Ind. Eng. Chem. Res. 1997, 36, 1171-1175
1171
Kraft Lignin-Poly(DADMAC) Precipitate Formation Raymond E. Lappan, Robert Pelton,* Ian McLennan, Jasmin Patry, and Andrew N. Hrymak Department of Chemical Engineering, McMaster Centre for Pulp and Paper Research, McMaster University, Hamilton, Ontario, Canada L8S 4L7
The precipitation of kraft lignin with poly(diallyldimethylammonium chloride), poly(DADMAC), was characterized as a function of pH. At a mass mixing ratio of poly(DADMAC) to lignin of 0.53 at pH 12.6, approximately 80% of the added lignin was removed, whereas the precipitate contained less than 35% of the added poly(DADMAC). Lignin precipitated with as much as 75% of the charges not associated with poly(DADMAC). Lower pH solutions required less poly(DADMAC) for lignin precipitation. Colloidal complex formation was measured as a function of time by dynamic light scattering, and the results could be fitted by a diffusion-controlled aggregation model. Introduction Cationic polyelectrolytes are heavily used in the paper industry to increase the deposition efficiency of colloidal fillers and fines onto wood pulp fibers, to remove soluble anionic polyelectrolytes from process streams, and to improve wastewater-treatment efficiency. In these applications, the cationic polymers function by forming molecular complexes with wood-based anionic polymers, and the complex either precipitates or adsorbs onto a solid substrate. Complex formation between oppositely charged aqueous polymers is well-known (Michaels, 1965) and well-documented in an excellent review article (Burkart et al., 1989). Recently, we have shown that cationic polyelectrolytes can improve the displacement washing efficiencies of brownstock washers in the kraft pulping process (Li and Pelton, 1992b; Pelton and Grosse, 1994; Lappan et al., 1996). The improved washing depends upon the formation of precipitate in wood-fiber pads when cationic polymer is mixed with kraft black liquor which contains partially degraded kraft lignin, an anionic polyelectrolyte. Described in this paper is an investigation of complex formation between aqueous kraft lignin and poly(diallyldimethylammonium chloride) (poly(DADMAC)). The objectives were to determine the compositions of the poly(DADMAC)/kraft lignin precipitate as a function of pH and to determine if the rate of precipitate formation was consistent with diffusioncontrolled models. Experimental Section Poly(diallyldimethylammonium chloride) (poly(DADMAC)), MW ) 1.0 × 105, was supplied by Allied Colloids Inc. (Percol 1697), Brampton, Canada, as a liquid concentrate. The mass fraction of poly(DADMAC) determined by quintuplet gravimetric analysis of freezedried samples was 0.4283 ( 0.0111 (99.5% confidence). Indulin-C, a powdered mixture of the sodium salt of kraft lignin (80-90%) and sodium carbonate (10-15%) purchased from Westvaco, SC, was used as received in all experiments. Note that all kraft lignin solutions were prepared from the same bulk Indulin-C. The average charge per gram of kraft lignin was determined by colloidal titration (Lindstro¨m, 1991), * To whom correspondence should be addressed. Phone: 905-625-9140, ext. 27045. Fax: 905-528-5114. E-mail:
[email protected]. S0888-5885(96)00460-5 CCC: $14.00
using a Rank Brothers Charge Analyzer II, Cambridge, England, using a reciprocating piston streaming current detector. The procedure started with the addition of either 0.5 or 1.0 mL of 1.0 g/L kraft lignin solution to 85 mL of deionized water. Then a few drops of either 0.1, 0.01, or 0.001 N NaOH was added to the solution to adjust the pH before the titration. The titrant was 0.001 polybrene (Aldrich), MW ) 7800. The pH dependence of polybrene equivalent weight was measured by calibration experiments with sodium poly(vinyl sulfate) as the primary standard. The kraft lignin solubility as a function of solution pH was determined by the addition of 10 g of kraft lignin to 100 mL of water, with final pH adjustment by the addition of 1.0 N HCl or 1.0 N NaOH. Samples were stirred overnight