Progress 12/15/01 to 12/31/05
Outputs
Our work examines redox reactions between quinones and iron. Quinones, which can exist in fully reduced (dihydroxybenzene), intermediate (semiquinone radical), and fully oxidized (benzoquinone) forms, are major redox-active components of soil organic matter. Some quinones, e.g. caffeic acid, juglone, and lawsone, are released by plants into soils. Lignin breakdown generates other quinones. Iron is the most abundant redox-active metal in soils. Chelating agents released by roots, e.g. oxalate and malonate, alter iron speciation, and hence indirectly affect quinone-iron reactions. Our first manuscript based upon this work (M. Uchimiya and A.T. Stone, 2006, Geochim. Cosmochim. Acta, vol. 70, pages 1388-1401) presents thermodynamic calculations for systems containing quinones, iron, and chelating agents. Reduction potentials decrease as we move from benzene- to naphthalene- and finally to anthracene-based quinones. Reduction potentials are raised by
electron-withdrawing-substituents, or lowered by electon-donating ones. By coordinating Fe(III) in preference to Fe(II), chelating agents lower the reduction potential of the iron half reaction. Our calculations enable us to identify pH windows within which reaction can take place, e.g. for the oxidation of juglone by FeOOH(goethite). Our second manuscript (under re-review by Environ. Sci. Technol.) examines rates of redox reactions between benzoquinone oxidants and dihydroxybenzene reductants with dissimilar functional groups and structures. Soils receive inputs of quinones from different sources, and hence cross reactions of this kind are commonplace. Quinones that lack electron-withdrawing substituents exhibit reversible reactions below pH 7. Initial rates are inversely proportional to the hydrogen ion concentration, attributed to the greater reactivity of monoprotonated versus diprotonated dihydroxybenzene molecules. Reversible reactions are generally faster for pairs having
higher thermodynamic driving force. Concentrations in reversible reactions eventually reach plateaus, which coincide with calculated equilibrium values. If a reactant benzoquinone possesses an electron-withdrawing substituent, reaction progress falls short of expected values. If a product benzoquinone possesses an electron-withdrawing substituent, reaction progress extends beyond what is thermodynamically predicted. Both observations are attributable to irreversible side reactions, most likely Michael Addition. Our final two manuscripts (in preparation) examine the oxidation of Fe(II) by benzoquinones. Time course plots reveal a short lag period followed by first-order consumption of both reactants. Electron-withdrawing substituents and addition of oxalate and malonate raise reaction rate and broaden the pH range where reaction can take place. Adding small amounts of dihydroxybenzene product at the onset of reaction accelerates reaction rate. Both in the presence and absence of
oxalate or malonate, reaction rates increase as the pH is increased. Rate constants for both forward and reverse reactions are required to quantitatively model time course plots.
Impacts
The soil surrounding plant roots is termed the rhizosphere. Redox reactions within the rhizosphere control the oxidation state and speciation of iron, which in turn control iron bioavailability. Redox reactions also control the bioavailability of other important nutrients and contaminants. Our work explores the participation of soil organic matter in rhizosphere redox reactions. Quinones are believed to be the most important redox active components of soil organic matter, and therefore serve as the focus of our research. Thermodynamic calculations indicate the extent of reaction progress achievable when one quinone reacts with another, and when quinones react with iron. Kinetics studies in the laboratory allow us to compare timescales and rates of these processes. Under conditions where quinone-quinone electron exchange reactions are fast relative to reactions with iron, the entire pool of constituent quinones are simultaneously reduced (or oxidized). Under conditions
where quinone-quinone electron exchange reactions are slow, one or two quinone structures are consumed by reaction while others are left undisturbed. Irreversible side reactions observed when quinones possess electron-withdrawing ring substituents likely age organic matter over time, especially in neutral and alkaline soils. Our work assists efforts to understand how soil properties (pH, organic matter content) and biochemicals released by plants and other soil organisms affect iron oxidation state and speciation, and hence iron bioavailability.
Publications
- Uchimiya, M. and A.T. Stone. 2006. Redox reactions between iron and quinones: thermodynamic constraints. Geochimica et Cosmochimica Acta 70(6): 1388-1401.
