Study On The Analytical Methods And The Removal Mechanism Of The Degradation Intermediates Of Organic Contaminants In The Constructed Wetlands Treating Wastewater

Constructed Wetland(CW) is one of wastewater treatment systems developed since 50 to 60's in the 20th century.Since the appearance of the root zone theory,which was firstly proposed by Kickuth in 1972,the constructed wetland as a new wastewater treatment technology has entered into the field of water pollution control.CW promises high efficiency for removal of contaminants in wastewaters,easy operation,low cost for operation and maintenance.And it is especially suitable for wastewater treatment in the area of the small town and countryside.However,due to the complication and the"black box"mechanism involved in contaminant removal in CW,knowledge regarding the fate and dynamics of contaminants and their fate in constructed wetland system is highly limited.And great efforts have been paid to modification of CW technology.Currently,the available information on the performance of these systems is limited to common contaminants,such as chemical oxygen demand(COD),biochemical oxygen demand(BOD₅).Very little study on the chemical constitutes of COD,BOD₅,and specific degradation intermediates of organic contaminants in the operation process of constructed wetlands.It is well known that the dissolved organic matter plays an important role in the process of biochemical wastewater treatment.Therefore,it is essential to systematically study the organic matter and its degradation intermediate products in the constructed wetland system in order to improve its performance.Solid-phase microextraction is a green extraction technology derived from the solid-phase extraction.Solid-phase microextraction is organic solvent-free.Using solid-phase microextraction can greatly simplify the analytical processes.It is suitable to analysis and determination of organic compounds and their degradation intermediates in complex wastewaters,and especially for the analysis of volatile organic compounds. Under the financial support of the Natural Science Foundation of China(No.20477033),this dissertation focused on the degradation and mechanism of the organic contaminants in the subsurface constructed wetlands,and the main contents are as follows:(1) Headspace solid-phase microextraction in combination with GC-FID for quantification of the volatile free fatty acids in wastewater from constructed wetlands has been developed.It is based on the selective adsorption of VFAs onto polyacrylate(PA) fiber.(2) Effect of design and operation parameters on removal of volatile fatty acids in the subsurface constructed wetlands were investigated through the statistic analysis of parameters effects.(3) Effect of organic loading rate,sulfate,temperature and hydraulic retention time on the behavior of volatile short chain fatty acids was investigated in a pilot-scale constructed wetlands system.The removal mechanism of degradation intermediates of organic contaminants in the constructed wetland has also been studied.(4) A new method for the determination of small molecule organic sulfur such as alkanethiols has been developed based on the headspace solid-phase microextraction combined with gas chromatography-flame photometric detector(FPD).The optimized conditions for the analysis of alkanethiols were obtained.The major conclusions are summarized as follows:1.Solid-Phase Microextraction in Combination with GC-FID for Quantification of the Volatile Free Fatty Acids in Wastewater from Constructed WetlandsA headspace solid-phase microextraction(HS-SPME) followed by gas chromatography coupled to flame ionization detector(GC-FID) has been developed for the direct determination of volatile fatty acids(VFAs) in wastewater from constructed wetlands.Through the investigation on extraction behavior of different coatings for VFAs and the chromatographic behavior of VFAs in Stabilwax-DA capillary column(30m×0.32mm i.d.,0.25μm film thickness) using nitrogen as carrier gas,the polyacrylate(PA) fiber was selected as the satisfactory coating for VFAs analysis, and optimum chromatographic conditions were obtained.The oven temperature was programmed at 70℃for 2 min,then a 4℃/rain rate until 180℃,holding 2 min,and then a 20℃/min rate until 200℃,holding the final temperature for 3 min.The various parameters including extraction