Picloram adsorption on clays.

J L MARCO BROWN; R. M. TORRES SÁNCHEZ; M DOS SANTOS AFONSO

Granada España

Congreso; Interfaces Against Pollution 2006; 2006

Pesticides are widely used for control of broad-leaf weeds and other vegetation. The behavior of pesticides in soil is a largely dynamic phenomenon. After application, they may pass into streams, rivers and lakes or leach through soil with the possibility of contaminating subsurface waters. In a first approximation, soils can be considered as a combination of the minerals that compose them. Thus, pesticides also may be adsorbed onto soils components such as clays. In particular, montmorillonites have shown a great capacity of adsorption of both metallic cations and organic substances due to their small particle size (<2 µm), high surface area and isomorphic substitutions in their structure. The study of the interaction parameters associated with adsorption and desorption will provide a thermodynamic database that will be useful to predict the environmental fate and behavior of pesticides.  Four types of clays were choose due to the different structural characteristics, montmorillonites (Wyo and 428), nontronite, illite and kaolinite and they were physical-chemical characterized by several methods. The purity determined by the Rietveld method is higher than 98% for all the samples confirming the superficial analyses carried out by EDAX. The morphology of the particles observed by electronic microscopy corresponded to the suitable ones in the bibliography of the topic. The specific interlayer surface of the clays was calculated as the difference between the total surface measured by water adsorption (Sw) and the external surface determined by N2 adsorption (BET method, SN2).  The specific interlayer surface determined for Wyo, 428, nontronite, illite and kaolinite were 611, 496, 548, 25 and 15 g/m2 respectively.  The IEP values obtained for Wyo, 428, nontronite, illite and kaolinite were, 3.2, 1.0, 5.1, 7.1 and 2.5 respectively. XRD of the dry and wet samples (P/P0=0.47) were also carried out, The influence of the water content in the interlayer, was measured by a d(001) reflection parameter shift, indicating the interlamellar space size for the expandable samples like montmorillonites and kaolinite.  Due to the influence of the pH on the montmorillonites structures XRD were also carried out at different pH values, not being evidenced of notorious crystalline modifications.  The values of CEC were 306; 305; 294; 214 and 39 meq/100g for Wyo, 428, nontronite, illite and kaolinite, respectively. Picloram, 4-amino-3,5,6-trichloropicolinic acid is a herbicide from the picolinic acid family and is used for broadleaf weeds control in wheat, barley  crops and woody plants on rights-of-ways. Picloram adsorption isotherms were measured by UV-Vis spectrophotometry at 223 nm.  Surface coverage was calculated by the difference between initial concentration and the concentration remained after 24 hs equilibrium.  The adsorption dependence with pH was also measured and the thermodynamic adsorption parameters were calculated.