The following data were reported by Roy et al (2001) for the adsorption of herbi

ID: 718009 • Letter: T

Question

The following data were reported by Roy et al (2001) for the adsorption of herbicide (2,4 dichlorophenoxy acetic acid designated 2,4-D) on a typical Louisiana soil. They equilibrated 5 grams of soil with 200 mL of aqueous solution containing known concentrations of 2,4-D and shook vigorously in a constant temperature shaker bath for 48 hours. The final equilibrium concentrations of 2,4-D in water were measured using a liquid chromatograph. Final Equilibrium Aqueous Concentration, mg/L Initial Spike Aqueous Concentration, mg/L 1.0 2.9 6.1 14.8 45.3 101.9 148.0 239.5 1.1 3.1 6.2 15.0 48.9 105.3 152.8 255.0 Based on appropriate statistical methods, determine which of the three isotherms, linear, Freundicd, or Langmiuir, providés thebetrepresentation of the datia? Show your work clearly. Tabulate the results from all three isotherms on a separate page.

Explanation / Answer

Given the feed volume (V) = 200 ml = 0.2 L

Mass of adsorbent used (m) = 5 gm

Equilibrium time = 48 hrs

Final equilibrium aqueoous concentrations are Ce,mg/L

Initial spikeaqueous concentrations are Ci, mg/L

Now calculating, adsorbent capacity qe mg/g = (Ci - Ce) V/m

Linear Isotherm: Linear isotherm is a plot of qe vs. Ce (Fig. 1).

Fig. 1: Linear Isotherm (qe vs. Ce).

Freundlich Iostherm: Freundlich isotherm can be represented as;

= 1

and its linear form is log qe = log Kf + 1/n log Ce.

Fig. 2 represents a plot of linear form of Freundlich isotherm.

Fig. 2: Freundlich Isotherm (log qe vs.log Ce).

Langmuir Isootherm:

The linear form of Langmuir Isotherm can be given as:

Fig. 3 represents a plot of linear form of Langmuir Isotherm.

Fig. 3: Langmuir Isotherm (1/qe vs. 1/Ce).

The plotted data was correlated via linear regression and R2 values were obtained as:

Isotherm

R2 Value

Linear

0.8849

Freundlich

0.8675

Langmuir

0.5162

As depicted, Linear isotherm has represented the best fit for the given data with R2 value of 0.8849.