Microwave technology for organic wastewater treatment
At present, the treatment methods of organic wastewater mainly include physical, chemical and biological methods. The physical method only transfers the pollutants in the wastewater from phase to phase, but does not eliminate the pollutants fundamentally. The biological method is only suitable for the treatment of biodegradable organic wastewater; There are many kinds of chemical methods, including air wet catalytic oxidation, supercritical oxidation, ozone catalytic oxidation, electrocatalytic oxidation, etc., but there are also shortcomings such as high investment and operating costs and harsh operating conditions.
Microwave is an electromagnetic wave with a wavelength of 1mm~1m and a frequency of 300MHz~300GMHz, which can interact with chemical media and produce a variety of application effects, and has been rapidly developed in recent years. More and more researchers have applied microwave technology to water treatment technology and achieved good treatment effect.
1. Degradation mechanism of microwave wastewater treatment technology
The energy of microwave radiation alone is low, not enough to break the chemical bonds of organic molecules, and it is difficult to directly degrade organic pollutants. Microwave radiation combined with other technologies can effectively improve the ability to treat pollutants. At present, the principle of microwave radiation combined with other technologies to remove organic matter mainly includes microwave thermal effect, hot spot effect and free radical theory.
1.1 Thermal effect
The phenomenon of microwave energy being absorbed into the medium material and converted into heat energy is called thermal effect. In the process of microwave-enhanced Fenton reaction, the thermal effect of microwave can reduce the chemical bond strength of molecules and accelerate the formation of ·OH, which is conducive to promoting the oxidation reaction. However, too high temperature or too fast warming in the reaction process will also affect the effective utilization rate of H2O2.
1.2 Hot Spot effect
Substances with strong wave-absorbing properties (such as activated carbon, transition metals and oxides) after microwave radiation, the surface point and microwave energy strongly interact, so that the surface point selectively quickly rise to a very high temperature, forming the active center is "hot spot"; After the organic matter in the wastewater is in contact with the "hot spot", it is either accelerated adsorption, high temperature pyrolysis, or degraded due to catalytic oxidation. On the other hand, the "hot spot" can cause the vibration of atoms and molecules, reduce the activation energy required for the reaction, and is conducive to the reaction.
1.3 Free radical theory
It is generally believed that the addition of oxidants will produce ·OH, SO4·- and other active substances, which play an oxidation role in the process of degrading organic pollutants. Oxygen or other oxidants play the role of electron acceptor in the microwave catalytic oxidation system, which is essential for the degradation of pollutants. There is also a theory that under microwave irradiation, the catalyst supported metal or metal oxide will also produce ·OH, O2·-, h+ and other active substances, without the need to add additional oxidants.
2. Application of microwave technology in the field of organic wastewater treatment
2.1 Microwave combination Fenton technology
Fenton technology is a classic advanced oxidation treatment process, which has a good treatment effect, especially for organic pollutants that are difficult to degrade in sewage. However, Fenton process also has shortcomings such as long reaction time, large footprint, and large amount of sludge. In order to seek an economical and efficient water treatment technology, researchers combined microwave technology and Fenton process to strengthen oxidation reaction, which has a high treatment efficiency and a good application prospect.
A microwave-Fenton technique was used to treat antibiotic wastewater. The results show that microwaving Fenton technology can effectively improve the biodegradability of antibiotic wastewater, and is superior to Fenton technology in both COD removal rate and biodegradability. The reaction time is short, the footprint is small, it is easy to optimize operation, and it has high economic feasibility. The microwave assisted rapid Fenton combined process is used to pretreat complex wastewater from wastewater treatment plant, which can reduce the COD of wastewater from about 7,000 mg/L to 2,500 mg/L, and the COD removal rate can reach more than 65%. Meanwhile, the biodegradability of wastewater is improved. The methylene blue in aqueous solution is removed by microwave-enhanced Fenton process. After 1min of microwave irradiation, the degradation rate of methylene blue is as high as 93%, and the treatment efficiency is much higher than that of traditional Fenton process. NannanWang et al. used microwave enhanced Fenton technology to treat p-nitrobenzene in water. The ·OH produced by microwave enhanced Fenton method within 7min is 2.8 times higher than that produced by traditional Fenton method. Under the best conditions, the degradation rate of p-nitrophenol is as high as 92.3%.
2.2 Microwave combination Fenton-like technology
In view of the shortcomings of Fenton technology in practical application, researchers have developed various Fenton technologies on the basis of conventional Fenton method. The combination of Fenton-like technology and microwave has a high treatment effect on organic pollutants in sewage.
