Effect Of Degradation Intermediates Of Organic Pollutants On The Generation And Degradation Of Alkyl Sulfur In The Subsurface Flow Constructed Wetlands

The dimethyl sulfide (DMS), dimethyl disulfide (DMDS) and methyl mercaptan (MT) are the main volatile alkyl sulfides and the major source of one of odors in the constructed wetlands. These sulfides show significant impact on global warming, acid rain formation.The constructed wetland is an important integral part of the land wetland. As a unique and new sewage treatment technology, it is widely used in the many countries because of its high efficiency, low operating costs and simple operation. However, rescent works on the constructed wetland are focused on the design of wetlands and its performance for treating sewage. The wetlands are often ignored as the source of alkyl sulfur on land, very few studies have dealt with the mechanism of alkyl sulfur formation in the internal of wetland and the linkages between the intermediate degradation products of organic pollutants and the formation of alkyl sulfur. Therefore, the object of this thesis is to evaluate the production, transformatin and degradation of alkyl sulfur in the constructed wetland, and the main contents are as follows:1. Effect of commonly found degradation intermediates of organic pollutants, such as alcohols, aldehydes, acids and ketones, on production and release of DMS was studied by using sludge from the constructed wetlands. On this basis, methyl donor for DMS formation was discussed.2. The effect of sulfate, nitrate on the formation and the transformation of DMS, DMDS, MT in constructed wetlands was studied.3. A fast, efficient and environment-friendly method for degradation of DMS was investigated.The major conclusions through our study are summarized as follows: 1. In order to explore the source of DMS in the subsurface flow constructed wetland, we simulated the constructed wetland environment and studied the mechanism of DMS production by assuming that the soluble organic matter containing methyl are the methyl donor of DMS. The experimental results suggested that DMS was produced by sulfide biomethylation and degradated by microbial at the same time. Ethanol, acetic acid, acetone can inhibit the activity of methanogenic bacteria, leading to increase of DMS production. However, ethanol, acetic acid, and acetone did not significantly affect the production of DMS. The concentration of DMS produced in experiment group at the 48 h, and 72 h were close to control groups, which indicated that they are not real methyl donor. Aldehydes impact on the DMS significantly (p<0.001) and it has higher linear correlation (R2=0.937-0.984) between aldehydes and DMS. DMS production increased with increasing aldehyde concentration from 0-40 mmol/L, up to 153 ppb DMS could be produced at 40 mmol/L aldehyde level. The above results suggested that aldehyde may reaction with sulfide as methyl donor in constructed wetlands. The reaction of DOM and sodium sulfide in water suggested that only 5.32 ppb DMS can be produced when using the same formaldehyde concentration as those in above experiment. This showes DMS production by chemical methylation of sulfide, but biological pathway is the most important way. pH value has more significant on the production of DMS, the DMS production is larger under acidic conditions, but significantly reduced to disappear in alkaline conditions.2. It was found that the presence of nitrate is in favor of the production of DMDS and DMS. Nitrite can promote the production of MT and DMS. Sulfate can be reducted by SRB to sulfide, which is as S-donor for the production of DMS, thus contributing to the production of DMS. Only copper ions can promote the production of DMS, and other metal ions had no significant effect on the production of DMS. The effect of nano-materials on the role of alkyl sulfur is closely related to its nature and size. Iron oxide nano-materials can significantly affect the production and release of DMS, but carbon nanotubes can not.3. In Fenton degradation reaction, when concentration of Fe2+ is less than 1 mg/L, concentration of H2O2 is less than 5 mg/L, DMS degradation efficiency can reach the 100%, pH value has little effect on this oxidation reaction. If H2O2 is used alone, 100% DMS degradation can be reached under conditions of 1000 mg/L of H2O2, temperature of 60℃. The Degradation products are Fe3+ and SO42- and others. The activation energy of the reaction is 39.182 kJ mol/L with index of 0.4998, and 0.2284 for Fe2+, and H2O2, respectively, suggesting that this reaction is more dependent on the concentration of Fe2+.