1 Treatment process
1.1 Overview of Fenton Oxidation
The essence of the Fenton method is the chain reaction between divalent iron ions (Fe2+) and hydrogen peroxide to catalyze the formation of hydroxyl radicals, with a strong oxidation capacity, and its oxidation potential is second only to fluorine, up to 2.80V. The inorganic chemical reaction process is that the mixed solution of hydrogen peroxide (H2O2) and divalent iron ions (Fe2+) oxidizes many known organic compounds such as carboxylic acids, alcohols, and esters into an inorganic state.
In addition, hydroxyl radicals have high electronegativity or electrophilicity, and their electron affinity can be as high as 569.3kJ with strong addition reaction characteristics, so fenton reaction has a high ability to remove refractory organic pollutants, and is widely used in wastewater treatment such as printing and dyeing wastewater, oily wastewater, phenolic wastewater, coking wastewater, nitrobenzene wastewater, diphenylamine wastewater and so on.
1.2 Oxidation mechanism
Fenton oxidation is a method in which H2O2 generates hydroxyl radicals with strong oxidation capacity under acidic conditions (· OH), and trigger more other reactive oxygen species to achieve degradation of organic matter, whose oxidation process is a chain reaction. Among them, the OH is generated as the beginning of the chain, while other reactive oxygen species and reaction intermediates constitute the nodes of the chain, each reactive oxygen species is consumed, and the reaction chain is terminated.
The reaction mechanism is more complex, these reactive oxygen species are only available to organic molecules and mineralized into inorganic substances such as CO2 and H2O. This makes Fenton oxidation one of the important advanced oxidation technologies.
When Fenton discovered Fenton's reagent, it was unclear exactly what oxidant had reacted hydrogen peroxide with divalent iron ions to produce such a strong oxidation capacity. More than twenty years later, it was hypothesized that hydroxyl radicals might have been produced in the reaction, otherwise the oxidation would not have been as strong.
Therefore, a more widely cited chemical reaction equation was later used to describe the chemical reactions that occur in Fenton's reagent: Fe2++ H2O2→Fe3++ OH-+ OH• (1) As can be seen from the above formula, 1mol of H2O2 and 1mol of Fe2+ react to form 1mol of Fe3+, accompanied by the formation of 1mol of OH- plus 1mol of hydroxyl radicals. It is the presence of hydroxyl radicals that give Fenton's reagents a strong oxidation capacity. It is calculated that in a solution with pH = 4, the oxidation potential of the •OH radical is as high as 2.73 V.
In nature, oxidation capacity is second only to fluorine gas in solution. Therefore, persistent organic compounds, especially aromatic compounds and some heterocyclic compounds that are difficult to oxidize by the usual reagents, are all degraded by indiscriminate oxidation in front of Fenton's reagents. In 1975, walling C, a famous American environmental chemist, systematically studied the types of various free radicals in Fenton's reagent and the role fe plays in Fenton's reagent, and came up with the following chemical reaction equation:
H2O2+ Fe2+→ Fe3++ O2• + 2H +(2) O2+ Fe2+→ Fe3++ O2•
(3) It can be seen that in addition to the production of 1mol of OH radicals in Fenton reagent, it is also accompanied by the generation of 1mol of peroxygen radical O2•, but the oxidation potential of peroxy radicals is only about 1.3 V, so the main oxidation role in Fenton reagent is OH•free radicals.
1.3 Brief description of the Fenton system process flow
In the second sedimentation well with Fenton feeding pumped to the Fenton oxidation tower, the refractory pollutants in the wastewater oxidation degradation, Fenton oxidation tower effluent self-flow to the neutralization pool, in the neutralization pool to add liquid alkali, the wastewater neutralization to neutrality; neutralization pool wastewater self-flow to the degassing tank, by blowing air stirring, a small amount of bubbles in the wastewater is removed; the degassing tank effluent self-flow to the coagulation reaction tank, in the pool is added to the flocculant PAM and fully reacted, so that the iron sludge in the wastewater flocculation After the coagulation reaction, the wastewater flows from the water to the final sedimentation tank, and the iron sludge in it is precipitated, and the supernatant is discharged according to the standard. The final sink iron sludge is pumped by the sludge to the original sludge treatment system for treatment.
