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Preserving Inorganic Arsenic Speciation in Reducing Groundwater


High levels of inorganic arsenic (iAs) have been detected in groundwater across more than 70 countries, posing a health risk to over 100 million people, a situation often referred to as "the biggest calamity of the world." This public health crisis necessitates testing all domestic well water for arsenic worldwide. If elevated levels of iAs are found, speciation analysis is crucial to determine whether the arsenic is in the reduced (arsenite, iAs(III)) or oxidized (arsenate, iAs(V)) form, which is essential for selecting the most appropriate treatment technology.

Figure: Groundwater arsenic contamination areas

However, such speciation analysis is challenging due to the instability of arsenic species, with transformations occurring during sampling, transportation, and storage. This issue is particularly problematic in reducing groundwater, where iAs is frequently found as iAs(III) and can oxidize before laboratory analysis, leading to an underestimation of iAs(III).

To prevent species interconversion, on-site measurements or at least on-site separation of species and various sample preservation techniques have been proposed. Sample preservation is the most commonly selected solution due to the difficulties of conducting highly selective speciation analysis in the field.

For sample preservation, filtration with 0.45 μm membrane filters, refrigeration to 4 °C, and storage in opaque bottles, with or without preservatives, have been investigated. Different preservatives, including various acids (HCl, HNO3, H2SO4, H3PO4) and complexing agents like ethylene diamine tetraacetic acid disodium salt (EDTA) or EDTA with acetic acid (HAc), have been recommended, though no consensus exists on a universally effective preservative. Despite the lack of consensus, different preservation methods have been approved by regulatory authorities.

New Study Findings:
The new study by Chinese researchers aims to resolve the ongoing debate about the effectiveness of common preservation methods. They conducted a systematic comparison using 62 reducing groundwater samples representing different aquifer conditions in northwestern and southern China. They tested 14 preservation methods, including three strong acids, various concentrations of EDTA and HAc, three storage temperatures, and two types of sample bottles.

Preservation was achieved by complexing redox-active elements such as Fe and Mn with EDTA, lowering the pH from 7.0 ± 0.5 to 5.2 ± 0.4, and eliminating photooxidation. Adding EDTA to a final concentration of 20 mmol/L and storing the samples in opaque bottles in the dark at 4 °C preserved iAs species for up to 60 days.

The authors recommend that users start by adding EDTA on-site, adjusting the amount based on observed Fe and Mn concentrations and the final pH. They urge regulatory authorities to re-examine their preservation protocols in standard methods for speciation analysis.

The original publication

Yanhua Duan, Jingyu Liu, Yuxia Yang, Long Han, Zengyi Li, Yuehong Gu, Yunjie Ma, Alejandro Palomo, Xiaobao Tuo, and Yan Zheng, Large Quantity EDTA Addition and Cold Storage in Dark Recommended for Preserving Inorganic Arsenic Speciation in Reducing Groundwater, ACS EST Water, 4 (2024) 2031−2043. DOI: 10.1021/acsestwater.3c00619

Related Studies (newest first):

D. Tao, C. Shi, W. Guo, Y. Deng, Y. Peng, Y. He, P.K.S. Lam, Y. He, K. Zhang, Determination of As species distribution and variation with time in extracted groundwater samples by on-site species separation method. Sci. Total Environ., 808 (2022) 151913. DOI: 10.1016/j.scitotenv.2021.151913

Sarah J. Stetson, Melinda L. Erickson, Jeffrey Brenner, Emily C. Berquist, Christopher Kanagy, Susan Whitcomb, Caitlyn Lawrence, Stability of inorganic and methylated arsenic species in laboratory standards, surface water and groundwater under three different preservation regimes, Appl. Geochem., 125 (2021) 104814. DOI: 10.1016/j.apgeochem.2020.104814.

C.-C. Brombach, S.T. Lancaster, M.A. Dexter, K.C. Thompson, W.T. Corns, Stability, preservation and storage of As(iii), DMA, MMA and As(v) in water samples. Anal. Methods, 13/28 (2021) 3154−3159. DOI: 10.1039/d1ay00743b

D.B. Wu, T. Pichler, Preservation of co-occurring As, Sb and Se species in water samples with EDTA and acidification. Geochem. Exploration, Environ. Anal., 16/2 (2016) 117-125. DOI: 10.1144/geochem2015-369

Orhan Gunduz, Hakan Gurleyuk, Ayse Cakir, Alper Elci, Alper Baba, Celalettin Simsek, Sample Collection into Sterile Vacuum Tubes to Preserve Arsenic Speciation in Natural Water Samples, J. Environ. Eng., 139/8 (2013) 1080-1088. DOI: 10.1061/(ASCE)EE.1943-7870.0000717.

