Synthesis of porous carbons for the fixing of cationic molecules in aqueous solution

Abstract

• A pre-treatment with soda and then an H 3 PO 4 -activation under moderate heat was used to valorise the Hibiscus sabdariffa stems. • Several models (B.E.T., Rouquerol, 2D-NLDFT, …) were used to characterize the textural properties of the porous carbons obtained. • The charcoals with a predominantly microporous surface adsorbed more than 90 % of methylene blue at different concentrations. In Africa, the calyxes of the Hibiscus sabdariffa are widely used to produce a fresh fruit juice, known in Benin as bissap. The stems from this production have no potential use and are therefore an environmental problem. The parietal composition of these stalks revealed a high level of lignin (31 ± 1 %) and cellulose (35 ± 1 %). They can therefore be converted into activated carbon for various applications. In this study, the stems were transformed into activated carbons at moderate temperature using orthophosphoric acid (H 3 PO 4 ) as the activating agent. The physicochemical, structural, and textural properties were determined. Adsorption kinetics and isotherms were studied and modelled. Porous carbons give specific surface areas equal to 1066 and 1053 m 2 .g −1 respectively for 2:1 and 4:1 ratios according to the B.E.T-Rouquerol model. Similarly, the said porous carbons give specific surface areas equal to 991 and 975 m 2 .g −1 respectively for 2:1 and 4:1 ratios according to the 2D-NLDFT model. Using the latter model, the pore size distribution is predominantly microporous, with the Vµ/VT (%) ratio increasing to 56 %. Thermogravimetric analysis of the porous coals showed favourable fixed carbon levels (≈ 60 %). The adsorption kinetics of methylene blue on activated carbons is governed by the pseudo-second order kinetic model indicating that the dye adsorption process is globally controlled by chemisorption. The results of the modelling of the methylene blue adsorption isotherms shown that the Langmuir model is the most credible model that best describes these experimental results, with 588 and 526 mg.g −1 . These activated carbons are good candidates for the development of bio-sourced filters for the treatment of industrial wastewater.