An integrated sampling/speciation method for inorganic arsenic in soil solution

Abstract

SummaryArsenic mobility, bioavailability, and toxicity in soil-water systems are strongly affected by species distribution. In spite of species-selective analytical tools, results may be biased by postsampling oxidation or reduction of compounds. The scope of this study was to delineate a speciation method for inorganic arsenic integrated with pore-water sampling to prevent problems related to postsampling species transformation. Experiments were performed with exchange cartridges packed with quaternary amine (nitrate-form) to separate anionic arsenate from uncharged arsenite. Standard solutions with varied As(III) : As(V) ratios as well as groundwater and soil-solution samples were studied. Analyses were performed by ICP-MS using either extraction cartridges directly coupled to the instrument or samples obtained by offline speciation tests. Results showed that As(III) passes the packed bed with the rinsing solution, while As(V) is retained on the column and can be released by elution with 0.25 M NH 4 NO 3 .Recoveries between 98% and 116% were insensitive to pH variation (pH 3-8) and competing anions. Groundwater samples from a contaminated site yielded recoveries between 92% and 125% when the sum of As(III) and As(V) was compared to total As. Integration of the speciation scheme with pore-water sampling was tested in a soil-column experiment. Soil material from a fen site with elevated geogenic As contents was subjected to a varying moisture regime. Reducing conditions during prolonged saturation caused a marked increase of As(III) concentrations. Conversely, when the soil was drained, total As levels decreased and were dominated by As(V). Overall, extraction results with standard solutions and natural samples indicate reliable performance of the method in experimental investigations and field studies.

Arsenic release and speciation in a degraded fen as affected by soil redox potential at varied moisture regime

“…To our knowledge no indications exist for further inputs of As. Arsenic maxima in the soil coincide with patchy enrichment zones of pedogenic Fe (hydr)oxides (Weigand et al, 2007).…”

“…Nitrate and SO 4 2− were determined by ion chromatography. Separation of tri-and pentavalent As species in suction cup samples was carried out by on-line solid phase extraction with quaternary amine as described by Weigand et al (2007). Quantification limits of the method were 2 μg l − 1 for As(III) and 1 μg l − 1 for As(V).…”