Formation of Carbonyl and Carboxyl Groups on Cellulosic Pulps: Effect on Alkali Resistance

Abstract

Ozone bleaching generates carbonyl groups on the cellulose polymer when applied to unbleached kraft pulps. This suggests that pulp fully bleached with a totally chlorine-free (TCF) sequence may contain more oxidized groups than standard elemental chlorine-free (ECF) bleached pulp. A fully bleached pulp was treated with sodium hypochlorite to form oxidized groups (mostly carbonyls) on the pure carbohydrates, which were investigated during subsequent alkaline treatment. Carbonyl groups had a strong impact on color development during alkaline treatment. Among the carbonyls, the keto groups were the most active. This was confirmed by the behavior of carbohydrate model compounds that contained aldehyde, keto, and/or carboxyl groups when subjected to alkaline conditions. A subsequent hydrogen peroxide (P) stage effectively decreased the carbonyl content, which reduced yellowing during alkaline treatment. However, the oxidized cellulose was severely depolymerized. The addition of magnesium sulfate (Mg) into the P stage minimized depolymerization while maintaining some of the carbonyls in the carbohydrates. It is proposed that Mg cations can hinder alkaline β-elimination, possibly by forming a complex with the carbonyl groups. Keywords INTRODUCTIONInterest in cellulose-based materials has recently increased because cellulose is a renewable resource available in considerable quantities. Among the most active areas of development is the production of dissolving pulp for textiles, plastics, and various additives (Eichinger 2012). Most of the cellulose available today is produced from wood pulped by the kraft process and subsequently bleached by an oxidative process. The conventional bleaching sequences use chlorine containing oxidants, primarily chlorine dioxide, but also in some cases hypochlorite and even chlorine. They all generate chlorinated organics, which are released in the effluents and may also remain on the cellulose. Whether or not this represents an environmental issue has been a matter of debate for decades (NCASI 2013). Nevertheless, developing alternative chlorine-free processes has been an active subject of research for many years. Totally chlorine-free (TCF) bleaching processes have been proposed, which generally include the use of oxygen, ozone, and hydrogen peroxide (Lachenal and Nguyen-Thi 1994;Metais et al. 2013). The implementation of TCF presents several advantages: (1) no chlorinated chemicals should be formed; (2) less organic materials are released in the mill effluent since a portion of the bleaching effluent, which contains only background levels of chloride ions, can be burnt; and (3) less water is required to run the mill. However, the development of TCF bleaching has so far been very PEER-REVIEWED ARTICLE bioresources.com Perrin et al. (2014). "Alkaline yellowing of oxidized pulps," BioResources 9(4), 7299-7310. 7300 limited. The primary reason is due to the oxidation and depolymerization of cellulose by the reagents. Among them, ozone is considered to be responsible for most of the oxidati...

Protein and cell wall polysaccharide carbonyl determination by a neutral pH 2,4-dinitrophenylhydrazine-based photometric assay

“… 5% Phenol solution : Dissolve 5 g in 100 ml ddH 2 O and store frozen. Hypochlorite (NaClO)-carbonylated carboxymethyl cellulose (CMCellu NaClO-carbnylated ) stock solution: Since carbonyls generated on cellulose pulp by ozone and hypochlorite bleaching [74] , [75] , the present study tested the ntrDNPH assay on carbonyls which were artificially generated on CMCellu by an extensive modification of the NaClO-based method described elsewhere [75] . Specifically, 15 mg CMCellu is dissolved in 3 ml 0.2 M acetate buffer, pH 4.8 in a glass tube, and to the resulting 0.5% (w/v) CMCellu solution 3.34 mM active ClO − (or 5% of the weight of CMCellu) is added, followed by incubation for 1 h at 75 °C.…”

“…This is justified by the fact that cell wall polysaccharide constituents (such as cellulose) are known to oxidize to monocarbonyls, diketones and aldehydes (besides other oxidation forms) [73] . Moreover, carbonyls are known to form on cellulose pulp upon bleaching with the well-known ROS generators hypochloride and O 3 [74] , [75] . The methods currently available for the determination of carbonyl groups in cell wall polysaccharides (e.g., cellulose) are based on their condensation with various reagents (such as oxime formation by carbonyl reaction with hydroxylamine [76] ), their oxidation or reduction (to carboxyls or hydroxyls, respectively) [76] , their titration with cyanide and its subsequent quantification by silver nitrate [77] , and their determination via the copper number (with Cu 2+ salts, which are then reduced and determined titrimetrically [78] ).…”

Chemical characteristics and stability of eucalyptus kraft pulps bleached with tertiary amine catalyzed hypochlorous acid

“…The presence of carbonyl groups in cellulose leads ultimately to its partial depolymerization and deterioration of the strength (Beyer et al 2006). In addition, the carbonyl groups in cellulose and hemicelluloses are often mainly responsible for the brightness reversion of fully bleached chemical pulps (Röhrling et al 2002;Sevastyanova et al 2006;Jullander and Brune 1957;Lewis and Epstein 1962;Zhou et al 2011;Perrin et al 2014). The structure and origin of the carbohydrate derived chromophores have been studied in details (Sevastyanova et al 2006;Forsskahl et al 2009;Theander and Nelson 1988;Potthast et al 2005).…”

Yellowing and brightness reversion of celluloses: CO or COOH, who is the culprit?

“…If the latter was true and no macromolecular effects were active, then the same previous effects of cellulose should be observable when working with model compounds representing the (oxidized) anhydroglucose units of cellulose. In this regard, it is important to note that the literature offers abundant examples of chromophore generation studies that use low-molecular-weight carbohydrates (e.g., Perrin et al 2014), but only a few of these model compounds represent cellulose sufficiently. Here, a 4-O-substituent is imperative to resemble the chemical behavior of the b-1,4-glycosidically linked glucopyranose units in cellulose closely.…”