STUDIES ON ENVIRONMENTAL ASPECTS OF ENZYMATIC PREBLEACHING OF AGRORESIDUE PULPS

Abstract

The impact of commercial xylanase (Pulpzyme HC) on soda and oxygen delignified soda pulps of wheat straw (Triticum aestivium) and sarkanda (Saccharam munja) were studied on different bleaching sequences. The bleaching sequences studied for non oxygen pulps were CEH, D50C50ED and DED and for oxygen delignified pulps were CEH and D50C5oEpDED. Experiments were also perfOrmed on wheat straw and oxygen delignified wheat straw pulps to analyze the data on enzymatic prebleaching and to understand the manner by which the xylanase works on the substrate (xylan). A target brightness of 80% ISO is achievable for both wheat straw and sarkanda pulps using CEH, D50C50ED, DED and OCEH sequences. Higher brightness (87% ISO) can be achieved in both the pulps using the OD50C50EpDED sequence. Total bleach chemical consumption for different sequences followed the order DED>CEH>D50C50ED. BOD and COD also follow the same order but the AOX is lowest for DED sequence and the highest for CEH sequence. BOD/COD (Biodegradability of effluents) remain at the same level. The CEH sequence shows the lowest strength properties. Bleaching of oxygen delignified bleached pulps using D50C5oEpDED sequence yields pulps, which are weaker in comparison to oxygen delignified CEH bleached pulps for both the raw materials. The BOD, COD and the AOX of the combined effluents were higher for D50C50EpDED sequence The xylanase prebleaching is possible for both oxygen delignified and non oxygen wheat straw and sarkanda pulps. The optimized conditions of xylanase prebleaching are different for different pulps. Oxygen delignified pulps required lower xylanase dose compared to their respective non-oxygen pulps. There is a minor decrease in kappa number, small increase in pulp viscosity and brightness just after xylanase pretreatment for both the pulps. There is a substantial decrease in bleach chemical consumption of xylanase pretreated pulps compared to untreated pulps to obtain 80% and 87% ISO pulp brightness. The savings in bleach chemicals of xylanase pretreated pulp for a five-stage sequence (OD50C50EpDED) was lower compared to the three stage sequences. At the same bleach chemical dose, xylanase pretreatment gave pulps of higher brightness as compared to pulps without xylanase pretreatment. AOX values of combined effluent from xylanase pretreated pulps are lower for different bleaching sequences. The BOD, COD and BOD/COD of combined effluents are higher for xylanase pretreated pulps compared to non treated pulps. The xylanase pretreated bleached pulps show marginal increase in tensile, tear and burst index at same target brightness and a marginal decrease in these parameters when bleached at the same bleach chemical dose. If the cost of AOX reduction is also taken into consideration, then bleaching via xylanase route will be competitive The studies of fractionation and mechanical peeling suggest that surface xylan may not be the sole target substrate for xylanase aided bleaching though it appears that xylanase tends to hydrolyze surface xylan more due to its higher accessibility. The oxygen delignified pulp responds better to xylanase aided bleaching. This may be due to the redistribution of xylan and lignin to the outer fiber surface. The decrease in absorption coefficient after xylanase pretreatment give a evidence of xylanase attack on chromophoric groups which is further confirmed by a minor decrease in kappa number, presence of lignin in xylanase pretreated effluents by infrared ii spectroscopy and increase in UV absorbing materials determined by measuring absorbance at 280 nm. No change in light scattering coefficient and tensile index in xylanase pretreated pulps suggest that xylanase does not cause a change in fiber bonding. The water retention value which is a measure of fiber swelling show a small decrease just after xylanase pretreatment suggesting that the xylanase does not cause fiber swelling. The infrared and UV spectroscopy studies also indicate xylanase attack on LCC or lignin-hemicellulose bond. iii