(349e) Competitive Adsorption Experiment: Application to Wastewater Pollutant Removal

Dye pollution of wastewater is a major problem worldwide.  Research is ongoing for economic and effective substrates to adsorb dye components.  Generally, wastewater contains many different pollutants, and their adsorption isotherms and kinetics vary.  Modeling the process and predicting the outcome is a challenge.

An inexpensive experiment was designed to help students apply statistical techniques to understand both the process and the analysis of competitive adsorption.  First, students measured the spectrophotometric absorption of various concentrations of three dyes individually, each at the three optimum wavelengths.  Next, a two-way ANOVA was performed to determine which factors (dye or absorption wavelength) affect the optical density.  Finally, the students used the data to determine a linear model (and statistical analysis of the parameters) for each dye at each wavelength.

The parameters were used to form a matrix, K, where k_ij is the absorptivity of dye i at wavelength j (i.e., the k values are the slopes of the linear models).  Using the additive property of absorbance, (A = K C, where A is the vector of measured absorbances and C is the vector of dye concentrations) this matrix was used to find the concentration of the dyes in binary and trinary mixtures from measurements of the absorbance at each wavelength.  The calculated concentrations compared to the known values had errors of 4 – 10%.

To demonstrate an adsorption application, the students also measured adsorption kinetics of the dyes individually and in combinations.  A known amount of adsorbant was added to the dye or mixture.   Liquid samples were removed periodically (every 1 – 3 minutes for 20 minutes) and the sample absorbance measured at each of the three wavelengths.  The rate data for each dye was analyzed for both pseudo-first order and second order kinetics.  The measure of competitiveness between dyes was found by comparing the kinetic parameters.  Eventually, different adsorbants could be compared for different dye mixtures to optimize the clean up of polluted wastewater.

An assessment of student learning shows that the students who perform the experiment can apply a two-way ANOVA and interpret the results correctly, and have a better understanding of how the chemistry of the solute and adsorbate affect the competitive kinetics.