Effect of salinity on kinetics of adsorption of Allura Red Dye

How does the pseudo first order rate constant of adsorption (measured in min^-1) of the dye –Allura Red (Carmoisine-a food color) from a solution of it in aqueous NaCl by activated charcoal (as adsorbent) depends on the molar concentration of the NaCl used, determined using colorimetry?

Inquiry leads to production of knowledge and the process gains optimum output when that is connected to the real-life observation of a knower – This concept that I studied in TOK class has always evoked me to ask questions about any fact and observations that I come across and use my best scientific judgements to justify them. Chemistry was not an exception either. Though the global pandemic has brought an end to the aspirations of life in mankind but it has healed the nature as well. I realized this while coming across the fact that the turbidity of River Yamuna in the city of Delhi, India has decreased during the times of pandemic which made the issue of accumulation of domestic waste polluting the water bodies more prominent. One of the most effective way to clean these biological waste or domestic waste from water bodies is adsorption. In within the water bodies. Off-late recent research by an environmentalist in Japan led to the introduction of a material prepared from the waste non-biodegradable plastics which acted as an adsorbent of the toxic and carcinogenic organic products involved. However, the same method was not found to be effective enough when used for Arabian Sea in the Mumbai belt of India. Investigations and analysis showed that there were several factors related to the differences in composition of river water and sea water which can be accounted for this. This brought me to the question, by any chance is salinity the factor that makes the adsorption process as a cleansing method less effective for adsorption. Does the presence of a foreign ionic impurity like NaCl will have an effect on the adsorption kinetics of a dye? The chemical waste products present in sea water are mostly heterocyclic organic compounds or azo compounds. To mimic that in a laboratory set up, the food color-Allura Red dye, an azo dye was chosen. Activated charcoal is one of the most widely used adsorbent due to its highly porous surface. Thus, I finally arrived at the research question stated above.

Allura Red dye is a food color popularly known as Carmoisine Red (E 122). It has the molecular formula – C₁₈H₁₄N₂Na₂O₈S₂. It is a synthetic azo dye where the term azo indicates the presence of the N = N. It is made by the coupling reaction of 6 - hydroxy naphthalene - 2 - sulphonic acid and 4 - Amino - 5 -methoxy - 2 - methylbenzene sulfonic acid. At room temperature, it appears as a red amorphous powder with a melting point of 300.00⁰C⁸. This dye is used as a food colour in various soft drinks, chocolates and even preparations of processed meat products. As indicated from the molecular structure given below, the dye consists of two anionic sulphonic acid groups – SO₃ - which makes an ion pair with Na⁺ ions and thus the dye exists as a disodium salt. Despite having three hydrophobic phenyl rings, the dye is highly soluble in water due to the presence of a phenolic OH group and the O^- atoms of the SO₃- groups which can make intermolecular H bond with the water molecules.

Adsorption is a surface phenomenon where the molecules of a substance (adsorbate) adhere to the surface of another substance (adsorbent). In this investigation, the food coloring dye- Allura Red Dye (ARD) is the adsorbate and activated charcoal is the adsorbent. Activated charcoal acts as a potential adsorbent because it is highly porous in nature. Adsorption finds applications in various fields like delivery of drugs, chelation therapy where removal of heavy metals by complex ligands, blood coagulation. There are two types of adsorption – physisorption and chemisorption9. In physisorption, the adsorbate molecules are bonded with the adsorbent molecules via physical forces of attractions like – Van derWaal forces of attraction, dipole-dipole forces, Hydrogen bonds and so on. In chemisorption, the adsorbate molecules are bonded to the surface of the adsorbent via covalent bonds. Physisorption is a monolayered phenomenon while chemisorption is a multilayered phenomenon. Adsorption of Allura Red Dye (ARD) by activated charcoal is an example of chemisorption.

