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

## TL;DR

A new eco-friendly composite material effectively removes both a textile dye and toxic chromium from wastewater, with high efficiency and reusability.

## Contribution

A novel dual-function composite using graphene and aminated lignin is introduced for simultaneous removal of Congo red and Cr(vi) with mechanistic and reusability insights.

## Key findings

- The composite achieved maximum adsorption capacities of 121.3 mg g−1 for Congo red and 196.4 mg g−1 for Cr(vi).
- The material retained 80.89% efficiency for Congo red and 85.31% for Cr(vi) after five regeneration cycles.
- Adsorption mechanisms include electrostatic attraction, π–π interactions, and hydrogen bonding.

## 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.

A sustainable graphene–aminated lignin (MG–AL) composite achieves dual removal of Cr(vi) and Congo Red from wastewater via synergistic amine functionalization, defect-rich graphene, high chemisorption, and excellent reusability.