- Uchimiya, M. and A.T. Stone. 2006. Aqueous oxidation of substituted dihydroxybenzenes by substituted benzoquinones. Environmental Science & Technology. (Under re-review)
|
Progress 10/01/02 to 09/30/03
Outputs
Redox reactions of iron in soils are important for several reasons. Although Fe(III) is a weak oxidant in a thermodynamic sense, it serves as an electron acceptor for important sub-oxic and anerobic bacteria. Fe(II) is a strong reductant in both a thermodynamic and kinetic sense, and is known to reduce nitroaromatic and oxime-carbamate organic contaminants. Redox reactions between iron and quinones serve as the focus of our work. Quinones are arguably the most redox-active moieties within soil natural organic matter. Our experiments to date have established several key points: (i) Fe(III) reaction with reduced quinones occurs primarily under acidic conditions, while Fe(II) reaction with oxidized quinones occurs primarily under neutral and alkaline conditions; (ii) Most naturally-occurring chelating agents coordinate Fe(III) more strongly than Fe(II). As a consequence, chelating agent addition shifts the pH range where Fe(II) oxidation can take place towards more
acidic conditions. Chelating agent additions also dramatically accelerate Fe(II) oxidation rates; (iii) Although it is commonly assumed that redox reactions involving quinones are reversible, side reactions under neutral and alkaline conditions (e.g. Michael Addition by hydroxide ion) ultimately convert quinones into unreactive products. Quinones with high reduction potentials yield benzoquinones that are especially short-lived. The presence of ring substituents that block Michael addition may stabilize quinone structures. (iv) Structurally realistic quinone compounds (e.g. those with carboxylate and other Lewis Base-containing side groups found in natural organic matter) react in distinctive ways, unlike simple quinones (e.g. hydroquinone and p-benzoquinone). Considering these points together reveals that oxidation/reduction and Lewis Base moieties within natural organic matter exert strong controls on iron speciation in soils. Iron, in turn, affects the character of the natural
organic matter moiety pool through it's influence on source and sink terms for particular moieties.
Impacts
Soil redox reactions affect the health of plants and soil microorganisms, and play a role in the degradation of agricultural chemicals. The work just described will provide soil scientists with a more accurate portrait of soil redox status.
Publications
- No publications reported this period
|
Progress 10/01/01 to 09/30/02
Outputs
Reactions between iron and quinones are currently under investigation. Quinones exist in fully reduced forms (as hydroquinones and catechols), as intermediate radical species (as semiquinones), and in fully oxidized forms (the benzoquinones). It has been hypothesized that quinones released by soil bacteria, such as cinnaquinone and menaquinone, facilitate Fe(III) respiration. A number of allelochemicals (e.g. juglone and lawsone) possess quinone moieties. The fungicides dichlone, chloranil, and dithanon are all quinones. The high quinone content of soil natural organic matter is believed to arise from inputs of lignin degradation products. The thermodynamics of quinone redox processes are a strong function of molecular structure. Single aromatic ring quinones such as hydroquinone can only reduce Fe(III) (hydr)oxides at pHs below approximately 5.0. Quinones with three aromatic rings, in contrast, can reduce Fe(III) (hydr)oxides throughout the pH range of soils. The
back reaction is also potentially important; single ring quinones such as p-benzoquinone can oxidize Fe(II) at pHs above pH 5.0. Experimental investigation of the back reaction indicates that the reaction rate increases dramatically as the pH is increased. A lag period in the reaction is observed, which can be eliminated by small additions of the hydroquinone reaction product. A conproportionation reaction, in which p-benzoquinone and hydroquinone react to form semiquinone radical, may play a role in the presence or absence of a lag period. The dicarboxylate ligand malonate lowers the reduction potential of the Fe(III)/Fe(II) half-reaction, allowing p-benzoquinones to oxidize Fe(II) under a wider range of acidic conditions. Direct electron transfer between different quinone structures is also potentially important. It is interesting to consider, for example, about rates of equilibration between different quinone moieties within organic matter macromolecules. Our experiments with one-
and two-ring quinones indicate that rates of these redox equilibration reactions are inversely proportional to the proton concentration. The work just described is of considerable practical interest; quinones are believed to be dominant organic participants in soil redox reactions, while Fe(III) and Fe(II) are believed to be the dominant inorganic participants in soil redox reactions.
Impacts
Soil redox reactions affect the health of plants and soil microorganisms, and play a role in the degradation of agricultural chemicals. The work just described will provide soil scientists with a more accurate portrait of soil redox status.
Publications
- No publications reported this period
|
|