| Pesticides are widely used to protect crops from the grass harm and insect pests. However, the prolonged and excessive uses of pesticides result in their residues in various environmental matrices. Alachlor (2-chloro-2’,6’-diethyl-N-(methoxymethyl) acetanilide), an important chloroacetanilide herbicide, has been widely used in the control annual grasses and broadleaf weeds in corn, peanut, soybeans, as well as sorghum. As present, alachlor has been detected in soil, surface water and even groundwater around the world because of its high solubility in water (240 mg/L at 25℃) and stability. The half-lives of alachlor in soil and water are over 70 and 30 days, respectively.as a highly toxic endocrine disrupting chemical, alachlor may hurt eyes, liver, kidneys, spleen, and even induce cancer with long-term exposure. In fact, it has been classified as a B2 carcinogen by the Environmental Protection Agency of the United States with a maximum contaminant level of 2μg/L in drinking water. Because of the high toxicity and chemical stability, alachlor could not be effectively degraded by conventional biological remediation processes. Therefore, it is of great importance to develop effective and environmental friendly methods to remove alachlor.Fenton (Fe(Ⅱ)/H2O2 or Fe(Ⅲ)/H2O2) systems, which could generate ·OH, have been extensively studied in view of their high efficiency, simplicity and environmental benignancy, as well as its potential applications in the degradation of persistent organic pollutants. Although Fenton systems have been utilized to degrade organic compounds since the 1890s, there are still some drawbacks limiting their further application in industrial wastewater treatment. The rapid reaction between Fe(Ⅱ) and H2O2 could exhaust Fe(Ⅱ) after 30 s, resulting in the low utilization efficiency of H2O2. Moreover, ferric ions are quickly accumulated during the Fenton reactions, because the reaction rate of Fe(Ⅲ) and H2O2 is much slower than that of Fe(Ⅱ) and H2O2. The formed ferric ions are inclined to precipitate as iron hydroxides, which would block the Fe(Ⅲ)/Fe(Ⅱ) cycle and thus slow the Fenton reactions. It was reported that aminopoly(carboxylic acid)s (e.g. ethylenediaminetetraacetic acid (EDTA) and ethylenediamine-N,N’-disuccinic acid (EDDS)) were able to prevent iron precipitation for their chelation ability with ferric ions. However, the wide application of EDTA may cause some adverse environmental consequences because of its poor biodegradability. Moreover, EDTA and EDDS could not accelerate the Fe(Ⅲ)/Fe(Ⅱ) cycle to promote the Fenton oxidation.This dissertation mainly focused on the utilization of protocatechuic acid (PCA) and hydrothermal carbons to realize the effective Fe(Ⅲ)/Fe(Ⅱ) cycle and then improve the oxidation performance of Fenton system. As H2O2 and Fe(Ⅲ) are ubiquitous in natural water environments in spite of tiny amount, Fe(Ⅲ)/H2O2 Fenton like reaction might take place in various natural aquatic environments and thus participate in the transformation of organic contaminants. Therefore, the widely present phenolic acids and hydrothermal carbons are highly likely to promote the conversion of organic contaminants in natural aquatic environments via Fenton oxidation processes. The results were summarized as follows.1. In this study, we demonstrated that PCA possesses remarkable chelation ability with ferric ions and reduction ability to reduce Fe(Ⅲ) to Fe(Ⅱ) at acidic pH. It was found that the addition of protocatechuic acid could increase the alachlor degradation rate by 10 000 times in this Fenton oxidation system at pH=3.6. This dramatic enhancement of alachlor degradation was attributed to the complexing and reduction abilities of protocatechuic ligand, which could form complexes with Fe(Ⅲ) to prevent the their precipitation of Fe(Ⅲ) and also realize the effective Fe(Ⅲ)/Fe(Ⅱ) cycle to enhance the OH generation. Meanwhile, the alachlor degradation efficiency in Fe(Ⅲ)/PCA/H2O2 system was higher than that of Fe(Ⅲ)/EDTA/H2O2 and Fe(Ⅲ)/EDDS/H2O2 systems at near natural pH even in the case of PCA concentration as low as 0.1 mmol/L. More importantly, both alachlor and PCA could be effectively mineralized in this Fenton system, suggesting the environmental benignity of PCA/Fe(Ⅲ)/H2O2 Fenton like system. We employed gas chromatography-mass spectrometry to identify the degradation intermediates of alachlor and then proposed a possible alachlor degradation mechanism in this novel Fenton oxidation system.2. FeCl3·6H2O and protochatechuic acid (PCA) was used as raw materials to synthesize Fe-PCA metal-organic frameworks through a simple solvothermal method. Systematical characterizations were applied to investigate the element distribution, element composition, and the surface functional groups of Fe-PCA. It was found that the obtained Fe-PCA exhibited favourable ability to decompose H2O2 to produce ·OH for the degradation of organic pollutants. During the reaction of Fe-PCA and H2O2, the structure of Fe-PCA was destroyed. So partly of iron ions and PCA were released to the reaction solution. The released PCA could reduce Fe(Ⅲ) to Fe(Ⅱ), and thus improve the oxidation performance of the Feton system. Though the structure of Fe-PCA was destroyed, the degradation percentage of alachlor still reached 83.3% after four cycles.3. Hydreothemal carbons were synthesized by a facile hydrothermal approach to simulate the natural coalification process. Subsequently, the effects of hydrothermal carbon on the Fe(Ⅲ)/H2O2 Fenton like reaction and the subsequent alachlor degradation were systematically investigated. It was found that hydrothermal carbon could enhance the alachlor degradation with Fe(Ⅲ)/H2O2 by promoting the Fe(Ⅲ)/Fe(Ⅱ) cycle via electron transfer from hydrothermal carbon to ferric ions. The electron spin resonance spectra analysis revealed that hydrothermal carbon was of abundant carbon-centered radicals to act as the electron donor. Meanwhile, hydroxyl groups on the surface of hydrothermal carbon also played an important role in the enhanced alachlor degradation. The decrease of surface hydroxyl group significantly depressed the alachlor degradation. On the basis of these results, we proposed that Fe(Ⅲ) could complex with surface hydroxyl groups of hydrothermal carbon to favor the electron transfer from the hydroxyl groups to Fe(Ⅲ) and the subsequent Fe(Ⅱ) generation. The reusability and the stability of HTC were also studied. There was no obvious decrease in degradation efficiency after seven recycles, suggesting the reusability of HTC.4. To realize the effective use of natural resources, the compost products of chicken manures were utilized to synthesize hydrothermal carbons. Though the hydrothermal carbons synthesized with the compost products of chicken manures could not enhance the alachlor degradation in the Fe(Ⅲ)/H2O2 Fenton like system, the hydrthemal carbons modified with glucose could effectively accelerate the alachlor efficiency in the Fe(Ⅲ)/H2O2 Fenton like system. Moreover, the alachlor degradation rate increased with the increasing of glucose modification amount. As indicated the electron paramagnetic resonance and attenuated total reflectance-fourier transform infrared spectroscopy spectra, the glucose modification could significantly increase the amount of carbon-centered radicals and oxygen containing surface functional groups in the hydrothermal carbons, and thus improve the alachlor degradation in the Fe(Ⅲ)/H2O2 Fenton like system. The glucose modified hydrothermal carbons also exhibited excellent reusability. There was no obvious decrease in degradation efficiency after seven recycles... |
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