2.2.1 H2O2 is an oxidizing agent
The degradation of perfluorooctanoic acid (PFOA) in aqueous solution was studied by combining microwave with H2O2 using Pb-BiFeO3/rGO as catalyst. The results show that the combined process has a high removal efficiency for PFOA. When the initial mass concentration is 50mg/L and the reaction time is 5min, the removal rate can reach 99.2%. XinliangLiu et al. prepared an activated carbon iron-carrying catalyst. Under the synergic action of microwave - catalyst -H2O2, the removal rates of phenol and TOC were 91% and 48%, respectively. The microwave-assisted Fenton-like system was used to treat p-nitrophenol (PNP) wastewater, and its reaction mechanism was discussed. The results show that for PNP solution with initial mass concentration of 50mg/L, the removal rate of PNP is 99.41% and the removal rate of TOC is 77.9% after 6min of reaction under static optimum process conditions. The removal rate of PNP remained above 80% and the removal rate of TOC was 71.2% under the dynamic optimal process conditions for 20min. The introduction of microwave and catalyst can promote the formation of H2O2 ·OH, and significantly increase the removal rate of PNP. Using fly ash as catalyst, the removal effect of Rhodamine B by microwave combined Fenton process was studied. The results show that metal oxides (Fe2O3, MnO2) in fly ash can rapidly absorb microwave to form active centers, catalyze H2O2 to generate ·OH, and then remove Rhodamine B. Under the conditions of 2mmol/L H2O2 dosage, 15g/L fly ash dosage, pH 3, microwave power of 0.1kW and microwave irradiation time of 20min, the degradation rate of Rhodamine B reached 91.6%. The microwave-catalyzed oxidation of Rhodamine B was studied using Fe3O4 as catalyst. The results showed that under the conditions of 0.4g/L catalyst dosage, n (H2O2) : n (Rhodamine B) = 1:1 and pH 2.4, Rhodamine B with an initial mass concentration of 300mg/L was completely degraded within 7min, the TOC removal rate was 97.66%, and the catalyst could be reused for 7 times. ·OH plays an important role in the degradation of Rhodamine B. The microwave catalytic oxidation and degradation of phenol was studied by using Cupric based silicon carbide (Cu/SiC) as catalyst. The results showed that microwave assisted Cu/SiC could effectively degrade phenol with H2O2 or persulfate, and ·OH and SO4·- were the main active components.
2.2.2 Sulfate is an oxidizing agent
The degradation of 4-nitrophenol in Fe3O4/ activated sulfate/microwave system was studied. The results showed that Fe3O4/ activated sulfate/microwave system could effectively degrade 4-nitrophenol. Under the conditions of 0.1g/L Fe3O4 dosage, m (activated sulfate) : m (4-nitrophenol) 15, microwave temperature 80℃ and reaction time 28min, the degradation rate of 4-nitrophenol was as high as 98.2%. When the initial pH was 3, the dosage of persulfate was 8g/L and the microwave power was 600W, the COD removal rate and decolorization rate were 82.29% and 77.89%, respectively. The active substances in the reaction process are ·OH and SO4·-, of which SO4·- plays a major role in the oxidation process. The degradation of PNP by Fe3O4/ activated sulfate/microwave system was studied. The initial mass concentration of PNP was 20mg/L, the dosage of Fe3O4 was 2.5g/L, and n (activated sulfate) : n (PNP) =15: 1. When the reaction temperature is 80℃ and the microwave irradiation time is 28min, the removal rate of PNP is as high as 94.2%.
2.3 Microwave bonded absorbing materials
The wave-absorbing material can strongly absorb microwave energy, forming a "hot spot." After contact with the "hot spot", the organic matter in the sewage is degraded by adsorption, pyrolysis, or the oxidation of the generated active substances (h+, ·OH, O2·-) by microwave-excited electron-holes.
2.3.1 Activated carbon based catalyst
Phenol in wastewater was degraded by CuO/AC as catalyst. The results showed that under the optimal experimental conditions of CuO/AC dosage of 30g/L, microwave power of 600W, reaction time of 18min and initial mass concentration of phenol of 500mg/L, the phenol removal rate could reach 99.42% and the corresponding TOC removal rate was 90.4%. ·OH plays an oxidant role in the process of microwave catalytic oxidation, and no additional oxidant is required in the reaction process. The removal of phenol from water by microwave irradiation combined with activated carbon was studied. The results show that microwave irradiation can enhance the adsorption of activated carbon and increase the removal rate of phenol by 16.6%~29.5% without any oxidant. Microwave /Fenton/ activated carbon system was used to degrade phenol solution. The experimental results show that after microwave treatment, the average particle size of activated carbon becomes significantly smaller, surface micropores increase, and impurities decrease, which strengthens the adsorption effect and increases the reactive site.
