Copper and cadmium sorption onto kraft and organosolv lignins

The relative metal sorption ability of kraft and organosolv lignins was examined as a function of solution chemistry (pH, ionic strength (I), sorbate-to-sorbent ratio) and reaction time. The surface charge characteristics and functional group composition of these lignins, especially kraft lignin, are favorable for metal sorption. Sorption of Cu and Cd increased with increasing pH and decreasing I. Description of sorption isotherms required the more complex Sips equation, as compared to the simpler Langmuir and Freundlich formulations, indicative of the presence of binding sites with varying affinities on these lignin biosorbents. Sorption capacity varied in the following order: softwood organosolv lignin < hardwood organosolv lignin < hardwood kraft lignin < softwood kraft lignin with sorption maximum of 21.5, 40, 66.7, and 80.6 μmol/g, respectively, for Cu, and 8.2, 18.3, 25.2, and 28.7 μmol/g for Cd. Both Cu and Cd sorption kinetics were rapid with equilibrium levels attained within 80 min and faster uptake was noticed for Cu. Strong competitive effects exhibited by H+ and Na+ in limiting Cu and Cd sorption are suggestive of the occurrence of weak ion-exchange type interactions involving the carboxylic and phenolic functional groups. Additional pretreatment and surface modifications of these biosorbents might be required to increase metal sorption capacity and enhance their use in waster/wastewater treatment.