Speciation of chromium on a straw lignin: adsorption isotherm, EPR, and XAS studies

The binding ability of a lignocellulosic substrate extracted from wheat straw with chromium(III) ion was investigated. The study was carried out at macroscopic and microscopic scales. The adsorbent used was first characterised using X-ray diffraction and electron microscopy. Then, macroscopic studies were conducted using batch adsorption experiments, at room temperature, as a function of time, pH, and metal concentration. The results obtained indicate that several successive phenomena take place at the substrate surface. Indeed, sorption, co-precipitation, and precipitation processes arise depending on the experimental conditions. The speciation of Cr was also investigated, at the atomic scale, by EPR, extended X-ray absorption fine structure (EXAFS), and X-ray absorption near edge structure (XANES). This study revealed that chromium(III) surface complexes have an octahedral geometry. Chromium ions are held in inner-sphere complexes, and are coordinated to six oxygen atoms at an average distance of 1.90 Å.