In situ monitoring of electroactive species by using voltammetry at microelectrodes

[摘要] In recent years, there has been considerable interest in the development of probes and analytical methods for in situ analysis.1-5 In situ analysis is attractive because it allows, for instance, elimination of many artefacts due to sample handling and real time analysis for the rapid detection of pollutant inputs in environmental samples. In general, microsensors and microanalytical systems are key steps in the development of in situ analytical procedures. To this purpose voltammetric methods are competitive,6 with respect to other instrumental analytical techniques, and offer several advantages as, for instance, simple and compact low cost apparatus, reliable automatic measurements, high sensitivity and speciation capabilities.1 Moreover, with the advent of microelectrodes,7,8 the analytical applications have been extended to resistive media,9 not accessible to conventional voltammetry with macroelectrodes (i.e., electrodes with radii > 0.1 mm). Moreover, the use of microelectrodes reduces the requirement of well defined cell geometry, thus leading to the simplification of the electrochemical apparatus. A two-terminal system is needed,7,8 in place of the classical three electrode cell, while the rapid achievement of steady-state conditions provides a means to avoid forced convection when preconcentration steps are needed to increase the sensitivity for trace analysis measurements.10,11 The purpose of this paper is to show the performance of mercury microelectrodes for the in situ detection of some key electroactive species in samples of environmental interest. Here, in situ is synonymous with measurements taken directly in the sample with no or minimum pretreatment. Either homogeneous or "semisolid samples", as soils and sediments, and the electroactive species oxygen, sulphide and heavy metal ions are investigated.   Experimental ApparatusVoltammetric experiments were carried out by using an EG&G Mod. 283 potentiostat/galvanostat (PAR, Princeton, N.J., USA), controlled with a personal computer via the EG&G PAR 270 electrochemical software. All measurements were carried out in a two-electrode cell, located inside a Faraday cage made of sheets of aluminium. The cell apparatus employed for measurements in the sediment samples was an air-tight vessel made of glass, which contained a small Teflon cup 1.5 mL volume, into which the solid sample was placed. A micromanipulator was used to hold the microelectrode in place. The reference electrode was either an Ag/AgCl saturated with KCl or a pseudo reference silver wire of 0.5 mm radius. The platinum microdisk electrode, which served as substrate for mercury deposition, was prepared by sealing a 25 mm diameter platinum wire (Goodfellow Metals, Cambridge, UK) into glass. Before use, the disk surface was polished mechanically with aqueous suspensions of graded alumina powder of different sizes (1, 0.3 and 0.05 mm) supported on a polishing microcloth (Buehler, Lake Bluff, IL, USA). Sphere-cap12 mercury microelectrodes were prepared by ex situ deposition of mercury, under potentiostatic control, onto the platinum microdisk, as reported elsewhere.10,13,14 The height, h, of the sphere cap was calculated on the basis of the plating charge spent during the electrodeposition step. 10,13,14 The steady-state diffusion limiting current Id at the platinum disk or at the sphere-cap mercury microelectrodes can be predicted by:10-15 where k is a parameter that depends on the electrode geometry (for the disk k = 4,15 while for the sphere cap, it depends on the h/r ratio10,12-14), Cb is the bulk concentration of the electroactive species and r is the radius of the microdisk. This equation was employed for the determination either of the effective radius of the microdisk or the k values of the various sphere cap mercury microelectrodes. The steady-state limiting current was obtained from a 1 mmol L-1 Ru(NH3)Cl3 in Milli-Q water containing 0.1 mol L-1 NaClO4. In this way, each sphere cap microelectrode employed for the calibrationless quantification (see later) was well characterised. Calculated k values for the various mercury microelectrodes employed here varied over the range 6.18 - 6.45. Unless otherwise stated, the experiments were carried out at room temperature that varied between 15 and 36 °C (spring and summer periods).Reagents and samplesAll the chemicals employed were of analytical-reagent grade. Unless otherwise stated, the solutions were prepared with water purified with a Milli-Q purification system (Millipore, USA), and deaerated with nitrogen (99.99%) supplied by SIAD (Bergamo, Italy). Sulphide stock solutions, approximately 1x10-2 mol L-1, were prepared by dissolving large sodium sulphide crystals, which were previously washed with Milli-Q water and dried with acetone to remove any oxidised surface layer,16 in a 1 mol L-1 NaOH well deaerated solution. The stock solution was kept, under nitrogen atmosphere, in a tightly closed flask, and was standardised by potentiometric measurements with a sulphide ion-selective electrode, adopting the standard addition method. For the measurements in sand samples, typically, 10 g of sand were mixed with 3.5 mL of water solutions and the mixture was allowed to equilibrate for 24 h. The sampling of the sediment cores were performed in the Lagoon of Venice and taken with Plexiglass core tubes approximately 30 cm long and 6 cm wide. Stoppers were placed in the bottoms of the tubes and then taped immediately after the samples were taken, to minimise the sample exposure to air. Samples were then brought to the laboratory and the analysis performed on sections taken at different depth and put in the small Teflon cup of the voltammetric cell (see above). The measurements in soil samples were made with the mercury microelectrode immersed directly into them. Pore-water was extracted from the sediments by squeezing of core sections and the analyses were performed in the unfiltered samples. The sediment samples were squeezed with a Reeburgh-type17 squeezer in air atmosphere. The natural pH of the pore-water was, on average, equal to 8.5 ± 2.   