Peter R. Teasdale

Synthesis and characterisation of a polyacrylamide–polyacrylic acid copolymer hydrogel for environmental analysis of Cu and Cd

Abstract A polyacrylamide–polyacrylic acid copolymer hydrogel was prepared by the controlled hydrolysis of polyacrylamide in an alkaline solution of 10% sodium hydroxide. The structure and composition of the resulting copolymer hydrogel was approximately two acrylic acid units for every acrylamide unit. The capacity of the copolymer hydrogel to bind various metal ions was tested under a range of uptake conditions, with varying uptake time, pH and ionic strength. Ions such as Cu 2+ and Cd 2+ were bound more strongly to the copolymer hydrogel than the competing ions of Na + , K + , Ca 2+ and Mg 2+ , particularly at pH>5, largely due to the increased acidity of these transition metal ions. Metals bound to the copolymer hydrogel were efficiently (>94%) eluted in 2 M HNO 3 solution. The copolymer was found to have a p K a of ∼4.5 and had an equilibrium swelling ratio of 120 at pH>6 with an ionic strength equivalent to 0.01 M NaCl, and a swelling ratio of 265 at low ionic strengths. This material should be of use for the recovery and separation of Cu 2+ and Cd 2+ ions, and trace environmental analysis applications such as the diffusive gradients in thin films (DGT) technique.

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.

New Diffusive Gradients in a Thin Film Technique for Measuring Inorganic Arsenic and Selenium(IV) Using a Titanium Dioxide Based Adsorbent

A new diffusive gradients in a thin film (DGT) technique, using a titanium dioxide based adsorbent (Metsorb), has been developed and evaluated for the determination of dissolved inorganic arsenic and selenium. As(III), As(V), and Se(IV) were found to be quantitatively accumulated by the adsorbent (uptake efficiencies of 96.5-100%) and eluted in 1 M NaOH (elution efficiencies of 81.2%, 75.2%, and 88.7%). Se(VI) was not quantitatively accumulated by the adsorbent (<20%). Laboratory DGT validation experiments gave linear mass uptake over time (R(2) >or= 0.998) for As(III), As(V), and Se(IV). Consistent uptake occurred over pH (3.5-8.5) and ionic strength (0.0001-0.75 mol L(-1) NaNO(3)) ranges typical of natural waters, including seawater. Field deployments of DGT probes with various diffusive layer thicknesses confirmed the use of the technique in situ, allowing calculation of the diffusive boundary layers and an accurate measurement of inorganic arsenic. Reproducibility of the technique in field deployments was good (relative standard deviation <8%). Limits of detection (4 day deployments) were 0.01 microg L(-1) for inorganic arsenic and 0.05 microg L(-1) for Se(IV). The results of this study confirmed that DGT with Metsorb was a reliable and robust method for the measurement of inorganic arsenic and the selective measurement of Se(IV) within useful limits of accuracy.

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.

Survey of users and providers of recycled water: quality concerns and directions for applied research

Users and providers of recycled water in Queensland, Australia, were canvassed to ascertain concerns about recycled water quality and directions for applied research. Some 79% of respondents had concerns about recycled water quality including microbiological components, such as viruses, parasites and bacteria, salinity related components, aggregate components, such as pH and solids, nutrients, organic components, including pesticides and endocrine disruptors, as well as quality variability. Relative differences in the proportion of concern for some quality characteristics were observed between the areas of different population density with those from the low population areas being more concerned than those from medium or high areas. Some users and providers observed changes in recycled water quality during distribution including nutrients, chlorine, solids, colour and odour. Some 52% of providers and 19% of current users propose to expand their usage and 30% of non-users propose to commence doing so in the next 5 years. Recycled water quality characteristics identified for applied research included microbiological components, organic components, nutrients and salinity.

