title: DOM-Flocculation: A suitable approach for separating free and DOM-bound herbicides?
abstract: Dissolved organic matter (DOM) in soil solution is considered to interact with herbicides enhancing their mobility and promoting subsequent leaching.Batch experiments were conducted to test if free and DOM-bound herbicides can be separated by a DOM-flocculation technique with Cu as a coagulant.DOM was extracted from the H and A horizons of two soils (Terric Histosol, Cumuli-Calcaric Cambisol) and from the O horizon of a forest soil (Humic Cambisol). DOM-solutions (100 mL) were fortified with the herbicides terbuthylazine and pendimethalin (100 microg active ingredient each) and equilibrated for 14 hours. After DOM-flocculation with Cu (addition of 0.5 mM CuCl(2)) herbicide recovery was determined in the supernatant solutions and in the precipitate of Humic Cambisol-DOM, respectively.Recovery of the herbicides from pure water was 85-99% and was not influenced by the addition of Cu. At low pH (4.8-5.3) DOM-flocculation of different DOM-extracts was insufficient and varied in a range of 18 - 90%. Herbicide recovery from DOM-solutions decreased moderately for terbuthylazine (60-90%) and strongly for pendimethalin (5 - 30%). In general, the addition of Cu caused no further reduction of herbicide recovery from supernatant solutions, except for Humic Cambisol-DOM. The effects of Cu-addition were most evident for pendimethalin (strongly reduced concentration in the supernatant solutions) and were considered to be caused by a flocculation of DOM-bound moieties. Flocculation of Humic Cambisol-DOM increased from 18 - 24% at pH 5 to > 95% at pH 8. However, at this pH the formation of Cu(OH)(2) as a sorbing subcomponent of the flocculated matter lead to an overestimation of DOM-bound pendimethalin. Calculating this side effect 6% of pendimethalin added was DOM-bound. Only traces of terbuthylazine (< 1%) were found in the solid matter of flocculated Humic Cambisol-DOM.To sum up, the new approach to separate freely dissolved herbicides from DOM-bound moieties not fully corresponded to our expectations. DOM-flocculation was found to depend strongly on pH-environment influencing not only DOM-herbicide interactions but also the clear separation of DOM-bound herbicides from herbicides in solutions.
authors.affiliation: Institute of Soil Science (310), Department of Soil Chemistry, University of Hohenheim, D-70593, Stuttgart, Germany.
authors.forename: H
authors.initials: H
authors.lastname: Giessl