Results and Discussion Detection of oxygen in the soil liquidsThe soil liquids (the water phase) are held by the pores of the soil system, in the vadose and the saturated zone.18 In the former, they are partly filled with water and partly with air. The oxygen content in the water phase is important parameter telling on the extent to which oxidation processes may occur at the water-solid interface.19,20 For instance, the major changes that occur in redox conditions between oxic waters and anoxic sediments can have profound influences on the speciation and bioavailability of many trace metals.21 In order to ascertain the performance of the mercury microelectrode for the direct determination of the oxygen concentration by voltammetry in the water phase of soils, a series of measurements was performed in sand samples that were equilibrated with water. Figure 1 shows typical voltammograms recorded with a mercury microelectrode, at 10 mV s-1, on a sand sample equilibrated with air-saturated (curve a) or purged with a stream of pure nitrogen (curve b) aqueous solutions, containing 0.5 mol L-1 NaCl. In the presence of oxygen, the voltammogram displays two well defined waves that follow the expected sigmoidal shape, typical for voltammetry with microelectrodes under steady-state conditions.7,8 These waves are due to the irreversible reduction of oxygen to hydrogen peroxide and water (first and second waves, respectively).22 The involvement of oxygen in the reduction process is confirmed by the lack of waves in the voltammogram recorded in the samples equilibrated with purged aqueous solutions (curve b). To investigate on the reproducibility of the current responses, the oxygen reduction voltammogram was cycled with the same mercury microelectrode immersed in the soil over a period of about 90 min. It was found that the steady-state limiting current of the first wave was constant within 3%, thus indicating good electrode stability under the experimental conditions employed here.   In real samples, the distribution of air and water in the vadose depends on the climate (temperature, rainfall and evaporation).18 To simulate these different conditions, voltammograms were recorded in sand samples equilibrated with water kept at different temperatures over the range 15 - 35 °C. Moreover, the current signal of the first plateau was monitored for an entire working day (about 8 h) at 31 °C, room temperature in a summer day. For the determination of the oxygen concentration in soil, the height of the first wave of curve a in Figure 1 along with equation (1) was employed. In the calculation, the following parameters were used: n=2, the diffusion coefficient of O2 at 25 °C, D = 2.12 x 10-5 cm2 s-1,23 was corrected for temperature effect by taking into account the Stokes-Einstein equation;24 k values were those evaluated for the specific mercury microelectrode, as described in the experimental section. Figure 2 shows the experimental data thus obtained. The oxygen concentration decreases with temperature, while it is almost constant with time although some water evaporation from the soil-water mixtures occurred. The trends observed clearly reflect the solubility of oxygen in water. In fact, the experimental data fall within the theoretical values of oxygen solubility from air saturated aqueous solutions,25 that are also included in Figure 2. These results indicate that the oxygen content in soils can be determined by a simple linear sweep voltammetry and exploiting the steady-state limiting current equation that holds for sphere-cap mercury microelectrodes.   Detection of sulphide ionsInorganic sulphur compounds have been the subject of great laboratory and field interest in the environmental literature.26-33 Characterisation and monitoring of such species are in fact critical for understanding of sulphur and metal cycling in suboxic sedimentary environments. For the detection of sulphide ions in aqueous solution, the following electrode process occurring at a mercury electrode is usually exploited:26,34 Figure 3 shows typical cyclic voltammograms recorded at 50 mV s-1 in a 0.02 mmol L-1 Na2S + 0.5 NaCl solution, adjusted to pH=12 by addition of a proper amount of a 1 mmol L-1 NaOH solution. Starting from -0.8 V, where no process occurred, the potential was reversed at -0.4 V. The small anodic process is due to the formation of mercury sulphide onto the electrode surface.34-38 The cathodic wave is due to the reduction of the HgS present onto the electrode surface, according to the inverse of reaction 2. Both position and height of this wave depended on the sulphide concentration in the solution as well as on the amount of HgS accumulated onto the electrode surface. In particular, it was found that the peak potential shifted towards less negative values by 30 mV for a tenfold decrease of the sulphide concentration, while the peak shifted towards a more negative potential as the amount of HgS accumulated on the mercury surface increased.  It must be noticed that cathodic peaks were in any

[学科分类] 化学（综合）