and centrifuged at 6700 g for 30 min to remove the solids and known volumes of supernatant dried to determine the dissolved mass. Poly(DADMAC) and kraft lignin reactant solutions were prepared by dissolving a known mass of each reactant in deionized water, followed by a pH adjustment with 1.0 N HCl or 1.0 N NaOH. Precipitation experiments consisted of mixing equal volumes of each reactant for a minimum duration of 5 min before any further sample manipulation. Unreacted kraft lignin, in the supernatant, was measured spectrophotometrically at λ ) 280 nm after solids separation by centrifugation at 6726 g for 30 min. Precipitate mass was determined gravimetrically after drying the samples at 105 °C for 24 h. Additional solids analysis was conducted by solidstate 13C NMR. Sample preparation consisted of forming precipitate at optimum mixing ratios, as determined by UV measurements, at pH 8, 9, 10, 11, and 12.6 followed by water removal by freeze drying. The relative concentrations of kraft lignin to poly(DADMAC) in each sample were determined by measuring the relative lignin aliphatic peak areas to the poly(DADMAC) methyl peak areas using a Bruker UNIX 200 spectrometer operating at 50 MHz. Typical acquisition parameters were spin rate 6 kHz, 90° pulse of 3.9 µs, a contact time of 1 ms, a relaxation delay of 4.0 s, a sweep width of 40 kHz, and 1K time domain points zero filled to 16K. High-power 1H decoupling was employed in all NMR experiments. The spectra were acquired over 2-3 h. The rates of poly(DADMAC)/kraft lignin precipitate growth were studied by dynamic light scattering (DLS) using a Brookhaven Instrument Corporation Model BI200SM, fitted with an argon laser (λ ) 514.4 nm). Kraft © 1997 American Chemical Society
1172 Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997
Figure 1. Charge density and solubility of lignin as a function of pH.
Figure 3. Supernatant absorbance as a function of reactant mixing ratio at pH 12.6.
Figure 2. Poly(DADMAC) diameter in pH 12.6 deionized water as determined by DLS. Data analysis by Brookhaven software packages CONTIN and EXPSAM. Note that the intensity is peak normalized.
lignin solution (1.0 g/L) was diluted with pH 12.6 adjusted distilled water (by NaOH addition) in a 20mL vial to give a final concentration of 0.312 mg/L. The experiment started after temperature equilibrium at 25 °C with the addition of poly(DADMAC) solution (final concentration 0.172 mg/L) to the vial and the simultaneous start of the data acquisition system. Data were acquired once every 2 mins for a 1-h period, starting 30 s after the program was activated, with delay times ranging from 2 to 40 000 µs and correlation times of 30 s. These procedures were repeated for multiple scattering angles and reactant concentrations. Results Reactant Properties. Kraft lignin charge densities and solubility characteristics are important reactant parameters that determine the properties of poly(DADMAC)-kraft lignin precipitates. Figure 1 presents the average charge density, determined by colloid titration, and kraft lignin solubility as a function of pH. The charge density increased with pH, reflecting the dissociation of phenolic groups above pH 10. The hydrodynamic size of poly(DADMAC) was measured by DLS. Figure 2 presents the apparent poly(DADMAC) molecular size at pH 12.6 for data collected over a 1-h period where over 100 000 autocorrelation samplings were taken. The mean apparent size was found to be between 1 and 10 nm depending on the analysis technique used to deconvolute the autocorrelation function. The results in Figure 2 were calculated by the CONTIN v4.0 (nonnegatively constrained least squares: regularized) and EXPSAM v3.0 programs provided by Brookhaven Instrument Corp. Kraft lignin measurements by DLS, however, did not yield any meaningful data. Precipitate Formation. Precipitate formation was measured either by monitoring the change in superna-
Figure 4. Precipitate mass as a function of lignin and poly(DADMAC) concentration when mixed at equal volumes. Solution pH 12.6.