ARD (adsorbate) + AC (adsorbent) ---------→ ARD - AC (adsorbed complex)

ARD = Allura Red Dye

AC = Activated charcoal

The rate of this adsorption reaction is found to be of overall second order. The orders with respect to ARD and AC are 1.

rate = k [ARD][AC]

If the concentration of AC is taken in huge excess of ARD, the reaction can be considered to be of pseudo first order with respect to ARD.

if [A] ≫ [ARD]

[AC] ≅ constant

rate = k' [ARD] where k'' = pseudo first order rate constant = k × [AC]

\(rate=k'[AC]\frac{-d[ARD]}{dt}=k'[ARD]\)

\(\frac{-d[ARD]}{[ARD]}k'dt\)

\(\displaystyle\int\frac{-d[ARD]}{[ARD]}\displaystyle\int k'dt\)

- ln [ARD] = k't + c.................equation (1)

At time (t) = 0 [ARD] = initial concentration of ARD = [ARD]₀

- ln [ARD]₀ = c

- ln [ARD] = k't - ln [ARD]₀

- ln [ARD]₀ - ln [ARD]₀ = k't

ln \(\frac{[ARD]_0}{[ARD]}=k't\).................equation (1)

According to Beer-Lambert law

Absorbance (A) = ∈ × c × l

If the same sample is taken, the molar absorptivity constant ∈ will remain constant and the same colorimeter is used, the path-length (l) will also remain same.

Therefore, Absorbance (A) ∝ molar concentration (c)

Thus, equation-2 can be re-written as:

\(ln\frac{A_0}{A}k't\)

A_o = initial absorbance of ARD

A = absorbance of ARD at time t

Thus, if ln \(\frac{A_0}{A}\) against time(t), the gradient of the curve obtained is the pseudo first order rate constant.

In a research paper titled – “Effectiveness of Alkali-Acid Treatment in Enhancement the Adsorption Capacity for Rice Straw: The Removal of Methylene Blue Dye” by Nady Fathy it was reported that the removal of the dye methylene blue by rice straw used as adsorbent was found to decrease from 90 % to 82% as the molar concentration of NaCl increases from 0.05 moldm^-3 to 0.20 moldm^-3. This shows that the increase in the concentration of NaCl has caused the adsorption of methylene blue by rice straw to occur at a slower rate.

The experiment was executed using the least possible amount of chemicals.

All waste materials were disposed safely.

Any toxic gases were not evolved.

• A 100 cm³ glass beaker was taken.
• A watch glass was placed on a top pan digital mass balance and the reading was tared to 0.00 ± 0.01 g.
• Solid NaCl was transferred to the watch glass from the reagent bottle to a watch glass using a spatula until the digital mass balance reads 0.58 ± 0.01 g (0.01 moles).
• The weighed mass of NaCl was exactly transferred to the glass beaker.
• 4.96 ± 0.01 g (0.01 moles) of Allura Red Dye (ARD) was weighed using the digital mass balance and added to the same beaker.
• Distilled water was added to the same beaker using a graduated measuring cylinder till the mark of 100 cm³.
• 1.20 ± 0.01 g of activated charcoal was weighed using the digital mass balance and added to the same beaker.
• The weighed mass of activated charcoal was transferred to the beaker slowly along a glass rod so that it settles down at the bottom.
• As soon as the activated charcoal settles down at the bottom, the stop-watch was started.
• A graduated pipette was used to extract 1.00 ± 0.05 cm³ of the supernatant liquid and transferred to a cuvette. The absorbance of the solution was recorded using a digital colorimeter setting the wavelength at 504 nm. The colorimeter was calibrated using distilled water at the same wavelength and the reading was adjusted to 0.000 ± 0.001 abs.
• As soon as the stop-watch reads 5.00 ± 0.01 mins, step-10 was repeated.
• Step 11 was repeated at 10.00 ± 0.01 mins, 15.00 ± 0.01 mins, 20.00 ± 0.01 mins and 25.00 ± 0.01 mins.
• Steps 10-12 were repeated for three more times.
• Steps 1-13 were repeated using 1.16 ± 0.01 g, 1.74 ± 0.01 g, 2.32 ± 0.01 g and 2.90 ± 0.01 g of NaCl.

The red color of the solution started to fade out with the progress of time. As the concentration of NaCl has increased, the disappearance of color with time was slower.

Formula used: Pseudo first order rate constant (k’) in min^-1 = Gradient = Co-efficient of x in the equation of trend line

As the molar concentration of NaCl increases from 0.10 moldm^-3 to 0.50 moldm^-3, the pseudo first order rate constant decreases from, the pseudo first order rate constant decreases from 26.90 × 10^-3 min^-1  to 4.40 × 10^-3 min^-1. This shows that as the NaCl solution used as medium is more concentrated, the rate at which the dye molecules are adsorbed by the surface of the activated charcoal decreases.

