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Reactions of aldehydes with chlorous acid and chlorite in chlorine dioxide bleaching
Holzforschung
Abstract
The role of chlorine (III) compounds, i.e. chlorous acid (HClO(2)) and chlorite (ClO(2)(-)), in chlorine dioxide bleaching were investigated by treating different pulps with Cl(III). It was discovered that in addition to its fully inorganic reactions, chlorous acid is consumed by organic structures present in the pulp. These structures were assumed to be aldehydes. The aldehydes might exist already in unbleached pulps, but chlorine dioxide bleaching was also found to generate new aldehyde groups. The reactive groups were concluded to originate both from carbohydrates and lignin.
... [44] According to research, ·OH is produced by the reaction of ClO 2 with H 2 O (Equations (1)- (7)). [44][45][46][47] Moreover, it is clear that ·OH is efficient in degrading benzene, toluene, and xylene [48][49][50][51][52]. Therefore, it could be speculated that the ·OH produced by ClO 2 with H 2 O plays a vital role in removing BTEX. ...
With the development of the chemical industry, benzene, toluene, ethylbenzene, and xylene (BTEX) have gradually become the major indoor air pollutants. Various gas treatment techniques are widely used to prevent the physical and mental health hazards of BTEX in semi-enclosed spaces. Chlorine dioxide (ClO2) is an alternative to chlorine as a secondary disinfectant with a strong oxidation ability, a wide range of action, and no carcinogenic effects. In addition, ClO2 has a unique permeability which allows it to eliminate volatile contaminants from the source. However, little attention has been paid to the removal of BTEX by ClO2, due to the difficulty of removing BTEX in semi-enclosed areas and the lack of testing methods for the reaction intermediates. Therefore, this study explored the performance of ClO2 advanced oxidation technology on both liquid and gaseous benzene, toluene, o-xylene, and m-xylene. The results showed that ClO2 was efficient in the removal of BTEX. The byproducts were detected by gas chromatography-mass spectrometry (GC-MS) and the reaction mechanism was speculated using the ab initio molecular orbital calculations method. The results demonstrated that ClO2 could remove the BTEX from the water and the air without causing secondary pollution.
... Whatever the underlying phenomena, it is clear that HCl gas manages to increase the amount of colloidal material in the filtrate with all polymorph samples (Fig. 11) but the cellulose within the colloidal particles is clearly more susceptible to hydrolysis to glucose with cellulose III samples, particularly with cellulose III II (Fig. 8). The presence of chlorite may play a role in this as chlorite has been shown to quantitatively affect the reactivity of cellulose (Hubbell & Ragauskas, 2010;Javed & Germgard, 2011) with many side reactions in acidic environment, resulting in, e.g., chlorine dioxide formation (Lehtimaa, Kuitunen, Tarvo, & Vuorinen, 2010). Full treatise of the role of different chlorine-containing species in the system is, however, beyond the scope of this initial study. ...
As cellulose is the main polysaccharide in biomass, its degradation into glucose is a major undertaking in research concerning biofuels and bio-based platform chemicals. Here, we show that pressurized HCl gas is able to efficiently hydrolyze fibers of different crystalline forms (polymorphs) of cellulose when the water content of the fibers is increased to 30–50 wt%. Simultaneously, the harmful formation of strongly chromophoric humins can be suppressed by a simple addition of chlorite into the reaction system. 50–70 % glucose yields were obtained from cellulose I and II polymorphs while >90 % monosaccharide conversion was acquired from cellulose IIIII after a mild posthydrolysis step. Purification of the products is relatively unproblematic from a gas-solid mixture, and a gaseous catalyst is easier to recycle than the aqueous counterpart. The results lay down a basis for future practical solutions in cellulose hydrolysis where side reactions are controlled, conversion rates are efficient, and the recovery of products and reagents is effortless.
... It is a gas with good solubility and relatively high stability in acidic aqueous media, and thus it is applied this way at a pH around 3. ClO 2 is a strong oxidant for many organic compound classes, such as phenols, aldehydes, unsaturated structures, or amines. There are three typical initial reactions of ClO 2 -hydrogen atom abstraction, one-electron transfer, and radical addition reactions to double bonds-which are typically followed by subsequent reactions of ClO 2 -derived chlorine species (Leigh et al. 2014;Aguilar et al. 2014;Lehtimaa et al. 2010;Napolitano et al. 2005;Hull et al. 1967). Because of the different reaction modes and many side reactions, its application in synthetic organic chemistry is rather limited, but it is a favored oxidant in pulp bleaching because of its high selectivity: it mainly attacks residual lignin, with its aromatic and quinoid structures, while leaving the carbohydrate structures in cellulose and hemicelluloses largely unchanged. ...
2,5-Dihydroxy-[1,4]-benzoquinone (DHBQ, 1) is the most prominent representative of cellulosic key chromophores, which occur almost ubiquitously in all types of aged cellulosics. The degradation of DHBQ by chlorine dioxide under conditions of industrial pulp bleaching (“D stage”) was studied, i.e. in moderately acidic medium (pH 3) at temperatures between 50 and 90 °C. The degradation in the presence of excess ClO2 generates rhodizonic acid (RhA, 5,6-dihydroxycyclohex-5-ene-1,2,3,4-tetrone, 2) as a secondary chromophore which is even more stable and more potent as a chromophore than the starting DHBQ, especially in the form of its salts. At least a threefold ClO2 excess is needed for complete DHBQ consumption. The reaction from DHBQ to RhA involves pentahydroxybenzene (PHB, I) as an intermediate which is either readily further oxidized to RhA by excess ClO2 or slowly reconverted to DHBQ in the absence of ClO2. The RhA yield after 30 min reaction time had a maximum of 83% at a DHBQ/ClO2 molar ratio of 1:5, and decreased with increasing ClO2 charge, reaching 38% at a DHBQ/ClO2 ratio of 1:8 and above. Degradation of DHBQ by ClO2 is 42 times faster than that of RhA (50 °C, pH 3). RhA is present in aqueous medium in the form of its dihydrate, 2,3,5,5,6,6-hexahydroxycyclohex-2-ene-1,4-dione, which contains two pairs of geminal diols at C-5 and C-6. At pH 5 and above it forms an aromatic C6O6²⁻ dianion, so that the RhA salts are very stable. These salts are intensively colored, not only the ones with transition metal cations, but also those with monovalent (Na⁺, K⁺) and especially divalent (Ca²⁺, Mg²⁺) main group metals, and usually have very low solubility so that they precipitate on the pulp fibers. It was demonstrated that the inferior ClO2-bleachability of some pulps is due to the conversion of DHBQ into colored RhA and its respective salts.
