STUDIES ON THE USE OF LIGNIN OBTAINED FROM BLACK LIQUOR (A PAPER INDUSTRY WASTE) FOR THE REMOVAL OF SOME INORGANIC POLLUTANTS

Abstract

Water is one of our most precious resources, vital for development as well as in day-to-day living. Our growing population and rapidly diversifying and expanding needs make water an increasingly scarce resources. At the same time, the discharge of partially treated waste water in ever increasing volumes has led to the degradation of water ways. Several toxic elements are found in polluted waters, the most dangerous amongst these are heavy metals. Toxic effects of heavy metals are quite well documented. Of the various treatment methods adsorption appears to offer the best overall treatment especially for those which cannot be removed by other techniques. Activated carbon adsorption is an important process used to remove pollutant from waste waters. However, the drawback of the process is the high cost involved in its manufacture and its "In situ" regeneration. As such it has become necessary to look for other economically feasible adsorbents. Lignin is a waste material present in the black liquor of pulp and paper industry and it causes problems for its disposal. Lignin in pure form, is an amorphous, aromatic biopolymer and second in natural abundance only to cellulose. About 50x10 ton/year of lignin is produced from woody plants at pulp mills worldwide. Of this huge quantity, only a very small fraction is recovered as chemicals and steam and the organic materials in black liquor are burnt in the recovery units of the mills. Studies have shown that lignin can adsorb a number of dyes, xenobiotics and other organic and inorganic pollutants from their aqueous solutions. As such investigations were planned to extract lignin from black liquor with a view to make it safe for disposal and also study the adsorption of heavy metals to explore the utility of the material (extracted from paper waste) for the removal of toxic metals from waste waters. The heavy metals chosen for study are mercury, lead, chromium, zinc and iron. Lignin was extracted from black liquor by lowering the pH with dilute HCl till the precipitation was complete. For purification, 10% solution of lignin was prepared in dioxane, filtered and added dropwise from a burette into a vigorously stirred anhydrous ether to reprecipitate the compound. The precipitate were washed with ether, bengene and finally with petroleum ether and dried in vacuum. Lignin thus obtained is tan coloured, quite stable in acidic, mildly alkaline and salt solutions. It has been characterized by elemental, infrared and scanning electron microscopic studies and is found to be of Guiacyl type. To determine the extent of pollution created by pulp and paper mills, samples were collected from large as well as small paper mills and analyzed for some physico-chemical parameters. The results show that small mills generate a high pollution load in comparison to large mills. In the absence of chemical recovery units, the small mills discharge a variety of organic residues (lignins) and cooking chemicals. However, in large mills, the black liquor is subjected to burning as a fuel to recover heat energy and chemicals. The extent of adsorption at equilibrium is dependent on the physical and chemical characteristics of the adsorbent, adsorbate and the experimental system. Laboratory scale batch studies have been performed to established optimum conditions for the adsorption of metal 11 ions. The parameters evaluated include hydronium ion concentration, temperature, adsorbent dose and competitive Influence of other metals plus some salts etc. It is found that the adsorption of heavy metals is an endothermic process and the uptake increases with temperature. Adsorption of heavy metal ions does not follow any set pattern. Mercury, lead and chromium are taken up in large quantities, zinc is fairly well adsorbed but the uptake of iron is quite low. The optimum pH is found to vary for various metal ions and it is found that the adsorption is maximum at a pH which precedes hydrolysis. The uptake of Hg +, Pb 4 ,Cr and Zn in the presence of other metal ions plus some salts presents interesting data. The adsorption of mercury, lead and zinc is significantly decreased in the presence of chromium. A slight lowering in the uptake of mercury, lead, zinc and chromium is also observed in the presence of sodium. The results show a positive adsorption in all adsorption isotherms. These are regular and concave to equilibrium concentration axis. The removal of metal ions occurs in two phases. In the first phase, the uptake of solute is fast while in the second phase rate of removal becomes quite slow and subsequent removal of solute continues over longer period