Question

You are an environmental geochemist working at a contaminated site. How would you evaluate possible heavy metal contamination in the soil? Which method(s) would you use? Which steps would you take to ensure proper sample handling? How would you prepare samples for the methods of analysis you would use in the lab?

Answer 1

Evalution of heavy metals in soil samples most accurately can be done by the use of ICP-OES (Inductively Coupled Plasma- Optical Emission Spectroscopy), which is a most advance method to detect the concentrations of heavy metals present in the samples. I myself have worked with this project so i have a knowledge how to perform this.

It is based on the Principle of absorption of light photons by the metals and gets excited to the higher energy levels and then relaxed to its ground state. On relaxation the amount of energy released is detected by the detector and shown as a absorption graph. Intensity of peaks tells us the amount of concentraion which is given by the software used in the analysis of ICP-OES

Method of sample preparetion-

For the ICP-OES, it is mandatory that the solution is 2-5 % acidic with Supra pure HNO3. Hence all the solution are prepared in Supra pure NItric Acid and triple distill water.

Step -1 - Dry the soil samples on 60-100 oC inthe Oven and grind this to very find powdered form.

Step 2 - Weigh few mg of samples in teflon beaker. (say 0.2 g)

Step 3 - Add Supra pure HNO3, HClO4, and HF in the ration of 3:3:4 maximaum to 10 mL and put the teflon beakers on hot plate for ashing. Keep checking the sample till it completely got burned and only dried ash is remaining. remove from hot plate.

Step 4 - After cooling this, add 1 mL of supra pure HNO3 to this and rinse with Distilled water.

Step 5 - Filter this to valumeteric flask of 50 mL grade 45 whatman paper. and make up this upto 50 mL with distill water.

Step 6 - After shaking , transfer the solution to sample bottles. here you go. Sample are ready for testing in ICP-OES.

Proper sampling is most important for this analysis. Samples should be taken from various sites of location at distance of 100-500 meter distance or whichever is suitable for your analysis. It should be taken from 1-5 meter depth.