Cellulose (2021) 28:10183–10201
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there are many contradictory reports regarding the effects of changing variables on adsorption results. Adsorption conditions include temperature, time, hemicellulose dosage, pH, salt addition, fibre consis- tency and agitation rate. Temperature and time are the most commonly examined factors for hemicellulose adsorption, but the reported effects vary greatly. Adsorption of partially methylated LBG on bleached sulfite pulp increased with temperature from 5 to 61 C (Russo 1959). However, Gruenhut (1953) concluded that LBG adsorption to kraft fibre increased with decreasing temperature; maximum adsorption was observed at 4.2 C. Most (1957) and Leech (1953) reported a greater amount of hemicellulose was retained by pulp fibre with increasing time, and further concluded that adsorption equilibrium was not obtained even after 10 days. However, Swanson et al. (1949) obtained 76–96% LBG adsorption to bleached sulfite pulp and reported that equilibrium was reached within 30 min. Salts, process chemicals and pH also strongly influence the process (Hedborg and Lindstro¨m 1993; Shirazi et al. 2003; van de Steeg 1989; van de Steeg et al. 1993a, b; Zakrajsˇek et al. 2009) since adsorption occurs by electrostatic interaction of polyelectrolytes (polymers with electrolyte groups) with negatively- charged cellulose fibres (Niskanen 1998; van de Ven 2000; Sjostrom 1989). Cellulose fibres are negatively charged due to carboxyl groups and hydroxyl groups. Addition of salts decreases the attractive electrostatic forces between cationic starch and cellulose fibre thus adsorption decreases (van de Steeg 1989; van de Steeg et al. 1993a, b; Hedborg and Lindstro¨m 1993). When pH increases, carboxyl groups deprotonate and gen- erate more negative charge on fibre surface (Hedborg and Lindstro¨m 1993). As a result, adsorption of cationic polymers increases with rising pH (Shirazi et al. 2003; van de Steeg 1992; van de Steeg et al. 1993a, b). However, for polymers with negative charge, low pH facilitates adsorption by converting carboxyl groups to their undissociated state (Scallan 1983). High pH leads to a high electrostatic repulsion between fibres and negatively charged polymer, thus reducing the adsorption. Gruenhut (1953) concluded that LBG adsorption to kraft pulp fibre was higher at pH 4 than at pH 6.5. Keen and Opie (1957) found that maximum guar gum adsorption to bleached kraft pulp was obtained at pH 6.7 and minimum adsorption occurred at pH 11.5. In contrast, Most (1957) found
hemicellulose from slash pine adsorbed more to bleached sulfite pulp at pH 10 than at pH 4.5. Finally, Hannuksela et al. (2002) reported that adsorption of guar gum on bleached kraft pulp was independent of refining severity, pH, temperature and salt concentra- tion. de Jong and van de Velde (2007) determined that the charge density, defined as mol negative charge/mol of monosaccharide, of native LBG was less than 0.3. Thus, LBG is a weakly, negatively charged polymer, and salts might have a relatively low impact on adsorption. The results of the limited number of studies on the influence of ionic strength and pH on hemicellulose adsorption to cellulose are contradictory. Mass transfer, another important adsorption condi- tion, is influenced by agitation rate and fibre consis- tency. Turbulence, created by strong agitation, reduces mass transfer resistance by disrupting the boundary layer at the interface of the fibre and bulk solution (Russo 1959; Zakrajsˇek et al. 2009). Fibre consistency negatively correlates to extent of adsorption. Zakraj- sˇek et al. (2009) and Most (1957) showed low fibre consistency increased adsorption of starch and hemi- cellulose to pulp fibres due to high concentration gradient and greater fibre surface availability. Fibre properties such as surface area and fines content change the availability of adsorption sites (Zakrajsˇek et al. 2009). Several scholars attributed the increase of adsorption as a function of refining due to fibrillation, generation of fines, increased surface area and total pore volume (Zakrajsˇek et al. 2009; Russo 1959; Keen and Opie 1957; Hannuksela et al. 2002). To elucidate the contradictory effects of factors, fundamental analysis including adsorption isotherms and kinetics are needed. Adsorption isotherms describe adsorption of a substance to a solid surface from an aqueous phase under isothermal conditions (Foo and Hameed 2010). Langmuir isotherms and Freundlich isotherms are commonly used to describe dye or chemical adsorption to cellulosic fibres (Lang- muir 1916; Li et al. 2018; Roy et al. 2013; Urruzola et al. 2013; Vucˇurovic´ et al. 2012; Zakrajsˇek et al. 2009). Adsorption kinetics describe the variation of amount adsorbed with time and can guide how to most effectively apply additives during papermaking (Zakrajsˇek et al. 2009). Adsorption rates of polymers are related to the collision rate. For small particles, collision rate is dependent on Brownian motion, while
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