Perfluorosulfonated Ionomer-Modified Diffusive Gradients in Thin Films: Tool for Inorganic Arsenic Speciation Analysis

In-situ measurement of dissolved sulfide in surface sediment porewater using diffusive gradients in thin films (DGT) coupled with digital imaging

Dissolved sulfide in sediment porewater significantly influences aquatic ecosystems. Conventionally, sulfide determination in sediment porewater relies on ex-situ analytical methods, susceptible to measurement errors due to sulfide oxidation and volatilization during sample analysis. In this study, we introduced an innovative in-situ method for assessing dissolved sulfide in surface sediment porewater, leveraging the integration of diffusive gradients in thin films (DGT) with digital imaging. The DGT device effectively concentrates sulfide in sediment porewater, inducing observable color changes in the binding gel. Recordings of these changes, captured by imaging equipment, facilitated the establishment of calibration curves correlating grayscale value alterations in the binding gel to sulfide concentrations. Under optimal conditions, the developed method demonstrated a linear detection range of 3.0-200 μmol L-1 at 20 °C, particularly when the exposure time exceeded 180 min. The developed method is insensitive to salinity and suitable for measuring sulfide concentrations in various natural water environments. Compared to traditional ex-situ methods, our approach circumvents challenges linked to intricate pre-treatment, prolonged analysis duration, and significant systemic errors. This proposed method presents a real-time solution for sulfide concentration assessment in surface sediment porewater, empowering researchers with an efficient means to monitor and study dynamic sulfide levels.

Novel in situ method based on diffusive gradients in thin-films with lanthanum oxide nanoparticles for measuring As, Sb, and V and in waters

Lanthanum oxide nanoparticles (nano-La₂O₃) was used to develop a novel binding gel within an in situ passive sampler based on diffusive gradients in thin-films technique (NL-DGT) for measuring As(V), Sb(V), and V(V). Performance characteristics of NL-DGT were independent of pH (pH: 3.1-7.9 for As, 3.1-8.5 for V, and 3.1-6.5 for Sb) and ionic strength (0.1-500 mmol L^-1 for As and V, and 0.1-200 mmol L^-1 for Sb). No obvious competition effects among As, Sb, and V with different concentration ratios were found for NL-DGT measurement. Long term storage (8-188 d) of the nano-La₂O₃ gels in 0.01 mol L^-1 NaNO₃ at 4 °C did not affect their performance. During the field deployments in Yangtze and Jiuxiang River, NL-DGT measured concentrations of As and V were similar to those measured by the grab samples, while some differences were found for Sb between DGT and grab sampling because higher pH (∼8.0) in the studied rivers caused the performance deterioration of NL-DGT. Generally, the newly developed NL-DGT is suitable for monitoring As and V in freshwater from acidic to light alkaline and Sb in acidic and neutral water.

Development of cerium oxide-based diffusive gradients in thin films technique for in-situ measurement of dissolved inorganic arsenic in waters

A diffusive gradients in thin films (DGT) method using a new high-capacity cerium oxide (CeO₂) binding gel, for the first time, was developed for measuring dissolved inorganic arsenic in freshwater and seawater. The capacities of the new CeO₂ binding gel were 682 μg and 375 μg for As^III and As^V, respectively. The masses of As^III and As^V accumulated by CeO₂-DGT device increased linearly with time and agreed well with the theoretical value calculated by DGT equation. The arsenic accumulation by CeO₂-DGT was independent of pH (4.05-9.04) and ionic strength (0.1-750 mM), and common anions including CO₃^2-, SO₄^2-, Cl^- and PO₄^3- had no obvious interference. CeO₂-DGT showed excellent long-term deployment performance in freshwater and synthetic seawater. Field trials with CeO₂-DGT achieved successfully the time-weighted-average concentrations of total inorganic arsenic in reservoir water (1.38 ± 0.09 μg/L) and coastal seawater (0.45 ± 0.06 μg/L). The results were comparable to those measured by grab sampling. The proposed method was reliable and robust for in-situ measurements of dissolved inorganic arsenic in environmental waters.

