Adsorption characteristics of carboxymethylated lignin at a hydrophobic solid/water interface

Carboxymethylated lignin (CML), a water-soluble lignin derivative with more carboxylic groups introduced by chemical modification, can be used as dispersant in industry. In this paper, using the methods of zeta potential measurement, X-ray photoelectron spectroscopy and UV spectroscopy, natural graphite (NG) and high-purity graphite (HPG) were selected to investigate the adsorption characteristics of CML at the hydrophobic solid/water interface. The adsorption isotherm of CML on HPG was more suitable for the Langmuir model than the Freundlich model, whereas the opposite result was obtained for the adsorption of CML on NG. Moreover, the amounts adsorbed of CML on HPG increased with higher ionic strength and lowering solution pH. However, the uptake of CML on NG increased dramatically with adding NaCl under acidic condition, while the ionic strength had no obvious influence on the adsorption under neutral condition. With increasing solution pH, the amounts of CML adsorbed on NG were dropped under acidic condition followed by increases under alkaline condition. It is suggested that the interactions between CML and the minor metallic impurities on NG surface probably contribute considerably to the differences between adsorption behaviors of CML on NG and HPG in varying solution conditions. Moreover, the results showed that the adsorption mechanism of CML at graphite/water interface could be described by Lennard-Jones potential.