Adsorption of MCPA on goethite and humic acid-coated goethite

Interactions Between Ionic Pesticides and Model Systems for Soil Fractions

“…As regards the negative charge of ionized MCPA molecules in solution, interaction with the goethite is only expected to be effective at pH values below PZC=9.4, at which the surface of the mineral oxide is positively charged. Experimental results (Iglesias et al, 2010a) indicate that the adsorption of MCPA on goethite decreases as the pH increases. At pH 4 there is a strong electrostatic interaction between both substances, but the adsorption decreases linearly with increasing pH because of the decrease in the positive surface charge of the goethite, so that at pH values close to the PZC, there is little interaction between the organic anions and the surface.…”

“…The adsorption isotherms for MCPA on goethite consist of two different regions (Figure 4a). The first corresponds to an S-shaped isotherm (Giles et al, 1960;Celis et al, 1999;Inacio et al, 2001;Iglesias et al, 2010a), which presents an initial region of low affinity between the solid surface and the herbicide, so that adsorption of a certain amount of adsorbate will be necessary to facilitate the adsorption process (Grant et al, 1998) and after this initial section, a Langmuir-shaped portion is observed in the isotherm. At acidic pH, MCPA may be sorbed through its -COO -groups oriented toward the positively charged iron oxide surface, which favours lateral van der Waals interactions between the aromatic rings of the MCPA in the first adsorption layer (Celis et al, 1999).…”

“…A rigorous interpretation of this interaction, using the CD-MUSIC model, requires knowledge about the parameters that determine the electrostatic potential at the solidsolution interface of HA-coated goethite. Since this information is not available, interpretation of the adsorption of MCPA onto HA-coated goethite was based on parameters previously obtained for bare goethite, with some modifications (Iglesias et al, 2010a). As in the case of the bare goethite, the formation of inner-and outer-sphere complexes between MCPA and singly coordinated groups of the coated mineral surface was assumed for the modelling calculations.…”

“…Pesticide-pesticide interaction can also be established between the first layer of adsorbed MCPA and the subsequent molecules of pesticide added, so that a second adsorption layer may be formed. Among the experimental results that correspond to the interaction between the pesticide and the surface groups of goethite, the part of the isotherm which displays Langmuir behaviour ( Figure 4b) has been interpreted with the CD-MUSIC model (Iglesias et al, 2010a). It can be assumed, as for other simple organic acids (Filius et al, 1997), that the carboxylic groups of the organic molecules can form inner-sphere and outer-sphere surface complexes with the singly coordinated surface groups (≡FeOH -0.5 ), and that the triply coordinated surface groups (≡Fe 3 O -0.5 ) do not take part in the MCPA binding ( Figure 5).…”

Adsorption of paraquat on goethite and humic acid-coated goethite

“…grade. Preparation and characterization of the goethite are described elsewhere [10]. A specific surface area of 67.9 m 2 g −1 was determined by the BET-N 2 adsorption method.…”