Abstract

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 precipitates (Fe oxides and/or hydroxides designated by “FeOOH”). These precipitates (ppts) can form during the transport of the sample to the laboratory for arsenic speciation analysis. The analysis of the ppt indicates considerable 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(III) indicate that the resulting “FeOOH” ppt contained a mixture of As(III) and As(V) with near quantitative removal of the As_aq in 18 h. The corresponding aqueous fraction after filtration through a 0.45 µm filter was composed primarily of As(V). The formation of “FeOOH” ppt and the loss of As_aq to 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 produced 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 As(III)/As(V) ratios. The native distribution of As(III)/As(V) was stabilized 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 dramatic losses (a factor of 2–5) of As_aq in less than 1 d. These results indicated that EDTA preservation treatment can be used to preserve As_aq in waters where the predominant species is the reduced form [As(III)] or in waters which the predominant species is the oxidized form [As(V)]. This preliminary investigation of EDTA to preserve As species in Fe-rich waters indicates stability can be achieved for greater than 14 d.