Preservation of As Ⅲ and As Ⅴ in surface and ground waters

• Published in: Chinese journal of geochemistry Vol. 25; no. B08; p. 197
• Main Author: Gwendy E.M. Hall Judy E. Vaive Mike B. Parsons
• Format: Journal Article
• Language: English
• Published: 2006
• Subjects: 地下水; 地表水; 水文化学; 稳定性
• ISSN: 1000-9426; 1993-0364

The literature on preservation procedures for the stabilization of As redox species in waters is controversial. A paper by McCleskey et al. （App1. Geochem., 2004） states that any field collection procedure that filters out microorganisms, adds a reagent that prevents dissolved Fe and Mn oxidation and precipitation, and isolates the sample from solar radiation, will preserve the As Ⅲ/Ⅴ ratio. They state that reagents such as HCl, H2SO4 and EDTA will prevent Fe oxidation and precipitation but warn that high concentrations of EDTA （e.g. 12.5 mM） are necessary for samples high in dissolved Fe content. The USGS recommended preservation procedure is to make the surface or ground water 1.25 mM in EDTA and to use a medium of 12.5 M EDTA only for Fe-rich acid mine drainage. Earlier studies by the GSC （Hall et al., J. Anal. At. Spectrum, 1999） showed that HCl （at 0.1%） did indeed effect an immediate change on the species distribution of As in several natural waters. Further work has shown that making up a calibration standard containing As Ⅲ in an HCl medium can also be problematic. For example, standards （0.5 or 5.0： g · L^-1 of As Ⅲ/Ⅴ in 0.1 M HCl） are not stable when the solution is diluted with H20 and mixed by shaking： all the As Ⅲ is converted to As V. However, if the solution is mixed by vortexing, whereupon less air is introduced to the solution, stability of the two As species is maintained. A GSC study using well waters from the West Bengal region showed that 1.25 mM EDTA did not prevent the oxidation of As III to As V and indeed, in some waters, total As showed a significant decrease in the EDTA medium compared to the HCl^- and HNO3^- preserved counterparts. These waters do not contain high amounts of Fe （〈10 mg · L^-1）; rather they are highly alkaline with high concentrations of Ca （80-120 mg · L^-1）. The poor performance of EDTA is due to the instability of the Fe-EDTA complexes and preferential binding of EDTA by Ca, resulting in both oxidation of As Ⅲ to As Ⅴ and precipitation of Fe in some waters, removing arsenate （and arsenite） from solution also. Clearly, the higher concentration of EDTA （12.5 mM） is needed not only for Fe-rich waters,