Oxidationreduction Reactions Of Nitrogen Foods

Life processes involve electron transport. Specifically, the mitochondrion and the chloroplast are the sites of this electron movement in the eucaryotes. In the procary-otes, this function is embedded in the sites of the cytoplasmic membrane. As far as electron movement is concerned, life processes have similarity to a battery cell. In this cell, electrons move because of electrical pressure, the voltage difference. By the same token, electrons move in an organism because of the same electrical pressure, the voltage difference. In a battery cell, one electrode is oxidized while the other is reduced; that is, oxidation-reduction occurs in a battery cell. Exactly the same process occurs in an organism.

In an oxidation-reduction reaction, a mole of electrons involved is called the faraday, which is equal to 96,494 coulombs. A mole of electrons is equal to one equivalent of any substance. Therefore, a faraday is equal to one equivalent.

Let n be the number of faradays of charge or mole of a substance participating in a reaction, and let the general reaction be represented by the half-cell reaction of Zn as follows:

The couple Zn/Zn2+ has an electric pressure between them. Now, take another couple such as Mg/Mg2+ whose half-cell reaction would be similar to that of Equation (15.10). The couples Zn/Zn2+ and Mg/Mg2+ do not possess the same voltage potential. If the two couples are connected together, they form a cell. Their voltage potentials are not the same, so a voltage difference would be developed between their electrodes and electric current would flow.

Let the voltage between the electrodes of the above cell be measured by a potentiometer. Designate this voltage difference by AE. In potentiometric measurements, no electrons are allowed to flow, but only the voltage tendency of the electrons to flow is measured. No electrons are allowed to flow, therefore, no energy is dissipated due to friction of electrons "rubbing along the wire." Thus, any energy associated with this no-electron-to-flow process represents the maximum energy available. Because voltage is energy in joules per coulomb of charge, the energy associated can be calculated from the voltage difference. This associated energy corresponds to a no-friction loss process; it is therefore a maximum energy—the change in free energy— after the entropic loss has been deducted.

Let n, the number of faradays involved in a reaction, be multiplied by F, the number of coulombs per faraday. The result, nF, is the number of coulombs involved in the reaction. If nF is multiplied by AE, the total associated energy obtained from the previous potentiometric experiment results. Because the voltage measurement was done with no energy loss, by definition, this associated energy represents the free energy change of the cell (i.e., the maximum energy change in the cell). In symbols,

The ± sign has been used. A convention used in chemistry is that if the sign is negative, the process is spontaneous and if the sign is positive, the process is not spontaneous.

As mentioned, the battery cell process is analogous to the living cell process of the mitochondrion, the chloroplast, and the electron-transport system in the cyto-plasmic membrane of the procaryotes. Thus, Equation (15.11) can represent the basic thermodynamics of a microbial system.

In the living cell, organic materials are utilized for both energy (oxidation) and synthesis (reduction). Microorganisms that utilize organic materials for energy are called heterotrophs. Those that utilize inorganics for energy are called autotrophs. Autotrophs utilize CO2 and HCO- for the carbon needed for cell synthesis; the het-erotrophs utilize organic materials for their carbon source. Autotrophs that use inorganic chemicals for energy are called chemotrophs; those that use sunlight are phototrophs. The bacteria that consumes the nitrogen species in the biological removal of nitrogen are chemotrophic autotrophs. Algae are phototrophic autotrophs.

Somehow, in life processes, the production of energy from release of electrons does not occur automatically but through a series of steps that produce a high energy-containing compound. This high energy-containing compound is ATP (adenosine triphosphate). Although ATP is not the only high energy-containing compound, it is by far the major one that fuels synthesis in the cell. ATP is the energy currency that the cell relies upon for energy supply.

The energy function of ATP is explained as follows: ATP contains two high-energy bonds. To form these bonds, energy must be obtained from an energy source through electron transfer. The energy released is captured and stored in these bonds. On demand, hydrolysis of the bond releases the stored energy which the cell can then use for synthesis and cell maintenance.

ATP is produced from ADP (adenosine diphosphate) by coupling the release of electrons to the reaction of organic phosphates and ADP producing ATP. ATP has two modes of production: substrate-level phosphorylation and oxidative phospho-rylation. In the former, the electrons released by the energy source are absorbed by an intermediate product within the system. The electron absorption is accompanied by an energy release and ATP is formed. The electron-transport system is simple.

Fermentation is an example of a substrate-level phosphorylation process that uses intermediate absorbers such as formaldehyde. Substrate-level phosphorylation is inefficient and produces only a few molecules of ATP.

In the oxidative phosphorylation mode, the electron moves from one electron carrier to another in a series of complex reduction and oxidation steps. The difference between substrate-level and oxidative phosphorylation is that in the former, the transport is simple, while in the latter, it is complex. For a hydrogen-containing energy source, the series starts with the initial removal of the hydrogen atom from the molecule of the source. The hydrogen carries with it the electron it shared with the original source molecule, moving this electron through a series of intermediate carriers such as NAD (nicotinamide adenine dinucleotide). The intermediate that receives the electron-carrying hydrogen becomes reduced. The reduction of NAD, for example, produces NADH2. The series continues on with further reduction and oxidation steps. The whole line of reduction and oxidation constitutes the electron-transport system. At strategic points of the transport system, ATP is produced from ADP and inorganic phosphates.

The other version of oxidative phosphorylation used by autotrophs involves the release of electrons from an inorganic energy source. An example of this is the release of electrons from NH-, oxidizing NH+ to NO-, and the release of electrons from NO-, oxidizing NO- to NO-.

The transported electrons emerge from the system to reduce a final external electron acceptor. The type of the final acceptor depends upon the environment on which the electron transport is transpiring and may be one of the following: for aerobic environments, the acceptor is O2; for anaerobic environments, the possible - -2 -acceptors are NO3, SO4 , and CO2. When the acceptor is NO3, the system is said to be anoxic.

The values of free energy changes are normally reported at standard conditions. In biochemistry, in addition to the requirement of unit activity for the concentrations of reactants and products, pressure of one atmosphere, and temperature of 25°C, the hydrogen ion concentration is arbitrarily set at pH 7.0. Following this convention, Equation (15.11) may be written as

The primes emphasize the fact that the standard condition now requires the {H+} to

be 10 moles per liter.The subscript 0 signifies conditions at standard state.

In environmental engineering, it is customary to call the substance oxidized as the electron donor and the substance reduced as the electron acceptor. The electron donor is normally considered as food. In the context of nitrogen removal, the foods are the nitrites, nitrates, and ammonia. Equation (15.10) is an example of an electron donor reaction. Zn is the donor of the electrons portrayed on the right-hand side of the half-cell reaction. On the other hand, the reverse of the equation is an example of an electron acceptor reaction. Zn+2 would be the electron acceptor. McCarty (1975) derived values for free energy changes of half-reactions for various electron donors and acceptors utilized in a bacterial systems. The ones specific for the nitrogen species removal are shown in Table 15.2.

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