Speciation of Inorganic Arsenic in Groundwater as Molybdoarsenate by On-Site Solid-Phase Extraction and Graphite Furnace Atomic Absorption Spectrometry

Solid-phase extraction was used for on-site speciation and preconcentration of inorganic arsenic in groundwater involving the retention of molybdoarsenate on activated carbon. A total of 0.25 mu g of As(V) in 50-500 mL of sample was selectively retained on 100 mg of activated carbon in a mini-column as molybdoarsenate at pH 2. A total of 0.1 mL of 0.26 mol/L ammonium molybdate was introduced for every 50 mL of the sample. As(III) was quantitatively retained with As(V) by adding 0.01 mL of 0.02 mol/L KMnO4 solution for every 50 mL of the sample prior to the addition of ammonium molybdate. The retained As(V) was quantitatively eluted by circularly passing 3 mL of 1 mol/L NH4OH through the column for at least 5 min with determination by graphite furnace atomic absorption spectrometry. A palladium chemical modifier was used to provide a detection limit and blank equivalent concentration of 0.047 and 0.19 mu g/L, respectively (n = 6, sample volume of 50 mL). Concomitant ions in seawater or river water did not cause interferences. The method was used for the preconcentration of arsenic in groundwater from Chiba, Japan. The recoveries of fortified As(V) (2 mu g/L) were from 89 to 98%. Concentrations of 0.35-1.6 mu g/L of As(V) were determined with a relative standard deviation less than 3.7% for five replicates. The recoveries of 2 mu g/L fortified As(III) were from 92 to 103%.