The Solution Behavior Of Lignosulfonate And Its Adsorption At The Air/Liquid And Solid/Liquid Interfaces

Making full use of natural renewable resources and producing environmentally friendly green materials through green catalytic processes have become the prime frontier research in the linked fields of chemistry and chemical engineering. Lignin is the main component of the plant cell wall; it exists widely in nature, and it is the second most abundant biopolymer (after cellulose) on earth. Lignosulfonates come from the byproducts of the pulp and paper industry; they are renewable, low cost, nontoxic and abundant. As a kind of anionic surfactant, lignosulfonates can be widely used as a coal-water slurry additive, a concrete water-reducing agent, a pesticide dispersant, a ceramic grinding agent and so on. The application performance of lignosulfonates is closely related to their physicochemical properties, such as their surface activity and adsorption performance (which is affected by the solution behavior of lignosulfonates) at the interface. Thus, study on the solution behavior of lignosulfonates and its adsorption mechanism at interface has an important meaning for improving its application performance. But it is difficult to obtain regularity results from the studies on the solution behavior and interface adsorption mechanism of lignosulfonates as the uncertaincy structure and composition complexity of lignosulfonates. In this thesis we report on studies of the influence of ionic strength and pH on the solution behavior of lignosulfonates and their adsorption at the air/liquid and solid/liquid interfaces. The structure of lignosulfonates in aqueous solution and their adsorption mechanism at interfaces are analyzed. This research can provide theoretical instruction for industrial applications of lignosulfonates. This research has an important theoretical significance and application value for the utilization of lignosulfonates as resources.Firstly, commercial sodium lignosulfonate (SL) was purified with the ionic-resin method, ultrafiltration, long chain aliphatic amine extraction and solvent extraction. The structure, composition and molecular weight distribution of raw SL and purified SL were characterized by infra-red spectroscopy, elementary analysis and gel chromatography. Experimental results indicated that solvent extraction did not purify raw SL efficiently. The ionic-resin method, ultrafiltration and long chain aliphatic amine extraction removed impurities with molecular weight less than 1000; after purification, the content of lignosulfonate was raised from 59.0% to 90.0%; the number average molecular weight and weight average molecular weight of purified SL increased. Ultrafiltration is the most favourable purification method, as measured by the purified SL yields, the purification efficiency and the purification technology. Consequently, in this thesis we chose the purified SL (PSL) obtained from the ultrafiltration method with 2500 ultrafiltraion membrane as our research sample for studing the solution behavior of PSL and the adsorption of PSL at air/liquid and solid/liquid interfaces.Secondly, the solution behaviors of PSL at different ionic strengths and pH values have been investigated by means of acid-base titration, surface tension, viscosity, fluorescence spectrometry and dynamic light scattering (DLS) experiments. Results showed that the critical aggregate concentration (CAC) of PSL was 0.05g/L; when the concentration of PSL was below CAC, there were a few of PSL aggregates in solution; when the concentration was above CAC, the number of PSL aggregates increased sharply. The average dimensions of PSL molecules and PSL aggregates in aqueous solution were about 8 nm and 80 nm, respectively. The surface of PSL aggregates was mainly covered by sulfonic groups and a few of phenolic hydroxyl groups, and the core of PSL aggregates was generated by hydrophobic chains. The hydrophobic core was loose and contained many weakly ionized groups such as carboxyls and phenolic hydroxyl groups. Increasing the PSL concentration favoured aggregation of PSL. While along with the increasing of the pH value, the number of ionic groups in the hydrophobic core also increased, and the PSL hydrophobic core became looser; thus the diameter of PSL aggregates increased as the pH value increased. The electrostatic repulsion between PSL molecules can be shielded by addition of salt, so that the diameter of PSL aggregates increased as the ionic strength increased.Thirdly, the influence of ionic strength and pH on the adsorption behavior of PSL at the air/liquid interface was studied by a dynamic tension meter and an LB film balance. The results showed that the adsorption of PSL at the air/liquid interface was a slow process. The surface tension of PSL solutions decreased as the ionic strength increased and the pH value decreased. PSL could form unstable and soluble films at the air/liquid interface; furthermore, the stability of the PSL film at the air/liquid interface decreased as the concentration of PSL and ionic strength increased and the pH values decreased. The results indicated that the PSL with higher molecular weight was preferably adsorbed at the air/liquid interface, and then the PSL with lower molecular weight was adsorbed at the air/liquid interface when the concentration of PSL increased. In addition, the arrangement of PSL at the air/liquid interface turned from disorderly to orderly as the concentration of PSL increased.Fourthly, the influences of temperature, pH, ionic strength and addition of urea on the adsorption of PSL at TiO₂/water interface were investgated. The molecular weights of PSL before and after adsorption were determined. The experimental results showed that the adsorption rate of PSL at TiO₂/water interface increased as the temperature increased and decreased as the pH value increased, while it decreased first and increased later with the ionic strength increased. The adsorption amount increased as the temperature and ionic strength increased, but decreased with the pH value increased and addition of urea. The PSL with higher molecular weight was first adsorbed at TiO₂/water interface. The adsorption isotherms of PSL at TiO₂/water interface conformed to the Langmuir adsorption model at different temperatures, pH values and with addition of urea, but with addition of NaCl, the adsorption isotherms of PSL at TiO₂/water interface corresponded to the Freundlich adsorption model.Fifthly, the influences of temperature, pH, ionic strength and addition of urea on the adsorption of PSL at active carbon/water were researched. The molecular weights of PSL before and after adsorption were determined. The experimental results showed that temperature had little influence on the adsorption of PSL at active carbon/water interface. The adsorption rate and adsorption amount of PSL increased with increasing of ionic strength increased and decreasing of pH value, but the adsorption amount of PSL decreased with addition of urea. The PSL with lower molecular weight was first adsorbed at the active carbon/water interface. The adsorption isotherms of PSL at active carbon/water interface conformed to the Langmuir adsorption model at different temperatures, pH values and with addition of urea; however, it corresponded to the Freundlich adsorption model with addition of NaCl.Based on the experimental exploration, it was concluded that PSL absorbed at TiO₂/water and active carbon/water interfaces by means of electrostatic interaction, hydrogen bonding and hydrophobic interaction. There had some differences between the two adsorbent processes. Firstly,the adsorption rate and amount of PSL at TiO₂/water interface increased as temperature increased, while the temperature did not have an effect on the adsorption of PSL at active carbon/water interface. Secondly, the adsorption amount of PSL at the active carbon /water interface was 10 times as the TiO₂/water interface under the same conditions. Thirdly, the PSL with higher molecular weight was preferably absorbed to the TiO₂/water interface, while the opposite happened at active carbon/water interface because of their rich porous structure and higher specific surface area. The decrease of the pH value and the increase of the ionic strength had the same effect on the adsorption behavior when PSL were adsorbed on the two adsorbents. With addition of urea, the adsoption amount of PSL on the two adsorbents decreased, and the reduced degree on active carbon surface is bigger than that on TiO₂ surface. It indicated that the hydrogen interaction was larger when PSL adsorbed on active carbon than that on TiO₂ surface. The results could provide a significant insight in understanding the adsorption and application of PSL on the metallic oxide and porous nonpolar particles.