Soil—A Natural Filter

Introduction

The ability of soil to bind and store nutrients and chemicals is investigated by filtering two organic dyes—a positively charged and a negatively charged dye—through soil and sand.

Concepts

• Soil quality and texture
• Cation exchange capacity

Background

Soil or “dirt” is something most of us see every day. However many people may not realize that the U.S. Department of Agriculture (USDA) has identified more than 15,000 soil types in the United States alone! What is thought of as dirt is actually a complex mixture of organic matter, inorganic materials, water, air and living organisms. Roughly, soil is composed of 50% water and air. The USDA classifies soil particles into three main categories—clay, sand and silt.

Soil acts as a natural filter by absorbing chemicals that may be applied to the soil from outside sources. One of the most important functions of soil is to store nutrients and water for exchange with plant roots. This is primarily provided by the clay and humus (organic material from decaying organisms) in soil. Clay consists of negatively charged colloidal particles that selectively adsorb positively charged cations. The ability of soil to bind these ions is called cation-exchange capacity or CEC. Cation exchange provides the mechanism for the uptake of potassium, calcium, magnesium, and other trace metals required by plant life.

The ability of the soil to stand up against runoff and ground water contamination depends heavily on the composition of the soil. Sandy soils easily drain water and do not bind nutrients well. Due to this, sandy soils often do not support root growth or many microorganisms. Soil containing lots of clay is dense and readily holds water but has a tendency to become waterlogged. Excess water excludes air and the soil creates an anaerobic environment. Microorganisms that are a necessary part of healthy soil may die off. However, clay easily binds nutrients. Silt falls somewhere between sand and clay retaining a fair amount of water without becoming waterlogged and binds well to nutrients. The best soil for planting crops contains a balance of sand, silt and clay.

Materials

Safety Precautions

Methylene blue solution and eosin Y may stain skin and clothing. Wear chemical splash goggles and chemical-resistant gloves—do not handle soil with bare hands. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The dye solutions and filtrates may be disposed of down the drain according to Flinn Suggested Disposal Method #26b. Syringes may be rinsed out with water and saved for future demonstrations. Sand and soil may be thrown in the regular trash according to Flinn Suggested Disposal Method #26a.

Prelab Preparation

To prepare a 0.2% eosin Y solution, pour 10 mL of the 1% eosin Y solution into a 100-mL beaker and add 40 mL of distilled water. To prepare a 0.2% methylene blue solution, pour 10 mL of the 1% methylene blue solution into a 100-mL beaker and add 40 mL of distilled water. Obtain a soil sample two days prior to the lab and allow it to air dry by spreading the sample out on newspaper.

If available, four different support stands may be used—one for each syringe. Otherwise, one support stand with four clamps, each positioned 90 degrees apart, adjusted to different heights, will also work.

Procedure

1. Remove the plunger from all four 60-mL syringes.
2. Place a small piece of cotton into the tip of each syringe to block the openings.
3. Add approximately 20 cm³ of air-dried soil to two of the syringes and pack the soil down by tapping the side of the syringe. Clamp each syringe to a support stand, tip down, and place a flask or beaker under the syringe tip (see Figure 1).

4. Add approximately 20 cm³ of fine sand to the remaining two syringes.
5. Clamp these syringes to the support stand with a flask or beaker placed under each tip.
6. Using a graduated cylinder, add 20 mL of methylene blue solution to one of the syringes containing the air-dryed soil sample. Allow the solution to pass through the soil and collect the filtrate in the flask or beaker below.
7. Using a graduated cylinder, add 20 mL of eosin Y solution to the other syringe containing the air-dryed soil sample. Allow the solution to pass through the soil and collect the filtrate in the flask or beaker below.
8. Repeat steps 6 and 7 for the two syringes containing the sand samples.
9. Observe the volume and the color of the four filtrates. Students should record this information in the Data Table on the worksheet.

Student Worksheet PDF

12621_Student1.pdf

Teacher Tips

• This kit contains enough chemicals to perform the demonstration seven times: 100 mL of eosin Y, 1%, 100 mL of 1% of methylene blue solution, 500 g of fine sand, 10 cotton balls and four 60-mL syringes.
• As a follow-up, have students bring in soil samples from different locations for testing.
• The structures of methylene blue, an organic redox dye and eosin Y (also called eosin red), a fluorescent biological indicator stain, are shown below. Methylene blue is blue in the oxidized form and the chromophore (colored part of the structure) is positively charged. Eosin Y, is red but has a strong green fluorescence. The chromophore is negatively charged (anionic).
  
  {12621_Tips_Figure_2}
• Related websites for more information
  http://www.soilinfo.psu.edu/index.cgi?soil_data&conus&data_cov (Earth System Science Center at Pennsylvania State University) (accessed July 2018)
  http://websoilsurvey.nrcs.usda.gov/app/ (Natural Resource Conservation Service) (accessed July 2018)

Sample Data

Answers to Questions

1. Did the sand samples and/or the soil samples absorb water? Which absorbed the most water?
   

   Yes, all samples absorbed some water. The soil samples absorbed approximately two times as much water as the sand samples.

2. Compare the result of the dyes after filtering through the soil samples. Which dye was readily absorbed by the soil sample? Explain.
   

   The methylene blue dye was absorbed by the soil sample. The methylene blue solution was dark blue originally and colorless once passing through the soil. This indicates that the soil filtered out the dye molecules. The eosin Y however was not filtered out by the soil sample since the filtrate was the same red color as the original solution before being passed through the soil.

3. Why was one dye retained by the soil while the other moved through the soil unaffected?
   

   Soils bind cations, not anions. Methylene blue is a cationic dye, whereas eosin Y is an anionic dye. Only the cationic methylene blue was filtered out of the soil sample.

4. What purpose did the sand serve in this demonstration?
   The sand samples served as controls. Due to the large particle size, low surface area, and lack of charge, sand does not bind to cations or anions.

References

This activity was adapted from Chemistry in the Environment, Flinn ChemTopic™ Labs, Volume 22; Cesa, I., Ed.; Flinn Scientific: Batavia, IL, 2006.