Speciation of dissolved inorganic arsenic by diffusive gradients in thin films: selective binding of AsIII by 3-mercaptopropyl-functionalized silica gel

A diffusive gradients in thin films (DGT) technique for selectively measuring As(III) utilizes commercially available 3-mercaptopropyl-functionalized silica gel. Deployment of the new technique alongside the Metsorb-DGT for total inorganic arsenic allows the calculation of As(III) directly and As(V) by difference. Uptake of As(III) by mercapto-silica was quantitative and elution with a mixture of 1 mol L(-1) HNO(3) and 0.01 mol L(-1) KIO(3) gave a recovery of 85.6 ± 1.7%. DGT validation experiments showed linear accumulation of As(III) over time (R(2) > 0.998). Accumulation was unaffected by varying ionic strength (0.0001-0.75 mol L(-1) NaNO(3)) and pH (3.5-8.5). Deployment of mercapto-silica DGT and Metsorb DGT in seawater spiked with As(III) and As(V) demonstrated the ability of the combined approach to accurately quantify both species in the presence of potential competing ions. Ferrihydrite DGT, which has been previously reported for the measurement of total inorganic arsenic, was evaluated in seawater and shown to underestimate both As(III) and As(V) at longer deployment times (72 h). Reproducibility of the new mercapto-silica DGT technique was good (relative standard deviations < 9%), and the average method detection limit was sufficiently low to allow quantification of ultratrace concentrations of As(III) (0.03 μg L(-1); 72 h deployment).

Titanium dioxide-based DGT for measuring dissolved As(V), V(V), Sb(V), Mo(VI) and W(VI) in water

A titanium dioxide-based DGT method (Metsorb-DGT) was evaluated for the measurement of As(V), V(V), Sb(V), Mo(VI), W(VI) and dissolved reactive phosphorus (DRP) in synthetic waters. Mass vs. time DGT deployments at pH 6.06 (0.01 mol L(-1) NaNO3) demonstrated linear uptake of all analytes (R(2) ≥ 0.994). Diffusion coefficients measured using a diffusion cell were in reasonable agreement with diffusion coefficients measured using DGT samplers (DCell/DDGT=0.82-1.10), although a systematic difference was apparent. The Metsorb-DGT method was independent of ionic strength (0.001-0.7 mol L(-1) NaNO3 at pH 7.1) for the measurement of all analytes (CDGT/CSol=0.88-1.11) and, with the exception of V(V), the method was independent of pH (3.98-8.24, 0.01 mol L(-1) NaNO3), indicated by CDGT/CSol values in the range 0.88-1.13 for short-term deployments (up to 10h). For V(V) at pH 3.98, Metsorb-DGT underestimated the solution concentration by 17%, presumably due to weak binding of the VO2(+) species. The Metsorb-DGT and ferrihydrite-DGT (in situ precipitated ferrihydrite) methods were compared by deploying samplers in synthetic freshwater (pH 7.20, conductivity 223 μS cm(-1)) and synthetic seawater (pH 8.3. salinity 34.6) for up to four days. For synthetic freshwater, CDGT/CSol values between 0.87-1.17 were obtained for all analytes measured by the Metsorb-DGT method over the deployment period. For ferrihydrite-DGT, CDGT/CSol values between 0.97-1.23 were obtained for As(V), V(V), W(VI) and DRP. However, Mo and Sb(V) showed reduced uptake and CDGT/CSol values were in the range 0.18-1.14 and 0.39-0.98, respectively. In synthetic seawater deployments, Metsorb-DGT was capable of measuring As(V), V(V), Sb(V), W(VI) and DRP for up to 4 days (CDGT/CSol=0.89-1.26), however, this method was not capable of measuring Mo for deployment times >4h (CDGT=0.27-0.72). For ferrihydrite-DGT, CDGT/CSol values in the range 0.92-1.16 were obtained for As(V), V(V) and DRP, however, Mo(VI), Sb(V) and W(VI) could not be measured to within 15% of the solution concentration (CDGT/CSol 0.02-0.83).

