In Situ Voltammetry at Deep-Sea Hydrothermal Vents - ACS Publications

Chapter 3

Downloaded by NANYANG TECHNOLOGICAL UNIV on June 3, 2016 | http://pubs.acs.org Publication Date: February 14, 2002 | doi: 10.1021/bk-2002-0811.ch003

In Situ Voltammetry at Deep-Sea Hydrothermal Vents 1,*

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DonaldB.Nuzzio ,Martial Taillefert , S. Craig Cary , Anna Louise Reysenbach , and GeorgeW.Luther, III 3

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Analytical Instrument Systems, Inc.,P.O.Box 458, Flemington,NJ08822 College of Marine Studies, University of Delaware, 700 Pilottown, Road, Lewes,DE19958 Department of Biology, Portland State university, Portland,OR97201 2

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There is a need to build instrumentation and sensors that can measure in situ chemical changes in dynamic environments. Hydrothermal vents are arguably the most dynamic aqueous systems on earth. The orifice of a vent approaches 360 °C and spews vast quantities of dissolved hydrogen sulfide and iron into ambient seawater at 2 °C. These chemical species fuel incredible deep-sea (micro)biological communities, which may be a model for life on other planets. Here we describe an in situ submersible analyzer and electrodes for the measurement of aqueous chemical species found near hydrothermal vents. A standard three-electrode arrangement is controlled by a voltammetric analyzer that is deployed from the deep-sea submersible, Alvin. Real time measurements for a variety of redox species under flow conditions were made with a 100 μm Au/Hg solid-state working electrode at a depth of 2500 m. The solid-state working electrode was used to detect dissolved O , S(-II), Fe(II) and FeS molecular clusters. Our in situ data show that significant changes can occur in chemical speciation 2

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© 2002 American Chemical Society Taillefert and Rozan; Environmental Electrochemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

41 and analyte concentration when waters are sampled and then measured aboard ship.


Introduction Voltammetric (micro)electrode techniques have been used in a variety of geochemistry and marine chemistry applications. Recently, in situ voltammetric measurements have received increasing attention (1). Both direct (2-4) and on line (5-/7, flow cell) type arrangements have been used for trace metal and major chemical species determinations. However, voltammetric instrumentation has only been deployed in shallow waters to date. There is a need to make measurements in the deep-sea, and this is particularly true of deep-sea hydrothermal vents that have an incredible microbial and macrofaunal community fueling itself via chemosynthesis (12,13). Changes in temperature can be dramatic in this environment and electrodes must be able to respond precisely in waters of lower pH, high pressure, high temperature and high water flow rates. When high temperature waters mix with low temperature waters, their chemistries can differ dramatically . Because organisms live in these dynamic and extreme conditions, it is critical to understand how that chemistry drives or influences biology. In this paper we describe in situ electrochemical instrumentation and the initial deployment of it at 9 °N East Pacific Rise (EPR; at a 2500 meter water depth; 250 atm of pressure) and at Guaymas Basin, Gulf of California, 2000 meter water depth (200 atm of pressure). A companion paper in this volume (11) demonstrates that current-concentration curves are affected by water flow rates but not by pressure. At high flow rates and reasonable scan rates (~ 1 V s* ), the current-concentration curves become independent of water flow rate for the target redox species. Deep-sea hydrothermal vents can have high concentrations of iron and sulfide. In this paper we demonstrate the use of voltammetry to measure Fe and S species. Using a solid-state gold amalgam (Au/Hg) working electrode, we show the simultaneous detection and quantification of several sulfur and iron dissolved species. In previous work, dissolved chemical species (0 , H 0 , S 0 ", S *, HS", Γ, Fe(II), Mn(II), organically complexed Fe(III), and FeS clusters) have been simultaneously measured in situ in sediments and natural waters (6,14,15). 1

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Taillefert and Rozan; Environmental Electrochemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Experimental Methods


