the filtrate is measured.
If filtration is insufficient, fine activated-carbon particles may remain in the filtrate, causing adsorption to continue during measurement or turbidity to affect absorbance measurement.

On the other hand, if the adsorbate is adsorbed onto the filter paper or container, the concentration may be measured lower than the actual value.
Adsorption onto filter paper is particularly important for dyes and hydrophobic substances.
Because the filtration operation directly affects calculation of the adsorption capacity, the conditions must be standardized.

Causes of Error in Adsorption Isotherm Experiments

Causes of error in adsorption isotherm experiments include errors in concentration preparation, errors in activated-carbon mass, insufficient contact time, insufficient stirring, temperature changes, insufficient filtration, adsorption onto filter paper, variation in activated-carbon particle size, errors in the calibration curve, and turbidity during absorbance measurement.
Because adsorption capacity is calculated from the concentration difference, errors in concentration measurement directly affect the adsorption capacity.

Under low-concentration conditions in particular, the concentration difference is small, so measurement errors have a large effect.
Under high-concentration conditions, the concentration may exceed the linear range of the calibration curve or the activated-carbon surface may approach saturation and the change may become small.
Causes of error are easier to organize when divided into preparation, adsorption operation, separation operation, concentration measurement, and analysis.

When the Results Can Be Considered Good

An adsorption isotherm experiment can be considered to have produced good results when the equilibrium concentration decreases below the initial concentration after activated carbon is added and the adsorption capacity changes systematically as the initial or equilibrium concentration increases.
In addition, if the graph of adsorption capacity versus equilibrium concentration shows a smooth curve and can be explained using the concepts of Langmuir-type or Freundlich-type adsorption, the results can be considered reasonable.

If the adsorption capacity tends to approach saturation in the high-concentration region, this may indicate that the number of adsorption sites is finite.
If a linear relationship is obtained in a log-log plot, the results can be discussed as Freundlich-type adsorption onto a heterogeneous surface.
Even if the results do not completely agree with the ideal equation, it is sufficient if they can be explained by considering the heterogeneity of the activated-carbon surface and measurement errors.

Example Discussions When the Experiment Did Not Go Well

When an adsorption isotherm experiment does not go well, possible causes should be considered from results such as almost no decrease in concentration, a negative adsorption capacity, failure of the adsorption capacity to increase in concentration order, large scatter in the graph, or disagreement with literature values.
Organizing the causes according to the amount of activated carbon, concentration preparation, contact time, filtration, absorbance measurement, calibration curve, and temperature makes the discussion easier.

How to Write Points for Improvement

In a discussion of an adsorption isotherm experiment, writing not only the causes of error but also points for improvement makes the report easier to organize.
Points for improvement can be organized according to solution preparation, adsorption operation, separation operation, concentration measurement, and analytical method.

Improvements to Solution Preparation

• Prepare the initial concentration accurately
• Set the concentration series appropriately
• Measure the solution volume accurately
• Keep pH and temperature constant
• Dissolve the adsorbate completely
• Perform a blank test

Improvements to the Adsorption Operation

• Measure the mass of activated carbon accurately
• Standardize the particle size of the activated carbon
• Allow sufficient contact time
• Keep the stirring conditions constant
• Confirm the time required to reach adsorption equilibrium
• Eliminate temperature differences among samples

Improvements to Separation and Measurement

• Perform sufficient filtration
• Check for adsorption onto the filter paper
• Prevent fine activated-carbon particles from remaining in the filtrate
• Measure within the linear range of the calibration curve
• Avoid turbidity and bubbles during absorbance measurement
• Perform multiple measurements and calculate the average value

Improvements to Analysis

• Clearly state the unit of qe
• Create a graph of equilibrium concentration Ce versus adsorption capacity qe
• Consider a Langmuir plot
• Consider a Freundlich plot
• Check the causes of outliers
• When comparing with literature values, match the conditions

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of an adsorption isotherm experiment, simply writing that “the concentration decreased” or “activated carbon adsorbed the substance” results in a superficial discussion.
A good discussion relates concentration changes, adsorption capacity, equilibrium concentration, adsorption sites, adsorption-isotherm equations, and the surface structure of activated carbon.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of adsorption isotherm experiments.
Adjust the necessary parts according to your own experimental results.

• Adsorption is a phenomenon in which molecules or ions in a solution are retained on a solid surface.
• Activated carbon is effective as an adsorbent because it is porous and has a large specific surface area.
• The decrease in concentration after activated carbon was added indicates that the adsorbate was adsorbed onto the activated carbon.
• Adsorption capacity can be determined from the difference between the initial concentration and equilibrium concentration.
• As the equilibrium concentration increases, the adsorption capacity per unit mass tends to increase.
• If the increase in adsorption capacity becomes more gradual in the high-concentration region, saturation of adsorption sites may be considered.
• The Langmuir equation assumes monolayer adsorption onto uniform adsorption sites.
• The Freundlich equation empirically represents adsorption onto a heterogeneous surface.
• If the contact time is too short, adsorption equilibrium may not be reached and the adsorption capacity may be underestimated.
• Insufficient filtration or contamination by fine activated-carbon particles causes errors in concentration measurement.

Points to Check When Discussing Adsorption Isotherms

Checking the following points before writing the report makes the discussion easier to write.

• Is the definition of adsorption explained?
• Is the reason why activated carbon is effective as an adsorbent described?
• Are the differences between the initial concentration and equilibrium concentration organized?
• Is the concentration decrease related to adsorption capacity?
• Are the equation and unit for calculating qe clearly stated?
• Are adsorption percentage and adsorption capacity distinguished?
• Is it considered whether adsorption equilibrium was reached?
• Is the shape of the adsorption isotherm explained?
• Is the relationship with Langmuir-type or Freundlich-type adsorption considered?
• Are the effects of contact time, particle size, stirring, and temperature considered?
• Are errors in filtration and absorbance measurement considered?
• Do the points for improvement correspond to the causes of error?

Summary

An adsorption isotherm experiment is an experiment that investigates how much adsorbate in solution is taken up by an adsorbent such as activated carbon.
When activated carbon is added, the adsorbate is adsorbed onto the activated-carbon surface and into its pores, causing the concentration in the solution to decrease.
From the difference between the initial concentration and equilibrium concentration, the adsorption capacity qe per unit mass can be determined.

An adsorption isotherm shows the relationship between equilibrium concentration Ce and adsorption capacity qe at a constant temperature.
As the equilibrium concentration increases, the adsorption capacity increases, but at high concentrations the adsorption sites on the activated-carbon surface may approach saturation and the increase in adsorption capacity may become more gradual.
This behavior can be discussed in relation to the Langmuir adsorption isotherm equation and the Freundlich adsorption isotherm equation.

In a report, rather than simply writing that “the concentration decreased,” organize and discuss the pore structure of activated carbon, concentration changes, adsorption capacity, equilibrium concentration, adsorption equilibrium, the shape of the adsorption isotherm, Langmuir-type and Freundlich-type adsorption, contact time, particle size, filtration operation, causes of error, and points for improvement.
Adsorption isotherm experiments are important experiments for quantitatively connecting surface phenomena with concentration changes.