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ABSTRACT
Water quality assessment was conducted on the Ruiru River, a tributary of an important tropical river system in Kenya, to determine baseline river conditions for studies on the aquatic fate of N-methyl carbamate (NMC) pesticides. Measurements were taken at the end of the long rainy season in early June 2013. Concentrations of copper (0.21–1.51 ppm), nitrates (2.28–4.89 ppm) and phosphates (0.01–0.50 ppm) were detected at higher values than in uncontaminated waters, and attributed to surface runoff from agricultural activity in the surrounding area. Concentrations of dissolved oxygen (8–10 ppm), ammonia (0.02–0.22 ppm) and phenols (0.19–0.83 ppm) were found to lie within normal ranges. The Ruiru River was found to be slightly basic (pH 7.08–7.70) with a temperature of 17.8–21.2°C. The half-life values for hydrolysis of three NMC pesticides (carbofuran, carbaryl and propoxur) used in the area were measured under laboratory conditions, revealing that rates of decay were influenced by the electronic nature of the NMCs. The hydrolysis half-lives at pH 9 and 18°C decreased in the order carbofuran (57.8 h) > propoxur (38.5 h) > carbaryl (19.3 h). In general, a decrease in the electron density of the NMC aromatic ring increases the acidity of the N-bound proton removed in the rate-limiting step of the hydrolysis mechanism. Our results are consistent with this prediction, and the most electron-poor NMC (carbaryl) hydrolyzed fastest, while the most electron-rich NMC (carbofuran) hydrolyzed slowest. Results from this study should provide baseline data for future studies on NMC pesticide chemical fate in the Ruiru River and similar tropical water systems.
Acknowledgments
We are grateful to the Towson University School of Emerging Technologies and Fisher College of Science and Mathematics for research grants to support this work, and acknowledge the Department of Chemistry at Kenyatta University for supporting this research.
Supplementary Materials
Colorimetric Reactions
The following are the chemical reactions occurring in vials provided by the manufacturer, and used to measure each of the analytes in this study.
| a. | Copper: Total soluble copper reacted with bathocuproine ligand to form an orange colored chelate in direct proportion to the copper concentration. | ||||
| b. | Ammonia: Free ammonia reacts with hypochlorite to form monochloramine which further reacts with salicylate in the presence of sodium nitroferricyanide to form a green colored complex called 5-aminosalicylate in direct proportion to ammonia concentration. | ||||
| c. | Phenols: Phenols react with 4-aminoantipyrine in alkaline solution to produce a red colored complex in direct proportion to phenol concentration. This reaction is initiated by potassium ferricyanide, which is coated on the tip of the ampoule. | ||||
| d. | Nitrate: Nitrates are reduced to nitrite with cadmium. The nitrite diazotizes with a primary aromatic amine in an acidic solution, then couples with another organic molecule to produce a highly colored azo dye in direct proportion to nitrate concentration. | ||||
| e. | Phosphate: Phosphates react with ammonium molybdate in an acidic solution to form molybdophosphoric acid. This acid is then reduced by stannous chloride to the intensely colored molybdenum blue in direct proportion to phosphate concentration. | ||||
| f. | Dissolved Oxygen: In an acidic solution, oxygen oxidizes the yellow-green colored leuco-form of indigo carmine to generate a highly colored blue dye in direct proportion to oxygen concentration. Table 1S. Photomoteric measurements using certified standards. Table 2S. Measurements along Ruiru River in ppm. Table 3S. Replicate measurements at site 5 over four-day period. Concentrations in ppm. Temperature in oC. Table 4S. Duplicate measurements (in ppm) at odd-numbered sites along The Ruiru River. Figure 1S. UV-vis spectra of carbofuran, carbaryl and propoxur.  | ||||
Notes
1 The method used involves the reaction of phenol and other phenolic compounds with 4-aminoantipyrine in alkaline solution to produce a red-colored complex. This reaction is initiated by potassium ferricyanide which is coated on the tip of the ampoule. This chemistry detects ortho-and meta-substituted phenols and under certain pH conditions will detect phenols substituted in the para-position with a carboxyl, halogen, hydroxyl, methoxyl or sulfonic acid group. Results are expressed in ppm (mg/L) “equivalent phenol” as C6H5OH and represent the minimum concentration of phenolic compounds in the sample.