Submitted to: Humic Substances and Organic Matter in Soil and Water Environments; Charact
Publication Type: Book / Chapter
Publication Acceptance Date: 7/21/1996
Publication Date: N/A
Citation: Interpretive Summary: A porous membrane method was used to evaluate the binding strength of a 'weed killer' herbicide (napropamide) on the humus fraction of organic matter extracted from soils, peats, and waters. Binding strength may be related to solubility and movement of the herbicides to surface and ground waters. Frequency of changing the water outside the membrane affected the binding of the herbicide inside the porous membrane. A radio-isotope labeling technique and the extractable organic concentrations were used to compare the samples. About 1.5 parts per million of pesticide was bound to the soil humic fraction under our experimental conditions. Results showed that the soil organic fraction was stronger in binding compared with the peat and water samples. Amino acids, and elemental contents from the three sets of samples were then analyzed for a possible explanation. It seemed that the higher nitrogen and phosphorus contents and higher neutral water-resistant amino acids in the soil humic fraction contributed to the stronger binding strength. These results will help us understand how to prevent herbicides and other polluting chemicals from entering surface and ground water supplies.
Technical Abstract: Equilibrium and continuous dialysis methods, followed by gel filtration, were used in evaluations of humic acid-herbicide complexations. Complexes isolated by Sephadex G-25 gel filtration yielded up to 1.5 mg of napropamide adsorbed per gram of humic acid (HA). Studies using dialysis procedures suggest that napropamide was more strongly held by soil HA than by the HAs from peats and waters. Iso-electrofocusing of soil HA complexes showed that the **14C-label of the sorptive was retained in the HA-sorbate complex, whereas the pure napropamide (with its **14C label) disappeared from the gel loading area. Elemental analyses data showed that soil HAs had relatively higher nitrogen and phosphorus, and generally lower oxygen contents than those from most peats and waters. The contents of neutral hydrophobic amino acids in the soil HAs were 2 to 5 times greater than those for peats and waters. Such compositional differences may reflect a greater contribution from hydrophobic bonding in the cases of the soil HAs, compared with the HAs from peats and waters.