Room-temperature one-step synthesis of amine-functionalized lignin with ultra-high nitrogen content for efficient adsorption of Hg(II) and Congo red from wastewater.

Abstract

Lignin has attracted attention in water treatment due to its extensive sources, complex structure, environmental friendliness, and functionality. Mannich modification is an effective method for enhancing the lignin's active sites, but it typically requires high temperatures and long reaction times, which limits its scalability for practical application. This study presents a simple, one-step Mannich reaction at room temperature for synthesizing aminated lignin (NAL) with high yield and ultra-high nitrogen content (17.34 %), using triethylenetetramine (TETA) as a modifier. The method effectively leverages TETA's symmetrical polyamine properties and glutaraldehyde (GDA)'s dual crosslinking functionality. Density functional theory (DFT) revealed that the potential and the discrepancy of reactive groups' energy gap (E_gap) were the key factors influencing the synthesis. The as-designed NAL exhibited excellent adsorption capacities of 1088.71 mg/g and 909.09 mg/g at 318 K for Hg(II) and Congo red (CR), respectively, that were superior to most reported modified lignin-based adsorbents. NAL also demonstrated robust resistance to ion interference, good reusability, and practical applicability. Notably, the adsorption performance of CR in the Hg(II)-CR binary system was enhanced with an adsorption capacity ratio (R_q) reaching 2.09. XPS, Zeta potential, and DFT calculations revealed that NAL's superior adsorption properties result from multiple interactions, including coordination, π-π interactions, hydrogen bonding, and electrostatic forces. Overall, NAL represents a green and highly effective material for environmental remediation with significant research and practical application value.