Graphic abstract
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... Chlorine dioxide is a strong oxidant for variety of organic compounds, such as phenols, aldehydes, unsaturated structures, or amines. It undergoes hydrogen atom abstraction, one-electron transfer, and radical addition reactions to double bonds (Leigh et al. 2014;Aguilar et al. 2014;Lehtimaa et al. 2010;Napolitano et al. 2005;Hull et al. 1967). It is a favored oxidant in pulp bleaching because of its high selectivity, meaning that residual lignin, with its aromatic and quinoid structures, is attacked while the carbohydrate structures of cellulose and hemicelluloses remain largely unharmed. ...
5,8-Dihydroxy-[1,4]-naphthoquinone (DHNQ) is one of the key chromophores occurring in all types of aged cellulosics. This study investigates the degradation of DHNQ by chlorine dioxide at moderately acidic (pH 3) conditions, corresponding to the conditions of industrial bleaching (“D stage”). The degradation involves three major pathways. As initial reaction, a hydrogen transfer from DHNQ to chlorine dioxide via a PCET mechanism occurs to form a radical DHNQ· and chlorous acid. DHNQ· is then attacked by water to give a pentahydroxynaphthalene radical PHN· that is stabilized by strong delocalization of the non-paired electron into its aromatic ring. PHN· immediately disproportionates to give the observable intermediate 1,2,4,5,8-pentahydroxynapththalene (I), which was comprehensively confirmed by NMR and MS (path A). In the presence of excess ClO2, I is immediately further oxidized into acetic acid, glycolic acid, oxalic acid and CO2 as the final, stable, and non-colored products (path C). In the absence of excess ClO2, elimination of water from I regenerates DHNQ (path B), so that at roughly equimolar DHNQ/ClO2 ratios ClO2 is fully consumed while a major part of DHNQ is recovered. To avoid such DHNQ “recycling” under ClO2 consumption—and to completely degrade DHNQ to colorless degradation products instead—ClO2 must be applied in at least fivefold molar excess relative to DHNQ.
Graphical Abstract
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... Cl 2 O solution was prepared as described earlier. 35 Synthesis of [DBNH]OAc. 1,5-Diazabicyclo[4.3.0]non-5-ene ...
In continuation of previously reported results, the ionic liquid 1,5-diazabicyclo[4.3.0]non-5-ene-1-ium acetate was also found to be a powerful non-derivatizing solvent for cellulosic waste such as paper and cardboard. The ionic liquid could dissolve all the present bio-polymers (cellulose, hemicellulose, and lignin) in high concentrations, resulting in solutions with visco-elastic properties that were suitable for dry-jet wet fiber spinning. The cellulosic raw materials were refined gradually to identify the influence of residual components on the spinnability of the respective solution. Polymer degradation and losses in the spinning process could be avoided nearly entirely. With the exception of virtually unrefined cardboard, all the samples showed excellent spinnability, resulting in fibers with high tensile strength. Prototype textiles were produced to validate the quality of the fibers and demonstrate the possibility of using residual lignin in cardboard as a natural dye.
Chlorine dioxide (ClO2) is a strong oxidizing agent and an efficient disinfectant. Due to its broad-spectrum bactericidal properties, good inactivation effect on the vast majority of bacteria and pathogenic microorganisms, low resistance to drugs, and low generation of halogenated by-products, chlorine dioxide is widely used in fields such as water purification, food safety, medical and public health, and living environment. This review introduced the properties and application status of chlorine dioxide, compared the action mode, advantages and disadvantages of various disinfectants. The mechanism of chlorine dioxide inactivating bacteria, fungi and viruses were reviewed. The lethal target of chlorine dioxide to bacteria and fungi is to destroy the structure of cell membrane, change the permeability of cell membrane, and make intracellular substances flow out, leading to their death. The lethal targets for viruses are the destruction of viral protein capsids and the degradation of RNA fragments. The purpose of this review is to provide more scientific guidance for the application of chlorine dioxide disinfectants.
The chromaticity coordinates and luminance factor are important indicators of the fluorescent dyeing performance of fluorescent dyed fabric. However, with the current bleaching pretreatment process, the chromaticity coordinates and luminance factor of fluorescent yellow dyed modacrylic/cotton blended fabric usually do not meet the requirements of standard ISO 20471. Here we report a new type of bleaching process method via sodium chlorite to solve the problem of yellowing of the background color of the modacrylic/cotton blended fabric. Compared with hydrogen peroxide bleaching, the whiteness of the modacrylic/cotton blended fabric was increased by 13.52% with sodium chlorite bleaching. The color performance analysis showed that the chromaticity coordinates and luminance factor of fluorescent yellow dyed modacrylic/cotton fabric after exposure to xenon light or washing cycles could meet the standard ISO 20471 requirements. Moreover, the fluorescent yellow dyed fabric has good color fastness, such as rubbing resistance, perspiration, laundering at 60°C, and dimensional stability. In addition, the fluorescent yellow dyed blended fabric also has flame retardant performance. Our work may provide a method for the preparation of high-visibility fluorescent yellow fabric with flame retardant performance. It could be used for multifunctional personal protective equipment.
