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unrefined and refined NBSK pulp are investigated; refining and LBG dosage are varied.
for large particles or systems with flow motion, the rate is dependent on flow conditions (van de Ven 1994). Pseudo-first-order and pseudo-second-order models are commonly used to describe the kinetics of adsorption of dyes or chemicals to pulp fibres. They were proposed (Lagergren 1898; Ho 1995, 2006; Ho et al. 1996; Blanchard et al. 1984; Ho and McKay 1998, 2000) and applied in many cellulose/fibre adsorption studies (Li et al. 2018; Roy et al. 2013; Vucˇurovic´ et al. 2012; Pan et al. 2016). However, reports on application to polysaccharides adsorption to pulp fibre are rare. Currently, starch is the most widely used strength additive thus its adsorption is well studied (Hedborg and Lindstro¨m 1993; Wa˚gberg and Bjorklund 1993; van de Steeg 1989; van de Steeg et al. 1993a, b; van de Steeg 1992; Shirazi et al. 2003; Zakrajsˇek et al. 2009). However, hemicelluloses recovered from wood, such as O-acetyl-galactoglucomannans (GGM), could also be used as strength additives. These could be isolated from process streams in the pulp and paper mills as part of an integrated biorefinery. There is limited fundamental understanding of application of hemicel- lulose polysaccharides such as GGM, or the closely- related LBG. GGM recovered from pulp mill wastes has high polydispersity (Chen et al. 2020), are contaminated with other biomass components (e.g. extractives) and thus are is not well-suited to studies investigating fundamental adsorption mechanisms. In contrast, LBG is a well-defined, commercially avail- able galactomannan-type polysaccharide: a backbone of (1–4)- b -D-mannopyranosyl units with side chains of (1–6)- a -D-galactopyranosyl units having a 1:4 ratio of galactose to mannose (BeMiller and Whistler 2012; Roller and Jones 1996). The composition, molecular structure and charge density of starch and LBG differ considerably thus past starch research cannot be transferred to LBG adsorption. The goals of this work are to identify the favorable adsorption conditions for LBG on NBSK pulp, understand the underlying mechanisms, and the resulting effects on paper properties. LBG adsorption is analyzed using pseudo-first-order and pseudo-sec- ond-order kinetics with respect to LBG concentration. The adsorption isotherms are analyzed using both the Langmuir model and the Freundlich model. The effects of temperature, refining, sodium chloride addition, and pH on LBG adsorption are investigated. Changes in paper strength due to LBG adsorption to
Materials and methods
Materials
Never dried NBSK pulp was supplied by Canfor Pulp Products. The NBSK pulp was washed with deionized water until the UV–vis absorption of filtrate was less than 0.005 abs at 200 nm wavelength before use. LBG with purity greater than 90% was purchased from Sigma Aldrich. Sodium chloride ( [ 99%), potassium chloride ( [ 99%), hydrochloric acid (37%), sodium acetate ( [ 99%), acetic acid ( [ 99%), sodium bicar- bonate ( [ 99%), sodium carbonate ( [ 99%), potas- sium chloride ( [ 99%) and sodium hydroxide ( [ 98%) were purchased from Sigma Aldrich. Sodium phosphate monobasic ( [ 98%) and dibasic ( [ 99%) were purchased from Fisher Scientific. Sulfuric acid (98 wt%) was purchased from Sigma Aldrich and diluted to desired concentration. The carbohydrates kit (CAR10-1KT) used to calibrate the high-performance liquid chromatography (HPLC) was purchased from Sigma Aldrich and contained mannose, glucose, galactose, xylose and arabinose. The purity of the standards was greater than 98%.
LBG adsorption
LBG powder was hydrolyzed in deionized water at a concentration of 0.5 wt% at 98–100 C for 45 min with continuous agitation to produce a solution of polysaccharides with a narrow molar mass distribu- tion. Undissolved gum particles in hydrolyzed LBG stock solution were removed by two rounds of vacuum filtration. The filtrate was further diluted and cen- trifuged twice at 3500 r.p.m. for 15 min, and the supernatant was recovered for adsorption experi- ments. The weight-average molar mass of hydrolyzed LBG was measured by a Waters Alliance HPLC coupled with refractive index (RI) detector and Ultrahydrogel 120, 250, and 1000 columns. The calibration standard was a pullulan standard kit (WAT034207). The weight-average molar mass of LBG was 1215 kDa ( ± 89 kDa standard deviation) in this work.
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