Controlled Release Of Avermectin From Attapulgite-Based Composites

Controlled release of agrochemicals is a newly-developed technique withwide and highly potential applications, which features less abrupt and slowerrelease of active agents from formulations compared to conventional ones bymeans of scientific design. As a result, the formulations can continually serveas a drug supplier, and their impact can last for a longer time, killingpesticides and weeds efficiently. Controlled-release formulations prepared byadsorption can be easily industrialized and is worth much and deep research.In this work, avermectin is selected as the drug for release, and is loadedon porous attapulgite-based materials by means of adsorption. Threecontrolled release systems are discussed: silica gel-attapulgite(SA),attapulgite-silica gel(AS), and activated carbon-attapulgite(AA). XRD andFT-IR analysis imply that combination between the drug and substrate isphysical. The substrate used for drug loading is mesoporous for compositescontaining silica gel, and microporous for composites containing activatedcarbon, which is proved by pore size distribution calculated by DensityFunctional Theory (DFT) method.In addition, adsorption of avermectin on attapulgite-based composites is discussed, and kinetic analysis shows that pseudo second order model is betterfor description of the process than pseudo first order model, but intraparticlediffusion model shows the best correlation coefficient.A better sustained release curve can be obtained by increasing theconcentration of silica gel (or activated carbon) or diameter of theformulations, or decreasing drug loading. Kinetic analysis shows that releaseof avermectin from the three attapulgite-based systems is a process which canbe divided into first abrupt release stage, second decelerate stage, and thirduniform release stage. And release kinetics can be expressed in the form ofRitger-Peppas model and explained in Fickian diffusion mechanism.Attapulgite-silica gel controlled release system with85%of silica gel,0.5%ofavermectin, and particle size of10to20mesh has abrupt release of33%andhalf life of308.1h, while for activated carbon-attapulgite controlled releasesystem with45%of activated carbon,0.5%of avermectin, and particle size of20to40mesh they are44%and1.8h, respectively.