Issue



End-of-Life Leaching of Lead and Other Elements


10/01/2005







By Bev Christian, Alexandre Romanov, and David Turner

There is much concern about what will happen when consumer electronics being stored in basements, garages, and attics reach municipal landfills. This article uses the EPA Method 1311, Toxicity Characteristic Leaching Procedure (TCLP) as the testing basis for leaching studies. The well-known EPA leaching protocol was used to determine end-of-life leaching of lead and other elements for whole portable electronic devices, cut-up circuit packs, and finely-ground circuit packs. Some individuals have expressed concern about this test because of the extreme solubility of lead acetate, as opposed to more common salts of lead (Table 1). This test is meant to determine the absolute worst case for leachate formation, and not what actually leaves the landfill as leachate.


Table 1. Solubility of some lead salts.
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In the present work, the protocols called out in the EPA method have been adhered to for testing. However, additional testing was also done using the same method, except for the form in which samples were introduced to the leaching medium. These additional samples were either whole units of portable electronic devices; whole, cut circuit packs; or circuit packs ground to different degrees of fineness. These tests compared how the surface area of the samples affect metal concentrations in the leachate. While finely ground electronic devices will not be landfilled, some whole devices that have not been disassembled inevitably will find their way into landfills.

This study also examines what effects introducing sulfide (S-2) or carbonate (CO3-2) has on the solubility of the elements of interest leached into the solution. Finally, leachate of known amount and concentration of dissolved material was percolated through soil to show what would happen if leachates were released from a landfill and found their way into the soil. This work is preliminary.

Experiment

Samples used were portable electronic units soldered with Sn36Pb2Ag. Depending on the portion of the study, whole units, the plastic casings or whole circuit boards (as is, cut, or ground) were used. Except in the case of leaching whole units, LCD modules and RF shields were not subjected to further testing.

The analytical protocol used is EPA method 1311, TCLP. Samples were cut to EPA-specified size using either a band saw or a cross-sectioning cut-off saw. Initial work showed that extraction fluid #1 was necessary (acetic acid/NaOH/ASTM Type-II water). A grinder usually equipped with a tungsten-carbide blade was used after initial experiments. Samples were ground for 20 minutes. The samples were mixed end-over-end in a tumbler at 30 rpm for 18 hours. Analysis for metal contamination was conducted with an ICP-OES unit. The ICP-OES was calibrated for each element and calibrations were checked by verification standards for quality control. Recovery of the verification standard for each element was between 90% and 110%.

Results

The first three groups to be leached were: whole units, populated circuit boards cut into pieces, and ground circuit boards (three samples each, nine samples total). When actual electronic devices are disposed of, only whole units or crushed or damaged units are likely to make it into a landfill. The leaching of ground circuit packs is for worst-case research purposes and is not likely representative of actual waste conditions.

Where there was no metal found, less than method-detection limits (<MDL) was recorded. Beryllium, bismuth, cadmium, indium, palladium, platinum, and selenium were not found in any leaching samples. Results from the leached samples reveal high levels of five metals (Figure 1).


Figure 1. Mean concentrations of Cu, Fe, Ni, Pb, and Zn.
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Once the second sample in the grind and sieve through #80 group was leached and filtered, it had a light blue color, while other samples had a light brown color. The blue color suggests that the sample had a higher concentration of copper. This assumption was confirmed, as the concentration of copper was 502 ppm. There is no explanation for this difference.

It seems intuitive that the concentrations of metals in the leachate should increase as samples go from whole units to cut boards to coarse and fine grinds. Of the five major elements detected, all but iron follow this pattern. An exception is that whole units leach about three and four times the amounts of nickel and zinc as the cut units produce, respectively.

One result of note was the high concentration of iron from the cut circuit packs compared to the ground samples (Figure 2). Iron concentrations range from 250 to 304 ppm for the cut samples to 106 and 51 ppm for the sample portions going through sieves #35 and #80, respectively. This contradicts the expected trend of ground samples leaching higher concentrations of metals. As iron concentrations decrease, concentrations of lead and copper increase.


Figure 2. Significant elements found.
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Leaching concentrations of silver, arsenic, gold, antimony, and yttrium were found in small amounts (<1/3 of a ppm for each). These were found in leaching solutions for ground circuit boards, not whole or cut samples. At these levels, it is immaterial whether electronic devices would be ground before disposal.

The data show that adding either carbonate or sulfide has a profound effect on the amounts of heavy metal ions that remains in the solution. The effect of the addition of carbonate depends on the ion in question. There is little effect on zinc or nickel, halving the concentration of copper - more than halving the concentration of lead - and producing a concentration reduction of iron by up to eightfold. For the sulfide addition, the soluble-ion concentrations are cut to almost zero, except in the case of iron where it was cut by more than a factor of ten. These results were not unexpected.

Finally, a spike with 500 mg of iron, silver, copper, nickel, zinc, barium, and lead (major elements of the leachate) in 2% nitric acid were passed through soli samples. A bag of commercially available topsoil was bought, air-dried at about 30°C, and loosely packed into each of six small chromatographic columns with an ID of about 1 cm. The soil was washed repeatedly with deionized water (~200 ml total per column) to remove entrapped air and settle the soil in the columns. Two columns were used as blanks (5 ml of 2% nitric acid was put on each), and the remainder received the 5-ml spike. One portion of 15 ml and later six portions of 20-ml Milli-Q water were pushed through the first three columns (Blank, Spike 1, and Spike 2). Each portion of the eluant was collected and analyzed separately. For the other three columns, six portions of 20-ml EPA leaching solution #1 followed one portion of 15-ml elemental spike. Again, each portion of the eluant was collected and analyzed separately. The capacity of the soil samples to hold metal ions is significant. Little came off the column - on the order of single-digit ppm values or less. Further work is needed.

This study confirmed that at least for a particular, intact handheld electronic device, the intact nature of the equipment can prevent high levels of elements from leaching from the unit in short periods of time (if disposed-of in a municipal landfill under worst-case leaching conditions).

Whole units leached five elements: copper, iron, nickel, lead, and zinc. Copper, iron, and lead contamination was reduced greatly when the unit was left intact. Nickel and zinc contamination was three times higher for whole units as opposed to cut and ground samples.

Ground circuit boards leached high concentrations of copper, lead, and iron. A surprising finding was the high concentrations of iron from cut samples (~284 ppm) compared to ground samples (~94 ppm). No explanation can be offered for this; however, it may be tied to an inverse relationship to the concentration of copper. Topsoil also has a significant effect on cation concentrations tested, resulting in much-reduced amounts escaping capture in a column of soil. Ultimate soil capacity was not determined. It would appear that the possibility of significant amounts of heavy metals escaping from modern, well-maintained landfills is low.

This article is excerpted from a paper presented at the International Lead-free Conference; Toronto, May 2005. For information, e-mail: bchristian@rim.com.

BEV CHRISTIAN, Ph.D., manager, Materials Interconnect Lab, Research in Motion, may be contacted at (519) 888-7465, ext. 2468; e-mail: bchristian@rim.com. ALEXANDRE ROMANOV, chemist, Materials Interconnect Lab, Research in Motion; (519) 888-7465, ext. 3178; e-mail: aromanov@rim.com; DAVID TURNER is a mechanical engineering student, University of Waterloo, Ontario, Canada.