of time. The data for the adsorption of heavy metal ions fit well into the Freundlich and Langmuir models. The Freundlich constants signify favorable adsorption of all the adsorbate except mercury, where the data does not fit into the Freundlich model. It also reflects a considerable degree of non-linearity between adsorbate concentration in solution and the extent of adsorption. Langmuir parameter Q° ( the amounts of metal ions necessary for forming a complete monolayer) is higher than the adsorption maxima observed ill experimentally for all the metal ions. This may be attributed to the fact that in batch process a complete utilization of the sorption capacity is never achieved. However, the Q values especially for lead and zinc on lignin are quite higher than the Q values reported on waste tea leaves [108]. The values of constant "b", which reflects the energy of interaction with the surface are also in the same order in which 3+ these metal ions are adsorbed on lignin. (except that of Cr ). The influence of the shape of isotherm on whether the adsorption is 'favourable' or 'unfavourable' has been considered using the method of Weber and Chakraborti [114]. A comparison of dimensionless constant separation factor indicates that the adsorption is favorable for all the heavy metal ions. Thermodynamic parameters of the process have also been evaluated. The Gibbs free energy values for all the heavy metal ions are negative thereby indicating the spontaneous nature of the process. The positive AH values indicate the process to be endothermic and the positive entropy of adsorption (AS) also reflects the feasibility of the process under investigation. Sorption kinetics provide very useful information in determining the mechanism of rate controlling step and In providing a quantitative know how of the adsorption process. Batch methods, due to their simplicity and ease of evaluation of adsorption parameters, have been used for studying the kinetic aspect of the process. Preliminary Investigations on the rate of adsorption of mercury, lead, chromium and zinc on lignin indicate that adsorption occurs quite rapidly in initial stages. About 40 to 45 percent of the ultimate adsorption occurs within the first hour of contact. This initial rapid stage of adsorption then slows down as it approaches to equilibrium. The Bt various t plots for IV mercury, lead and chromium, are linear and pass through origin at and -4 above 5x10 M concentration while in the case of zinc same type of -5 linearity and origin oriented curves are obtained at and above 5x10 M concentration. However, at lower concentrations, the curves deviate from linear behaviour. This shows that the adsorption of metal Ions is particle diffusion controlled at higher concentration, while at low soluble concentrations, the film diffusion also affects the uptake. The adsorption rate is found to increases with Increase in temperature and with decreasing particle size. This fact further supports the particle diffusion mechanism as the rate controlling process. Increase in mobility of the sorbate species, at higher temperature results in an enhancement in the sorption rate. The values of effective diffusion coefficient (D.) are in the same l 2+ order in which these heavy metal ions are adsorbed on lignin i.e., Hg > Pb > Cr' > Zn' . The energy of activation (E ) has been evaluated a form the linear behaviour of log D. Vs 1/T plots and D values have been l o used to calculate AS, entropy of activation. A negative AS, for all the metal ions indicates that these do not. cause any structural change in the sorbent material during diffusion. Column studies to design a fixed bed absorber with this material have also been undertaken. The total time (t ) involved in the the primary zone to establish itself, move down the length of the column of lignin and out of the bed, the time (t ) required for the movement of s the zone down its own length in the column after it has been established; the fractional capacity (f) of the lignin in the adsorption zone at break point to continue to remove solute from solution; the length of the adsorption zone (5); and the percentage saturation at break point have been evaluated for columns of mercury. The poor column capacity observed In the case of mercury is mainly due to higher flow rate and increased particle size. Preliminary studies carried out for the desorption of mercury using mineral acids, NH OH and NaNO , reveal that 1M HNO is an appropriate evaluating agent. After desorption, the column was again loaded with mercury and the cycle was repeated for four times. A significant fallin adsorption capacity of the column was observed after the first cycle and there after the subsequent cycles do not record much change in it.