Detecting heavy metals in soil using spectroscopy

[Sareena Avadhany]

Hi!

My project is looking at a bay area river that may contain heavy metal contaminants. I plan on collecting samples and analyzing them to detect heavy metals and at what quantity. I would then like to create a math model.

I would need to test the soil for heavy metals using spectroscopy, but I have been unable to find methods of how to isolate these heavy metals from the soil.

From my research, I've decided to use reflectance spectroscopy. I want to detect Mercury, Arsenic and lead.

Thanks!

Sincerely,
Sareena

[ChrisG]

Hi,
It sounds like a very interesting and challenging project. I have not used these methods, but from what I read it seems you do not need to isolate the metals to use reflectance spectroscopy. However, there may be limitations in the reflectance spectroscopy method to quantify lead and arsenic as discussed below. Here is info from one paper. These authors have published other papers that might have additional info.

Title: Possibilities of reflectance spectroscopy for the assessment of contaminant elements in suburban soils
Author(s): Wu YZ, Chen J, Wu XM, Tian QJ, Ji JF, Qin ZH
Source: APPLIED GEOCHEMISTRY 20 (6): 1051-1059 JUN 2005

From abstract: "The objective of this study was to examine the possibility of using soil reflectance spectra as a rapid method to simultaneously assess contaminant metals (Ni, Cr, Cu, Hg, Pb, Zn) and As in the suburban soils of the Nanjing area. One hundred and twenty soil samples were collected for chemical analyses and spectral measurements. Prediction of contaminant elements was achieved by a partial least-square regression (PLSR) approach. According to their relationships with Fe, the seven contaminant elements could be categorized into two groups. The results showed that the prediction accuracy for Group II (Ni, Cr, Cu and Hg) was higher than that for Group I (Pb, Zn and As)."

From methods: "The sample preparation followed the procedure
from Balsam and Deaton (1991). Ground samples
were made into a slurry on a glass microslide with distilled
water, smoothed, and dried slowly at low temperature
(<40C). Data, reflectance intensity relative to a
white spectralon standard, were written directly to a
computer disk."

From
Title: SEDIMENT DISPERSAL IN THE ATLANTIC-OCEAN - EVALUATION BY VISIBLE-LIGHT SPECTRA
Author(s): BALSAM WL, DEATON BC
Source: REVIEWS IN AQUATIC SCIENCES 4 (4): 411-447 1991

abstract: "Samples were prepared by pulverizing, then making a thick slurry on a glass microslide, and slowly drying at < 50-degrees-C. Diffuse reflectance of the samples relative to a white, barium sulfate standard was determined with a spectrophotometer."

I don't have access to the full text of Balsam and Deaton.

I hope that helps. Good luck!

[Sareena Avadhany]

Hi Chris,

Thanks for the information. Spectroscopy is very new to me, so I just assumed a clean environment was necessary to run samples.

I talked with my chemistry teacher a few days ago, and she suggested to maybe place the soil in water, and centrifuge it, and then analyze the water for heavy metal contaminants. Would that be a feasible idea? I thought it might simulate heavy metals could seep into water. This would support my hypothesis that if high levels of heavy metals exist in the soils that border the river, it would be extremely harmful to the river ecosystems.

For sampling, I was thinking of conducting 3 trials, each with 30 samples. Each sample would be 1/4 cup of loose soil. Is 1/4 of a cup too small a sample size?

Thanks!

Sincerely,
Sareena

[Craig_Bridge]

Is 1/4 of a cup too small a sample size?

That depends on how sensitive your experimental setup is. Nobody can answer this for you. You are going to have to run some process controls yourself.

You need to run an experiment with several known contaiminated soil samples that have different known parts per million of one contaiminant. Send these control samples through your process and determine if your process will extract enough of the contaiminant to measure with the equipment you have.

Note: You will have to do this with each contaiminant that you are interested in to determine how sensitive or insensitive your process and measurement equipment is with respect to each contaiminant.

You probably need to be working with somebody who has experience in dealing with the difficulties of process hygene to keep from contaiminating your measurement equipment. Quantitative analysis of small PPM samples is a difficult art.

