Two decades of chemical imaging of solutes in sediments and soils – a review

Abstract

The increasing appreciation of the small-scale (sub-mm) heterogeneity of biogeochemical processes in sediments, wetlands and soils has led to the development of several methods for high-resolution two-dimensional imaging of solute distribution in porewaters. Over the past decades, localised sampling of solutes (diffusive equilibration in thin films, diffusive gradients in thin films) followed by planar luminescent sensors (planar optodes) have been used as analytical tools for studies on solute distribution and dynamics. These approaches have provided new conceptual and quantitative understanding of biogeochemical processes regulating the distribution of key elements and solutes including O2, CO2, pH, redox conditions as well as nutrient and contaminant ion species in structurally complex soils and sediments. Recently these methods have been applied in parallel or integrated as so-called sandwich sensors for multianalyte measurements. Here we review the capabilities and limitations of the chemical imaging methods that are currently at hand, using a number of case studies, and provide an outlook on potential future developments for two-dimensional solute imaging in soils and sediments.

In situ observation of localized, sub-mm scale changes of phosphorus biogeochemistry in the rhizosphere

“…pH-sensitive planar optode (PO) sensors were used for the imaging of pH distributions in the soil and rhizosphere (Larsen et al 2011; Santner et al 2015). Planar optodes are a fluorescent assay that uses reversible fluorophores immobilized in a thin layer of an analyte-permeable matrix, in this case a polyurethane-based hydrogel (Hydromed D4, Advan Source biomaterials, Massachusetts, US).…”

“…The spatial resolution achieved for the PO images was ~ 60 μm × 60 μm (Santner et al 2015). The acquired PO images were split into their red, green and blue color channels.…”

“…where M / A is the mass of analyte bound per unit gel area, which is the result obtained from LA-ICPMS analysis, and t is the gel application time, is more appropriate for interpreting chemical images (Santner et al 2015). Therefore all calibrated DGT images in this work are shown as fluxes towards the DGT gel in pg cm −2  s −1 .…”

Assessing the remobilization of Antimony in sediments by DGT: A case study in a tributary of the Three Gorges Reservoir

“…The diffusive gradients in thin films (DGT) technique has been developed as a passive sampling technique for in situ measurement of trace metals and anionic species and their remobilization flux from the solid phase in water, sediment and soil (Davison et al, 1997;Davison and Zhang, 2012;Santner et al, 2015). As an in situ sampling technique, DGT has an advantage compared to the traditional bioavailable measurement methods (e.g., sequential extraction) in eliminating the risk of speciation changes during the transportation, storage and analysis of samples ( € Osterlund et al, 2010).…”

Ammonia/Ammonium Toxicity Root Symptoms Induced by Inorganic and Organic Fertilizers and Placement

“…Tremendous advances have occurred in the imaging and sensing of soil chemical-root interactions (Blossfeld et al, 2013;Santner et al, 2015). High resolution root imaging with inexpensive CCD flatbed scanners have advanced rhizotron-type imaging of developing root systems, whereby interspecies species variation in root hair density and lengths were discerned in response to soil conditions (Hammac et al, 2011).…”