Computer Imaging Densitometry Research Articles

Detecting antimony(III) on-site using novel gel-based techniques: Colorimetric diffusive equilibrium in thin films for two-dimensional imaging and surface-enhanced Raman scattering for sensitive quantification

Antimony (Sb) pollution has raised increasing public concerns and its rapid on-site screening is central for the risk assessment. Herein, we proposed two gel-based methods based on colorimetric diffusive equilibrium in thin films (DET) and surface-enhanced Raman scattering (SERS), for two-dimensional imaging and sensitive detection of Sb(III) by revisiting the phenylfluorone (PhF) complexation reaction. PhF was well dispersed in the polyvinyl alcohol (PVA) hydrogel and reacted with Sb(III) in the DET gel to form a strong PhF-Sb(III) complex. The distribution of Sb(III) was easily visualized at a submillimeter resolution using computer imaging densitometry, with a detection limit (LOD) of ∼100 nmol L−1. Field application in the Sb mine area reveals limited dissolved Sb(III) penetrating the redox barrier below the sediment-water interface by 20 mm in rivers and tailing pond sediments. To improve the detection sensitivity and apply the principle to trace Sb quantification, a SERS platform was established by anchoring PhF on the hydrogel-stabilized Ag nanoparticles via C–O–Ag bonding to specifically detect Raman-inactive Sb(III). Benefiting from the high SERS activity of PhF and enrichment ability of hydrogel, Sb(III) was quantified with a LOD of 1.2–10.7 nmol L−1 depending on the sample volume. The coexisting ions at a 100-fold higher concentration than Sb(III) resulted in only 3.3–10.4 % variation in SERS intensity, indicating a negligible interference on the SERS platform. The platform exhibited a RSD of 6.6–13.1 % and acceptable recoveries for various environmental matrices, highlighting its promise in on-site application.

An In Situ Analyzer for Two-Dimensional Fe(II) Distribution in Sediment Pore Water Based on Ferrozine Coloration and Computer Imaging Densitometry.

A novel integrated analyzer was developed for the in situ determination of two-dimensional (2D) dissolved Fe(II) distributions in sediment pore water. The analyzer utilized gel enrichment and optical imaging techniques. An image probe mainly consisting of a gel holder and portable document scanner was designed to be inserted into sediment. The gel holder exposed to the sediment was made to hold a polyacrylamide gel strip (diffusive gel) and polyacrylamide gel strip impregnated with C18 and coated with ferrozine (concentrating gel). The concentrating gel strip could accumulate the dissolved Fe(II) in pore water and produce a magenta-colored Fe(II)-ferrozine compound on the gel strip in two dimensions. The portable document scanner sealed in a transparent box and stuck onto the back of the gel holder could record gel images from the back of the concentrating gel strip. Gel images with grayscale intensities were acquired and analyzed using ImageJ software, and Fe(II) concentration was determined based on a deployment time related calibration curve established in the laboratory. The measurement accuracy and precision were investigated. The quantitative range reached up to 200 μmol L–1. The method and analyzer exhibit distinct characteristics of in situ enrichment and measurement; they were successfully applied to determine the 2D Fe(II) distribution in lake and marine sediment pore waters.

In situ, high-resolution measurement of labile phosphate in sediment porewater using the DET technique coupled with optimized imaging densitometry

Obtaining two-dimensional distributions of reactive phosphorus in sediment porewater is very important for understanding fine-scale phosphorus mobilization and sequestration processes in sediments. In this study, the diffusive equilibrium in thin films (DET) measurement based on computer imaging densitometry (CID) was studied in detail with optimal conditions described. This study focuses on evaluating the two-dimensional colorimetric DET method coupled with CID (DET-CID method) for porewater labile phosphate measurements. The result shows that the red channel filter is the optimum channel for sensitivity to process the image. Additionally, staining time and temperature have great influence on the method, and 20 min staining time and ≥25 °C staining temperature were recommended. The minimum detection limit of labile phosphate of this method was 0.300 mg P/L, and the maximum detection limit could reach 50.00 mg P/L. The DET-CID technique can be used to measure labile phosphate in a wide range of acidic and alkaline water bodies (pH = 2–10 and water hardness from 0 to 2000 mg/L as CaCO3). The linear regression analysis shows that this technique presents very similar results compared with other two existing methods (R2 = 0.999). Our results would give insights into the precisely measurements of labile phosphate in field applications.