Chemicals and solutions Chemicals used in laboratory experiments were analytical gradefromFisher Scientific Co. Milli-Q quality (Millipore) was used for all reagents. Laboratory measurements were carried out in a 0.55 M NaCl solution or in seawater. Mn(II), Fe(II) and S(-II) standards were prepared from MnCl · 4 H 0 , ferrous ammonium sulfate, and Na S«9H 0. The mercury plating solution was prepared as 0.1 Ν Hg(N0 ) in 0.05N HN0 . 2

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Electrodes Gold amalgam PEEK™ electrodes were made as described by Luther et al (6) by fixing 100 μπι-diameter Au wire soldered to the conductor wire of a BNC cable within a body of 0.125"-diameter PEEK™ tubing, which is commercially available as standard HPLC high-pressure tubing. The metal is fixed within the tubing with West System 105 epoxy resin and 206 hardener. A portion of the black outer coat and braid of the BNC wire are removed to expose the teflon shield and Cu conductor wire so that the Au wire soldered onto the Cu conductor can be inserted into the PEEK™. The epoxy is injected into the PEEK™ tubing which contains the gold wire that was previously soldered to the conductor wire of the BNC cable. Then the teflon is inserted into the PEEK™ tubing until the black coating of the BNC wire fits against the PEEK™ tubing, and the assembly is held with epoxy, which has a moderate setting time (~1 hr), and does not drain out the lower open side. On setting, the epoxy seals the tip and the lower end can be refilled with epoxy if necessary. Then the top end is coated with Scotchkote (3M) electrical coating and Scotchfil (3M) electrical insulation putty. PEEK™ and high-purity epoxy fill permit the determination of metal concentrations without risk of contamination, and at temperatures as high as 150 °C. Pt counter and solid Ag/AgCl reference electrodes were made similarly but 500 μπι diameter wire was used for each. These PEEK™ electrodes could be used as is or mated with standard HPLC fittings from Upchurch, Inc for insertion into a flow cell (11). Once constructed the working electrode (Au) surface was sanded, polished and plated with Hg by reducing Hg(II) from a 0.1 Ν Hg / 0.05 Ν H N 0 solution, for 4 minutes at a potential of -0.1 V, while purging with N . The mercury/gold amalgam interface was conditioned using a 90-second -9 V polarization procedure in a 1 Ν NaOH solution (16). The electrode was then run in linear 3

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43 sweep mode from -0.05 to -1.8 V versus a Saturated Calomel Electrode (SCE) or Ag/AgCl electrode several times in oxygenated seawater to obtain a reproducible 0 signal. For DSVAlvin work, four Au/Hg electrodes can be controlled by the analyzer. The reference electrode was Ag/AgCl and the counter electrode was Pt wire, both of which were mounted on the basket of DSV Alvin so that they would not enter sulfidic waters (15). For hydrothermal vent work, the Ag/AgCl reference was silver wire, which was oxidized in seawater at +9 V for 10 sec to form a AgCl coating. This electrode was used as a solid-state electrode in the seawater medium (1=0.7) so that no pressure effects on filling solutions would hinder electrode performance. Peak potentials of the analytes measured in situ and aboard ship were the same and similar to those for a saturated calomel electrode (SCE). All laboratory and shipboard analyses were carried out using an Analytical Instrument Systems (AIS) DLK-100A potentiostat controlled by a microcomputer using software provided by the manufacturer. A DLK-SUB-1 was used for all in situ work (see below).


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Field Experiments For hydrothermal vent work, two working electrodes as well as a thermocouple sensor and tubing that lead into a flow cell inlet and a discrete syringe sampler system were placed in a sensor or wand package (15) that can be held by a manipulator (arm) of Alvin. The wand was held over the vent orifice and areas along the length of vent chimneys. These latter areas are termed diffuse flow because water temperatures can range from 8 to 125 °C and do not emanate from the vent orifice. A flow cell (11,15) was fixed in the submersible's basket that was bathed by waters at 2 °C. A submersible electrochemical analyzer (DLK-SUB I) from Analytical Instrument Systems, Inc. was used for data collection (see below). The electrochemical package was deployed during cruises to 9 °N East Pacific Rise (May, 1999) and Guaymas Basin, Gulf of California (January 2000). Separate discrete samples were taken with a gas tight syringe sampler (11,16)fromthe same waters for comparison with the flow cell measurements. The discrete samples were measured aboard ship by voltammetry (11). We typically made three to five replicate measurements per sample with the flow cell system. Electrodes were calibrated at different temperature (17) as well as for flow (11).


