Abiotic process

FIGURE 17.34 Vertical distribution of dissolved reactive phosphorus in soil pore water and water column at impacted and unimpacted sites in the WCA-2A of the Everglades. (Reddy, K. R., Unpublished Results, University of Florida.) 

Detailed information on phosphorus cycling can be found in the following papers Davis 

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Isotope effects also play an important role in the distribution of sulfur isotopes. The common state of sulfur in the oceans is sulfate and the most prevalent sulfur isotopes are (95.0%) and (4.2%). Sulfur is involved in a wide range of biologically driven and abiotic processes that include at least three oxidation states, S(VI), S(0), and S(—II). Although sulfur isotope distributions are complex, it is possible to learn something of the processes that form sulfur compounds and the environment in which the compounds are formed by examining the isotopic ratios in sulfur compounds. 

The global nitrogen cycle is often referred to as the nitrogen cycles, since we can view the overall process as the result of the interactions of various biological and abiotic processes. Each of these processes, to a first approximation, can be considered as a self-contained cycle. We have already considered the biological cycle from this perspective (Fig. 12-1), and now we will look at the other processes, the ammonia cycle, the cycle, and the fixation/denitrification cycle. 

The principal abiotic processes affecting americium in water is the precipitation and complex formation. In natural waters, americium solubility is limited by the formation of hydroxyl-carbonate (AmOHC03) precipitates. Solubility is unaffected by redox condition. Increased solubility at higher temperatures may be relevant in the environment of radionuclide repositories. In environmental waters, americium occurs in the +3 oxidation state oxidation-reduction reactions are not significant (Toran 1994). 

Table 20.5 also indicates whether a process is biotic (mediated or initiated by organisms in the environment), abiotic (not involving biological mediation), or both. Biotic processes are limited to environmental conditions that favor growth of mediating organisms. Abiotic processes occur under a wide range of conditions. Adsorption, precipitation, complexation, and neutralization are abiotic all other processes in Table 20.5 may be either. 

Full understanding and control of transition-metal species in biotic and abiotic processes is still in its relative infancy. In the ongoing twenty-first-century exploration of this fascinating domain, ab initio theory can be expected to play an increasingly important role. 

A half-life of about 40 days was reported for hexachloroethane in an unconfined sand aquifer (Criddle et al. 1986). Laboratory studies with wastewater microflora cultures and aquifer material provided evidence for microbial reduction of hexachloroethane to tetrachloroethylene under aerobic conditions in this aquifer system (Criddle et al. 1986). In anaerobic groundwater, hexachloroethane reduction to pentachloroethane and tetrachloroethylene was found to occur only when the water was not poisoned with mercury chloride (Roberts et al. 1994). Pentachloroethane reduction to tetrachloroethylene occurred at a similar rate in both poisoned and unpoisoned water. From these results, Roberts et al. (1994) suggested that the reduction of hexachloroethane to tetrachloroethylene occurred via pentachloroethane. The first step, the production of pentachloroethane, was microbially mediated, while the production of tetrachloroethylene from pentachloroethane was an abiotic process. 

Similarly to N, most S pools are found in organic form in forest floor and soil humus. However, unlike nitrogen, there are important abiotic processes, especially sulfate sorption processes, which play a critical role in regulating sulfate dynamics in forest ecosystems. An example of this type of exposure pathway was shown in the Habbard Brook whole-tree harvesting experiment, where the decrease in sulfate output from the watershed was attributed to sulfate adsorption, which was enhanced by soil acidification from nitrification (see above). 

Sorption/desorption is the key property for estimating the mobility of organic pollutants in solid phases. There is a real need to predict such mobility at different aqueous-solid phase interfaces. Solid phase sorption influences the extent of pollutant volatilization from the solid phase surface, its lateral or vertical transport, and biotic or abiotic processes (e.g., biodegradation, bioavailability, hydrolysis, and photolysis). For instance, transport through a soil phase includes several processes such as bulk flow, dispersive flow, diffusion through macropores, and molecular diffusion. The transport rate of an organic pollutant depends mainly on the partitioning between the vapor, liquid, and solid phase of an aqueous-solid phase system. 

Hydrocarbon-degrading microorganisms are ubiquitous in most ecosystems [32,117] however, it is often very difficult to prove that transformation, degradation, and mineralization actually occur in the environment because it is difficult to distinguish contributions from biotic and abiotic processes under uncontrolled conditions in the natural environment [338]. In contrast, laboratory assays can provide definitive evidence for microbial degradation, and sterilized samples can be used to determine abiotic losses. Thus, contributions from microbial degradation can be differentiated from abiotic loss by a mass balance... 

Because weathering and other abiotic processes simultaneously occur and contribute to changes in the concentrations of PAHs in the field, laboratory microbial degradation and the determination of a target transformation metabolite appear to be useful to evaluate the possibility of microbial transformation in any contaminated environment. Such case studies follow ... 

A small fraction of POM is created via abiotic processes, all of which involve transformation of DOM into the particulate phase. As already noted, destabilization of colloidal DOM can lead to the formation of gels. Increasing salinity destabilizes colloids, so flocculation of DOM is common in estuaries. 

Ngim, K.K. and Crosby. D.G. Abiotic processes in influencing fipronil and desthiofipronil dissipation in California, USA. rice fields. Environ. Toxicol. Chem., 20(5) 972-977, 2001. 

Subsurface transformations of contaminants comprising multiple components are reflected in the composition of the residual contamination products, which may have different physical and chemical properties than the original pollutant. Differential partitioning-dissolution and volatilization of component mixtures are the main abiotic processes leading to alteration of the original pollutant. 

For example, biotransformation of naphthalene in an operating actiyated sludge treatment system (after correction for abiotic processes) was modelled a priori by an elementary first-order (in naphthalene concentration) rate equation (24). The complex actiyated sludge system was perturbed by induction of sinusoidal naphthalene feed concentrations for eight sinusoidal frequencies while the naphthalene in the reactor offgas was measured eyery ten minutes. Abiotic fates (stripping, and sorption) were accounted for and... 

The principal abiotic processes that may transform thorium compounds in water are complexation by anions/organic ligands and hydroxylation. The increase in the mobility of thorium through the formation of soluble complexes with CQs, humic materials, and other anions or ligands and the decrease in the mobility due to formation of Th(OH)4 or anionic thorium-hydroxide complexes were discussed in Section 5.3.1.2. In a model experiment with seawater at pH 8.2 and freshwater at pH 6 and pH 9, it was estimated that almost 100% of the thorium resides as hydroxo complexes (Boniforti 1987). 

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