Adsorption of Metformin on Activated Carbon Produced from the Water Hyacinth Biowaste Using H 3 PO 4 as a Chemical Activator

Water hyacinth biomass was used for the synthesis of activated carbons in the process of chemical activation with H 3 PO 4 , followed by controlled carbonization. The study investigates the effect of various impregnation weight ratios of H 3 PO 4 and dry hyacinth (0.5–3.0), as well as different carbonization temperatures (T = 400–800 °C), on the surface characteristics of the produced activated carbons (AC). The activated carbon obtained with an impregnation ratio of 1.5 and a carbonization temperature of 600 °C (1.5 AC/600) showed the highest values of specific surface area of 1421 m 2 g −1 , representing a selected adsorbent for metformin removal. The chosen sample was characterized by elemental analysis, adsorption–desorption isotherms of nitrogen at −196 °C, via FTIR spectroscopy and the SEM method. The modeling of the experimental adsorption data showed that metformin adsorption: (i) can be best described by the Langmuir isotherm model, with the value of q max = 122.47 mg g −1 ; (ii) led the pseudo-second order kinetic model; and (iii) is a spontaneous (Δ G ° = −3.44 kJ mol −1 ) and endothermic (Δ H ° = 8.77 kJ mol −1 ) process. A desorption study has shown that 92% of metformin was successfully desorbed in the presence of a 0.1 MHCl/ethanol mixture (volume ratio 2:1). The recovery of the adsorbent of 84%, after five successive cycles, indicated that the 1.5 AC/600 has potential to be applied in the real systems for water treatment.