Production of paper grade pulp is still relying on the use of chlorine dioxide in ECF bleaching sequences in spite of the AOX pollutants this process releases in the environment and in paper products. The shift towards the use of TCF bleaching sequences will be necessary and is already made possible by the total substitution of chlorine dioxide for ozone, a greener and more powerful oxidant. It is shown in this thesis that an ozone TCF bleached pulp can reach brightness and strength properties equivalent to that of a conventional ECF pulp. Furthermore, a UV resonance Raman spectroscopy study demonstrates the superiority of ozone (associated with hydrogen peroxide) in stabilising the brightness, compared to the joint use of chlorine dioxide and peroxide. Analysis by means of direct dissolution of cellulosics in DMAc / LiCl (8 %) and SEC indicate that the strength of the TCF pulp is preserved. This is because the cellulose depolymerisation taking place in the various ozone stages is uniform within the fibres. Replacement of chlorine-based bleaching agents with oxygen, hydrogen peroxide and ozone allows for substantial savings on caustic soda in the sequence. In fact, in the absence of chloride ions, oxidised white liquor can be used as an alkali source and recycled in the kraft process with the alkaline bleaching effluent. Although effluent recycling induces an overconsumption of ozone, results presented here show that this may be the most cost-effective scenario. This paves the way for the development of highly competitive TCF bleaching sequences for the production of bleached kraft pulp.Production of paper grade pulp is still relying on the use of chlorine dioxide in ECF bleaching sequences in spite of the AOX pollutants this process releases in the environment and in paper products. The shift towards the use of TCF bleaching sequences will be necessary and is already made possible by the total substitution of chlorine dioxide for ozone, a greener and more powerful oxidant. It is shown in this thesis that an ozone TCF bleached pulp can reach brightness and strength properties equivalent to that of a conventional ECF pulp. Furthermore, a UV resonance Raman spectroscopy study demonstrates the superiority of ozone (associated with hydrogen peroxide) in stabilising the brightness, compared to the joint use of chlorine dioxide and peroxide. Analysis by means of direct dissolution of cellulosics in DMAc / LiCl (8 %) and SEC indicate that the strength of the TCF pulp is preserved. This is because the cellulose depolymerisation taking place in the various ozone stages is uniform within the fibres. Replacement of chlorine-based bleaching agents with oxygen, hydrogen peroxide and ozone allows for substantial savings on caustic soda in the sequence. In fact, in the absence of chloride ions, oxidised white liquor can be used as an alkali source and recycled in the kraft process with the alkaline bleaching effluent. Although effluent recycling induces an overconsumption of ozone, results presented here show that this may be the most cost-effective scenario. This paves the way for the development of highly competitive TCF bleaching sequences for the production of bleached kraft pulp.
The chapter analyzes the new types of natural fibers and explores their prospects for varied applications and possible substitutes for existing fibers. Lotus fibers extracted from lotus stem has been characterized by various methods. The crystallinity, crystallite index, cellulose content, and moisture regain of the fiber are determined. The initial modulus, yield point, and specific work before and after the yield point of palm and ramie fibers are also determined. Palm fiber showed remarkable yield characteristics. The crystallinity index of ramie fibers was higher than that of palm fibers, which considerably influenced the mechanical behavior of the fibers. The distinctive tensile properties of palm fiber may endow its potential application in industrial areas. The surface and cross-sectional features of pig hair fibers have been evaluated using scanning electron microscopy. The cross-section of pig fiber is modeled into an ellipse and the elliptical features of the fiber are correlated with its tensile properties. Attempt has been made to extract sunhemp fiber from stalks of sumhemp plants which are considered as agricultural waste after harvesting seeds. Urea-treated sunhemp stalks give relatively longer, finer, and stronger fibers than the compost culture treated and control stalks. Comprehensive characterization of spadix fiber has been done to examine its morphological, physical, mechanical, chemical, and thermal properties. High cellulose content of the fiber provides better strength and ensures better bonding with the matrix. Moreover, the low density of the fiber makes it alternative for hazardous synthetic fiber. The new fibers explored show prospects for varied applications and also offer possible substitutes for some commercially used fibers and are a step toward sustainability.
Lotus stems were treated with sodium hydroxide followed by sodium chlorite to prepare lotus fibers. The lotus fibers were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectrometry (FTIR), X-ray diffraction (XRD), and thermogravimetric (TG) analysis. The crystallinity and crystallinity index of lotus fibers are 62.11 and 62.29%, respectively. The content of cellulose in lotus fibers after being purified with sodium chlorite is improved and reaches 69.74%. The moisture regain of lotus fibers was studied and the results show that moisture regain of purified lotus fibers is 6.87%. The influences of concentration of sodium chlorite on tensile strength and breaking elongation of lotus fibers were investigated. The tensile strength and breaking elongation of lotus fibers both decrease as the concentration of sodium chlorite increases. The results are expected to provide valuable guidance for preparing lotus fibers from lotus stems through chemical treatment.
The mechanism of chlorine dioxide (ClO2) decomposition in an alkaline medium has been investigated. The formation of radicals and chlorinated species was studied in aqueous solutions containing ClO2 and simple model compounds of lignin or cellulose (vanillin, vanillyl alcohol, veratryl alcohol, methylglucoside and cellobiose) at acidic and alkaline pHs. Because hypochlorite (ClO⁻) is an intermediate occurring in the course of the reaction mechanism, similar experiments were carried out with solutions of sodium hypochlorite (NaClO) at alkaline and acidic pHs. Electron paramagnetic resonance (EPR) spectroscopy based on the spin-trapping technique revealed the presence of hydroxyl radicals (HO˙) at alkaline pH with ClO2 alone or with model compounds. At the same pH, only a small amount of HO˙ was detected with ClO⁻. Chlorite (ClO2⁻) and chlorate (ClO3⁻) ions were dosed with iodometric titrations, both during ClO2 alkaline decomposition and during reactions with model compounds. Vanillin and vanillyl alcohol were oxidized by ClO2. The intermediate ClO2⁻ was either inert or reacted with the aldehyde function of vanillin. Cellobiose was attacked only in an alkaline medium, either directly by ClO2 or indirectly by HO˙ radicals. This resulted in the formation of glucose, which was then degraded by ClO2⁻ ions. The generation of HO˙ could be one reason for cellulose degradation by ClO2 at alkaline pH, but possibly not the unique one, as it was not proved in this article, whether or not ClO2 is able to directly attach the OH functions of anhydrosugars at alkaline pH.
