The influence of chitosan coating on the controlled release behaviour of zinc/aluminium-layered double hydroxide-quinclorac composite

Controlled release formulations of herbicides have been recognised to have potential in reducing the adverse effects of conventional herbicides on our environment. In this work, the controlled release behaviour of quinclorac (QC) from zinc/aluminium-layered double hydroxide-quinclorac (Zn/Al-LDH-QC) and its chitosan-coated composite Zn/Al-LDH-QC-Chi into aqueous solutions of Na3PO4, Na2SO4 and NaCl were studied. The physicochemical properties of both composites were studied initially, using powder X-ray diffraction (PXRD), Fourier transform infrared (FTIR), thermogravimetric and derivative thermogravimetric analysis (TGA/DTG) and field emission scanning electron microscopy (FESEM) instruments, so that the effect of chitosan coating on their functional groups, thermal stabilities and surface morphology could be observed thoroughly. The results from the release studies showed that the presence of chitosan as a coating enhances the electrostatic attraction between the positively charged amino groups on the backbone of chitosan and the negatively charged QC anions, thus assisting in prolonging the time of release of QC. The kinetic studies revealed that both Zn/Al-LDH-QC and Zn/Al-LDH-QC-Chi composites obey second-order mechanisms, thus signifying that the release of QC into the release media by both composites took place via dissolution and ion exchange. These results therefore indicate the promising potential of Zn/Al-LDH-QC and Zn/Al-LDH-QC-Chi to overcome the drawbacks of the over-use of herbicides in paddy cultivation areas.