2.3.2 New material catalyst
Using MgFe2O4-SiC as catalyst, the microwave catalytic oxidation and degradation of azo dye DirectBlackBN were studied. The removal rate of DirectBlackBN and TOC was as high as 96.5% and 65% in 5min. The degradation mechanism is that SiC can strongly absorb microwave due to its dielectric property, and MgFe2O4 loaded on SiC surface forms many hot spots, producing a large number of active sites and holes, and electron-hole pairs react with O2 or H2O to produce active substances such as ·OH. The catalyst was prepared with sepiolite as the carrier and NiFe2O4 as the active component. Under microwave irradiation, water molecules in wastewater decompose into ·OH and ·H after contact with hot spots, and O2 in wastewater reacts with ·H to form ·OH and O2·-. In addition, NiFe2O4 forms electrons and holes under microwave irradiation, the migrating electrons react with O2 to produce O2-, and h+ oxidizes water to ·OH. ·OH, O2·-, h+ and other active components are the key to the degradation of organic pollutants. A series of catalysts were prepared with montmorillonite and perovskite. The results show that LCCOM0.2 has A significant effect on the removal of bisphenol A (BPA), and the degradation rate of BPA with an initial mass concentration of 50mg/L is as high as 99.8%. The degradation mechanism is that the surface active site of the catalyst is stimulated by microwave to produce strong oxidizing ·OH. With magnetically separated NiCo2O4-Bi2O2CO3 catalyst, the degradation rate of 4-nitrophenol can reach 94.7% when the reaction time is 1min, and the degradation rate of 4-nitrophenol is 97.27% when the reaction time is 4min. After repeated use of NiCo2O4-Bi2O2CO3 for 5 times, the degradation rate of 4-nitrophenol can still reach 97.31%. A BiO2CO3 composite catalyst was developed for the degradation of nitrophenol. It was found that O2·- is the main active substance produced by BiO2CO3 matrix composite, which has very good catalytic activity for the degradation of ortho-nitrobenzene and p-nitrobenzene. Under optimal conditions, the removal rate of nitrobenzene is as high as 98.96%.
2.4 Microwave-assisted photocatalytic oxidation technology
Photocatalytic oxidation is a kind of technology with strong oxidation ability. The combination of microwave and photocatalytic oxidation technology is one of the hotspots in the field of photocatalysis. It has been pointed out that microwave irradiation can improve the ability of the catalyst to absorb light, promote the production of ·OH, and improve the effect of photocatalytic treatment.
The microwave-assisted photocatalytic oxidation system was used to degrade 2, 4-dichlorophenoxyacetic acid. It was found that there was a synergistic effect between ozone, microwave and ultraviolet photocatalysis. Compared with the microwave /UV photocatalytic oxidation system, the addition of ozone greatly improved the degradation effect of 2, 4-dichlorophenoxyacetic acid. Microwave radiation plays an important role in ozone-assisted photocatalysis of organic pollutants in water. The rate and mechanism of free radical generation in microwave combined ultraviolet system were studied. It is believed that the degradation of organic matter is mainly caused by the oxidant producing a large number of free radicals under the action of microwave/ultraviolet lamp, which significantly improves the degradation efficiency of organic matter. The degradation of 4-chlorophenol was carried out in a microwave electrodeless light catalytic oxidation reactor. The degradation effect of 4-chlorophenol under single process and combined process was investigated. The results showed that the degradation rate of 4-chlorophenol was the highest under microwave irradiation /TiO2/H2O2. The bactericidal ability of single O3, single microwave and microwave UV /O33 processes on Escherichia coli and Bacillus subtilis was compared. The results show that the microwave UV lamp /O3 has the best effect, the sewage sterilization rate can reach 99.99%, and the effluent TOC and COD are reduced.
3. Conclusion and prospect
Microwave wastewater treatment technology has the advantages of high degradation efficiency and short treatment time. With the continuous optimization of materials, structures and other aspects, this technology has excellent application prospects, but the following problems still need to be solved
(1) At present, the mechanism of microwave degradation of organic matter in wastewater mainly includes hot spot effect and free radical theory, but the reaction between microwave and absorbing materials, oxidants and organic matter is a complex process, and its degradation mechanism needs further study.
(2) Researchers have selected and developed new and efficient catalyst carriers and active components, but they are limited to small trials at present; Studies on the stability of catalysts are limited to a small number of repeated experiments, and large-scale stability verification experiments need to be carried out.
(3) Most of the research on microwave wastewater treatment technology is carried out under static conditions, and dynamic experimental research needs to be carried out to provide theoretical basis and reference for future engineering applications.
(4) Microwave wastewater treatment technology is still in the development stage, the form of microwave reactor, catalyst arrangement, microwave and other oxygen
