Abiotic processes pathways

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). 

Environmental organic matter is a composite of humic and nonhumic substances, which is formed through operation and interactions of various biotic and abiotic processes. Humic substances are formed through both selected preservation (residue) and catalytic synthesis mechanisms. Both enzymatic and mineral catalyses contribute to the formation of humic substances in the environment. The relative importance of these catalytic reactions would depend on vegetation, microbial population and activity, enzymatic activity, mineralogical composition and surface chemistry of environmental particles, management practices, and environmental conditions. Selective preservation pathways would play a more important role in humification processes in poorly drained soils and lake sediments, compared with more aerated environmental conditions. 

Following release to the environment, synthetic chemicals may be degraded by biotic and abiotic processes. The degradation of the chemical can follow a plethora of pathways and a range of other substances can be formed via these different pathways (e.g. [1]). A munber of terms have been used for these substances including metabolites, degradates and transformation products -in this book we use the term transformation products. While we often know a lot about the environmental properties and effects of the parent synthetic chemical, we know much less about the transformation products. 

Basically there are two families of polymers synthetic and natural. Polymers formed by abiotic process by laboratory or industrial synthesis form the family of synthetic polymers and the polymers synthesized in biological systems by specific metabolic pathways form the family of natural polymers. 

A reduction of nitro substituent, under both aerobic and anaerobic conditions, seems to be a common enzymatic mechanism in the environment [5,29]. Such reduction has been demonstrated in various organisms which utilize the nitro compotind as an electron acceptor. The activity of nitroreductases, many of which have a broad substrate specificity, has been demonstrated in cell-free systems, and some enzymes have been purified and characterized [5,19,29]. The resulting aromatic amines are often further transformed into persistent azo compounds or polymers by biotic or abiotic processes [1,30,31]. In the second pathway, the nitro substituent is directly removed as nitrite [24,32,33], with the formation of catechol. 

Both biological and chemical (abiotic) pathways exist for the reduction of chromate and uranyl however, the reaction kinetics for the two broad classes differ appreciably. Reduction of chromate will probably occur through chemical means, albeit that the chemical reactant may result from microbial processes, in anaerobic environments ferrous Fe will dominate the reduction of chromate at mildly acidic to alkaline pH values while sulfide will dominate at lower pH values given equal availability. The microbial reduction of Fe (hydr)oxides promotes reduction of Cr(Vl) to Cr(Ill)— the result of a coupled, two-step, biotic-abiotic reaction pathway in which Fe(ll) produced during Fe respiration catalyzes the reduction of Cr(Vl). Thus, attenuation of chromate in saturated soil environments may be in large part attributable to dissimilatory Fe reduction. Enhancing Fe reduction may therefore promote the reductive stabilization of Cr. 

The third most common biodegradation pathway for chlorinated hydrocarbons is cometabolism via enzymatic reactions occurring fortuitously with oxidation of compounds such as methane or toluene. These three pathways are, of course, complicated by the fact that 1) each can only occur under specific chemical conditions 2) the dominant pathway changes as the source of electron donor and/or acceptor (both anthropogenic and indigenous) is reduced and 3) There are several different isomers, and other compounds of chlorinated hydrocarbons, that can enter these pathways from biotic or abiotic processes. In other words, it is a complicated process. 

As mentioned previously, treatment of chlorinated solvents in groundwater can occur via biotic and abiotic reductive processes. In the past, it was assumed that biotic and abiotic reductive pathways are independent. As a result, the research in these two areas was... 

An example of a process based on the causative relationship is one referenced earlier in this chapter, named Biogeochemical Reductive Dechlorination (BiRD). This method can be viewed as an alternative method to MRD for destructing CAHs (Brown et al., 2009 Kennedy et al., 2006). The main concept in BiRD is based on this fact that carbon is used by bacteria for growth and for producing energy via different reductive metabolic pathways that can affect the abiotic processes. Reduced minerals, in this case, mostly iron sulfides (FeS and FeS2) are produced through bacterial respiration these minerals aid the abiotic reductive dechlorination process (Kennedy et al., 2006 Lee and Batchelor, 2002). BiRd occurs via three steps ... 

Degradation of a herbicide by abiotic means may be divided into chemical and photochemical pathways. Herbicides are subject to a wide array of chemical hydrolysis reactions with sorption often playing a key role in the process. Chloro-j -triazines are readily degraded by hydrolysis (256). The degradation of many other herbicide classes has been reviewed (257,258). 

The most important removal pathways of PhACs during wastewater treatment are biotransformation/biodegradation and abiotic removal by adsorption to the sludge. The efficiency of their removal at WWTP depends on their physico-chemical properties, especially hydrophobicity and biodegradability, and process operating parameters (i.e., HRT, SRT, and temperature). For certain NSAIDs (e.g., ibuprofen, acetaminophen), high removals (>90%) are consistently reported in literature... 

Photolysis Abiotic oxidation occurring in surface water is often light mediated. Both direct oxidative photolysis and indirect light-induced oxidation via a photolytic mechanism may introduce reactive species able to enhance the redox process in the system. These species include singlet molecular O, hydroxyl-free radicals, super oxide radical anions, and hydrogen peroxide. In addition to the photolytic pathway, induced oxidation may include direct oxidation by ozone (Spencer et al. 1980) autooxidation enhanced by metals (Stone and Morgan 1987) and peroxides (Mill et al. 1980). 

Parathion (0,0-diethyl 0-/7-nitrophenyl phosphorothioate) is degraded in the near subsurfaee aerobie environment via hydrolysis, where two degradation products are observed, diethylthiophosphoric acid and p-nitrophenol, according to the schematic pathway described in Fig. 16.35. Abiotic hydrolysis of parathion in the subsurface is a result of a surface-mediated transformation (see Sect. 16.1) or a biodegradation process. 

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