Pectin hydrogels modified with montmorillonite: Case studies for the removal of dyes and herbicides from water

Abstract

Hydrogels were synthesized with pectin and modified with montmorillonite for case studies involving the removal of dyes and herbicides from water. Methylene blue and 2,4-dichlorophenoxyacetic acid (2,4-D) were used as model pollutants. Fourier-transform infrared spectroscopy performed before and after the pollutant sorption studies enabled the identification of chemical groups in the structure of the hydrogels and interactions with each pollutant in the aqueous medium. Thermogravimetric analysis, differential thermogravimetry and differential scanning calorimetry were employed to determine thermal events in the polymeric networks of the hydrogels before and after pollutant sorption. Scanning electron microscopy was employed to evaluate the hydrogel morphologies before and after the sorption studies. The water diffusion mechanisms and point of zero charge in each hydrogel varied according to the type of aqueous medium. Moreover, the pollutant sorption capacity of the hydrogels varied as a function of the sorption time, pH and temperature of the aqueous solution as well as the initial concentration of the pollutant. The Redlich-Peterson and Sips isotherms had the best fit to the experimental data, indicating that the sorption of methylene blue and 2,4-D tends to occur by simultaneous multi-site interactions and the formation of a monolayer. The thermodynamic and kinetic data complemented the isotherm data and assisted in the definition of real-world application strategies. Composite hydrogels with lower montmorillonite content are potentially better removers of drugs, food dyes, sugar substitutes and herbicides from water compared to conventional hydrogels not modified with montmorillonite. Overall, these hydrogels could be reused for three successive cycles of sorption after the recovery with either hydrochloric acid or ethanol.