CE 170: Environmental Engineering

Lab Procedures

Adsorption

Objectives

Our purpose is to demonstrate the phenomenon of adsorption and compare the different adsorptive behaviors of several soil types. We will run a simple batch adsorption test using methylene blue (MB), a cationic organic dye, and several different soil adsorbents (coarse sand, medium sand, fine sand, silt and clay).

Procedures

Methylene blue will be measured using a spectrophotometer. A spectrophotometer can sense the absorbance of light at specified wavelengths.

Absorbance = log(I0/I)

where I0 and I are the intensities of the light entering and leaving the test sample, respectively. Light absorbance at a wavelength of 655 nm has been found to work well for detecting methylene blue. The absorbance can be related to the concentration of the light-absorbing substance through use of a linear calibration curve.

Calibration curve procedure

  1. Your instructor will provide a series of standard methylene blue solutions.
  1. Measure the absorbance of the standard your instructor gives you and report that value to the class to be recorded on a spreadsheet.
  1. When all the groups have reported their standard measurements, use the linear regression routine in Excel or on your calculator to pass a line through the data. What we want is a calibration curve of the form:

C = mA + b

where

C = concentration in mg/L

A = absorbance

m = slope, and

b = intercept from the linear regression.

  1. Use the calibration equation to calculate methylene blue concentrations from absorbance values in the rest of the lab.

Measurement of Partitioning Coefficient

  1. Your instructor will assign your group an adsorbent and will instruct you on how much use. Weigh out the specified amount and add it to a 80- or 100-mL beaker. It's OK if you don't hit the specified amount exactly; just be sure to record the mass you use.
  1. Place 50 ml of methylene blue stock solution (10 mg/L) into the beaker.
  1. Swirl or stir the beaker for at least 15 minutes.
  1. Let the contents of the beaker settle for a couple of minutes and then transfer the top approximately 25 mL of your volume (i.e., the supernatant) to a centrifuge tube. Give the tube to your instructor for centrifugation. (It’s all right if some of the solids get into the centrifuge tube, but try to minimize them.)
  1. After centrifugation, CAREFULLY remove your tube. (You don’t want to stir up the contents.) Use a pipet to remove about 10 mL of the supernatant, and place it into a small test tube for the spectrophotometer. Do this step very carefully. Suspended solid material will interfere with your absorbance reading.
  1. Put the test tube into the spectrophotometer and read the absorbance.

Data Analysis

  1. Calculate the mass of MB adsorbed per mass of adsorbent (x/m)

x = mass of MB adsorbed

= (mass of MB in beaker at start) - (mass of MB at end)

= (Ci*VMB)start - (Ce*VMB)end

m = mass of adsorbent (g)

  1. Calculate the linear partitioning coefficient expressing the relationship between x/m and Ce:

Kp = partitioning coefficient = (x/m)/Ce

  1. Compare your results with those of your classmates to see how the different materials act.