Facile synthesis of iminated lignin for enhanced free radical and lead ion scavenging capabilities

Kraft lignin (KL) holds significant potential as a renewable resource for the development of innovative materials that are currently not fully utilized. In this study, a novel iminated lignin (IL) was synthesized by grafting primary amine lignin (N-KL) onto salicylaldehyde. The effects of the dosage and reaction temperature on the nitrogen content of N-KL were evaluated. The maximum nitrogen content in N-KL reached to 3.32 %. Characterization by spectroscopy techniques (FT-IR, XPS, and NMR), elemental analysis, and gel permeation chromatography confirmed the imination of lignin. Additionally, the antioxidant activity of the lignin samples was investigated using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging ability. Moreover, the DPPH radical scavenging capacity of IL-6 (IC₅₀ = 38.6 ± 3.9 μg/mL) was close to that of commercial antioxidant butylated hydroxytoluene (BHT) (IC₅₀ = 37.7 ± 4.5 μg/mL). Furthermore, the adsorption equilibrium results indicated that IL-6 had a maximum uptake of 115.6 mg/g Pb²⁺, which was 3.2-fold higher than that of KL. Kinetic adsorption experiments suggested that IL-6 adsorption follows a pseudo-second-order model. Therefore, the synthesized iminated lignin is a promising candidate for the development of environmentally friendly materials with applications as an antioxidant and lead-ion adsorbent.