Carbon sorbents from peracetic lignin
Authors: Halysh Vita , Deykun Iryna , Trus Inna , Radovenchyk Vіacheslav , Gomelya Mykola 
Published: September 2022

The transition to non-traditional types of plant raw materials for the production of cellulose and materials based on it is becoming more and more relevant for for scientists and industry around the world. The most expedient for this is the use of waste from the agro-industrial complex in the form of straw or stalks. During the delignification of non-wood lignocellulosic raw materials by the organosolv method, which is quite environmentally friendly, spent solutions with a high soluble lignin content are formed. From an ecological point of view, it is important to create a complex, practically waste-free technology for processing lignin into target products. The paper investigated the possibility of using lignin precipitated from the spent cooking solution after organosolv delignification of rapeseed straw with the following carbonization for obtaining sorbents for the removal of dangerous synthetics dyes from water solution.

At the end of cooking, the cellulosic product was separated from the spent solution by filtration. The spent solution was collected in a separate container and treated with 5 volumes of water, resulting in lignin condensation. Condensed lignin was separated from the cooking solution by centrifugation at 5500 rpm for 10 min and washed with distilled water, dehydrated by centrifugation and dried at a temperature of 80 oC until a constant moisture content of 7-8% was reached.

Then lignin was treated with a 20 wt.% solution of orthophosphoric acid or 20 wt.% solution of sodium hydroxide, after which it was carbonized in a muffle furnace.

Carbonization of lignin leads to a decrease in the average pore size to 2.191-4.421 nm and to an increase in the pore volume by 1.8 times when treated with sodium hydroxide and 2.0 times when treated with orthophosphoric acid, which allows to increase the sorption capacity of the material.

The influence of the pH of an aqueous solution on the process of pollutant sorption is decisive. Oxygen-containing functional groups give the surface of sorption materials a positive charge in an acidic environment, which leads to the formation of electrostatic repulsion forces with the dye cation, which also has a positive charge. When the pH is increased to slightly alkaline, dissociation of functional groups occurs, therefore the surface of the sorbent acquires a negative charge, which leads to the emergence of forces of electrostatic attraction with the dye. Absorption of cationic dye increases with increasing pH from 2.0 to 6.0, further increase in pH does not affect the efficiency of the process.

Carbonization of lignin leads to an increase in the porosity of the sorbent, so it takes more time to reach full sorption equilibrium. During the first 80 minutes, the maximum absorption rate of methylene blue is observed, after 180 minutes of contact, full sorption equilibrium is reached.

Using the Langmuir equation, the sorption isotherm of dye absorption on the obtained sorbents was estimated, and it was established that the vacant adsorption sites on the lignin surface are homogeneous and the dye adsorption is monolayer. It was established that the sorption kinetics is described by a pseudo-second-order model.

The development of an effective method of peracetic lignin utilization as carbon sorbent allows to create resource-saving technologies.