Synthesis, characterization and environmental application of an original adsorbent: polyaniline-coated luffa cylindrica

Abstract

The coating of the fibrous skeleton of luffa cylindrica (LC) with polyaniline (PANI) was carried out by in-situ polymerization of aniline. A thin PANI layer in its emeraldine-salt form (ES) got bound on to the LC surface, as revealed by the green color the latter developed, procuring more roughness to the luffa fibers. The surfaces of both materials, i.e. the purified original luffa (LC) and the PANI-coated luffa (PANI/LC) were characterized by FTIR, completed by the Raman range, SEM/EDX and XRD. In particular, the presence of N–H moieties in the FTIR spectrum and the appearance of the following bands: C = N stretching of the quinoid di-imine units, C = C stretching of the quinoid ring and C–N.+ band of the radical-cation in the Raman spectrum confirmed the effectiveness of the coating of LC. Dedoping, as determined by solid addition method, completed at around pH 9.5. After characterization, PANI/LC was tested for its capacity to adsorb hexavalent chromium anionic species. First, the emergence of the chromium characteristic peak in the EDX spectrum brought experimental evidence for the effective sorption of chromium onto the PANI/LC surface. Then, the novel material displayed a high efficiency to retain the metallic pollutant, going up to ~ 300 mg.g−1. Thus, after the PANI-grafting, this light and cheap agricultural by-product should compete advantageously with commercial anionic exchangers, at least in terms of efficiency. Besides, adsorption parameters were examined. The Cr(VI) uptake process was found to follow second order kinetics, with a pseudo-second order rate constant equal to 0.7144 × 10–3 g.mg−1.min−1. At the lower end of concentrations, the adsorption efficiency decreased with increasing temperature. The computed thermodynamic parameters (namely ΔH°, ΔS° and ΔG°) indicate that adsorption is exothermic, with an unfavourable entropy change, and confirm its spontaneity. None of some classical models fitted the equilibrium sorption data, a result that expresses the sorption mechanism complexity.