•The functionalized lignin (FL) successfully fabricated from industrial black liquor.•High quality FL was deeply characterized.•FL was effective for the unprecedented adsorption characteristics.•Results were verified by classical kinetics, isotherms, and thermodynamic studies.•Strong chemical interaction occurred between FLA and Reactive Green 19 dye. The availability of lignin in significant quantities as a byproduct from the paper industry from industrial black liquor is still an issue. On the other hand, its potential for use as a green, low-cost adsorbent for the efficient removal of harmful organic dye from industrial effluent has attracted attention, although the challenge remains difficult. In this way, the goal of this study was to see if it was possible to remove the reactive green dye from the functionalized lignin-based adsorbent (FLA) using an acidic approach. The kinetic analysis revealed that the adsorption process followed a pseudo-second-order kinetic model with intra-particle diffusion. The FLA also demonstrated multi-layer adsorption, according to the isotherm data. Meanwhile, FTIR and SEM studies revealed that electrostatic interaction, hydrogen bonding, - stacking, and physical adsorption were the RG-19 adsorption processes. Thermodynamic characteristics were also examined, and the results revealed that RG-19 adsorption on FLA is an exothermic and spontaneous process. Molecular dynamics (MD) simulations in the atomic aspect have proven the response behind the higher adsorptive of FLA toward RG-19 under the force of interaction. FLA has the potential to be employed as a promising sustainable and economic use of biomass as a renewable, low-cost, and natural biomass material, inspiring more researchers to explore solutions to the problem of polluted rivers and streams.