fenton process
Fenton’s reagent is a solution of hydrogen peroxide and an iron catalyst used to oxidize pollutants and wastewater. Fenton’s reagent can be used to destroy organic compounds such as trichlorethylene (TCE) and tetrachlorethylene (PCE).Iron(II) is oxidized to iron(III) by hydrogen peroxide, forming hydroxyl radicals and hydroxide ions in the process. The iron(III) is then reduced back to iron(II) by another hydrogen peroxide molecule, forming a hydroperoxyl radical and a proton. The net effect is the disproportionation of hydrogen peroxide to produce two different oxygen radical species and water (H+ + OH-) as a byproduct.
Fe2+ + H2O2 → Fe3+ + HO• + OH-
Fe3+ + H2O2 → Fe2+ + HOO• + H+
Free radicals generated by this process cause secondary reactions. For example, hydroxyl is a strong non-selective oxidant. Oxidation of organic compounds by Fenton’s reagent is rapid and exothermic, and contaminants are oxidized primarily to carbon dioxide and water.
Oxidation by ozone
Ozone is a highly oxidizing agent that reacts with organic substrates either by slow, selective reactions or by fast, non-selective radical reactions that produce H2O. Ozone decomposition in aqueous solutions has been demonstrated to form HO, especially when initiated by OH.
This pathway is initiated in different ways by HO2-, HCOO-, Fe2+, humic substances, or primarily HO-. This is why ozonation is in principle more efficient in alkaline media, with an optimum value around pH 9. Ozonation converts complex organics into aldehydes, ketones, or carboxylic acids, all of which are readily biodegradable compounds. Ozonation can also be used in combination with other conventional or AOPs.
However, from an operational perspective, there are limitations associated with gas-liquid ozone mass transfer. Therefore, this process requires an efficient reactor design to maximize the mass transfer coefficient of ozone. Increase the area of the contact interface. It may also be effective to install a large bubble column to increase the residence time in the reactor or to increase the pressure to several atmospheres to increase the solubility of ozone.
Adding H2O2 to an ozonation system enhances the oxidizing capacity of the process through secondary reactions. Hydrogen peroxide initiates O3 decomposition through electron transfer.
O3 + H2O2 → H2O• + O2 + HO2•。
This process is expensive but quick and can treat organic contaminants at very low concentrations (ppb) and pH 7-8. The optimal O3/H2O2 molar ratio is 2:1. One of the main areas of application for this process is the degradation of pesticides. It is also effective for post-treatment of chlorinated water because it can decompose trihalomethane and related compounds.
photochemical oxidation process
When ozone combines with ultraviolet light, several processes occur. Ozone irradiation in water results in the quantitative formation of H2O2. Photolysis of H2O2 by UV irradiation produces two hydroxyl radicals per molecule of hydrogen peroxide. The reagent also reacts with 03.H2O2 Similarly, ozone also reacts with hydroxyl radicals to form superoxide radicals.
Since ozone has a higher absorption coefficient than H2O2, this combined AOP can be used to treat water with a high UV absorption background. The ability to use UV-B light (280-315 nm) avoids the use of quartz reactors, and the efficiency is higher than that of Oz or direct UV.
When UV-C irradiation is used, additional H2O• is produced by photolysis of O3. and other oxidizing agents improve efficiency.
Advanced Oxidation Process (AOP)
In its broadest sense, AOP refers to a set of chemical treatment procedures designed to remove organic (and sometimes inorganic) substances in water and wastewater by oxidation through reaction with hydroxyl radicals (OH). A chemical process that uses ozone (O3), hydrogen peroxide (H2O2), and/or UV light. One such type of process is called in situ chemical oxidation.
AOP relies on the in situ generation of highly reactive hydroxyl radicals (OH). These reactive species are the most powerful oxidizing agents that can be applied to water and can oxidize virtually any compound present in the water matrix. As a result, once formed, OH reacts nonselectively, and contaminants are rapidly and efficiently fragmented and converted into small inorganic molecules. Hydroxyl radicals are generated with the help of one or more primary oxidants (e.g., ozone, hydrogen peroxide, oxygen) and/or energy sources (e.g., ultraviolet light) or catalysts (e.g., titanium dioxide).
AOP procedures are particularly useful for treating biologically toxic or non-degradable substances such as aromatics, pesticides, and petroleum components present in wastewater. Additionally, AOPs can also be used to treat secondary treatment wastewater effluents, referred to as tertiary treatment. Pollutants are largely converted into stable inorganic compounds such as water, carbon dioxide, and salts. That is, they undergo petrification.
In general, the chemistry of AOPs can be basically divided into three parts.
Formation of OH
Initial attack on the target molecule by OH and its decomposition into fragments.
Subsequent attacks by OH until final petrification.
Ozonation, UV/H2O2, and photocatalytic oxidation rely on different mechanisms for OH production.
UV/H202:
H2O2 + UV → 2-OH
Ozone-based AOP:
O3 + H2O – HO2 + O2
O3 + HO2 → HO2: + 03
03: + H+ – HO3.
H03. → OH + O2
Photocatalytic oxidation with TiO2:
TiO2 + UV + e +h+
Ti(IV) + H2O = Ti(IV)-H20
Ti(IV)-H2O +h+ = Ti(IV)–OH + H+
advantage
Organic compounds in the aqueous phase can be effectively removed rather than collecting or transferring contaminants to another phase.
Due to the remarkable reactivity of OH, it reacts almost indistinguishably with almost all aqueous contaminants. Therefore, AOP may be applicable to many, if not all, scenarios where many organic pollutants are expected to be removed simultaneously.
Some heavy metals can also be removed in the form of precipitated M(OH)x.
Disinfection can also be achieved in some AOP designs, making AOP an integrated solution to some of the water quality issues.
The complete reduction product of OH is H2O, so AOP theoretically does not introduce new harmful substances into the water.
Cons
Most notably, the cost of AOP is too high, as most AOP systems require continuous input of expensive chemical reagents to maintain operation.
Some technologies require pretreatment of wastewater to ensure reliable performance, which can be potentially costly and technically demanding. For example, the presence of bicarbonate ions (HCO3-) can significantly reduce the concentration of •OH due to scavenging processes that produce the much less reactive species .CO3- with H2O. As a result, bicarbonate must be purged from the system or AOP will be compromised.
Given the potential costs, AOPs may not be able to treat large volumes of wastewater independently. Instead, AOP should be introduced at the final stage after most contaminants have been removed by primary and secondary treatments.