Tetracycline adsorption on chitosan: A mechanistic description based on mass uptake and zeta potential measurements

Adsorption phenomena occurring at the solid/liquid interface of chitosan particles are of extreme importance in the kinetics of drug release/upload as well as in effluent treatment by adsorption. In this work, equilibrium and kinetic aspects of protonated tetracycline adsorption on chitosan are explored using classic solution depletion method and zeta potential measurements. Equilibrium experiments showed that for solutions with tetracycline initial concentration of ca. 1.2 g L−1, corresponding to a pH around 3, chitosan structure disrupted, as indicated by an increase in magnitude of tetracycline sorption. Adsorption and zeta potential isotherms before disruption suggested that the process of adsorption had a Langmuir character up to a point at which subsurface was exposed to adsorption; at this point, a second mode of sorption began: zeta potential tended to an equilibrium value, following Sips isotherm and tetracycline sorption had a linear dependence on its continuous phase concentration. The kinetics of tetracycline sorption suggested that sorption of tetracycline was divided between the sorption of protonated and non-protonated tetracycline; tetracycline in its non-protonated form seemed to rule the sorption of tetracycline.

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► Tetracycline sorption on chitosan may occur by diffusion (absorption) or adsorption.
► A discontinuity in sorption isotherm indicates the point that absorption prevails.
► Zeta potential may be used to follow chitosan surface charging during adsorption.
► Influences of protonated and nonprotonated tetracycline can be separately analyzed.
► Equilibrium and kinetic expressions using depletion method confirm the results.