Picloram, adsorption

Surfactant effects on adsorption of herbicides on to soil have been investigated and suggested to be a factor to be considered in the overall effect of surfactant on toxicity towards the plant. The degradation, mobility and uptake of one such compound, picloram [4-amino-3,5,6-trichloropicolinic acid] (pK = 3.4) is affected by adsorption-desorption processes in solids. Picloram adsorption on to soils at pH 5 was reduced by 1 % anionic surfactant [284]. The mechanism involved in picloram adsorption included protonation of the molecule, metal-ion bridging and interaction with metal ions. Picloram adsorption was enhanced by cationic surfactants, suggesting that hydrophobic adsorption of the cationic monomers on to the soil provides a cationic surface for interaction of the anionic picloram. Different soils with different pH values resulted in some variations in these effects which are presented in Table 10.29. 

Table 10.29 Picloram adsorption on Oregon soils from aqueous and cationic, anionic and non-ionic surfactant solutions containing 21 picloram...

Van der Waals forces were considered to be involved in the physical adsorption of Carbaryl, Parathion, Alachlor, Picloram, and 2,4-D by SPHs [17,25,152, 160-162]. 

Figure 11 shows the relative adsorption of 14 different herbicides by soil organic matter. The acidic herbicides, dinoseb, picloram, 2,4-D, and dicamba, were adsorbed in relatively low amounts compared with the basic and cationic herbicides, and the adsorption amount was inversely related to the water solubilities of the acidic compounds (Table IV). The weakly acidic phenol, dinoseb pKa = 4.40), was adsorbed more than the stronger acids, picloram pKa = 1.90), 2,4-D pKa = 2.80), and dicamba pKa = 1.9). Since the organic matter was an acidic muck... 

Check samples were prepared for Cases 1-4. These samples were duplicates of the exposed samples except that they were shielded from the light. Bioassay analyses of these check samples detected no loss of picloram. This result eliminates microbiological or adsorptive processes as significant factors contributing to the loss of picloram in these experiments. 

Grover, R. Adsorption of Picloram by Soil Colloids and Various Other Adsorbants, Weed Sci., 19(4) 417 18 (1971). 

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