1. GEOCHEMICAL MOBILIZATION OF ARSENIC TO GROUND WATER Sara Baldvins CHEM 4101 December 9, 2011

2. 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.

3. 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.

4. Species Separation Methods TechniqueAdvantagesDisadvantages Sequential Extraction Low matrix interference, high yields, can be automated, inexpensive Time consuming, potentially large user error CEHigh 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.

5. Sample Prep: Sequential Extraction Once the soil is ground to the appropriate particle size the reagents will be applied stepwise as follows: StepTarget Geologic Phase MgIonically bond As PO 4 Strongly adsorbed As HCl As co-precipitated w/AVS, oxides, and very amorphous Fe oxyhydroxides Ox As co-precipitated w/amorphous Fe oxyhydroxides HNO 3 As co-precipitated w/pyrite and amorphous As 2 S 3 Hot HNO 3 Orpiment and remaining recalcitrant As minerals

6. Analytical Techniques TechniqueAdvantagesDisadvantages HG-AASLeast 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-AFSGreat LOD with HG, inexpensive Some species hard to detect, large matrix interferences ICP-MSGood 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

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

8. Atomic Absorption Spectrometer For the PerkinElmer PinAAcle 900T:  With HG precision ≤ 4.5% for As  LOD ≤ 5 ppb  LOQ approx. 1 ppb

9. XANES  X-ray Absorption Near-Edge Structure  The 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.

10. XANES

11. 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.

12. 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.