Two-dimensional dissolved ferrous iron distributions in marine sediments as revealed by a novel planar optical sensor

Abstract

A new, single use planar optical sensor was developed for measuring high resolution, two-dimensional Fe2+ distributions in marine sediments. Ferrozine (3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-p,p′-disulfonic acid monosodium salt hydrate) was used as the Fe2+ indicator, and was covalently immobilized onto a transparent poly(vinyl alcohol) membrane through a water soluble poly(N-isopropylacryl amide) polymer chain, backed by a polyester sheet. When the colorless and transparent sensor membranes are deployed in Fe2+ solutions or natural marine sediment samples, a violet-red color with maximum absorption wavelength at 562nm develops due to the formation of ferrozine-Fe2+ complex in the sensor membrane. The absorbance of the sensor at 562nm shows excellent linear relationships versus Fe2+ concentrations in the range of 0–200μM, with a lower detection limit of 4.5μM. The response time of the sensor film varies with Fe2+ concentration, following Langmuir kinetics, with a typical practical range of 10–30min at room temperature (~22°C). The absorbance attained is largely independent of temperature, oxygen exposure before or after development, salinity and pH changes. No interferences from other major components and trace metal ions in seawater have been observed. The sensor is simple, stable (irreversible after removal), and precise, and has been successfully used to measure virtually continuous two-dimensional Fe2+ distributions in intertidal flat and subtidal sediment samples. Images are readily obtained from the membrane by using a scanner or inexpensive LED excitation and commercial grade digital cameras, with a typical pixel resolution of ~50×50μm over areas >150cm2. The complex heterogeneous distribution patterns of Fe2+ associated with both inhabited and abandoned biogenic structures and other natural diagenetic heterogeneity are readily revealed and can be related directly to the corresponding visible images of sedimentary features. These patterns demonstrate directly that the average concentrations typically measured in traditional vertical profiles can be a misleading indicator of the microenvironmental Fe2+ concentrations controlling sediment–water fluxes and authigenic mineral formation–dissolution.