|Alternative Title||Mechanism and application of vacuum ultraviolet/ultraviolet for degradation of typical pesticides in drinking water|
|Place of Conferral||北京|
|Keyword||农药,Vuv/uv,辐射强度,降解机制,经济可行性 Pesticide, Vuv/uv, Fluence Rate, Degradation Mechanism, Economic Feasibility|
水在185 nm VUV辐射下会裂解生成羟基自由基（HO?），从而进一步生成H2O2。H2O2的生成量随反应时间的增加而增加，反应5 h后达到最大值（约41 μM）。在反应前10 min内，H2O2的生成很好地符合伪零级反应动力学，且生成速率与VUV辐射强度成明显线性相关（r2 = 0.993）。H2O2生成的表观光量子产率被测定为0.024。随着溶液pH或者反应体积增加，H2O2的生成速率逐渐下降。溶液氧和循环流速对H2O2生成速率的影响可忽略不计。由此，本文开发了一种新的VUV辐射强度测定方法，相对于传统的测定方法，新方法具有安全无毒、灵敏度高、检测程序简单、适用性广等优点。
5种农药，包括涕灭威（ALD）、甲草胺（ALA）、地茂散（CHL）、灭虫威（MET）和莠去津（ATR），都能够有效地被VUV/UV降解，且降解反应都符合伪一级反应动力学。反应10 min后（VUV辐射剂量为12 mJ cm?2），它们的去除率分别达到了100%、100%、95.5%、98.2%和91.8%。由于VUV能够光解水产生强氧化剂——HO?， VUV/UV光降解农药的效率要比单独UV直接光降解效率高得多。ALD、ALA、CHL、MET和ATR与HO?的二级速率常数分别被测定为7.57 × 109、4.89 × 109、2.57 × 109、3.81 × 109和2.38 × 109 M?1 s?1。pH对不同农药的影响都不一样，无机离子（NO3?、Cl?、HCO3?）和天然有机物（NOM）会通过捕获HO?来抑制农药的VUV/UV光降解，而SO42?则表现出稍微的促进作用。反应30 min后，农药的矿化率仅为19.0%到42.7%之间，明显低于相应的去除率，但它们溶液的毒性明显下降。
乐果（DMT）在VUV/UV处理过程中的伪一级反应速率常数为0.38 min?1，反应10 min后的降解率达到了97.8%；而在UV处理同样时间后，DMT的降解率仅为5.2%。当初始浓度小于或大于5 mg L?1时，DMT的VUV/UV光降解分别符合伪一级或伪零级反应动力学。当初始浓度大于50 mg L?1时，DMT的零级反应速率常数不再受到初始浓度的影响，由此可以测定DMT的VUV表观光量子产率为0.19。使用气相色谱/质谱联用（GC/MS）测定了9种DMT在VUV/UV降解过程中生成的中间产物，并提出了DMT的VUV/UV光降解机制。由于DMT的硫原子容易遭受攻击，SO42?的释放速率在降解反应开始时比PO43?高，但在反应6 h后两者的释放速率都达到了81%以上，含氮无机离子（NH4+、NO3?和NO2?）的释放率则达到了90.5%。
在中试反应过程中，农药的VUV/UV降解也符合伪一级反应动力学，随着流速的增加，农药的去除率逐渐下降。无机离子和NOM同样会对农药的VUV/UV降解产生明显的抑制作用，但对于不同的农药表现出选择性的影响。水源的水质对VUV/UV降解农药有明显的影响，以源水作为进水时农药的去除率要明显低于砂滤水和炭后水。在砂滤水中VUV/UV降解农药的EEO（Electrical Energy-per-Order）值在0.27和1.52 kWh m?3 Order?1之间，而且不受溶液流速的影响。VUV/UV中试反应器连续运行80天后，所有农药去除率的下降幅度都小于10%。低EEO值和长期运行的稳定性验证了VUV/UV工艺在小规模供水中降解农药污染物的技术和经济可行性。
Pesticides have been used worldwide to increase agricultural production. Their pollution of natural water bodies therefore has been identified as a major problem in various countries due to the persistence of pesticides in aquatic environment and their potential adverse health effects. Therefore, as an advanced oxidation process, vacuum ultraviolet/ultraviolet (VUV/UV) process has recently gained increasing attention in water treatment, for it has advantages of efficient contaminant degradation, no chemical addition, simple reaction system, and easy operation and maintenance. Many previous studies have demonstrated the potential of the VUV/UV process in treating water contaminated by organic compounds. However, little research has been conducted to assess the technical and economic feasibility of its application to pesticide removal in small-scale water treatment. Therefore, this study aimed to gain an insight into the pesticide degradation by VUV/UV in drinking water, which can hopefully provide theoretical basis for its practical application.
