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How to Leech for Metals

High levels of certain metals in waste can cause it to be classified as hazardous by governmental agencies. Hazardous waste policies require a test for leachable toxic metals if they are present in solid waste at levels above certain thresholds. The ability of metals to move out of the waste by leaching is an important environmental concern. In some cases, the chemicals are immobilized within the waste material. Analysts in certified laboratories perform a test to leech for metals and organic compounds in the United States, called the Environmental Protection Agency (EPA) "Toxicity Characteristic Leaching Procedure (TCLP)," or method 1311.

Things You'll Need

  • Zero-headspace extraction (ZHE) vessel made of glass, Teflon, polypropylene, polyethylene,
  • polyvinylchloride or stainless steel
  • Glass fiber filter
  • Filter holder of no less than 300 ml
  • pH meter
  • TEDLAR bags, or gas-tight syringes made of Teflon or stainless steel
  • Laboratory balance, accurate within 1/100 g for measurements of 0.1 g
  • 500ml beaker or Erlenmeyer flask
  • Watchglass for covering beaker or Erlenmeyer flask
  • Magnetic stirrer
  • Magnetic sirrer bar
  • Thermometer
  • Compressed air supply and tubing
  • Reagent water having undetectable amounts of the metal(s) being analyzed
  • Hydrochloric acid, 1 Normal, ACS reagent grade
  • Nitric acid, 1 Normal, ACS reagent grade
  • Sodium hyroxide, 1 Normal, ACS reagent grade
  • Glacial acetic acid, ACS reagent grade
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Instructions

  1. Sample Evaluation and Preparatory Steps

    • 1

      Make extraction fluid No. 1. Mix 5.7 ml of glacial acetic acid with 0.5 liters of reagent water. Add 64.3 ml of the 1 Normal (N) sodium hydroxide solution. Dilute this mixture to a final volume of 1 liter. The pH should be 4.93 +/- 0.005. Extraction fluid No. 2 is made by diluting 5.7 ml of acetic acid to a final volume of 1 liter. The pH of this should be 2.88 */- 0.05.

    • 2

      Do a preliminary evaluation on 100 g of the material. This involves (1) determining if there are insignificant solids or finding the percent solids, (2) evaluating if the solid part needs to have reduced particle size (if it passes through a 9.5 mm sieve it needs no reduction), and (3) finding the appropriate extraction fluid. The percent solids is determined by the proportion of solids retained on a pre-weighed filter after filtration. The formula for calculation of the percent dry solids is: % dry solids = ((weight of the dry waste + filter) - (tared weight of the filter)) / (initial weight of the waste ) x 100. If there is less than 0.5 percent dry solids, the extraction can be done directly on the waste. If there is more than this amount, you need to determine the type of extraction fluid.

    • 3

      Determine the appropriate extraction fluid for samples that have more than 0.5 percent dry solids. Place a 5-g sample (having a particle size less than 1 ml) in a 500-ml beaker or Erlenmeyer flask containing 96.5 ml of reagent water. Stir this for 5 minutes on a magnetic stirrer. If the pH of this water is less than 5.0, use extraction fluid 2. If not, add 3.5 ml of 1 N hydrochloric acid, cover the container with a watch glass, and heat to sustain a temperature of 50 degrees Centigrade for 10 minutes. If the pH is less than 5.0, use extraction fluid 1, otherwise use extraction fluid 2. If there is less than 0.5 percent dry solids, acidify the filtrate to a pH of less than 2 and directly analyze it as the TCLP extract.

    Using the Zero-Head Extraction Device

    • 4

      Load the ZHE device once with 25 g or less of the waste filtrate of the appropriate particle size (or liquid if there is < 0.5 percent solids). The weight of waste to be added is determined by the formula: Weight to charge the ZHE = (25 g / percent solids) x 100. On the top flange of the ZHE, place the filter and support screens, then secure the flange to the ZHE container. Secure all fittings and orient it vertically with the gas flange below. Connect the gas line to the flange valve on the bottom and, keeping the top flange valve open, apply up to 10 psi to force all air in the head space out. When liquid appears, immediately close the outlet valve. For mostly solid materials, slowly raise the pressure to 50 psi to force out most of the air.

    • 5

      Connect the filtrate collection vessel to the outlet valve and open it. Increase the pressure to 10 psi to force out liquid for 2 minutes, and repeat at 20, 30, 40 and 50 psi. Increases to the next stage occur after no liquid leaves at each increment of 10 psi, with a waiting period of 2 minutes. Close the valve and weigh the collection vessel that contains the filtrate. This fraction can be analyzed immediately or kept refrigerated at 4 degrees Centigrade until the other part is ready for analysis.

    • 6

      Calculate the weight (in grams) of the extraction fluid that needs to be added to the ZHE by the formula: Weight = (20 x % solids x weight filtered waste) / 100. Add the extraction fluid (after the pressure is released) to the vessel through the inlet port. Close the port and rotate the vessel end-over-end three times. Orient it vertically, then bleed out any head space while under a chemical fume hood. Apply pressure of 5 to 10 psi to the vessels and check the fittings for secure connections.

    • 7

      Put the ZHE container in a rotary shaker with a rotations of 30 revolutions per minute for 16 to 20 hours. Maintain an ambient temperature of 21 to 25 degrees centigrade. If the pressure was not maintained during this period, the procedure needs to be done again. If the pressure was maintained, collect the TCLP extract in TEDLAR bags or gas-tight syringes.

    • 8
      Computers attached to the spectrometer collect the data on metals concentration.

      Analyze the samples in four volume-equivalent portions. Add the metal standard to three of the fractions in different concentrations at a level of 50, 100 and 150 percent of the expected concentration in the unknown test sample. These aliquots are used to develop a linear regression analysis that helps determine the concentration of the leached metal in the unknown. The plot is made by putting the values of the external standard test result concentrations on the y-axis, then on the x-axis the concentration of metal plus standard in the four portions of the TCLP extract. The unknown concentration is the intercept value of the abscissa on the x-axis. The instruments that can be used to determine the heavy metals concentrations in the TCLP extract include inductively coupled plasma--optical emission spectroscopy (ICP--OES) and flame atomic absorption spectroscopy.

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