tant kraft lignin concentration after precipitation or by determining the total mass of dried complex. Combining the results from both methods permits calculation of both the kraft lignin and poly(DADMAC) contents in the precipitates. Examples of the results from the supernatant method are shown in Figure 3, which shows the results of two sets of experiments at pH 12.6 for the reaction of poly(DADMAC) with kraft lignin. These data are the result of mixing 15 mL of various concentrations of poly(DADMAC) with 15 mL of kraft lignin solution. As demonstrated in the figure, the optimum dosage (0.53 g of poly(DADMAC)/g of kraft lignin) is estimated to be the point where the absorbance of the supernatant does not decline any further when subjected to higher poly(DADMAC) dosage. Direct precipitate mass measurements were made for 24 samples in triplicate, and the results are summarized Figure 4, which is a three-dimensional plot of the precipitate mass as a function of the concentrations of kraft lignin and poly(DADMAC) (pH 12.6). The results in this figure are the averaged amounts of precipitate produced from triplicate precipitation experiments. The ridge which represents the maximum amount of precipitate for a given kraft lignin or poly(DADMAC) concentration corresponds to a reactant dosage ratio of 0.53 g of poly(DADMAC)/g of kraft lignin. Note that this ratio is equal to the results of the supernatant kraft lignin analysis method shown in Figure 3. Figure 5 is a two-dimensional plot of the data in Figure 4. Kraft lignin concentrations are expressed as an excess quantity defined as the total kraft lignin concentration minus the amount required to complex with poly(DADMAC) assuming a mixing ratio of 0.53. Similarly, the mass of the precipitate was normalized
Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997 1173
Figure 7. Precipitate composition as a function of pH. The numbers beside the gravimetric data points are the percentage of the added lignin (L) and poly(DADMAC) (P) present in the precipitate. Figure 5. Normalized precipitate mass as a function of the excess lignin concentration. Graph based on data in Figure 4 assuming optimum ratio is 0.53 g of poly(DADMAC)/g of lignin. The numbers beside the curves are the total poly(DADMAC) concentrations in g/L.
Figure 6. Optimum reactant mixing ratio of lignin and poly(DADMAC) reactants as a function of pH as determined by UV measurement. Also shown are estimate mass ratios based upon lignin charge contents determined by colloid titration.
by dividing the mass by the calculated maximum possible mass assuming an optimum poly(DADMAC)/ kraft lignin ratio of 0.53. Therefore, the lines in Figure 5 correspond to constant poly(DADMAC) concentration slices through Figure 4, with zero excess kraft lignin denoting the kraft lignin concentration corresponding to the ridge in Figure 4. The results in Figure 5 show that the maximum dimensionless precipitate mass was about 0.8 and independent of poly(DADMAC) concentration. Although the optimum poly(DADMAC)/kraft lignin ratio was independent of poly(DADMAC) concentration, the curves were broader for higher poly(DADMAC) concentrations. Effect of pH on Precipitate Formation. Since the charge content of kraft lignin is pH dependent, the corresponding amount of poly(DADMAC) required for precipitation is also a function of pH. Figure 6 shows the optimum mixing ratio of poly(DADMAC) to kraft lignin solution concentrations determined from kraft lignin supernatant measurements as a function of pH (see Figure 3 for examples of data). Also shown in Figure 6 are the estimated ratios assuming that all the charges on kraft lignin, determined by colloid titration, are satisfied by poly(DADMAC). Both curves show that the amount of poly(DADMAC) required increases with
