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Sara Baldvins - Analytical Problem: Geochemical Mobilization of Arsenic to Ground Water

Arsenic concentrations in ground water in many west-central Minnesotan wells are currently above the U.S. federal drinking water standard of 10 μg/L. This is a naturally occurring phenomenon that has been associated with the glaciated regions in the upper Midwest. In order to re-mediate these high concentrations to comply with the public health standards a better understanding of the mechanisms that cause arsenic release from solids and into ground water are needed.

Arsenic exposure from contaminated drinking water is a significant environmental cancer risk, equivalent to environmental tobacco smoke and home radon exposure. With an approximate 150,000 to 250,000 Minnesotans getting their drinking water from contaminated wells, the public health standards need to be met.
The general hypothesis is that there are certain hydrogeochemical conditions prevalent in glaciated regions that make arsenic more mobile. Primarily, that the presence of certain arsenic species that are more mobile in the particular hydrogeochemical conditions of the region result in the higher concentrations of arsenic in the ground water.

The analyte is Arsenic in its varying forms across the species that exist in the soil. Arsenic exists in a few hundred common minerals. For example, in reducing environments arsenic is present in iron sulfides such as arsenopyrite (FeAsS), realgar (AsS), and orpiment (A2S3). It can also be found as a contaminant in pyrite (FeS). In oxidizing environments, arsenic is found in arsenic trioxides (arsenolite: As2O3 and claudetite: As2O3). Under a wider range of geological conditions, arsenic has also been associated with minerals such as iron oxides (Fe2O3), iron hydroxides (FeOOH), and other metal oxides and hydroxides (such as Al). In natural fresh waters, arsenic, if present, is generally an inorganic species. In reduced environments, the form of Arsenite (As(III) as H3AsO3 and its dissociation constituents) is predominate . Arsenate (As(V) a H3AsO4 and its dissociation constituents) predominates in oxic environments.

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

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

Similar Analytical Problem

a. Boron in MN Groundwater, Justin Michael is trying to determine what is the most effective and cost-efficient way to remediate Boron contamination. Boric Acid and Boron Anions are the analyte and the matrix they are found in is the Groundwater. This is a health concern.

b. We will both need to take a wide variety of samples to get an idea of how the system is working and responding. We will both need to test water samples.

c. Justin is concerned with cleaning up the Boron where I am more interested in finding our where the Arsenic is coming from so Justin will treat his samples in lab to determine the effectiveness of a technique then apply that technique in the environment and monitor its affect. I will basically be surveying soils and water samples to gather more information about how Arsenic is present in the soil to see if I can draw any conclusions.


Studies Needed

1. Need to identify the different types of arsenic found in the soil. Use multiple soil sources both near contaminated wells and near clean wells. In addition to analyzing various locations also analyzing different depths.

2. Quantify how much arsenic exists in each state in each soil sample.

3. Take into account other environmental conditions such as reducing environments and water pH.

4. Using GIS modeling input all factors and see if trends exist and if we can come to any conclusions.

I expect contaminate levels in the ppm

Fluorescence Techniques

I found a very useful paper describing arsenic measurements using Atomic Fluorescent Spectroscopy (AFS). According to their study the strongest emission signal was obtained at a wavelength of 2350 angstroms (235 nm) with a corresponding absorption signal at 1890 angstroms (189 nm) . One thing to note is that at concentrations above 200 ppm the calibration curves begin to curve dramatically. This is because when there is a larger density of As atoms causes self-absorption of the emitted resonance radiation.

Signal strength seems to be greatly impacted by the excitation source and a hydride generation technique seems to work best for low concentrations.

http://www.sciencedirect.com.ezp1.lib.umn.edu/science?_ob=MiamiImageURL&_cid=271360&_user=616288&_pii=0039914068801523&_check=y&_origin=search&_coverDate=31-Jul-1968&view=c&wchp=dGLzVlk-zSkWb&md5=570a4763747d1f53728925b692df0eba/1-s2.0-0039914068801523-main.pdf

Chemical Structure and Standards

Structures for Realgar, Tetraarsenic Oxide, and Arsenic Trioxide:

View image

Structure of Arsenopyrite:

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Standards:

o2si offers Arsenic standards for making a calibration curve but I also think it would be a good idea to test our entire method by sorbing arsenic in known concentrations onto geothite or another mineral. An example of a procedure was found in the following paper:

http://www.qi.fcen.uba.ar/materias/cqi/2011/infosuplementaria/practicas/practica1/As_en_magnetita,%20%20%20%20%20_hematita_y_goethita%5B1%5D.pdf

Atomic Spectrometry

Yes we can use atomic spectrometry to detect arsenic. Perkin Elmer recommends using 193.7 nm for the wavelength. A Flow Injection Analysis System (FIAS) - Atomic Absorption Spectrophotometer is hydride generation technique that is commonly used for quantifying arsenic. Using a hydride generation technique is a important to separate hydride forming metals, such as As, from a range of matrices and varying acid
concentrations. This analytical technique can improve detection limits by a factor of approximately 3000 times that of flame detection limits and typically have less interference than graphite furnace techniques. This is important when I am working with concentrations in ppb.