A new colorimetric DET technique for determining mm-resolution sulfide porewater distributions and allowing improved interpretation of iron(II) co-distributions.

Measurement of sulfide in pore waters is critical for understanding biogeochemical processes, especially within coastal sediments. Here we report the development of a new colorimetric DET (diffusive equilibration in thin films) technique for determining mm-resolution, two-dimensional sulfide distributions in sediment pore waters. This colorimetric sulfide DET method was based on the standard spectrophotometric methylene blue assay, but modified to allow quantitation of sulfide by computer imaging densitometry. The method detection and effective upper measurement limits of the optimised technique were 3.7 and 1000μmolL-1, respectively. The optimised sulfide DET method was combined with the colorimetric iron(II) DET method to obtain co-distributions in coastal seagrass (Zostera muelleri) colonised sediment under light and dark conditions. In the dark, seagrass sediments were more reduced than in the light, with large areas being dominated by high porewater sulfide concentrations. These co-distributions were compared with those obtained using the previously described DET-DGT (diffusive gradients in thin films) method for measuring iron(II) and sulfide co-distributions. There was less overlap of iron(II) and sulfide distributions using the sulfide DET as the two DET methods are influenced most by the later hours of deployment, whereas the sulfide-DGT measurement integrates concentrations over the whole deployment period. Overlap was most apparent in very dynamic sediment zones, such as burrow wall sediments.

Release mechanism and kinetic exchange for phosphorus (P) in lake sediment characterized by diffusive gradients in thin films (DGT)

Diffusive gradients in thin films (DGT) technique has been newly designed for the identification of formation mechanisms of “internal phosphorus (P)-loading” and the numerical simulation of P exchange at DGT/sediment interface in Lake Dianchi. The primary mechanism was Fe-redox controlled P release from Fe-bound P in sediments, which was revealed by CDGT (P and Fe), total P (Fe) and P (Fe) fractions in NH4Cl and BD phases in sediments and their relationships at sites (N-T). The breakdown of algae biomass in the top layer of sediments at sites (O-T) and the coupled P/Fe/sulfur reactions at two depths at site N played a minor role in P release. The “internal P-loading” was calculated to be 19.23ta−1, which was 3.0% of the “entering P-loading”. At sites (1–9), DGT induced flux in sediments (DIFS) model for P was used to derive curves (i) the resupply parameter (R) against deployment time and (ii) the dissolved/sorbed concentrations against the distance at DGT/sediment interface, the variation characters of which were controlled by kinetics and sediment-P pool. Sulfide microniches in sediments related to P release were evaluated by computer imaging densitometry (CID). DGT-DIFS-CID should be a reliable method to reveal P mobilization in lake sediments.

Zr oxide-based coloration technique for two-dimensional imaging of labile Cr(VI) using diffusive gradients in thin films

A novel Zr oxide diffusive gradients in thin films (DGT)-based measurement technique for high-resolution imaging of labile Cr (VI) is introduced in this study. The method is based on the diphenylcarbazide coloration technique for Cr (VI) combined with computer-imaging densitometry, which provides a relation between the accumulated mass of Cr (VI) and the grayscale intensity. The Zr oxide gels show good performance in reflecting the accurate measurement of Cr (VI), independent of the effects of pH, ionic strength, and PO43− concentration. Settling of the Zr-oxide gel was identified as a simple and effective method to suppress leaching of the claret-colored compound out of the gel. In addition, the optimal volume of added coloration reagent was 125 times that of the binding gel and 30min was selected as the optimal time for the chromogenic reaction. The relationship between the accumulated Cr (VI) and grayscale intensity was analyzed under the optimized conditions. The Zr oxide DGT technique could also obviously reflect the heterogeneity of sediment. Consequently, Zr oxide DGT-based coloration is certified to be a robust and suitable tool for providing effective and high-resolution information on bioavailable Cr (VI).