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Voltammetry The three-electrode configuration (working, reference and counter) was used to determine the concentration of the species present in natural waters. Linear sweep voltammetry (LSV), cyclic voltammetry (CV) and square wave voltammetry (SQW) were used for analyses. The following conditions were generally applied during the LSV and CV scans: scan rate - 200, 500 or 1000 mV s" , scan range = -0.1 to -1.75 V, equilibration time = 5 s. Square wave voltammograms were conducted under the same conditions with a pulse height of 24 mV, 1 mV scan increment and 50 mV s" scan rate. To prevent memory effects, caused by the accumulation of sulfide and metal species on the mercury surface, conditioning steps were applied to the working electrode as per Brendel and Luther (17). To reoxidize metals (Mn, Fe) that are reduced at the amalgam, a potential of -0,1 V was applied over 10-30 seconds before each scan. When sulfide was present, conditioning at -0.9 V for 10 s was employed since the metals and sulfide are not electroactive at that potential (14, 17). All LSV and CV scans shown in the figures below are actual data; i.e., no software smoothing routines are used to enhance the quality. 1


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Results & Discussion

Instrumentation A schematic of the analyzer, electrodes and sensor package system is provided in Figure 1. The DLK-SUB-1 is an electrochemical analyzer built by Analytical Instrument Systems, Inc. The instrument is designed to perform all of the standard voltammetric analyses that would be available in a shipboard or land-based laboratory. The following standard voltammetric techniques are possible using the DLK-SUB-1: linear sweep, cyclic, normal pulse, differential pulse, square wave. Chronoamperometry as well as stripping techniques for monitoring trace levels of analytes can also be performed. This versatility provides great flexibility to the researcher for in situ experiments. The instrument utilizes the 24 V DC power availablefromDSV Alvin for its power source. A l l waterproof connections were made with connectors from Impulse, Inc. and the aluminum housing (1 meter length; 20 cm outside diameter) of the instrument was rated for operation at full ocean depth (-6000 m; 600 atm). The instrument is a complete stand-alone package capable of being deployed for long periods of time from Alvin, or remotely operated vehicles (ROV) with tethers up to 1500 meters without the need for signal amplification.


45 Electrode Wand

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Working Electrodes (Au/Hg) Counter Electrode (Pt)

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Hull of Alvin


Reference Electrode (Ag / AgCl)

Laptop Computer In Alvin

AIS, DLK-SUB-1 Electrochemical Analyzer

Figure 1. Schematic of the analyzer, electrodes and cable communication through the hull ofAlvin. The voltammetry hardware is linked to an IBM compatible computer inside the housing. The internal computer communicates with another computer through the hull of Alvin via a 15-m RS 232 cable and is controlled by an operator, who can reprogram waveforms to respond to the radically different environments found at vents. A separate 1-meter cable is used to make connections with the working, counter, and reference electrodes and the pressure housing. This cable has four inputs for working electrodes (that can be selected one at a time via a multiplexer), one input for the counter electrode and another for the reference electrode. Another input for grounding the reference electrode from the submersible insured signal integrity. The electrode wand is constructed of Delrin and has a stainless steel handle so that the manipulator (arm) of Alvin can hold and deploy the electrodes without breaking them.

Hydrothermal vent measurements The major aqueous species, which are found near hydrothermal vents, react at the Au/Hg electrode according to the following electrode reactions (17,18). +

0 + 2 H + 2e"-> H 0 H 0 + 2 H + 2e" -» H 0 HS" (H S) + Hg -» HgS + H + 2 e" HgS + H* + 2 e" ο HS" + Hg FeS + 2e" + H -> Fe(Hg) + HS" Fe + Hg + 2e Fe(Hg) 2

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-0.30 V (la) -1.30 V (lb) ads. onto Hg

In Situ Voltammetry at Deep-Sea Hydrothermal Vents - ACS Publications

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