COLLOIDAL AND SURFACE PROPERTIES OF LIGNIN FROM EUCALYPTUS WOOD

Abstract

Lignin is the second most abundant renewable organic source after petroleum which constitutes a large percentage „ of wood. It cements cellulose fibres in the plants and is obtained as a bye product in the form of black liquor in the manufacture of paper. Lignin and its derivatives have found use as binder, tanning agents, emulsifying agent in oil well drilling muds, resins, electrolytic refining, storage batteries, flame retardant etc. Lignin offers unlimited challange for future research. Since the discovery of lignin in plants Kbout 150 years ago, enormous work has been reported on the structural investigations andc-olour reactions of this product. Efforts are also under way to understand the mechanism of delignification reactions and dis colouration of lignin and lignin pulps. A beginning has also been made to understand the bio-degradation reactions and gene tic chemistry of lignin. Besides these, it offers unlimited scope for research, in economic production of bulk chemicals as substitutes for petro chemical products. The colloid chemical and surface properties of this product are yet to be fully explored and have attracted little atten tion. Few papers have appeared on the colloidal properties of lignins and these too have been done on lignins derived from soft woods. The paper industry has been using soft woods for the manufacture of paper. With the growing urbanisation, the forest area is depleting very fast and the paper mills have -vnow started using increasing amounts of hard woods along with soft woods. In view of this, it was thought worthwhile to investigate the colloidal and surface properties of Eucalyptus (a hard wood used in the manufacture of paper), which have not been investigated so far. The contents of the thesis are as follows '.- First chapter deals with the isolation and characteriza tion of lignin (section A) and lignosulphonate (section B). For the isolation of thiolignin, the pulping of Eucalyptus chips has been carried out under the following conditions. Amount of chemicals (NaOH+Na S): 2(# ( per cent based on O.D. chips) Sulphidity :25fr Cooking temperature I l6Cr& Bath ratio I 1 I 4 Cooking period :4 hrs(including 1 ^ hrs to raise the temperature to maximum) The thio lignin was recovered from the cooked material by acidification and was purified by Ahlm's method. The thio lignin so obtained was characterized by determining the carbon, hydro gen and sulphur content, methoxyl content, klason lignin content and total hydroxyl content. The high klason lignin content shows that the thiolignin obtained is a pure form of lignin as the carbohydrates associated are quite small. For the isolation of the ligno sulphonate, sulphonation of the Eucalyptus chips has been carried out under the follow ing conditions. -viulphurdioxide * 10$ Wood ' liquor ratio : 1* 6 Cooking temperature : 170°C Cooking period '. 4 hours( including 1- hours to raise the temperature to maximum) Crude ligno sulphonate was isolated from the cooked liquor by precipitation with ethyl alcohol. Lignosulphonate was puri fied by Aries and Pollak method. The lignosulphonate was charac terized by determining the carbon, hydrogen, oxygen and sulphur content, methoxyl content and hydroxyl content. These results indicate that lignosulphonates contain combined sulphur and no free sulphur. Second chapter deals with the colloidal properties of the lignin. The lignin sol has been prepared by solvent replace ment method. Section A describes the electrokinetic studies. These studies show that lignin sol is negatively charged and has zeta potential of -27.7 mV., which is comparable with other hydrophobic sols. The zeta potential decreases with increasing amounts of coagulating electrolytes. A low value of critical zeta potential supports that the charge on the colloidal particle originates mainly from structural characteristics. Section B describes the rheological properties of the lignin sol. The viscosity of the sol increases with increasing electrolyte concentration and this effect is more pronounced with ions of higher valency which is coherent with the effect on zeta potential and coagulation. Bchulz-Blaschke equation -viihas been utilized to evaluate the intrinsic viscosity [rjj and interaction index (k). The axial ratio has been determined using Kuhn's equation. The increase in intrinsic viscosity and the axial ratio with increasing electrolyte concentration indicates the formation of bigger aggregates of higher dissymmetry. The decrease in the values of interaction index indicates a decrease in either or both the hydro dynamic and electrical interactions between suspended units as more and more of electrolytes get adsorbed. Section C describes the coagulation studies of the lignin sol with electrolytes. The sol obeys Hardy-Schulze rule. The validity of the Bhattacharya's equation -g~g = - ,t ♦ - ( where c is the concentration of the electrolyte added, a the critical stability concentration, m and n added are emperical constants and t is time of coagulation) has been tested and is found to hold true for lignin sol at two concen trations viz., 0.75 and 0.375 g/1. The values of the empirical constants a,m and n has also been evaluated. The values of a and m decrease with increasing valency of the coagulating ions. Third chapter deals with the use of ligno sulphonate as an emulsifying agent. Section A covers the interfacial tension and electrical conductivity measurements carried out with kerosene oil/water and turpentine oil/water emulsions. The interfacial tension decreases with increase in lignosulphonate concentra tion, electrolyte concentration, temperature and carbon-carbon alcohol chain. The electrical conductance increases with -viiiemulsifier concentration, emulsification time and temperature and carbon-carbon alcohol chain length and phase volume ratio. Section B covers the evaluation of spreading coefficients, Girifalco-Good's constants and liquid-liquid phase interaction from the interfacial tension data. The values of the spreading coefficients show that higher lignosulphonate concentration and temperature increases the spreading nature while the higher salinity increases the non-spreading nature. The values of the Girifalco-Good's constants show that higher lignosulphonate concentration and salinity favours hydrogen bonding between oil and lignosulphonate solutions, while the temperature disfavours this phenomenon. This is in agreement with the fact that for the hydrogen bond formation in both liquids, Girifalco-Good's constant must be greater than 0.84. Fourth chapter deals with the sorption of two cationic dyes, methylene blue and crystal violet and two metal ions, Cd2* and pb2+ on this material. Section A describes the adsorp tion of the dyes on the lignin. The isotherms are regular and concave to the equilibrium concentration axis. The adsorption of the two dyes increase with increasing pH of the dye solu*- tion and decreasing particle size of the lignin. The adsorption data fits well in the Langmuir model in the entire adsorbate concentration range. The values of the constants r and b have also been evaluated. The specific surface area has also been evaluated. The specific surface area is far less than reported by Jodl from the adsorption of water vapour on rjLJP* cuoxam spruce lignin. These two dyes do not seem to be suitable for specific surface area determination of Eucalyptus lignin. The adsorption increases with increase of temperature in case of crystal violet and process is exothermic but in case of methylene blue, the adsorption decreases with the increase of temperature and the process is endothermic. The heat cf adsor ption vary linearly with the amount of the dye adsorbed on lig nin. The desorption of these two dyes has also been tried by a number of mono, bi and trivalent cations and the desorption process has been found to follow the lyotropic series of the ions. Moreover, it was not possible to recover more than 5% of the dye loaded on lignin surface. 2+ Section B describes the removal of two metal ions Cd and Pb from aqueous solution by lignin. The uptake of the ions increase with increasing pH of the solution and decreasing particle size of adsorbent material. The adsorption isotherms are regular and concave to the equilibrium concentration axis thereby indicating a positive adsorption. The adsorption data fits well in Langmuir model. The sorption of Cadmium is more than that of lead but the maximum uptake of the two metal ions is much higher than that of dyes. These metal ions are not desorbed by acids or salt solutions.