GEOCHEMICAL MOBILIZATION OF ARSENIC TO GROUND WATER

Sara Baldvins

CHEM 4101

December 9, 2011

Greatest Mass Poisoning in History

Naturally occurring arsenic (As) contamination in ground water is causing widespread health problems.

35 million in Bangladesh and 6 million in Bengal are at risk.

As poisoning has also been reported in China, Argentina, Chile, Mexico, Thailand, and Taiwan.

Analytical Problem

Hypothesis

The speciation of arsenic in soils impacts how mobile the arsenic is which contributes to the high concentrations found in the ground water of some regions.

Problem Summary

Certain soils easily mobilize As to the ground water.

In these soils certain hydrological, geological, and chemical conditions make arsenic more mobile.

Species Separation Methods

Technique Advantages Disadvantages

Sequential Extraction

Low matrix interference, high yields, can be automated, inexpensive

Time consuming, potentially large user error

CE High resolution, great separations, quick run-times

Significant matrix interference, low reproducibility without considerable efforts, small sample size

Ion Chrom. Recovery better than 92% for most species, good resolution

Loses resolution in presence of other anions, multiple interferences between the As compounds.

Sample Prep: Sequential Extraction

Once the soil is ground to the appropriate particle size the reagents will be applied stepwise as follows:

Step Target Geologic Phase

Mg Ionically bond As

PO4 Strongly adsorbed As

HCl As co-precipitated w/AVS, oxides, and very amorphous Fe oxyhydroxides

Ox As co-precipitated w/amorphous Fe oxyhydroxides

HNO3 As co-precipitated w/pyrite and amorphous As2S3

Hot HNO3 Orpiment and remaining recalcitrant As minerals

Analytical Techniques

Technique Advantages Disadvantages

HG-AAS Least expensive, LODs good with HG, most commonly used for As detection

One species at a time, slow run time, large amount of sample preparation

HG-AFS Great LOD with HG, inexpensive

Some species hard to detect, large matrix interferences

ICP-MS Good LOD, multiple species at one time, short running time, less sample preparation required

Requires standards, expensive, hard to run large amount of extraction sample through

Hydride Generator

The are large interferences when using AAS to detect As so a Hydride Generator must be used.

Atomic Absorption Spectrometer

For the PerkinElmer PinAAcle 900T:With HG precision ≤ 4.5% for AsLOD ≤ 5 ppbLOQ approx. 1 ppb

XANES

X-ray Absorption Near-Edge StructureThe absorption edge corresponding to the

liberation of a core electron from an element will exhibit several identifiable features which change depending on the chemical environment of the element being probed. 

Answers questions about the oxidation state, coordination, and spin state of the probed element.

XANES

Conclusions

Multiple methods are generally required for speciation analysis.

Method choice is highly dependant on operating budget since so many samples are needed.

HG-AAS is cheap and effective and a good method for bulk analysis.

XANES adds validity to the HG-AAS findings by providing an exact picture of the structures in the sample.

References

1. Berg, J. A., 2008. Hydrogeology of the Surficial and Buried AquifersRegional Hydrogeological Assessment, RHA-6, part B, Plates 1-6. State of Minnesota, Department of Natural Resources, Division of Waters.

2. Erickson, Melinda L., and Randal J. Barnes. "Glacial Sediment Causing Regional-Scale Elevated Arsenic in Drinking Water." Ground Water 43(2005a): 796-805.

3. Keon, N.E., C.H. Swartz, D.J. Brabander, C. Harvey, and H.F. Hemond. "Validation of an Arsenic Sequential Extraction Method for Evaluating Mobility in Sediments." Environmental Science and Technology. 35(2001): 2778-2784 .

4. Smedley, P. L. and Kinniburgh, D. G., 2002. A review of the source, behaviour and distribution of arsenic in natural waters. Applied Geochemistry 17, 517-568.