Electric attachment of iron filings to flocs and its catalytic effect on the reaction. The coagulation of battery reaction products, adsorption of new flocs and bed filtration are the comprehensive results. Among them, the main functions are redox and electric agglomeration. The main components of scrap iron are iron and carbon. When it is immersed in electrolyte solution, there is an electrode potential difference of 1.2v between Fe and C, so numerous micro-battery systems will be formed and electric fields will be formed in their action space. The anodic reaction will generate a large amount of Fe2+, which will be oxidized to Fe3+ to form a flocculant with high adsorption and flocculation activity. Cathode reaction produces a lot of new ecology [H] and [O]. Under acidic conditions, these active components can undergo redox reaction with various components in wastewater, leading to chain-breaking degradation of organic macromolecules, thus eliminating the chromaticity of organic substances, especially printing and dyeing wastewater, improving the biodegradability of wastewater, consuming a lot of H+ and generating a lot of OH- by cathodic reaction, and also improving the pH value of wastewater.
When wastewater comes into contact with iron and carbon, the following electrochemical reactions occur:
Anode: Fe-2e —→ FeEO (Fe/Fe) = 0.4.
Cathode: 2H++2e—→H2 Eo(H+/H2)=0V.
When oxygen exists, the cathode reaction is as follows:
O2+4h++ 4e——→2H2O Eo(O2)= 1.23v
O2+2H2O+4e——→4OH-Eo(O2/OH-)= 0.4 1V
In some experiments, H2O2 is added after iron-carbon reaction, and Fe2+ generated by anode reaction can be used as a catalyst for subsequent catalytic oxidation treatment, that is, Fe2+ and H2O2 form Fenton reagent oxidation system. The new ecology [H] produced by the cathodic reaction can undergo redox reaction with various components in the wastewater, destroying the chromophoric groups (such as azo groups) in the intermediate molecules of dyes and decolorizing them. Through the iron-carbon aeration reaction, a large number of hydrogen ions are consumed, which improves the pH value of wastewater and creates conditions for subsequent catalytic oxidation treatment.
According to the principle of catalytic oxidation, adding proper amount of H2O2 solution and Fe2+ in wastewater to form reagent has strong oxidation ability, and is especially suitable for the treatment of refractory organic wastewater. Fenton reagent has strong oxidation ability, because HO is decomposed by Fe to produce OH (hydroxyl radical).
Mechanism of biochemical performance improvement and chroma removal
Micro-electrolysis has obvious effect on chroma removal. This is because the new ecological divalent iron ions produced by electrode reaction have strong reducing ability, which can reduce the chromophores nitro -—NO2 and nitroso -NO of some organic compounds to amino -—NH2, and the biodegradability of other amino organic compounds is obviously higher than that of nitro organic compounds. The divalent iron ions in the new ecology can also open the double bonds of some unsaturated chromophores (such as carboxyl -—COOH and azo -N=N-), so that the chromophores are destroyed, the chromaticity is removed, and some refractory cyclic and long-chain organic compounds are decomposed into biodegradable small molecular organic compounds, thus improving the biodegradability. In addition, bivalent and trivalent iron ions are good flocculants, especially the newly born bivalent iron ions have high adsorption and flocculation activity. Adjusting the pH value of wastewater can turn iron ions into flocculent precipitation of hydroxide, adsorb suspended or colloidal particles and organic polymers in wastewater, further reduce the chromaticity of wastewater, remove some organic pollutants and purify wastewater.
Since its birth, micro-electrolysis treatment of wastewater has attracted the attention of environmental researchers at home and abroad, and has done a lot of research! There are many patents and many practical technological achievements. In recent years, micro-electrolysis treatment of industrial wastewater has developed very rapidly, and has been applied to industrial wastewater treatment projects such as printing and dyeing, electroplating, petrochemical, pharmaceutical, gas washing, printed circuit board production and arsenic-containing and fluorine-containing wastewater, and has received good economic benefits and environmental protection effects. Micro-electrolysis has a good treatment effect on wastewater decoloration, and its operating cost is low, so it will have a good industrial application prospect in China.
At present, micro-electrolysis equipment at home and abroad are all fixed beds, which are characterized by simple structure and good flow-pushing performance, but there are many practical problems: first, the efficiency is not high and the reaction speed is not fast; Second, the bed is easy to harden, resulting in short circuit and dead zone; Third, iron filings supplement the labor intensity.
Problems in treatment of industrial wastewater by internal electrolysis
The mechanism of internal electrolysis on dyes with different structures and properties is different, so it is necessary to further explore the mechanism of decoloration and pollution reduction and the best treatment process. According to the characteristics of various dyes, especially when treating high concentration wastewater, it is necessary to find a suitable process combining coagulation, biochemical method and aeration oxidation method to effectively overcome the shortcoming of low removal rate of this method.
