Unraveling the adsorption mechanism of methylene blue onto selective pH precipitated Kraft lignins: Kinetic, equilibrium and thermodynamic aspects.

Lignin has been used on its crude or modified forms for adsorption purposes. This work evaluated the influence of selective pH precipitation of Kraft lignins (KLs) on their adsorptive performance for removing methylene blue (MB). The alkaline and acid KLs (KL A and KLB, respectively) were characterized by FTIR, P NMR, GPC and pH analyses. The effects of biosorbent and adsorbate concentrations, pH, ionic strength, contact time and temperature on the MB adsorption were evaluated. The equilibrium, kinetic and thermodynamic parameters were calculated by Langmuir and Freundlich isotherms, pseudo-first and second order and Van't Hoff and Gibbs models, respectively. KL A and KL B presented peculiar structural features, mainly hydroxyls concentration and M values, which have influenced on the removal efficiency of MB and the adsorptive capacities of KL A (>80 %; ≥80 mg g) and KL B (>90 %; ≥20 mg g), respectively. The equilibrium, kinetic and thermodynamic parameters have shown that MB adsorption presented different mechanisms for each KL, but it only has driven by chemisorption for KL B. Therefore, KL A and KL B can be considered as potential novel biosorbents obtained through a clean, fast and simple route for textile wastewater treatment.