The drainage of filtrates during the mat formation in a brown stock washer directly impacts the washer's productivity and indirectly affects its efficiency. In this paper, we demonstrate the role of entrained air on mat formation and filtration as they occur in brown stock washing. We further show how chemical treatments, such as defoamers and wash aids, and fiber surface chemistry influence air entrainment and thereby washer productivity and efficiency. With this knowledge, washer operations can be improved through the design and selection of better treatments.
Chlorine dioxide gas is effective at cleansing fruits and vegetables of bacterial pathogens and(or) rot organisms, but little data are available on chemical residues remaining subsequent to chlorine gas treatment. Therefore, studies were conducted to quantify chlorate and perchlorate residues after tomato and cantaloupe treatment with chlorine dioxide gas. Treatments delivered 50 mg of chlorine dioxide gas per kg of tomato (2-h treatment) and 100 mg of gas per kg of cantaloupe (6-h treatment) in sealed, darkened containers. Chlorate residues in tomato and cantaloupe edible flesh homogenates were less than the LC-MS/MS limit of quantitation (60 and 30 ng/g respectively), but were 1319 ± 247 ng/g in rind + edible flesh of cantaloupe. Perchlorate residues in all fractions of chlorine dioxide-treated tomatoes and cantaloupe were not different (P > 0.05) than perchlorate residues in similar fractions of untreated tomatoes and cantaloupe. Data from this study suggest that chlorine dioxide sanitation of edible vegetables and melons can be conducted without the formation of unwanted residues in edible fractions.
The oxidation of pyruvic acid by hypochlorous acid was studied in the pH 1.0 – 11.0 range. The main path in the redox process is the reaction between the pyruvate ion and HOCl. It was shown that the reaction is second order in both reactants and kHOCl = 2.17 ± 0.04 M−1s−1(I = 1.0 M (NaClO4), T = 25.0 °C) with ∆HHOCl≠ = 34.6 ± 1.2 kJmol−1 and ∆SHOCl≠ = −120.3 ± 3.6 Jmol−1K−1 for this reaction step. The relatively large negative entropy of activation is consistent with an O atom-transfer mechanism. DFT calculations support this conclusion by predicting a concerted rearrangement of the activated complex which includes the reactants and the solvent water molecule. It was also confirmed that the hydration of pyruvic acid has significant contribution to the observed kinetic features under acidic condition. The hydration reaction is a proton catalyzed process and the pseudo-first order rate constant can be interpreted by considering the protolytic and hydration equilibria of pyruvic acid. The rate constants were determined for the non-catalytic and the proton catalyzed path: kh0 = 0.24 ± 0.01 s−1 and kh1 = 5.5 ± 0.2 M−1s−1.
An investigation of the kinetics and mechanism for epoxidation of styrene and para-substituted styrenes by chlorite at 25 °C in the pH range of 5-6 is described. The proposed mechanism in water and water/acetonitrile includes seven oxidation states of chlorine (-I, 0, I, II, III, IV, and V) to account for the observed kinetics and product distributions. The model provides an unusually detailed quantitative mechanism for the complex reactions that occur in mixtures of chlorine species and organic substrates, particularly when the strong oxidant chlorite is employed. Kinetic control of the reaction is achieved by the addition of chlorine dioxide to the reaction mixture, thereby eliminating a substantial induction period observed when chlorite is used alone. The epoxidation agent is identified as chlorine dioxide, which is continually formed by the reaction of chlorite with hypochlorous acid that results from ClO produced by the epoxidation reaction. The overall stoichiometry is the result of two competing chain reactions in which the reactive intermediate ClO reacts with either chlorine dioxide or chlorite ion to produce hypochlorous acid and chlorate or chloride, respectively. At high chlorite ion concentrations, HOCl is rapidly eliminated by reaction with chlorite, minimizing side reactions between HOCl and Cl2 with the starting material. Epoxide selectivity (>90% under optimal conditions) is accurately predicted by the kinetic model. The model rate constant for direct reaction of styrene with ClO2(aq) to produce epoxide is (1.16 ± 0.07) × 10(-2) M(-1) s(-1) for 60:40 water/acetonitrile with 0.20 M acetate buffer. Rate constants for para substituted styrenes (R = -SO3(-), -OMe, -Me, -Cl, -H, and -NO2) with ClO2 were determined. The results support the radical addition/elimination mechanism originally proposed by Kolar and Lindgren to account for the formation of styrene oxide in the reaction of styrene with chlorine dioxide.
Measurements of kinetic isotope-effects (k*/k) for the oxidation of ten aldoses-1-t with sodium chlorite in acid solution gave values ranging from 0.56 to 0.75, with an average of 0.66. The values show that the C1-H* bond is not ruptured in the rate-determining step. For the reaction, a mechanism is proposed which accounts for this fact and for the dependence of the rate of reaction on the concentration of chlorous acid. The mechanism involves formation of a chlorous acid-aldehyde intermediate; this decomposes, giving the aldonic acid and hypochlorous acid. The latter then reacts with more chlorous acid, affording the chlorine dioxide and hydrogen chloride found experimentally.
Chlorine dioxide delignification (D0) modifies kraft residual lignin by oxidizing phenolic groups to both quinonoid and muconic structures. Results from various in situ analyses (i.e., kappa/Klason lignin, methanol, and methoxyl group analyses) demonstrated that the D0 stage preferentially oxidizes phenolic entities to quinones over muconic structures in ∼2:1 ratio (1.7 mmol/g lignin vs. 0.8 mmol/g lignin), respectively. The level of quinones detected by these in situ methods was significantly higher than those determined by the NMR analysis of isolated D0 residual lignin. It was discovered that the acidic procedure used to isolate the oxidized lignin for NMR analysis chemically eliminated the majority of quinonoid structures, which resulted in lower amounts of quinones detected. This study clearly indicates that quinones are a significant reaction product formed during the D0 stage. Dedicated to Drs. Donald R. Dimmel and Thomas J. McDonough on their retirement from the Institute of Paper Science and Technology.