pH, reflecting the increasing kraft lignin charge content. However, for every pH value, kraft lignin removal by precipitate formation occurred with poly(DADMAC) concentrations approximately half that required to balance all the charges on kraft lignin. Note that kraft lignin charge densities were determined by colloid titration using low-molecular-weight polybrene (7800) which may have accessed more kraft lignin charges than did the high-molecular-weight poly(DADMAC) (105). The compositions of the precipitates, expressed as a ratio, are given Figure 7 both as poly(DADMAC)/kraft lignin mass ratios determined gravimetrically and by NMR. The percentages beside the gravimetric data give the fraction of added kraft lignin and poly(DADMAC) which was present in the precipitate. The NMR ratios are based on the measurements of the mole ratio of methyl protons, from poly(DADMAC), to aromatic kraft lignin protons. The kraft lignin repeating unit, needed to convert from a mole ratio to a mass ratio basis, was assumed to have three aromatic protons and the same molecular weight as coniferyl alcohol. In all cases, the NMR estimates were higher than the gravimetric results, although the trends were the same. Comparison of the amounts of material added (see “direct measurement” mixing ratio in Figure 6) with the precipitate composition (Figure 7) reveals that greater than 80% of the added kraft lignin was present in the precipitate. According to NMR estimates, similar fractions of the poly(DADMAC) were removed, whereas the gravimetric results indicate that greater than 65% the added poly(DADMAC) did not form precipitate. The NMR estimates were complicated by interferences from the kraft lignin methyl groups and by using coniferyl alcohol as a structural model. By contrast, the estimated error in the gravimetric ratios was 13% (95% confidence), so we believe the gravimetric ratios are the best estimates. Aggregation Kinetics. In applications such as paper making and polymer-enhanced brownstock washing, kraft lignin precipitation must be fast to be compatible with the process. The kinetics of precipitate formation with very dilute kraft lignin solutions (0.3 mg/ L) were followed by DLS, and the results, collected at scattering angles of 30 and 90°, are summarized in Figure 8. These data were calculated from the first cumulant and thus represent intensity-weighted averages. The apparent particle size was a weak function of the scattering angle since most of the 90° data match the 30° data with only distinct separation of size occurring as apparent particle size exceed 200 nm in
1174 Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997
Figure 8. Apparent aggregate diameter as a function of time as determined by DLS (0.312 mg/L of lignin, 0.172 mg/L of poly(DADMAC), pH 12.6).
diameter. At 30°, qRa was less than 1, whereas at 90° qRa was greater than 1, where q is the standard definition of the scattering vector and Ra is the apparent intensity-weighted radius as calculated from the first cumulant and the Stokes-Einstein equation. The DLS results were analyzed using a model initially developed by Pusey et al. (1982) and refined by Vermold and Hartl (1983). We assumed that poly(DADMAC) initially reacted with all the kraft lignin to form primary complex particles with radius R0 and a number concentration of n0. A key assumption was that primary complex particle formation was instantaneous and that the DLS measurements reflected the diffusion-controlled aggregation of the complex particles into larger agglomerates. A similar assumption was used by Ahrabi et al. (1986) in their analysis of stopped-flow experiments of lignin sulfonate precipitation with cationic polymers. The equations given by Vermold and Hartl (1983) are
p j 5/3(t) D h (t) + D h 0 6/3 p j (t)
(1)
r
p j n/6(t) )
∑ pn/3np(t)/n0
n ) 5, 6
(2)
p)1
(t/τ)p-1 np(t) ) n0 (1 + t/τ)p+1
p ) 1, 2, ..., r
(3)
where D h 0 ) diffusion coefficient of the primary complex particles, D h (t) ) average diffusion coefficient of the aggregate, t ) time, np ) number concentration of the aggregate containing p complex groups, n0 ) number concentration of the complex formed, and τ ) Smoluchowski’s rapid coagulation half-life constant [τ ) 1/(4πD0R0n0)]. The diffusion coefficient data as functions of time collected at a scattering angle of 30° were fitted to the model. The model was fitted to the data with a leastsquares technique with the inverse of time squared as the weighting factor. The purpose of this weighting technique was to bias the analysis toward the initial data where the Smoluchowski’s approximation for the aggregation kernel (DijRij ) 2D0R0) is most applicable. The results predicted by the model are compared with the data in Figure 9. The best fit was obtained for a primary complex radius, R0, of 25 nm and coagulation half-life τ ) 34.5 s. The corresponding n0 value is 9 × 1015 m-3, which is 110 times less than the concentration of poly(DADMAC) molecules. The dynamic light scattering experiments involved very dilute poly(DADMAC)
Figure 9. Hard-sphere diffusion model as fitted to 30o DLS data (R0 ) 25 nm, τ ) 34.5 s).