It solves the contradiction that the electrochemical attenuation rate of acidic wastewater is high, while the electrode adsorption of neutral acidic wastewater and the hydrolysis and flocculation of new iron ions are good. Screening effective catalysts and additives to give full play to the best effects of electrochemical attenuation candle and flocculation adsorption in a wide PH range. Especially in acid wastewater, although the decolorization rate is high, the iron dissolution and sludge amount are large. Effective measures should be taken to minimize the amount of sludge, reduce the water content of sludge and avoid secondary pollution. Choosing a suitable activation method of iron filings and designing a reasonable filter bed can solve the shortcomings of easy passivation and caking of iron filings, and improve the treatment efficiency.
Problems and countermeasures
As a wastewater treatment device, iron bed needs to be further improved in theory and practice. In actual operation, filler passivation, hardening and "color reversion" of effluent often occur, which must be properly solved in practical engineering.
Passivation of filler by 1)
After the iron bed runs for a period of time, a passive film will be formed on the surface of the filler, and the suspended particles in the wastewater will also be partially deposited on the surface of the filler, which will hinder the effective contact between the filler and the wastewater and lead to the reduction of the treatment effect of the iron bed. The operation period of iron bed should be determined according to the actual operation situation, generally about 20 d, and the soaking activation time can be 2-3 h.
2) About the problem of packaging hardening
The hardening of iron bed packing not only leads to the deterioration of wastewater flow pattern in iron bed, reduces the treatment effect, but also greatly increases the difficulty of packing replacement.
By adding appropriate auxiliary materials into the iron bed filler, the phenomenon of packing hardening can be effectively avoided, and at the same time, it is beneficial to the full contact of gas, liquid and solid ink stone and improve the treatment effect. The auxiliary material can be X50 polyethylene polyhedral hollow sphere.
The use of fluidized bed device can also solve the hardening problem of iron bed packing. However, the application of this device is limited by high investment cost, operation cost and operation management requirements.
After the iron-carbon internal electrolysis column runs for a period of time, the iron chips are easy to agglomerate and appear channeling, which greatly affects the treatment effect. At present, Wu et al. can effectively prevent the agglomeration of iron filings by using high-frequency pore-forming technology, but this technology needs further research and improvement.
Iron-carbon fluidized bed reactor is used to pretreat dye wastewater, which overcomes the shortcomings of fixed-bed iron-carbon reactor, such as easy passivation of surface, easy caking of filler and gradual decrease of operation effect with the extension of operation time.
After the internal structure of the reactor is properly adjusted, the traditional fixed bed process can be easily transformed into fluidized bed process. This can not only improve the pretreatment effect, but also greatly facilitate the operation and operation management of facilities.
3) "Color reversion" of iron bed effluent.
After decolorization by iron bed, the color of some dye wastewater gradually deepened in a short time. As for the reason of this "color reversion" phenomenon, it is generally believed that the iron bed filler reacts with wastewater, which destroys the chromophoric or chromophoric groups of dye molecules, but the dye molecules are only converted into colorless small molecular organics, which still exist in wastewater, and these small molecular organics have a certain reverse reaction trend. However, the author found through experiments that for some types of dye wastewater, when the pH value of neutralization precipitation is 8-8. 5. This phenomenon of "color reversion" is not only manifested in the gradual deepening of the color of the wastewater, but also in the gradual turbidity of the wastewater. After standing for a long time, a small amount of darker sediments will appear. After analysis, this is Fe (OH)3 precipitation. This phenomenon can be easily explained: Fe2+ is oxidized to Fe3+, and the solubility product constants of their hydrolysates Fe(OH )2 and Fe(OH) 3 are different by more than 102 1 times.
Based on the above analysis, the author thinks that completely removing Fe2+ will aggravate this "color reversion" phenomenon to some extent. Therefore, to solve the problem of "color reversion" of iron bed effluent, it is necessary not only to completely remove the color matrix in the subsequent treatment process, but also to adjust the pH value to above 9 in the neutralization and precipitation process, so as to completely precipitate Fe2+ or add appropriate oxidants (such as O2, H2O2 and O3) to rapidly oxidize Fe2+ into Fe3+, and then precipitate it in the form of Fe (0H)3 colloid.
4) Iron-carbon method is usually carried out under acidic conditions. However, under acidic conditions, the amount of iron filings dissolved is large, and there are many precipitates generated during neutralization with alkali, which increases the burden of dehydration process and makes waste residue treatment a difficult problem. At present, the waste residue is generally sent to an ironmaking plant for treatment or mixed into building materials.
Matters needing attention in iron-carbon micro-electrolysis:
1. Microelectrolytic packaging should be waterproof and anticorrosive before use. Once in operation, it should always be protected by water. Should not be exposed to the air for a long time, so as not to be oxidized in the air and affect the use.
2. Proper aeration should be paid attention to during the operation of micro-electrolysis system, and repeated aeration for a long time is not allowed;
3. Micro-electrolysis system should not run under alkaline conditions for a long time;
4. Other preventive measures can be based on the basic principle of micro-electrolysis. Oily wastewater must be separated from oil first.
5. For some special wastewater, the iron-carbon micro-electrolysis process can only break the chain, that is, break the macromolecular chain into smaller small molecular chain substances, and the COD will rise instead of falling. In this case, Fenton process will be used as a supplement to achieve better electrolytic effect.