http://www.perkinelmer.com/CMSResources/Images/APP_PinAAcleToxicMetalsinWaterbyHGCVAA.pdf

Mass Spectrometry

a. Arsenic(V): 74.92
b. ICP
c. quadrupole ion trap

View image

Chromatographic Techniques

1. If we had a high enough fraction of organic As compounds we could use GC, because these compounds tend to be very volatile, but I am predicting mainly inorganic As which would not benefit from GC. There is not enough difference in polarity of the inorganic As species for HILIC to be effective. Affinity Chromatography is mainly for biochemical mixtures. Most of the species I am looking at do not have chiral centers so Chiral Chromatography will be ineffective. One big separation we have to be able to accomplish is separating the different oxidation states (AsIII and AsV). Size Exclusion Chromatography is more designed for macromolecules so isn't able to accomplish what we need.

2. Ion exchange would be my first choice. Anion Exchange Chromatography (AEX) separates the common As species but Cation Exchange Chromatography (CEX) does not retain the two most toxic species (AsIII and AsV). In most cases AEX does a good enough job at separating all the species but sometimes using a mixed-mode anion-cation exchanger yields better results.

3. The Alltech Anion HC column (Part #: 269036 4.6 x 100 mm) reported good results for this analysis. It is packed with a polystyrene divinylbenzene-based anion-exchanger, having a quaternary amine functional group. It is capable of operating over a pH of 2-12, which is essential since most As compounds are only stable at pH < 3 and pH > 9. Particle size of 12 microns with a capacity of 3 meq/g. No information on back-pressure was reported but this particle size is withing normal operating sizes of many columns so back-pressure should not be too much of an issue to overcome.

4. Several concentrations of both ammonium dihydrogen phosphate and sodium hydroxide have been evaluated in both isocratic and gradient methods. A gradient elution involving 10mmol/L ammonium dihydrogen phosphate solution and water had the shortest analysis time with good resolution.

5. ICP MS is my choice of a detector because we are working with many different species in concentrations in the ppb and it is the most versatile and sensitive method.

6. NA

7. Khalid H. Al-Assaf, Julian F. Tyson, & Peter C. Uden. (2009). Determination of four arsenic species in soil by sequential extraction and high performance liquid chromatography with post-column hydride generation and inductively coupled plasma optical emission spectrometry detectionThis article is part of a themed.. JAAS (Journal of Analytical Atomic Spectrometry), 24(4), 376-384. Retrieved from http://search.ebscohost.com/login.aspx?direct=true&db=aph&AN=37142308&site=ehost-live

Ricci, G. R., Shepard, L. S., Colovos, G., & Hester, N. E. (1981). Ion chromatography with atomic absorption spectrometric detection for determination of organic and inorganic arsenic species. Analytical Chemistry, 53(4), 610-613. doi:10.1021/ac00227a012

Kozak, L., Niedzielski, P., & Szczuciński, W. (2008). The methodology and results of determination of inorganic arsenic species in mobile fractions of tsunami deposits by a hyphenated technique of HPLC-HG-AAS. International Journal of Environmental Analytical Chemistry, 88(14), 989-1003. doi:10.1080/03067310802183852

Problems using similar techniques: Blog 10
1. HG-AAS
2. Matthew Marah - AAS

Capillary Electrophoresis

1.CE has proven to be an excellent technique for separating the various Arsenic species found in the soil. The various oxidation states of the inorganic compounds and the differences in polarity of the organic species make separation ideal for CE. CZE is the most commonly used and the separation works great so while MEKC and CGE can be used it is rather unnecessary. In some cases using isoelectric focusing has improved results.

2.Using CZE will be the best method because an ideal separation can be achieved without any additional effort or techniques.

3. One buffer used for these analytes can be prepared by mixing 50 mM formic acid and 50 mM ammonium formate, adjusted to pH 2.9-5.0. Separations of samples could be accomplished by applying a 120 kV potential. One type of capillary found to be used is an untreated fused-silica capillary (50 mm id, 365 mm od, 40-70 cm total length) obtained from Polymicro Technologies, Phoenix, AZ, USA.

4.Because of the low concentrations in the samples UV detection is generally insufficient due to a short optical path length. As a result MS is the most favorable detector. ICP-MS is the most common but more recently ISI-MS has been getting looked into for this particular analytical problem.

Kitagawa, F., Shiomi, K. and Otsuka, K. (2006), Analysis of arsenic compounds by capillary electrophoresis using indirect UV and mass spectrometric detections. ELECTROPHORESIS, 27: 2233-2239. doi: 10.1002/elps.200500614

Comments

Blog 13? -1 pt

Blogs 9, 10, and 11. Good answers.

Blog 8? (-1 pt)

Good answers for both Blogs 6 and 7.

Good answers for BLOGS 4 and 5.

BLOG 2? (questions 1-5)? (posting for question 6)? (-0.85 pt)

Great job describing your analytical problem.