Kinetic modeling and rate constant determination of Vacuum-UV oxidation of BAM in Danish groundwater
Author
Breinholt, Lars
Term
4. term
Education
Publication year
2020
Abstract
This thesis investigates the removal of 2,6-dichlorobenzamide (BAM), a widespread pesticide degradation product in Danish groundwater, by Vacuum-UV (VUV) oxidation and develops a kinetic model of the process. A 1.74 L Ultraaqua reactor with a low-pressure mercury lamp emitting at 185 nm served as the reference system, and BAM’s hydroxyl radical rate constant was determined experimentally via competition kinetics and theoretically using a group contribution method (2.34·10^9 and 4.88·10^9 M−1 s−1, respectively). The model accounts for photon absorption, radical generation, and competing reactions in groundwater and was used to assess the effects of water chemistry and co-contaminants. In Esbjerg groundwater, simulations indicated that bicarbonate and NVOC scavenge hydroxyl radicals to a similar extent, while chloride and bicarbonate absorb most 185 nm radiation (chloride about five times more than bicarbonate). At realistic BAM levels (0.2–0.5 μg L−1), less than one pass through the reactor was needed to meet the drinking water threshold, regardless of the rate constant used. The presence of 2-methyl-4-chlorophenoxy acetic acid (MCPA) only noticeably inhibited BAM removal at ≥100 μg L−1. Comparing waters from Esbjerg, Middelfart, and Hvidovre showed that the number of equivalent reactors needed to reduce 0.5 μg L−1 BAM below the threshold increased from 0.33–0.65 (Esbjerg) to 0.71–1.53 (Middelfart) and 1.90 to above 4 (Hvidovre). This was reflected in energy use (EEO): 0.69–1.49 kWh m−3 order−1 for Esbjerg, 1.56–3.28 for Middelfart, and 4.12–8.69 for Hvidovre. Due to COVID-19, experimental work was curtailed, and the study is primarily based on theoretical modeling.
Dette speciale undersøger, hvordan 2,6-dichlorbenzamid (BAM), et udbredt pesticidnedbrydningsprodukt i dansk grundvand, kan fjernes ved hjælp af Vacuum-UV (VUV) oxidation, og opstiller en kinetisk model for processen. En 1,74 L Ultraaqua-reaktor med lavtryks-kviksølvlampe ved 185 nm blev anvendt som reference, og BAM’s reaktionshastighed med hydroxylradikaler blev bestemt både eksperimentelt via konkurrencemetodik og teoretisk med en gruppebidragsmetode (henholdsvis 2,34·10^9 og 4,88·10^9 M−1 s−1). Modellen inkluderer lysabsorption, radikaldannelse og konkurrerende reaktioner i grundvand og blev brugt til at vurdere indflydelsen af vandkemi og medforurening. I Esbjerg-grundvand påviste simuleringerne, at bicarbonat og NVOC i samme størrelsesorden tilbageholder hydroxylradikaler, mens chlorid og bicarbonat absorberer hovedparten af 185 nm-strålingen (chlorid omtrent fem gange mere end bicarbonat). Ved realistiske BAM-niveauer (0,2–0,5 μg L−1) krævedes mindre end ét pass gennem reaktoren for at komme under grænseværdien, uafhængigt af den anvendte hastighedskonstant. Tilstedeværelse af 2-methyl-4-chlorphenoxyeddikesyre (MCPA) påvirkede først BAM-nedbrydningen mærkbart ved ≥100 μg L−1. Sammenligninger af vand fra Esbjerg, Middelfart og Hvidovre viste, at antallet af ækvivalente reaktorer for at reducere 0,5 μg L−1 BAM under grænsen steg fra 0,33–0,65 (Esbjerg) til 0,71–1,53 (Middelfart) og 1,90 til over 4 (Hvidovre). Dette afspejledes i energiforbruget (EEO): 0,69–1,49 kWh m−3 order−1 for Esbjerg, 1,56–3,28 for Middelfart og 4,12–8,69 for Hvidovre. På grund af COVID-19 blev omfanget af eksperimentelt arbejde reduceret, og arbejdet er overvejende teoretisk modelbaseret.
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