solutions (0.3 mg/L) compared with 1 g/L used in our previous displacement washing experiments (Pelton and Grosse, 1994). The above model predicts τ to be 6 µs if n0 is assumed to be proportional to the concentration of poly(DADMAC). In summary, the kinetics of colloidal complex growth (Figure 8) from very dilute solutions was explained by a diffusion-limited coagulation mechanism (Figure 9); this result is consistent with studies of lignin sulfonate precipitation (Ahrabi et al., 1986). Discussion Lignin is a cross-linked phenolic polymer concentrated in the regions between cellulose fibers in wood. In the kraft pulping process, lignin is partially decomposed at high temperatures and pHs to release the cellulose fibers from the wood. The kraft lignin released to the black liquor in the kraft pulping process is a complex mixture of polyphenolic species. Favis et al. (1984) argue, based on the topochemistry of the softwood fiber cell wall, that kraft lignin species are disks with aspect ratios of approximately 7 and equivalent average spherical diameters of about 10 nm. Light scattering results also indicate that kraft lignin has structures more compact than a random coil (Nyman et al., 1986). Thus, kraft lignin could be viewed as a suspension of microgel particles swollen in alkali (Lindstro¨m, 1979). Native lignin is hydrophobic, and as much as 30% of the kraft lignin can be partitioned into organic solvents (Nyman et al., 1986). In addition to breaking down the lignin network, the kraft pulping process introduces carboxyl groups into the lignin, which gives stable aqueous solutions/dispersions at high pH values. Lowering the pH of aqueous lignin below 10 causes precipitation (Ale´n et al., 1979). This work involves precipitating kraft lignin at high pH values by cationic polymer addition. Most related previous work involved cationic polymer interactions with lignin sulfonates (Stro¨m and Stenius, 1981; Ahrabi et al., 1986) at neutral pH values. Lignin sulfonate forms stable aqueous solutions/dispersions at neutral pH, and higher molecular weight components behave like microgels (Stro¨m and Stenius, 1981). Low-molecular-weight cationic polymers form colloids when mixed with lignin sulfonates, and the stability of the colloids depends upon the magnitude of their ζ potential. By contrast, high-molecular-weight cationic polymers cause lignin sulfonates to precipitate over a wide range of mixing stoichometries (Stro¨m and Stenius, 1981). The kinetics of lignin sulfonate interactions with cationic polymers was studied by stopped-flow techniques and appears to be mass transport controlled with no indication of a significant activation energy (Ahrabi et al., 1986).
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The influence of cationic polymer structure on kraft lignin precipitation at high pHs has been reported (Li and Pelton, 1992a). The major findings were that between 80 and 90% of kraft lignin could be precipitated by cationic polymers. The higher the cationic charge density, the more effective the polymer, whereas the molecular weight of the cationic polymer did not seem to be important. In all cases, the colloidal stable polymer/lignin complex was obtained if insufficient cationic polymer or kraft lignin was present. An interesting observation in this work was that the mixing ratio of poly(DADMAC) to lignin giving the maximum amount of precipitate was approximately independent of the absolute concentrations (see Figures 4 and 5). Furthermore, the amount of poly(DADMAC) present at the optimum mixing ratio was far less than that required to neutralize all the electrical charges on lignin (Figure 6). Indeed, a comparison of the amount of cationic polymer added (Figure 6) with the corresponding composition of the precipitate (Figure 7) reveals that at the optimum mixing ratio, most of the added poly(DADMAC) remains in solution. Two processes seem to be operative. In the first, about 20% of the kraft lignin, as measured by UV spectroscopy, and 60-70% of the added poly(DADMAC) do not precipitate. Therefore, we propose that in the first process, the more hydrophilic lignin fraction forms water-soluble complexes with most of the added poly(DADMAC). It has been estimated that about 85% of the lignin solvent extracted from spruce meal contains grafted carbohydrate (Forss et al., 1989). Koshijima and co-workers (1989) have published evidence that the amphipathic lignin-carbohydrate polymers form aggregates (micelles) in aqueous solution. In the second process, the more hydrophobic lignin fraction forms precipitate upon interaction with poly(DADMAC). Since the amount of poly(DADMAC) in the precipitate increases with pH (Figure 7), partial charge neutralization may be part of the precipitation mechanism. Conclusions The following conclusions can be made for the above equilibrium and kinetic experiments and agglomeration model: 1. Poly(DADMAC) precipitates from 80 to 90% of the kraft lignin at pH 10-12.5; however, only a fraction (
