Investigation of trace metal binding properties of lignin by diffusive gradients in thin films

The binding behavior of lignin for Pb, Cu, Co, Mn, Cd and Ni was studied using the diffusive gradients in thin films technique (DGT). Samplers with different structures of diffusive gel were used in the well-stirred systems containing known concentrations of metals along with (a) 10, 20 and 40 μM lignin and; (b) 0.64 and 6.47 μM Suwannee river fulvic acid + 40 μM lignin at an ionic strength of 0.01 M (NaNO₃) and pH = 7. Diffusion coefficients of lignin complexes in acrylamide gels were estimated and found to be less than 5% of the equivalent coefficients for the uncomplexed metal ions. These values were used to calculate concentrations of labile metals from DGT measurements in solutions, where lignin could discriminate metals in the order of Pb⁺² > Cu⁺² > Cd⁺² > Ni⁺² > Co⁺² > Mn⁺². Stability constants (Log K) were calculated using Visual MINTEQ II and WHAM V software. The K values were compared with the stability constants from titration of Pb and Cd with 10 μM lignin aqueous samples and with those of humic substances in natural waters. The constants obtained from measurement of complexing capacities might bias the real corresponding values unless two line regression analyses on titration data are considered. The DGT study of fractionation of metal species at varying ratios indicated that the proportion of organic complexes decreased with increasing ratios and gradually more metals were exchanged with inorganic phases. Speciation of Pb and Cd is affected by the concentrations of FA, Cd is dominantly bound with FA while Pb is evenly partitioned between the ligands. The comprehensive knowledge of metal-lignin complexes sheds some light on in situ operational speciation information that can be achieved by DGT.