Representative measurement of two-dimensional reactive phosphate distributions and co-distributed iron(II) and sulfide in seagrass sediment porewaters

The high degree of heterogeneity within sediments can make interpreting one-dimensional measurements difficult. The recent development and use of in situ techniques that measure two-dimensional distributions of porewater solutes have facilitated investigation of the role of spatial heterogeneity in sediment biogeochemistry. A colourimetric diffusive equilibration in thin films method has been developed that allows two-dimensional, high-resolution measurement of reactive phosphate in sediment porewaters. A method detection limit of 0.22 μM, an effective upper limit of ~1000 μM and relative standard deviations typically below 5% were achieved. This method was evaluated by deployment in seagrass (Zostera capricorni) colonised sediments, as part of combined probes with similar colourimetric methods for sulfide and iron(II). The two-dimensional, high resolution distributions obtained provide a highly representative measurement of the co-distributions of porewater solutes, allowing heterogeneous features and biogeochemical processes to be observed and interpreted. Microniches of high phosphate concentration >100 μM were observed throughout the distributions and were interpreted to be due to localised zones of rapid organic matter mineralisation, possibly using electron acceptors other than iron(III) oxyhydroxides (e.g. aerobic respiration) as often they did not correspond with microniches of higher Fe(II) concentration.

Trace metal speciation measurements in waters by the liquid binding phase DGT device

The speciation measurements of trace metals by the diffusive gradients in thin-films technique (DGT) using a poly(4-styrenesulfonate) (PSS) aqueous solution as a binding phase and a cellulose dialysis membrane (CDM) as a diffusive layer, CDM-PSS DGT, were investigated and showed good agreement with computer modelling calculations. The diffusion coefficients of ethylenediaminetetraacetic acid (EDTA) complexes with Cd(2+) and Cu(2+) were measured and compared with those of the inorganic metal ions. CDM-PSS DGT device was tested for speciation measurement in sample solutions containing EDTA, tannic acid (TA), glucose (GL), dodecylbenzenesulfonic acid (DBS) and humic acid (HA) as complexing ligands forming organic complexes with varying stability constants. Lower percentages of DGT labile copper concentrations over total filterable copper concentrations obtained from the deployments in freshwater sites indicated that copper complexes with organic matter were basically not measured by the devices.

Comparing dissolved reactive phosphorus measured by DGT with ferrihydrite and titanium dioxide adsorbents: Anionic interferences, adsorbent capacity and deployment time

Two adsorbents (Metsorb and ferrihydrite) used in binding layers with the diffusive gradients in a thin film technique were evaluated for the measurement of dissolved reactive phosphorous (DRP) in synthetic and natural waters. Possible interferences were investigated with Cl(-) (up to 1.35 mol L(-1)) and SO(4)(2-) (up to 0.056 mol L(-1)) having no affect on either DGT binding layer, and HCO(3)(-) (up to 5.7 mmol L(-1)) having no effect on Metsorb-DGT, over 4 days. However, HCO(3)(-) interfered with the ferrihydrite-DGT measurement at concentrations typical of many natural waters (≥0.7 mmol L(-1)) after a deployment period of 1-2 days. The capacity of the Metsorb binding phase for DGT response was ∼37,000 ng P, whereas the capacities of a low-mass (17.8 mg of adsorbent per DGT sampler) and high-mass (29.2mg of adsorbent per DGT sampler) ferrihydrite binding phase were substantially lower (∼15,000 ng P and ∼25,000 ng P, low-mass and high-mass, respectively). Increasing the capacity of the ferrihydrite adsorbent allowed the ferrihydrite-DGT to be utilized for up to 3 days before interference by HCO(3)(-) was observed. Seawater deployments demonstrated that even high-capacity ferrihydrite-DGT devices underestimated the DRP concentration by 37%, whereas Metsorb-DGT measurements were accurate. The Metsorb-DGT is superior to the ferrihydrite-DGT for determining DRP over deployment times greater than 1 day and in waters with ≥0.7 mmol L(-1) HCO(3)(-). Based on the experience obtained from this detailed validation process, the authors propose a number of key requirements that need to be considered when developing new DGT binding layers, with testing the performance over longer deployment times being critical.