Lignin produced from two processes, 1. An acid hydrolysis process: sequential acid hydrolysis of Paulownia elongata wood powder (Klason lignin); 2. Kraft pulping process: black liquor (Kraft lignin), were studied for their heavy metal adsorption affinities. Both lignin samples were effective in removing Pb(II), providing the industrial application potential of lignin in treating lead contaminated wastewater. Hot water treatment of woody biomass is a typical biorefinery process that can result in lignin extraction for potential applications. Hot-water treatment was thus evaluated for its impact on the heavy metal adsorption affinities of Klason lignin. It was found hot water treatment can either enhance or weaken the lignin adsorption capacity depending on the severity of the treatment. Samples with long duration of treatment encountered a substantial loss in the adsorption ability. Depolymerization and condensation lignin reaction schemes under acidic and high temperature environment were summarized and applied to explain the affinity changes. The adsorption mechanism was further studied with Kraft lignin. The Pb(II) adsorption affinity of Kraft lignin was found to follow an “S” dependency on the environmental pH, indicating the existence of more than one ion-exchanging functional groups involved in the adsorption process. NMR characterization of Kraft lignin discovered phenolic hydroxyl groups and carboxyl groups as ion-exchanging functional groups. Other chemical structures in Kraft lignin such as aldehyde groups, ketone groups, ether bond, and aromatic rings are also considered as adsorption functional group because of their potential to complex with heavy metal cations. In order to explain the process from a mechanistic point of view, an novel adsorption theory named “1-n cooperative adsorption theory” was proposed which considered the existence of multiple active sites and the interaction of one adsorbate to multiple sites. The derived model evaluated the effect of temperature and pH on the adsorption affinity, which achieved a significant improvement compared to the Langmuir model. The simulation results show the binding affinity towards Pb(II) is significantly higher than Cd(II) and Ni(II). The new theory also has wide application range to other multivalent interactions including adsorption, flocculation, chelation, and filtration
