An investigator conducted a study to evaluate how different types of dissolved organic matter (DOM) influence the rate of photochemical degradation of a synthetic pesticide, Pesticide , in natural sunlight. Pesticide degrades when exposed to ultraviolet (UV) light, but dissolved organic matter can either shield the pesticide from light (attenuation) or sensitize it by generating reactive oxygen species (sensitization).
*Experiment 1*
Four identical quartz tubes were prepared, each containing a aqueous solution of Pesticide . To three of the tubes, a different type of DOM (humic acid, fulvic acid, or amino acids) was added at a concentration of . The fourth tube received no DOM. All four tubes were exposed to natural sunlight for . The percentage of Pesticide degraded in each tube was measured. The results are shown in Table 1.
| Tube | DOM Type Added | DOM Concentration () | Percentage of Pesticide Degraded |
|---|---|---|---|
| 1 | Humic acid | 5 | 42% |
| 2 | Fulvic acid | 5 | 58% |
| 3 | Amino acids | 5 | 74% |
| 4 | None | 0 | 65% |
*Experiment 2*
To determine whether the degradation was driven specifically by UV light rather than thermal decomposition (since the sun also heats the samples), the investigator prepared two additional quartz tubes. Each tube contained a aqueous solution of Pesticide and of humic acid. One tube was exposed to sunlight (Tube 5), while the other tube was wrapped in aluminum foil to block all light and placed adjacent to the first tube in the same outdoor environment (Tube 6). After , the percentage of Pesticide degraded was measured. The results are shown in Table 2.
| Tube | Wrapped in Foil? | Percentage of Pesticide Degraded |
|---|---|---|
| 5 | No | 42% |
| 6 | Yes | 3% |
Based on the design of Experiments 1 and 2, which of the following options correctly identifies the control group for the presence of DOM in Experiment 1, and the control group for light exposure in Experiment 2, along with the correct justification for their selection?
- ATube 4 in Experiment 1, because it lacks DOM, allowing the investigator to isolate the effect of DOM on Pesticide degradation; and Tube 5 in Experiment 2, because it represents the standard environmental setup containing both light and heat.
- BTube 1 in Experiment 1, because humic acid serves as the baseline DOM for comparison; and Tube 6 in Experiment 2, because the foil prevents any chemical interactions from occurring.
- Tube 4 in Experiment 1, because it lacks DOM, allowing the investigator to isolate the effect of DOM on Pesticide degradation; and Tube 6 in Experiment 2, because it lacks light exposure, isolating the effect of light from temperature changes.Answer
- DTube 4 in Experiment 1, because it acts as a baseline showing that Pesticide does not degrade in the absence of organic matter; and Tube 5 in Experiment 2, because it allows the investigator to measure degradation in the presence of both heat and light.