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Effect of using ozone before elemental chlorine free (ECF) bleaching of oxygen delignified wheat straw pulp produced following soda-anthraquinone pulping on bleaching effluent and pulp properties was studied. The effluent generated during bleaching of pulp contains high amount of adsorbable organic halogens (AOX), chemical oxygen demand (COD), biochemical oxygen demand (BOD) and highly toxic chlorophenolic compounds. This study is aimed to utilise green chemistry approach during bleaching of one of the abundantly used agro residue wheat straw for improving the quality of bleaching effluent and pulp properties. Introducing ozone stage before ECF bleaching resulted in significant reduction of chloriphenolic compounds like chlorocatechols by 48.9 %, chloroguaiacols by 33.3 %, chlorovanillins by 28.4 % and chlorophenols by 26.7 % in the effluent compared to those of control. Incorporation of ozone before ECF bleaching BOD, COD, AOX and colour were reduced by 40.0 %, 41.1 %, 46.7 % and 57.8 %, respectively, as compared to control. Optical properties like brightness and whiteness of the pulp bleached using ozone were also improved by 2.5 units and 4.0 units, respectively as compared to that with control.

Recently, a new type of bleaching sequence, Elemental Chlorine Free (ECF) light with one D stage, has been developed. It combines the efficiency and high selectivity of chlorine dioxide (ClO2) bleaching with more environmental friendly oxygen based bleaching chemicals. This work examines the effect of pH on the formation of adsorbable organically bound halogens (AOX) in an intermediate D stage – a single ClO2 stage at the middle of an ECF light bleaching sequence. Carbon dioxide (CO2) is used to generate a bicarbonate buffer in situ, stabilizing the pH during the bleaching. Near-neutral pH is hypothesized to decrease the formation of strongly chlorinating species, so that the AOX formation is reduced. The results indicate that a near-neutral pH D stage can reduce the AOX content in the effluents with up to 30%. The ISO brightness was unchanged to a lower ClO2 consumption. The pulp viscosity was slightly higher after near-neutral pH D stage, but to its disadvantage a lesser delignification and removal of HexA was obtained. The degradation of HexA correlated well with the AOX, affirming earlier theories that HexA has a major impact on the AOX formation. The higher amounts of residual HexA and lignin resulted in more thermal yellowing of the pulps bleached with a near-neutral pH D stage.

The factors governing the brightness reversion (BR) of dissolving pulps under heat exposure are investigated. Carbonyl (CO) groups were artificially introduced on fully bleached pulp by sodium hypochlorite (NaClO) oxidation. It was demonstrated that the CO groups are responsible for loss of brightness stability (BS). These groups were partly eliminated by an alkaline extraction stage (E), which improved BS. However, an alkaline peroxide stage (P) was more efficient than E to improve BS, but without any additional CO loss. Moreover, an unbleached dissolving pulp was bleached in the laboratory by elemental chlorine free (ECF) and totally chlorine free (TCF) [ozone-based] sequences to the same brightness. The very low CO content was about the same in both cases. The ECF-bleached pulp showed substantially lower BS than the TCF pulp. These results are interpreted such that the chemistry of chromophores in the unbleached pulp also governs BS. In situ detection of phenolic and quinone chromophores in bleached dissolving pulp was performed by electron paramagnetic resonance (EPR) spectroscopy and ultraviolet resonance Raman (UVRR) spectroscopy. The content of these groups was bleaching-sequence-dependent, which may be related to the BS differences.

The effect of ECF and TCF bleaching on the surface chemical composition of kraft pulp as determined by ESCA