In situ measurement of dissolved Fe(ii) in sediment pore water with a novel sensor based on C18-ferrozine concentration and optical imaging detection

Abstract

A novel sensor system, which integrated sampling, enrichment, and in situ measurement of dissolved Fe(II) in sediment pore water, was developed. The sensor system, which included a C18-ferrozine based concentrating device and an optical imaging device, was installed in a piece of water-tight polyvinyl chloride cylinder. The concentrating device consisted mainly of a layer of diffusive hydrogel and a layer of C18 concentrating gel coated with ferrozine, which reacted with Fe(II) to form a magenta complex, but showed no reaction with Fe(III). The process of color formation was recorded using an optical imaging device fitted with an LED light source and a camera. When the sensor system was deployed in the sediment, the dissolved Fe(II) in the pore water diffused through the hydrogel layer and was concentrated on the C18-ferrozine layer. The color image was captured by the camera and in situ detection could be realized. The grayscale intensities of the color images, which were analyzed using Image-J software were used for quantifying the amount of dissolved Fe(II). The sensor could measure quantitatively in the range of 5–100 μmol L⁻¹. Sample salinity had a slight effect on the determination and could be corrected with a calibration curve using solutions with known salinity. The distinguishing features of the sensor system were its simplicity, low cost, and in situ enrichment and measurement, and it was successfully applied to measure the concentration of dissolved Fe(II) in lake and marine sediments.