Oxidation/Reduction in Aquatic Chemistry PDF

Environmental Chemistry CHEM204 Oxidation/Reduction in Aquatic Chemistry Dr. Nadiyah Alqazlan The Significance of Oxidation/Reduction in Aquatic Chemistry Understanding Redox Reactions in Aquatic Systems 1 Redox reactions play a crucial role in water, soil, sediment,...

Environmental Chemistry CHEM204 Oxidation/Reduction in Aquatic Chemistry Dr. Nadiyah Alqazlan The Significance of Oxidation/Reduction in Aquatic Chemistry Understanding Redox Reactions in Aquatic Systems 1 Redox reactions play a crucial role in water, soil, sediment, and living systems, influencing elemental cycles, mobility of aquatic species, biodegradation, sorption-desorption phenomena, precipitation- dissolution processes, and pollutant remediation. The Significance of Oxidation/Reduction in Aquatic Chemistry Understanding Redox Reactions in Aquatic Systems Redox reactions in natural waters and wastewaters 2 are often catalyzed by microorganisms. Bacteria play a key role in reactions such as organic matter oxidation, iron reduction, and ammonia oxidation. The Significance of Oxidation/Reduction in Aquatic Chemistry Understanding Redox Reactions in Aquatic Systems The concept of pE, analogous to pH, is used to express the degree of oxidation or reduction in an 3 aquatic medium. Low pE values indicate reducing conditions, while high pE values reflect oxidizing conditions.  THE ELECTRON AND REDOX REACTIONS Understanding Redox reactions involve the transfer of electrons Redox between species. They are key to understanding redox Reactions phenomena in natural waters. Changes in In redox reactions, changes in oxidation states occur. Oxidation States Some atoms lose electrons and become more positive (oxidation) Some atoms gain electrons and become more negative (reduction) https://byjus.com/chemistry/oxidation-and-reduction/  ELECTRON ACTIVITY AND pE Electron activity and pE pE is used instead of E to illustrate redox equilibria in aquatic systems over many orders of magnitude of electron activity, similar to pH. pH is a convenient way of expressing the activity of the hydrogen ion in manageable numbers, and pE serves a similar purpose for electron activity. Electron activities in water can vary over more than 20 orders of magnitude, making pE a convenient measure to express electron activity. It is easy to visualize the activities of ions in terms of concentration, BUT it is harder to visualize the activity of the electron, hence the use of pE.  THE RELATIONSHIP OF pE TO FREE ENERGY Thermodynamics and Role of Mediators Predicting Useful Energy Aquatic Systems Aquatic systems and Bacteria, fungi, and To predict useful the organisms that human beings act as energy that can be inhabit them must mediators or catalysts extracted from follow the laws of of chemical reactions chemical reactions in thermodynamics. in aquatic systems. an aquatic system, it is important to know the They derive their They play a crucial role free energy change energy from chemical in extracting useful (ΔG) for the redox reactions and extract a energy from these reaction. certain percentage of reactions. useful energy from This information can be them. obtained from pE Examples of Chemical Reactions In Aquatic Systems: Redox Reaction: two redox half-reactions are needed to obtain a complete redox reaction in water. Examples: microbially mediated oxidation of organic matter to CO2 and water. fermentation of organic matter to methane by anaerobic bacteria in the absence of oxygen.  THE LIMITS OF pE IN WATER Oxidizing Limit The pE value on the oxidizing side of water is limited by the oxidation of water. When water is oxidized, oxygen is produced, and the pE value cannot exceed the point at which oxygen has a pressure of 1.00 atm. This can be considered as the oxidizing limit of water. Reducing Limit On the reducing side of water, the pE value is limited by the reduction of water. When water is reduced, hydrogen is produced, and the pE value cannot go below the point at which hydrogen has a pressure of 1.00 atm. This can be regarded as the reducing limit of water. Temporary In certain cases, water may have temporary nonequilibrium pE Nonequilibrium values that are more negative than the reducing limit or more pE Values positive than the oxidizing limit. This