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The clay-alginate composites showed relatively excellent drug sustained-release properties. In order to enhance the sustained-release property of bentonite-alginate composite gels, the alachlor loaded chitosan quaternary modified bentonite-alginate composite gel was prepared. The underlying effect and mechanism of the structure of modified bentonite-alginate composite gels on the release behavior of alachlor were explored by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TG). The results showed that the release of alachlor from the composite gel was dominated by Fickian diffusion and closely related to the adsorption capacity and permeability of the matrix. The chitosan quaternary combined with the negative charge sites on the surface of bentonite nanoplatelets, which significantly enhanced the adsorption capacity of bentonite for alachlor, and the adsorption equilibrium constant (Ka ) of alachlor on bentonite and modified bentonite were 0.13 and 0.40, respectively. Also, the stacking aggregation of bentonite nanoplatelets and the permeability of the gel network were effectively reduced through the electrostatic interaction between chitosan quaternary and alginate, and the permeability of modified bentonite-alginate composite gel is 9.6 times that of pure alginate gel. The release kinetics of alachlor from the composites revealed that the release rate of alachlor from the composite gel first decreased and then increased as the content of clay gradually increased. The best-sustained release property of composite gel was obtained when the mass fractions of alginate, chitosan quaternary modified bentonite, alachlor, and deionized water were 1.6%, 0.2%, 0.2%, and 97.8%, respectively, under which the release time of alachlor from bentonite-alginate and modified bentonite-alginate composite gels was 4.4 and 8.3 times longer than the release from pure alginate gel, respectively. [ABSTRACT FROM AUTHOR]