2 Influencing factors
2.1 Temperature
Temperature is one of the important influencing factors of the Fenton reaction. General chemical reactions will speed up the reaction speed with the increase of temperature, and the Fenton reaction is no exception, and the temperature increase will accelerate · Oh's generation speed helps · OH reacts with organic matter to improve the oxidation effect and coD removal rate; however, for a complex reaction system such as Fenton reagent, the temperature increase not only accelerates the positive reaction, but also accelerates the side reaction, and the temperature increase will also accelerate the decomposition of H2O2, decomposing into O2 and H2O, which is not conducive to · Generation of OH.
There are also some differences in the optimal temperature of the Fenton reaction for different types of industrial wastewater. When treating aqueous polyacrylamide solutions, the optimal temperature is controlled at 30 °C to 50 °C.
When studying the treatment of washing glue wastewater, it was found that the optimal temperature was 85 °C. When treating trichloro(phenyl)phenol, when the temperature is lower than 60 ° C, the temperature helps the reaction to proceed, and conversely, when it is higher than 60 ° C, it is not conducive to the reaction.
2.2pH
In general, Fenton reagents react under acidic conditions, and Fe2+ cannot catalyze the oxidation of H2O2 in neutral and alkaline environments. OH, and will produce iron hydroxide precipitation and lose its catalytic ability. When the concentration of H+ in the solution is too high, Fe3+ cannot be smoothly reduced to Fe2+, and the catalytic reaction is hindered.
A number of studies have shown that Fenton's reagent has a strong oxidation capacity under acidic conditions, especially pH at 3 to 5, and the organic matter degradation rate at this time is the fastest, which can be degraded in just a few minutes. At this point, the reaction rate constant of the organics is proportional to the initial concentration of Fe2+ and hydrogen peroxide. Therefore, when using the Fenton process in engineering, it is recommended to adjust the wastewater to = 2 to 4, which is theoretically the best at 3.5.
2.3 Organic substrates
For different types of wastewater, the dosage and oxidation effect of Fenton's reagent are different. This is because different types of wastewater, the types of organic matter are different.
For carbohydrates such as alcohols (glycerol) and sugars, under the action of hydroxyl radicals, the molecules undergo dehydrogenation reactions, and then the C-C bond is broken; for the sugars of macromolecules, the hydroxyl radicals break the glycoside bonds in the sugar molecular chain, degrading to form small molecular substances; for water-soluble polymers and ethylene compounds, hydroxyl radicals make C=C bonds broken; and hydroxyl radicals can make the aromatic compounds open the ring and form fatty compounds, thereby eliminating the biotoxicity of reducing the wastewater of this type. Improve its biochemistry; for dyes, hydroxyl radicals can open the unsaturated bonds of functional groups in the dye, so that the dye can be oxidized and decomposed to achieve the purpose of decolorization and coD reduction.
Experiments on the degradation of chitosan with Fenton's reagent showed that when the pH of the medium is 3 to 5, the molar ratio of glycan, H2O2 and catalyst is 240:12~24:1~2, the Fenton reaction can break the glycosidal bonds in the chitosan molecular chain, thereby generating small molecule products.
2.4 Hydrogen peroxide and catalyst dosage
The Fenton process needs to determine the dosage and economy of the agent when treating wastewater. The amount of H2O2 is large, and the removal rate of wastewater COD will increase, but when the amount of H2O2 is increased to a certain extent, the removal rate of COD will slowly decrease. Because the amount of H2O2 dosing increased in the Fenton reaction, · The yield of OH will increase, and the removal rate of COD will increase, but when the concentration of H2O2 is too high, hydrogen peroxide will decompose and no hydroxyl radicals will be produced.
The amount of catalyst is also the same as the amount of hydrogen peroxide, in general, increase the amount of Fe2+, the removal rate of wastewater COD will increase, when Fe2+ increases to a certain extent. The rate of COD removal began to decline. The reason is because when the Fe2+ concentration is low, as the Fe2+ concentration increases, H2O2 produces · OH increases; when the concentration of Fe2+ is too high, it will also cause the ineffective decomposition of H2O2 and release O2.
Source: Eco Little Bee