The relative amount of chlorine dioxide produced by the disproportionation of sodium chlorite increases as the concentrations of chlorite and chloride ions increase. It passes through a minimum when the concentration of perchloric acid varies from 2 M to 0.01 M. The fast reaction between chlorite and hypochlorous acid is a part of the mechanism of this disproportionation and its stoichiometry was also investigated. The relative amount of chlorine dioxide produced depends on the method of mixing the reactants and on the acidity. It increases if the concentration of chlorite increases and can exceed the amount predicted by HClO + 2 HCIO2 → 2 ClO2 + Cl- + H+ + H2O. Ortho-tolidine reacts very rapidly with chlorine and with chlorine dioxide but not with chlorous acid. In perchloric acid solutions (pH < 2.5) the product of its oxidation has a considerable absorption with a maximum at 440nm ( = 59700 M-1 cm-I). Chlorine reacts more rapidly with ortho-tolidine than with chlorous acid. The kinetic investigation of the disproportionation of chlorous acid is thus simplified by the use of ortho-tolidine. With added chloride ions the rate determining step is HCIO2 + C1- + H+ → 2 HCIO with a rate constant k = 1.810-3 M-1 s-1. The most reliable values for the free enthalpy of formation of oxychlorine compounds are selected from the literature.
The disproportionation of chlorite was studied in 0.01 to 1 M perchloric acid solutions at 25°C and an ionic strength of 1 M. The results suggest at least three reaction paths. The first is catalysed by Cl- ions, the second gives a second-order rate law, and the third is catalysed by iron. Its rate law can be interpreted by the reversible reaction ClO2- + Fe3+ = ClO2 + Fe2+ followed by two rate-determining reactions Fe2+ + HCIO2 -> products, Fe2+ + ClO2- -> products. From this study and the former, made with added ortho-tolidine, we conclude that the second-order reaction proceeds by a radical chain mechanism.
Rates of reaction of chlorine dioxide with phenol and with hydroquinone were determined with a stopped-flow spectrophotometer in the pH range 4-8. Second-order rate constants increase with increasing pH, consistent with a mechanism in which both the free phenol and the more reactive phenoxide anion react with ClO 2. Removal of an electron from the substrate by ClO 2 to form a phenoxyl radical and ClO 2- ion is the rate-determining step. Subsequently, in the case of hydroquinone, ClO 2 removes another electron from the radical, forming p-benzoquinone and another ClO 2- ion. In the case of phenol, ClO 2 adds to the phenoxyl radical para to the oxygen, and p-benzoquinone is formed with concomitant release of HOCl. The mechanism for phenol reaction accounts for (i) the immediate formation of p-benzoquinone without apparent intermediacy of hydroquinone, (ii) the chlorination observed in solutions containing excess phenol, and (iii) the production of only 0.5 mol of ClO 2-/mol of ClO 2 consumed.
In ClO2 delignification and bleaching, in-situ generated HClO is believed to be the main reason for AOX and chlorate formation. Several studies have shown that HClO scavengers added to a ClO2 stage reduce AOX formation. The present study focuses on the effect of HClO scavengers on chlorate formation. Four different scavengers, sulphamic acid, dimethyl sulphoxide (DMSO), hydrogen peroxide and oxalic acid, were investigated. Unexpectedly, chlorate formation was not reduced by hydrogen peroxide and oxalic acid, and a reduction of only 30-35% was obtained with sulphamic acid and DMSO. The ineffectiveness of hydrogen peroxide and oxalic acid is ascribed to their lower reaction rates with HClO compared to that of chlorate formation, and it is suggested that the reaction rates of DMSO and sulphamic acid with HClO are of the same magnitude as that of chlorate formation.
Delignification processes comprising various pulping and bleaching reactions are reviewed in the light of a general concept dividing these reactions into nucleophilic and electrophilic categories. It is shown by numerous examples that pulping is due to the action of external nucleophilic species attacking electron-deficient centres in carbonyl and conjugated carbonyl (=enone) structures and to the presence of neighbouring nucleophilic groups attacking the β-carbon atom of side chains. On the other hand, bleaching is initiated by electrophilic species attacking electron-rich centres in aromatic nuclei and unsaturated, ring-conjugated side chains. These initial bleaching reactions are followed by nucleophilic reactions taking place during the same or during subsequent bleaching steps.
Rate constants as well as the products formed from the reaction between chlorine dioxide and the non-phenolic lignin model compound 1-(3,4-dimethoxyphenyl)ethanol have been determined in the pH interval 3-8. Spectroscopic data for the substrate and some products are given. The pH of the reaction mixture has a large influence on both the rate constant and the product distribution. Under technical chlorine dioxide bleaching conditions (pH 3, 40°C and an ionic strength of 0.3 mol kg-1 ) the rate constant has been found to be 0.12(2) M-1 s-1, which corresponds to a half-life of approximately 7 min in 0.01 M solution. The influence of temperature [Ea = 29(7)kJ mol-1] on the rate of reaction of this lignin model is small. At acidic pH (35) a large variety of both oxidized (quinones, lactones and ketones) and chlorinated products are formed. Above pH 8, one oxidation product, 3,4-dimethoxyacetophenone, predominates. No chlorinated products are found at a high pH. The mechanisms of formation of all these products are discussed.
The formation of organically bound chlorine (AOX) during ClO2 bleaching can be reduced by additives. Dimethylsulphoxide (DMSO) is particularly efficient. It reacts with HOCl generated in situ to give dimethylsulphone (DMSO2). Unfortunately, the capture of HOCl results in poorer delignification. Therefore, the use of DMSO must be limited to relatively low AOX abatement. The addition of FeSO4, which should regenerate ClO2 from the HClO2 formed during ClO2, has no positive effect on AOX. However, Fe2+ causes some viscosity loss, probably because it induces the formation of Cl• radicals. ClO2 splitting, coupled with an addition of alkali to raise the pH to neutrality at the start of each ClO2 phase, leads to AOX reduction without any negative effect on delignification.
