Quinones humic acid

Humic acid and the corresponding fulvic acid are complex polymers whose structures are incompletely resolved. It is accepted that the structure of humic acid contains oxygenated structures, including quinones that can function as electron acceptors, while reduced humic acid may carry out reductions. These have been observed both in the presence of bacteria that provide the electron mediator and in the absence of bacteria in abiotic reactions, for example, reductive dehalogenation of hexachloroethane and tetrachloromethane by anthrahydroquininone-2,6-disulfonate (Curtis and Reinhard 1994). Reductions using sulfide as electron donor have been noted in Chapter 1. Some experimental aspects are worth noting ... 

The N-labeled amines produced by partial and total reduction of the nitro groups in 2,4,6-trinitrotoluene reacted with carbonyl groups (quinones and ketones) in humic acid to produce a range of products (Thom and Kennedy 2002). 

This organism is able to oxidize acetate to CO2 under anaerobic conditions in the presence of Fe(III). A study of the intermediate role of humic and fulvic acids used ESR to detect and quantify free radicals produced from oxidized humic acids by cells of G. metallireducens in the presence of acetate. There was a substantial increase in the radical concentration after incubation with the cells, and it was plausibly suggested that these were semiquinones produced from quinone entities in the humic and fulvic structures (Scott et al. 1998). 

Quinone is very sensitive to alkalis, even to carbonate and ammonia, and is converted by them into a brown acid possibly identical with the natural humic acid of brown coal (lignite) (Eller). The mechanism of this reaction remains obscure. 

Anaerobic conditions often develop in hydrocarbon-contaminated subsurface sites due to rapid aerobic biodegradation rates and limited supply of oxygen. In the absence of O, oxidized forms or natural organic materials, such as humic substances, are used by microorganisms as electron acceptors. Because many sites polluted by petroleum hydrocarbons are depleted of oxygen, alternative degradation pathways under anaerobic conditions tend to develop. Cervantes et al. (2001) tested the possibility of microbially mediated mineralization of toluene by quinones and humus as terminal electron acceptors. Anaerobic microbial oxidation of toluene to CO, coupled to humus respiration, was demonstrated by use of enriched anaerobic sediments (e.g., from the Amsterdam petroleum harbor). Natural humic acids and... 

To vahdate mineralization of toluene to CO under anoxic quinone and humus-respiring conditions, Cervantes et al. (2001) performed additional experiments using emiched phosphate-buffered basal sediments from Amsterdam petroleum harbor. After two weeks of incubation, 85% of added C-labeled toluene was observed as CO. Emiched sediment converted C-labeled toluene to in media supplemented with AQDS or with humic acid (Fig. 16.34A). There was negligible recovery of in the endogenous and sterile controls. The conversion of C-labeled toluene to was coupled to an increase in electrons recovered as AH QDS or as reduced humus (Fig. 16.34B). However, there was no toluene reduction in autoclaved sediments. These results indicate that humic substances... 

Stepwise chemical reduction of humic acid caused a variation in spin content as shown in Figure 7. The initial rise in radical content is attributed to anion radical formation caused by sodium the following decrease in spin content with further addition of sodium is probably caused by the reduction of these anion radicals. The subsequent increase in radical content could be caused by the one-step reduction of the remaining quinone moieties. 

In a recent study S. E. Moschopedis (II) produced some direct evidence for the presence of quinones and hydroxyquinones in coal and humic acid, and infrared examination of products obtained in the course of the present investigation clearly indicated the existence of five-membered cyclic anhydrides in the reactant mixtures. 

Other reactions leading to NH4 removal from soil solution besides microbial nitrogen assimilation, metal-ammine formation, or adsorption onto mineral surfaces, involve NH3 fixation by incorporating it as NH2 in aromatic rings of humic acids (quinone) followed with aromatic ring condensation. 