The Simultaneous Measurement of Phosphorus, Sulfide, and Trace Metals by Ferrihydrite/AgI/Chelex-100 DGT (Diffusive Gradients in Thin Films) Probe at Sediment/Water Interface (SWI) and Remobilization Assessment

A new type of ferrihydrite/AgI/Chelex-100 DGT (diffusive gradients in thin films) probe was developed for simultaneous determination of three kinds of pollutants with a high resolution at exactly the same location of the SWI (sediment/water interface) of Dianchi Lake. Sediment porewater profiles (DGT concentration/flux) of eight metals and labile P at the SWI were measured by DGT and an HR-ICP-MS (high-resolution inductively coupled plasma mass spectrometry) instrument with a spatial resolution of 2 mm. Based on the DGT-CID (computer imaging densitometry) technique, the S(−II) (sulfide) profile and grayscale image with a spatial resolution of 0.0423 mm were obtained. By the investigation of sediment porewater profiles of labile metals, P, and S(−II), the remobilization feature (Fe/Mn reduction and co-release of pollutants) and “internal P loading” character were identified and assessed. The assessment of the scanned AgI gel indicated microniche peak fluxes and the distribution character of sulfide microniches in sediment.

A colorimetric DET technique for the high-resolution measurement of two-dimensional alkalinity distributions in sediment porewaters

Measurements of porewater alkalinity are fundamental to the study of organic matter mineralization in sediments, which plays an essential role in the global cycles of carbon and nutrients. A new colorimetric diffusive equilibration in thin film (DET) technique is described for measuring two-dimensional total alkalinity distributions in sediment porewaters at high resolution (1–2mm2). Thin polyacrylamide hydrogel layers (0.8mm) equilibrate with the porewater and, after removal, are immediately laid onto another hydrogel containing formic acid, which reacts with alkalinity-generating species, and the pH-indicator bromophenol blue. The resultant color change is quantified using computer-imaging densitometry. The lower limit of detection is 0.2meqL−1 and the upper measurement limit is 8meqL−1. Deployment in seagrass colonized sediment revealed high levels of spatial heterogeneity in the porewater alkalinity distribution, with concentrations ranging from 2.28meqL−1 in the overlying water to 5.13meqL−1 in some parts of the sediment. This is the first time that two-dimensional, high-resolution distributions of porewater alkalinity have been measured.

Gel-Based Coloration Technique for the Submillimeter-Scale Imaging of Labile Phosphorus in Sediments and Soils with Diffusive Gradients in Thin Films

We report a highly promising technique for the high-resolution imaging of labile phosphorus (P) in sediments and soils in combination with the diffusive gradients in thin films (DGT). This technique was based on the surface coloration of the Zr-oxide binding gel using the conventional molybdenum blue method following the DGT uptake of P to this gel. The accumulated mass of the P in the gel was then measured according to the grayscale intensity on the gel surface using computer-imaging densitometry. A pretreatment of the gel in hot water (85 °C) for 5 d was required to immobilize the phosphate and the formed blue complex in the gel during the color development. The optimal time required for a complete color development was determined to be 45 min. The appropriate volume of the coloring reagent added was 200 times of that of the gel. A calibration equation was established under the optimized conditions, based on which a quantitative measurement of P was obtained when the concentration of P in solutions ranged from 0.04 mg L(-1) to 4.1 mg L(-1) for a 24 h deployment of typical DGT devices at 25 °C. The suitability of the coloration technique was well demonstrated by the observation of small, discrete spots with elevated P concentrations in a sediment profile.

High-Resolution Simultaneous Measurements of Dissolved Reactive Phosphorus and Dissolved Sulfide: The First Observation of Their Simultaneous Release in Sediments

The reassessments of environmental processes in sediments rely upon capturing the heterogeneous features of elements at a small scale and at the same location. In this study, a diffusive gradients in thin films (DGT) technique was developed for the high-resolution simultaneous measurements of dissolved reactive phosphorus (DRP) and dissolved sulfide. A new binding gel was used in this DGT technique, which was prepared by incorporating AgI particles into the zirconium oxide binding gel previously used in the DGT measurement of DRP. The concentrations of the DRP and sulfide loaded into the binding gel were determined by a routine procedure and a computer-imaging densitometry (CID) technique, respectively. The performance of this DGT technique was tested under laboratory conditions and applied to in situ measurements in sediments of a shallow lake. Simultaneous release of DRP and dissolved sulfide was observed in a sulfide microniche with a diameter of ∼3 mm and in locally aggregated zones with a length over 1 cm, which was attributed to the simultaneous reductions of Fe(III) and sulfate and the associated release of Fe-bound P in the zones of the reactive organic matter in sediments. The good performance of this technique implies that there is a great potential for the development of new DGT techniques capable of simultaneous measurements of more analytes.