[ChrisG]

Hi Chris,

Thanks for the information. Spectroscopy is very new to me, so I just assumed a clean environment was necessary to run samples.

I talked with my chemistry teacher a few days ago, and she suggested to maybe place the soil in water, and centrifuge it, and then analyze the water for heavy metal contaminants. Would that be a feasible idea? I thought it might simulate heavy metals could seep into water. This would support my hypothesis that if high levels of heavy metals exist in the soils that border the river, it would be extremely harmful to the river ecosystems.

For sampling, I was thinking of conducting 3 trials, each with 30 samples. Each sample would be 1/4 cup of loose soil. Is 1/4 of a cup too small a sample size?

Thanks!

Sincerely,
Sareena

Hi, yes, that does sound like a reasonable plan. You can use more or less aggressive extraction procedures depending on your goals. These procedures should be widely available. Let us know if you have trouble finding references for your procedures.

Before you spend a lot of time on this, I'd ask a couple of questions:
(1) From a practical point of view, why do you expect there are high concentrations of heavy metals in these sediments?
(2) How would you propose to evaluate whether the metal in these soils is "extremely harmful" to the ecosystem? Are the processes in your lab experiments comparable to processes in the stream bed? If they are, and you find that a heavy metal leaches from the soils, does it go into the river or down into the ground water? If it does go into the river, how does it affect the multitude of species that exist there? These are tricky questions - I don't expect you to have answers, but they are worth considering as you design your experiment and construct your hypotheses.

[Louise]

Hi Chris,

Thanks for the information. Spectroscopy is very new to me, so I just assumed a clean environment was necessary to run samples.

I talked with my chemistry teacher a few days ago, and she suggested to maybe place the soil in water, and centrifuge it, and then analyze the water for heavy metal contaminants. Would that be a feasible idea? I thought it might simulate heavy metals could seep into water. This would support my hypothesis that if high levels of heavy metals exist in the soils that border the river, it would be extremely harmful to the river ecosystems.

For sampling, I was thinking of conducting 3 trials, each with 30 samples. Each sample would be 1/4 cup of loose soil. Is 1/4 of a cup too small a sample size?

Thanks!

Sincerely,
Sareena

Hi, yes, that does sound like a reasonable plan. You can use more or less aggressive extraction procedures depending on your goals. These procedures should be widely available. Let us know if you have trouble finding references for your procedures.

Before you spend a lot of time on this, I'd ask a couple of questions:
(1) From a practical point of view, why do you expect there are high concentrations of heavy metals in these sediments?
(2) How would you propose to evaluate whether the metal in these soils is "extremely harmful" to the ecosystem? Are the processes in your lab experiments comparable to processes in the stream bed? If they are, and you find that a heavy metal leaches from the soils, does it go into the river or down into the ground water? If it does go into the river, how does it affect the multitude of species that exist there? These are tricky questions - I don't expect you to have answers, but they are worth considering as you design your experiment and construct your hypotheses.

Chris raises two important points. I'd like to add a few additional comments.

1)I suggest you do a sample calculation to estimate some experimental parameters. For example, your experimental/spectrscopy technique may only detect metals that are in the >100 ppm range (for example... I have no idea the sensitive of your particular technique). Then, you can look up some information about the ion levels of soil in your area. (The local agricultural extension office should have this information.) How much metal ion is in 1/4 cup soil (or 100 grams or whatever). Suppose the water extracts 100%, 50%, 25%, whatever (you should be able to find some literature suggesting the efficiency level of different extraction protocols). With the volume you need for the spectrophotometer, will you have a high enough concentration of metal to measure?
2) Many colorimetric assays are not sensitive to specific metals. That is, you may be testing for Zn, but Fe will also lead to some false positives. You need to make sure your procedures are metal specific.
3) Originally you mentioned doing reflectance spectroscopy. You should double check that you can use that instrument for your liquid samples. Usually liquids are measured by absorbance (transmission), not reflectance.
4) Lastly, the pH of the soil is an important parameter to record. Metal solubility depends very much on the oxidation state of the metal.

Good luck.

Louise