A group of researchers from China have compared 14 different methods for preserving inorganic arsenic species in reducing groundwater. They recommend adding large amounts of EDTA and storing the samples cold and in the dark.
Background: 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.
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last time modified: June 11, 2024
