Sorption properties of organosolv lignin towards methylene blue

Abstract

Involvement of secondary raw materials such as plant wastes and by-products of plant materials processing for solving problems of environmental pollution is considered as a promising direction by scientists in many countries of the world. The purpose of the paper is to study the structure and sorption properties of lignin as a by-product of organosolv delignification of wheat straw. In this work lignin precipitated from the spent cooking solution after oxidative-organosolv delignification of wheat straw was used as the raw material. Different methods of analysis were used to study the structure and properties of the obtained lignin, including gravimetric, spectroscopy, and microscopy. The sorption capacity of the material with respect to methylene blue was studied using model solutions and a spectrophotometric method to determine dye concentrations.Investigation of the sample by the method of infrared spectroscopy and scanning electron microscopy showed that organosolv lignin is a highly dispersed material with particles of various shapes and sizes, and contains a small number of polysaccharides, the total pore volume of such material is 0.431 cm3. The sorption capacity of lignin towards methylene blue was investigated as a function of the pH of the dye aqueous solution, its initial concentration, and the duration of the process. It is shown that the sorption capacity is largely depends on the pH of the medium and increases with a change in pH from 3 to 6. However, a further increase in pH does not change the specified indicator. This dependence of the sorption capacity is related to the property of the surface of the plant material to change the charge from positive in acidic medium to negative in neutral and alkaline, due to the presence of oxygen-containing functional groups (mainly phenolic and aliphatic hydroxyls), which are easy can be protonated and deprotonated. The absorption kinetics of the dye is quite fast and the sorption equilibrium is reached within the first 120 min of contact. Pseudo-first and pseudo-second order models and intraparticle diffusion were used to study the sorption kinetics. Compared to the kinetic model of pseudo-first order (R2 = 0,985) and intraparticle diffusion (R2 = 0,856), the kinetic model of pseudo-second order (R2 = 0,999) better describes the kinetics of sorption and indicates that fixation of the dye on the surface of the material takes place with the involvement of different mechanisms. To study the adsorption equilibrium in the sorption of methylene blue, model solutions with initial dye concentrations of 30 to 500 mg/l were used. It was found that at an initial concentration of methylene blue in a solution below 100 mg/l, the sorption capacity increases linearly with increasing initial concentrations. Then the increase in the sorption capacity is slower. The maximum sorption capacity is 44.7 mg/g. In the case of the adsorption isotherm, the Langmuir equation (R2 = 0.996) is more suitable than the Freundlich equation (R2 = 0.955) to describe the absorption process of the cationic dye and indicates that the adsorption is a monolayer.In general, it was shown that organosolv lignin from wheat straw with sorption capacity slightly exceeds oreganoslv lignin from other plant raw materials. Creating new, inexpensive and effective materials is an urgent task for scientists. Developing new resource-saving technologies to solve the problem of environmental pollution is a priority for chemical technology and ecology worldwide. This is especially important for Ukraine, as efficient processing of agricultural plant waste with the use of environmentally friendly technologies will help to meet the needs of many industries and improve the environment. It is promising for further to study the structure of organosolv lignin to determine the content of various functional groups and to determine the possibility of its use for absorption heavy metal ions from aqueous solutions.