When water is exposed to 185 nm VUV, it is readily photolyzed into hydroxyl radicals (HO?) and hydrogen atoms (H?), and afterwards H2O2 is formed. The H2O2 concentration increased with increasing reaction time until it leveled off at a quasi-stationary concentration of 41 μM after 5 h exposure. The H2O2 production followed pseudo-zero-order reaction kinetics well over the first 10 min of VUV/UV exposure. A good linear correlativity (r2 = 0.993) was achieved between the VUV photon flux and the H2O2 production rate, and the apparent quantum yield for H2O2 production by 185 nm VUV irradiation of water was determined to be 0.024. As the solution pH or reaction volume increased, the H2O2 production rate decreased. Dissolved oxygen and recirculating rate had a negligible influence on the H2O2 production. Based on these results, a novel VUV fluence rate determination method has been developed. This novel method has advantages of non-toxicity, low detection limits, low costs and convenience, and thus can be used as a good alternative to traditional actinometers that commonly use organic solvents.
Five pesticide, including aldicarb (ALD), alachlor (ALA), chloroneb (CHL), methiocarb (MET) and atrazine (ATR), could be effectively degraded by VUV/UV, and the degradation process followed pseudo-first-order reaction kinetics. ALD, ALA, CHL, MET and ATR were degraded by 100%, 100%, 95.5%, 98.2% and 91.8%, respectively, after VUV/UV irradiation for 10 min (VUV fluence = 12 mJ cm?2). The VUV/UV process was much more efficient and energy-saving than the UV process for pesticide removal due to the highly oxidative HO? produced by VUV photolysis of water. The second-order reaction rate constants of ALD, ALA, CHL, MET and ATR with HO? were determined to be 7.57 × 109, 4.89 × 109, 2.57 × 109, 3.81 × 109 and 2.38 × 109 M?1 s?1, respectively. Solution pH had varying influences on the degradation of different pesticides by VUV/UV. Inorganic anions (Cl?, NO3?, HCO3?) and natural organic matter (NOM) inhibited the pesticide degradation by scavenging HO?, but SO42? promoted the pesticide degradation slightly. After VUV/UV treatment for 30 min, the pesticide mineralization rates ranged from 19.0% to 42.7%, which were obviously lower than their corresponding degradation rates, but the toxicity of the pesticide solution significantly decreased.
Dimethoate (DMT) degradation by VUV/UV followed pseudo-first-order reaction kinetics with a rate constant of 0.38 min?1. 97.8% of DMT was degraded after 10 min VUV/UV exposure, while only 5.2 % degradation was achieved by UV photolysis after the same exposure time. When the initial concentration was lower than 5 mg L?1, the DMT degradation followed pseudo-first-order reaction kinetics. It could be fitted with pseudo-zero-order reaction kinetics when the initial concentration was higher than 5 mg L?1. However, when the initial concentration was higher than 50 mg L?1, the pseudo-zero-order reaction rate constant was independent of initial concentration. The apparent quantum yield for DMT degradation by VUV was determined to be 0.19. Nine organic degradation intermediates were identified by a gas chromatograph/mass spectrometry, and a tentative mechanism for the DMT degradation by VUV/UV was proposed. The S=P bond of DMT could be easily attacked by HO?, so SO42? release rate was higher than that of PO43? at the beginning of the exposure reaction, but their release rates both reached over 81% after 6 h of exposure. The release rate of nitrogen containing inorganic ions, including NH4+, NO3? and NO2?, reached 90.5% after the same reaction time.
The pesticide degradation followed pseudo-first-order reaction kinetics as well in pilot-scale study. The removal efficiency of each pesticide decreased with increasing flow rates. Inorganic anions (Cl?, NO3?, HCO3?) and NOM also inhibited pesticide degradation by scavenging HO?, but had selective influence on different pesticides. The water matrices had a significant influence on the pesticide degradation by VUV/UV. The pesticide removal efficiency was lower with source water as the influent than that of sand-filtered water or active carbon filtered water. The Electrical Energy-per-Order (EEO) values for the removal of mixed pesticides in the sand-filtered water in the pilot reaction system ranged from 0.27 to 1.52 kWh m?3 Order?1, which were not affected by the flow rate. The removal efficiencies of all pesticides only decreased by<10% after a long-term continuous operation for 80 days. Low EEO values and stable performance confirm the technical and economic feasibility of VUV/UV process for pesticide removal in small-scale water treatment.
The developed novel VUV fluence rate determination method shows a practical significance in reactor design, reaction rate determination and operating cost calculation. Study on the kinetics, influencing factors and mechanism has gained a deeper insight into the pollutant degradation by VUV/UV process. The results obtained from the pilot-scale study conduce to the application of the VUV/UV process in practical small scale water treatment.
|杨腊祥. 真空紫外紫外降解饮用水中典型农药的机理和应用研究[D]. 北京. 中国科学院生态环境研究中心,2018.|
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