Acid/base and Cu(II) binding properties of natural organic matter extracted from wheat bran: modeling by the surface complexation model

A simple surface complexation model, the double-layer model (DLM), was used to describe the sorption of Cu(II) on a lignocellulosic substrate extracted from wheat bran. The solid was first characterized by IR and 13C-NMR and the binding of protons was studied by potentiometric titrations. The organic solid was represented by two acid sites. From the acid–base titration data, total concentrations of these sites and H+ binding constants (SiOH SiO−+H+) were obtained by using the computer program FITEQL 3.2. The binding of Cu(II) was then investigated. The thermodynamic approach was completed by two spectroscopic studies. A laser microspectrofluorimetry study was used to specify the adsorption phenomena. The square planar geometry of the solid/Cu complexes and the formation of inner-sphere complexes were suggested by an ESR study. After identifying the stoichiometry of the surface complexes by potentiometric titrations of the Cu2+/solid system, the surface complexation model was applied to describe Cu(II) binding by the lignocellulosic substrate. A single set of data was used; the model fit was good and covered a wide range of pH. The surface complexation constants derived from this single metal data set were then used to predict the sorption for increasing metal loading, different solid to solution ratios or different ionic strength. The predictions agreed well with the experimental data showing the capability for the DLM surface complexation model to describe Cu binding to natural organic matter with a minimum number of adjustable parameters (only five parameters were necessary to describe the data).