Sustainable Congo red (CR) and hexavalent chromium [Cr(vi)] removal using graphene functionalized aminated lignin composite: mechanistic and reusability insights

Abstract

This research aims at introducing a new composite capable of removing both a textile dye [Congo red (CR)] and a toxic metal, [hexavalent chromium Cr(VI)], from wastewater using a single material. The composite was developed through a facile and eco-friendly approach utilizing microwave-exfoliated graphene (MG) with bio-derived aminated lignin (AL). Unlike most studies that investigate either dyes or heavy metals separately, this work demonstrates the dual functionality of MG–AL in targeting both classes of contaminants within one framework, while also providing kinetic, isotherm, and thermodynamic insights. The synergistic combination of MG and AL, not previously reported, enhanced adsorption selectivity, capacity, and stability by preventing graphene aggregation and improving pollutant affinity through amine functionalization. Comprehensive characterization using FTIR, SEM, TGA, and XRD confirmed successful functionalization, enhanced stability, and improved crystallinity. Under optimal conditions, the composite achieved remarkable maximum adsorption capacities of 121.3 mg g⁻¹ for CR and 196.4 mg g⁻¹ for Cr(VI), surpassing many reported biomass-based adsorbents. Adsorption followed pseudo-second-order kinetics and Langmuir isotherm, indicating chemisorption and monolayer coverage. Thermodynamic parameters confirmed that the adsorption process was spontaneous (ΔG⁰ = −9.09 kJ mol⁻¹ for CR dye, and ΔG⁰ = −14.15 kJ mol⁻¹) and endothermic (ΔH⁰ = 53.02 kJ mol⁻¹ for CR dye, and ΔH⁰ = 49.08 kJ mol⁻¹ for Cr(VI)). Notably, the composite retained significant removal efficiency after five regeneration cycles [80.89% for CR dye and 85.31% for Cr(VI)], outperforming many graphene or lignin-based adsorbents typically tested for fewer cycles. Mechanistic studies indicate that electrostatic attraction between protonated amine groups of AL and anionic pollutants (HCrO₄⁻/Cr₂O₇²⁻ and dye sulfonate groups) plays the predominant role in adsorption, supplemented by π–π interactions with defect-rich graphene surfaces and hydrogen bonding between functional groups. This dual-function, reusable adsorbent advances lignin valorization and aligns with green chemistry principles, providing a scalable and versatile material for wastewater treatment.