Zirconium metal organic frameworks-based DGT technique for in situ measurement of dissolved reactive phosphorus in waters

In an effort to provide early warnings for the occurrence of eutrophication, it is highly desirable to develop an accurate and efficient technique to ensure continuous monitoring of dissolved reactive phosphorus (DRP) in the aquatic environment from the viewpoint of environmental management. Herein, a new diffusive gradient in thin film (DGT) technique was developed and evaluated for in situ measurement of DRP in waters, in which Zr-based metal organic frameworks (MOFs, UiO-66) were utilized as aqueous binding agent (abbreviated as UiO-66 DGT). As expected, the UiO-66 DGT demonstrated high uptake capacity towards phosphorus (20.8 μg P cm^-2). Meanwhile, an excellent linearity between the accumulated DRP mass and deployment time over 5 d (R² = 0.996) was obtained regardless of high or low phosphate solution. In addition, effective diffusion coefficients (D) of DRP increased exponentially with increasing ionic strengths (R² = 0.99). Based on the rectified D, the performance of the UiO-66 DGT was independent of solution pH (6.5-8.5) and ionic strengths (ranging from 0.01 to 100 mmol L^-1). Furthermore, field deployments of the UiO-66 DGT were undertaken in a natural eutrophic lake (Lake Chaohu, China). It was noteworthy that DRP could be continually accumulated by the UiO-66 DGT for more than 14 d and good agreements were obtained between the concentrations measured by DGT (C_DGT) and those by ex situ chemical extraction method in solution (C_sol), as reflected by C_DGT/C_sol of 0.9-1.1. In situ determination of DRP speciation was also carried out at different sites across Lake Chaohu. Overall, this study contributed to a better constructing of liquid binding phase DGT for the measurement of DRP in waters, facilitating the widespread application of the UiO-66 DGT as a routine monitoring technique and for large-scale environmental analysis.

Novel Fe3O4 nanoparticles-based DGT device for dissolved reactive phosphate measurement

The use of Fe₃O₄ nanoparticles as a liquid binding phase in DGT is demonstrated for the sampling measurement of dissolved reactive phosphate in natural water.

Novel Speciation Method Based on Diffusive Gradients in Thin-Films for in Situ Measurement of Cr(VI) in Aquatic Systems

Hexavalent chromium (Cr(VI)) is much more toxic and mobile than the trivalent species (Cr(III)) and consequently, in situ monitoring of Cr(VI) can improve the understanding of Cr biogeochemistry and toxicity in ecosystems. The passive diffusive gradients in thin-films (DGT) technique is a powerful tool for determining metal(loid) speciation, but a binding phase that absorbs only one specific species of Cr is needed. N-Methyl-d-glucamine (NMDG) functional resin was incorporated into the DGT binding phase for selective measurement of Cr(VI). This NMDG-DGT sampler exhibited a theoretically linear accumulation of Cr(VI), with negligible accumulation (<5%) of Cr(III), even after 72 h deployment. The good prediction of Cr(VI) concentration in synthetic freshwater with NMDG-DGT, even in the presence of 10-time more Cr(III), further indicated the sampler's reliability in selective detection of Cr(VI). Moreover, its high capacity for Cr(VI), which exceeded 230 μg cm(-2), facilitates measurement of Cr(VI) in both uncontaminated natural waters and in slightly and heavily contaminated (ppm level) waters. Field deployment of the NMDG-DGT sampler in such waters allowed accurate measurement of time-averaged Cr(VI) concentration, indicating its robustness for in situ measurements of Cr speciation and its potential for further application in the risk assessment of Cr.