The reaction of chlorite and formaldehyde was studied in basic and slightly acidic media. Though the expected product was CO2, the oxidation of HCHO, however, gave nearly quantitative formation of ClO2, the oxidation product of ClO2-. In excess HCHO the stoichiometry of the reaction was deduced as 3ClO2- + HCHO + 2H+ → HCOOH + 2ClO2(aq) + Cl- + 2H2O; but in high excess of ClO2- the stoichiometry was 6ClO2- + HCHO + 4H+ → CO2(g) + 4ClO2(aq) + 3H2O + 2Cl-. The reaction is autocatalytic in HOCl. The first step of the reaction produces HOCl, which catalyzes the formation of ClO2 and further oxidation of HCOOH to CO2. ClO2 was found to be relatively unreactive toward HCHO and HCOOH, and hence it accumulated rapidly.
The aim of this work was to demonstrate that dimethylsulfoxide (DMSO) is an excellent masking agent for aqueous chlorine in the determination of other oxychlorines. By the addition of excess DMSO, specific absorbance of free chlorine disappeared, and the oxidation of iodide to iodine by chlorine was completely prevented. Chlorine dioxide, chlorite and chlorate were not affected by the co-existence of excess DMSO, but chlorosulfamic acid, showed results comparable to free chlorine. By using ion chromatographic analysis of the mixed solution of free chlorine and DMSO, chloride was recovered as the only anionic species and its molar concentration was approximately twice the initial chlorine concentration. DMSO seems to reduce and mask chlorine completely, without affecting other oxychlorines. Chlorine and DMSO reacted in the molar ratio of 1:1 The reaction seemed to be second-order. The rate constant was larger at a lower pH, and it was dependent not on a concentration of total chlorine, but on that of hypochlorous acid. The redox potential of DMSO was higher at a lower pH, and only hypochlorous acid would have a redox potential high enough to oxidize DMSO in acidic conditions. These results suggest that DMSO reacts with hypochlorous acid stoichiometrically. In practical use, DMSO may be successfully used as a masking agent for aqueous chlorine.
This study investigated the effects of temperature, chlorine dioxide dosage, and filtrate circulation on various filtrate and pulp properties in chlorine dioxide prebleaching of birch kraft pulp. Also the effect of a preceding A-stage was examined. Kappa number, delignification, and hexenuronic acid removal were unaffected by the used dilution media and temperature. Increasing ClO2 charge resulted in a lower kappa number, increased hexenuronic acid removal, and more efficient delignification. With a preceding A-stage the kappa number was lower throughout the D0-stage, but the kappa number reduction and the removal of hexenuronic acid were slightly decreased. Chlorine dioxide consumption was practically independent of the tested variables; only dosage had an effect. The formation of chlorate and chloride was increased with higher dosages of ClO2, but the stoichiometry remained unchanged. The concentration of chlorite + chlorous acid during bleaching depended on temperature, used dilution medium, and the presence of an A-stage.
A mechanism involving HOCl, Cl—ClO2, and Cl2 as intermediates is proposed for the disproportionation of chlorous acid. In the absence of chloride, the reaction is controlled by two simultaneous processes, 2HClO2 → H+ + HOCl + ClO3− and HClO2 + ClO2− → HOCl + ClO3−. Chloride has a catalytic effect and an inhibiting effect as well on the formation of chlorine dioxide. The initial reaction rate passes through a minimum at a certain concentration of chloride at low acidities, which can be interpreted by the postulated mechanism. Under chloride catalysis, the reaction is controlled by the process H+ + Cl− + HClO2 → 2HOCl.
Fog in areas of southern California previously thought to be pollution-free has been shown to have a pH as low as 1.69. It has been found to be most acidic after smoggy days, suggesting that it forms on the aerosol associated with the previously exiting smog. Studies on Whiteface Mountain in the Adirondacks show that fog water is often 10 times as acidic as rainwater. As a result of their studies, California plans to spend $4 million on acid deposition research in the coming year. (JMT)
The reaction between ClO2•- and HOCl has been studied by spectrophotometrically monitoring the production of ClO2 at pH 5-6. In excess ClO2-, the reaction is first order in ClO2-, HOCl, and H+, and the stoichiometry is given by HOCl + 2ClO2- + H+ → 2ClO2 + Cl- + H2O. In excess HOCl and at higher pH's, ClO3- is produced, and the order of the reaction is between 1 and 2 for HOCl and between 0 and 1 for H+. By combining computer simulation and least-squares analysis, we obtain a mechanism in which the reaction 2HOCl + ClO2- → ClO3- + Cl2 + H2O (k = (2.1 ± 0.1) × 10-3 M-2 s-1) plays a key role in explaining the behavior at high [HOCl]/[ClO2-].
Chlorous acid is known to react with aldehydes. This study investigated the oxidation kinetics of different aldehydes by chlorous acid. Reaction rates with chlorous acid at 25 °C were obtained for formaldehyde (11.0 M−1 s−1), vanillin (0.59 M−1 s−1), veratraldehyde (1.00 M−1 s−1), benzaldehyde (5.6 M−1 s−1), glucose (3.39 × 10−3 M−1 s−1), glycolaldehyde (39.0 M−1 s−1), and 5-formyl-2-furancarboxylic acid (5.2 M−1 s−1). The activation energies varied between 26 and 63 kJ/mol. The rate-determining step was shown to be the addition of chlorous acid to the aldehyde structure. The practical significance of the Cl(III) reaction with aldehyde structures in chlorine dioxide bleaching was discussed. It was concluded that various aldehydes present in cellulosic pulp are at least partly responsible for the consumption of Cl(III) in chlorine dioxide bleaching.
The kinetics and stoichiometry of the reaction between hypochlorous acid and chlorous acid was studied under mildly acidic conditions (pH = 2.5−4.7). The experiments were conducted in dilute solutions (10−4−10−3 M) at T = 5−25 °C. External buffer compounds were not used. Chlorous acid, chlorine dioxide, and chlorate concentrations were monitored with titrimetric methods. Rate coefficients and activation energies were determined for the five main steps of the kinetic model (others were adopted from literature). Both hypochlorous acid and chlorine were found to react at an appreciable rate with chlorous acid/chlorite. Chloride ion was found to promote both the overall reaction rate and chlorine dioxide/chlorate ratio. The results may be exploited in optimizing the red-ox efficiency of technical chlorine dioxide bleaching processes.
