Adsorption of Methylene Blue on Organosolv Lignin from Rice Straw

Organosolv lignin (OL) is a relatively pure, unaltered and high-quality lignin, bearing high phenolic and aliphatic hydroxyl contents, which makes it a promising adsorbent in wastewater treatment. In this paper, as a byproduct of delignification of steam-exploded rice straw with the mixed solvents, OL was extracted. The characterization using FTIR, light microscopy and SEM showed that the obtained OL has the typical functional groups of gramineous lignin and a honeycomb-like structure. Batch adsorption studies were carried out for the adsorption of methylene blue (MB) onto the OL. The influences of experimental parameters such as pH, initial dye concentration and contact time were investigated. Results showed that the adsorption of MB on OL was strongly pH-dependent and gave good results in a wide range of pH values (about 5.0∼9.0). Compared with pseudo-first-order kinetic model (R2= 0.935), pseudo-second-order kinetic model (R2= 0.984) provides a better fit to the adsorption kinetic data. Furthermore, the calculated equilibrium adsorption capacity qe is 20.38 mg/g which is very similar to the actual amount of adsorption equilibrium (20.62 mg/g). Regarding the adsorption isotherms, Langmuir equation (R2= 0.999) was more suitable than Freundlich equation (R2= 0.920). The monolayer capacity of organosolv lignin at 20̊ was 40.02 mg/g, which is comparable with some other lignocelluloses adsorbents reported before.