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Dyes such as Congo red are often found in industrial wastewater and must be removed before the water can be discharged. Scientists performed 3 experiments to determine how much Congo red would be removed from a solution by binding to particles of bentonite (B), which is a type of clay, or to particles of a chemically modified bentonite (MB). In each trial of each experiment, Steps 1−5 were performed: 1. A 50 mL volume of an aqueous 300 mg/L Congo red solution having a particular pH was placed in each of 2 flasks. A specific mass of B was added to one flask, and the same mass of MB was added to the other flask. 2. The flasks were sealed and shaken at a speed of 200 revolutions per minute for a certain length of time at 25°C. 3. The contents of each flask were filtered to remove all solid material. 4. The concentration of Congo red remaining in the solution from each flask was measured. 5. The percent of Congo red that had been removed (% CR removed) from the solution was calculated for each flask. Experiment 1 In all trials, the pH of the Congo red solution was the same, and the shaking time was 360 min. From trial to trial, the mass of B and of MB was varied. The results are shown in Figure 1. Figure 1 Key B MB 100 d e vo m e r RC 40 % 0.000 0.050 0.100 0.150 0.200 mass of B or MB (g) Experiment 2 In all trials, the mass of B and of MB was 0.100 g, and the shaking time was 360 min. From trial to trial, the pH of the Congo red solution was varied. The results are shown in Figure 2. Figure 2 Key B MB 100 d e vo m e r RC 40 % 5 6 7 8 9 10 pH Experiment 3 In all trials, the pH of the Congo red solution was the same as it was in Experiment 1, and the mass of B and of MB was 0.100 g. From trial to trial, the shaking time was varied. The results are shown in Figure 3. Figure 3 Key B MB 100 d e vo m e r RC 40 % 0 30 60 90 120 150 180 210 240 270 shaking time (min) Figures adapted from M. A. Akl, A. M. Youssef, and M. M. Al-Awadhi, “Adsorption of Acid Dyes onto Bentonite and Surfactant-modified Bentonite.” ©2013 by M. A. Akl, et al.