Several pathways leading to the decomposition of chlorite and chlorous acid have been published. In this study, both experimental and computational approaches have been applied to clarify the authenticity of the different routes. The decomposition of chlorine (III), i.e. chlorous acid and chlorite, was monitored with iodometric titration at changing chloride concentrations, temperature, and existence of iron (III) at pH 1−3. Dimethylsulfoxide (DMSO) was used to prevent hypochlorous acid from reacting with chlorite. Chlorine dioxide was not formed in the absence of metals when hypochlorous acid and chlorine were trapped. The self-decomposition of Cl(III) proceeds only via the acidic form while chlorite is stable. Chloride ions enhanced the chlorous acid decomposition rate especially at low pH. Chlorite decomposes in the presence of Fe3+ ions. General kinetic parameters and their temperature dependencies were determined for chlorous acid self-decomposition.
The oxidation of iron(II) by chlorine(III) appears to occur via a one-electron-transfer mechanism. The reaction was studied at 10 and 25° under pseudo-first-order conditions with iron(II) being in excess. Chloride ion and phenol were added to the reaction mixtures in order effectively to eliminate complicating side reactions of the intermediate product chlorine(I). The reaction rates are essentially unaffected by chloride ion concentration. The hydrogen ion dependence is appropriate to the rate expression - d[Fe(II)]/dt = k1[Fe(II)][Cl(III)] + k2[Fe(II)][Cl(III)]/[H+] At 25° and 2.0 M ionic strength, k1 = (1.89 ± 0.09) × 103 M-1 sec-1 and k2 = 58 ± 10 sec-1. The results are discussed in terms of several possible one-electron-transfer mechanisms.
The disproportionation of chlorous acid was studied at an ionic strength of 2.0 M under a variety of hydrogen ion conditions from 1.2 × 10-3 to 2.0 M and with up to 0.1 M added chloride ion. In the absence of added chloride ion, in 1.2 and 2.0 M perchloric acid at 25°, the stoichiometry can be approximated as: 4HClO2 → 2ClO2 + ClO3- + Cl- + 2H+ + H2O. At the beginning of the reaction less chlorine dioxide than that predicted by the above equation is formed, and, as the reaction proceeds, the relative amount of chlorine dioxide produced increases. The relative amount of chlorine dioxide produced also varies with the hydrogen ion concentration. Chloride ion catalyzes the disproportionation of chlorous acid and also alters the stoichiometry to approximately 5HClO2 → 4ClO2 + Cl- + H+ + 2H2O. As the reaction proceeds in the presence of initial chloride ion, less chlorine dioxide than that predicted by the second equation is formed. Also, as the initial concentration of chloride ion is decreased, the relative amount of chlorine dioxide formed decreases. A mechanism which is consistent with these observations is proposed.
The kinetics and mechanism of the iron(III)-catalyzed decomposition of the chlorite ion have been investigated by using conventional batch, stopped-flow, stopped-flow-rapid-scan spectrophotometric, and quenched stopped-flow methods at 25-degrees-C and in 1.0 M NaClO4. The concentration vs time profiles were determined for chlorite ion, chlorine dioxide, and, in a few cases, chloride ion in the 40 ms-several minute interval. It was confirmed that the stoichiometry can be given as the appropriate combination of the following reactions: 4HClO2 = 2ClO2 + ClO3- + Cl- + 2H+ + H2O; 5HClO2 = 4ClO2 + Cl- + H+ + 2H2O. The proposed mechanism postulates that the catalytic decomposition is initiated by the formation of the FeClO22+ complex and the rate-determining step is the redox decomposition of this species. The mechanism was validated by model calculations based on the GEAR algorithm. The measured and calculated kinetic curves are in excellent agreement under a variety of experimental conditions. It was shown that the overall stoichiometry is kinetically controlled and ultimately determined by fast secondary reactions between various chlorine species. This work represents the first totally inorganic application of the quenched stopped-flow method. Several aspects of this technique are discussed.
On the basis of rapid-scan spectrophotometric and one-wavelength stopped-flow experiments, the formation of the FeClO2(2+) complex was confirmed in the iron(III)-chlorite ion system. The complex formation is associated with the appearance of a new charge-transfer band in the visible spectral region. The stability constant of FeClO2(2+) was calculated from the final absorbance value of stopped-flow traces recorded at 510 nm. log K = 1.14 +/- 0.02 and epsilon-510 = 636 +/- 10 M-1 cm-1 (I = 1.0 M (NaClO4), 25-degrees-C). It was demonstrated that the complex formation is kinetically coupled with catalytic decomposition of ClO2-. The proposed mechanism for the ligand substitution reaction includes the reactions of Fe3+ and Fe(OH)2+ and also a generalized pathway for the chlorite ion decomposition. The rate constant for the Fe3+ + ClO2- = FeClO2(2+) step, 269 +/- 55 M-1 s-1, is consistent with an associative interchange mechanism predicted by previous kinetic data for complex formation reactions of Fe(III).
Practical applications and unique kinetic phenomena have generated considerable interest in the redox chemistry of chlorine(III). The complex kinetic and stoichiometric features of the related reactions are well documented, but relatively little is known about possible interactions between chlorine(III) and transition metal ions. Recent studies confirmed that the chlorite ion forms weak complexes with common metal ions such as Cu(II), Fe(III) or Hg(II) and the formation of an inert chlorito complex was also reported. This paper reviews kinetic and equilibrium aspects of the formation of chlorito complexes as well as the role of transition metal ions in the redox reactions and the catalytic decomposition of chlorine(III). It will be shown that the metal ions enhance the reactivity of chlorine(III) by activating the OCl bond. Chlorito complexes may undergo either redox decomposition or promote the reactions of the coordinated chlorine(III) with other oxychlorine species. It is a common feature that reactive intermediates are formed, which are involved in subsequent fast reactions with the reactants and other intermediates. As a consequence, complex kinetic patterns ensue and the stoichiometries are kinetically controlled. Mechanistic aspects of these reactions will be discussed in detail.