Optimization of colorimetric DET technique for the in situ, two-dimensional measurement of iron(II) distributions in sediment porewaters

The recently developed colorimetric diffusive equilibration in thin films (DET) technique for the in situ, high-resolution measurement of iron(II) in marine sediments is optimized to allow measurement of the higher iron concentrations typical of freshwater sediment porewaters. Computer imaging densitometry (CID) is used to analyze the retrieved samplers following exposure to ferrozine, a colorimetric reagent selective for iron(II). The effect of ferrozine concentration, image processing parameters and ionic strength are investigated to improve the applicability of this technique to a wider range of aquatic systems than reported in the first publications of this approach. The technique was optimized to allow detection of up to 2000μmolL−1 iron(II), a four-fold increase on the previous upper detection limit of 500μmolL−1. The CID processing of the scanned color image was also optimized to adjust the sensitivity of the assay as required; by processing the image with different color channel filters, the sensitivity of the assay can be optimized for lower concentrations (up to 100μmolL−1) or higher concentrations (up to 2000μmolL−1) of iron(II), depending on the specific site characteristics. This process does not require separate sampling probes or even separate scans of the DET gels as the color filter and grayscale conversion is done post-image capture. The optimized technique is very simple to use and provides highly representative, high-resolution (1mm) two-dimensional distributions of iron(II) in sediment porewaters. The detection limit of the optimized technique was 4.1±0.3μmolL−1 iron(II) and relative standard deviations were less than 6%.

Direct colorimetric detection of copper(II) ions in sampling using diffusive gradients in thin-films

A novel binding phase was developed for use in diffusive gradients in thin-film (DGT) sampling for Cu(II) by employing methylthymol blue as a chelating and chromogenic agent. Methylthymol blue was adsorbed onto beads of Dowex 1 × 8 resin (200–400 mesh) and the resin beads were then immobilised onto an adhesive disc. Analysis of exposed binding discs by either UV–vis spectrophotometry or computer imaging densitometry provided robust quantification of adsorbed Cu(II) in the 0.2–1 μg cm −2 range, allowing detection at μg L −1 concentrations in the test solution (ca. 17 μg L −1 for a 24 h deployment), and in good agreement with established DGT theory. The method was shown to be a potential replacement for binding phases based on Chelex 100 where a colorimetric response to a specific metal is desired.

A novel gel‐based technique for the high resolution, two‐dimensional determination of iron (II) and sulfide in sediment

Many coastal marine sediments display highly heterogeneous biogeochemistry due to complex biological and chemical interactions. Existing measurement techniques are limited in their ability to characterize the distributions of reduced species at high resolution in two dimensions. To obtain more detailed information than existing methods, a novel technique for the simultaneous high‐resolution (1 mm), two‐dimensional determination of porewater iron (II), and sulfide using a gel‐based diffusive sampler was developed. A diffusive equilibration in a thin‐film (DET) hydrogel was colorized by Ferrozine reagent, imaged electronically, and analyzed using computer imaging densitometry. With the selected gel parameters, the method detection limit for iron (II) was 0.6 µmol L−1, with an upper calibration limit of 500 µmol L−1 and relative standard deviations below 10% across the calibration range. This method was integrated into the existing diffusive gradient in a thin film (DGT) method for sulfide determination. Field deployments of a prototype probe, which measured an area of 170 × 80 mm, in estuarine sediment revealed complex, fine‐scale, interlocking zones of iron (II) and sulfide. This confirms the need to assess more than a single dimension, at appropriately high resolution, to obtain an accurate representation of sediment geochemistry. Advantages of the technique include its high spatial resolution, minimal sample handling, flexible probe size, simultaneous measurement of two analytes at exactly the same location in the sediment, and the rapid production of data without requiring expensive analytical instrumentation.

Size and Density Distribution of Sulfide-Producing Microniches in Lake Sediments

The technique of diffusive gradients in thin films (DGT) with measurements by computer-imaging densitometry (CID) was used to study the in situ, two-dimensional distribution of sulfide-producing microniches in sediments from a eutrophic lake (Esthwaite Water, UK). The DGT-CID technique precipitates and immobilizes the net pore-water flux of dissolved sulfide as black Ag2S by reaction with a AgI binding gel. The mass of accumulated sulfide is then determined from a scanned grayscale image of this gel. DGT probes deployed in laboratory mesocosms of homogeneously mixed sediment, then analyzed at high spatial resolution (approximately 0.1 mm), showed that apparent niche areas (operationally defined by DGT-CID) were <1 mm2 for 30% of the niches. In eight DGT probes deployed in undisturbed sediment cores, the proportion of microniche-related sulfide flux was > or =1-8% of the total horizontal net pore-water flux of sulfide. This study suggests that microniches that introduce local redox gradients are common in sediments. As fluxes within these microenvironments can considerably exceed background values, consideration of the dynamics of the biogeochemical processes occurring at these sites is likely to be key to improving our understanding of early diagenesis. Measurement procedures and three-dimensional reaction-transport models should be designed with the aim of furthering understanding of the complexities associated with locally supplied particles of reactive organic matter.

