We present a new
approach combining diffusive equilibrium in thin-film
gels and spectrophotometric methods to determine the spatial distribution
of arsenite, arsenate, and phosphate at submillimeter resolution.
The method relies on the simultaneous deployment of three gel probes.
Each retrieved gel is exposed to malachite green reagent gels differing
in acidity and oxidant addition, leading to green coloration dependent
on analyte speciation and concentration. Hyperspectral images of the
gels enable mapping the three analytes in the 2.5–20 μM
range. This method was applied in a contaminated brook in the Harz
mountains, Germany, together with established mapping of dissolved
iron. The use of two-dimensional (2D) gel probes was compared to traditional
porewater extraction. The gels revealed banded porewater patterns
on a mm-scale, which were undetectable using traditional methods.
Small-scale correlation analyses of arsenic and iron microstructures
in the gels suggested active iron-driven local redox cycling of arsenic.
Overall, the results indicate continuous net release of arsenic from
contaminant particles and deepen our understanding of arsenate transformation
under anaerobic conditions. This study is the first fine-scale 2D
characterization of arsenic speciation in porewater and represents
a crucial step toward understanding the transfer and redox cycling
of arsenic in contaminated sediment/soil ecosystems.