Comparative study of toxic chlorophenolic compounds generated in various bleaching sequences of wheat straw pulp

This study compared the chlorophenolic compounds generated during the bleaching of wheat straw pulp. The six different bleaching sequences used in present study were: (i) chlorine-extraction-hypo (CEH); (ii) persulphate-peroxide-chlorine-extraction-hypo (Px-P-CEH); (iii) oxygen-chlorine-extraction-hypo (OCEH); (iv) chlorine dioxide-extraction- chlorine dioxide (DED); (v) persulphate-peroxide-chlorine dioxide-extraction- chlorine dioxide (Px-P-DED); and (vi) oxygen-chlorine dioxide-extraction- chlorine dioxide (ODED) bleaching. The concentrations of a large number of identified chlorophenolic compounds often exceed their threshold concentrations in CEH effluent. Therefore, the untreated effluent generated from the CEH bleaching sequence of wheat straw pulp can be considered toxic. Prebleaching (PxP) reduces the generation of chlorophenolic compounds by 36 % in the (PxP)CEH sequence. Oxygen prebleaching reduces the generation of chlorophenolic compounds by 86 % in the OCEH bleaching effluent. The PxP also reduces the generation of chlorophenolic compounds by 46 % in the (PxP)DED bleaching sequence and oxygen delignification substantially reduces the generation of the compounds by 78 %.