Sorption of an acidic herbicide on synthetic iron oxides and soils: sorption isotherms

The sorption behaviour of the new acidic herbicide quinmerac on a range of different synthetic and natural iron oxides was studied in order to determine the influence of pH and degree of crystallinity. The synthetic iron oxides chosen for the batch sorption experiments were 6-line ferrihydrite, goethite and hematite. In addition, natural soil substrates rich in iron oxides (cambic and spodic B horizons and ochreous sludge) were used for sorption experiments. The synthetic iron oxides were characterized by X-ray diffraction, transmission electron micrographs and by measuring their specific surface area. Sorption of quinmerac (pK_a2 = 4.3) on both synthetic and natural ferrihydrite was linear up to the water solubility of the herbicide at low pH values. Sorption decreases with increasing pH. At pH > 5.5 no sorption was observed. Sorption decreases with increasing degree of crystallinity. The isotherms on the well crystallized oxides goethite and hematite indicated saturation of binding sites at quinmerac concentrations of 1.8 10^-3 mg g^-1 (goethite) and 1.0 10^-3 mg g^-1 (hematite). Sorption on the soil samples depends on both the content of weakly crystalline iron oxides and on organic carbon content.