polyacrylamide diffusive gel and membrane filter at pH 5 using a diffusion
cell
Measurement of diffusion coefficients of AsV and AsIII through the diffusive gel and membrane filter were carried out by placing a piece of diffusive gel and a 0.025 µm cellulose nitrate membrane filter (Schleicher & Schuell) in the opening that separates the two sides of the diffusion cell. The membrane filter was placed on the side of the gel facing the solution containing As. This was done to emulate the set-up used when deploying DGT devices. The membrane filters were firstly soaked in Milli-Q water for 24 h and cleaned in 5 % v/v HNO3
Both sides of the diffusion cell contained 0.7 mL of 1 mol L-1 NaNO3, 1.75 mL of 1 mol L-1
sodium acetate (pH 5.0), and 66.85 mL of Milli-Q water; 0.7 mL of 1000 ppm AsVor 1000 ppm AsIII was added to compartment A of the diffusion cell to give an approximate AsV or AsIII concentration of 10 ppm. Concentrated HCl (70 µL) was added to compartment B of the diffusion cell to equal the concentration of HCl in compartment A due to As standards being prepared in 10 % v/v HCl. In addition, 0.63 mL of Milli-Q water was added to compartment B so that both sides of the diffusion cell contained the same volume. The final concentrations of NaNO3 and sodium acetate in both sides of the diffusion cell were 0.01 and 0.025 mol L-1,
respectively. The pH of the solution in both sides of the diffusion cell was adjusted to 5.0 using 1 mol L-1 NaOH. The pH was measured at the end of the experiment to make sure that it had not changed significantly. Both sides of the diffusion cell were stirred using small magnetic followers.
Samples of solution were removed from compartment B of the diffusion cell (1.1 to 0.1 mL for AsV experiments and 2.6 to 0.2 mL for AsIII experiments) every 15 mins so that the As concentration could be determined after appropriate dilution. An equivalent volume was removed from compartment A to maintain the same volume in each side of the diffusion cell. Replicate samples were removed and analysed from compartment B of the diffusion cell for at least 2 sampling times to determine reproducibility. The temperature in compartment A of the diffusion cell at the start and end of the experiment was measured. In addition, whenever replicate samples were removed for analysis, the temperature was also measured. It is the average of these temperature measurements that is reported in the results section.
Samples were removed from compartment A of the diffusion cell at the start and end of the experiment to determine the actual (rather than calculated) concentration of As initially present (it is this initial As concentration that is used to determine the diffusion coefficients) and to make sure that the As concentration did not change significantly during the experiment. For the AsV diffusion coefficient experiments, samples were removed for AsIII analysis to make sure that reduction of AsV had not occurred during the experiment. All samples removed during the experiments were analysed within 2 h.
For the determination of the AsIII diffusion coefficient, the diffusion cell experiment was carried out in duplicate, whereas for the determination of the AsV diffusion coefficient, only one experiment was carried out.
4.2.3 Measurement of As
Vand As
IIIdiffusion coefficients through
polyacrylamide diffusive gel and membrane filter at pH 5 using DGT
devices
The diffusion coefficients of AsV and AsIII through the diffusive gel and membrane filter were determined using DGT devices. This was carried out by deploying DGT devices in AsV and AsIII solutions for set times. The DGT devices were assembled and deployed as described in chapter 2. The membrane filters used for this work were 0.025 µm cellulose nitrate membrane filters (Schleicher & Schuell). The membrane filters were initially soaked in Milli- Q water for 24 h and cleaned in 5 % v/v HNO3 for 24 h. The membrane was then rinsed in
Milli-Q water and stored in 0.01 mol L-1 NaNO3.
The As deployment solutions were prepared by diluting 40 mL of 1 mol L-1 NaNO3, 100 mL
of 1 mol L-1 sodium acetate (pH 5.0), and 24 mL of either 10 ppm AsV or 10 ppm AsIII, to 4 L. The volume of solution was sufficiently large to avoid significant depletion of As by the DGT devices. Using this volume, the amount of As removed from solution was < 5 %. The final concentrations of NaNO3, sodium acetate, and As in the deployment solutions were 0.01
mol L-1, 0.025 mol L-1, and ~ 60 ppb, respectively. A small volume of 1 mol L-1 NaOH was added to adjust the pH to ~ 5.0. The pH of the As deployment solutions were measured at the start, end, and approximately half way through the experiments. The deployment solutions were left to stand for ~ 1 h before DGT devices were deployed. Fifteen DGT devices were deployed in the well-stirred solution (stirring rate was 620 rpm) and removed at various times to give deployment times from ~ 4 to 32 h. The exact deployment times were recorded. At each retrieval time, 3 DGT devices were removed so triplicate samples could be obtained. The temperature was recorded at the start and end of the experiment as well as each time DGT devices were retrieved. It is the average of these temperature measurements that is reported in the results section. After retrieval, DGT devices were placed in Milli-Q water and then washed thoroughly before disassembling and removal of the iron-oxide gel adsorbent. The same elution and analysis procedure was used as described in section 4.2.1.
Samples were removed from the As deployment solutions at the start and end of the experiment to confirm that the As concentration did not change significantly over this time. These samples were stabilized by adding concentrated HCl to give a final concentration of 1.1 mol L-1 HCl and analysed at the same time as the DGT eluents. The As concentrations
determined from these samples were used to calculate the diffusion coefficients. In addition, an AsIII analysis was carried out on the deployment solution at end of the experiment to make sure that reduction of AsV had not occurred (for AsV experiments) or oxidation of AsIII had not occurred (for AsIII experiments).
For AsV, this experiment was carried out in duplicate. The first experiment utilized diffusive gels that were supplied by DGT Research Ltd. (Lancaster, U.K.); the second experiment used diffusive gels that had been prepared in our laboratory. For AsIII, this experiment was carried out 4 times in total; all experiments used diffusive gels prepared in our laboratory.
When calculating AsV and AsIII diffusion coefficients using DGT devices, a 100 % elution efficiency was assumed.