Natural zeolite as adsorbent for metformin removal from aqueous solutions: Adsorption and regeneration properties

Drugs are being registered in water resources around the world, becoming a palpable environmental problem by altering the natural characteristics of water and, consequently, altering the environment of living organisms. Many techniques can be used to remove drugs present in contaminated waters, among them stands out the adsorption process. In this work, the adsorption of metformin, an antidiabetic drug, using a commercial natural zeolite as an adsorbent was investigated. The material was characterized by Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), X-ray diffractometry (XRD), X-ray Fluorescence (XRF), N2 adsorption/desorption analysis and zeta potential. The efficiency of metformin removal was evaluated under different pH conditions and adsorbent ratios. The most favorable conditions for adsorption were obtained for 1 g/L of zeolite and pH 7.0. The pseudo-second order had a better correlation with the experimental kinetic data, with equilibrium obtained after 6 h, reaching 76 % of removal and adsorption capacity of 6.8 mg/g. The Langmuir isotherm described well the equilibrium and the maximum adsorption capacity was 109 mg/g for 25 degrees C and 35 degrees C, and 130 mg/g for 45 degrees C. The Gibbs free energy for all temperatures studied was less than zero, which indicates a spontaneous process of metformin adsorption on the zeolite surface. NaOH solution showed the highest adsorbent regeneration efficiency, with approximately 36 % desorption in 6 h. It was possible to reuse the adsorbent for three consecutive cycles of adsorption/desorption. Afterward, there was a sudden decrease in the adsorption capacity and a collapsing effect on the material surface.