A correlation between reaction rates, molecular stmcture of the humic or fulvic acid, and content of reactive sites is more difficult to demonstrate. It has been hypothesized that the hydroquinone or quinone is the main reactive site for electron transfer during dechlorination reactions. Phenolic acidity, as based on the inflection point during titration of organic matter, is indicative of the hydroquinone content within humic materials. Published information indicates that the quinone content of humic acids is generally higher than for fulvic acid (Stevenson, 1994). 

In another study, ammonia fixation of N-labeled ammonium hydroxide with Suwannee River fiilvic acid, IHHS peat and leonardite humic acid were examined by solution NMR with the application of INEPT and DEPT pulse sequences.(23) Similar reaction of ammonia with all three samples is reported. Most of the nitrogen incorporated seems to be in the form of indole and pyrrole followed by pyridine, pyrazine, amide and aminohydroquinone nitrogen. The authors also suggest a possible reaction mechanism to explain the formation of the heterocyclic compounds identified in the spectra. They also claimed that these results need to be substantiated through further work with model compounds and experiments with the reaction conditions, i.e., in which phenols will undergo oxidation to quinones when reacted with ammonia. 

NMR Spectra of Unreacted Samples. Quantitative liquid phase NMR spectra of the unreacted samples are shown in Figure 2. Peak areas of the spectra are listed in Table I together with elemental analyses. Characteristically, the humic acid has a greater aromatic carbon and lesser carboxylic acid carbon content than the fulvic acid. The naturally occurring nitrogen contents are 2.68% and 4.18% for the fulvic and humic acids, respectively. Overlap of functional groups which may serve as substrate sites for nucleophilic addition by aniline occurs within the major peak areas of the spectra. Quinone carbons (190 to 178 ppm) overlap with ketone carbons from 220 to 189 ppm, amides and esters (174 to 164 ppm) overlap with... 

FIGURE 2. Dragunov s structure of humic acid as recorded by Kononova (1966) (1) Aromatic ring of the di- and trihydroxybenzene type, part of which has the double linkage of a quinone group. (2) Nitrogen in cyclic forms. (3) Nitrogen in peripheral chains. (4) Carbohydrate residue. 

FIGURE 4. Hypothetical structure of humic acid showing free and bound phenolic OH groups, quinone structures, oxygen as bridge units, and carboxyls variously placed on the aromatic ring. From Stevenson (1982). 

Volk and Schnitzer (1973) concluded that variations in the functional group components and spectral properties of humic acids from a group of Florida mucks indicated that higher rates of humification were related to (1) greater amounts of carboxyl, phenolic hydroxyl, quinone, and ketonic carbonyl groups (2) fewer alcoholic hydroxyl groups and aliphatic structures, as per IR evidence and (3) increments in EJEf, ratios and free-radical contents as revealed by ESR spectroscopy (Table 12). 

Spectra of fulvic acids must be interpreted carefully purification of this fraction leads to losses that can reach 50%. Comparison of spectra of different humic fractions of the same sample, such as a sediment from the Oman Sea (Fig. 6) illustrates several differences. Oxygenated functional groups are more important in fulvic and humic acids than in stable residues. Particularly important are the absorption bands at 3400 cm (OH from alcohols, acids, etc.), 1710 cm (C==0 from quinones, ketones, carboxylic acids), 1250 cm (C—O from alcohols, esters, ethers) and 1050 cm (C—O from carbohydrates). Absorption at 1050 cm is nearly absent in stable residues. Aliphatic content increases from fulvic acids to humic acids and stable residues (bands between 2870 and 2960 cm ) and the shape of the aliphatic bands (2900-2950 1450 1375) indicates that fulvic acids contain mainly CH groups. 

Reducing agents that accumulate in soils under anaerobic conditions, such as Fe ", sulfide, and phenols (from the reduction of quinones in humic acid) may react with certain classes of organic chemicals. For instance, Fe can reduce halogenated aliphatic compounds, an important class of organic pollutants in soils. The process initially involves the transfer of a single electron, for example. 

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