Measuring sulfide accumulation in diffusive gradients in thin films by means of purge and trap followed by ion‐selective electrode

A procedure for measuring in situ sulfide concentrations by coupling diffusive gradients in thin films (DGT) to solid-state ion-selective electrodes (ISE) is described and evaluated. Laboratory tests were performed to evaluate the coupling of these techniques, and these results were compared to the previously used methods of computer imaging densitometry (CID) and methylene blue. An average elution efficiency of 89%, as detected by ISE, was determined for a series of solutions containing amounts of sulfide ranging from 0.009 to 2.50 micromol. The validity of the standard mass-transfer equation for sulfide accumulation by DGT probes and subsequent detection by ISE was tested by measuring the mass of sulfide collected over time and with respect to varying diffusive gel thicknesses. Regressions of the sulfide mass accumulation versus the independent variables of time and inverse diffusive thickness proved to be linear. The use of DGT coupled to ISE provides a preconcentration method for sulfide that improves the detection limit (DL) over other techniques. This alternative method provides quantitative measurements of DGT-captured sulfide from solutions with a detection limit of at least 0.1 micromol/L for a 24-h deployment. The DGT technique also provides potential advantages in understanding sulfide speciation over existing methods.

In situ two-dimensional high-resolution profiling of sulfide in sediment interstitial waters.

A recently developed technique of diffusive gradients in thin films (D6T)-computer-imaging densitometry (CID) was improved to study in situ two-dimensional distributions of sulfide in sediment interstitial waters adjacent to the DGT device. The in situ profiler accumulates sulfide from the sediment interstitial water through a diffusive gel onto a AgI binding gel to form black Ag2S. The amount of sulfide bound in the binding gel is then determined from the gray scale density of the dried binding gel. New gel-making procedures were employed to produce stable, evenly dispersed AgI binding gels and to minimize the darkening of the AgI gel upon exposure to light The improved DGT-CID technique was used to profile the distribution of sulfide in sediment interstitial waters at Delta Marsh, a highly productive Prairie wetland on the south shore of Lake Manitoba, at a vertical and lateral resolution of < or = 0.4 mm. The in situ high-resolution microprofiles revealed unprecedented two-dimensional heterogeneity in sulfide concentrations in the sediment interstitial waters adjacent to the DGT device. The mosaic distribution of oxic and sulfidic microenvironments suggested not only the complexity and heterogeneity of the biogeochemistry of sulfur species and sulfide-binding metals (e.g., Cd, Cu, Pb, Hg, Zn) in sediments but also the capability of aquatic organisms for coping with the sulfidic environment

In situ, High-Resolution Measurement of Dissolved Sulfide Using Diffusive Gradients in Thin Films with Computer-Imaging Densitometry

The technique of diffusive gradients in thin films (DGT) has been developed for the measurement of dissolved sulfide. Sulfide species from the sampled waters diffuse through a polyacrylamide hydrogel and then react with pale yellow AgI((s)), incorporated at the surface of a second gel, to form black Ag(2)S((s)). The accumulated sulfide can be measured with a conventional purge-and-trap method followed by colorimetry (methylene blue). This enables the dissolved-sulfide concentration to be calculated under suitable conditions. Alternatively, the color change in the accumulating gel can be used to measure sulfide. A conventional flat-bed scanner, allied to imaging software, provided a densitometric measurement that was quantitatively related to the amount of sulfide accumulated. DGT measurements on synthetic solutions accurately determined the sulfide concentration (95% recovery), thereby confirming the unobstructed diffusion of HS(-) through the gel. The accumulated mass was inversely proportional to the diffusion-layer thickness as theoretically predicted. With the selected geometry, the limit of detection of the densitometric procedure for a 24-h deployment was 0.13 μmol L(-)(1), and the maximum concentration measurable was 60 μmol L(-)(1). When used in anoxic lacustrine waters, DGT provided sensible concentrations. It was also used to measure depth profiles at submillimeter resolution in estuarine surface sediments.