Sediment and water column applications of in situ voltammetry to further our understanding of biogeochemical processes in oceanography George W. Luther, III Maxwell P. & Mildred H. Harrington Professor of Marine Studies College of Marine Studies, University of Delaware, Lewes, DE Acknowledgments: Colleagues: Donald Nuzzio (AIS, Inc.); Martial Taillefert; Brian Glazer Funding: NSF (OTIC, Chem OCN, LeXen); NOAA (Sea Grant, NURP)

Single analyte devices - (micro)electrodes Potentiometric (V ~ [conc]) Membrane pH, pCO 2 AND solid state pS 2- (Ag/AgS) ISE’s for NH 3, NO 3 -, Ca, CO OK for freshwater but seawater salts interfere with NH 3, NO 3 - electrodes. Amperometric (I at constant V~ [conc]) – Clark O 2 sensor Includes Enzyme/bacterial sensors NO 3 - via bacterial reduction to N 2 O H 2 O 2 via enzyme H 2 S - based on reaction with Fe(CN) 6 3-

Voltammetric- Multiple analyte systems based on current (I) vs potential (V) curves at a single working electrode; similar to absorbance vs Wavelength or energy plots in spectroscopy. Detect Multiple Analytes AND Chemical Speciation At gold/amalgam electrodes O 2, H 2 O 2 Fe 2+, FeS aq, Fe(III) –organic complexes Mn 2+, I -, IO 3 - H 2 S/HS - as total S(-II), S x 2-, S 2 O 3 2- Trace metals (Cu, Cd, Pb, Zn) including (in)organic complexes All are measurable, if present, in one I vs V scan

Solid state (micro)electrodes for the analysis of biologically relevant compounds and ions Rationale for design and use Fine scale resolution - mm in sediments;  m in biofilms and mats determine sediment heterogeneity vs. homogeneity prospect for life forms; understand ecosystem health / ecology Uses sedimentary porewaters of bays, oceans and lakes water column; e.g., Chesapeake Bay, Black Sea Hydrothermal Vents

PEEK & Glass encased electrodes in marine epoxy 100  m diameter Au wire epoxy Sediment working electrode Water column / vent working electrode Tested to 2500 m and 120 o C

In situ comparison of O 2 Clark vs voltammetric Au/Hg in sediments from a ROV Excellent agreement No O 2, Mn Real time voltammetry of porewaters

VOLTS vs. Ag/AgCl Current (nA) depth (mm) Mn Fe(III) I-I- In Situ Voltammetric Porewater Data from Raritan Bay - 10 m water depth

The Black Sea – world’s largest body of water containing H 2 S (> 90% of the water mass)

AIS/UD Electrochemical analyzer (O 2, H 2 S) MBARI Pump profiler MBARI CTD Mount for UD electrodes IN SITU instrumentation – coupling of sensors

AIS Electrochemical analyzerMicrocat CTDSolid state electrodes (O 2, H 2 S) IN SITU instrumentation – coupling of sensors

170 m 184 m H 2 S 160 m O 2 2 m 69 m 114 m 106 m 145 m IN SITU voltammograms for O 2 and H 2 S over depth at station 9

Profiles vs depth Increase T, inc. O 2 or decrease H 2 S Mn catalytic cycle Mn 2+ + ½ O 2 + H 2 O -> MnO H + MnO 2 + HS H + -> Mn 2+ + S o + 2 H 2 O “fingering” a result of physical forcing

Tourquay Canal DE – July 10, 2000 benthic processes at their worst? Close-up End of canal meter normal depth with holes of 5 meters deep

Hydrothermal Vents at 9 o 50’ N East Pacific Rise DSV Alvin Over the stern

Fe, S chemistry Origin of Life, of organic compounds and a source of H 2 at HYDROTHERMAL VENTS FeS + H 2 S FeS 2 + H 2 (Wachterhauser’s hypothesis) Voltammetry can measure FeS aq (molecular clusters) and H 2 S Apply in situ solid state electrodes to look for (micro)organisms that can benefit from this reaction or the products of this reaction Apply in situ solid state electrodes to understand the chemical reason why organisms live in different ecological niches

General Block Diagram of IN SITU submersible Electrochemical Instrument Working Electrodes (Au/Hg) AIS, DLK-SUB-1 Electrochemical Analyzer Laptop Computer In Alvin Electrode Wand Reference Electrode (Ag / AgCl) Counter Electrode (Pt) Flow cell Alvin hull

Black Smoker Voltammetry Speciation Data- 0.5 m above vent chimney Major signals for Free H 2 S and FeS aq O 2 not detected 360 o C 25 o C 0.5 m Volts vs Ag / AgCl Current (A) Unseen part of the wand Noise from Alvin

Sulfur chemistry 0.5 m above a Black Smoker 360 o C 25 o C 0.5 m Electrical noise from Alvin

Near Plume of Riftia H 2 S/HS - and O 2 only No FeS aq polysulfides can be present chemoautotrophs require H 2 S Current (A) Volts vs Ag / AgCl

Pompeii Worm Habitat Characterization 81 ° C 20 ° C Major signal due to FeS aq + Fe 2+ Free H 2 S/HS - was not detected O 2 not detected Epibionts not chemoautotrophic Volts vs Ag / AgCl Current (A) Electrode helps prospect for life forms

In Alvinella tube, 80  20 o C atm In flow cell; 2 o C 250 atm Aboard ship lab 22 o C 1 atm H 2 S + Fe 2+  FeS aq + 2H + FeS aq + 2H +  H 2 S + Fe 2+ ANOTHER REASON FOR IN SITU SENSORS Change in chemical speciation at hydrothermal vents

Other working electrodes can be used for other chemical / biochemical species Delivery Systems – unattended with (micro)manipulators Landers – sediment work ROV’s - sediment and water column work AUV’S - water column work Moorings – water column work Manned submersibles;e.g., Alvin – vent work NEPTUNE – Tectonic plate observatory Some Future Work / Needs