Formation mechanisms natural soils

FORMATION MECHANISMS OF HUMIC SUBSTANCES IN THE ENVIRONMENT 2.6.4. Natural Soils... 

Th + or AP" " induced a precipitate to form in all Bradyrhizobium and Sinorhi-zobium cultures tested, which suggested a defense mechanism based on metal precipitation by extracellular polymers (Santamaria et al., 2003). Among the metals tested, only Fe " ", Ap+, and Th were able to induce the formation of precipitate. AP+ is probably the natural soil component against which this defence mechanism could be directed, and a different defence mechanism based on extracellular aluminium precipitation within a gelatinous residue has been described for P. fluorescens (Appanna and St. Pierre, 1996). However, tliis polymer was composed mainly of phosphatidylethanolamine. While metal binding to extracellular polymers and bacterial surfaces have been proposed as the reason for increased metal resistance of biofilm-growing bacteria, this proposed defense mechanism involved the physical removal of the capsule after metal binding (Santamaria et al., 2003). 

Therefore, in natural soil diverse mechanisms of binding of TAT must exist. These range from ionic interaction of the TAT monomer with the polyanion structures of clay minerals and humic substances up to very complex subsequent 02-dependent reactions that give rise to the formation of polymers. This latter process in particular is highly irreversible and resembles the formation of humic substances from monocyclic aromatic precursors (24). 

Figure 4. Copper complexation by a pond fulvic acid at pH 8 as a function of the logarithm of [Cu2+]. On the x-axis, complex stability constants and kinetic formation rate constants are given by assuming that the Eigen-Wilkens mechanism is valid at all [M]b/[L]t. The shaded zone represents the range of concentrations that are most often found in natural waters. The + represent experimental data for the complexation of Cu by a soil-derived fulvic acid at various metakligand ratios. An average line, based on equations (26) and (30) is employed to fit the experimental data. Data are from Shuman et al. [2,184]...

Although the term acid rain has been used extensively in the popular literature to describe the formation and deposition of acids at the earth s surface, the terminology acid deposition is more commonly encountered in the scientific literature. The reason for this is that deposition of acids can occur either as dry deposition or as wet deposition. The former refers to the direct transport of acidic gases or small particles to the surface, followed by adsorption, without first being dissolved in an aqueous phase such as rain, clouds, or fog. Wet deposition, on the other hand, refers to the transport of acids to, and deposition on, surfaces (including soil, trees, grass, buildings, etc.) after the acids have been dissolved in an aqueous medium. It should be noted that the surface itself can be either wet or dry the terms wet and dry deposition refer to the mechanism of transport to the surface, not to the nature of the surface itself. 

In model experiments with catechol, Fe(III), and chloride, and in soil emission studies, it is found that chloroethyne (chloroacetylene) (58) is produced (382). The natural formation of 58 parallels that of vinyl chloride, which is also found in these experiments. The in vitro and in vivo mechanisms are unknown, but the authors propose the path shown in Scheme 3.1 (382). Both chloroethyne and vinyl chloride are emitted from three soil types (coastal salt marsh, peatland, and a deciduous forest). 

During the weathering process, elements can disperse from source mineralisation by a variety of chemical processes. For reasons discussed below, electrochemical processes are increasingly thought to be the primary transport mechanism in environments of thick, young, exotic (i.e., transported) overburden. They are also likely to operate in other environments but their dominance as a transport mechanism is less certain. This chapter presents the principles behind electrochemical masj transport and discusses the role of natural geoelectrochemical processes in the formation of selective leach and conventional geochemical soil anomalies. 

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