A.R. Kumar, P. Riyazuddin, Preservation of inorganic arsenic species in environmental water samples for reliable speciation analysis. TrAC Trends Anal. Chem., 29/10 (2010) 1212-1223. DOI: 10.1016/j.trac.2010.07.009

Youn-Tae Kim, Hyeon Yoon, Cheolho Yoon, Nam-Chil Woo, An assessment of sampling, preservation , and analytical procedures for arsenic speciation in potentially contaminated waters, Environ. Geochem. Health., 29/4 (2007) 337-346. DOI: 10.1007/s10653-007-9091-3

G. Samanta, D.A. Clifford, Preservation and field speciation of inorganic arsenic species in groundwater. Water Qual. Res. J. Can., 41/2 (2006) 107−116. DOI: 10.2166/wqrj.2006.013

G. Samanta, D.A. Clifford, Influence of sulfide (S2-) on preservation and speciation of inorganic arsenic in drinking water. Chemosphere, 65/5 (2006) 847−853. DOI: 10.1016/j.chemosphere.2006.03.023

 Birgit Daus, H. Weiss, Jürgen Mattusch, Rainer Wennrich, Preservation of arsenic species in water samples using phosphoric acid - Limitations and long-term stability, Talanta, 69/2 (2006) 430-434. DOI: 10.1016/j.talanta.2005.10.012

P.K. Dutta, S.O. Pehkonen, V.K. Sharma, A.K. Ray, Photocatalytic oxidation of arsenic(III): Evidence of hydroxyl radicals. Environ. Sci. Technol., 39/6 (2005) 1827−1834. DOI: 10.1021/es0489238

Andrew G. Gault, Joydeb Jana, Sudipto Chakraborty, Partha Mukherjee, Mitali Sarkar, Bibash Nath, David A. Polya, Debashis Chatterjee, Preservation strategies for inorganic arsenic species in high iron, low- Eh groundwater from West Bengal, India, Anal. Bioanal. Chem., 381/2 (2005) 347-353. DOI: 10.1007/s00216-004-2861-1

G. Samanta, D.A. Clifford, Preservation of Inorganic Arsenic Species in Groundwater. Environ. Sci. Technol., 39 (2005) 8877-8882. DOI: 10.1021/es051185i

Patricia A. Gallagher, Carol A. Schwegel, Amy Parks, Bryan M. Gamble, Lar Wymer, John T. Creed, Preservation of As(III) and As(V) in Drinking Water Supply Samples from Across the United States Using EDTA and Acetic Acid as a Means of Minimizing Iron-Arsenic Coprecipitation, Environ. Sci. Technol.,  38/10 (2004) 2919-2927. DOI: 10.1021/es035071n

R.B. McCleskey, D.K. Nordstrom, A.S. Maest, Preservation of water samples for arsenic(III/V) determinations: an evaluation of the literature and new analytical results. Appl. Geochem., 19 (2004) 995-1009. DOI: 10.1016/j.apgeochem.2004.01.003

J.A. Smieja, R.T. Wilkin, Preservation of sulfidic waters containing dissolved As(III). J. Environ. Monit., 5/6 (2003) 913−916. DOI: 10.1039/b306567g

A. J. Bednar, J. R. Garbarino, J. F. Ranville, T. R. Wildeman, Preserving the Distribution of Inorganic Arsenic Species in Groundwater and Acid Mine Drainage Samples, Environ. Sci. Technol., 36/11 (2002) 2213-2218. DOI: 10.1021/es0157651

Birgit Daus, Jürgen Mattusch, Rainer Wennrich, H. Weiss, Investigation on stability and preservation of arsenic species in iron rich water samples, Talanta, 58/1 (2002) 57-65. DOI:10.1016/S0039-9140(02)00256-4

M.T. Emett, G.H. Khoe, Photochemical oxidation of arsenic by oxygen and iron in acidic solutions. Water Research, 35 (2001) 649-656. DOI: 10.1016/S0043-1354(00)00294-3

Patricia A. Gallagher, Carol A. Schwegel, Xinyi Wei, John T. Creed, Speciation and preservation of inorganic arsenic in drinking water sources using EDTA with IC separation and ICP-MS detection, J. Environ. Monit., 3/4 (2001) 371-376. DOI: 10.1039/b101658j

S.J. Hug, L. Canonica, M. Wegelin, D. Gechter, U. Von Gunten, Solar oxidation and removal of arsenic at circumneutral pH in iron containing waters. Environ. Sci. Technol., 35/10 (2001) 2114−2121. DOI: 10.1021/es001551s

M.J. Kim, J. Nriagu, Oxidation of arsenite in groundwater using ozone and oxygen. Sci. Total Environ., 247/1 (2000) 71−79. DOI: 10.1016/S0048-9697(99)00470-2

Torsten Lindemann, Andreas Prange, Walter Dannecker, Bernd Neidhart, Stability studies of arsenic, selenium, antimony and tellurium species in water, urine, fish and soil extracts using HPLC/ICP-MS, Fresenius J. Anal. Chem., 368/2-3 (2000) 214-220. DOI: 10.1007/s002160051368

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M. Borho, P. Wilderer, A reliable method for preservation and determomination of arsenate(III) concentrations in groundwater and water works samples, J. Water Supply: Res. Technol. - Aqua, 46/3 (1997) 138-143.

J.T. van Elteren, H. Hoegee, E.E. van der Hoek, H.-A. Das, C.L. de Ligny, J. Agterdenbos, Preservation of As(III) and As(V) in some water samples, J. Radioanbal. Nucl. Chem., 154/5 (1991) 343-355. DOI: 10.1007/BF02165490

Venghuot Cheam, Haig Agemian, Preservation of Inorganic Arsenic Species at Microgram Levels in Water Samples,  Analyst (London), 105 (1980) 737-743. DOI: 10.1039/AN9800500737

D.E. Tallman, A.U.J.A.C. Shaikh, Redox stability of inorganic arsenic (III) and arsenic (V) in aqueous solution. Anal. Chem., 53/1 (1980) 196−199.

last time modified: June 11, 2024


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