The decrease in the pseudo first order rate constant is not gradual in nature as the distance between the consecutive points are not the same. The major difference is found in between the values of pseudo first order rate constant at 0.30 moldm^-3 and 0.40 moldm^-3. Moreover, the trend line plotted using MS-Excel clearly indicates that the point at 0.30 mol dm^-3 is the most deviated point and this indicates that there is a systematic error in this investigation.

The equation of trend line has been obtained using MS-Excel and it indicates that the pseudo first order rate constant and the molar concentration of NaCl is related according to the equation: y = -56.10 x + 33.07 where y represents the pseudo first order rate constant and x indicates the molar concentration of NaCl.

The gradient as indicated in the equation of trend line is -56.10 and the negative value confirms that there is a negative correlation between the pseudo first order rate constant and the molar concentration of NaCl.

The value of regression coefficient has also been obtained in MS-Excel and the value is 0.9395 which confirms that there is a 93.95% correlation between the pseudo first order rate constant and the molar concentration.

As the molar concentration of NaCl increases, there are more number of Na⁺ and Cl- ions in the medium. Thus, most of the surface sites are occupied by the ions and there are none available for the dye molecules to bind with. As a result, less number of dye molecules are adsorbed on the surface of the activated charcoal, the decrease of absorbance slows down. This is why the decrease of absorbance within the same time frame is less. For example, at 0.10 moldm^-3 NaCl, the absorbance decreases from 0.964 ± 0.001 abs to 0.565 ± 0.001 abs while at 0.50 moldm^-3 NaCl, the decrease is from 0.964 ± 0.001 abs to 0.867 ± 0.001 abs. (Refer to Table - 1). This proves the claim that as the molar concentration of NaCl increases, more area in the surface are blocked by the foreign ions and thus there are less free surface sites available to make chemical bonds with the dyes which eventually decreases the rate of adsorption.

How does the pseudo first order rate constant of adsorption (measured in min^-1) of the dye –Allura Red (Carmoisine-a food color) from a solution of it in aqueous NaCl by activated charcoal (as adsorbent) depends on the molar concentration of the NaCl used, determined using colorimetry?

• As the molar concentration of NaCl increases from 0.10 moldm^-3 to 0.50 moldm^{- 3}, the pseudo first order rate constant decreases from, the pseudo first order rate constant decreases from 26.90 × 10^-3 min^-1 to 4.40 × 10^-3 min^-1.
• The decrease in the pseudo first order rate constant is not gradual in nature as the distance between the consecutive points are not the same.
• The gradient as indicated in the equation of trend line is -56.10 and the negative value confirms that there is a negative correlation between the pseudo first order rate constant and the molar concentration of NaCl.
• As the molar concentration of NaCl increases, more area in the surface are blocked by the foreign ions and thus there are less free surface sites available to make chemical bonds with the dyes which eventually decreases the rate of adsorption.
• The qualitative observation that the color of the solution fades with time supports the conclusion.

• The independent variable chosen has a wide range and is continuous in nature. The difference between the consecutive values are same. This allows a fair and accurate analysis.
• The data processing is coherent. For example, Figure - 8 to 12, the raw data table indicates that as the molar concentration increases, the decrease of absorbance within the specific time interval is lesser. Figure - 15 indicates that as molar concentration increases, the pseudo first order rate constant decreases which means that the number of dye molecules adsorbed within 25 minutes gets lesser and thus the fading of color is slow.
• The factors-temperature, pH which may affect the reliability of the data has been controlled.

Apart from presence of foreign impurities, there are other factors like pH which also has an impact on the rate of adsorption. I would like to perform an investigation to study the effects of pH on rate of adsorption. To do this, I would like to perform the adsorption of a dye in presence of solutions with different buffer tablets. The absorbance of the solution can be recorded as a function of time using a colorimeter. The scatter plot of absorbance versus time can give us the rate of adsorption from the value of gradient. Thus, the rate of adsorption at different pH values can be measured. This has a real life application as bio-sorbents used to remove toxic metals from wastewater depends on the pH at which it is carried out.

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