Mechanism of metribuzin herbicide sorption by humic acid samples from peat and vermicompost

Humic acids (one derived from vermicompost, originating from animal manure and treated with the earthworm Eisenia foetida or Lumbricus rubellus, and the other originating from peat) were characterized in respect of their major elemental UV-Vis spectroscopy and acidity. Humic acid, isolated from vermicompost (VHA), was compared with the humic acid sample extracted from peat (PHA) in respect of its chemical interactions and sorption with metribuzin herbicide (4-amino-6 t-butyl-3-methylthio)-1,2,4 triazine-5(4H)-one). The results showed that metribuzin was degraded by the humic acids in the 5.8-7.12 pH range. The nature and arrangement of functional groups in both humic acids are discussed on basis of the FTIR spectroscopy. The VHA spectrum shows a very strong band at similar to 1400 cm(-1). This band should have a different origin than the one in the same position present in the PHA spectrum, since it is not pH-dependent, while in the PHA the band at 1400 cm(-1) assigned to symmetric stretching of carboxylate group, is strongly pH-dependent. Since VHA shows a higher concentration of nitrogen, the band could be assigned mainly to v(C-N) of amine groups. A model of the sorption of metribuzin on humic acids was studied. A mechanism of interaction between metribuzin and humic acids was proposed. (C) 1998 Elsevier Science B.V.