Effects of Natural Attenuation, Organic Amendments and Metal Toxicants on 2, 4-Dichlorophenoxyacetic Acid Dissipation Dynamics in Soil

Abstract

The dissipation dynamics of the herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), in soil was studied under the influence of natural attenuation (NA), poultry manure (PM), swine manure (SM), urea (UR), and metal toxicants (Cd, Pb) in a laboratory microcosm. A sandy clay soil was characterized and used for the study. The effects of variables such as initial 2,4-D concentration (0-500 mg/kg), level of amendment (0-30 %w/w), amendment blends (binary and ternary), Cd/Pb loadings (0-200 mg/kg) and incubation time (0-30 days) on 2,4-D dissipation were investigated. In all the experiments, residual 2,4-D was recovered by solvent extraction, followed by reaction with chromotropic acid reagent for color development and assayed by UV-Vis spectrophotometry at λ max 565 nm. 2,4-D naturally attenuated in soil, however, addition of PM, SM or UR did enhance the extent and rate of degradation. Relative to NA, the degrading efficiencies, DE (%) of the amendments increased with increase in incubation time and level of amendment, but did not necessarily depend on initial 2,4-D concentration. Pseudo-first-order degradation constants (x10-2 /day) decreased with herbicide load and increased with amendment level; with corresponding half-lives (day) decreasing and increasing, respectively. DE values indicated that apart from the binary PM + SM amendment blend, the single amendments (especially PM and UR) were more effective at 2,4-D degradation in soil. The extent of 2,4-D degradation was reduced at increasing Cd and Pb loads, furnishing relatively low DE's (< 50%) for the amendments but more so in the presence of Cd).