Speciation and preservation of inorganic arsenic in drinking water sourcesusing EDTA with IC separation and ICP-MS detection

The native distribution of As(III) and As(V) in drinking water supplies can influence the treatment removal strategy. The stability of As(III) and As( V) in iron-rich drinking waters can be affected by the formation of Fe prec ipitates (Fe oxides and/or hydroxides designated by "FeOOH"). These precipi tates (ppts) can form during the transport of the sample to the laboratory for arsenic speciation analysis. The analysis of the ppt indicates consider able loss of the aqueous arsenic species (As-aq) to the solid phase "FeOOH" ppt. Studies of laboratory reagent water containing both As(III) and Fe(II I) indicate that the resulting "FeOOH" ppt contained a mixture of As(III) a nd As(V) with near quantitative removal of the As-aq in 18 h. The correspon ding aqueous fraction after filtration through a 0.45 mum filter was compos ed primarily of As(V). The formation of "FeOOH" ppt and the loss of As-aq t o the ppt can be virtually eliminated by the use of EDTA, which sequesters the Fe(III). Reagent water fortified with Fe(III), As(III) and EDTA produce d less than a 1 ppb change in the As(III)(aq) concentration over 16 d. The EDTA treatment was also tested on three well waters with different native A s(III)/As(V) ratios. The native distribution of As(III)/As(V) was stabilize d over a period of 10 d with a worst case conversion of As(III) to As(V) of 2 ppb over a 30 d period. All well waters not treated with EDTA had dramat ic losses (a factor of 2-5) of As-aq in less than 1 d. These results indica ted that EDTA preservation treatment can be used to preserve As-aq in water s where the predominant species is the reduced form [As(III)] or in waters which the predominant species is the oxidized form [As(V)]. This preliminar y investigation of EDTA to preserve As species in Fe-rich waters indicates stability can be achieved for greater than 14 d.