In the post-harvest treatment of pears and apples in the Argentinean Patagonia, several pesticides are used [1], including pyrimethanil (PRM), for the storage of fruits. The effluents released by the packing plants originate point source of contamination that should be treated prior to its discharge to natural water-courses. Montmorillonites (Mt) and their exchange products with quaternary ammonium salts, organo-montmorillonites (OrMt), have been used to remove this kind of contaminant [2] as well as metals [3], antibiotics [4], etc. The objective of this work was to study the adsorption of pyrimethanil on Mt and OrMt as well as the adsorption kinetics that manages the process. The adsorption isotherms were fitted to Langmuir, Freundlich and Dubinin-Radushkevitch (D-R) models. For kinetic analysis, three mathematical models were employed: pseudo first order (PFO), pseudo second order (PSO) and intra-particle diffusion model (IDM). The adsorption products were characterized by XRD and FTIR. The OrMt were prepared from didodecyldimethylammonium (DDAB) and octadecyltrimethylammonium (ODTMA) bromides at 150% of the cation exchange capacity (CEC). These adsorbents were named DMt150 and OMt149 for DDAB and ODTMA exchanged raw montmorillonite (Mt), respectively, where the number indicated the surfactant amount exchanged determined by total carbon analysis. The maximum adsorption of PRM was the following: 330, 240 and 156 μmol PRM/g clay, for DMt150, OMt149 and Mt, respectively. The large amount adsorbed by DMt150 sample was assigned to different conformational states of DDAB in the interlayer space [5], due to the presence of two hydrocarbon chains, allowing the entrance of PRM to the interlayer, and conferring greater hydrophobicity than that of OMt149. All adsorbents fitted well to the sorption models, except OMt149 sample that did not fit to Langmuir. The DMt150 sample attained the highest KF value (obtained from the Freundlich model) within the studied samples. The high PRM affinity for DMt150 sample indicated the presence of hydrophobic interactions between the PRM and the organic fraction of the OrMt. The Qmax parameter obtained with D-R model were higher than that obtained with Langmuir model for all samples. The adsorption energy values found by D-R model indicated a physisorption process for PRM adsorption on OrMt samples, supporting the hypothesis of hydrophobic or Van der Waals type interactions. The kinetics experiments indicated that the adsorption process was governed by pseudo-second order kinetics. The XRD analysis indicated an increase of the basal spacing from 1.25 to 1.39 nm for Mt sample after PRM adsorption, assigned to PRM molecules arrangement in a planar conformation. For OrMt, no changes were noticed in the interlayer space after PRM adsorption, which could indicated a shielding of PRM entrance by the presence of the surfactant. FTIR analysis showed that the secondary amino group of PRM was involved in the adsorption process. The appearance of a new band in the Mt-PRM spectra at 1579 cm-1 could indicate the formation of H-bonding through one of the pyrimidine ring nitrogen. Summarizing these results, PRM adsorption increased by the use of DMt150 and OMt149 compared to Mt due mainly to hydrophobic interactions between the PRM and the organic fraction of the OrMt samples.

[1] Lombardi B., Baschini M., Torres Sánchez R.M. (2003). Optimization of parameters and adsorption mechanism of thiabendazole fungicide by a montmorillonite of North Patagonia, Argentina. Applied Clay Science, 24:43- 50. [2] Sanchez-Martin M.J., Rodriguez-Cruz M.S., Andrades M.S., Sanchez-Camazano M. (2006). Efficiency of different clay minerals modified with a cationic surfactant in the adsorption of pesticides: Influence of clay type and pesticide hydrophobicity. Applied Clay Science. 31:216- 228. [3] Sarkar B., Naidu R., Megharaj M. (2013). Simultaneous Adsorption of Tri- and Hexavalent Chromium by Organoclay Mixtures. Water, Air & Soil Pollution. 224:1704. [4] Sun K., Shia Y., Chen H., Wang X., Lic Z. (2017). Extending surfactant-modified 2:1 clay minerals for the uptake and removal of diclofenac from water. Journal of Hazardous Materials. 323:567-574. [5] Sagitova E.A., Donfack P., Prokhorov K.A., et al. (2009). Raman spectroscopic characterization of the interlayer structure of Na+-montmorillonite clay modified by ditetradecyl dimethyl ammonium bromide. Journal of Physical Chemistry B. 113:7482-7490.

This research was funded by the Spanish Ministry of Economy and Competitiveness through the project CTM2013-42306-R supported by the European Regional Development Fund (FEDER ) and by the Argentine Ministry of Science, Technology and Productive Innovation, FONARSEC project, Nano FS-008/2010.

Comunicación oral presentada en la XVI International Clay Conference from the Oceans to Space 16th ICC – July, 17-21,Granada 2017

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