Getting the lead out
December 28, 2009 10:13 AM   Subscribe

Please help me with my science fair project.

I'm exploring safe ways to remove lead from contaminated soil. One of the techniques I want to use is electrokinetic remediation. From the research I've done, I understand that passing an electric current through the soil should separate lead particles. However, the only diagrams I've been able to find look like this and I don't understand what's going on there.

So if I want to build something simple to test this out, how do I go about doing so? I get a pot of soil with lead mixed throughout ... and then?
posted by estlin to Science & Nature (7 answers total) 2 users marked this as a favorite
Best answer: A few resources that might help. This PDF may be something you have already found, but seems pretty informative. It references a journal article, "Pb(II) Removal from Kaolinite by Electrokinetics" by Hamed et al. in Journal of Geotechnical Engineering, Vol. 117, No. 2, February 1991, pp. 241-271 -- MeMail me if you'd like a copy.

Basically it looks like you're trying to apply a DC voltage across the soil to try to get Pb+ to migrate, and then remove the soil on the high-concentration side. The diagram you link to seems to be a system for doing that and includes e.g. the pumping apparatus for moving contaminated fluid around, which is why it seems so convoluted.

One thing you're going to need to learn very quickly is (a) how sensitively you can test for lead, and (b) how effective the technique you're using might be in the best-case scenario. It's entirely possible that you would miss an effective technique because your detection is not sensitive enough to tell that you made a difference.

You can amplify the effects of the separator by stealing a trick from the Manhattan Project -- run the separator on a soil sample, then dig up the high concentration side and use that as fodder for another separator, repeat until you can detect a change.
posted by range at 10:32 AM on December 28, 2009

Brain damaged link.
posted by range at 10:32 AM on December 28, 2009

I think you need to ponder your lead-detection assay a little. Like range said, the sensitivity of your assay may be the limiting factor here. I'm not a heavy metals chemist, but the methods I know of for detecting metal ions tends to be pretty involved (mainly atomic absorption spectroscopy and/or cell culture toxicity).

In a few minutes of Googling the only assays I could find were for bulk amounts of lean, like for testing ore samples from a mine or something. In your case, you need a more sensitive assay. I couldn't find anything practical for a school science fair (assuming you don't have access to a real analytical lab).

I'd suggest giving the assay side of things some more thought before proceeding. Would you have any money for sending samples out to be tested? Galbraith Labs will test a sample for Pb for $69. There are other analytical labs that do this kind of thing; maybe you could find one that would run your samples for free/cheap since you're a student.

If you have any ideas for Pb testing, maybe you could post them here and we could offer some additional help. Good luck!
posted by Quietgal at 11:48 AM on December 28, 2009

you know I meant lead
posted by Quietgal at 11:57 AM on December 28, 2009

Response by poster: Thanks for that link, range. Reading it now.

And Quietgal, we were actually planning on using distillation to measure the amount of lead left in the soil after passing the charge through. My chem teacher said it'd work; do you have any concerns that it wouldn't? [My chem teacher has supplies of bits of lead that we can use, so that isn't an issue.]
posted by estlin at 12:45 PM on December 28, 2009

Best answer: Estlin, I'm not personally familiar with distillation for measuring heavy metals but after searching a bit it seems that the distillation process is for purification before analysis. In other words, you get rid of the other junk that might interfere with your assay by distilling the sample first. But you still need a method to measure how much lead you recover after distillation.

I think your teacher is planning to do a gravimetric analysis (a fancy way of saying you plan to weigh the sample). If you have a lot of lead, this would work. But I'm guessing that your analytical balance requires at least 10 mg of sample to get an accurate weight. Some balances are even less sensitive. Keep this in mind when you design your experiments.

I realize this is a school project so I don't want to go all SCIENCE! on your ass, but it's a good introduction to the complexities of doing research. In your case, one of the first things to consider is "How much lead is it OK to have in soil?" From there you decide how sensitive your assay needs to be: let's say it's OK to have 10 mg Pb per kg dry soil (I'm just making that up for the sake of convenient numbers). Therefore your assay needs to be able to detect less than 10 mg/kg so you know if your soil is safe or not.

Of course, you have to extract the Pb from the soil, and you may not be able to take a full kg sample of soil. But let's say you can get 100 g samples. Now you need an assay that can detect less than 1 mg of Pb. There are pricey analytical instruments that can do this easily, but there are more things to consider.

A big problem in analytical chemistry is all the junk in a sample that will interfere with your assay. Whatever method you pick, something will interfere with it. Guaranteed. So you need to clean up your sample, getting rid of the junk while retaining the stuff you want to analyze. But no separation method is perfect - you will always lose some of your analyte and retain some of the junk. So you need to set up a series of so-called standards and run them through the entire process. In your case, you would make several soil samples with various known amounts of Pb, run them through your cleanup process (distillation), then measure how much lead you recover. Compare that to the known amount you put in, and that's the overall recovery for your process.

Let's say your average recovery is 50%. Now you know your assay has to be able to detect less than 0.5 mg of lead. That's starting to get below the range of most analytical balances that a high school would have. Unless you have a flame atomic absorption spectrometer lying around, or a few grand to buy a used one (and a safe place to set up the acetylene tank), you're probably looking at a contract lab for assaying your samples.

I won't even go into precision and accuracy, specificity (e.g., how do you know it's lead and not zinc or something else?), interference and signal suppression (e.g., the presence of certain contaminants may prevent the Pb from giving a signal, which is not a problem with gravimetric assays but can be a real bitch with spectroscopic assays), and other problems that make analytical chemists tear their hair out.

Of course, you could just decide that a science fair project doesn't need to be all that realistic, and a "yes/no" result is good enough. Meaning, you can detect Pb before electrophoresis but not afterwards (and it doesn't matter what your sensitivity and recovery are because this is just high school and the fact that you're doing this project instead of hanging out at the mall is already pretty damn good).

Good luck and let us know if you need anything else!
posted by Quietgal at 3:19 PM on December 28, 2009

posted by Jinx of the 2nd Law at 8:18 PM on December 28, 2009

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