Iron and manganese surface complex formation with extracted lignin. Part 1: Adsorption isotherm experiments and EPR spectroscopy analysis

Abstract

The adsorption of iron(III) and manganese(II) onto lignin extracted from wheat straw was investigated at 20 °C using batch adsorption experiments. Two solid substrates were studied : the Ligno-cellulosic Substrate (LS), which resulted from successive acido-basic treatments, and the Cell Wall Residue substrate (CWR), which was obtained by solvent extraction. Comparisons between these two lignin-containing substrates were made. Experiments were conducted as a function of the pH for a wide range of metal concentrations. The percentage of metal adsorbed increased with the pH and the results indicate that Fe(III) was strongly adsorbed at low pH (<3) while Mn(II) formed far less stable surface complexes from pH = 8.0. Moreover, in contrast to Fe(III), Mn(II) adsorption was significantly affected by the presence of calcium and carbonate, which are abundant ions in the soils of the Champagne-Ardenne region. The presence of calcium ions decreased the manganese adsorption by 25% to 40%. EPR spectroscopy was used to investigate the geometrical environment of the metal in the surface complexes. As the metal was adsorbed, redox processes took place, which could be observed by the change in the amount of quinonic species present in the lignin surface. Then, the evolution of quinonic species amount as a function of the metal concentration was examined by EPR measurements considering the signal centred at g = 2.0025. Mechanisms involved in the electronic transfer between the metal ion and the surface organic radicals are discussed.