Sediment redox potential, Eh.
Source: ICRAM
Organic enrichment of sediments usually leads to reduced conditions which equate to “bad” sediment quality, wherein natural benthic communities undergo substantial changes. The oxidation-reduction (redox) conditions in surface sediments depend on the degree of organic enrichment and can be assessed by measuring the vertical redox potential profile (expressed in mV) in the top 15 cm (Zobell, 1946). The redox state of sediment is the result of the combined effect of biological and chemical processes of reversible and/or irreversible nature and is therefore difficult to define. It has been pointed out that the concentrations of the components reacting reversibly are, in most cases, too small to get reliable results from redox measurements. Thus, what is really measured is a mixed potential which is not useful for chemical equilibrium calculations (Bågander, 1978). However, Eh profiles still provide useful information, since the decrease in Eh with the depth is related to the decrease in the dissolved oxygen concentration in the pore water. Negative Eh values are therefore associated with anoxic conditions, in which the degradation of the organic matter is performed by anaerobic bacteria. In marine sediments, these bacteria mainly use sulphate as the electron acceptor, which is reduced to hydrogen sulphide (Porrello et al., 2005; Chamberlain, 2002; Danovaro et al., 2004).
Redox potential is measured by profiling an electrode down a sediment core to as deep as is necessary to detect the redox discontinuity layer (normally 10 or 15cm), i.e. the point at which redox values change abruptly from highly negative values to either less negative, or to positive values. Redox potential will be measured in replicate undisturbed cores at each station.
Required:
Surface sediment in cores, at least 0-20 cm depth
Ruler (to measure sediment height)
Portable pH/Eh meter
Redox electrode, with shaft >15cm long, preferably as thin as possible, with Platinum ring indicator
Double junction silver/silver chloride reference electrode
Palmer stand or equivalent to ensure controlled profiling
Redox electrode calibration:
The ZoBell solution is used as a reference; this solution (0.003 M potassium ferricyanide, 0.003 M potassium ferrocyanide, and 0.1 M potassium chloride) has an Eh value of +430 mV at 25°C.
Measurement:
The redox potential should be measured by carefully pressing the redox electrode into the intact sediment core (in the tube) as soon as possible after collection (the longer the sediment sits in the collection tube, the more it changes due to biogeochemical processes that directly affect redox values), at depth intervals of 1cm from the surface to 15cm depth. Measuring redox by inserting electrodes into holes in the side of the cores leads to artifacts (see Hionchey and Schaffner 2005) and is therefore not recommended. Moreover, slicing the sediments to measure redox in these is not recommended as exposure to the air will rapidly and drastically alter the redox potential values. It is essential that the measurements be done using either a combination redox electrode or a redox electrode that is connected to a reference electrode that is in contact with the sediment porewater. The measurements will be recorded in mV when the meter readings stabilize at each depth.
References
Bågander, L.E. 1978. An evaluation of the use of redox measurements for characterizing recent sediments. Estuarine and coastal Marine Science, 6: 127-134.
Chamberlain, J., 2002. Modelling the environmental Impacts of Suspended Mussel (Mytilus edulis L.) Farming. Ph-D Thesis, Napier Univeristy, Edimburgh.
Danovaro, R., Gambi, C., Luna, G.M., Mirto, S., 2004. Sustainable impact of mussel farming in the Adriatic Sea (Mediterranean Sea): evidence from biochemical, microbial and meiofaunal indicators. Marine Pollution Bulletin, 49: 325-333.
Hinchey, E.K. and L. C. Schaffner. 2005. An evaluation of electrode insertion techniques for measurement of sediment redox potential in estuarine sediments. Chemosphere 59:703-710.
Porrello, S., Tomassetti, P., Manzueto, L., Finoia, M.G., Persia, E., Mercatali, I., Stipa, P., in press. The influence of marine cages on the sediment chemistry in the Western Mediterranean Sea. Aquac.
Zobell, C. E., 1946. Studies on redox potential of marine sediments. Bulletin of the American Association of Petroleum Geologists 30, 477-511.
