Fenton and Fenton-like processes are efficient and inexpensive advanced oxidation processes for the purification of water and wastewater containing organic pollutants. The Fenton process is a reaction between peroxide and ferric ions to produce reactive oxygen species that oxidize organic or inorganic compounds in wastewater. Hydrogen peroxide and ferrous ions in combination are often referred to as Fenton's reagent, which is capable of treating a wide range of wastewaters including phenol, formaldehyde, benzene, and complex wastes such as pesticides, specialty chemicals, and dyes.
When hydrogen peroxide reacts with excess ferrous ions, it is oxidized to ferric ions within seconds, decomposing hydrogen peroxide to hydroxyl radicals in the process. The reaction rate of the Fenton process is very high.
Fe2+ + H2O2 Fe3+ + OH- + *OH
Under the action of excess peroxide and free radicals, ferric ions can be reduced back to ferrous ions. This reverse reaction is called a Fenton-like process and is slower in comparison to the Fenton process.
Ferrous ions react with hydroxide ions to form iron-hydroxy complexes. The coagulation ability of the Fenton reagent depends on this conversion. Dissolved solids in wastewater settle out. Small flocs are observed during this process and these flocs take a long time to settle out. Therefore, during treatment, chemical coagulation needs to be provided to reduce the settling time of flocs. Depending on the properties of the compounds, the following reactions may occur:
Free radical addition: Free radical organic compounds are formed when hydroxyl radicals are added to unsaturated aromatic compounds.
Hydrogen abstraction: By removing hydrogen atoms, water and free radical organic compounds are generated. A chain reaction is initiated, which reacts with oxygen to form peroxide radicals.
Electron transfer: Electron transfer produces high-valent ions. Oxidation of monovalent anions leads to the formation of atoms.
Free radical combination: One hydroxyl radical reacts with another hydroxyl radical to form stable products.
For optimal operating conditions, the temperature must be around 10-40 oC to ensure good degradation efficiency and pH 3 to obtain the highest oxidation activity of hydroxyl radicals. In addition, higher concentrations of Fe2+ and H2O2 are favorable due to higher degradation rates. Higher H2O2 is harmful to microorganisms and increases degradation efficiency. Finally, after Fenton oxidation, a chemical coagulation tank must be used to keep the soluble iron within the specified limits. Overall, the reaction takes several hours, depending on the pollutant, and the progress of the reaction can be observed by color changes.
The Fentons process uses inexpensive reagents and the reactor layout is also simple, which is its biggest advantage in operation and construction. Compared with other systems, this reaction is not negatively affected by mass transfer and has good application prospects in the removal of polypropylene copolymers (PPCP) from wastewater. It has the advantages of fast reaction speed, low chemical dosage, and controllable and easy generation of hydroxyl radicals. The reaction can be carried out using homogeneous and heterogeneous catalysts. But the core chemical of this process: hydrogen peroxide is very expensive and may hinder operation. As water regulations continue to strengthen, the main challenges to improve Fenton technology are also increasing. Finally, in order for the radical to be stable, it should meet the required acidic conditions.
Technical Advantages
The reactor system allows most of the Fe³ produced by the Fenton process to crystallize and attach to the surface of the fluidized bed Fenton carrier. The crystallized Fe³ iron salts then form a synergistic effect with the Fenton system, converting back to Fe² in a cyclical manner. This significantly reduces the amount of chemical sludge produced by traditional Fenton methods. Hydrogen peroxide usage decreases by over 20%, Fe² addition reduces by over 50%, and sludge production decreases by over 40%, resulting in an effective COD removal rate improvement of over 10% and a reduction in operational costs by 30% to 50%.
Typical Applications
1. Applied in the deep treatment of industrial wastewater from industries such as dyeing and printing, chemicals, petrochemicals, leather, paper-making, and pharmaceuticals.
2. Biochemical pretreatment of difficult-to-degrade, high COD wastewater.
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