Fenton’s reagent is a solution of hydrogen peroxide and an iron catalyst that is used to oxidize contaminants or waste waters. Fenton’s reagent can be used to destroy organic compounds such as tri-chloroethylene (TCE) and tetra-chloroethylene (PCE).
Iron (II) is oxidized by hydrogen peroxide to iron (III), forming a hydroxyl radical and a hydroxide ion in the process. Iron (III) is then reduced back to iron (II) by another molecule of hydrogen peroxide, forming a hydroperoxyl radical and a proton. The net effect is a disproportionation of hydrogen peroxide to create two different oxygen-radical species, with water (H+ + OH-) as a byproduct.
Fe2+ + H2O2 → Fe3+ + HO• + OH–
Fe3+ + H2O2 → Fe2+ + HOO• + H+
The free radicals generated by this process then engage in secondary reactions. For example, the hydroxyl is a powerful, nonselective oxidant. Oxidation of an organic compound by Fenton’s reagent is rapid and exothermic and results in the oxidation of contaminants to primarily carbon dioxide and water.
Oxidation with Ozone
Ozone is a highly oxidizing agent & can react with an organic substrate, through a slow and selective reaction below or through a fast and non-selective radical reaction producing HO•. It has been demonstrated that ozone decomposition in aqueous solution forms HO•, especially when initiated by OH-.
The route can be initiated in different ways, by HO2– , HCOO–, Fe2+, humic substances or principally by HO-. This is why, in principle, ozonation is sensibly more efficient in alkaline media, presenting an optimum around pH 9. With ozonation complex organics are transformed into aldehydes, ketones or carboxylic acids, all easily biodegradable compounds. Ozonation is also versatile to be combined with other conventional or AOPs.
However, from the operational point of view, there are limitations associated with the gas-liquid ozone mass transfer. Consequently, the process requires efficient reactor design in order to maximize the ozone mass transfer coefficient; increasing the interfacial area of contact. In addition, increasing the retention time in the reactor by large bubble columns, or increasing the solubility of ozone by increasing the pressure to several atmospheres, may be effective.
Addition of H2O2 to the ozonation system enhances the oxidation capacity of the process through secondary reactions. Hydrogen peroxide initiates O3 decomposition by electron transfer.
O3 + H2O2 → HO• + O2 + HO2•.
The process is expensive but fast, and can treat organic pollutants at very low concentrations (ppb), at pH between 7 and 8; the optimal 03/H2O2 molar ratio is 2:1. One of the principal fields of application of this treatment is in the degradation of pesticides. It is also effective for the posttreatment of water previously treated with chlorine because it can decompose tri-halo- methanes or related compounds.
Photochemical Oxidation Process
When ozone is combined with UV radiation, several processes take place. Irradiation of ozone in water leads to quantitative formation of H202 : Photolysis of H2O2 by UV radiation yields two hydroxyl radicals per each molecule of hydrogen peroxide; the reagent also reacts with 03:
As well as H2O2, ozone 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 high UV absorption background. The efficiency is higher than that of Oz or direct UV, because UV-B light (280-315 nm) can be used, avoiding the use of a quartz reactor.
When UV-C irradiation is used, photolysis of O3 generates additional HO•. and other oxidants, with the subsequent increase of the efficiency.
Advanced oxidation processes (AOPs)
AOPs in a broad sense, refers to a set of chemical treatment procedures designed to remove organic (and sometimes inorganic) materials in water and wastewater by oxidation through reactions with hydroxyl radicals (OH). In such chemical processes that employ ozone (O3), hydrogen peroxide (H2O2) and/or UV light. One such type of process is called in situ chemical oxidation.
AOPs rely on in-situ production of highly reactive hydroxyl radicals (OH). These reactive species are the strongest oxidants that can be applied in water and can virtually oxidize any compound present in the water matrix. Consequently, OH reacts unselectively once formed and contaminants will be quickly and efficiently fragmented and converted into small inorganic molecules. Hydroxyl radicals are produced 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).
The AOP procedure is particularly useful for treating biologically toxic or non-degradable materials such as aromatics, pesticides, petroleum constituents etc present in waste water. Additionally, AOPs can be used to treat effluent of secondary treated wastewater which is then called tertiary treatment. The contaminant materials are converted to a large extent into stable inorganic compounds such as water, carbon dioxide and salts, i.e. they undergo mineralization.
Generally speaking, chemistry in AOPs could be essentially divided into three parts:
- Formation of OH
- Initial attacks on target molecules by OH and their breakdown to fragments.
- Subsequent attacks by OH until ultimate mineralization.
Ozonation, UV/H2O2 and photocatalytic oxidation rely on different mechanisms of OH generation:
- H202 + UV → 2-OH
- Ozone based AOP:
- O3 + HO – 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)-H20 +h+ = Ti(IV)–OH + H+
- It could effectively eliminate organic compounds in aqueous phase, rather than collecting or transferring pollutants into another phase.
- Due to the remarkable reactivity of OH, it virtually reacts with almost every aqueous pollutants without much discrimination. AOPs could therefore be applicable in many, if not all, scenarios where many organic contaminants are expected to be removed at the same time.
- Some heavy metals could also be removed in forms of precipitated M(OH)x.
- In some AOPs designs, disinfection could also be achieved, leading AOPs to an integrated solution to some of the water quality problems.
- Since the complete reduction product of OH is H20, AOPs theoretically do not introduce any new hazardous substances into the water.
- Most prominently, costs of AOPs are too high, since a continuous input of expensive chemical reagents are required to maintain the operation of most AOPs system.
- Some techniques require pre-treatment of wastewater to ensure reliable performance, which could be potentially costly and technically demanding. For instance, presence of bicarbonate ion (HCO3-) can appreciably reduce the concentration of •OH due to scavenging processes that yield H20 and a much less reactive species, .CO3-. As a result bicarbonate must be wiped out from the system or AOPs are compromised otherwise.
- Given the potential costs, AOPs may not individually handle a large amount of wastewater; instead, AOPs should be deployed in the final stage after primary and secondary treatment have successfully removed a large proportion of contaminants.