Redox cline

As illustrated in Fig. 9.13 the Fe(II), forming complexes with these hydroxy and carboxyl ligands, encounter in their upward diffusion the settling of Fe(III)(hydroxides and interact with these according to the catalytic mechanism, thereby dissolving rapidly the Fe(III)(hydr)oxides. The sequence of diffusional transport of Fe(II), oxidation to insoluble Fe(OH)3 and subsequent settling and reduction to dissolved Fe(II) typically occurs within a relatively narrow redox-cline. 

NO 3-Reducing. Fig. 9.15 shows data on groundwater below agricultural areas. The sharp decrease of 02 and NO3 at the redox cline indicate that the kinetics of the reduction processes are fast compared to the downward water transport rate. Postma et al., 1991 suggest that pyrite, present in small amounts is the main electron donor for NO3 reduction (note the increase of SOJ immediately below the oxic anoxic boundary). Since NO3 cannot kinetically interact sufficiently fast with pyrite a more involved mechanism must mediate the electron transfer. Based on the mechanism for pyrite oxidation discussed in Chapter 9.4 one could postulate a pyrite oxidation by Fe(III) that forms surface complexes with the disulfide of the pyrite (Fig. 9.1, formula VI) subsequent to the oxidation of the pyrite, the Fe(II) formed is oxidized direct or indirect (microbial mediation) by NO3. For the role of Fe(II)/Fe(III) as a redox buffer in groundwater see Grenthe et al. (1992). 

A large range of home-made equipment is used to sample at interfaces to the sediment and the atmosphere or across internal density discontinuity layers or redox (chemo-)clines e.g.. Bale and Barrett, 1995 Eversberg, 1990 Hallberg et al, 1977 Schwedhelm et al, 1988 ... 

---