Determination of total arsenic using a novel Zn-ferrite binding gel for DGT techniques: Application to the redox speciation of arsenic in river sediments

A new laboratory-made Zn-ferrite (ZnFe2O4) binding gel is fully tested using Diffusive Gradient in Thin films (DGT) probes to measure total As [including inorganic As(III) and As(V), as well as MonoMethyl Arsenic Acid (MMAA(V)) and DiMethyl Arsenic Acid (DMAA(V))] in river waters and sediment pore waters. The synthesis of the binding gel is easy, cheap and its insertion into the acrylamide gel is not problematic. An important series of triplicate tests have been carried out to validate the use of the Zn-ferrite binding gel in routine for several environmental matrixes studies, in order to test: (i) the effect of pH on the accumulation efficiency of inorganic As species; (ii) the reproducibility of the results; (iii) the accumulation efficiency of As species; (iv) the effects of the ionic strength and possible competitive anions; and (v) the uptake and the elution efficiency of As species after accumulation in the binding gel. All experimental conditions have been reproduced using two other existing binding gels for comparison: ferrihydrite and Metsorb® HMRP 50. We clearly demonstrate that the Zn-ferrite binding gel is at least as good as the two other binding gels, especially for pH values higher than 8. In addition, by taking into consideration the diffusion rates of As(III) and As(V) in the gel, combining the 3-mercaptopropyl [accumulating only As(III)] with the Zn-ferrite binding gels allows for performing speciation studies. An environmental study along the Marque River finally illustrates the ability of the new binding gel to be used for field studies.

Measurement of dissolved reactive phosphorus in water with polyquaternary ammonium salt as a binding agent in diffusive gradients in thin-films technique

Diffusive gradients in thin-films (DGT) sampler with a polyquaternary ammonium salt (PQAS) aqueous solution as a binding phase and a dialysis membrane as a diffusive phase (PQAS DGT) was developed for the measurement of dissolved reactive phosphorus (DRP) in water. The performance of PQAS DGT was not dependent upon pH 3-10 and ionic strength from 1 × 10(-4) to 1 mol L(-1). The effective binding capacity of PQAS DGT containing 2.0 mL of 0.050 mol L(-1) PQAS solution was estimated as 9.9 μg cm(-2). The measurement of DRP in a synthetic solution by PQAS DGT over a 48 h deployment period demonstrated high consistency with the concentration of DRP in the synthetic solution measured directly by the ammonium molybdate spectrophotometric method. Field deployments of PQAS DGT samplers allowed for accurate measurement of the DRP concentration in situ. The advantages of PQAS DGT include no requirement of the elution steps and direct concentration measurements of the binding phase.

Rapid on-site separation of As(III) and As(V) in waters using a disposable thiol-modified sand cartridge

The rapid redox transformation of arsenic (As) species in waters presents a great environmental challenge in the accurate determination of its concentration and toxicity. The motivation of the present study was therefore to develop a method for rapid on-site separation of As(V) and As(III) in various aqueous matrices. The authors synthesized a thiol-modified sand (T-sand) that selectively removed As(III) but did not adsorb As(V). The novel application of this T-sand in a disposable cartridge was able to successfully separate As(V) (37-970 µg L(-1) ) and As(III) (not detected to 488 µg L(-1) ) in 23 groundwater samples collected in areas with naturally occurring As. The As speciation results determined with T-sand separation in the field were consistent with those obtained using high-performance liquid chromatography-atomic fluorescence spectrometry. Furthermore, the T-sand cartridge was applicable in a wide variety of matrices, including groundwater, leachants of the toxicity-characteristic leaching procedure, and extracts from the California waste extraction test; sequential extraction test; and in vitro gastrointestinal extraction. This easy-to-use separation method is especially suitable for routine field monitoring of As speciation.

Selective sampling and measurement of Cr (VI) in water with polyquaternary ammonium salt as a binding agent in diffusive gradients in thin-films technique

A diffusive gradients in thin films (DGT) device with polyquaternary ammonium salt (PQAS) as a novel binding agent (PQAS DGT) combined with graphite furnace atomic absorption spectrometry (GFAAS) was developed for the selective sampling and measurement of Cr (VI) in water. The performance of PQAS DGT was independent of pH 3-12 and ionic strength from 1 × 10(-3) to 1 molL(-1). DGT validation experiments showed that Cr (VI) was measured accurately as well as selectively by PQAS DGT, whereas Cr (III) was not determined quantitatively. Compared with diphenylcarbazide spectrophotometric method (DPC), the measurement of Cr (VI) with PQAS DGT was agreement with that of DPC method in the industrial wastewater. PQAS-DGT device had been successfully deployed in local freshwater. The concentrations of Cr (VI) determined by PQAS DGT coupled with GFAAS in Nuer River, Ling River and North Lake were 0.73 ± 0.09 μg L(-1), 0.50 ± 0.07 μg L(-1) and 0.61 ± 0.07 μg L(-1), respectively. The results indicate that PQAS DGT device can be used for the selective sampling and measurement Cr (VI) in water and its detection limit is lower than that of DPC method.

