Thermodynamics and kinetics of pretilachlor adsorption: Implication to controlled release from organobentonites

Abstract

To get a further insight into the mechanism of controlled release from organobentonites, thermodynamics and kinetics of pretilachlor adsorption on organobentonites modified with dodecyltrimethyl ammonium chloride (DTMA) was conducted. The structural effects of organobentonites on the interaction with pretilachlor and the diffusion of the herbicide in interlayer space were investigated and related to the sustained release of pretilachlor from organobentonites. The adsorption of pretilachlor was a spontaneous process (ΔG < 0) dominated by hydrophobic interaction with an unfavorable enthalpy (ΔH > 0) and favorable entropy change (ΔS > 0). The entropy change dropped with increasing surfactant loading of organobentonites, corresponding to a decrease in the degree of freedom of pretilachlor molecules due to the enhanced structure order of the surfactant in the interlayer gallery. However, the increase in surfactant loading generally resulted in an enhancement of pretilachlor adsorption due to the reduction of enthalpy change. As compared to the pseudo-first-order model, the pseudo-second-order model fitted better to the kinetics of pretilachlor adsorption on organobentonites, of which the rate constant k2 decreased and level off with increasing surfactant loading, exhibiting a similar trend to the entropy change for pretilachlor adsorption. The release of pretilachlor from organobentonites was predominated by Fickian diffusion, which could be understood from adsorption thermodynamics and kinetics. The time taken for 50% of active ingredient to be released, T50, from organobentonites with DTMA loading level ranged from 40 to 150% of the cation exchange capacity of the bentonite was 6–10.6 times that for the control formulation and exhibited a good linear relationship with the relative value of the adsorption equilibrium constant Ka to the rate constant k2 from pseudo-second-order model.