time,extraction temperature,pH value,ion strength,sample volume and desorption conditions were investigated for optimization of HS-SPME performance for VFAs in the wastewater.The experimental results show that the linear dynamic ranges were 10-45000μg/L,and over two to four orders of magnitude depending on different acids.The detection limit(3σ) for VFAs were low toμg/L levels and the RSDs were less than 10%,and the recoveries were between 85%and 117%. By using 2-ethylbutyric acid as internal standard,the proposed method has been successfully applied to determination of VFAs in wastewater from the constructed wetlands.2.Effect of Design and Operation Parameters on Removal of Volatile Fatty Acids in Subsurface Flow Constructed WetlandsA pilot-scale subsurface constructed wetland wastewater treatment system was sampled for one year to study the effect of bed aspect ratio,substrate medium size,water depth,HLR(hydraulic loading rate) and temperature(season) on removal of VFAs.The yearly experimental results demonstrated that the system showed good performance for VFAs removal in wastewater under different HLR ranging from 12 cm d^-1 to 86 cm d^-1.The system removed 64%of acetic,86%of propionic acid,respectively.The results showed that HLR and water temperature(season) were major factors that controlled the system performance for the target analytes.According to ANOVA test,the HLR caused significant differences(p＜0.05) on the average acetic,isopentanoic, hexanoic acids effluent concentrations.And temperature caused significant differences(p＜0.05) on the average effluent concentrations of all target analysts.Deep water depth caused better removal for acetic and propionic acids.However,bed aspect ratio,substrate medium size and water depth did not cause significant differences(p＞0.05) on the average VFAs effluent concentrations. The survey of dissolved oxygen and redox potential(ORP) indicated that the constructed wetlands system showed strong reduced condition.On the basis of investigations of electron acceptors(such as SO₄^2-,NO₃^- and NO₂^-) and dissolved organic pollutants(such as TOC) concentrations along with the length of constructed wetlands,it can be concluded that anaerobic biology processes such as sulfate reduction and methanogenisis were the main mechanism for VFAs removal in constructed wetland beds.3.Effects of Organic Loading Rate,Temperature,Sulfate,Hydraulic Retention Time On the Removal of the Volatile Short Chain Fatty Acids in a Pilot-Scale Constructed WetlandsThis study investigated the effect of organic loading rate(OLR),temperature,sulfate concentration and hydraulic retention time(HRT) on the removal of volatile short chain fatty acids (VFAs) in a pilot-scale subsurface constructed wetland wastewater treatment system.The results indicated that OLR and COD/SO₄^2- ratio are two important factors affecting the VFAs removals. Higher OLR,higher temperature,and relative longer retention time(i.e.2.6 d) favored the VFAs removal in the studied wetland during the sampling period.Influent COD/SO₄^2- ratio was about 6, suggesting that methanogensis was probably a dominant pathway for acetic acid degradation in the present SSFW system.The possible mechanism for VFAs removal in SSFW was also discussed.4.Headspace Solid-Phase Microextraction Combined with Gas Chromatography -Flame Photometric Detection for the Analysis of Alkylthiols in WastewaterA headspace solid-phase microextraction combined with gas chromatography/flame photometric detection has been developed for the analysis of alkylthiols in wastewater from constructed wetlands.The effect of various extraction parameters such as extraction coatings, extraction time,temperature,sample volume,salt effect,sample pH value on the extraction of alkylthiols,namely,EtSH,Me-PrSH,1-PrSH,2-PrSH,1-BuSH and CycloPeSH were investigated and optimized.The enriched alkylthiols were separated by DB-VRX(60m×0.25 mm×1.40μm) capillary column and detected by FPD.Under the optimized conditions,the linear dynamic ranges for EtSH,Me-PrSH,1-PrSH,2-PrSH,1-BuSH were in the range of 0.12～16.21μg/L.The detection limit(3σ) for the studied five alkylthiols were 0.97～7.89 ng/L with RSD(n＝11) ranging from 2.8%to 7.5%.By using CycloPeSH as internal standard,the proposed method has been applied successfully to the analysis of alkylthiols in wastewater from constructed wetlands with satisfactory results.