**2. Conclusion**

kinetics with a first order in each oxidant (HbO2) and reductant ([FeII

<sup>3</sup>, [FeIII(CN)6]

**1.5 Advanced oxidation processes for water treatment**

The oxidation of organic compounds by a number of oxidants either of inorganic

nature or organic nature has been of interest. These redox reactions are usually catalyzed by transition metals. The kinetics of the oxidation of pyridinecarbaldehyde isonicotinoyl hydrazone to isonicotinoyl picolinoyl hydrazine was studied, and the mechanism was proposed in the view of results obtained in aqueous solution [34]. The reaction was catalyzed by iron(III). Advanced oxidation processes (AOPs) are used to remove pollutants/contaminants such as organic and inorganic compounds from water and wastewater by oxidation of these unwanted compounds. The process involves a number of chemical reactions consisting of

[CoIII(4,7-DPSphen)3]

*Redox*

complexes [33].

**8**

distorted tetrahedral arrangement.

respectively. The results declared that the structures of the reactants such as protein and external donor control the kinetics of the electron transfer with an inner-sphere mechanism that involves direct electron transfer from the aquopentacyanoferrate(II) to bound dioxygen that yields peroxide, subsequently. Another study surfaced the effect of binding sites and protonation on the kinetics of the electron transfer reaction (s) of blue copper proteins [30]. The oxidants with different binding sites such as

parsley plastocyanin. In each reaction, regardless of the binding sites, and prior to electron transfer, a strong association between protein and complex occurs. The variation in the binding sites varied the reduction potential and affected the rate of electron transfer, consequently. The reductant (plastocyanin) is a copper protein that consisted of type 1 copper, which is involved in electron transport from photosystem II to photosystem I at the surface of the thylakoid membrane. A single copper here utilizes oxidation states I and II. The structure of poplar plastocyanin PCuII contains Cu(II) coordinated with two histidines, one cysteine, and one methionine in a

It has always been of interest to probe the details of the transfer of electron(s) and proton(s) because of successfully unveiling strategies of energy conversion in both of the fields, biology and chemistry. The energies as well as mechanism are strongly influenced by the coupling of electron and proton transfer. This defines the need to build up multiple redox equivalents to carry out those reactions that involve multielectrons. This also explains those mechanistic pathways through which electron and proton transfer occur simultaneously to avoid intermediates of high energy [31]. The theoretical background of the proton-coupled electron transfer reactions in solutions and proteins and electrochemistry was reviewed and discussed [32]. The theoretical treatment was based on the calculations of multistate continuum theory wherein the solvent provides dielectric continuum, the solute is treated as a multistate valence bond model, and quantum mechanical approach is used for transferred proton or hydrogen nucleus. The rate expression of electronically nonadiabatic electron transfer and proton-coupled electron transfer depends upon the reorganization energies of solute (inner-sphere) and solvent (outer-sphere) and also upon electronic coupling. For proton-coupled electron transfer, this is the average of the proton vibrational wave functions of the reactants and the products. The compensation of the smaller outer-sphere solvent reorganization energy for proton-coupled electron transfer by the larger energy needed to coupling for electron transfer appears with a similar rate for both electron transfer and protoncoupled electron transfer in calculations. A comparative theoretical study supported the reviewed outcomes through the proton-coupled electron transfer, single proton transfer, and single electron transfer reactions in iron bi-imidazoline

<sup>3</sup>, and [CoIII(phen)3]

(CN)5H2O]3),

3+ were used to oxidize

This concise review of the redox reactions and their applications surfaced the crucial role of redox processes. The importance of redox processes is undoubtedly tremendous. The applications encompass energy production, technological development to treat and maintain water resources, and advances in materials chemistry. These advances may lead the life to its standard in an economic and cost-effective way. Redox reactions are also an important facet of biological and biochemical world to carry out life and its routine practices. For example photosynthesis, respiration and digestion are among the common ones. Precisely, we can sum up with one sentence that "redox" is basically the key to sustaining life on this planet.