DGT measurement of dissolved aluminum species in waters: comparing Chelex-100 and titanium dioxide-based adsorbents

Aluminum is acutely toxic, and elevated concentrations of dissolved Al can have detrimental effects on both terrestrial and aquatic ecosystems. Robust analytical methods that can determine environmentally relevant Al fractions accurately and efficiently are required by the environmental monitoring community. A simple, robust passive sampling method, the diffusive gradients in thin films (DGT) technique, was evaluated for the measurement of dissolved Al species in freshwater and marine water using either Chelex-100 or Metsorb (a titanium dioxide-based binding agent) as the adsorbent. Mass vs time DGT deployments at pH 5.05 (Al(3+) and Al(OH)(2+) dominate) and 8.35 (Al(OH)(4)(-) dominates) demonstrated linear uptake of Al (R(2) = 0.989 and 0.988, respectively) for Metsorb. Similar deployments of Chelex-DGT showed linear uptake at pH 5.05 (R(2) = 0.994); however, at pH 8.35 the mass of Al accumulated was 40-70% lower than predicted, suggesting that Chelex-100 is not suitable for Al measurements at high pH. The Metsorb-DGT measurement was independent of pH (5.0-8.5) and ionic strength (0.001-0.7 mol L(-1) NaNO(3)), whereas the Chelex-DGT measurement was only independent of ionic strength at pH 5.0. At pH 8.4, increasing ionic strength led to considerable underestimation (up to 67%) of Al concentration. Deployments of Metsorb-DGT (up to 4 days) in synthetic freshwater (pH range 5.4-8.1) and synthetic seawater (pH 8.15) resulted in linear mass uptakes, and the concentration measured by DGT agreed well with solution concentrations. Conversely, deployment of Chelex-DGT in synthetic seawater and freshwater (pH ≥7.7 Al(OH)(4)(-) dominant species) resulted in a decrease in accumulated mass with increasing deployment time. In situ field evaluations in fresh, estuarine, and marine waters confirmed that Metsorb-DGT was more accurate than Chelex-DGT for the measurement of dissolved Al in typical environmental waters.

Titanium dioxide-based DGT technique for in situ measurement of dissolved reactive phosphorus in fresh and marine waters

A new diffusive gradients in a thin film (DGT) technique for measuring dissolved reactive phosphorus (DRP) in fresh and marine waters is reported. The new method, which uses a commercially available titanium dioxide based adsorbent (Metsorb), was evaluated and compared to the well-established ferrihydrite-DGT method (ferrihydrite cast within the polyacrylamide gel). DGT time-series experiments showed that the mass of DRP accumulated by Metsorb and ferrihydrite was linear with time when deployed in simple solutions. Both DGT methods showed predictable uptake across the pH (4.0-8.3) and ionic strength (0.0001-1 mol L(-1) NaNO(3)) ranges investigated, and the total capacity of the Metsorb binding phase (∼40,000 ng P) was 2.5-5 times higher than the reported total capacity of the ferrihydrite binding phase. The measurement of DRP in synthetic freshwater and synthetic seawater by Metsorb-DGT over a 4 day deployment period showed excellent agreement with the concentration of DRP measured directly in solution, whereas the ferrihydrite-DGT method significantly underestimated (23-30%) the DRP concentration in synthetic seawater for deployment times of two days or more. Field deployments of Metsorb-DGT samplers with various diffusive layer thicknesses allowed accurate measurement of both the diffusive boundary layer thickness and DRP concentration in situ. The Metsorb-DGT method performs similarly to ferrihydrite-DGT for freshwater measurements but is shown to be more accurate than the ferrihydrite method for determining DRP in seawater.