The kinetics of decomposition of aqueous chlorous acid has been reinvestigated at pH 0.7-1.9, ionic strength 1.0 M (HSO 4 -/SO 4 2-), and temperature 25.0 (0.1 °C. Optical absorbances were collected in the 240-450 nm wavelength range for up to ∼90% decomposition for time series lasting as long as 2 days. The number of absorbing species was investigated by matrix rank analysis; no absorbing intermediate was formed in significant concentration during the decomposition. Of the many mechanistic models tested, the one that fit best included the following reactive intermediates: HOCl, Cl 2 O 2 , Cl 2 O 3 , • ClO, • OH. The stoichiometric ratio of ClO 2 produced to Cl(III) consumed varies with pH and [Cl -]. Reaction of Cl 2 O 3 with Cl(III) yields chlorate exclusively. Reaction of Cl 2 O 3 with Cl -favors ClO 2 over chlorate, but does not entirely exclude chlorate, because it is produced by hydrolysis of Cl 2 O 2 . Invoking Cl 2 O 3 explains the variation in stoichiometric ratio as well as the maximum observed in the initial rate of ClO 2 formation as a function of pH. The kinetics of chlorous acid decomposition cannot be quantitatively fit through the last stages of the reaction without postulating a first-order decomposition. Scission of chlorous acid to give short-lived hydroxyl and chlorine-(II) monoxide is a plausible route for this process. A set of best-fit and literature-derived parameters is presented for the complete mechanism.
The affect of phenolic hydroxyl groups on the reaction efficiency during chlorine dioxide pre-bleaching of a soft-wood kraft pulp was investigated. The removal of phe-nolic hydroxyl groups via pulp methylation did not adversely affect the chlorine dioxide bleaching efficiency or the amount of chlorate formed during exposure to chlorine dioxide. Ion analysis of the reaction systems revealed that the formation of chloride and chlorite ions during the bleaching process were very similar between the kraft and methylated kraft pulps. These results indi-cate that the kinetic rates of lignin oxidation by chlorine dioxide and its reduction products, chlorite and hypo-chlorous acid, are much faster than the rate of inorganic reactions leading to chlorate formation.
The hypochlorous acid formed intermediately during the bleaching of an oxygen-prebleached kraft pulp with pure chlorine dioxide (a D0-stage) was captured as N-chlorosulfamic acid by addition of sulfamic acid to the bleaching liquor. The amount of hypochlorous acid captured corresponded to about 50 mol% of the consumed chlorine dioxide. The amount of chlorite formed (20 to 30 mol%) was less than the amount of hypochlorous acid captured. The excess of hypochlorous acid over chlorite suggests that chlorine dioxide is reduced initially not only by a one-electron mechanism to chlorite but also by a two-electron mechanism to monochlorine monoxide, which is then reduced by lignin or by chlorine dioxide to hypochlorous acid. The routes for the further reactions of chlorite, monochlorine monoxide and hypochlorous acid are discussed.
The kinetics of the aqueous phase reaction between molecular chlorine and chlorite ion was studied experimentally by measuring the enhancement of chlorine transfer from the gas phase. The results were interpreted by using the penetration theory of mass transfer, in conjunction with the assumption that the reaction is second order overall, being first order with respect to both chlorine and chlorite. The second-order rate constant was determined as a function of temperature and solution ionic strength. These experiments indicated that the reaction rate constant at 293 K and zero ionic strength is 1.62 × 104 M-1·s-1. The reaction rate constant increased with increasing temperature, having an activation energy of 39.9 kJ·mol-1, and decreased with increasing ionic strength. A collision theory interpretation of the observed rate indicates that the steric factor is on the order of unity, suggesting that nearly every collision of sufficient energy results in reaction, regardless of the relative orientation.
Rates of reaction of chlorine dioxide with phenol and with hydroquinone were determined with a stopped-flow spectrophotometer in the pH range 4-8. Second-order rate constants increase with increasing pH, consistent with a mechanism in which both the free phenol and the more reactive phenoxide anion react with ClOâ. Removal of an electron from the substrate by ClOâ to form a phenoxyl radical and ClOââ» ion is the rate-determining step. Subsequently, in the case of hydroquinone, ClOâ removes another electron from the radical, forming p-benzoquinone and another ClOââ» ion. In the case of phenol, ClOâ adds to the phenoxyl radical para to the oxygen, and p-benzoquinone is formed with concomitant release of HOCl. The mechanism for phenol reaction accounts for (i) the immediate formation of p-benzoquinone without apparent intermediacy of hydroquinone, (ii) the chlorination observed in solutions containing excess phenol, and (iii) the production of only 0.5 mol of ClOââ»/mol of ClOâ consumed.
The kinetics and mechanism of the reaction between Cl(2) and ClO(2)(-) are studied in acetate buffer by stopped-flow spectrometric observation of ClO(2) formation. The reaction is first-order in [Cl(2)] and [ClO(2)(-)], with a rate constant of k(1) = (5.7 +/- 0.2) x 10(5) M(-)(1) s(-)(1) at 25.0 degrees C. Nucleophilic attack by ClO(2)(-) on Cl(2), with Cl(+) transfer to form ClOClO and Cl(-), is proposed as the rate-determining step. A possible two-step electron-transfer mechanism for Cl(2) and ClO(2)(-) is refuted by the lack of ClO(2) suppression. The yield of ClO(2) is much less than 100%, due to the rapid reactions of the metastable ClOClO intermediate via two competing pathways. In one path, ClOClO reacts with ClO(2)(-) to form 2ClO(2) and Cl(-), while in the other path it hydrolyzes to give ClO(3)(-) and Cl(-). The observed rate constant also is affected by acetate-assisted hydrolysis of Cl(2). The rate of Cl(2) loss is suppressed as the concentration of Cl(-) increases, due to the formation of Cl(3)(-). In excess ClO(2)(-), a much slower formation of ClO(2) is observed after the initial Cl(2) reaction, due to the presence of HOCl, which reacts with H(+) and Cl(-) to re-form steady-state levels of Cl(2).
- Jeanes A.