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heterogeneous surface. The calculated adsorption constant ( n ) is5.00( ± 0.04, 95% confidence interval), and K f the equilibrium constant is 2.57 mg g - 1 (mg L - 1 ) - 1/n ( ± 0.01 mg g - 1 (mg L - 1 ) - 1/n , 95% confidence interval). This change in adsorption isotherm relative to the unrefined NBSK pulp is likely a result of the heterogeneity of the fibre surface created by fibrillation during refining.
Adsorption–desorption equilibrium is a dynamic process. Surface site coverage will vary with adsorp- tion conditions. The Langmuir equilibrium constant ( K e ) was determined to be 0.16 L mg - 1 ( ± 0.06 L mg - 1 , 95% confidence interval) at 35 C, which is lower than K e = 2.82L mg - 1 at 25 C. This decrease could be caused by lower concentration of adsorbed LBG ([ C e S e ], Eq. 8) and/or increased surface site concentration ([ S e ], Eq. 8). From Fig. 5a, the equilib- rium adsorption amount ( q e ) increased with increasing temperature at higher dosage levels, indicating that the adsorbed LBG concentration ( C e S e ) increases with increasing temperature. Thus, the lowered K e value at 35 C could due to the increased surface site concentration. Site coverage on unrefined NBSK pulp increased rapidly at LBG dosages less than 0.2wt% at 25 C (Fig. 5b). However, the dependence on dosage dimin- ishes as full coverage is approached. At 35 C, the reduced dependence on dosage could result from increased maximum adsorption capacity ( Q max ). Past studies have shown paper tensile strength improve- ment is not proportional to hemicellulose dosage, and the increase in paper tensile strength diminishes as dosage is increased. Hannuksela et al. (2004) reported GGM dosage of 0.8 wt% o.d. fibre increased tensile strength by 13% but a higher dosage of 1.6 wt% o.d. fibre caused only a small additional increase in tensile strength. The most significant improvement in tensile strength was achieved at GGM dosage less than 0.1 wt% dried fibre (Hannuksela et al. 2004). Our results help explain Hannuksela et al. (2004)’s observations. When LBG dosage was approximately 0.12 wt%, the coverage of fibre sites at 25 C was 0.52 (Fig. 5b). Full coverage was obtained when the dosage was increased to 2.12 wt% of o.d. pulp fibre. As the fibre surface becomes saturated, it is likely that improvement in inter-fibre bonding will be limited leading to small gains in paper tensile strength, even after addition of excess LBG.
Effect of salt addition
LBG adsorption to NBSK pulp fibre in response to sodium chloride addition was investigated at 25 Cfor 10 min with LBG dosage 0.2 wt% relative to o.d. fibre; sodium chloride concentration was varied from 0–1 mol L - 1 (Fig. 6). The LBG adsorption fraction was approximately 85–100% and independent of sodium chloride concentration at concentrations from 0–1.0 mol L - 1 (Fig. 6). The small effect of sodium chloride might due to the small electrostatic repulsion between LBG and fibre and the low negative charge density on LBG (de Jong and van de Velde 2007). This trend is consistent with Hannuksela et al. (2002)’s research that guar gum and GGM adsorption were unaffected by addition of less than 0.1 mol L - 1 sodium chloride.
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Effect of refining
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Table 4 and Fig. 4c clearly demonstrate that the adsorption to lightly refined pulp at 25 C is best described by the Freundlich model. Adsorption to unrefined pulp at 25 C was best described by the Langmuir model. The Freundlich model is used to describe multi-layer adsorption on a non-uniform and
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0.000 0.005 0.010
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NaCl concentration (mol/L)
Fig. 6 The adsorbed fraction of LBG on NBSK pulp fibre with varying sodium chloride concentration 0–1.0 mol L - 1 after 10 min at 25 C, LBG dosage 0.2 wt% of o.d. fibre
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