is possible in the presence of certain catalysts or chemical reactions. Cont. There are pH-dependent limits to the pE values at which water is thermodynamically stable. Water may be both oxidized: 2H2O ←→ O2 + 4H+ + 4e- (1) or it may be reduced: 2H2O + 2e- ←→ H2 + 2OH- (2) These two reactions determine the limits of pE in water. On the oxidizing side (relatively more positive pE values), the pE value is limited by the oxidation of water, Half-reaction (1) The evolution of hydrogen, Half-reaction (2), limits the pE value on the reducing side.  pE VALUES IN NATURAL WATER SYSTEMS Obtaining accurate pE values through direct potentiometric Direct measurements in natural aquatic systems is generally not Potentiometric possible. However, in principle, pE values can be calculated Measurements from the species present in water at equilibrium. To calculate pE values, it is necessary to consider the Calculation of species present in water at equilibrium. pE Values By understanding the equilibrium reactions and the concentrations of the species, the pE value can be estimated. pE–pH DIAGRAMS What are pE–pH diagrams? Complexity of pE–pH diagrams pE–pH diagrams are pE–pH diagrams can be highly graphical representations complicated due to the that show the relationships numerous species that can be between pE and pH in water. formed. They depict the regions of They require careful analysis stability and the boundary and interpretation to lines for various species in understand the behavior of water. different species in water. They help in understanding pE–pH diagrams are useful the redox and acid-base tools for studying behavior of species in water. geochemical processes and environmental conditions. pE–pH DIAGRAMS Introduction to Eh-pH diagrams https://www.youtube.com/watch?v=8TSPK2I87j8  HUMIC SUBSTANCES AS NATURAL REDUCTANTS Humic substances are products that result from the decomposition Introduction to of plant and animal residues. Humic They are oxidation-reduction-active species that play a significant Substances role in chemical and biochemical processes in natural water and wastewater systems. Reducing Agents? A substance which loses electrons to other substances in a redox reaction and gets oxidized to a higher valency state.  HUMIC SUBSTANCES AS NATURAL REDUCTANTS Role of Humic Soluble humic substances have been found to act as reducing Substances as agents in the reduction of species in water by solid reductants, Reducing such as solid iron(II) species. Species Humic substances act as electron shuttles. They transfer electrons Humic to iron(III) during the microorganism-mediated bioreduction of Substances as iron(III) to iron(II) in water. Electron The presence of carboxyl (–CO2H) groups in humic substances Shuttles enhances this process. PHOTOCHEMICAL PROCESSES IN OXIDATION/REDUCTION Introduction Superoxide Ion Hydrogen Peroxide Light absorption can Superoxide ion (O2 -) is Hydrogen peroxide energize chemical an oxidizing agent in (H2O2) is an species, making them sunlit water, produced intermediate in more reactive. by photochemically photochemically excited organic matter induced oxidation on dissolved O2. processes in water, These species can generated when O2 - It can oxidize participate in reacts with water. inorganically oxidation-reduction complexed metals. processes in water. Using Redox Reaction In Water Treatment: Oxidation-reduction in water treatment removes dissolved minerals, such as iron and manganese from water. This process can also remove organic matter, such as bacteria, from water. Oxidation-reduction is usually accomplished by adding chemicals, such as chlorine or ozone, to water. CORROSION Definition Factors Affecting Corrosion Corrosion is the destructive Corrosion is influenced by: alteration of metal through 1. Type and concentration of interactions with its oxidants surroundings. 2. Release rate of metals It's a redox phenomenon leading to the formation of 3. Corrosion scale properties more stable ions, salts, oxides, and hydroxides. Oxygen and bacteria can significantly affect corrosion processes. CORROSION Impacts & Costs Corrosion leads to significant costs due to equipment and structure damage. 1. It introduces metals into water systems 2. It damages pollution control equipment 3. It destroys waste disposal pipes.