Chlorpyrifos desorption from montmorillonite and its thermal and mechanical treatment products

M. PANTANETTI; M. DOS SANTOS AFONSO; R.M. TORRES SANCHEZ

Antalya

Congreso; Euroclay 2011; 2011

Soc. Turca de arcillas

Chlorpyrifos (CPF) is a pyridine organothiophosphate insecticide of direct application and widespread use, its half-life of 60 to 120 days indicating its high permanence and impact in the environment. Montmorillonite clay, a soil component, due to its high specific surface, have been used as adsorbent of various agrochemicals and may arise as a theoretical model for understanding the adsorption of these contaminants in the soil and consequently their lixiviation to water bodies. Thermal and milling treatments can modify the interlayer spacing and alter the crystal structure of clays, changing their surface sites and reactivity, with the consequent variation of adsorption/desorption capacities The aim of this study is to characterize a raw montmorillonite and its thermal and mechanical treatment products and determine the kinetics of CPF desorption to assess pollution of water bodies and possible use in controlled release. A montmorillonite (Cast) from the Province of Neuquén, Argentina, and its treatment products (3h at 550 °C, Cast550C and 300sec ground, Cast300s) were characterized by XRD, specific total surface measurements determined by adsorption of water vapor (Sw), apparent diameter (Dapp), zeta potential and SEM. The CPF adsorption and desorption were evaluated in batch conditions at room temperature and pH = 6. The XRD patterns showed the montmorillonite characteristic peaks in the Cast sample, with a reflection peak d(001) of 12.6 Å which shift to 7.3 Å after thermal treatment and peak d(060) showed an intensity decrease after mechanical treatment. These behaviors were assigned to the collapse of the interlayer by dehydration and change in the ab plane, respectively. After both treatments, the Sw values decreased to half (from 621 to around 300m2/g) and the Dapp values increased, indicating agglomerates formation. The Z-Potential curves, showed flat negative curves in all pH range (2-10), revealing higher negative surface charge for Cast than Cast550C and Cast300s samples. The particles morphology, observed by SEM corroborated the agglomerates formation indicated by surface values. CPF adsorption fit with Langmuir isotherms for all samples, indicating an affinity constant (KL) order: Cast.> Cast300s> Cast550C. XRD patterns of different amount of CPF adsorbed samples indicated the entrance of CPF into the clay gallery of all samples by shifting the reflection peak d(001) until 19.6 Å. The CPF maximum desorption values follow the order: Cast550C > Cast300s> Cast. (2.3, 2.0 y 1.7 mg/g) evidencing the different affinity of the adsorbent for the adsorbate found in the adsorption processes.