H a r v est ing En er g y f r om t h e Ma r in e Sedi men t -Wa t er I n t er f a ce C L A R E E . R E I M E R S * College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331 L E O N A R D M . T E N D E R * Center for Biomolecular Science and Engineering - Code 6900, Naval Research Laboratory, Washington, D.C. 20375 S T E P H A N I E F E R T I G Nova Research, Inc., 1900 Elkin Street, Alexandria, Virginia 22308 W E I W A N G Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey 08901 Pairs of pl at inummesh or graphi t e f iber-based el ectrodes, one embedded in marine sediment (anode), the other in proximal seawat er (cathode), have been used to harvest low- l evel power from natural , mi crobe est ablished, vol t age gradi entsat marine sediment-seawat er int erf aces in l aboratory aquari a. The sust ained power harvest ed thus f ar has been on the order of0.01 W/m 2 of el ectrode geometri c area but is dependent on el ectrode design, sediment composi t ion, and t emperature. It is proposed that the sediment/anode-seawat er/cathode conf igurat ion const i tut es a mi crobi al fuel cell in whi ch power resul ts from the net oxidat ion of sediment organi c matt er by dissolved seawat er oxygen. Considering typi cal sediment organi c carbon cont ents, typi cal f luxes of addi t ional reduced carbon by sediment at ion to sea f loors < 1000 m deep, and the proven vi abili tyof dissolved seawat er oxygen as an oxidant for power generat ion by seawat er batt eri es, i t is cal cul at ed that opt imized power suppli es based on the phenomenon demonstrat ed here could power oceanographi c instruments deployed for rout ine long-t ermmoni toring operat ions in the coast al ocean. Introduction Microbial decomposition of marine sediment organic matter consumes a succession of oxidants based on energy of reaction and regulatedby supply across the sediment-water interface (1). In organic-rich sediments it is common to observe oxygen reduction atthe sediment surface, nitrate, manganese, and iron reduction within the next few centi- meters, and sulfate reduction over another meter or so (2). As each oxidant issuccessively exhausted and its byproducts generated, distinct depth-dependent chemical profiles de- velop in sediment pore waters resulting in a voltage drop as large as 0.75 V within the top few centimeters of the sediment column (3, 4). Here we show thatthis potential gradient can sustain low-level power for prolonged periods if tappedby a simple fuel cell-like device consisting of an electrode shallowly embedded in marine sediment (anode), connected through an externalload to a second electrode in seawater (cathode). Unlike conventional fuel cells or seawater batteries (5-8) the describeddevice is intended tooperate in an environment (i.e., marine sediment/seawater interface) in which fuel (sediment organic detritus) and oxidant (dissolved seawater oxygen) are both natural and continually renewed resources. Experimental Section Marine sediment-seawater interfaces (i.e., ocean floors) were simulated in our laboratories in aquaria containing seawater and marine sediment (collected from either a salt marsh near Tuckerton, NJ, U.S.A. at approximately 39°30.5′N, 74°19.6′W or an estuarine site within Raritan Bay, NJ, 40°27.5′N, 74°04.4′W). After equilibration of the sediment at 22 °C, under circulating aerated seawater (salinity ) 34 psu), multiple pairs of coplanar, electrochemically cleaned (9) 20 or 100-cm 2 (geometric area) 52-mesh platinum electrodes (Alfa Aesar) were positioned aboutthe sediment-seawater interfaces (Figure 1) and held in place by Plexiglas, poly- carbonate or glass frame and rod rigs (not illustrated). Electrical contactto each electrode was made with a 20- gauge marine insulated wire (Ancor). Each wire terminus wassilver epoxied (Epotek) to the platinum, and the contact was encapsulated in insulating water-resistant epoxy (Torr- Seal). Carbon fiber-based anodes and cathodes of 100 cm 2 geometric area were also examined. These consisted of 27 g of loose-packed 8-micron diameter, 0.25-in. long, carbon fiber (Alfa-Aesar) sandwichedbetween fiberglass screens with Plexiglas frames. Electrical contactto each electrode was made by a 9-cm 2 platinummesh electrode wired as above and inserted between the fiberglass screens in physical contact with the carbon fiber. Resistances on the order of 5 ohms were measured across various positions on each electrode when dry suggesting that fiber-fiber contact provided conductivity throughouttheelectrode. For both electrode types initial placement of the anode (sediment embedded electrode) disrupted the interface-homogenizing local sediment and collapsing the voltage gradient. Monitor- ing the open circuit potential between theelectrodes (i.e., when theexternal circuit resistance was greater than 1 × 10 6 Ohm so that no appreciable current was observed) showed partial reestablishment of the voltage difference to ap- proximately 0.3 V to 0.4 V (anode negative voltage with respect to cathode) within minutes after anode placement, and complete reestablishment of the voltage gradientto ap- proximately 0.7 V on the order of 1-2 days. Once the voltage gradient stabilized, current flow through an external circuit was initiatedby connecting the two planar electrodes through a variable load causing a reduction in the voltage difference. Voltage and current sustainedby the two-electrode devices under variable loads were monitored for experimental periods ranging fromminutes to months using a data-logging, high- impedance multimeter (HP 34970A) or a high input imped- ance multimeter (Keithley 916) as an ammeter and poten- tiostat (PAR 263 A) as a voltmeter. Finally, the sediments wereelectrochemically characterizedby profiling (adjacent to the devices) with Pt-redox (Microelectrodes Inc.), volta- mmetric (10)(yielding O2 data), and pH (11) microelectrodes (the Rartian Bay sediment only) and sampled with 7.5 cm diameter core tubes. The cores were sectioned into vertical intervals of 0.25-0.5 cm under N2, and pore waters were extractedby centrifugation. Analytical methods applied to separated and then filtered pore water samples included * Corresponding authors phone: (541)867-0220; fax: (541)867- 0138; e-mail: clare.reimers@hmsc.orst.edu (C.R.) and phone: (202)- 404-6029; fax: (202)404-7946; e-mail: lmt@cbmse.nrl.navy.mil (L.T.). Env i ron. Sci . Te c hno l . 2001, 35, 192-195 192 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 1, 2001 10.1021/es001223s CCC: $20.00  2001 American Chemical Soci et y Publi shed on Web 11/16/2000