The thesis (dissertation) is devoted to the synthesis and investigation of the structure and physicochemical properties of plant by-products and lignocellulosic (LC) sorbents.
Schemes for modification of plant raw materials have been elaborated. Also, new technologies for obtaining LC materials from plant by-products, that are competitive and ecofriendly and exclude separating plant by-products into high-molecular components, were developed. Based on experimental data and mathematical modeling, the optimal conditions for acid-base modification of plant raw materials for their usage as adsorbents of toxic substances have been established.
The chemical structure and physicochemical characteristics of plant by-product (buckwheat seed husk, rice and sunflower husk, cocoa, crushed corn cobs, fruit pits (apricot, cherry, peach), hazelnut and walnut shells) have been studied. Analysis shows that the main components of such by-products are cellulose, hemicellulose, and lignin, which are bounded in biopolymer complexes. The morphology and molecular organization of LC-complexes, the influence of various modification factors that lead to the specific surface area, and the content of functional groups (sorption centers) increasing, have been studied. It was concluded that examined samples have lignin of guaiacyl-syringyl type.
Modification of the investigated samples leads to the lignin content increasing with simultaneously cellulose content decreasing. This fact was confirmed by TPD-MS analysis, which showed isolation of a significant part of hemicellulose from the vegetable raw materials after modification. Such structure changing indicates the destruction and washing out of hemicellulose. The structure transformation after sample modification also was proved by the 13C NMR analysis, which showed the signals of the hemicellulose groups disappearing, whereas the signals the cellulose and lignin groups increasing. The NMR spectra of the modified samples differ from the spectra of the original samples by a significant decrease in the methyl and ethyl signal intensity, as well as alkyl and carboxyl groups of hemicellulose. The bond’s destruction of lignin, cellulose, and hemicellulose was confirmed by IR spectra, which show a decrease in the intensity of those bands that are associated with a lowering in the energy of inter-and intramolecular H-bonds of polymer.
The SEM analysis of the original and modified sample revealed structural changes in the LC-complexes, as well as lignin and cellulose isolation from the modified raw materials. It should be noted that separation (or purification) of the plant raw materials without intensive chemical exposure does not lead to the synthesis of lignin-free cellulose samples. However, acid-base modification LC-matrix allows to separation "pure" lignin from cellulose samples.
It was concluded that the type of adsorbent (its composition, spatial structure, size, molecular weight, association abbility, and the presence of functional groups is a key factor that determines the efficiency of absorption. It has been proved that the sorption capacity of plant materials is significantly increasing after chemical modification without prior separation of LC matrices into constituent biopolymers. It was established that the obtained sorbents have a lot of oxygen-containing functional groups (-COOH, -CON, -OH) with high metabolic and sorption capacity. Considering the sorption of heavy metal ions Ni2+, Cd2+, Pb2+, Cu2+, Sr2+, it could be concluded that besides their own high sorption capacity, modification of the raw plant by-products leads to its increasing in 15-35%, and recovery could be reached 90%. The adsorption of the 137Cs from multicomponent saline solution under static conditions on the LC materials also have been examined. The extraction effect of radionuclides was from 63 to 97%, which allowed registering trademark "Celisorb". The dynamics of changes in the amount of 137Cs the child bodies were studied. It has been shown that extraction effects were from 27 - 49.2 %.
Key words: lignin, LC materials (complexes, sorbents, matrices), acid-base modification of plant raw materials, sorption of organic dyes, heavy metal ions and radionuclides.
