Natural bio-adsorbent from avocado skin for the removal of a model cationic dye, methylene blue

Abstract

The industrialization of avocado (Persea Americana) has brought great challenges. One of them is the comprehensive management of skin and seed waste that, due to the amount generated and its inadequate management, represents a risk to human health and ecosystems. This research presents the development of a bio-adsorbent from avocado peel waste and its application in the removal of a model cationic dye, methylene blue, AM. The bio-adsorbent was developed by a hydrothermal method at 70°C under mild acidic conditions. It was characterized by X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and scanning electron microscopy. The experimental design evaluated the effect of the concentration of aqueous solutions of the material to be adsorbed AM (2.5, 5, 10, 20 and 40 mg L^-1), the temperature (293, 308 and 323 K), and the pH value (2 , 4, 6, 8 and 10) in the adsorbent capacity using commercial activated carbon (CAC) as a control. The bio-adsorbent material obtained from avocado peel has a characteristic cellulose structure and IR spectroscopy confirms the presence of functional groups associated with this material. It turned out to be effective in removing the dye, reaching 99 to 76% removal with 5 and 40 mg L^-1 of dye, respectively, at pH of 7, and temperature of 293 K. The experimental results were fitted to two diffusion kinetic models and an intra-particle diffusion model. They also showed that the rate constant decreased with dye concentration. The thermodynamic parameters of the process were evaluated, determining that the adsorption mechanism is by chemisorption and that it is exothermic and spontaneous. The reuse study of the bio-adsorbent material indicate that, after 8 continuous cycles, by increasing the concentration of the dye, the bio-adsorbent quickly saturates, removing only 20% of the dye in the last 4 cycles.